Two experiments were conducted to evaluate effects of strain [five in Experiment (Exp.) 1 and six in Exp. 2)] and age (29, 47, and 57 wk in Exp. 1 and 29, 41, and 52 wk in Exp. 2) of commercial broiler breeders on incubation time and chick weight. Highly significant differences in egg weight were found among strains in both Exp. After adjusting for effects of egg weight, significant effects of strain, age, and their interactions were found on incubation time, egg weight at transfer, and chick weight at hatch in Exp. 1, but not in Exp. 2. Mean incubation times varied among strains from 496.6 to 498.8 h in Exp. 1 and from 499.3 to 501.9 h in the second experiment. In Exp. 1, incubation time decreased from 498.6 h when breeders were 29 wk to 494.8 at 47 wk, whereas in Exp. 2, it decreased from 510.5 h at 29 wk to 495.1 h at 41 wk. This decrease also resulted in a negative correlation between egg weight and incubation time. Differences due to strain and age were found for yolk and albumen percentage and yolk: albumen ratio. Percentage yolk was 27.2 and 32.7% and percentage albumen was 60.1 and 55.9% in eggs from 29 to 52 wk breeders, respectively. Shell percentage was significantly affected by strain. Strain by age interactions were found for each response in Exp. 1 but only for set and chick weight in Exp. 2. Differences among incubators were found only for incubation time; interactions of incubation time and strain and age were also detected. Results indicate that genotype, age of the female breeder, and incubator should be considered along with their interactions to obtain optimum hatching performance.

Three experiments were conducted, from January until September 2001, to estimate the optimized age to apply feed restriction to control mortality from ascites, with no negative effects on production and carcass characteristics of broilers. For each experiment, 1,200 1-d-old mixed Ross x Peterson chicks were reared in floor pens (50 chicks in each) and fed commercial feed. Feed restriction was applied for 8 h/d for 14 d at 21 or 28 d of age in experiment 1, 14 or 21 d in experiment 2, and 7 or 14 d in experiment 3. In experiments 2 and 3, a microelement supplement (without or with) was tested; the control groups received feed ad libitum and no supplement. Body weight gain, feed conversion, total mortality, and mortality from ascites, leg problems, and carcass characteristics were considered at the end of each experiment. The data were analyzed as a completely randomized design, or as a 2 x 2 factorial to estimate main and interaction effects (experiments 2 and 3). Additional analyses, including the control, were done; means comparisons were by orthogonal contrasts. The production and carcass characteristics of the restricted groups were lower than the control but were not statistically different in experiments 2 and 3, although the optimized age for feed restriction was at 7 d. Total mortality and mortality from ascites decreased by restriction, but leg problems increased without supplement. The results indicated that quantitative feed restriction and microelement supplementation at 7 d of age reduced mortality from ascites and leg problems and permitted compensatory growth sufficient to equal the production characteristics of the control group at 49 d of age. However, it is necessary to determine the specific microelements to be supplemented and to estimate the effects of season and genetic line.

A series of experiments was conducted to estimate phenotypic correlations between incubation characteristics, and to evaluate the effects of cold stress and genotype during incubation on chick weight, egg weight loss, hatching time, and embryonic mortality. Eggs were cooled at 18 or 24 C, for 12, 24, 36, 48, or 72 h beginning on Day 8, 12, 14, 16, or 18 of incubation. Other eggs were cooled intermittently for 6 h every 48 h or 12 h every 96 h. A control group in each experiment was not cold stressed. Results indicated a low and negative correlation between hatching time and chick weight, and a low and positive correlation between hatching time and weight loss from transfer to hatching when variability due to egg weight was removed. Chick weights at hatching were lower in chicks from cooled eggs than those of chicks from eggs incubated under normal temperature. The chicks from cooled eggs were more susceptible to dehydration during holding in the hatcher. Incubation times were delayed approximately as long as the times of embryonic cooling. Embryonic mortality was significantly increased under continuous (single period) cold stress, but not under intermittent cooling (6 h every 48 h). Significant genotype by environment interactions were found in the response of embryos of various strains to cold stress. Exposure for 36 h or longer had detrimental effects on chick weight and embryo viability, but these effects were modified by interactions among the factors involved. The results indicated that embryos from cooled eggs lose more weight during incubation and that the neonatal chicks are more susceptible to dehydration during holding time, and have a longer incubation period, and a greater embryonic mortality.

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