Prediction of feed intake in ruminants

Introduction

This paper presents some results from a study of literature on feed intake regulation in ruminants that was made as part of a research project aimed at analysing and predicting ruminant productivity in semi-arid regions.

Feed intake is probably one of the most important constraints on ruminant productivity irrespective of type of production system. Therefore prediction of intake becomes a critical issue in modelling such systems. Under semi-arid conditions the feed intake of ruminants is close to maintenance level for most of the year, resulting in weight stasis, small cumulative weight gain arising from even smaller daily gains, or weight loss. In such a situation small differences between energy intake and energy requirements for maintenance may tip the balance between production and loss. So, our ultimate interest is in the difference between two figures of approximately equal magnitude: total digestible energy intake and digestible energy requirements for maintenance.

Viewed in this way it appears almost impossible to attain any precision in predicting ruminant production from basic processes like feed consumption. However, it is certainly not a useless task, as only such efforts will reveal true limitations on productivity and effective ways to overcome them.

Predicting intake in ruminants requires an understanding of the mechanisms that regulate intake and their sensitivity to particular feed parameters. Over the last three decades much research has been devoted to elucidating intake-regulating mechanisms, with little success. As far as roughages are concerned, the typical diet of ruminants to which this paper will be restricted, emphasis has been on the existence of physical restrictions to intake. To quote Weston and Hogan (1973), it is generally assumed that, 'The ease with which the organic matter of the forage can be removed from the rumen is the most important dietary characteristic determining forage intake. The capacity of the rumen is limited and the rate of entry of feed organic matter into the rumen (rate of feed intake) cannot exceed its rate of removal. Hence the complex structure and function of the rumen, which obviate rapid removal of feed particles, can place a limit on the rate of feed consumption. It follows that forages with organic matter highly resistant to removal from the rumen are consumed in smaller amounts than those more readily degraded.' This has been repeatedly stated in slightly different forms in many articles dealing with intake regulation. Evidence for such a physical theory of intake control will not be reviewed. Here we will consider the consequences of such a theory for intake prediction given the diversity of roughages available to ruminants.

In order to predict voluntary feed consumption from feed quality two different approaches can be envisaged: either (1) detailed simulation models have to be built of those ruminal processes which are thought to limit intake, or (2) empirical relationships between intake and those feed parameters which should express the rumen filling effect at best must be used.

Currently only the second approach is widely used, although a number of detailed simulation models of ruminal processes have been developed (Mertens, 1973; Baldwin et al, 1977; Black et al, 1980; Beever et a1, 1980; France et al, 1982). Simulation studies have yielded one important, though perhaps disappointing, result: if studied at the level of ruminal dynamics, intake appears to be much more sensitive to animal parameters than to feed parameters. For instance, flow rates of water through the rumen and rumen space have a much larger effect on intake estimates than potential digestibility and rate of digestion of the feed (Mertens, 1973; Mertens and Ely, 1979; Poppi et al, 1981a; 1981b; 1981c). As neither variation in water flaw rates nor variation in rumen space can be truly explained in current models, these have to be related to the feed which is actually consumed: what was intended as an explanatory model of intake regulation has then become a description of processes related to the ingestion of feed. The finding that intake predictions are more sensitive to animal parameters than to feed parameters confirms what has long been known: animals given the same feed show substantial differences in intake. For instance, lactating animals may eat 1.5 times as much as growing, non-lactating animals. The conclusion that appetite is more important than feed quality would be justified, yet it does not solve the question of intake control. In the absence of truly explanatory models of intake regulation, empirical relations between intake and feed characteristics remain the sole instrument for predictive purposes. How accurate and reliable are they? Do we understand their nature? These aspects are briefly discussed below. The conclusion is that, at present, no simple and accurate formulas exist to predict digestible energy intake for a given category of animals. In addition, doubt is being expressed as to the usual explanation of intake-feed quality relations, i.e. as an expression of physical restrictions on the ingestion of fibre-rich diets.

A different analysis of the same data set as used for intake-fibre relations is given: instead of relating intake to certain feed parameters, intake of energy (cf. digestible matter) was related to intake of nitrogen for certain groups of feeds. Patterns of energy and nitrogen intake observed were analysed using information on protein metabolism in the rumen and the host animal. From that analysis it was concluded that the role of physical factors in intake control may be over-emphasized and that nutrient relations deserve more attention than has been given hitherto. Although shedding new light on old facts that analysis still leaves important questions unanswered. Having found that ruminants, when confronted with different roughages, appear to regulate both energy and nitrogen intake, the main question is why intake responses to supplementation with nitrogen and true protein are so unpredictable. Concluding this paper possible causes of this discrepancy are discussed.

Relationships between feed intake and feed characteristics

Numerous examples may be found in the literature of studies which correlate voluntary feed consumption of sheep and cattle to single or multiple properties of feeds. These may be of physical, chemical or plant morphological origin. However, most studies have a strong tendency to relate intake in some way to the division of each feed present between soluble, digestible or otherwise readily available or fragmentable components on the one hand and insoluble, indigestible or slowly degradable components on the other hand. Digestibility is perhaps the most frequently used parameter to assess the ingestibility of feeds. Although essentially referring to the absorbable part, the positive association between digestibility and intake has generally been considered as evidence for a physical restriction on intake. In other words digestibility has received the connotation of carrying more or less indigestible matter. The relationship with feed intake would then express a difficulty of ruminants to cope with less digestible feeds. Blaxter and his coworkers (Blaxter et al, 1961; Blaxter and Wilson, 1962) were probably the first to establish such a relationship by comparing different dried forages. Their work formed the basis for the use of digestibility as an index of feed intake in ARC handbooks (ARC, 1965; 1980). Many other researchers working with both fresh and dried forages have confirmed the positive correlation between intake and digestibility. However, individual regression formulas differ substantially.

Mertens (1973), in a major compilation of intake trials with sheep, used plant cell wall concentration as a predictor of intake (Figure 1). His interpretation of the negative correlation between intake and cell wall concentration is grossly in line with the usually accepted model of feed-intake regulation. High concentrations of cell wall in feeds would create a physical embarrassment to the animal, because they enhance the filling effect of the feed in the rumen: cell walls are slowly and only partially degraded, unlike cell contents which readily and completely disappear from the rumen by digestion and absorption. It was observed, however, that when cell wall intake was regressed upon cell wall concentration, the intake of cell wall material appeared to be lowest at both low and high concentrations and highest at intermediate levels (Figure 2).

