PHOTOPERIODIC EFFECT ON LIVE-WEIGHT GAIN OF BULL CALVES

The effects of day length, of the daily change in day length and of he at load, together with age effect, on live-weight gain of Holstein-Fri esian 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 4 50 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 quadrati c. Three heat load indexes were calculated, accounting for day tempera tures above 27 degrees C, or night temperatures above 18 degrees C, or both, and their effect was assumed to be linear also. Random effect o f calf and fixed effect of the year were also accounted for by the reg ression analysis. The mean gain was 1.274 kg/day. The effect of day le ngth tons 0.027 (s.e, 0.003) kg/day per h, and effect of day length ch ange was 0.042 (s.e. 0.003) kg/day per min/day both effects being high ly significant (P < 0.0001). The effects of heat load according to eac h of the three indexes were either not significant, or tended to be po sitive, which implies increased gain with increasing heat load. Effect of age was positive (P = 0.0005), and of age(2) was negative (P < 0.0 001). Based on the regression model that did not include heat load eff ect, 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 t rough 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 heav ier 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.