ABSTRACT
Pigs from one farrowing group in which gilts were bred to farrow pigs that would be either 14 or 21 d of age at weaning, were divided into older and younger age groups (108 pigs per group) and penned 12 pigs per pen in a wean-to-finish facility. At the end of the nursery phase, half the pigs in each age group were removed, rerandomized, and commingled for the finishing phase. The other half remained in their original pens. Pigs were fed common Phase 1 (d 0 to 14) and Phase 2 (d 14 to 35) nursery diets, and a common 4-phase program diet during growing/finishing, with transitions at 45, 68, and 90 kg of BW. The study ended when the lightest weight block averaged 107 kg. Blood was obtained on d 0, 2, 10, 27, 37, 44, and 65 after weaning to determine leukocyte concentrations. In addition, behavior was monitored during the nursery period at weaning (d 0), on d 7, 14, and 27 after weaning, and during the growing/finishing phase on d 35 (after commingling following the nursery phase), 38, 44, and 65 after weaning. Older pigs were heavier (P < 0.001) throughout the nursery period, and the BW difference between younger and older pigs increased from 2 to 6.5 kg at the start and end of the nursery period, respectively. Older pigs had a greater concentration of white blood cells (P < 0.05) and lymphocytes (P < 0.10) on d 0, 2, and 10 after weaning than younger pigs. Younger pigs spent less (P < 0.05) time resting on the day of weaning, and more (P < 0.05) time active during the overall nursery phase. During Phase 3 and in the overall finishing phase, younger pigs had greater (P < 0.01) ADG and G:F than older pigs. Moreover, during Phase 3, ADFI (as fed) decreased (P < 0.05) when older pigs were commingled compared with older pigs that were not commingled. There was no difference in ADFI of younger pigs, regardless of commingling (interaction; P < 0.10). Results of this study indicate that weaning age affects growth performance in a wean-to-finish facility, as well as behavioral and immunological responses to weaning and commingling after the nursery phase. Management strategies should be further explored to optimize these benefits without the detrimental effects on health observed during the nursery period in this study.
