A biometrical evaluation of weight of beef cows and performance of their progeny

Abstract

Data from Experimental Station herds in Alabama, Florida, Georgia, Louisiana, North Carolina, South Carolina and Texas were included in this cooperative Southern Regional Beef Cattle Breeding Project (S-10) study. Records form straightbred Angus, Brahman, Brangus, Herefords and Santa Gertrudis were studied. Analytical methods included the least-squares method of fitting constants and auxiliary methods for estimation of various parameters. Computer programs were written for these methods. Primary objectives of this study were to partition variance in cow weight and to evaluate linear and quadratic relationships between cow weight and progeny preweaning performance. Two measures of cow weight were available: weight after parturition (PCW) and weight at weaning (WCW). Progeny traits were birth weight (BW), weaning weight (WW) and average daily gain from birth to weaning (ADG). Sources of variation evaluated for cow weight included location, breed, sire, year of record, age in years, previous parity and calving month. Paternal half-sib estimates of heritability determined from combined location data were 0.96 for PCW and 0.74 for WCW. Combined location estimates of heritability from the regression of daughter's average record on dam's average record were 0.42 for PCW and 0.44 for WCW. Repeatability estimates indicateed that temporary environmental effects accounted for less than forty percent of the variation in PCW or WCW. The sources of variation evaluated for the progeny traits were location, breed, sire, year, sex, birth month and age, wright and previous parity of dam. Age of calf in days was included in the model for WW. Age and weight of dam tended to account for similar sources of variation in the progeny traits. Dam weight appeared more closely related to variation in BW than did dam age; however, dam age appeared more closely related to variation in ADG and WW than did dam weight. The quadratic relationships between the two weights of dam and the three progeny traits were calculated and plotted for each location. Even though the degree of curvilinearity of the dam weight-progeny trait regression relationship varied considerable across locations, they were generally positive. Sex of calf consistently accounted for an important portion of variation in the progeny traits. Males were heavier at birth and weaning and had higher ADG's than females. Calves raised by dams parous the previous year were lighter at birth and weaning and had lower ADG's. Estimates of heritability for the progeny traits ranged from low to moderate and phenotypic, genetic and environmental correlations among these traits were generally high.