Aharoni, Y., Department of Beef Cattle, Agricultural Research Organization, Newe Ya'ar Research Center, PO Box 90000, Haifa 31900, Israel
Brosh, A., Department of Beef Cattle, Agricultural Research Organization, Newe Ya'ar Research Center, PO Box 90000, Haifa 31900, Israel
Holzer, Z., Department of Beef Cattle, Agricultural Research Organization, Newe Ya'ar Research Center, PO Box 90000, Haifa 31900, Israel

The effects of day length, of the daily change in day length and of heat load, together with age effect, on live-weight gain of Holstein-Friesian bull calves, were studied using 8205 gain records of 1019 calves that were maintained in the experimental herd at Newe Ya'ar during a 5-year period (1991 to 1995). The age range of the calves was 150 to 450 days. Effects of day length (h) and of day length change (min/day) were assumed to be linear and effect of age was assumed to be quadratic. Three heat load indexes were calculated, accounting for day temperatures above 27°C, or night temperatures above 18°C, or both, and their effect was assumed to be linear also. Random effect of calf and fixed effect of the year were also accounted for by the regression analysis. The mean gain was 1·274 kg/day. The effect of day length was 0·027 (s.e. 0·003) kg/day per h, and effect of day length change was 0·042 (s.e. 0·003) kg/day per min/day both effects being highly significant (P < 0·0001). The effects of heat load according to each of the three indexes were either not significant, or tended to be positive, which implies increased gain with increasing heat load. Effect of age was positive (P = 0·0005), and of age2 was negative (P < 0·0001). Based on the regression model that did not include heat load effect, the peak gain was obtained on 19 May, 33 days before the longest day and the trough was obtained on 18 November, with a difference of 0·206 kg/day (proportionately 0·15 of the peak gain) between peak and trough gains. It was calculated by the regression coefficients for the photoperiod effects, that a calf that enters the feedlot at the age of 150 days and a live weight of 180 kg on 1 January, will be 23 kg heavier at the age of 350 days than a calf that enters the feedlot at the same age and weight on 1 July. This difference is reduced to 10 kg at the age of 450 days.