Figure 1. Relationship between intake of digestible dry matter (DT, g/kg W^0.75/d) and cell wall concentration of the feed (I_cw/I_T, g/g) for sheep fed grasses and legumes ad libitum

Figure 2. Relationship between intake of cell wall material (I_cw, g/kgw^0.75/d) and cell wall concentration of the feed (I_CW/I_T, g/g) for sheep fed grasses and legumes ad libitum

As the bulk density of cell walls increases with age, a similar weight of cell wall material in older plant material would occupy less space. Consequently one would not expect a depression of cell wall intake at higher cell wall concentrations if rumen fill was most affected by volumetric characteristics of a feed. Thus the problem is whether the filling effect of feed and digesta has to be considered on a volumetric or weight basis, which is apparently still unsolved in physical models of intake. It is noteworthy that a similar phenomenon is observed if the intake of indigestible matter is regressed upon digestibility. This is shown in Figure 3, using data from a much larger sample of roughages, which also indicates the amount of ballast (indigestible material) an animal would be willing to consume. This can be used to predict intake if feed digestibility is known. Even in a very homogeneous sample of feeds (Figure 4), all of which were fed in the fresh form to animals of similar age, weight, sex and breed, variation in indigestible matter consumption is considerable and clearly related to feed digestibility. Using a constant faecal output to predict intake frog feed digestibility, as in several cattle production models (Sanders and Cartwright, 1979a; 1979b; Konandreas and Anderson, 1982; Kahn, 1982), seems very unsatisfactory in view of these data from sheep.

Figure 3. Relationship between faecal output of dry matter (F_T, g/kg W^0.75/d) and dry-matter digestibility of the feed (D_T/I_T, g/g) for sheep fed. grasses and legumes ad libitum

Figure 4. Relationship between faecal output of organic matter (F_O, g/kg W^0.75/d) and organic-matter digestibility of the feed (Do/Io, g/g )for Texel sheep fed fresh grasses and legumes ad libitum

For this practice of estimating intake reference is usually made to the work of Conrad et al (1964) , who fed lactating cows ad libitum rations consisting of all roughage or roughage-grain mixtures. Feed digestibility ranged from 52 to 80%. Digestible dry-matter intake was measured and related to digestibility of the ration. The pattern observed differed from the one presented in Figure 5 for sheep fed all roughage diets that there appeared to be no further increase in digestible dry-matter intake for rations with dry-matter digestibility above about 67%. Statistical analysis revealed that intake of rations with digestibility of less than 67% could be explained from a constant faecal output per kg body weight. With rations with more than 67% digestibility, differences in milk production influenced intake. So they concluded that feed intake regulation charred from physically limited below 67% to metabolically limited above 67% digestibility of rations.

Figure 5. Relationship between intake of digestible organic matter (Do,g/kg W ^0.75/d) and organic-matter digestibility of the feed (Do/Io, g/g) for Texel sheep fed fresh grasses and legumes ad libitum

However their conclusions are subject to criticism. As shown by Mertens (1973) their statistical model was inappropriate, essentially in that intake was regressed upon digestibility and faecal output. This yields very little of interest, as intake always is a true mathematical function of the chosen variables: if feed digestibility and faecal output are known, intake can be calculated. The authors do not list individual data of the experiments, so re-analysis by others is not possible, which is unfortunate in view of the efforts invested in such feeding trials. Neither is it possible to check the constancy of faecal output suggested by the authors.

Relationships between intake and feed characteristics will not be further explored. However, two conclusions are particularly relevant. First, it is obvious from Figures 1 and 3 that using single variables like cell wall percentage or digestibility can give only a very crude estimate of digestible energy intake. Real variation in intake of feeds of a given cell wall percentage or digestibility often covers a two-fold range. Such a variation will be unacceptable in many cases, especially if intake changes from below to above maintenance within that range. Second, relationships between intake and feed characteristics invariably have generated the hypothesis that the presence of structural components in feeds (cell wall material, fibre or any other analogue) produces a filling effect in the rumen, which in turn limits voluntary feed consumption in the wage-fed ruminant. As long as no alternative explanation is available to override the importance of a filling effect, it remains a viable hypothesis but nothing more than that: a close correlation between intake and cell wall concentration of feeds is, of course, also a close correlation between intake and cell soluble concentration and the interpretation is left to the biologist. In the meantime some scepticism may be expressed as to the likelihood of such a hypothesis. The interpretation of intake-fibre relations as presented above is unsatisfactory for several reasons:

• As Figures 5 and 6 show, intake of roughages, expressed either as organic-matter intake or as digestible organic matter intake, is a continually rising function of digestibility rather than a satiation function. This suggests that the limitation imposed by digestibility applies equally over the whole range of digestibility values, and that intake of even very immature forages would then be physically restricted although intake of indigestible matter is relatively low with these forages (Figure 4).
• The digestibility of roughages is usually between 40 and 80%. To get the same intake of digestible matter from the lowest-quality feed the animal would have to consume twice as much DM as with the highest quality feed. Restricting the range to feeds of 60-80% digestibility an increase of only 33% is required. Is this outside the adaptational capacity of ruminants?

• Under certain conditions animals are able to increase the intake of the same roughage, for instance during cold stress or lactation. In the latter instance, increases of up to 50% have been noted. Intake would then always appear to be a compromise between demand for nutrients and the discomfort (increased rumen distension) associated with intake, a view advocated by many workers (see McClymont, 1967). Thus, production must often be inefficient, both from a biological and an economic point.

• This inefficiency would be greatly decreased if ruminants had bigger stomachs. Especially in natural environments with strong seasonal fluctuations in forage quality and availability, high intakes and rapid growth would improved chances for survival tremendously. Relative to their body weight ruminants apparently do not have a larger gut size than non-ruminant herbivorous species (Demment and van Soest, 1983), from which one is forced to conclude that selection for this trait has not been possible during their evolutionary development, a conclusion which is hardly believable in view of the immense morphological and anatomical diversity between animal species.

Figure 6. Relationship between intake of organic matter (Io, g/kg Wa^0.75d) and organic-matter digestibility of the feed (D_O/I₀, g/g) for Texel sheep fed fresh grasses and legumes ad libitum

Patterns of energy and nitrogen intake in sheep

In addition to parameters like cell wall percentage and digestibility, the use of nitrogen content in estimating intake of digestible matter (which will be considered here a measure of energy intake) has also been suggested. Elliott and Topps (1963), examining a range of feeds of low to medium quality, found dry-matter intake to be mare closely correlated with nitrogen concentration than with digestibility of the feed. The same would apply to intake of digestible dry matter. Siebert and Hunter (1977) proposed a regression equation between digestible organic-matter intake of growing cattle fed mixtures of spear grass and luoerne and nitrogen concentration to be used to estimate intake of grazing cattle from oesophageal sample analysis.

