Effects of heat load and photoperiod on milk yield and composition in three dairy herds in Israel

Effects of hear load and of photoperiod on lactation performance were evalu ated using milk test data of three Israeli Holstein herds over a period of 3 years, from 1994 to 1996. All together 2209 cows, with 28029 milk records , were included. Photoperiod effects were examined as associated with day l ength and daily changes in day length and heat load index was formulated as a function of the seasonal day and night ambient temperatures, to account for the heat load effect. The regression model included effects of cow, her d, year, lactation number and days in milk in addition to the seasonal effe cts. The dependent variables were milk yield and fat, protein and lactose c oncentrations. Milk yield was affected by both photoperiod and hear load, w ith the peak photoperiod effect in May and amplitude of 3.1 (s.d. 0.91 kg/d ay and negative heat load effect of -1.8 (s.d. 0.4) kg/day at its peak. Pro tein concentration was affected by photoperiod, with the peak effect in Jan uary (amplitude of 1.7 (s.e. 0.51 g/kg) but not by heat load (-0.2 (s.d. 0. 6) g/kg). Far concentration was affected primarily by heat load (-3.4 (s.d. 0.7),g/kg), with a photoperiod effect which peaked in October (amplitude o f 1.8 (s.d. 0.8) g/kg). Lactose concentration was affected by both environm ental factors to a lesser extent (photoperiod amplitude of 0.6 (s.d. 0.2) g /kg and heat load effect of -0.03 (s.d. 0.16) g/kg). Comparison of the pred icted seasonal effects on milk yield and composition with the annual punctu ation in the national herd showed a good match of the predicted effects wit h the national observations. It is concluded that while heat load relief ma y be beneficial, manipulation of the photoperiod may induce adverse effects on milk yield and composition.