Effect of maternal heat stress during the dry period on development of immune system of offspring
Maternal heat stress during the dry period affects calf immune response during postnatal life, but it is still unknown how in utero heat stress affects calf’s immune system development. The objective was to evaluate the effects of in utero heat stress on distribution of different immune cell types in blood and primary and secondary lymphoid tissues of the calf. Cows were dried off 60 d before expected calving and randomly assigned to one of two treatments: heat stress (HT) or cooling (CL). During the dry period, all cows were housed in a freestall barn with fans over the feed line and stalls, but only the feeding area for CL cows was equipped with soakers. Heat stress was moderate compared with other studies, as HT cows had only 0.1 oC increase in rectal temperature and 8 breath/min increase in respiration rate compared with CL cows. Immediately after birth, singleton calves (HT: n=200; CL: n=188) were weighted and then fed 3.8 L of colostrum (score: bull<80; heifer>80) within one hour after birth. Blood samples were collected from a subset of heifers (HT: n=12; CL: n=10) at birth before colostrum feeding, d 3, 28 and 56 of age to evaluate the proportion of blood T and B lymphocytes, granulocytes, monocytes, and γδ-T cells by flow cytometry. Additionally, a subset of bull calves (5/treatment/day) were randomly selected and slaughtered at birth (without colostrum feeding), 1 and 2 d after birth. Thymus and spleen were weighed and then a sample was excised, homogenized and assayed using flow cytometry to determine the proportion of different immune cell types. No difference was observed between treatments for calf birth weight (CL: 41.0; HT: 40.6 kg; P = 0.32). However, the thymus of CL bull was proportionally heavier (0.18 vs. 0.14 % of body weight, respectively; P < 0.05) compared with HT calves. Preliminary analyses indicate that treatments had no impact on the proportion of different immune cells of calf blood during the preweaning period. Thus, we conclude that the slight difference in heat strain on HT and CL cow during the dry period has no significant impact on general fetal growth during the dry period and blood immune cell profile during the preweaning period in current study; however, it seems that late gestation maternal heat stress influences fetal primary lymphoid tissue development.
Keywords: heat stress, dry period, dairy calf