Comparing different relationships reveals that important differences exist and that the use of such relationships has to be restricted to the range of forages from which they were obtained. Thus the question arose as to whether, for a given type of stock, some upper limit of intake of digestible matter could be set for any given nitrogen concentration in the feed. From published data on feed intake of growing cattle it became apparent that such an upper limit indeed does exist but that it is of limited use. It was also soon evident that it is mare informative to relate intake of digestible matter to nitrogen intake. A first analysis of this relation dealt with data from growing, non-lactating cattle (Ketelaars, 1983). That analysis comprised only a limited number of intake data from a wide range of breeds and weights of animals.

Many more data on intake are available for wether sheep, which are commonly used in trials to compare the ingestibility of a range of feeds. In addition, much experimental work on intake regulation has also been confined to sheep, and wether sheep of 30–70 kg body weight are probably the best-studied ruminant animals as far as feed intake and intake regulation are concerned. So it was decided to concentrate on data for sheep to further develop the conceptual model, of which a crude representation was given by Ketelaars (1983).

Sample description

Data on voluntary intake of roughages by wether sheep were collected from the literature. The criteria of selection were that, apart from intake, nitrogen concentration of the feed, in vivo digestibility and weight or metabolic weight of the animals were known. Only data from roughages will be used to present patterns of energy and nitrogen intake. These include grasses and legumes, fed either alone or as mixtures of both. Supplements allowed were minerals and vitamins. Feeds supplemented with protein or nitrogen source were excluded as were ground and pelleted roughages.

Data were obtained from Demarquilly and Journet (1967), Demarquilly and Weiss (1970), Heany et al (1963), INRA (1978), Mertens (1973), Milford (1960; 1967), Milford and Minson (1968a; 1968b), Minson (1967; 1973), Minson and Milford (1967; 1968), and Minson et al (1964).

Data were available on 766 different feeds. In reality the number of trials will have been larger as the French data (Demarquilly and Weiss, 1970; INRA, 1978) are averages for a certain feed tested in several trials. Of the total of 766, 32 (less than 5%) were discarded because they did not fit the general pattern of intake shown later. In 21 cases intake of digestible dry matter was substantially less than expected for the digestibility class in question, and for the remaining 11, intake was unusually high.

Of the 734 feeds which were left, 573 were grasses and 161 legumes. Mixtures of grasses and legumes were classified according to the dominant fraction. Rations were fed in the fresh or dried form, long or chopped. Together they comprise a large number of species of temperate, tropical and subtropical origin.

In selecting data no distinction was made as to breed of animal used. Breed was not always recorded but the range spanned typical wool breeds like the Merino and typical meat breeds like the Texel. Exact weights were not always given. Mean weight of the animals used in these trials will probably fall between 40 and 50 log. Actual weights will have shown wider variation, probably between 30 and 70 kg liveweight.

Special attention must be drawn to the data collected by French workers for fresh pasture grasses and legumes (Demarquilly and Weiss, 1970; INRA, 1978), which have been used in this paper to illustrate various trends in intake. It is the largest homogeneous subsample of the total data available for which essential additional information could be found on protein metabolism in the rumen and the host animal. Therefore these data will be analysed separately but are also included in the analysis of the total sample. Figures 7 and 8 show the relationship between in vivo dry-matter digestibility and nitrogen concentration of the feeds for the total sample, and the relationship between in vivo organic-matter digestibility and nitrogen concentration of the feeds for the subsample. For most of the data digestibility was reported on a dry matter basis, and digestible dry matter intake has been chosen as the parameter to be used in the presentation of the total sample data. French workers, however, consistently reported digestibility on an organic-matter basis, and this is preferred as it is a more precise parameter of energy intake. Conversion between organic-matter and dry matter digestibility is not straight-forward, as it depends on ash concentrations and digestibility of ash and these are often not reported. Where conversion was necessary, e.g. for inclusion of the French data in the total sample, information was used from data given by Troelsen and Campbell (1969) . In their trials with 36 hays of temperate grasses and legumes the regression equation between in vivo dry-matter digestibility (D_T/I_T, g/g) and in vivo organic-matter digestibility (D_O/I_O, g/g) appeared to be: D_T/I_T = 1.00 D_O/I_O – 0.0146 with r = 0.99. So, if necessary, D_T/I_T has been estimated from D_O/I_O by subtracting 0.015 g/g.

Figure 7. Relationship between in vivo dry-matter digestibility (D_T/I_T, M) and nitrogen concentration in the dry matter (I_N/I_T, g/g) of grasses and legumes fed to wether sheep of various breeds

Figure 8. Relationship between in vivo organic-matter digestibility (Do/Io, g/g) and nitrogen concentration in the dry matter (I_N /I_T, g/g) of fresh pasture grasses and legumes fed ad libitum to wether sheep of the Texel breed

Using the same conversion and assuming a mean ash concentration of 0.10 g/g of dry matter it can be calculated that each gram of digestible dry matter will be roughly comparable to 0.92 gram of digestible organic matter.

Results of the total sample

Patterns of intake of digestible dry matter and nitrogen for the total sample are presented in Figure 9, 10 and 11. Figure 9 shows all the data. Figure 10 shows data for the 0.59–0.61 g/g digestibility class, which comprised the largest number of feeds, with the linear regression line. Figure 11 shows the regression lines for digestible dry-matter intake as related to nitrogen intake after feeds had been grouped according to digestibility class.

Figure 9. Relationship between intake of digestible dry matter (D_T, g/kgW^O.75/d) and intake of nitrogen (I_N, g/kg W ^0.75/d) for wether sheep of various breeds fed grasses and legumes ad libitum

Figure 10. Relationship between intake of digestible dry matter (D_T,g/kg W^0.75/d) and intake of nitrogen (I_N,g/kg W ^0.75/d) for wether sheep of various breeds fed grasses and legumes with dry-matter digestibilities (D_T/I_T ;g/g) in the range of 0.59 to 0.61

Figure 11 . Regression lines relating intake of digestible dry matter (D_T ,g/kg W ^0.75/d) to intake of nitrogen (I_N ,g/kg W ^0.75/d) at different levels of dry-matter digestibility of the feed (D_T/I_T, g/g )

Digestibility classes were first chosen so as to get a reasonable number of data in classes with average dry matter digestibility of 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75 and 0.80 g/g, without too much variation in digestibility within classes. In view of the experimental variation in in vivo digestibility measurements, digestibility classes seem sufficiently narrow to relate the results to the mean of the classes. As a consequence digestibility classes in between the first mentioned are narrower. Regression equations are listed in Table 1.

