Feeding of thyroprotein to lactating dairy cows in hot environments

Abstract

A review of the literature revealed that the use of thyroproteins in hot environments was contra-indicated, but that there was little experimental evidence to support this contention. Four lactating Friesian cows were exposed to two 3 week periods of heat stress; 37°C (70 - 90% RH) for six hours by day, and 21°C or less by night. They were fed to requirement, and during one period of heat exposure 7.5g iodinated casein was added daily to each cow’s feed.

During thyroprotein supplementation milk production was increased by approximately 8% (p < 0.05) but severe liveweight losses incurred. On cessation of thyroprotein administration milk production declined, but not to levels significantly below normal (p > 0.05), and liveweight was rapidly regained.

Although metabolic heat production was increased when feeding thyroprotein (p < 0.05), clinical parameters indicated that the cows were not more distressed by the hot environments whether fed thyroprotein or not. Dry matter digestibility investigations were inconclusive, and measurements of sweat rate confirmed that iodinated casein did not exacerbate hyperthermia. Rectal temperatures and respiratory rates were correlated to various heat stress indices, viz. Wet bulb temperature, Bianca’s wet bulb: dry bulb equation, Equivalent temperature, Temperature humidity index, and Heat absorption capacity. The best correlation was between dry bulb temperature per se and respiratory rate. Routine haematology was carried out, and radioimmunassays of plasma thyroid hormones were done. Heat stress clearly depressed thyroid function, but feeding iodinated casein elevated plasma and levels, as well as the FTI. It was concluded that the use of thyroproteins in hot environments is not necessarily to be condemned, and that field trials feeding low levels of iodinated casein to lactating dairy cows in the tropics would be justified.