Aharoni, Y., Department of Beef Cattle, Agricultural Research Organization, Newe Ya'ar Research Center, P.O. Box 1021, Ramat Yishay 30095, Israel
Brosh, A., Department of Beef Cattle, Agricultural Research Organization, Newe Ya'ar Research Center, P.O. Box 1021, Ramat Yishay 30095, Israel
Ezra, E., Israel Cattle Breeders Association, P.O. Box 3015, Qesarya Industrial Park 38900, Israel

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 and 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 database to analyze the effects of the day length and the daily change in day length 3 wk prepartum. The prepartum day length effect was negatively related to milk yield and to milk fat and protein percentages. The daily change in day length was negative for milk yield and lactose percentage and positive 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 after 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.