Table 1. Regression equations relating intake of digestible dry matter (D_T, g/kg w^0.75/d) to intake of nitrogen (I_N, g/kg w ^0.75/d) for sheep of different breeds fed grasses and legumes of different dry matter digestibility (D_T/I_T,g/g)

*Number of feeds

Discussion

From Figure 7 it is evident that the total sample comprises the entire range of roughage qualities ranging from less than 0.30 g/g dry matter digestibility and less than 0. 005 g/g nitrogen in the dry matter to more than 0.80 g/g dry matter digestibility and 0.045 g/g nitrogen. A second important observation is that the correlation between in vivo dry matter digestibility and nitrogen concentration of the feed is very weak: in other words, the often rather close association between these parameters observed in aging plant material of the same type and from the same location almost disappears if feeds of widely different origin are studied.

The data from the French subsample shown in Figure 8 are typical for the diet of ruminants during the main grazing season in western Europe, with immature grasses in spring having up to 0.036 g/g nitrogen (22.5% protein) and over 0.80 g/g organic-matter digestibility. Even when flowering, grasses have nitrogen concentrations above 0.01 g/g and organic-matter digestibility over 0.55 g/g.

At first sight, the nature of the relationship between intake of digestible dry matter and nitrogen as depicted in Figure 9 is not very clear. Fitting some kind of saturation function curve would have been possible but would give little help in the interpretation. The pattern becomes much clearer if feeds are grouped on the basis of digestibility as shown in Figures 10 and 11. Orderly increases in intake of digestible dry matter with nitrogen intake became visible at each level of digestibility, although there is still considerable scatter of the data points. Linear relations have been fitted to these increases as they appear, on average, to be the best representation. Only data for feeds of between 0.50 and 0.55 g/g digestibility showed a more curvilinear trend. As slopes of the regression equations for feed intake below maintenance level (approximately 30 g D_T/kg W^0.75/d) tend to be higher than above it, such curvilinearity might result from the charge in feeding level within this data group. If so, a broken line would perhaps be more appropriate.

Individual regression lines for legumes and grasses separately are not shown. Inspection of the data in Figure 10 reveals that the higher intake of legumes as compared with that of grasses of similar digestibility can be attributed to the higher nitrogen concentration of the former. Within both grasses and legumes similar increases in intake of digestible dry matter with nitrogen intake are present.

The main conclusion that can be drawn from the total sample is that if roughages are classified according to digestibility then voluntary intake of digestible dry matter by sheep increases linearly with nitrogen intake within such classes. For predictive purposes this means that high protein feeds are eaten in greater amounts than low-protein feeds, irrespective of digestibility level. For our understanding of intake regulation, clearly the most important question is what does digestibility mean to the ruminant animal? This question will be examined with the help of data from the subsample.

Results of the subsample

Data from the subsample for sheep or the Texel breed fed fresh pasture grasses and legumes are given in Figures 12, 13 a-g, and 14 and Table 2. Figure 12 shows the complete data set, a total of 137 feeds. Also shown is a curve representing the maximum intake of digestible organic matter for a certain range of nitrogen intakes (abbreviated D_{O max} –curve). Its derivation is discussed below.

Figure 12. Relationship between intake of digestible organic matter (D_O, g/ kg W ^0.75/d) and intake of nitrogen (I_N,g/kg W ^0.75/d) for wether sheep of the Texel breed fed fresh pasture grasses and legumes ad libitum. The solid line shows the calculated maximum intake of digestible organic matter above maintenance as related to nitrogen intake. The broken line shows the estimated maximum intake of digestible organic matter for each level of nitrogen intake below maintenance level

Figure 13. Relationships between intake of digestible organic matter (D_O ,g/kg W ^O.75/d) and intake of nitrogen (I_N, g/kg W^0.75/d) for Texel wethers fed fresh pasture grasses and legumes with organic-matter digestibilities (Do/Io, g/g) of (a) 0.61–0.63; (b) 0.64–0.66; (c) 0.67–0.69 ; (d) 0.70–0.72 ; (e) 0.73–0.75; (f) 0.76–0.78 ; (g) 0.79–0.81. Regression lines are based only on data for grasses

Sources: Demarquilly and Wesis(1970) and INRA (1978).

Figure 14. Regression lines relating intake of digestible organic matter (Do,g/kg W^0.75/d) to intake of nitrogen (I_N, g/kg W^0.75/d) at different levels of organic-matter digestibility of the feed (D_o/I_o,g/g ). This is a composite of the regression lines shown in figures 13 a–g. Also shown is the curve for maximum intake of digestible organic matter from figure 12

Table 2. Regression equations relating intake of digestible organic matter (D_O,g/kgw ^0.75/d) to intake of nitrogen (I_H, g/kg w ^0.75/d) for sheep of the Texel breed fed fresh grasses of different organic matter digestibility (D_O/I_O,g/g).

*number of feeds

Figures 13 a – g show data for individual classes of digestibility. Only data for grasses were used in the calculation of the regression lines. Figure 14 gives a composite picture of the regression lines together with the D_{O max}–curve. Finally, Table 2 includes the details of the regression equations.

Discussion

The pattern of linear increases in intake of digestible matter with intake of nitrogen observed in the total sample is confirmed by the subsample. For the grasses, linearity is present at each level of digestibility and scatter of points is reduced. The latter is partly the result of excluding variation due to breed, form of feed, location etc, and partly due to the fact that these data are averages for a certain feed tested in a number of trials. Some sources of variation will be analysed later.

Another observation is that slopes of the regression lines become smaller at higher digestibility levels. With regard to the difference between legumes and grasses, it now appears that intake of highly digestible legumes (i.e. above 0.72 g/g organic-matter digestibility) can be predicted from intake of grasses taking into account differences in nitrogen concentration, but for lower-quality legumes discrepancies exist: intake of these legumes is less than would have been expected on the basis of intake of grasses and their nitrogen concentration.

The observed pattern of energy and nitrogen intake in sheep resembles similar patterns in young monogastric animals, in which low-protein feeds usually depress energy intake. Analysing a number of feeding trials with rats, dogs and chickens, it was found that if intake of energy is depressed by low protein/energy ratios in the feed, increasing this ratio by adding protein to the feed results in linear increases of both energy intake and nitrogen intake. At the same time liveweight gain and nitrogen retention show similar proportional increases. From these observations it was hypothesised that the intake-depressing effect of low protein energy rations in monogastric animals may be functionally explained by an inability of the animal to produce liveweight gain if the diet contains less than a certain concentration of protein. As maintenance requirements are mainly composed of energy and liveweight gain requires relatively more protein, intake of energy above maintenance is dependent on increased protein energy ratios of the feed. If this hypothesis is correct, maximum allowed intake of energy as a function of protein intake can be calculated from (1) maintenance requirements for protein and energy, (2) the minimum protein energy ratio in liveweight gain and (3) the efficiency of use of feed protein and feed energy by the animal. The intake data for sheep show intake of energy (cf digestible matter) for most feeds, if not for all, below the maximum that can be attained with these animals. Apparently, intake of energy was limited by some factor and for the moment it will be assumed that this factor was availability of true protein. With this assumption, potential energy intake versus nitrogen intake can be calculated according to above mentioned hypothesis, as explained below.

