Effects of restricted feeding in the growing and laying periods on the performance of White Leghorn by Australorp crossbred and White Leghorn strain cross chickens.

1. The effects of restricted feeding in the rearing and laying periods on the performance of three White Leghorn cockerel by Australorp pullet crossbred strains and one White Leghorn strain cross were studied in two experiments. 2. After feeding ad libitum in the rearing period, mean body weights of the three crossbred strains at 20 weeks of age were 1.75, 1.66 and 1.55 kg and that of the White Leghorn strain cross was 1.40 kg. The differing restrictions during rearing reduced mean body weight at 20 weeks by 14 to 33% and increased the time to sexual maturity by 8 to 23 d. 3. All restriction regimes during rearing increased mean egg weight and tended to reduce mortality in the laying period. Optimum hen-day egg number over 64 weeks of lay was obtained following mild restriction (14% body weight reduction) of the two heavier strains or ad libitum feeding of the lighter strains. Over 48 or 64 weeks of lay, egg number per hen housed (at 18 to 20 weeks of age) was optimised after moderately restricting (14 to 21% body weight reduction) the crossbred strains or feeding the White Leghorn strain cross ad libitum. 4. Over 64 weeks of lay, mortality was reduced from 19.3% to 10.5% in the lightest crossbred strains, in which a high proportion of deaths were associated with Marek's disease and lymphoid leucosis, by restriction during rearing. 5. Restricting food intake by 7 or 8% throughout the laying period reduced hen-housed egg number and mean egg weight of all four strains. The reduction in hen-day egg number associated with food restriction during lay was less for the heavier strains that had also been restricted during rearing. 6. Metabolisable energy intakes required to support maximum production over 64 weeks of lay were 1.23 to 1.36 MJ/d.