Evaluation of Merit of the Optimum Index-Based Selection to Improve Ratio Traits Using the Japanese Black Cattle Data

Abstract

Shojo, M. and Iwaisaki, H. 1999. Evaluation of merit of the optimum index-based selection to improve ratio traits using the Japanese black cattle data. J. Appl. Anim. Res., 16: 153–162. Ratio traits are widely applied for evaluating and selecting for energy, growth or production efficiency in domestic animals. In this study, relative to direct selection for a ratio trait, the efficiency of selection based on the optimum linear index to achieve the genetic improvement of the ratio trait was evaluated numerically, assuming one cycle of truncation selection and using the estimates of phenotypic and, genetic parameters for ratio component traits in the Japanese Black cattle. Ratio traits studied were TDN efficiency (TDNE) defined, as the ratio of weight gain to TDN intake and roughage intake ratio (RIR) expressed as roughage intake divided by total feed intake. According to the parameter situations of the component traits, or the numerator and denominator traits of TDNE and RIR, selection pressure put on the ratio traits in the direct selection was shown to act on the corresponding component traits by a disproportionate fashion. The expected correlation between the optimum and ratio criteria ranged from .97 to .89 and from .55 to .34 for TDNE and RIR, respectively, when the proportion selected was assumed to be 1% to 50%. For improving TDNE and RIR in the current population, selection based on the optimum index was expected to be approximately 6% to 15% and twice to three times more efficient than direct selection, respectively. However, the ratio genetic responses to the optimum index-based selection were expected to be achieved, by differentially decreased population means of the component traits, indicating that the optimum index-based selection, as well as the ratio-based selection, is not able to control the directions and the magnitudes of the genetic changes in the component traits. Therefore, it seems that merit of using the optimum index is limited, if the goal is to bring about the genetic improvement of a given ratio trait with controlling the genetic responses of the component traits in biologically efficient, specific directions.