Availability of true protein is taken as the amount of non-ammonia nitrogen which reaches the duodenum. This amount can be quite different from the quantity of protein ingested. For many feeds this will be lower, due to losses of protein in the rumen, while for others it will be more, due to conversion of endogenous nitrogen (mainly ureum in saliva) into bacterial protein. Such changes in availability of protein between mouth and duodenum clearly hamper the interpretation of intake patterns in ruminants. One way to handle this problem is to calculate the D_{o max}–curve, for which it is assumed that efficiency of conversion of ingested protein into true protein reaching the duodenum is maximal. This is done in the following manner.

• Requirements of energy far maintenance have been set at 425 kJ M_E/kg W ^{0. 75}/d, the value proposed by Ψrskov (1982). Using an efficiency of 0.81 for conversion of digestible energy to metabolisable energy and an energy value of 19.1 kJ/g digestible organic matter, energy requirements are equivalent to the ingestion of 27.5 g D_o/kg W ^0.75/d. However, energy requirements for maintenance vary between breeds, ages and seasons. Effects of such differences will be analysed later.

The energy value of digestible organic matter is the mean calculated for fresh pasture grasses using data from INRA (1978).

• Requirements of protein for maintenance continue to be a matter of debate as they are difficult to measure in ruminants. Here they have been set at 0.4 g ingested N/kg W^O.75/day. As Figure 9 shows, this amount is the minimum that allows consumption of 30 g " D_T/kg W^O.75/d, which is equivalent to 27.5 g D_O/kg W ^O.75/d. In addition, a value of 0.4 g on a metabolic–weight basis compares favourably with recent estimates of protein requirements obtained with intragastric infusion techniques. From nitrogen excretion of ruminants on nitrogen-free infusates, Ψrskov (1982) estimated minimum needs for protein in tissue maintenance to be about 300–400 mg N/kg W ^O.75/d; the higher value would apply to younger animals, the lower value to older animals. Intake of 0.4 g feed nitrogen produces a flow of non-ammonia nitrogen into the duodenum of approximately 0.6 g. Assuming efficiency of nitrogen use to be similar for maintenance and productive processes, i.e. 0.54 (see below) this would provide 324 mg net amino-acid nitrogen to the tissues.
• Efficiency of conversion of ingested nitrogen into animal products has been based on data from infusion experiments with rumen bacterial mass in lambs (data cited by Ψrskov, 1982). Net efficiency of total microbial nitrogen appeared to be 0.54. Incremental efficiency of ingested nitrogen will, therefore, never be more than 0.54 unless undergraded plant protein has a higher biological value. As there is no evidence, for the latter, the value of 0.54 will be retained. The fact that this figure was obtained with lambs may introduce some error. Synthesis of wool protein is more important in older sheep than synthesis of body protein. If efficiency of use of absorbed protein is less for synthesis of wool, overall efficiency would shift to lower values.

It is noteworthy that Boekholt (1976), in N-balance experiments, found a maximum incremental efficiency of 0.75 for retention of digestible nitrogen in lactating dairy cows. As true digestibility of feed nitrogen was 0.83 g/g, maximum incremental efficiency of ingested nitrogen would amount to 0.62, a value 15% higher than the value of 0.54.

• Composition of production above maintenance, i.e. liveweight gain + wool, was not known for the specific animal breed and physiological stage. Instead, data on nitrogen retention were available from a paper of Grenet and Demarquilly (1977). They measured nitrogen retention in the same type of animals (wether sheep of the Texel breed) given 23 different grass samples and 3 lucerne samples ad libitum, all in the fresh form, data from which are directly compatible with the data on intake. Only information from the grass samples is used here. Grenet and Demarquilly (1977) related nitrogen retention to various parameters in single- and multiple-regression analyses. Using single parameters, intake of digestible organic matter was found to be the best predictor of nitrogen retention. Th regression equation between nitrogen retention (R_N, kg/d) and intake of digestible organic matter (D_O, kg/d) was;

                  RN = 0.01875 D_O – 0.0104 (r = 0.86.)

Multiple regression with other parameters (digestibility of organic matter, concentration of crude-fibre and soluble carbohydrates in the feed and nitrogen intake) could not improve this prediction equation substantially.

From this finding it is concluded that, irrespective of level of ad libitum intake of digestible organic matter, animals produced a product which required a fixed ratio of digestible energy and net protein. As Tolkamp (personal communication) showed, utilisation of metabolisable energy for net energy is constant in ad libitum fed animals (i.e. 0.60) and metabolisable energy is a fairly constant proportion of digestible energy (i.e. 0.81), and it is concluded that composition of total product had a constant ratio of protein to energy. It should be noted that exactly the same conclusion was reached for monogastric animals under conditions of protein-limited intake of energy.

The regression equation cited above is expressed on a per animal basis. Assuming 55 kg as the mean weight of the animals, R_N equals zero at an intake of digestible organic matter of 27 . 5 g/kg W ^0.75/d which coincides with the proposed maintenance requirements for energy.

The slope of the regression line gives the increase of retained nitrogen per gram of extra digestible organic matter ingested. Conversely, it also incorporates the maximum incremental increase in intake of digestible organic matter as related to nitrogen intake: 28.8 g ^D_O/g R_N. The latter value is obtained by dividing the maximum efficiency of conversion of ingested nitrogen into retained nitrogen (0.54) by the incremental increase of retained nitrogen per gram of digestible organic matter intake (0.01875 g/g).

Summarising, the function giving maximum intake of digestible organic matter for intakes of nitrogen equal to or more than 0. 4 g/kg W^0.75/d can be written as:

^D_{O max} = 16.0 + 28.8 I_N, I_N > 0.4

with ^D_{O max} as maximum intake of digestible organic matter in g/kg W ^0.75/d and I_N as intake of nitrogen in g/kg W^0.75/d. This would apply up to the satiation level for intake of digestible organic matter.

