Physiologic markers and a genomic polymorphism as predictors of reproductive traits in Angus and Brangus heifers

The hypothesis genotypes that yield variation in metabolic hormone concentrations have potential as predictors of puberty traits was tested in two studies. Postweaning, Brangus heifers had greater (P < 0.05) area under the LH response curve (LHAUC) than Angus heifers, however, LHAUC was not a predictor of reproductive traits in Angus heifers, but tended ( P = 0.09) to be a source of variation of age of first observed estrus in Brangus heifers. Sire was significant (P ≤ 0.05) in prediction of LHAUC and serum IGF-I. The IGF-I gene underlies a QTL proposed to regulate twinning and unequal distribution of genotypic frequencies for a C/T SNP in the IGF-I gene was detected among Angus and Brangus heifers. Genotype was a significant (P = 0.01) predictor of serum concentrations of IGF-I; Angus heifers with TT genotype and Brangus heifers with CT genotype secreted greater (P = 0.01) concentrations of serum IGF-I than other genotypes. Lack of variation in categorical reproduction traits prevented analyses of these traits in Angus heifers. Genotype and serum IGF-I were significant (P < 0.02) predictors of estrus at synchronization in Brangus heifers. A greater (P < 0.01) percentage of heifers with CC genotype expressed this estrus than heifers of CT or TT genotypes, and serum concentrations of IGF-I were 11.5% greater (P = 0.01) in heifers observed in estrus versus heifers not observed in estrus at synchronization, but values were not significant predictors of reproductive traits.; In a 91-d case study, Brangus heifers born of a sire balanced for growth and scrotal circumference (Balanced) achieved puberty ∼1.4 mo earlier (P = 0.02) than heifers of a moderate growth-large scrotal circumference sire or a large growth-moderate scrotal circumference sire. Total and residual feed intake, and serum insulin were lower (P < 0.06) in heifers of Balanced sire than other sire groups.; Results from these studies suggest acceptance of the hypothesis if tested in Brangus heifers. Studies suggest LHAUC was not effective for predicting reproductive traits in NMSU Angus and Brangus heifers. Variation among sires for residual feed intake exists in the Brangus herd, and single gene locus association involving IGF-I (i.e., CC genotype) appeared reliable for predicting serum concentrations of IGF-I and estrus at synchronization.