Effect of stocking rate on the botanical composition and nutritive value of diets selected by West African Dwarf goats on farmer-managed stylo fodder banks in central Nigeria

R.M. Njwe¹, A. 0. Ikwuegbu², G. Tarawali² and D.A. Little²

¹University of Dschang, B.P. 222, Dschang, Cameroon
²ILCA, PMB 2248, Kaduna, Nigeria

Abstract

Smallholder farmers in the subhumid zone of Nigeria keep small flocks of goats, which need to be tethered or otherwise confined during the growing season to avoid trespass and damage to crops. Being constrained for six months or more, reduces mobility and feed intake resulting in lower goat productivity. To solve this problem, some farmers established small stylo-enforced fallows on which they confined their goats during the cropping period. For these fallows to retain sufficient stylo over several years, appropriate stocking rates of goats need to be determined.

This paper reports on a on-farm stocking rate trial, involving four farmer-managed stylo-fallows of 0.10 to 0.16 in size, which were stocked at 24, 42, 52 and 81 goats per hectare over a 3-month period from August to November 1992. Pasture components (stylo, grasses, forbs) were determined on three dates by plot sampling. On the same dates, diet composition and its nutrient content were derived from extrusa of oesophageally fistulated goats. Data on the composition of pasture and diet allowed the calculation of a selectivity index for the three pasture components.

Average sward composition was 54% grasses, 32% stylo and 13% forbs. Overtime, forbs increased and grass declined from 60% in August to 45% in late October, while stylo content rose slightly across stocking rates. Stylo was preferred at all times, rising from 50% of the diet in early September to 80% in mid October but declined when leaf fall started. Throughout the period, the selectivity index of stylo was unity, averaging 2.44 across stocking rates and sampling dates.

Crude protein content in the diets declined with stocking rate from 14.8 to 12.8% and with time (P<0.05) from 16.2 to 12.8%. Degradability averaged 73%, and decreased (P<0.05) with increasing stocking rate but not with time. Given these results, the optimum stocking of 40 goats/ha (800 kg LW/ha) was recommended which would guarantee a high proportion of stylo in the fallow sward and in the diet.

Introduction

Smallholder farmers at Abet keep mainly goats as part of their crop-oriented production system. During the cropping season goats are tethered on small parcels of pasture or fallow in order to avoid crop damage; this tends to affect their productivity adversely. In response to this problem several farmers have sown Stylosanthes hamata in fallow to create small fodder banks that aim at improving crop yields through maintaining soil fertility and at the same time ensure adequate feeding of small ruminants during the cropping period (Ikwuegbu and Ofodile 1992). Under these on-farm conditions, stocking rates vary greatly and some fodder banks are overstocked implying that animals grazing them may not have sufficient feed during the wet season. Overstocking of fodder banks may have adverse effects on botanical composition and nutritive value of the selected diet. Understocking tends to encourage nitrophilous grasses thereby ousting the more nutritious legumes. The objective of the trial was therefore to assess the effect of stocking rate on the botanical composition of the sward and on the nutritive value of the diet selected by goats grazing farmer-managed fodder banks.

Materials and methods

Four fodder banks (A, B, C and D) — 0.12, 0.11, 0.16 and 0.10 ha in size — were grazed during three months (August through to early November) at average stocking rates of 24, 42, 52 and 81 adult goats/ha, corresponding to 480 to 1620 kg liveweight or 1.9 to 6.5 TLU per ha; across the grazing period rates varied somewhat, as farmers removed or added animals to their flocks.

During September and October, diet composition of grazing goats was sampled by three oesophageally fistulated animals at two-weekly intervals. Prior to sampling, they were fasted for 16 hours (i.e. as from 5 pm). The fistulated goats were allowed to graze for 30 minutes on each fodder bank. After the bag was detached and emptied, the extrusa sample from each animal was weighed, transferred into a clean muslin cloth, gently squeezed to remove excess saliva (where necessary), and then mixed thoroughly with a spatula. A subsample of about 5 g was removed for estimation of proportions of stylo, grass and forbs, using a modified version of the microscope point technique described by Hamilton and Hall (1975). The subsample was again washed several times with water to remove excess saliva, and then transferred into muslin cloth in order to eliminate water. The resulting biomass was spread thinly at 100 linear plots for identification of grass, stylo and forbs in the diet using a binocular microscope at 15Χmagnification. A frequency distribution chart was made for each plant species from which the relative frequency in the diet was calculated as a percentage.

The botanical composition of the fodder bank was analysed each time the oesophageal fistula samples were collected except on the 64th and 78th day of sampling when due to logistic reasons no samples were collected. In each paddock, 6 quadrats (1Χ1 m²) were thrown at random, in which total herbage was cut and separated in the three components. These were weighed and subsampled for DM content. Proportions were expressed as a percentage of total DM weight. The selectivity index of grass, stylo and forbs was estimated as follows: Selectivity Index: Proportion in the diet (%) divided by proportion in the sward (%).