The course of the ^D_{O max} –curve below maintenance level could not be established in a similar way as for above maintenance: the concept of a fixed composition of weight gain cannot be readily interpreted for a situation of increasing weight losses. It is also evident that, below maintenance, intake measurements are more time dependent: intake of feeds which do not sustain maintenance often declines with time. Extrapolation of the D_{O max} –curve below maintenance level has therefore been based on information from Figures 9 and 15. Figure 9 suggests that intake of digestible organic matter and nitrogen both converge towards zero with decreasing feed quality. The outer border of the scatter of points in Figure 15, which relate intake of dry matter to nitrogen concentration of the feed, would also indicate that animals refuse feeds with nitrogen concentrations below 0.003 g/g. The fact that feeding nitrogen-free diets usually results in cessation of eating within one week (Orskov, 1982) was another reason to extrapolate the curve ^D_O max towards the origin. In view of the functional meaning of this curve it would mean that sheep voluntarily stop consumption of feed energy in the absence of feed protein, not because of disturbance of digestive processes.

Figure 15. Relationship between dry-matter intake (I_T, g/kg W ^0.75/d) and nitrogen concentration of the feed (I_N/I_T, g/g) for wether sheep of various breeds fed grasses and legumes ad libitum

The ^D_{O max} –curve has been included in Figures 12 and 14. From Figure 12 it is clear that intake of digestible organic matter is close to this curve for only a few feeds and that the majority of intake data are to the right of it. From the assumptions made above the most likely cause is that efficiency of conversion of ingested nitrogen into retained nitrogen is less than maximal for most feeds. This explanation is supported by data on levels of ruminal ammonia in the animals that supplied the nitrogen retention data. These are shown in Figure 16 with the voluntary intakes of digestible organic matter for the respective feeds. Information in this graph is extremely useful as it allows a large number of conclusions:

• Lowest values for ruminal ammonia concentrations are found close to the ^D_{O max} –curve, almost approaching zero for data on the curve itself. As the presence of ammonia in the rumen inevitably is accompanied by losses of nitrogen from the ruminal system, due to the rapid passage of ammonia across the ruminal wall, feeds with intake on the ^D_{O max} –curve are obviously those with maximal efficiency of nitrogen conversion.
• Intake of nitrogen far most feeds will overestimate true protein availability for the host animal due to variable losses of feed protein as ammonia in the rumen.

• The ^D_{O max} -curve was constructed assuming that intake is protein limited in a similar fashion as was observed in growing monogastric animals, not hindered by bulk limitations to intake. Consumption of feeds with measured intake of digestible organic matter close to this curve can therefore be considered protein-limited. However these are also the feeds for which availability of ruminal nitrogen has been generally considered the limiting factor on rates of digestion and hence on feed intake. From this presentation of data there appears to be sufficient evidence to state that the limitation to intake of such feeds is a true protein limitation of the host animal and not a physical limitation on the through-put of feed across the rumen.

• Increases in intake of digestible organic matter along the ^D_{O max}–curve are made possible by changes in the ratio in which digestible organic matter and true protein are absorbed by the host animal. For ruminant animals this ratio will also reflect the ratio in which organic matter is digested and microbial protein concurrently synthesised. The latter is usually referred to as the efficiency of microbial protein synthesis, being the amount of microbial protein synthesised per gram of apparently digested or ruminally-fermented organic matter. The expression based on ruminally-fermented organic matter is the most common and will be used here also.
• The organic matter apparently fermented in the rumen is a constant fraction of the total digested organic matter (Ulyatt and Egan, 1979). For roughages ranging between 0.45 and 0.85 g/g digestibility of organic matter it appeared to be on average 0.61 g/g of total digested matter.

Figure 16. Rumen ammonia levels (mg NH₃–N/100 ml rumen fluid) of wether sheep of the Texel breed fed fresh pasture grasses and legumes ad libitum. Ammonia levels are shown next to data points. Compare figure 14 with figure 16 for interpretation of nitrogen metabolism in the rumen and related intake patterns

Assuming a certain fraction of feed protein to be rumen-degradable (i.e. available for microbial protein synthesis) for the feeds with intake at the D_{O max} –curve then the increased intake of digestible organic matter along this curve must be accompanied by an increase in efficiency of microbial protein synthesis; in other words more feed nitrogen is converted to microbial protein flowing from the rumen per gram of fermented organic matter. This is illustrated by data in Table 3 which shows the concentration of nitrogen in feed at which rumen ammonia starts to accumulate. Concentrations were estimated for feeds with assumed intakes at the intersection points of the regression lines with the ^D_{O max}–curve (for instance points B₁ and C₁ in Figure 17). The nitrogen concentration at which ammonia starts to accumulate shows a two-fold increase for feeds between 0.62 and 0.80 g/g organic matter digestibility.

Table 3. Nitrogen concentration of feed and intake of nitrogen (I_N, g/kg w^0.75/d) and digestible organic matter (^D_O, g/kg W ^0.75/d) at point of ammonia accumulation for increasing organic matter digestibility (^D_O/I_O, g/g).

^a per cent nitrogen in the digestible organic matter.

^b per cent nitrogen in the organic matter.



Figure 17. Graphical representation of intake patterns of Texel wethers fed fresh pasture grasses ad libitum

• Feeds with intake at the ^D_{O max}–curve belong to a group differing in digestibility and nitrogen concentration. In addition they are characterised by differences in efficiency in synthesis of microbial protein in the rumen. The interrelationship between these parameters will now be examined.
• Figure 16 shows that rumen ammonia levels increase parallel to the ^D_{O max}–curve. As far as synthesis of rumen microbial protein is concerned, values between 2 and 5 mg NH₃ N/100 ml appear to be critical. According to the work of Satter and Roffler (1981) efficiency of microbial protein synthesis does not increase further once this critical level is attained. For non-roughage feeds higher values may apply. Comparing Figures 16 and 14 it can now be concluded that, within each group of feeds with similar digestibility, there is one particular feed nitrogen concentration which allows maximum efficiency of microbial synthesis. Higher concentrations of feed nitrogen show immediate increases in levels of ruminal ammonia above this critical level. Therefore they do not induce higher efficiency of synthesis of microbial protein within this group of feeds.

• The fact that intake of digestible organic matter continues to increase for higher concentrations of feed protein within the same class of digestibility must mean that availability of true protein to the host animal also increases along this axis: only changes in the ratio of digestible organic matter and true protein availability would allow such increases in energy intake. If, microbial protein synthesis in the rumen does not contribute to this greater availability of true protein then the only source which remains is undegraded feed protein. As feed protein is never completely degradable the undegraded part can provide the animal with relatively more duodenal non-ammonia nitrogen once the capacity for microbial protein synthesis is saturated. The slope of the regression lines relating intake of digestible organic matter to intake of nitrogen for feeds of similar digestibility will therefore in some way reflect the response of sheep to an increased availability of undegraded plant protein.

