Effect of Mixing at Weaning and Dietary Nutrient Density on Growth Performance and Welfare of Pigs

Abstract Mixing pigs at weaning compromises animal welfare and may reduce growth performance (GP). Reducing nutrient density of the nursery diet is a strategy to avoid gastrointestinal problems in newly weaned pigs. However, the potential for compensatory growth in subsequent stages and the impact on welfare are still unknown. This study investigated the effect of mixing and reducing dietary nutrient density on GP, body lesions (BL), and behavior in pigs from weaning to slaughter. Forty-eight litters [554 pigs, 11-12 pigs/litter; Danish Duroc × (Large White × Landrace)] were included in the trial. At 28 days of age, pigs were housed in the nursery rooms as litter pens (INTACT, n = 24) or mixed with other litters by body weight to reduce variability within pen (MIXED, n = 24). A dietary regimen meeting the nutritional requirements (CON) of the pigs and a low-density regimen (LOW; -10% energy and protein) completed a 2x2 factorial arrangement (Mixing x Diet, n =12). On day 73, pigs moved to a growing-finishing room without further mixing and all pigs received the CON dietary regimen. At the beginning and at the end of each production phase, the number of BL and the frequency of aggressions and damaging behavior were determined. Data were analyzed using general linear mixed models. Mixing increased FCR by 4.0 % during the nursery period (P = 0.003; Table 1). Pigs fed LOW had decreased GP during the nursery period but better feed conversion ratio (FCR; P = 0.008) during the growing-finishing period, when all groups were fed the same diet. However, pigs fed LOW showed a lower slaughter weight (-2.6 kg; P = 0.025). Initial differences in the coefficient of variation (CV) between MIXED (10.4%) and INTACT (17.6%; P < 0.001) were reduced in CON pens (6.9 % to 0.5 %) but not in LOW pens (7.9 % to 4.5 %; interaction P = 0.025) at the end of the nursery period. Pigs in MIXED pens showed 358% more BL (P < 0.001), more aggressions (P = 0.005) and damaging behavior (P = 0.042) at weaning, and more aggressions (P = 0.003) after being moved to growing-finishing rooms. Animals fed LOW showed more aggressions at weaning (P < 0.001) and more damaging behavior at the end of the nursery period (P < 0.001). When their diet was changed to CON, they showed 14% more BL (P = 0.009) but less aggressions (P = 0.004). Mixing animals at weaning had little impact on GP, but lesions, aggressions and damaging behavior towards pen mates indicate a detrimental effect on welfare and strategies to avoid mixing should be studied. The provision of a low-density diet during the nursery period had negative effects on welfare and growth was not totally compensated during the growing-finishing period.