Immune responses of piglets to weaning stress: impacts of photoperiod.

An acute stress response can be provoked by abrupt social, nutritional, and environmental changes associated with weaning, and this may disrupt homeostasis and thus compromise well-being. Manipulating environmental factors, such as photoperiod, might provide a simple way to reduce the physiological consequences that piglets experience due to weaning stress. The objective of this study was to evaluate the impacts of photoperiod manipulation across various weaning ages on leukocyte populations, lymphocyte proliferation, natural killer cytotoxicity (NK), chemotaxis, phagocytosis, and immunoglobulin G, cortisol, and BW of piglets during the nursery phase. Sixty-eight crossbred piglets were obtained from sows kept on a short-day (8 h of light/d) photoperiod from d 90 of gestation until weaning. Piglets were weaned at 14, 21, or 28 d of age and kept on a short or long (16 h of light/d) photoperiod until 10 wk of age. Piglet BW and blood samples were collected at weaning and at 6, 8, and 10 wk of age. Pigs weaned at 28 d had reduced neutrophil counts (P < 0.001), phagocytosis (P < 0.001), and lymphocyte proliferation (P < 0.05) at weaning compared with those weaned at 14 and 21 d. Pigs weaned at 21 d tended to have lower (P = 0.08) lymphocyte counts than did pigs weaned at 14 or 28 d. Pigs weaned at 14 d had reduced (P < 0.01) NK relative to those weaned at 21 or 28 d. Photoperiod also influenced pig BW and immune status. Generally, those pigs on the long-day photoperiod and weaned at 28 d were heavier (P < 0.001) than their counterparts weaned at 14 or 21 d. At 6 wk of age, NK was greater (P = 0.002) in pigs kept on a long day and weaned at 14 or 21 d than in pigs weaned at 28 d. Phagocytosis was less (P = 0.005) at 6 wk of age, but was greater at 8 wk, in piglets kept on the long day and weaned at 28 d than in long-day pigs weaned at 14 or 21 d. These results suggest that photoperiod differentially influences immune responses in piglets weaned at different ages and indicate an inverse relationship between growth and immune status. Here, weaning at 28 d and a long-day photoperiod was the treatment combination that was most physiologically beneficial to piglets, whereas a 14-d weaning and short-day photoperiod was least physiologically beneficial.