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A Genetic Analysis of Maturing Patterns in Straightbred and Crossbred Hereford, Angus and Shorthorn Cattle^{1 ,2,}

Texas Agricultural Experiment Station, College Station 77843 and U.S. Department of Agriculture, Clay Center, Nebraska 68933

Abstract

Relationships among body weights, absolute growth rate (AGR), absolute maturing rate (AMR), relative growth rate (RGR) and degree of maturity of body weight (u) were examined for 208 straightbred and reciprocally crossed Hereford, Angus and Shorthorn cows by 16 Hereford, 17 Angus and 16 Shorthorn sires, u is that fraction of mature weight (adjusted for condition) attained at any age (i.e., u = weight/ mature weight). Heritability of weight and u was respectively .68 and .73 at birth, .59 and .64 at 200 days, .87 and .55 at 396 days, .82 and .21 at 550 days, .41 and —.26 at 3 1/3 yr, and .44 and .40 at puberty. The genetic correlations among weights were large. Heritability of age at puberty was .64 ± .31. The genetic correlation between age and weight at puberty was .67 ± .24.

Animals growing most rapidly during the preweaning interval, in absolute terms (AGR) and relative to both current (RGR) and mature weight (AMR), tended to grow more slowly at later ages. Animals more mature at one age were generally more mature at all other ages. Variation in mature weight accounted for only 19% of the genetic variation in u through 550 days. Heritabilities were generally larger for RGR than for AMR. On average, 50% of the variation in AGR was independent of mature weight. Correlations between all measures of growth in the same interval were positive but were generally negative between different intervals. Thus, selection for increased growth rate over any age interval, in either absolute or relative terms, would tend to alter the shape of the growth curve over the interval of selection. However, selection for AGR over any interval would increase weight at all ages, while selection for preweaning RGR would tend to decrease birth weight and weight subsequent to 550 days, but increase 200-, 396- and 550-day weights.

¹ The cooperation of the Nebraska Agricultural Experiment Station, University of Nebraska, Lincoln, is acknowledged.

² The authors gratefully acknowledge the assistance of J. N. Wiltbank and R. D. Humphrey in the collection of the data used in this study.

³ U.S. Meat Animal Research Center, North Central Region, Agricultural Research Service, U.S.D.A.

⁴ Present address: Winrock International Livestock Research and Training Center, Petit Jean Mountain, Morrilton, Arkansas 72110.

⁵ Department of Animal Science, Texas A&M University, College Station 77843.

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