SELECTION FOR REPRODUCTION AND PIGLET SURVIVAL

Transcription

1 SELECTION FOR REPRODUCTION AND PIGLET SURVIVAL Egbert F. Knol 1 Arjan Neerhof 2 1 Geneticist, Ph.D., IPG, Institute for Pig Genetics, PB AA Beuningen, the Netherlands; Egbert_Knol@IPG.NL 2 Geneticist, M.Sc., TOPIGS International, PB ZH, Helvoirt, the Netherlands; Arjan.Neerhof@TOPIGS.com 1 Introduction Selection for litter size in the pig is possible. The review of Haley et al. (1988) can be regarded as the end of the discussion. Over time genetic parameters have been calculated for different lines and different parities (e.g. Irgang et al., 1994), all refining the idea that selection for litter size is possible. Selection for litter size is not without problems, however. One of the best examples is the classic selection experiment in Nebraska (Johnson et al, 1999), where 14 generations of selection to increase litter size were performed, mostly on an index of ovulations and prenatal survival. This resulted in a sizeable response in number of ovulations, in an interesting and significant increase in number of live born piglets per litter and in a (non significant) decrease in number weaned per piglet, indicating a concomitant increase in mortality. If this experiment is exemplary for selection programs of breeding companies then litter size will increase, but problems in piglet mortality also and the net result might be negative. An important explaining factor for the net zero result in the selection experiment is the negative genetic correlation between litter size and piglet survival. E.g. Hanenberg et al. (2001) found positive genetic correlations between litter size and number stillborn and negative correlations between litter size and mothering ability, results being consistent over parities. In this article we would like to give an update on current research results and possible approaches to realise a substantial and sustainable genetic trend in reproduction. 2 Definitions and complications Important genotypes for reproduction are the genotype of the sow, of the boar, of the piglet and of the nurse sow. Genetic analysis of litter size is mostly done using the genotype of the sow, but the genotype of the service sire is known to be important. A stillborn piglet is a piglet found dead and wet behind the sow, but it is rarely known if it died before, during or after the real expulsion process. A piglet might well be alive at birth, but crushed within a few minutes and consequently be addressed as stillborn. In Brazil the percentage of stillborn piglets appears to be lower than in other countries, probably due to the labour situation. Crossfostering is a non-random process; small and/or weak and/or heavy and/or strong piglets are crossfostered in order to maximise 1

2 the average survival of the piglet. A sow with a high mortality record might have produced low quality piglets or might have received low quality piglets or might have a low mothering ability. In this article we will use vitality (%) for the survival quality of a piglet, mothering ability (%) for the potential of a sow to raise piglets, farrowing survival (%) as the complement of stillbirth, and similarly preweaning survival (%) as the complement of preweaning mortality. Finally, piglet survival is the survival probability of a piglet from late gestation to weaning and is thus the mathematical product of farrowing survival and preweaning survival. Farrowing survival is the percentage of fully formed foetuses that survive the farrowing process. 3 Birth weight and combined selection for litter size and birth weight The relation between birth weight and survival has been studied and reported by many authors (e.g. Fireman and Siewerdt, 1997). There is a fair agreement that survival chances of a piglet drop quite dramatically below 1.00 kg, with an exception of piglets born from Chinese sows. Figure 1 is just an example and taken from our own work (Knol, 2001). It is almost impossible not to come to the conclusion that piglet survival will benefit from a genetic increase in birth weight. Or in other words, this very strong, clear and often repeated phenotypic relation between birth weight and survival must result in a similar genetic relation. This will make a selection strategy for increased litter size and, simultaneously, increased birth weight necessary. Birth weight is heritable and should be modelled with a direct/maternal model. In such a model most of the genetic variation by far stems from the maternal effect (Roehe, 1999), indicating that the sow is responsible for the weight of the piglets and that the service sire, the father of the piglets is quite irrelevant for the size of the piglets. We derived genetic para-meters for a number of reproduction traits, which were found consistent with literature values. Then we simulated a 9 year 50%/50% selection strategy (strat. 1) on litter size and birth weight (Knol, 2001) and found a zero expected trend for litter size, a zero trend for survival (!), and a 300 g increase in birth weight (Table 1), and concluded that selection for birth weight will not increase survival, but will counteract the selection for litter size. 4 Genetic variation in survival traits Piglet survival is a difficult trait to analyse because of the binary nature of the trait. There are only two options, a piglet is alive at weaning or it is dead. A piglet can have good genes for survival but be crushed by accident by the sow, or, another, piglet can have bad genes, but can be saved by a loving caretaker. Therefore large numbers of observations and good statistical models are necessary for a proper analysis at individual piglet level. After applying these models, genetic variation in survival traits appears to exist, with low heritabilities, but interesting genetic variation, indicating that selection for increased survival is a realistic possibility. 2

