Kambiz Ghotouri - M.Sc.student, Dept. of Animal Sciences, University of Gonbad Kavoos, POBox ۱۶۳, Golestan Province, Iran

Heat stress at the initiation of lactation negatively impacts the total milk production.Climatic conditions appeared to have maximum influence during the first 60 days oflactation when high-producing cows are in negative energy balance and make up for the deficit by mobilizing body reserves. High yielding cows are affected more than low yielding ones because the upper critical temperature shifts downwards as milk production, feed intake and heat production increase. Heat stress causes the rostral cooling centre of the hypothalamus to stimulate the medial satiety centre which inhibitsthe lateral appetite centre and consequently lowers milk production. The heat-stressed cows depend on glucose for body energy needs; therefore, less glucose is directedtowards the mammary gland, and there is a decline in milk production. Negative energy balance is associated with various metabolic changes that are implemented to support the dominant physiological condition of lactation. Marked alterations in both carbohydrateand lipid metabolism ensure partitioning of dietary-derived and tissue originating nutrients towards the mammary gland, and many of these changes are mediated by endogenous somatotropin which is naturally increased during periods of negative energybalance. Milk constituents are greatly affected by hyperthermia. The ability to usepowerful new tools in genomics, proteomics and metabolomics to evaluate geneticdifferences between animals in their response to thermal stress will yield important new information in the next quarter-century and will permit the selection of cattle for resistance to thermal stress.

Heat stress, milk, production, dairy cows