Marshall D M, Minqiang W, Freking B A
Dept. of Anim. & Range Sci., South Dakota State University, Brookings 57007.
J Anim Sci. 1990 Jul;68(7):1812-7. doi: 10.2527/1990.6871812x.
Relative date of first calving of beef heifers was studied in relation to production efficiency and subsequent reproductive performance. Crossbred heifers were managed in a drylot environment for 1 yr with feed intake measured through weaning of the first calf. Mean heifer age at entry into drylot was 572 d. Production traits were evaluated by calving group (CG), where CG1 included records of heifers calving (and calves born) in the first 21 d of the calving season for a particular year, CG2 included those calving from 22 through 42 d and CG3 included those calving after 42 d. Calving groups did not differ significantly for preweaning calf ADG, but weaning age differences resulted in heavier weaning weights for CG1 compared with CG2 and CG3. An earlier relative calving date was associated with increased cumulative feed energy intake of heifers and their calves during the 1-yr test period. In terms of production efficiency, the weaning weight advantage of earlier calving was offset only partly by the increased feed energy intake of the dam-calf unit, resulting in 6.3 Mcal (12.4%) less ME per kg calf weaning weight for CG1 vs CG3 for the 1-yr period. Within a limited calving season, earlier calving dams tended to be biologically and economically more efficient because a greater proportion of their annual production cycle was in a productive (lactating) mode, diluting maintenance costs as a fraction of all costs. Heifers in CG1 also tended to calve earlier than CG3 heifers for the second calf. Calving interval was a biased measure under the management conditions of a limited breeding season and culling of open cows.
研究了肉牛小母牛首次产犊的相对日期与生产效率及后续繁殖性能之间的关系。杂交小母牛在舍饲环境中饲养1年，饲料摄入量测量至第一头小牛断奶。进入舍饲时小母牛的平均年龄为572天。按产犊组（CG）评估生产性状，其中CG1包括特定年份产犊季节前21天内产犊（及所生小牛）的小母牛记录，CG2包括在第22至42天产犊的小母牛，CG3包括在42天后产犊的小母牛。断奶前犊牛的平均日增重（ADG）在各产犊组间无显著差异，但断奶年龄的差异导致CG1的断奶体重高于CG2和CG3。较早的相对产犊日期与1年测试期内小母牛及其犊牛累积饲料能量摄入量增加有关。就生产效率而言，较早产犊的断奶体重优势仅部分被母犊单位增加的饲料能量摄入量所抵消，导致CG1在1年期间每千克犊牛断奶体重比CG3少6.3兆卡（12.4%）的代谢能（ME）。在有限的产犊季节内，较早产犊的母牛在生物学和经济上往往更高效，因为它们年度生产周期中更大比例处于生产（泌乳）模式，从而降低了维持成本在所有成本中的占比。CG1中的小母牛第二胎产犊也往往比CG3中的小母牛早。在繁殖季节有限且淘汰空怀母牛的管理条件下，产犊间隔是一个有偏差的指标。
