In pig breeding the uniformity has a high importance for both reproduction (maternal) traits of sows and production traits. Uniformity in maternal traits is favorable for welfare and management of the sows and piglets (Baxter et al., 2013;
121 Rutherford et al., 2013) and it can lead to lower variation at weaning, which decreases the labor required in later stages of production (Merks et al., 2012). Uniformity in production traits is desired since at slaughter penalties are given for carcasses outside a specified weight range (Kanis et al., 2006), but also extending the period to obtain the desired slaughter weight for the lighter pigs creates additional costs (Merks et al., 2012). A more uniform pig production, reflected in reduced variation in the reproduction and production traits is, therefore, related to a higher farm profit.
6.2.1 Uniformity of sow reproduction traits
Efficient pig production aims to obtain high number of slaughter pigs per sow per year (Spötter and Distl, 2006; Dekkers et al., 2011; Rutherford et al., 2013). Therefore, in pig breeding genetic selection continues to increase the litter size. The annual genetic trend for litter size in different pig breeding programs was shown to be +0.16 (Merks et al., 2012; Tomiyama et al., 2011), +0.25 (Vidović et al., 2012), and even up to +0.5 (Taylor, et al., 2005) piglets per year per sow. Still, the variation in this trait is large. Poor reproductive performance remains one of the most common reasons for culling (30%) of the sows (Stalder et al., 2005; Hoving et
al., 2010; de Jong et al., 2014). At the same time, in the extreme cases litter size
goes above 25 piglets, which exceeds the physiological capacity of the sow to provide for the litter during gestation and post-farrowing. Sows with the large litters can also experience welfare issues such as high energy demands during gestation (Rutherford et al., 2013) and shoulder sores during lactation (Zurbrigg, 2006; Herskin et al., 2011). Moreover, an increasing litter size affects also welfare and survival of the piglets.
Selection for larger litters increased the ovulation rate in sows. During gestation, uterine capacity and its blood supplies are the physiological factors limiting possibility for all the fetuses to fully develop (Foxcroft et al., 2006; Pardo et al., 2013; Jang et al., 2014). The ovulation rate is genetically correlated (0.34) with number of stillborn (Johnson et al., 1999). This correlation is a consequence of intra-uterine competition between embryos, which affects piglets’ survival pre- farrowing.
Litter size is also closely related to the piglets’ birth weight, which is considered a sow reproduction trait (Roehe, 1999; Damgaard et al., 2003; Canario et al., 2010). From a welfare and management perspective high birth weight is a very important trait for piglets’ survival till weaning (Foxcroft et al., 2007; Foxcroft et al., 2009; Knol et al., 2010; Rutherford et al., 2013; Baxter et al., 2013). Larger litters, however, phenotypically tend to have higher variation in birth weight (Van der
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Lende and de Jager, 1991; Milligan et al., 2002; Foxcroft 2006). Thus the pre- weaning survival is decreased by increased competition between the piglets resulting from higher variation of within-litter birth weight observed in the large litters (Foxcroft et al., 2006; Kapell et al., 2011; Rutherford et al., 2013). To manage the largest litters it is often necessary to apply nursing techniques such as: cross- fostering (moving a piglet to a different sow with more milk), the use of nurse sow systems, which are either life sows or artificial rearing boxes, and early weaning (Baxter et al., 2013). These management techniques are, however, labor intensive and disturb nursing behavior of the sows and piglets.
Despite affecting the survival of the piglets, larger litters will still result in higher number of weaned piglets and slaughter pigs than small litters (Beaulieu et al., 2010). Thus currently in pig breeding, the goal is towards more sustainable production that will increase the piglet’s survival regardless of increasing litter size (Beaulieu et al., 2010; Kapell et al., 2011; Merks et al., 2012; Nielsen et al., 2013). To facilitate sustainable pig production a good balance needs to be found between sows’ and piglets’ welfare and the economic result. Improved uniformity of litter size (and ovulation rate) can reduce the number of extremely large litters with accounting for the physiological capacity of the sow. This will also improve piglets’ welfare and survival with maintaining high litter size.
6.2.2 Uniformity of production traits
Breeding for uniformity in production traits in pigs starts with birth weight of the piglets. Birth weight is one of the most important traits for the entire productive life of the pig (Bee, 2004; Gondret et al., 2005; Foxcroft et al., 2006; Rehfeldt et al., 2008; Fix et al., 2010; Schinckel et al., 2010). Besides its effect on piglet’s survival, it can also affect muscle development (Bee, 2004; Fix et al., 2010; Schinckel et al., 2010) and postnatal growth performance (Gondret et al., 2005, Foxcroft et al., 2006; Rehfeldt et al., 2008, Schinckel et al., 2010).
The piglets with low birth weight have slower pre- and post-weaning growth than heavier piglets. Merks et al. (2012) showed that the piglets smaller by about 30% than the average at birth are on average 5% lighter at slaughter and spend 10 days more than average in grow-finish pens. In contrast, piglets 30% heavier than average at birth reach a higher slaughter weight (by 3%) 4 days earlier than average. The difference in growth rate between the heaviest and the lightest grow- finish pigs creates additional costs and labor for farmers. These additional costs and labor are needed to deliver batches of equal-weight pigs to avoid penalties for carcasses outside the range (Kanis et al., 2006).
123 Variation in production traits and in time to achieve desired weight between animals contributes to price per kg of the carcass, which is highly relevant from the economic and management perspective of the farm (“all in, all out”). Lowering the variation in production traits is possible with the selection for improved uniformity of birth weight. It is necessary, however, to simultaneously maintain desired direction of selection for the mean birth weight since this trait affects the post- weaning growth of the pigs.