Effect of hen age, body weight, and age at photostimulation. 2. Embryonic characteristics of commercial turkeys

Turkey hens from two BW groups (which averaged 11.8 and 12.9 kg, Normal and Heavy, respectively) were photostimulated at either 29 or 31 wk of age to determine how changes in egg weight and egg component weights with hen age affect subsequent embryonic growth and yolk sac lipid mobilization. At 2-wk intervals during the first 10 wk of lay, all eggs were collected, individually weighed, and incubated. A subsample of eggs from each photostimulation (PS) age and BW group were randomly selected for yolk and albumen weight determinations and embryo weight, liver weight, and yolk sac measurements at 21 and 25 d of incubation. Yolk and yolk sac lipid measurements were done on similar sized eggs selected at 4 to 6 and 12 to 14 wk after PS. Yolk-free embryo weight, liver weight, and yolk sac weight at 21 and 25 d of incubation increased during the first 10 wk of lay. Neither hen age nor BW at PS had any consistent effects on yolk-free embryo weight, liver weight, or yolk-sac weight. When similar-sized eggs (80 to 85 g) were selected for analyses, yolk lipid content did not change with hen production age. The lipid content of the yolk sac was 0.97 g greater in 21-d embryos from hens 12 to 14 wk after PS than from hens 4 to 6 wk after PS. Differences in yolk sac residual lipid and lipid subclass characteristics were not evident after 25 d of incubation. In conclusion, hen BW at PS or age at PS had minimal affects on embryonic growth during the last week of incubation, and most differences in embryonic growth were attributed to differences in yolk sac lipid mobilization between hen production ages independent of egg size.