Effects of Heat Stress on Daily Milk Production of Dairy Cows in Different Lactation Periods

The effects of heat stress on the daily milk yield of dairy cows in different lactations were investigated in a largescale dairy farm at Huanggang city.The results showed that the mean daily milk production of early lactation cows in nonheat stress period and heat stress recovery period were significantly higher than those in moderate heat stress period(P 0.01).The mean daily milk production of middle lactation cows in non-heat stress period was significantly higher than those in moderate heat stress period(P0.05).Compared with the non-heat stress period,the mean daily milk production of early,middle and late lactation cows in moderate heat stress period decreased by 0.46,0.37 and 0.30 kg cattle,respectively,when the temperature-humidity index(THI) increased by one unit.The mean daily milk production of early,middle and late lactation cows in heat stress recovery period were higher than those in moderate heat stress period by 0.45,0.16 and 0.05 kg cattle,respectively,when THI decreased by one unit.It is indicated that the degree of heat stress affecting early lactation cows was greater than middle and late lactation cows.It is recommended that the mating and calving time of cows should be reasonable arranged.Mating in October,November and December,and calving in July,August and September should be avoided to avoid the early and middle lactation of them under heat stress period.