Environmental Heat Stress Response and Adaptation in Ruminants

Abstract

Environmental heat stress represents a worldwide problem for ruminant production, negatively impacting areas such as lactation, fetal development, and muscle growth. The aims of this dissertation were to determine the impact of environmental heat stress during gestation on the lactation performance of ewes, and the consequences of maternal heat stress on the post-natal performance of their offspring. Further, we tested the effect of supplementation with ß-adrenergic agonists on the growth performance of beef cattle exposed to environmental HS. To determine the influence of maternal heat stress (HS) on milk production, nineteen Columbia-Rambouillet pregnant ewes carrying singleton were exposed to hot ambient environment (n=7) or thermoneutral (TN) conditions during mid-gestation. Dry matter intake (DMI) and water intake (WI) were measured daily. At lambing, placental weight and colostrum production were measured, and for 21 days, daily milk yield was evaluated. Heat stressed ewes had lighter placental and fetal weight than TN ewes; however, colostrum production was not different. TN ewes had a similar total milk production than HS ewes. Daily milk production was higher on TN ewes on days 2, 5, and 6. HS during gestation negatively affects placental mass and milk production, impacting the nutrient sources for the fetus and causing growth restriction. In order to determine the impact of maternal heat stress on the offspring's growth performance and insulin response, twenty-one Columbia Rambouillet crossbred lambs (TN=12 and FGR=9) were used. Milk, feed, and body weights were recorded daily for eight weeks. Body measurements were taken weekly for eight weeks. Glucose tolerance tests were performed at four and eight months of age to evaluate the insulin response. HS fetuses were growth-restricted because HS lambs were 27% lighter than TN lambs at birth, indicating maternal HS caused FGR. Body weight of FGR lambs was lighter than that of TN lambs for the next eight weeks or until weaning. However, the average daily gain (ADG) and gain to feed (G:F) were not different between groups. FGR lambs presented smaller body measurements than TN lambs, except for head length (HL), which was similar between group treatments. After the intravenous glucose challenge, insulin concentrations were lower in FGR lambs than in TN lambs at four and eight months of age. FGR lambs presented asymmetric growth restriction in utero and had abnormal insulin secretion response. The use of ß-adrenergic agonists was evaluated with twenty-four Brahman steers exposed to two environmental conditions, TN and HS, and supplemented with or without Zilpaterol hydrochloride for 21 days. DMI, water intake, respiratory rate (RR), and rectal temperature (RT) were measured daily. After 21 days of supplementation, Brahman steers were relocated to the feedlot and harvested at an average weight of 544 kg post supplementation/environmental treatment, and carcass merit was evaluated. RR was higher on Brahman exposed to HS. Non-supplemented Brahman steers exposed to HS presented higher RT than TNZL steers on day eight. Heat-stressed steers supplemented with ZL had higher RT on day eleven than TNCN steers. ADG and G:F did not present significance nor carcass merit. No negative impact on growth and carcass characteristics was seen on Brahman steers exposed to HS. Zilpaterol supplementation did not affect the carcass merit, demonstrating an option to mitigate HS's adverse effect on Bos indicus. Environmental heat stress is a global problem that affects all stages of ruminant production and impacts the economy. Maternal heat stress during gestation results in a smaller placenta, decreasing the nutrients available to the fetus and fetal birth weights, causing asymmetric growth and a decrease in insulin secretion concentration. Maternal heat stress also affects milk production in lactated ewes, directly impacting the availability of milk nutrients. Similarly, HS postnatally decreases muscle growth in ruminants.