by Yadav Sharma Bajagai
Reduction in milk production is one of the major economic impacts of climatic stress in dairy cattle. Decrease in milk yield due to heat stress is more prominent in Holstein than in Jersey cattle . Decreased synthesis of hepatic glucose and lower non esterified fatty acid (NEFA) level in blood during heat stress [2, 3, 4] causes reduced glucose supply to the mammary glands resulting low lactose synthesis which in turn ensues low milk yield . Reduction in milk yield is further intensified by decrease in feed consumption by the animals to compensate high environmental temperature [5, 6].
Reduced milk production due to heat stress is attributable only partly to decrease in feed intake . Actually 35% of reduced milk production is due to decreased feed intake while remaining 65% is attributable to direct effect of heat stress . Other factors resulting reduced milk production during heat stress are decreased nutrient absorption, effect in rumen function and hormonal status and increased maintenance requirement resulting reduced net energy supply for production [8, 9].
Milk production in cow has been found to be reduced when ambient temperature and temperature humidity index increases above critical threshold [10, 11]. Heat stress during 60 days prepartum period negatively affects postpartum milk production  and cows parturated during summer produce less milk as compared to other season . Similarly, quantity of milk protein and solid not fat (SNF) have been found to be reduced during heat stress in dairy cattle [7, 13, 14]. Mallonee  reported 20% less milk yield in cattle kept in sun than milk yield in cattle kept in shed. Similarly, Roman-Ponce  found 10.7% higher milk production in cows kept in shed than that in cows kept in sun during hot weather.