The other subsample of the extrusa was dried in a forced air oven at 60°C, ground in a laboratory mill fitted with a 1 mm sieve and preserved for chemical analysis. Extrusa samples were analysed for nitrogen using the Kjeldahl method (AOAC 1965) while neutral detergent fibre (NDF) was determined according to the procedures of Van Soest and Wine (1967). Calcium and magnesium was determined by atomic absorption spectrophotometry as described by Pinta (1980). Extrusa dry matter disappearance was estimated using the nylon bag technique as described by Mehrez and Ørskov (1977).

Data on botanical composition (per cent grass and stylo) were transformed to arcsin and analysed according to the general linear model procedure (SAS 1985); the independent variables were grazing period and stocking rate. Duncan's multiple range test (Steel and Torrie 1960) was applied to the transformed means to compare treatment and then transformed back to the original means. Values of nitrogen, NDF, Ca and Mg content and dry matter degradability of extrusa samples were analysed using the general linear model procedure without transformations.

Results

The average floristic composition across fodder banks consisted of 54% grasses, 32% stylo and 13% forbs (Table 1). Fodder banks differed somewhat, bank B having the highest stylo content (45%) and bank C the highest forbs content (19%). Changes over time showed a rapid increase of forbs, whereas grass content declined from about 60% in August to 45% in late October. Stylo became slightly more important towards the end of the season (Table 1).

Table 1. Botanical composition of pasture swards and diets of goats at four stocking rates on farmers' stylo fodder banks at Abet, Nigeria.

Diet composition of grazing goats was variable (Table 1). However, average values showed that grass content increased with rising grazing pressure, whilst stylo in the diet decreased (Table 2). Across time, trends were less clear. Grass content in the diet were high when in the early stage of growth, decreased during mid season with a upturn towards the end. The average content was 23% ranging from 44% to 16%. A reverse trend was observed for stylo. Early in the season 50% of the diet consisted of stylo rising rapidly to almost 80% in early October to decline when the start of the dry season caused extensive leaf fall (Table 3).

Table 2. Effect of stocking rate on the mean proportion of grass, stylo and forbs in extrusa of goats grazing stylo fodder banks of farmers at Abet, Nigeria.

Table 3. Effect of grazing period on the proportion of grass, stylo and forbs in extrusa of West African dwarf goats grazing stylo fodder banks at Abet, Nigeria.

The data of the selectivity index were rather erratic (Table 4), indicating that the relationship between feeds on offer and in the grazed diet was not strong. Preference for grasses was relatively high at low and high stocking rates (0.90), and low at medium stocking rates (0.30). No obvious trend across stocking rates was apparent in the stylo ratings; the average scores ranged from 1.7 to 3.5; goats preferred stylo at all times except in bank D at the start and in bank A at the end of the grazing season (Table 4). On average, the rating of forbs reached unity, but preferences were erratic, indicating that goats went for specific species at a defined stage of growth.

Table 4. Selectivity indices for grass, stylo and forbs in farmer-managed fodder banks grazed by goats at different stocking rates during the growing season at Abet, Nigeria.

Crude protein declined somewhat with increasing grazing pressure but more steeply from 16.2% to 12.8% with increasing maturity of the herbage (Table 5). The average NDF was 55% and changed little across time or between fodder banks. Dry matter degradability declined with grazing pressure from 74.5 to 67.5% but did not fall with increasing herbage maturity. The upsurge on day 46 (to 77.8%) appears to coincide with a high stylo in the diet of 78% (Table 3). Calcium and magnesium contents were not affected by grazing pressure or date of sampling.

Table 5. Effect of stocking rate and grazing period on the nutritive value of diets selected by goats grazing stylo fodder banks at Abet in the subhumid zone of Nigeria (% DM).

Values with the same superscript are not significantly (P>0.05) different.

DMD = dry matter degradability; SR = stocking rate; GP = grazing period.

Discussion

At all stocking rates goats consistently preferred stylo to grass, probably because legumes contain more protein than grasses. At the higher stocking rate of 52–81 head/ha, goats manifested a higher selectivity for stylo and forbs than for grass (Table 4). Mellado et al (1991) reported similar preferences for goats grazing in a semi-arid region in Mexico.

Langlands and Bennet (1973) reported from studies of Merino sheep grazing pastures that the digestibility of dry matter declined by 4% when the stocking was increased in steps of 20 sheep per hectare. In contrast, the present study shows that there was an initial increase in degraded extrusa by 4% when the stocking rate was increased from 24 to 42 goats/hectare; stocking at 68–98 goats/hectare resulted in a significant decline of over 10% in degradability. However, higher stocking rates resulted in small reductions in the crude protein content in the diets goats, similar to the observation made by Langlands and Bennet (1973).

