Heat stress in dairy animals and current milk production trends, economics, and future perspectives: the global scenario

Exogenous bovine somatotropin and mist-fan cooling synergistically promote the intramammary glucose transport for lactose synthesis in crossbred Holstein cows in the tropics

“…This implies that the re-radiation heat load at 14.00-15.00 h could be reduced by installing the MF. Although the MF barn area had a significantly lower mean AT (p<0.05) than the NS barn, this may not be sufficient to eliminate heat stress in cows because the range for THI measured in the daytime in the MF barns throughout the experimental periods remained higher than the THI threshold level (72) for a comfort zone [4]. Indeed, the THI in both NS and MF barns ranged from 80 to 85, similar to the findings in a previous report [18].…”

“…However, the THI might not accurately reflect the heat stress in the crossbred lactating MF cows, as although the MF cooling system, which delivers a pressurized spray with considerable fanned air movement in the barn, results in higher humidity, it also causes a cooling effect. The MF cows had significantly lower RR and RT than the NS cows, indicating partial alleviation of the heat stress by the MF system, especially in the afternoon [4,18].…”

“…It has been reported that glucose and electrolytes are secreted across the mammary epithelium from the blood side to the milk side through the membrane route. The mammary gland can generate and maintain steep Na + , K + , and Cl − gradients between milk and plasma, where the K + concentration in milk, in particular, is considerably higher than that in plasma [4]. Indeed, K + channels have been shown to be localized at the apical membrane of mammary epithelial cells and are associated with milk-borne negative feedback regulation of milk secretion [4].…”

“…The regulation of biosynthetic capacity within the mammary gland during prolonged rbST treatment demonstrated that the proportion of utilized glucose in the mammary gland that was metabolized Available at www.veterinaryworld.org/Vol.14/May-2021/24.pdf for lactose synthesis was reduced, causing a decline in milk yield with advanced lactation [3]. These studies did not establish a relationship between the mechanism of action of bST and the effects of a high environmental temperature, including which factors were the cause and which factors were the effects of such change, although a high environmental temperature would be another factor affecting milk production in dairy cows in the tropics [4]. Several investigations have reported inconsistent observations about the action of bST treatment in dairy cows and goats kept at a high environmental temperature.…”

“…Other investigators reported that a high ambient heat load reduced the response to bST treatment and conferred low milk yields [6], while heat-stressed cows had an elevated rectal temperature (RT) and respiratory rate (RR), which was exacerbated in cows receiving bST treatment [7]. However, these lines of evidence were not definitive since another study in dairy cows showed that the reduction in milk yield during heat stress was modulated by a negative regulatory feedback system present in milk [4]. Against this background, the question arises of whether low milk production in crossbred dairy cows is due to the effect of a lower bST level per se, or from the combination of the effect of a high environmental temperature modulating a negative regulatory feedback system, where these combined effects on the mammary gland function remain obscure.…”