Genetic and phenotypic time trends of litter size, piglet mortality, and birth weight in pigs

Abstract

IntroductionLitter size in pigs has increased steadily since 1990. Because of unfavorable genetic correlations with piglet mortality, breeding goals should include survival traits next to litter size. Unbalanced breeding programs that neglect this requirement have produced increased mortality levels, attracting negative public attention. Balanced breeding does not have this disadvantage, but the general public is largely unaware of this.MethodsWe present long-term time trends as realized in commercial breeding. The data includes (i) phenotypes of litter size, piglet birth weight, and piglet mortality, as used in routine breeding value estimation; and (ii) the genomic Best Linear Unbiased Prediction (gBLUP) estimated breeding values thus obtained. Piglet mortality (2001–2022) and birth weight (2009–2022) phenotypes were related to litter size by recording year. Estimated breeding values (EBVs) for the mortality traits were regressed on those for litter size by birth year (2012–2022).ResultsAverage litter size is very weakly correlated to the mortality (R2 ≤ 0.06) and birth weight (0.07 ≤ R2 ≤ 0.26) traits, and those correlations are unfavorable (antagonistic) within each year. However, all traits analyzed here show favorable simultaneous phenotypic and genetic trends over time: the antagonisms are neutralized by balanced breeding. Above the annual mean litter size level, farrowing and lactation mortality rates increased with increasing litter size in every year (unfavorable), but the annual intercepts and the slopes decreased from 2001 to 2022 (favorable). Average litter birth weight decreased with litter size in every year (unfavorable), but the annual intercepts increased and the slopes decreased from 2009 to 2022 (favorable). The within-litter birth weight variation coefficient increased with litter size in every year (unfavorable), but the annual intercepts decreased from 2009 to 2022 (favorable). The proportion of low birth weights (i.e.,< 0.9 kg) for a given litter size is decreasing over time, and the critical birth weight level (below which lactation mortality increases strongly) is clearly population dependent and changes over time too.DiscussionThe increases in litter size and piglet survival rates due to balanced breeding policies lead to reduced total numbers of dead piglets (i.e., per country, per year) coinciding with a certain pig production volume (i.e., with a certain total number of weaned piglets).

Effects of the Mean Weight of Uniform Litters on Sows and Offspring Performance

“…However, the selection for more uniform litters can have negative impacts both on the litter size and/or on the mean weight of the piglets [23,24]. Increased litter size has been a goal of genetic selection progress in the last decades and, according to recent evaluations, it can continue to be a strategic goal in pig genetic improvement programs because the negative effects on preweaning mortality of piglets can be controlled [1]. The possible negative impacts on the mean weight of the piglets can be also problematic because lighter piglets have low survival rates [28][29][30] and, due to the positive relationship between birth weight and weaning weight [30][31][32], piglets could be weaned with less body weight, resulting in augmented difficulties to cope the weaning transaction challenges [33,34].…”

“…The number of weaned piglets per year is a key indicator in swine breeding, determining its profitability. During the last decades, genetic selection has led to a significant litter size increase [1]. However, larger litters are associated with higher piglet pre-weaning mortality [2][3][4] which is a major problem in commercial pig production, both from an economic and welfare point of view [5].…”

Potential negative effects of genomic selection

“…Adopting the second stage slowed or even reversed the decline of fitness traits that were part of the selection index ( Brito et al, 2021 ). Multi-trait selection became common in pigs ( Knap, 2022 ; Knap et al, 2023 ) and chickens ( Neeteson et al, 2023 ), which reversed the decline of many traits. While many traits can be easily recorded, some traits are difficult to measure, creating a possibility of a decline.…”

“… Cheverud (1984) showed that improving two traits was five times faster when the genetic correlation was 0 instead of −0.8. In a pig breeding scheme, where the number of piglets born is negatively correlated (unfavorable) with birth weight, it took 9 yr of balanced selection under GS to increase both traits by 2.8 piglets/litter and 0.27 kg, respectively, while reducing the number of stillborn piglets by 0.53 ( Knap et al, 2023 ). Thus, progress can be made in a well-designed breeding program, although it may be slow.…”

A bio-economic model for estimating economic values of important production traits in the black soldier fly (Hermetia illucens)

“…In production animals, selective breeding has resulted in a tremendous increase in e.g. milk production of dairy cattle (NAV trends, 2023;Oltenacu and Broom, 2010), litter size in pigs (Knap et al, 2023), and improved growth rate in fish (Gjedrem and Robinson, 2014). Selective breeding of insects has not been widely implemented yet.…”