3 Preweaaning survival, % ,0 0,5 1,0 1,5 2,0 2,5 3,0 Birth weight, kg Figure 1 Phenotypic relation between birth weight and piglet survival. Problems arising from the binary nature of piglet survival are smaller when analysed at the litter level, heritability of survival is then around 0.05 (see e.g. review of Rothschild and Bidanel, 1999). Other proof of the heritability of survival traits can be found in Siewerdt and Cardellino (1996), Grandinson et al. (2000) and Lund et al. (2000). 5 Combined selection for litter size and survival Currently the TOPIGS data set on piglet survival consists of some observations on individually weighed and crossfostered piglets and is analysed with a direct/foster sow model, which results in breeding values for vitality for the piglets and breeding values for mothering ability for the sows. Since both breeding values are estimated simultaneously they are corrected for one another, taking care of the non random nature of crossfostering (a gilt with an excellent udder might receive a lot of small and weak piglets and wean most of them. If some of the piglets die the breeding value of the gilt for mothering ability will drop, unless the quality of the piglet is accounted for). In Figure 2 a large group of piglets is divided into a high and a low group on the basis of their pedigree breeding value for survival. The genetic high group of piglets survived significantly better than the low group at almost every birth weight. Interestingly there is some evidence that selection for increased survival will lead to fewer very heavy piglets, therefore increasing uniformity. This decrease in variation in birth weight can be found also in a frequency decrease of very heavy placentas in sows carrying piglets of a high genetic merit. 3

4 Preweaning survival, % ,0 0,5 1,0 1,5 2,0 2,5 3,0 Birth weight, kg Figure 2 Phenotypic relations between birth weight and piglet survival for a genetic high survival and a genetic low survival group of piglets. Back to the primary question of substantial and sustainable genetic trend. A selection strategy was simulated for an index based on litter size and survival (see Table 1). This strategy (strat. 2) will result in a moderate increase in total born per litter, a decrease in number stillborn, a decrease in preweaning mortality and in a sizeable increase in number weaned per litter. Birth weight will decrease, but uniformity in birth weight will increase. 6 Some challenge in finishing traits Leenhouwers (2001) studied the biological aspects of genetic differences in piglet survival and found increased stomach and intestinal weights in piglets with a high genetic merit for survival. We ourselves (Knol, 2001) found indications of increased feed intake, increased gain and somewhat increased lipid deposition during finishing when selecting for increased survival, which would fit the increased stomach findings very well. More indications of a relation between increased survival and higher fatness can be found in literature (e.g. Mersmann et al., 1984; McKay, 1993; Kerr and Cameron, 1995). This would be a second challenge, after the increased litter size discussion, in realising a substantial and sustainable genetic trend in reproduction. Leenhouwers (2001) found, as the most striking difference between high and low genetic merit animals, a highly significant difference in cortisol in piglets of 112 days of gestation. Differences in cortisol are known to affect lung maturation and glycogen synthesis, both important for early survival. 4

5 Table 1 Current production level in the Netherlands, expected level following selection for litter size and birth weight (Strat. 1) and following selection for litter size and survival (Strat. 2). Current Strat. 1 Strat Total number born Live born Stillborn % preweaning mortality Piglet survival Weaned per litter Birth weight Variation in birth weight Litter weight Understanding the mechanism and knowing the genetic parameters simulation of selection on a balanced index revealed good possibilities of a simultaneous genetic improvement of finishing traits and piglet survival. 7 Current use of molecular genetics Worldwide, a huge effort in time and money is put into understanding the animal genome. Techniques and hypotheses from this work are also tested in the field of pig reproduction. Two interesting approaches in this field come from the group who performed the 14-generation selection experiment in Nebraska. Their first approach was to test 6 candidate genes in the field of reproduction (Linville et al., 2001) and their second to perform a marker scan for reproduction traits (Cassady et al. 2001), both using the high/low selection line animals. Both approaches did not yield evidence that a few simple genes might explain a substantial part of the variation in reproduction. Reversibly, two candidate genes with a high expectation for reproduction, ESR and PRLR, have been studied in great detail (van Rens, 2001). It was concluded that ESR is probably not the causative mutation, but a marker for litter size and that PRLR is probably a major gene for ovulation rate than for litter size.... Further more, results of the present thesis are an example of marker alleles having positive and negative effects at the same time, making it difficult to use the marker for selection. The favorable PRLR allele for litter size for example, appears to be the unfavorable allele for age at first estrus and litter average of teat number of the piglets. This problem seems to be a biological reality animal scientist will have to live with. It clearly demonstrates the importance of physiological research parallel to and coherent with the search for QTLs and markers for any trait. (van Rens, 2001) 5