Among these conclusions two elements are clearly of great interest: the relationship between efficiency of microbial protein synthesis and feed digestibility, and the response of animals to undegraded plant protein as compared with bacterial protein. Both aspects have been quantified. In principle, efficiency of microbial protein synthesis could be directly calculated from the ^D_{O max}–curve if it was known what proportion of feed protein is rumen-degradable. Only this and some endogenous nitrogen may contribute to microbial protein synthesis. However, degradability of plant protein and microbial protein synthesis are still little understood. Therefore, instead of assuming some average value for protein degradability another approach was chosen: it was assumed that irrespective of source of protein, whether of microbial origin or of plant origin, both would have a similar effect on the intake of digestible organic matter. In other words it was assumed that at the duodenal level a unique relationship exists between intake of digestible organic matter and non-ammonia nitrogen flow, each increment of non-ammonia nitrogen provoking the same increment in intake of digestible organic matter. This allowed estimates to be made of both efficiency of microbial protein synthesis and protein degradability. The way these have been derived will be shown elsewhere. Here only the results are given, summarized in Table 4.

Table 4. Microbial protein synthesis (g/l00g FOM^a) and protein degradability (g/g) calculated for feeds of increasing organic matter digestibility (^D_O/I_O ,g/g).

a/ FOM: organic matter apparently fermented in the rumen.

The data in Table 4 clearly show an increase in efficiency of microbial protein synthesis with higher feed digestibility: for highly digestible roughages relatively more microbial protein per gram digested organic matter becomes available to the host animal. Degradability of feed protein also appears to increase from low to high digestibility feeds, which means relatively greater losses of feed protein.

The theoretical calculations in Table 4 could not be adequately checked against values reported in the literature. According to a comparison made by Orskov (1982), most protein evaluation systems currently available use average values for microbial protein synthesis. Values range between 1.25 and 1.38 g N/MJ M_E or approximately 19–21 grams of protein per 100 grams of ruminally-fermented organic matter. Such values are in the middle of the range estimated here. A major cause of this lack of differentiation between feeds is the absence of sufficient reliable measurements. Data for any well-defined category of feeds, such as fresh forages fed ad libitum, are few and obtained by different techniques. The latter prevents any real comparison, as different methods in the same situation yield widely diverging results. However, it is noteworthy that Hagemeister et al (1981), working with lactating dairy cows, reported consistently lower values for microbial protein synthesis with high than with low-roughage rations.

A similar situation applies to the values calculated for protein degradability. Although protein degradability of fresh forages is generally assumed to be high, techniques to measure this parameter independently from the contribution of microbial protein in total duodenal protein flow are not available.

Despite this lack of validation it seems unlikely that the trend in efficiency of microbial protein synthesis is very different from that shown in Table 4. On theoretical grounds one also may expect a lower efficiency with increased concentration of indigestible matter in the rumen. As indigestible feed particles serve as the major carrier of microbial mass in the rumen, retention times of microbes in the rumen will tend to increase with less digestible feeds, resulting in increased turnover of microbial matter within the rumen and lower efficiency of microbial protein synthesis. Therefore, to return to our basic question, 'what does digestibility mean to the ruminant animal?', perhaps a more important aspect than its physical meaning may be the close association between digestibility and relative availability of microbial protein. That would also explain why animals which produce a product containing less-protein for instance, lactating dairy cows - may eat more of a given feed per kg of metabolic weight than animals producing a high-protein product, e.g. wether sheep.

Reasoning along this line it is tempting to suggest that the opportunities to utilise a certain protein energy ratio will set the limit for intake of roughages. If so, one would expect that supplemental protein should induce consistent changes in intake. But, although intake responses to protein supplementation have been found, they are not consistent. For example, Egan (1977) experimented with infusions of casein into the duodenum of sheep fed a range of different feeds. For same feeds moderate increases in intake were found as a consequence of casein infusion, but these were generally feeds which did not support maintenance without supplementation. Above maintenance, responses were almost negligible.

A similar situation exists with regard to additions of protein to the diet. Although more difficult to analyse, because of the uncertain effects on true protein availability in the duodenum, responses are inconsistent, ranging from nil to substantial.

So, the question clearly is: why are energy intake patterns so easily interpreted as responses to protein availability, whereas in practice such responses cannot be satisfactorily reproduced by manipulating true protein availability?

No clear answer has yet been found. One could hypothesise that the effect of protein availability on intake of digestible dry matter is determined by other factors. As ruminants receive a large proportion of their protein in bacterial mass, a whole array of substances will present themselves at the duodenum along with the protein. Also, plant protein which reaches the duodenum undegraded will certainly be mixed with non-protein or even non-nitrogen substances. Many of these substances may have very different nutritive properties to proteins or amino-acid mixtures. Thus, any relationship between digestible dry matter intake and nitrogen intake in ruminant nutrition is inevitably very unclear and potentially misleading.

Summarising, the following conclusions can be drawn. Intake data which are usually presented in a form to support the idea of physical regulation of intake fit equally well into a picture of a metabolic regulations. The latter would imply that ruminants, like monogastric animals, regulate both energy and nitrogen intake when given a range of feeds. Such a regulation, of course, must be reflected in the composition of animal products. It is tempting to suggest that the requirements of the animal for a certain balance of nutrients set limits to the intake of most roughage rations. That animals are sensitive to the balance, or imbalance, of nutrients absorbed from the gut is evident from intake responses to supplementation with various nutrients. However, such responses do not follow a pattern to be expected from nutrient relations in the intake of roughages. Thus, further research is needed to clarify the nature of nutrient relations in roughage feeding and supplementation trials. It would be extremely valuable to understand where, when and to what extent intake and thus production can be increased with definite inputs of essential nutrients, especially for many extensive livestock-production systems in semi-arid regions. To be able to predict such increases in productivity would make modelling livestock production systems much more meaningful.

Summary

Feed intake is a major determinant of ruminant productivity. Accurate prediction of intake is therefore important in modelling ruminant livestock production systems. Prediction of intake of roughages by ruminants has usually been based on a concept of physical restrictions to the ingestion of fibre-rich diets. However, explanatory models of intake incorporating this concept are not available. Use of empirically-based equations to predict intake from feed characteristics is inaccurate.

Re-analysis of numerous intake trials reported in the literature, both with sheep and with cattle, suggests that physical factors in intake regulation have been overemphasised. An important finding is that ruminants, given a wide range of roughages ad libitum, appear to regulate both energy and nitrogen intake. Above maintenance level this results in a proportional excess of energy and true protein available to the host animal, which, in turn, leads to a relatively constant composition of animal products under such feeding conditions. This suggests that intake is a response to relative nutrient availability from the gut and therefore can be manipulated by dietary nutrient additions. However, a consistent framework of intake responses to supplementation with essential nutrients is lacking. Possible causes of this discrepancy are discussed.

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Discussion

Statement – When you turn your graph around, you have the biological relationship of nitrogen retention as a function of TDN intake.

