MacNeil M D, Urick J J, Decoudu G
Fort Keogh Livestock and Range Research Laboratory, USDA, ARS, Miles City, MT 59301, USA.
J Anim Sci. 2000 Sep;78(9):2292-8. doi: 10.2527/2000.7892292x.
Simultaneous selection for low birth weight and high yearling weight has been advocated to improve efficiency of beef production. Two sublines of Line 1 Hereford cattle were established by selection either for below-average birth weight and high yearling weight (YB) or for high yearling weight alone (YW). Direct effects on birth weight and yearling weight diverged between sublines with approximately four generations of selection. The objective of this study was to estimate genetic trends for traits of the cows. A three-parameter growth curve [Wt = A(1 - b0e(-kt))] was fitted to age (t, d)-weight (W, kg) data for cows surviving past 4.5 yr of age (n = 738). The resulting parameter estimates were analyzed simultaneously with birth weight and yearling weight using multiple-trait restricted maximum likelihood methods. To estimate maternal additive effects on calf gain from birth to weaning (MILK) the two-trait model previously used to analyze birth weight and yearling weight was transformed to the equivalent three-trait model with birth weight, gain from birth to weaning, and gain from weaning to yearling as dependent variables. Heritability estimates were 0.32, 0.27, 0.10, and 0.20 for A, b0, k, and MILK, respectively. Genetic correlations with direct effects on birth weight were 0.34, -0.11, and 0.55 and with direct effects on yearling weight were 0.65, -0.17, and 0.11 for A, b0, and k, respectively. Genetic trends for YB and YW, respectively, were as follows: A (kg/generation), 8.0+/-0.2 and 10.1+/-0.2; b0 (x 1,000), -1.34+/-0.07 and -1.16+/-0.07; k (x 1,000), -14.3+/-0.1 and 4.3+/-0.1; and MILK (kg), 1.25+/-0.05 and 1.89+/-0.05. Beef cows resulting from simultaneous selection for below-average birth weight and increased yearling weight had different growth curves and reduced genetic trend in maternal gain from birth to weaning relative to cows resulting from selection for increased yearling weight.
为提高牛肉生产效率，人们主张同时选择低出生体重和高周岁体重。通过选择低于平均出生体重且高周岁体重（YB）或仅选择高周岁体重（YW），建立了1系赫里福德牛的两个亚系。经过大约四代选择后，两个亚系在出生体重和周岁体重上的直接选择效果出现了分化。本研究的目的是估计母牛性状的遗传趋势。对存活至4.5岁以上的母牛（n = 738）的年龄（t，天）-体重（W，千克）数据拟合了三参数生长曲线[Wt = A(1 - b0e(-kt))]。使用多性状限制最大似然法，将得到的参数估计值与出生体重和周岁体重同时进行分析。为了估计母牛从出生到断奶对犊牛增重的母体加性效应（MILK），之前用于分析出生体重和周岁体重的双性状模型被转换为以出生体重、出生到断奶的增重以及断奶到周岁的增重为因变量的等效三性状模型。A、b0、k和MILK的遗传力估计值分别为0.32、0.27、0.10和0.20。A、b0和k与出生体重直接效应的遗传相关分别为0.34、-0.11和0.55，与周岁体重直接效应的遗传相关分别为0.65、-0.17和0.11。YB和YW的遗传趋势分别如下：A（千克/代），8.0±0.2和10.1±0.2；b0（×1000），-1.34±0.07和-1.16±0.07；k（×1000），-14.3±0.1和4.3±0.1；以及MILK（千克），1.25±0.05和1.89±0.05。与选择高周岁体重的母牛相比，同时选择低于平均出生体重和增加周岁体重所产生的肉牛母牛具有不同的生长曲线，且从出生到断奶的母体增重遗传趋势降低。