6 Molecular genetics will, in the future, result in genetic improvements, but the limiting factor at present might be our insufficient understanding of the reproductive processes, making it difficult to search for the proper candidate genes. For now we have to settle for a marker approach with the tedious work of establishing the marker-trait relations for each population and repeating this analysis every now and then. 8 Summary and conclusions Reproduction traits are heritable, even piglet vitality and mothering ability. Genetic progress can be made for each trait. The challenge lies in the unfavourable genetic correlations, among which litter size and piglet survival and piglet survival and finishing traits. Quantitative genetics using a properly designed index will realise substantial and sustainable genetic trend. In the future molecular genetics will help to increase this trend. Current findings in molecular genetics and probably our current understanding of the interactions between traits are not sufficient to expect large steps from molecular genetics within the next few years. We expect, for the coming 10 years, a sound genetic trend of a quarter piglet weaned per sow per year, with lower mortality and stronger piglets if quantitative selection is practised on an index of litter size and piglet survival. 9 References CASSADY, J. P., JOHNSON, R. K., POMP, D. ROHRER, G. A., VAN VLECK, L. D., SPIEGEL, E. K. AND GILSON, K. M Identification of quantitative trait loci affecting reproduction in pigs. Journal of Animal Science 79: FIREMAN, F. A. T., AND F. SIEWERDT, Efeito do peso ao nascer sobre a mortalidade de leitoes do nascimento até 21 dias de idade. R. Bras. Zootec. 26: GRANDINSON, K., L. RYDHMER, E. STRANDBERG, AND M. S. LUND Estimation of genetic parameters for mortality and causes of death in piglets. EAAP 2000, The Hague. JOHNSON, R. K., M. K. NIELSEN, AND D. S. CASEY Responses in ovulation rate, embryonal survival, and litter traits to 14 generations of selection to increase litter size. J. Anim. Sci. 77: KERR, J. C., AND N. D. CAMERON Reproductive performance of pigs selected for components of efficient lean growth. Anim. Sci. 60: KNOL, E. F Genetic aspects of piglet survival. PhD Thesis. Wageningen. LEENHOUWERS, J. I Biological aspects of genetic differences in piglet survival. PhD Thesis. Wageningen. LINVILLE, R. C., POMP, D. JOHNSON, R. K., AND ROTHSCHILD, M. F Candidate gene analysis for loci affecting litter size and ovulation rate in swine. Journal of Animal Science 79:

7 LUND, M. S., M. PUONTI, L. RYDHMER, AND J. JENSEN Relationship between litter size, perinatal, and preweaning survival in Landrace pigs. EAAP 2000, The Hague. MCKAY, R. M Preweaning losses of piglets as a result of index selection for reduced backfat thickness and increased growth rate. Can. J. Anim. Sci. 73: MERSMANN, H. J., W. G. POND, R. T. STONE, J. T. YEN, AND R. N. LINDVALL Factors affecting growth and survival of neonatal genetically obese and lean swine: cross fostering experiments. Growth 48: ROTHSCHILD, M. F. AND J. P. BIDANEL Biology and genetics of reproduction. In: M.F. Rothschild and A. Ruvinsky (ed) Genetics of the pig. p 323. CAB International, Oxon, UK. ROEHE, R., Genetic determination of individual birth weight and its association with sow productivity traits using bayesian analyses. J. Anim. Sci. 77: SIEWERDT, F., AND R. A. CARDELLINO Genetic parameters of piglet mortality from birth to 21 days of age in the Landrace breed. Revta Soc. Bras. Zootéc. 25: ROTHSCHILD, M. F. & BIDANEL, J. P., Biology and genetics of reproduction. In: The genetics of the pig. Eds. M.F. Rothschild & A. Ruvinsky. CAB International. Pp VAN RENS, B. T. T. M, Physiological aspects of two candidate genes for litter size in pigs. PhD Thesis. Wageningen. 7