Kobek-Kjeldager, C., Moustsen, V. A., Theil, P. K. et al. 2020. Effect of litter size, milk replacer and housing on behaviour and welfare related to sibling competition in litters from hyper-prolific sows. Applied Animal Behaviour Science 230, 105032.

Litter sizes where piglets are supernumerary to teats leads to the practice of fostering to nurse sows or to artificial rearing systems, which is associated with compromised welfare. An alternative strategy is to leave the supernumerary piglets with their own dam and provide milk replacer in the farrowing pen. To provide sufficient space around the sow’s udder, loose housing could be an alternative to the use of farrowing crates. However, large litters have been associated with increased teat fighting, abrasions on sows and piglets and signs of hunger. The aim of this study was to investigate whether provision of milk replacer and/or loose housing could alleviate signs of hunger and sibling competition at the udder. Ninety-eight litters were studied in a 2 × 2 factorial design. The sows were housed in crates or loose (CRATE/LOOSE), and litters had access to milk replacer or no milk replacer (+MILK/-MILK). The litter size was either 14 or 17 piglets on day 1 postpartum but decreased as piglets died. Recordings included drinking bouts of milk replacer, teat fighting, udder massage and abrasions on sow and piglets. On day 3, at least one observation of milk replacer consumption was observed in 90% of the litters in +MILK but at less than five bouts per litter/12 h in all except two litters. On days 7, 14 and 21, 59-65% of the piglets in +MILK consumed milk replacer at least once with a mean frequency of 0.4-1.9 bouts/h/piglet. Increasing litter size resulted in more piglets involved in teat fighting, having low suckling success and snout abrasions (P < 0.05). The risk for all indicators were generally higher for smaller compared to larger piglets. Access to milk replacer reduced risk of piglet snout abrasions (P < 0.05), but not of teat fighting (P> 0.1). Loose housing reduced the risk of udder abrasions (P < 0.05) and tended to decrease udder massage (P < 0.10). Thus, milk replacer and loose housing minimally reduced litter competition in litters from hyper-prolific sows, indicating that supplementing large litters with milk replacer can be associated with compromised welfare. The welfare consequences of using milk replacer appear comparable to when surplus piglets are fostered by nurse sows or artificially reared. Before supplementing milk replacer to large litters is further applied under commercial conditions, research and innovation is needed to improve quality of milk replacer and cup design to alleviate the negative impact on welfare.