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Repeatabilities of Annual Fleece and Body Records of Rams

U. S. Department of Agriculture¹

Abstract

When selecting among rams of various ages, use of the weighting factor
where n is the number of records and r is their repeatability, can be decisive. It is, therefore, desirable to obtain estimates of the correlations among repeated records of the same individual and to check the validity of the assumptions underlying the use of this formula in each case to avoid misapplication. Data on nine traits collected over 13 yr. include observations on 1,034 Rambouillet, 742 Targhee and 412 Columbia rams having at least two and up to five annual records. Comparisons of pooled intra-year variances of unselected records made at different ages revealed that variances of many traits were nonhomogeneous over the five ages represented. Correlations (repeatabilities) among records were estimated as linear regressions of later upon earlier record to minimize effects of selection. The presence of a few estimates exceeding 1.0 indicated that in some instances certain correlations (for weight, face cover, fleece weights and grade) were overestimated. Correlations between pairs of records with differing numbers of intervening years frequently were different, often tending to be smaller as the number of intervening years increased. The occurrence of real differences among the correlations indicates that at least some basic assumptions involving the use of the formula are invalid and that other suggested formulas may be more suitable. In cases where the variances of the records are importantly different, however, the regression of "true producing ability" on phenotypic average cannot be expressed precisely by any simple formula involving only n and r as above.

¹ From the U. S. Sheep Experiment Station, Sheep and Fur Animal Research Branch, Animal Husbandry Research Division, A.R.S., Dubois, Idaho, in cooperation with the University of Idaho, Moscow, Idaho.