TRAITS OF ECONOMIC IMPORTANCE
CURRENT STATUS AND FUTURE PROSPECTS
CROSSBREEDING AND SELECTIVE BREEDING
FEEDING SYSTEM DURING TEST
A. Shoniwa and K. Dzama
Pig Industry Board, Box HG297, Arcturus, Zimbabwe and Department of Paraclinical Veterinary
Studies, University of Zimbabwe, P. O. Box MP 167, Mt Pleasant, Harare, Zimbabwe.
The Pig Industry Board is responsible for national evaluation of pigs. Pigs are tested using their own performance for litter size at birth, feed conversion efficiency and backfat thickness. A selection index is used to rank the animals. The accuracy of selection in the present system is low and it can be improved by using progeny testing and statistical techniques such as Best Linear Unbiased Prediction to compute estimated breeding values for the traits in question. Introducing progeny testing may also require more extensive use of Artificial Insemination so that boars can be tested widely. Under the current performance testing scheme, testing of animals should be done under the same environment they will experience on the farm.
In pig production there are many traits of economic importance and improving these traits requires the improvement of the environment together with genetic improvement. The genotype of an animal sets a ceiling to which the animal can be improved through manipulation of the environment. It is, therefore, important to compliment advances made in management practices by improving the pigs genetically.
The traits of economic importance include growth, carcass traits, reproductive traits and feed conversion efficiency (FCE).
Producers need animals with high growth rates. Fast growing animals result in reduced maintenance feeding and this has a positive effect on the producer's margin. Fast growth, while being desirable for many other reasons, also results in a lower housing charge per pig. Fast growing animals have, however, been reported to show a high incidence of leg problems (English, et al., 1988). It is, therefore, important when selecting breeding stock to take into account the undesirable correlation between growth rate and leg weakness. Good strong legs are essential for the breeding animals. A highly prolific sow is of no use if it cannot stand for the boar and a good boar is of limited use if it cannot mount a sow on heat.
The market demands leaner carcasses so the producer and breeder should strive to produce leaner pigs. Not only are leaner carcasses desirable from the consumer point of view but they are also cheaper to produce. Very lean carcasses should, however, be avoided. Genetically very lean pigs may have problems when they enter the breeding herd because they,will farrow with little fat reserves. Sows farrowing with very little fat reserves are more likely to have problems during and after lactation (Mullan and Williams, 1989). A sow suckling a large litter and with little fat reserves will lose a lot of condition during lactation and is more likely to have rebreeding problems. It has been reported that the success achieved in reducing the amount of fat in pigs has been done at the expense of appetite (English et al., 1988). Part of the mechanism by which breeders have achieved a reduction in carcass fat content has been by subconsciously selecting pigs with lower appetites. English et al. (1988) also reported that some genotypes with high lean and low carcass fat contents are unable to grow lean tissue at a sufficiently fast rate because of limited appetite. Breeding females with limited appetite will have problems meeting the requirements of lactation especially if they are nursing large litters. It is therefore important when selecting breeding stock to ensure that the animals selected have reasonable appetites.
Some genotypes which are extremely lean have been reported to have a higher incidence of porcine stress syndrome which causes pale, soft and exudative meat (English et al., 1988). Apart from having meat quality problems these strains of pigs also have lower feed intake and poorer reproductive performance. Care should be taken when importing semen.
Because of the low heritability of reproductive traits eg litter size, little effort has been put to try to improve these traits through selection. Traits of low heritability, have been improved through a combination of crossbreeding and manipulation of the environmental factors. Today, heterosis and management influences have been exploited extensively to improve reproductive performance. It is now wise to try to improve reproductive performance through selective breeding. When selecting highly prolific sows it is important to select animals with desirable teat spacing and numbers. Unless the extra piglet produced by the highly prolific sow is able to get a teat to suckle the benefits of (such highly prolific) sows will be lost. For highly prolific lines it may be wise to select animals with 14 or more functional teats.
In pig production feed accounts for about 75% of the total production costs. It is, therefore, important to target FCE for genetic improvement. A more efficient animal is desirable because it has the potential to reduce production costs. In addition more efficient feed converters tend to produce leaner carcasses.
Currently the Pig Industry Board (PIB) has the mandate for central testing of pigs in Zimbabwe. Pigs are evaluated based on their own performance. The animals sent for testing are preselected for litter size at birth and at weaning by the breeder. At the end of the on-station test, an index incorporating FCE and backfat thickness is used as a basis for selection. Reproductive traits in general, and litter size in particular have low heritabilities (.10). As such the accuracy of predicting the breeding value of an animal for litter size is fairly low. (ie x.10 = .32) The accuracy of predicting the breeding value of an animal for a trait of heritability .10 and repeatability .25 using the animal's own performance and an infinitely large number of its records is 63% (Van Vleck et al., 1987). The accuracy of selection for FCE (heritability 0.3) is .55 and the accuracy for selecting for backfat thickness (heritability .5) is .71 under the PIB performance testing programme. Clearly there is need to incorporate litter size into the index and to improve the accuracy of predicting the breeding values of the animals for the traits selected for. For the traits emphasised in this programme, progeny testing of boars and sows lends itself as a method of choice to increase accuracy of selection.
