|Title||Genetic aspects of piglet survival|
|Source||Wageningen University. Promotor(en): J.A.M. van Arendonk; T. van der Lende. - S.l. : S.n. - ISBN 9789058084187 - 121|
Animal Breeding and Genomics
|Publication type||Dissertation, internally prepared|
|Keyword(s)||varkens - dierveredeling - selectief fokken - biggen - mortaliteit - neonatale sterfte - genetica - pigs - animal breeding - selective breeding - piglets - mortality - neonatal mortality - genetics|
Piglet mortality is high. In the USA nearly 20% of the piglets do not survive between late gestation and weaning; 7% of the piglets die during farrowing and some 13% are lost during lactation. These statistics from the USA are no exception to the norm. Selection for increased piglet survival, if possible, could have an important economic impact.
Litters and sows
Data on some 33.000 litters and some 400.000 piglets from a commercial breeding program (TOPIGS, Vught) were available for analysis. The heritability estimate for litter size of 0.09 from this data fits nicely with literature estimates. For the trait litter size good genetic progress is possible at a rate of 0.15-0.25 piglets per litter per year, given a proper information and selection infrastructure.
It is therefore reassuring that the heritability of piglet survival is 0.06, not too far from 0.09. Heritability of uniformity in birth weight is 0.07. The genetic correlation of uniformity with survival is clearly positive (0.3), indicating that selection for more uniformity will increase survival and vice versa. Heritability of average birth weight of a litter is relatively good (0.3), with remarkably little influence from the sire of the litter (0.02). Genetic correlation of birth weight with piglet survival was very low.
The data were reanalyzed on an individual piglet basis, with ample consideration for crossfostering. Cross-fostering of piglets is not a random process. Cross-fostered piglets are, on average, smaller than non-cross-fostered piglets. Cross-fostering is supposed to increase the average survival probability of piglets. Light piglets are moved to sows with an expected high mothering ability, proven in previous lactations or with an excellent udder. These sows, however, have an increased possibility of loosing piglets, since they get the small and weak piglets. In the survival analyses a simultaneous estimation of the effects of birth weight, biological mother, piglet's genotype and nurse sow effect was performed. Unfortunately, this analysis is too demanding for the current state of the technique. The discrimination between the effects of the biological mother and the nurse sow was especially difficult, since the number of piglets that are cross-fostered (say 12 %) was limited. Less demanding analyses showed influences of all three genotypes (sow, nurse sow and piglet), very roughly in equal parts. A good and interpretable model is a model with the genotype of the piglet and the genotype of the nurse sow.
Does selection for increased survival work?
Low heritabilities and intermediate genetic variation were found. Breeding values were estimated for litters of gestating sows, subsequently realized survival was recorded and compared with the estimates. Results were very good; the top fifty percent of the litters had an expectation of 4-5 % higher survival, compared with the bottom fifty percent. Realized survival in the low group was 79 % in a dam line and 81 % in a sire line, while survival in the high group was 84 % and 84 %, respectively. In the sire line, average birth weight in the high group was 70 g lower than in the low group, explaining why the realized difference in the sire line was not the predicted 4-5 %.
In Figure 1 the relationship between birth weight and piglet survival is given for two groups of piglets, one group with a high genetic expectation for survival and one group with a low expectation. The form of the curve is similar for both groups, but the group with the high expectation is shifted to lower birth weights. At the same weight small piglets of the high group survive much better than small piglets of the low group.
Figure 1: Relationship between birth weight and survival in a group of piglets with a high genetic expectation for vitality () and a group with a low expectation (o-o).
Genetic correlations with finishing traits
The goal of pig production is to produce good quality lean meat as efficiently as possible. Therefore the relationships between piglet survival, birth weight and mothering ability on the one side and finishing traits on the other have been investigated. The conclusion was that the relationships are quite important. Vital piglets eat more, grow faster and produce more fat in their finishing phase. This means, looking at it the other way around, that genetically lean animals will be weaker in their perinatal phase. In literature, there are more indications of this phenomenon than originally expected. Body reserves of piglets seem to play an important role. Heavier piglets perform better during finishing and selection for lean gain will increase birth weight.
It is concluded that genetic variation for piglet survival exists and that selection for increased survival is possible. For a successful selection program it is necessary to record survival, sole recording of birth weight is not sufficient. It appears that advantage can be gained from a good attribution of the genetic variation to the sow, the nurse sow and the piglet. A proper attribution is only possible when cross-fostering percentage is high. However, if only the genotype of the piglet is considered, marked differences in piglet survival can already be seen between contemporary animals with a high and low genetic expectation for piglet survival.
Many traits have genetic relationships with piglet survival; especially traits in the growing phase have important genetic relationships with piglet vitality. The results from this thesis show that it is possible to combine selection for increased piglet survival with genetic improvement in other economically important traits.