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Genetic and Environmental Aspects of the Growth Curve Parameters in Beef Cows^1,2,

Colorado State University, Fort Collins 80523

Abstract

Brody's growth curve, a three-parameter function, and Richards' function, a four-parameter function, were fit to data from 233 inbred and linecross cows to study the genetic and environmental aspects of the growth curve parameters and to compare the two functions. Fitting Brody's curve was faster and less costly to compute than Richards' function, but Richards' function had smaller sums of squares and a better fit to actual data points. Year of birth had an effect on the A (P<.05), b (P<.01) and k (P<.01) parameters of Brody's curve. Parameter estimates from data of the youngest cows were at the extremes. The b parameter was the largest when estimated from data with no recorded birth weights. Year of birth was also an important effect for the b (P<.01), k (P<.05) and m (P<.01) parameters of Richards' function, but year effects were less interpretable. Mating system affected (P<.01) the A parameter of both functions; inbreds were lighter than linecrosses at maturity. Line of sire was an important source of variation (P<.01) for the A parameter of both functions. The heritability estimate of the A parameter from both functions was .44 ± .27; apparently the same trait in both curves. The estimate for the b parameter from Brody's curve was .39 ± .27, comparable with literature estimates of birth weight. The heritability estimate for k from Brody's curve was .20 ± .26. The heritability estimates for b, k and m parameters from Richards' function were .24 ± .26, .32 ± .27 and .21 ± .26, respectively. The genetic correlations between the A and k parameters in both curves indicated that cows with lighter mature weights reached that weight at younger ages.

¹ Published as Paper No. 2957, Journal Series, Colorado Agr. Exp. Sta., Ft. Collins.

² We gratefully acknowledge the cooperation of Dr. Duane J. Clow who provided the SAAM27 program used to calculate growth curves.

³ Present address: Dept. of Anim. Sci., Univ. of Arizona, Tucson 85721.

⁴ Dept. of Anim. Sci.

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