Conclusions

It has been shown that during the rainy season stylo pasture can be heavily stocked without markedly affecting the composition and nutritive value of the diet. However, the end of the growing season, stylo content of the sward declined from 46% in the two lower to 30% in the two higher rates (Table 1), an optimum rate of 40 goats or 800 kg livestock can safely be recommended. A similar recommendation emerged from other grazing trials, indicating that growth of goats and sustained forage production was assured in paddocks stocked at 800 kg/ha over three months (Ikwuegbu et al 1994).

References

AOAC (Association of Official Analytical Chemists). 1965. Official Methods of Analysis. 10th edition. AOAC, Washington, DC, USA.

Hamilton B.A. and Hall D.G.1975. Estimation of the botanical composition of oesophageal extrusa samples. LA modified microscope point technique. J. Brit. Grassld Soc. 30:28–34.

Ikwuegbu A.O. and Ofodile S.N. 1992. Wet season supplementation of West African Dwarf goats reared under traditional management in the subhumid zone of Nigeria. In: Ayeni A.O. (ed), Production System in the Humid Tropics. Proceedings of an International Workshop Held at Obafemi Awolowo University, Ile-Ife, 5–9 June 1992. Purdo, Wageningen. pp. 195–201.

Ikwuegbu O.A., Tarawali G. and Iji P. 1994. Sustaining a crop–livestock farming system in the subhumid zone of Nigeria by matching feed from improved Stylosanthes-based pastures and livestock production. In: de Leeuw P.N., Mohamed-Saleem M.A. and Nyamu A.M. (eds), Stylosanthes as Forage and Fallow Crop. ILCA (International Livestock Centre for Africa); Addis Ababa, Ethiopia. pp. 167–174.

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Mellado M., Foote R.H., Rodriquez A. and Zarate P. 1991. Botanical composition and nutrient content of diets selected by goats on desert grassland in Northern Mexico. Small Ruminant Research 6:141–150.

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Effet du taux de charge sur la composition botanique et la valeur nutritive des plantes sélectionnées par des chèvres naines d'Afrique de l'Ouest élevées sur des banques fourragères de stylo gérées par les paysans au Nigéria

Résumé

Les petits paysans de la zone subhumide du Nigéria élèvent de petits troupeaux de caprins qu'ils doivent maintenir au piquet ou en espace clos pendant la période de végétation pour les empêcher de s'introduire sur les cultures et de les endommager. Le confinement pendant six mois ou plus réduit la mobilité des caprins et leur consommation alimentaire, entraînant ainsi une baisse de leur productivité. Pour résoudre ce problème, certains paysans ont installé de petites jachères de stylo sur lesquelles ils maintiennent leurs chèvres pendant la période de végdtation. Pour permettre à ces jachères de produire des quantités suffisantes de stylo sur plusieurs années, il convient de déterminer le taux de charge approprié.

Cet article présente les résultats d'une étude du taux de charge conduite en milieu réel sur quatre jachères d'une superficie de 0,1 à 0,16 ha, exploitées par des paysans, sur lesquelles ont été parqudées 24, 42, 52 et 81 chèvres par hectare pendant la période de 3 mois allant d'août à novembre 1992. La composition du pâturage (stylo, graminées et autres herbacées) a été déterminée à trois dates différentes au moyen d'un échantillonnage des parcelles. A ces mêmes dates, la composition botanique et la valeur nutritive des aliments ingérés ont été déterminées à partir d'extrusas contenus dans des fistules introduites dans l'oesophage des chèvres. Les données de la composition du pâturage et des rations ont permis de calculer un indice de sélectivité des trois composantes du parcours.

La prairie était compos2e en moyenne de 32% de stylo, 54% de graminées et 13% d'autres plantes herbacées. Au fil du temps, la part de ces dernières a augmenté alors que celle des graminées a diminué de 60% en août à 45% fin octobre, et que celle du stylo a augmenté lentement indépendemment du taux de charge. Le stylo était à tout moment la plante préférée, sa part passant de 50% des aliments ingérés début septembre à 80% à la mi-octobre avant de baisser dès que la chute des feuilles a commencé. Au cours de la période étudide, Findice de sélectivité du stylo était égal à l'unité avec une moyenne de 2,44 pour l'ensemble des taux de charge et des dates d'échantillonnage.

La teneur en protéines brutes des rations baissait (P<0,05) avec l'accroissement du taux de charge (de 14,8 à 12,8%) et le temps (de 16,2 à 12,8%). Le degré de dégradation, qui atteignait en moyenne 73%, baissait (P<0,05) avec l'accroissement du taux de charge, mais pas au fil du temps. Au vu de ces résultats, un taux de charge optimal de 40 chèvres/ha (800 kg de poids vif/ha) a été recommandé car il permettrait de garantir une proportion élevée de stylo dans la jachère et dans la ration.