Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2006
Publication Date: 2/14/2007
Citation: Riley, D.G., Coleman, S.W., Chase, C.C., Olson, T.A., Hammond, A.C. 2007. Genetic parameters for weight, hip height, and the ratio of weight to hip height from random regression analyses of brahman feedlot cattle. Journal of Animal Science.85:42-52. Interpretive Summary: The purpose of this research was to assess the genetic control of body weight, hip height, and the ratio of body weight to hip height in Brahman steers and heifers across a feeding period of 170 d. Brahman steers and heifers (n = 724) were produced by mating 27 Brahman bulls to cows in Central Florida over 7 years. Each year, about one month after weaning at 7 months of age, calves were placed in feedlot pens and gradually adjusted to a high concentrate ration. Body weight and hip height were recorded at 28 day intervals until the average fat thickness of calves in a pen as measured by ultrasound was 10 mm. At that time, the entire pen was slaughtered commercially. New statistical modeling techniques were employed that permit the estimation of curves which represent the additive genetic and permanent environmental variances. From these curves, individual animal breeding values for these traits can be predicted at any point in the feeding period. Results indicated that the genetic control of these traits increases as time on feed increases. This means that selection for these traits late in the feeding period may more accurately sort sires for weight and weight:hip height ratio. It appears that selection for hip height may be equally accurate throughout the feeding period. Results also indicated that there was enough genetic variance for the ratio of weight to hip height to select animals to be parents with greater weights at lower hip heights. This may be desirable, as Brahman females with lower hip heights have been reported to have higher reproductive efficiency than taller Brahman cows.
Technical Abstract: The objective of this work was to use random regression analyses to estimate genetic parameters for weight, hip height, and the ratio of weight to hip height (n = 5,055) in Brahman cattle. A progeny test of Brahman sires (n = 27) generated records of Brahman steers and heifers (n = 724) over 7 yr. Each year after weaning, calves were assigned to feedlot pens where they were fed a high concentrate grain diet. Weights and hip heights were recorded every 28 d until cattle reached a targeted fatness level. All calves had records through 170 d on feed; subsequent records were excluded. Models included contemporary group (sex-pen-year combinations, n = 63), and age at the start of the feeding period as a covariate. The residual error structure was modeled as a random effect with two levels corresponding to two 85-d periods on feed. Linear random regression coefficients on Legendre polynomials of days on feed were selected to model additive genetic and permanent environmental effects. Estimates of heritability across the 170-d feeding period ranged from 0.32 to 0.56 for weight, from 0.41 to 0.51 for hip height, and from 0.22 to 0.58 for weight:hip height ratio. Estimates of heritability were similar to those obtained from single trait conventional (not random regression) analyses with the exception of two high estimates for hip height on days 85 (0.71 +/- 0.11) and 113 (0.64 +/- 0.11) of feeding. Estimates of the permanent environmental proportion of phenotypic variance ranged from 0.38 to 0.45 for weight, 0.09 to 0.15 for hip height, and 0.30 to 0.35 for weight:hip height ratio. Rank correlation coefficients of sire estimated breeding values (within trait) on seven different days on feed (at 28-d intervals) indicated substantial reranking of weight:hip height ratio estimated breeding values after 57 or more d on feed, but not for weight or hip height estimated breeding values. Rank correlation coefficients for different days on feed did not support an association of rankings for weight:hip height ratio and hip height estimated breeding values. The ratio of weight to hip height in cattle has previously been used as an objective measure of body condition score in cows or calves; it may offer a unique assessment of body dimension. Results indicated that there is substantial additive genetic variation for this trait, and it may be possible to use estimated breeding values to increase weight without increasing frame score in Brahman cattle.