Genetic relationships of female reproduction with growth, body composition, maternal weaning weight and tropical adaptation in two tropical beef genotypes

Abstract

The genetic relationships of female reproduction with growth and body composition, tropical adaptation traits and maternal weaning weight (descriptive of genetic potential milk production) were estimated in 1027 Brahman (BRAH) and 1132 Tropical Composite (TCOMP) females. Female reproduction was evaluated at puberty, as outcomes of the first and second annual mating periods (Mating 1 and Mating 2, which commenced when females averaged 27 and 39 months of age, respectively), as well as annual averages over up to six matings. Traits evaluated included age at puberty, Mating 1 and 2 pregnancy rate, weaning rate and days to calving, and lifetime annual calving and weaning rate. Traits describing growth and body composition (liveweight, hip height, ultrasound-scanned P8 fat depth and eye muscle area, subjective body condition score and blood IGF-I concentration) were measured in the animals as heifers (at ~18 months of age), and again at the start of Mating 2. Traits describing tropical adaptation included coat-length scores in both genotypes and, in BRAH, buffalo fly lesion scores. Previously reported analyses of these data identified heifer IGF-I and coat and buffalo-fly-lesion scores as potential genetic indicators for age at puberty in BRAH. The results of the present study found that exploiting these relationships would have no unfavourable genetic consequences for later female reproduction and, in some cases, may be indicators of female reproduction, when evaluated as outcomes of Matings 1 or 2, or as lifetime annual calving or weaning rates. For BRAH, heifer liveweight was a genetic indicator for Mating 1 weaning rate (rg = 0.70), and, while standard errors were high, there were also positive genetic correlations of heifer hip height, eye muscle area and blood IGF-I concentration with Mating 1 weaning rate (rg = 0.61, 0.58 and 0.43, respectively). For TCOMP, significant genetic relationships of heifer growth, body composition and tropical adaptation traits with female reproduction were virtually absent, suggesting that there is less opportunity to identify earlier in life measures as genetic indicators of reproduction for this genotype. Higher maternal weaning weight was significantly genetically related to lower lifetime annual weaning rate (rg = -0.50) in BRAH, and with lower Mating 2 calving and weaning rate (rg = -0.72 and -0.59, respectively) in TCOMP, which will need to be considered when making selection decisions that affect genetic milk in these genotypes. Importantly, the results presented revealed no strong genetic antagonisms of heifer growth and body composition traits with female reproduction, suggesting that selection could be undertaken to improve these simultaneously.