The Effects of Heat Stress on Metabolism, Energetics and Production in Growing and Lactating Cattle
Heat stress negatively influences multiple aspects of animal agriculture and is probably the global dairy industrys most costly issue. Evaluating the metabolic effects of heat stress (especially chronic hyperthermia) requires utilizing a pair-feeding experimental design to minimize or eliminate the confounding effects of dissimilar nutrient intake. Employing this type of approach is necessary to differentiate between the direct and indirect effects (e.g. reduced intake) of environmental-induced hyperthermia. Our studies indicate that heat-induced decreased feed intake only accounts for approximately 50％ of the reduction in milk yield from cows but almost fully explains the weight gain reduction in growing calves. Heat-stressed cows do not mobilize adipose tissue and have a much lower response to an adrenergic signal despite being on a lower plane of nutrition and having increased circulating levels of lipolytic signals (glucagon, cortisol, epinephrine etc.). Heat-stressed cows have decreased blood glucose levels and an increased rate of glucose clearance following a glucose tolerance test. In addition, although glucose production is decreased to a similar extent as pair-fed controls, a larger proportion of hepatic glucose production is utilized for non-milk synthesizing purposes. Basal and glucose stimulated insulin values are also higher in heat-stressed cows and calves. During thermal neutral conditions, increased insulin levels normally reduce liver gluconeogenesis, but do not in heat-stressed cows possibly due to down-regulation of hepatic insulin receptors. Furthermore, in stark contrast to adipose tissue, the liver remains responsive (with regards to stimulated glucose output) to adrenergic stimulation. In summary, our data indicate that heat-stressed cows fail to enlist glucose sparing mechanisms typically employed during periods of inadequate nutrient intake and glucoses contribution to whole animal energetics is increased. Consequently, glucose availability for mammary lactose synthesis is diminished and thus overall milk yield decreases more than would be expected based on reductions in nutrient intake.
Lance Baumgard Robert Rhoads
Iowa State University, Ames IA The University of Arizona, Tucson, AZ ,USA
国际会议
奶牛营养与牛奶质量国际研讨会(ist International Symposium on Dairy Cow Nutrition and Milk Quality)
北京
英文
102-107
2009-05-04（万方平台首次上网日期，不代表论文的发表时间）