Variations in milk production based on the temperature-humidity index and blood metabolic parameters in cows during exposure to heat stress

The temperature-humidity index (THI) is conventionally used to measure heat stress. The aim of this paper is to determine the influence of THI and THI+metabolic parameters in prediction of milk production. A total of thirty Holstein-Friesian cows were exposed to heat stress and the THI values, milk production and metabolite concentrations were measured on days 0 (in the thermoneutral period), 7 and 14 after the exposure. The average daily THI values obtained were 65 +/- 1.05 (day 0), 75 +/- 1.1 (day 7) and 77 +/- 1.4 (day 14). Heat-stressed cows were found to exhibit a decrease in milk production, contents of glucose and non-esterified fatty acids (NEFA) and the glucose-to-insulin ratio (G:I), whereas the levels of insulin, tumor necrosis factor-alpha (TNF-alpha) and the revised quantitative insulin sensitivity check index (RQUICKI) turned out to be elevated. THI can explain 37% of the milk production variance. The percent variance explained is significantly higher after the addition of metabolic parameters in the THI+glucose (58%) and THI+glucose+TNF-alpha (65%) models and non-significantly higher after the addition of other metabolic parameters. Partial correlation analysis showed that a correlation between milk production and THI is significantly dependent on glucose. TNF-alpha showed a tendency to regulate the above-mentioned correlation, while other metabolic parameters showed a non-significant effect on the correlation between THI and milk production. It was concluded that variation in milk production during heat stress could be better predicted when THI is used in combination with glucose and TNF-alpha as a metabolic predictor. The effect of glucose and TNF-alpha in milk production during heat stress would be investigated in the next stage of our research