Short communication: Prepartum photoperiod effect on milk yield and composition in dairy cows

In a previous paper we analyzed the effects of day length, the daily change in day length, and heat load prevailing on test days, and on milk yield an d composition of dairy cows in hot weather. For this analysis we used milk tests of three herds in Israel between 1994 and 1996. We used the same data base to analyze the effects of the day length and the daily change in day l ength 3 wk prepartum. The prepartum day length effect was negatively relate d to milk yield and to milk fat and protein percentages. The daily change i n day length was negative for milk yield and lactose percentage and positiv e for protein content and did not affect fat content. The difference of 4 h between the shortest and the longest day, plus the seasonal change in day length, accounted for the addition of 1.9 kg of milk/d for a cow calving af ter the shortest day compared with a cow calving after the longest day. The difference in milk composition between these two cows was estimated to be 0.27 and 0.08% of fat and protein, respectively.