Methods of estimating repeatability and most probable producing ability in beef cattle

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Virginia Polytechnic Institute and State University


The purpose of this study was to determine the most accurate method of calculating the most probable producing ability (MPPA) of beef cows based on one, two or three records. A preliminary analysis of preweaning records on 9,515 calves from 4 Angus herds enrolled in the Virginia Beef Cattle Improvement Association program was conducted on average daily gain, 205-day weight and index value. Adjusted 205-day weight was selected for further study. To obtain a realistic repeatability value of adjusted 205-day calf weight, needed for calculating MPPA, separate estimates were first obtained for each herd by intraclass correlation. These values were 0.34, 0.00, 0.35 and 0.14. The individual herd sums of squares and degrees of freedom were then pooled to obtain an overall estimate of 0.31. Repeatability was also calculated by the correlation of adjacent records (r = 0.48) and by the regression of the second record on the first (b = 0.51). The pooled intraclass correlation coefficient (0.31) was selected as the best for the calculation of all MPPAs. MPPAs were calculated by the method of Lush (1949), using the dam's progeny record: (1) as a deviation from the herd-year average, and (2) as a deviation from the sire progeny average within years. MPPAs were calculated by both methods based on each of one, tuo and three records and each of these correlated with the average of all subsequent records (maximum of six). The first, second and third MPPA values by the herd-year method yielded correlation coefficients of 0.18, 0.10 and 0.04 with the average of all subsequent records, respectively. Corresponding sire-year correlation coefficients were 0.23, 0.31 and 0.36, respectively. The differences between the correlation coefficients obtained by the two methods for MPPA one, two and three were significant at the level of P < .10, P < .05 and P < .0005, respectively, indicating that the sire-year method is more accurate. The variance was also lower for the sire-year method (427 ± 44.2 pounds) than for the herd-year method (428 ± 55.2 pounds).

Thus, the author has concluded from these results that the sire-year method of calculating most probable producing ability is a more accurate measure of the true productivity of beef cows.