SELECTION FOR HIGH AND LOW-THRESHOLD BODY-WEIGHT AT FIRST EGG IN BROILER STRAIN FEMALES .6. THRESHOLD WEIGHT FOR ONSET OF PHOTOSENSITIVITY IN FEMALES OF THE SELECTION LINES

A two-way selection program, tarried out for high and low threshold BW at onset of lay (TWL) resulted in a marked difference in TWL in the s elected lines. Later studies showed that the selection procedure had d ecreased photoperiodic drive in the high line (HL) as compared to the low line (LL), in this way increasing the number of days required from onset of photosensitivity to onset of lay in the HL as compared to th e LL. The present study was carried out to determine whether the diffe rence in photoperiodic drive was sufficient to account for the entire difference in TWL between the HL and LL, or whether it is also necessa ry to postulate a difference in threshold weight for onset of photosen sitivity (TWSM) as well. Birds of the two lines were raised under feed restriction to 22 wk reaching weights (1,881.8 g for LL, 2,034.8 g fo r HL), well below anticipated TWSM. They were then provided gradually increasing amounts of feed, resulting in daily gains of 3 to 5 g for t he LL and 8 to 9 g for the HL. Under this regimen, the HL entered lay an average of 64.4 d later and at a body weight 752.8 g greater than t he LL. On the basis of previous studies of these lines, of the 64.4 d delay in onset of lay in the HL as compared to LL, 21 d can be attribu ted to the difference in photoperiodic drive. The excess of 43 d must thus represent a difference in TWSM. During this period the HL were ga ining about 8.5 g/d, and at 22 wk HL body weight was already 153 g gre ater than LL body weight. In all then, TWSM of HL is estimated at 518 g greater than that of the LL. Thus, the selection procedure employed for TWL apparently affected TWSM as well as photoperiodic drive.