Crews D H Denny
Agriculture and Agri-Food Canada Research Centre, Lethbridge, Alberta, T1J 4B1, Canada.
Genet Mol Res. 2005 Jun 30;4(2):152-65.
Selection for the wide range of traits for which most beef breed associations calculate expected progeny differences focus on increasing the outputs of the production system, thereby increasing the genetic potential of cattle for reproductive rates, weights, growth rates, and end-product yield. Feed costs, however, represent a large proportion of the variable cost of beef production and genetic improvement programs for reducing input costs should include traits related to feed utilization. Feed conversion ratio, defined as feed inputs per unit output, is a traditional measure of efficiency that has significant phenotypic and genetic correlations with feed intake, growth rate, and mature size. One limitation is that favorable decreases in feed to gain either directly or due to correlated response to increasing growth rate do not necessarily relate to improvement in efficiency of feed utilization. Residual feed intake is defined as the difference between actual feed intake and that predicted on the basis of requirements for maintenance of body weight and production. Phenotypic independence of residual feed intake with growth rate, body weight, and other energy depots can be forced. However, genetic associations may remain when a phenotypic prediction approach is used. Heritability estimates for phenotypic residual feed intake have been moderate, ranging from 0.26 to 0.43. Genetic correlations of phenotypic residual feed intake with feed intake have been large and positive, suggesting that improvement would produce a correlated response of decreased feed intake. Residual feed intake estimated by genetic regression results in a zero genetic correlation with its predictors, which reduces concerns over long-term antagonistic responses such as increased mature size and maintenance requirements. The genetic regression approach requires knowledge of genetic covariances of feed intake with weight and production traits. Cost of individual feed intake measurements on potential replacements must be considered in implementation of national cattle evaluations for efficiency of feed utilization. These costs need to be compared to expected, and, if possible, realized rates of genetic change and the associated reduction in feed input requirements.
大多数肉牛品种协会计算预期后代差异所针对的广泛性状选择，侧重于提高生产系统的产出，从而提高牛在繁殖率、体重、生长速度和最终产品产量方面的遗传潜力。然而，饲料成本在牛肉生产可变成本中占很大比例，降低投入成本的遗传改良计划应包括与饲料利用相关的性状。饲料转化率定义为单位产出的饲料投入，是一种传统的效率衡量指标，与采食量、生长速度和成熟体型具有显著的表型和遗传相关性。一个局限性在于，饲料转化率直接或因生长速度提高的相关反应而产生的有利下降，不一定与饲料利用效率的提高相关。剩余采食量定义为实际采食量与根据维持体重和生产所需预测的采食量之间的差值。剩余采食量与生长速度、体重和其他能量储备之间的表型独立性可以被强制实现。然而，当使用表型预测方法时，遗传关联可能仍然存在。表型剩余采食量的遗传力估计适中，范围在0.26至0.43之间。表型剩余采食量与采食量的遗传相关性一直很大且为正，这表明改良会产生采食量下降的相关反应。通过遗传回归估计的剩余采食量与其预测因子的遗传相关性为零，这减少了对长期拮抗反应（如成熟体型增加和维持需求增加）的担忧。遗传回归方法需要了解采食量与体重和生产性状的遗传协方差。在实施全国牛群饲料利用效率评估时，必须考虑对潜在后备牛个体采食量测量的成本。这些成本需要与预期的以及如果可能的话实际的遗传变化率和相关的饲料投入需求减少进行比较。