Initially progeny testing was used to evaluate the genetic merit of pigs at PIB. Progeny testing was abandoned because it was considered costly and it also took more time to assess the animals compared to using the animal's own performance. Despite these demerits, progeny testing is the best method given the traits being selected for in the PIB test. It substantially increases the accuracy of selection for traits with low heritabilities like litter size. The accuracy of prediction of additive genetic value from records on 30 progeny would be twice as large (.66) as that of predicting breeding value from the animal's own record (.32). In addition progeny testing is important for evaluating traits which are sex limited eg litter size or traits where destructive sampling is involved eg carcass traits.
If progeny testing is reintroduced at PIB it may become necessary to promote extensive use of Artificial Insemination (AI). This will enable boars to be used faster and more widely thus generating larger databases for more accurate evaluations. Currently the use of AI in the industry is almost nonexistent. The economic benefits in terms of increased FCE, improved litter sizes and leaner carcass derived from accurately selecting pigs of high merit at national level far outweigh the disadvantages of progeny testing.
Progeny testing will result in generation of large data sets which will inevitably need to be stored on a powerful computer. Data on animals' progeny and relatives will not only be collected on station but also on farm through the existing PIB multiplication scheme. Even though data collection and record keeping at the P1B is meticulous, there is need for computerisation. The database created will be analysed with powerful statistical tools to generate estimated breeding values (EBV) for the traits of interest. An EBV is an estimation of the genetic value of an animal. It indicates its value as a parent. EBVs can be updated as more information on the animal's progeny and relatives become available. EBVs can be used to construct a selection index incorporating the breeders choice of traits. The current selection index in use at the PIB needs to be revamped along these lines.
One tool which can be used to compute EBVs is called Best Linear Unbiased Prediction (BLUP). These are mixed model equations which take into account the heritability of the trait, the amount of information available for each boar or sow, the genetic level of the herd, genetic trend and non-genetic factors such as management groups. Hoste (1994) reported that when BLUP assisted selection is compared with selection based on an animal's records, for traits of low heritability (.10) such as litter size, the response to selection is increased by up to 30% compared with improvements in selection response of 5 to 10 % for traits of moderate (.40) heritability.
Crossbreeding is widely utilised in the pig industry to utilise hybrid vigour and complementarity. However, unless properly planned schemes are put in place economical heterosis will not be achieved thus crossbreeding will be a financial liability (Dzama, 1994). There is evidence from PIB data that in poorly planned schemes where crossing is done haphazardly, the advantage of using crossbred sows to take advantage of maternal heterosis is very small or non existent (Mungate et al., 1995, unpublished). The PIB should develop and recommend various crossbreeding schemes to producers. EBVs obtained from the proposed progeny testing scheme should be used when selecting animals to cross. One way of doing it may be to develop specialised sire and dam lines. In trying to improve reproductive traits it may be beneficial to concentrate on improving reproductive performance in the dam lines while concentrating on improving growth, FCE and carcass traits in the sire lines.
In Zimbabwe selection of breeding stock on the performance testing programme is based on the restricted feeding system.. Most producers raise their fatteners on an ad libitum feeding system. It is therefore important to test animals under a feeding system which their progeny will experience on the different farms. When one tests animals under a restricted feeding system one will be limiting the genetic potential of some animals. Testing of animals on an ad libitum or "to appetite" basis has its disadvantages. As the pig proceeds to satisfy its appetite on an ad libitum system, its FCE will start to deteriorate and its lean content to decline because of increasing fat deposition (English et al., 1988). Thus selection for carcass lean content and FCE on ad libitum feeding will tend to favour pigs with a lower appetite. As reported earlier pigs with lower appetites have serious problems during and after lactation. An alternative system of testing which helps to prevent the reduction of appetite associated with feeding-to-appetite systems of testing is therefore desirable. English et al. (1988) reported a scheme first put forward by Professor Kielanowski and Dr Kotarbinska of Poland to be the most appropriate system upon which to base selection of breeding stock. This scheme is based on a time scale feeding system using a feed scale close to ad libitum. Under this system all pigs receive the same ration each day and are on test for the same period. The pigs which will have gained most weight by the end of the test will have the best combination of ability to consume their ration, FCE and lean tissue growth rate.
There is clearly a need to improve the reproductive performance, growth rates and carcass characteristics of pigs in Zimbabwe. Whether this goal is achieved depends on how accurately we select superior pigs for breeding. Certainly PIB provides a sound base from which to launch the proposed improvement programmes.
Dzama, K. (1994) J. Zimbabwe Soc. Anim. Prod. (in press).
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