15 The Trusted Source for Canada’s Pork Producers Better Pork | August 2024 preweaning survival in production. The perceived downside of this strategy is that genetic improvement on initial litter size has been slower than direct selection on total born or born alive. This is due to the antagonistic (unfavourable) genetic relationship between initial litter size and piglet survival. Therefore, to make more progress in number weaned, Hypor chose to improve all three traits simultaneously and more gradually, versus making progress in one trait quickly at the detriment of other important traits. Without balanced selection, overall maternal efficiency would have changed much slower. What about piglet quality? We cannot discuss litter size without a discussion on piglet quality. The prevailing thought is that as litter size increased, piglet quality deteriorated and directly caused the reduced survivability of piglets. As a result, many producers have shifted their focus to individual birth weights of piglets. Selection for litter size over the years has reduced the size of piglets at birth in many genetic lines. So why not put a lot of pressure on birth weights in place of selection for piglet survival? For many years, the strong phenotypic relationship between individual birth weight and piglet survival has been well established, with emphasis on the phenotypic. This is true – we know from data that small piglets will be at a disadvantage to compete and therefore, more likely to die in farrowing. This however ignores the fundamental fact that ‘small’ and ‘large’ are relative terms. Selection for individual birth weights would shift the entire distribution a small amount, say by adding 50 g (~0.1 lb) to every piglet born. Yet the issue of competition in very large litters remains. Smaller piglets, no matter their absolute weight, will be at a major disadvantage relative to heavier litter mates. This is an explanation as to why we see similar survival curves with any age of pig (e.g. weaning weights into nursery). The absolute value is much higher, yet we often still observe low weaning weight pigs perform worse in wean-to-finish compared to heavier contemporaries. The main problem is researchers and producers alike interpreting the well-known phenotypic correlation as a genetic correlation (i.e. correlation of breeding values). Some genetic correlations have shown to be as low as 0.14 between birth weight and piglet survival. This concept was first articulated 40 years ago by English and Morrison. “Attempts to improve piglet survival by achieving slight improvements in BALANCING LITTER SIZE Courtesy of Hypor % WEANED % BORN ALIVE TOTAL BORN BIRTH WEIGHT VARIATION IN BIRTH WEIGHT RUNT PIGLETS WEANED PIGS Figure 2. Hypor’s maternal selection strategy is to increase the number of weaned pigs.
RkJQdWJsaXNoZXIy NTc0MDI3