Reply – That's the usual way we present such a relationship, but I think that the animal reacts to the availability of protein. The energy intake is a function of protein intake, and that leads to a relationship between energy intake and nitrogen balance.

Question – For predictive purposes, would it not be better to correlate dry matter, digestible energy intake, or digestible dry matter with N% of the feed?

Answer – It would only be a trick. This relationship shows that if you characterise a feed by the ratio of digestibility and nitrogen content, the intake of such a feed will always develop along an almost straight line from the origin. If you hold digestibility constant, the intake of digestible material is a function of nitrogen percentage.

Question – Do you assume the same digestibility of protein as of total organic matter, or do you check the digestibility of protein independently? The two are not always the same.

Answer – I did not use digestibility figures for protein at all, because digestible protein is very difficult to interpret. You can use true protein concentrations, in combination with some measurement of rumen ammonia concentration, rather than digestible nitrogen because it is quite uncertain haw much of the digestible nitrogen is indeed digested by the animal.

Statement – Deficiency of amino acids can reduce digestibility. For instance, ground-nuts are not a good source of protein.

Reply – The effective amino acid profile supplied to the animal and the possible deficiencies could be a factor that explains differences in results of protein supplementation trials.

Questions – 1. Have you any particular reason for using metabolic weight, rather than liveweight in your measurements? It is used for maintenance but should it be used for intake?

2. You said that you found no experiments that showed faecal output was constant at different digestibilities. We have found that the faecal output at dry pasture and green pasture was identical, even though the intake was almost double in the green pasture.

3. How would you then predict intake, when you have ad lib availability of straw and poultry litter?

Answer – To start with your first question, there is no particular reason to use the exponent of 0.75. I use it to put animals of different weights on a similar scale.

If you stick to an explanation of requirements for production and maintenance, then intake should be related to the same exponent as maintenance requirements.

The second question regarding faecal output: I do not deny that you can find similar figures for faecal output in some cases, but I have looked at a wide range of data from sheep trials. There is a large variation and also a certain trend.

Your third question: At the moment we are not able to predict possible increases in intake due to non-protein N supplementation. Some people suggest using measurements of rumen ammonia as an indicator of the possible benefits, including non-protein nitrogen, but even if you know that the rumen ammonia concentrations are very low, say below 5 mg/100 ml, you can not say anything about the possible increases in intake that could be expected by including non-protein nitrogen.

Question – Would you expect the animal to obtain its requirements if it had ad lib access to these two feeds?

Answer – Yes, but the problem is to decide how much extra nitrogen can be effectively converted to bacterial protein. That depends on other characteristics of the ration, mainly on the potential digestibility of the total ration. The higher the digestibility the greater the quantity of nitrogen in the ration that can be effectively converted to microbial protein.

Question – The ILCA model usually over-estimates growth by about 100–250 kg. Would you predict that we would reduce the output if we put in your type of protein relationship?

Answer – I have not tried that so I can not specifically answer your question, but I have tried to convert the figures from sheep trials into intake figures for growing cattle using the idea that you construct the intake curve by looking at specific requirements for maintenance in terms of nitrogen and energy, and then by looking at the conversion of liveweight gain. The problem is that you have to find the intersection points, where the regression equation for a certain class of digestibility intersects this curve. It is also the point where rumen ammonia starts to accumulate. For every digestibility class there is one specific concentration of nitrogen below which feed nitrogen is converted to bacterial protein, and above which nitrogen is lost. The problem is that predictions are very sensitive to the slope of that curve and to assumptions about that critical protein content.

Question – It would seem reasonable to assume that some physical limit to processing rate must exist in ruminants. Would a combination of the traditional theory of some physical processing rate together with your energy/protein ratio theory explain the data that do not fit this hypothesis alone?

Answer – It is very difficult to combine both concepts. I can not see any way to do that.

Statement – Even if the feed has the correct protein/energy ratio for a given animal product and physiological state of the animal, and if you have a very low quality feed with a very low digestibility, an intake limit would be imposed by the physical processing potential of the animal.

Reply – It is very difficult to prove that such a limit exists. If you look at trials with monogastric animals, they have an enormous capacity to adjust for ballast in the ration. For instance you can feed rats with 50% indigestible matter as long as you can keep the protein/energy ratio in the digestible part at an appropriate level.

Comment – In the case of the ruminant, I think that there is quite good evidence (from Minson's lab, for example) that for the same level of digestibility and for the same nitrogen content, the feeds which are more easily and more rapidly broken down are in fact eaten in greater quantity. So I think the analogy of the ruminant and the ruminant is not a fair way to answer that last question.

Reply – It's true that feeds that are more rapidly comminuted are also eaten in greater quantities, but if you look at the work of Minson et al, their conclusion is that the rate of particle breakdown can not be a very severe limit to intake. That is illustrated by the fact that if you look at the particle size distribution in the rumen, there are many more small particles, even with low-quality rations. The water passage through the rumen could have a much mare profound effect on intake than the rate of particle breakdown. In principle, the animal should have a larger capacity to increase its intake by simply increasing the water flow through the rumen. Whatever the reason, some animals eat much more of a given feed than other animals, so the physical limit mist be very flexible.

Comment – Salt bushes for instance, have a high nitrogen content, but dry matter intake of these is low. The reason seems to be the low digestibility of the dry matter, which is something below 60%. True digestibility of protein is something like 90% so it seems that nitrogen is being used as an energy source by the microbes that need the correct proportionality between the available nitrogen and available carbon.

Reply – Yes, that is true, but the fact that an animal eats more of high-protein feeds than of low-protein feeds is ultimately due to an action of the animal itself. It is true that the microbes also need a certain ratio of protein and energy, but if you have feeds which supply sufficient nitrogen to the microbes, but not enough energy, the intake of such feed still increases. That can be explained by the fact that only a certain fraction of the total feed protein is degradable in the rumen by microbial action and there is always a certain undegradable fraction. That is the fraction that will raise the true protein availability to the host animal.

Comment – These bushes have a very high crude protein content, which is highly degradable in the rumen, and is used as an energy source. Because the energy is not readily available, intake is low.

Reply – The true protein available to the host animal is mainly a function of the amount of microbial protein. Where you have, like in the salt bush, relatively low dry matter digestibility, then the contribution of microbial protein will also be small, because there seems to be some functional relationship between the efficiency of microbial protein synthesis and the digestibility of the feeds. The lower the digestibility of the feeds, the smaller the amount of microbial protein which becomes available per gram of digestible organic matter. To say something about the additional value of increased protein in salt-bushes, you have to compare salt-bush material of different protein concentrations. Then I suppose you will find some increase in intake with higher N content because not all the protein will be rumen degradable.