316. Impact of Growth Trait and Dam Age on Immune Response to Weaning Stress in Beef Calves.

Abstract Intensive selection for economically important traits in beef cattle, particularly growth, may impact stress responsiveness and well-being. A higher growth rate may modify the stress response of the animal leading to an energetic trade-off between growth and immune function, thereby impacting health and well-being. Furthermore, maternal factors, including age of dam can also influence early-life outcomes. This study aimed to determine the effects of moderate versus high growth traits, dam age, and weaning stress on the immune phenotype of beef calves. A subset of calves sired by bulls exhibiting either moderate (MG, n = 40) or high growth (HG, n = 40) were used. Dams were classified by age into heifers (2 yr, n = 20), young (3-4 yr, n = 22), mid (5-6 yr, n = 16), and old (7-10 yr, n = 22). Body weights and blood samples were taken at weaning (D 0) and 14-, and 28-days post-weaning (DPW). Cortisol, complete blood counts, lymphocyte proliferation, and neutrophil chemotaxis were measured. Data were analyzed using PROC GLIMMIX (SAS 9.4). Least square means were computed for all main effects and interactions, with pairwise differences assessed using Tukey-Kramer adjustment for multiple comparisons. Significant interactive effects of dam age by day on body weight (P < 0.05) and plasma cortisol levels (P < 0.05) were found. Calves from heifer dams had increased (P < 0.05) body weight from 14 to 28 DPW compared to calves from dams in other age classes, whose body weights were similar. Notably, cortisol declined significantly at 28DPW compared to 14DPW in calves from both heifer and mid-aged dams. Conversely, cortisol levels were similar among calves from young and old-aged dams, but those from young dams had the highest peak at 14 DPW. There were also main effects of growth trait on body weight and dam age on immune measures. Specifically, calves sired by HG-bulls were heavier than those sired by MG-bulls. Calves from heifer-aged dams had lower (P < 0.05) neutrophil chemotaxis relative to calves born to dams in other age classes, while other immune measures were affected by weaning stress. LPS-induced lymphocyte proliferation indices peaked (P < 0.05) at 28DPW, whereas Con A-induced lymphocyte proliferation indexes were highest (P < 0.05) at 14DPW. The neutrophil-to-lymphocyte ratios were significantly higher at 14DPW (P < 0.05) and 28DPW (P < 0.05) relative to weaning (D0). Overall, these data indicate that dam age influences immune phenotype in response to weaning stress, while body weight is primarily affected by growth trait. Therefore, understanding the impacts of growth trait and dam age on stress responsiveness on immune phenotype can provide valuable insights into genotype-by-environment interactions and breeding strategies aimed at enhancing calf health and well-being.