Gestation length is highly heritable and could indirectly reduce birth weight without impacting the weaning weight of Australian tropically adapted Bos indicus beef cattle

Context: Gestation length is an important trait in beef cattle because it is associated with calf birth weight and dystocia. This paper presents the first genetic parameters for gestation length in Australian tropical breeds and the genetic relationships with birth and weaning weight. Aims: We investigated the genetic and non-genetic effects of gestation length, birth weight and weaning weight for three tropical beef breeds (Brahman, Droughtmaster and Santa Gertrudis) in northern Australia. Genotype by environment interactions were assessed for each trait. Methods: Animals were born between 2014 and 2022 from two herds, and after edits, there were 2,346, 7,044 and 6,248 records for gestation length, birth weight and weaning weight, respectively. Animals were by 245 sires, with an average half-sibling family size of 29. Genetic parameters were estimated from pooled breed and breed-specific datasets. To assess genotype by environment interactions, traits at each herd were considered separate traits and bivariate analysis was undertaken. Results: Breed, cohort and calf sex were the only non-genetic factors influencing gestation length; males were estimated to have longer gestation lengths of 3.1, 2.4 and 1.9 days for Brahman, Santa Gertrudis and Droughtmaster breeds, respectively. Gestation length was under high genetic influence, with a direct heritability of 0.79 (0.05). Birth weight and weaning weight direct heritability estimates were 0.52 (0.04) and 0.40 (0.04), respectively. Maternal heritability estimates for birth and weaning were 0.10 (0.02) and 0.15 (0.03), respectively. A positive genetic correlation of 0.30 (0.09) was estimated between gestation length and birth weight, while no significant genetic relationship was estimated between gestation length and weaning weight. No genotype by environment interactions were detected for gestation length, birth weight and weaning weight. Conclusions: Gestation length for three tropically adapted breeds in northern Australia was highly heritable and showed no genotype by environment interaction. Genetic selection of sires for gestation length will reduce gestation length and indirectly reduce birth weight and calving difficulties without any unexpected correlated effects on weaning weight. Implications: Selection for decreased gestation length can indirectly reduce calf birth weight without impacting weaning weight.