SIGNIFICANCE OF PHYSIOLOGICAL AGE OF LEGHORN PULLETS IN TERMS OF SUBSEQUENT REPRODUCTIVE CHARACTERISTICS AND ECONOMIC-ANALYSIS

Two experiments were conducted to note layers' response to light stimulation at different physiological or chronological ages. In Experiment 1, 40 pullets of mean body weight from within a population of some 1,000 pullets were moved to laying cages at 15 wk of age and the photoperiod changed from 8 to 14 h/day. An equal number of pullets were moved weekly to 21 wk of age and light stimulated at these times. In the laying house, birds were maintained in single cages with 8 replicate groups of 5 adjacently caged birds in each treatment. Conventional production data were collated with egg size classification, and economic analysis in terms of egg plus fowl income minus feed cost. Age at light stimulation had no effect on egg production or egg mass production to 329 days of age (P > .05). Earlier light stimulation resulted in production of fewer extra large eggs and more medium and small eggs (P < .01). Because there was no difference in egg pricing for large and extra large eggs, treatment had no effect on gross income minus feed cost (P > .05). In a second experiment, all birds were selected at 18 wk of age, moved to laying cages, and light stimulated (14 h/day) at this time. Experimental treatments involved varying body weight groups of from 1,060 to 1,543 g per bird, with the 6 groups each represented by 10 replicate groups of 5 individually and consecutively caged birds. Performance and economic analyses as described in Experiment 1 were undertaken to 518 days of age. Birds with smaller body weights at 18 wk of age did not exhibit any compensatory growth (P > .05). These smaller weight birds also ate less feed and produced eggs with reduced mean weight (P < .01), although egg production was not affected. Larger birds at maturity produced more extra large eggs (P < .01). However, there was no treatment effect on total eggs produced to 518 days of age or in egg plus fowl income minus feed cost (P > .05). For the larger birds, higher feed costs offset increased egg income related to increased egg mass output. The results of the two studies indicate the large effect that body weight at light stimulation can have on egg weight. The economic significance of this situation will vary dependent upon the price differential between large and extra large size eggs.