Influence of L‐carnitine and L‐arginine on protein synthesis and maturation of the semitendinosus muscle of lightweight piglets
Increased litter size, a large within‐litter variation and decrease in the average birthweight (BtW) of the litter are all consequences of selection for hyperprolific sows in the commercial pig production (Rutherford et al., 2013). Simultaneously in such sows, milk yield and number of functioning teats does no longer correspond to the number of piglets in the litter (Andersen, Naevdal, & Boe, 2011). This compromises the growth of the piglets in a way that they are unable to reach their full potential (Harrell, Thomas, & Boyd, 1993). Furthermore, lightweight (LW) piglets display impaired growth performance in the nursery (Deen & Bilkei, 2004) and in the post‐weaning growth phase (Madsen & Bee, 2015). In addition, LW piglets have been shown to exhibit a lower number of myofibres compared with their heavier littermates (Bérard, Pardo, Bethaz, Kreuzer, & Bee, 2010), partly contributing to their reduced post‐natal growth potential. The LW piglets also exhibit a lower survival rate compared with the litter average (Quiniou, Dagorn, & Gaudré, 2002), where the main cause of death across all BtW categories is documented to be crushing by the sow (Edwards, 2002). However, it is argued that due to undernourishment, LW piglets reside for longer periods in close proximity of the sow, thereby increasing the risk of crushing (Weary, Pajor, Thompson, & Fraser, 1996). In this context, early artificial rearing by relocating excess piglets to rescue decks and feeding them on milk replacer will naturally remove their risk of being crushed by the sow. Applying milk replacer to early‐weaned piglets has been shown to improve daily gain of medium and heavy birthweight piglets (Zijlstra, Whang, Easter, & Odle, 1996), suggesting it to be a viable alternative to insufficient sow milk yield. However, one important disadvantage of available commercial milk replacers is the suboptimal nutrient and amino acid composition compared with the sows’ milk (Mavromichalis, Parr, Gabert, & Baker, 2001; Theil, Kristensen, Jorgensen, Labouriau, & Jakobsen, 2007). Being important for growth (Kim & Wu, 2004), the semi‐essential amino acid arginine (ARG) is involved in the protein synthesis signalling pathway of the mammalian target of rapamycin (mTOR) (Yao et al., 2008). Its content is substantially lower in commercial milk replacer compared with sow milk (0.7 vs. 1.3 g/kg DM). L‐carnitine (CAR) is another compound which when fed to conventionally reared piglets, has shown to increase the number of myofibres in the early post‐natal period (Lösel, Kalbe, & Rehfeldt, 2009). In a recent study, we observed a greater ratio of lactate dehydrogenase to either citrate synthase with CAR and β‐hydroxyacyl‐CoA dehydrogenase activity with ARG‐supplemented milk replacers (Madsen, Kreuzer, & Bee, 2014). Thus, the relative importance of the glycolytic compared with the oxidative pathway was greater in the semitendinosus muscle of supplemented LW pigs suggesting that these supplements were beneficial for muscle maturation. However, because in that study milk replacer was offered restrictively several times a day based on previous recommendations (60 g DM/kg body weight (BW) (Kim & Wu, 2004)), weaning weights at day 28 were low and independent of the dietary treatment. In the present study, the hypothesis was to test the effect of L‐arginine and L‐carnitine supplements to milk replacer under ad libitum feeding conditions in combination with artificial rearing during the nursery phase on both muscle development and growth performance of LW piglets from hyperprolific sows.