Prediction of scrotal circumference expected progeny differences in limousin cattle
Keeton, Lyle L.
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In the first phase, 557 Limousin bulls were placed on a Canadian Limousin Association postweaning gain test over 4 yr (1987 to 1991). Bulls varied in age by 120 d. Weights (WT) and scrotal circumferences (SC) were collected at 28 d intervals from shortly after weaning to the end of the test on each bull (6 or 7 measurements per year). Hip heights (HT) were collected on the second and again on the final measurement dates each year. Age in days (AGE) was calculated for each measurement date. Age of dam (AOD) was also recorded for each bull on test. Correlations of SC with AGE, WT and HT were high and positive (P < .0 1). Year and AOD effects were present (P < .001). Quadratic regression equations were estimated for AGE, WT and HT on SC to develop adjustment equations. The resulting regression equation for the estimation of SC from AGE was: Y = -3.02 + .131(AGE)- .000114(AGE)2 (R2 = 74.8°/o, P < .001). Age-ofdam adjustment factors were determined by use of indicator variables. Analyses showed that age adjusted SC should be adjusted to a 4- to 10- year AOD equivalent by adding .51, .31 or .36 em for dams 2, 3 or> 11 years of age (P < .05). Adjustment of scrotal circumference in Limousin bulls to a constant, for subsequent use in national cattle evaluation, can best be achieved by removing some of the variation in scrotal circumference caused by environmental effects of age of dam and age, weight or height. Next, variance and covariance components were estimated for scrotal circumference and weaning weight from Limousin field data. Records of 8,226 bulls were used to evaluate 590 sires and 661 maternal grand sires. Data included all herd book records of bulls with a recorded scrotal circumference and their weaning contemporaries. Analyses were carried out by REML techniques employing the EM algorithm and fitting both single- and two-trait models. Scrotal circumference was first fitted in a single-trait, sire model to obtain starting values for variances for a later analysis. Likewise, weaning weight was fitted in a single-trait, sirematernal grandsire model to obtain starting values for (co)variances for a later analysis. Scrotal circumference and weaning weight were then fitted together in a two-trait model to estimate variance components. Estimates of variance components were calculated by equating (co)variances obtained from the models to their expectations. Estimates of heritability of scrotal circumference, direct weaning weight and maternal weaning weight were .46, .25 and .19, respectively. Estimates of genetic correlations between scrotal circumference and direct weaning weight, scrotal circumference and maternal weaning weight, and direct weaning weight and maternal weaning weight were .14, -.22 and -.44, respectively. The estimate of the environmental correlation between scrotal circumference and weaning weight was .59. Finally, the genetic parameters obtained were utilized to predict breeding values. Records on 9,618 bulls were available for the prediction of breeding values. Two-trait, reduced animal mixed model equations for a maternally influenced trait were utilized and solved by BLUP procedures to predict the breeding values. A genetic trend in scrotal circumference for animals born from 1971 to 1989 was not discernible in these data.