To evaluate the effect of breeder age and egg size on yolk absorption and embryo development, a total of 4,800 Ross 308 hatching eggs were subjected to 4 treatments arranged in a 2 × 2 factorial randomized complete block design using 2 breeder ages (29 and 53 wk of age, or young and old) and 2 egg sizes (57-61 g and 66-70 g, or small and large). A significant interaction between breeder age and egg size was found for egg composition. Yolk weight increased with flock age, whereas a larger egg size resulted in higher albumen content. A significant interaction between breeder age and egg size was found for yolk-free body (YFB) weight only at d 7. Until the fourteenth day of incubation, eggs from the old flock yielded greater YFB weight than did eggs from the young flock. At hatch, chicks of both age groups had comparable wet YFB weight, chick weight, wet and dry residual yolk weight, and chick length. Dry YFB weight of chicks from the old flock was higher than that of chicks from the young flock. Compared with the small eggs, embryos and chicks of the large eggs had greater YFB weight from d 14 to hatching. At hatch, these chicks were also heavier, longer, and had higher wet and dry YFB and residual yolk weight. Yolk absorption at d 18 and at hatch of embryos and chicks of the old flock was higher than that of the young flock, both in absolute values and percentages. Rates of absolute and percentage yolk absorption through d 18 and percentage yolk absorption at hatch were higher in the small eggs than in the large eggs. It can be concluded that egg size influences chick length at hatch and embryo development when expressed in terms of total and YFB weight. Although yolk availability and rate of absorption may have influenced dry YFB weight, they did not influence hatching chick length or total and YFB weight.

Two experiments were conducted to study the interaction between breeder age and egg size on the energy utilization (experiment 1) and heat production (experiment 2) of broiler embryos. In experiment 1, a total of 4,800 Ross-308 hatching eggs from 2 breeder ages (29 and 53 wk of age, or young and old) and, within each age, 2 egg sizes (57 to 61 g and 66 to 70 g, or small and large) were used. In experiment 2, a total of 240 Ross-308 hatching eggs from 2 breeder flocks at 29 (young) and 53 (old) wk of age, and which were selected from the same egg weight range (58 to 61 g), were tested in 2 replicate chambers. In experiment 1, it was shown that the amount of yolk relative to albumen was higher in the old flock eggs, and this effect was more pronounced in the large eggs. The old flock eggs, especially the larger egg size, contained more energy as a result of a greater yolk size. Energy utilization of the embryos was positively related to yolk size and the amount of energy transferred to yolk-free body (YFB) was largely determined by the available egg energy. The efficiency of converting egg energy into chick body energy (E(YFB)) was equal for both egg sizes and both breeder age groups. Chick YFB weight of young and old flock eggs was equal. However, dry YFB weight of chicks from old flock eggs was higher than in chicks from young flock eggs, which was associated with more protein and fat content and thus more energy accumulated into YFB. As a consequence, embryos derived from old flock eggs produced more heat from d 16 of incubation onward than those of the young flock eggs. In conclusion, the higher energy deposition into chick YFB of old flock eggs, leading to higher embryonic heat production, is the result of a higher amount of available energy in the egg and is not due to changes in E(YFB).

Breeder age and broiler strain can influence the availability of nutrients and oxygen, particularly through differences in yolk size and shell conductance. We hypothesized that these egg characteristics might affect embryonic responses to changes in eggshell temperature (EST). This study aimed to investigate the effect of breeder age, broiler strain, and EST on development and physiological status of embryos. A study was designed as a 2 × 2 × 2 factorial arrangement using 4 batches of 1,116 hatching eggs of 2 flock ages at 29 to 30 wk (young) and 54 to 55 wk (old) of Ross 308 and Cobb 500. EST of 37.8 (normal) or 38.9°C (high) was applied from incubation d 7 (E7) until hatching. The results showed that breeder age rather than broiler strain had an influence on yolk size (P = 0.043). The shell conductance was higher in Ross 308 than in Cobb 500 (P < 0.001). A high EST resulted in a higher yolk free body mass (YFBM) compared to the normal EST at E14 and E16, but at 3 h after hatch YFBM was lower when eggs were incubated at high EST compared to normal EST (all P < 0.001). Cobb 500 eggs yielded embryos with a lower YFBM at E14, E18, and 3 h after hatch (all P < 0.05) than Ross 308 eggs. Breeder age had no effect on YFBM, but the RSY weight was higher in embryos from the old flock compared to the young flock embryos at E14 and E16 (both P < 0.05). A 3-way interaction among breeder age, strain, and EST was found, especially for incubation duration, navel quality, and relative heart and stomach weights at 3 h after hatch (all P < 0.05). Based on the results obtained, we conclude that oxygen availability rather than nutrient availability determines embryonic development, and the egg characteristics affected embryonic responses to changes of EST, especially for variables related to chick quality.

