THE EFFECT OF HEAT STRESS ON MILK PROTEIN WITH RELEVANCE TO CHEESE MANUFACTURING

Heat stress has profound effects on the physiology of the cow, usually lowering voluntary feed intake which results in a sharp reduction in milk yield; at least this is what the farmer sees. It may also result in reduced fertility depending on the time frame the stress occurs. However, heat stress also affects other functions, amongst others milk composition with implications to cheese manufacturing. This is demonstrated in two studies, both published in the Journal of Dairy Science, Volume 98 of 2015; the one study by Dr U. Bernabucci and co-workers on pages 1815 to1827 with the title: Effect of summer season on milk protein fractions in Holstein cows and the other by Dr F.C. Cowley and colleagues on pages 2356 to 2368 with the title: Immediate and residual effects of heat stress and restricted intake on milk protein and casein composition and energy metabolism.

It is known that milk characteristics are affected by heat stress, but there is very little information about the changes in the milk protein fractions and their relationship with cheese-making properties of milk. Therefore, the main objective of the Bernaducci study was to evaluate the effect of hot season on milk protein fractions and cheese-making properties of milk destined for Grana Padano cheese production in Italy. The study was carried out in a dairy farm with a cheese factory. Temperature and relative humidity of the inside barn were recorded daily. During the experimental period, feed and diet characteristics, milk yield and milk characteristics were recorded in summer, winter and spring. Milk yield was recorded and individual milk samples were taken from 25 cows selected in each season during the afternoon milking. Milk fat, proteins, caseins (CN), lactose, SCC, acidity and milk rennet coagulation properties were recorded.

For all the main milk components (fat, protein, total solids, and solids-not-fat), the lowest values were observed in summer and the highest in winter. Casein fractions, with the exception of γ-CN, showed the lowest values in summer and the highest values in winter. The content of immunoglobulin and serum albumin was higher in summer than in winter and spring. A mild effect of season was observed for milk SCC, with higher values in summer than in winter and spring. A worsening of milk coagulation properties was observed in summer. The alteration of cheese-making properties during the hot season seems strictly linked with changes in the milk protein fractions, mainly associated with the decrease of αS-CN and β-CN and the increase in some other proteins.

In the Cowley study, the authors attempted to distinguish between the effects that are mediated by the reduced voluntary feed intake associated with heat stress, and the direct physiological and metabolic effects of heat stress. To that effect and to identify the effect on milk protein and casein concentration, they used 24 mid-lactation Holstein-Friesian cows. The cows were housed in temperature-controlled chambers and either subjected to heat stress [HS; temperature-humidity index (THI) ~78] or kept in an environment with a THI of less than 70. The latter cows were pair-fed with heat-stressed cows at a restricted intake for 7 days. A control group of cows was kept in the environment of THI less than 70, but was fed ad libitum. For the pair-fed groups, a subsequent recovery period of 7 days at THI of less than 70 and ad libitum feeding followed.

The results showed that intake accounted for only part of the effects of heat stress, since it reduced milk protein concentration, casein number and concentration, and increased urea concentration in the milk beyond the effects of restriction of intake. Under heat stress, the proportion in total casein of α_S1-casein increased and the proportion α_S2-casein decreased. Because no effect on milk fat or lactose concentration was found, these effects appeared to be the result of specific down-regulation of protein synthesis in the udder, and not a general reduction in udder physiological activity. No residual effects were found of heat stress or heat-stressed cows on restricted intake on milk production or composition after THI of less than 70 and ad libitum intake were restored. Heat-stressed cows had elevated blood concentrations of urea and calcium. Cows in heat stress at restricted intake had higher serum non-esterified fatty acid concentrations than cows in heat stress at ad libitum intake. It is clear that these two groups of cows mobilized different tissues as endogenous sources of energy. Apart from this effect, the two studies jointly showed that the protein fractions in the milk are substantially affected by heat stress, which could have major implications for the cost and composition of cheese made from milk of heat stressed cows.