Ricklefs R E
Poult Sci. 1985 Aug;64(8):1563-76. doi: 10.3382/ps.0641563.
Growth curves of selected and unselected lines of broilers and Japanese quail show that chicks respond to selection for 8-week or 4-week body mass, respectively, by increasing the exponential growth rate during the first 2 weeks after hatching. Comparative studies indicate that growth rate varies among species of birds according to adult body mass, developmental maturity of the chick at hatching, and the postnatal growth increments of the skeletal muscles of the legs. Differentiation of tissues leading to mature function apparently precludes embryonic function and decreases proliferation and growth. Precocial species, such as the domestic fowl, grow only one-quarter as fast, on average, as altricial species of the same size, whose chicks are less mature at hatching and depend more on parental care for food, warmth, and protection. Among precocial species, those whose legs have the smallest postnatal growth increments grow most rapidly overall. The domestic fowl is among the slowest growing of precocial species. Although diet quality may limit growth rate, this should not be a factor in selection programs, because high quality diets can be provided. Rate of food assimilation apparently also is limiting, but it can be increased experimentally by force feeding and presumably is sufficiently selectable so as not to limit breeding programs designed to increase growth rate. The response of broilers and quail to selection of body mass apparently affects the rate of proliferation of skeletal muscles during the early posthatching period. Muscle quality is not affected. There is inconclusive evidence of a shift in muscle cell nuclei from differentiated to proliferative pools in selected lines. Furthermore, selected broilers apparently are less capable than unselected chicks of generating body heat, which requires functional skeletal muscle during the first week posthatching. Growth performance of broilers might be increased beyond present levels by selecting directly for reduced or delayed maturity (e.g., thermogenesis, flight) of chicks during the first 2 weeks and by selecting smaller legs in adult birds. Any attempt to formulate a selection program on these ideas would require additional basic research and might be thwarted by economic considerations or by offsetting selection caused by phenotypic responses that strain certain functional relationships in the growing chick.
对肉鸡和日本鹌鹑选定品系与未选定品系的生长曲线研究表明，雏鸡分别针对8周龄或4周龄体重进行选择时，会在孵化后的前两周提高指数生长率。比较研究表明，鸟类的生长速率因成年体重、雏鸡孵化时的发育成熟度以及腿部骨骼肌的出生后生长增量而异。导致组织成熟功能的分化显然排除了胚胎功能，并降低了增殖和生长。早成鸟物种，如家禽，平均生长速度仅为相同大小的晚成鸟物种的四分之一，后者的雏鸡在孵化时不太成熟，在食物、温暖和保护方面更依赖亲代照顾。在早成鸟物种中，腿部出生后生长增量最小的物种总体生长速度最快。家禽是生长速度最慢的早成鸟物种之一。虽然日粮质量可能会限制生长速度，但在选择计划中这不应该是一个因素，因为可以提供高质量的日粮。食物同化率显然也有局限性，但通过强制喂食可以在实验中提高，并且推测其具有足够的可选择性，不会限制旨在提高生长速度的育种计划。肉鸡和鹌鹑对体重选择的反应显然会影响孵化后早期骨骼肌的增殖速率。肌肉质量不受影响。关于选定品系中肌肉细胞核从分化池向增殖池转移的证据尚无定论。此外，选定的肉鸡在产生体热方面的能力显然不如未选定的雏鸡，而在孵化后的第一周产生体热需要功能性骨骼肌。通过直接选择雏鸡在最初两周内降低或延迟成熟（例如产热、飞行）以及选择成年鸟较小的腿部，肉鸡的生长性能可能会提高到目前水平以上。基于这些想法制定选择计划的任何尝试都需要更多的基础研究，并且可能会受到经济因素或由表型反应引起的抵消性选择的阻碍，这些表型反应会破坏雏鸡生长过程中的某些功能关系。