Breeder age and broiler strain influence the availability of nutrients and oxygen through yolk size and eggshell conductance, and the effects of these egg characteristics on nutrient metabolism might be influenced by eggshell temperature (EST). This study aims to determine effects of breeder age, strain, and EST on nutrient metabolism of embryos. A study was designed as a 2 × 2 × 2 factorial arrangement using four batches of in total 4,464 hatching eggs of 2 flock ages at 29 to 30 wk (young) and 54 to 55 wk (old) of Ross 308 and Cobb 500. EST of 37.8 (normal) or 38.9°C (high) was applied from incubation day 7 (E7) until hatching. Wet yolk weight was determined mainly by breeder age (P = 0.043). Energy content in yolk (P = 0.004) and albumen + yolk (P = 0.005) were higher in old flock eggs than in young flock eggs, but did not differ between broiler strains. Eggshell conductance was higher in Ross 308 eggs than in Cobb 500 eggs (P < 0.001). Old flock embryos used more energy (P = 0.046) and accumulated more energy into yolk free body mass (YFBM; P = 0.030) than young flock embryos, whereas heat production (HP), energy lost, and efficiency of converting energy used to YFBM (EYFB) did not differ. Ross 308 embryos used more energy (P = 0.006), had a higher energy lost (P = 0.010), and a higher HP between E15 to E18 (P < 0.05) than Cobb 500 embryos. Energy content in YFBM did not differ between strains and EYFB (P = 0.024) was lower in Ross 308 than in Cobb 500. High EST resulted in higher HP than low EST from E11 to E15 (P < 0.05), but not after E15. Amount of energy used (P = 0.006) and energy accumulated in the YFBM (P < 0.001) was lower for embryos incubated at an EST of 38.9 than that of 37.8°C, whereas EYFB did not differ. In conclusion, breeder age, broiler strain, and EST differentially influence embryonic metabolism and particularly the availability of oxygen could have contributed to these differences.

Selection for production traits of broilers and layers leads to physiological differences, which may already be present during incubation. This study aimed to investigate the influence of strain (broiler vs layer) on egg nutrient availability, embryonic development and nutrient metabolism. A total of 480 eggs with an egg weight range of 62.0 to 64.0 g from Lohmann Brown Lite and Ross 308 breeder flocks of 41 or 42 weeks of age were selected in two batches of 120 eggs per batch per strain. For each batch, 30 eggs per strain were used to determine egg composition, including nutrient and energy content, and 90 eggs per strain were separately incubated in one of two climate respiration chambers at an eggshell temperature of 37.8°C. The results showed that broiler eggs had a higher ratio of yolk: albumen with 2.41 g more yolk and 1.48 g less albumen than layers. The yolk energy content of broiler eggs was 46.32 kJ higher than that of layer eggs, whereas total energy content of broiler eggs was 47.85 kJ higher compared to layer eggs. Yolk-free body mass at incubation day 16 and chick weight and length at hatch were higher in broilers compared to layers. Respiration quotient of broiler embryos was higher than layer embryos during incubation day 8 to incubation day 10. A 0.24 g lower residual yolk at the hatch of broiler embryos than for the layer embryos indicated that broiler embryos used more yolk and had a higher energy utilization and energy deposition in yolk-free body mass. Heat production of broiler embryos was higher than that of layer embryos from incubation day 12 to incubation day 18, but efficiency of converting egg energy used by embryos to form yolk-free body mass was similar. In conclusion, broiler and layer embryos have different embryonic development patterns, which affect energy utilization and embryonic heat production. However, the embryos are equal in efficiency of converting the energy used to yolk-free body mass.