Effect of ""effective temperature"" on milk yield of Holstein cows in hot and humid environments

This study was conducted to clarify the effects of ambient temperature and humidity on milk yield of Holstein cows in hot and humid environments. Milk yield and dry matter intake of 17 Holstein cows were determined from spring to fall, the years 1997 and 1998. Dry and wet bulb temperature was recorded for the period. Daily milk yield of the cows ranged from 22 to 44kg per day, when they were managed in comfortable conditions. The index of temperature and humidity (effective temperature) was calculated using the following formula ; Effective temperature=Dry bulb temperature (deg C)*0.35+Wet bulb temperature (deg C)*0.65. The expected milk yield is estimated by the two sets of data method to eliminate the influence of individual difference. The time series analysis and the segmented model of the non-linear regression analysis were used to determine the daily mean effective temperature (ET) where milk yield began decreasing. Milk yield began decreasing from the month of May, and a greater decline occurred in late June. The depression in milk yield per day increased with the rise of production level (P