Maternal heat stress reduces body and organ growth in calves: relationship to immune tissue development

Abstract Text: Maternal heat stress (HT) not only reduces fetal growth but also influences postnatal performance and immune function of the offspring. The objective was to evaluate the effect of in utero HT on overall fetal growth and organ development, particularly those associated with immune function. Dams were dried off 45 d before expected calving and randomly assigned to one of two treatments: HT or cooling (CL). During the dry period, all cows were housed under shade in a freestall barn, where the pen for CL cows was equipped with active cooling including water soakers and fans whereas the pen for HT cows had no soakers and fans. Based on rectal temperature (RT) and respiration rate (RR), heat stress was severe. Average RT in HT cows was 39.3 ^oC compared with 38.9 ^oC for CL cows, and HT cows had 66.7 breath/min respiration rate and 43.2 for CL cows. After birth all bull calves were immediately separated from their dams and weighed. Bull calves (n=30) were sacrificed at birth without colostrum feeding (5/trt) and 1 and 2 day of age (DOA, following colostrum feeding, 5/trt). Pooled colostrum (3.8 L) was fed within 4 h after birth to bulls slaughtered on 1 and 2 DOA. After slaughter, the small intestine was removed, weighed (1.0 to 1.4 kg), and dissected into duodenal, jejunal and ileal segments, and tissue samples from each section were fixed in 4% neutral formalin and then transferred to 70% ethanol for immunohistochemistry. Bull birth weight from HT dams was lower than bulls from CL dams (HT: 39.3; CL: 43.8 SEM = 1.1 kg; P < 0.01). The thymus, spleen, and heart weight of HT bulls was lower compared with the CL bulls (Thymus, HT: 107.7; CL: 138.0, SEM = 14.4 g; P = 0.02; Spleen, HT: 75.5; CL: 93.7, SEM = 6.9 g; P < 0.01; Heart, HT: 292.4; CL: 329.4, SEM = 19.6 g; P = 0.03). The liver weight of HT bulls tended to be lower compared with the CL bulls (HT: 811.4; CL: 914.0, SEM = 70.4 g; P= 0.09). We conclude that the acute difference in heat strain on HT and CL cows during the dry period has significant impact on general fetal growth and on immune tissue development, which may be associated with reduced immune function in early life.

Keywords: heat stress, bull, immune tissue