134 Estimating the Heritability of Nitrogen and Carbon Isotopes in the Tail Hair of Beef Cattle

Abstract

Abstract The natural abundance of nitrogen isotopes (δ15N) in animal tissues is related to the ability of cattle to lose less nitrogen in the urine and has been used to estimate nitrogen use efficiency and feed efficiency. Carbon isotope enrichment (δ13C) has also been used to estimate feed conversion ratio and grazing behavior in beef cattle. There are significant variations between the individual beef cattle in the same environment for δ15N and δ13C of the tail hair. Thus, the objective was to determine the heritability of δ15N and δ13C in the tail hair of tropically adapted beef cattle to further validate their use in genetic breeding programs. A total of 497 Bos indicus steers (269 Brahman and 228 Droughtmaster) were used, and the steers had reliable pedigree information for three generations as they were part of the beef cattle genetic evaluation system in Australia. Two single, mixed breed, contemporary groups of steers are used including the steers weaned in 2019 (n = 254) and steers weaned in 2020 (n = 243). Samples of tail switch hair representing hair segments growing during the dry season were collected and analyzed for δ15N and δ13C. Fixed effects fitted in the linear regression model were age, breed, year (as contemporary groups), and average daily gain during the different seasons. The heritabilities of δ15N and δ13C in the tail hair were estimated at 0.43 ± 0.14, and 0.41 ± 0.15, respectively, which indicates both are moderately heritable traits. The genetic correlation between δ15N and δ13C was negative at -0.78 ± 0.16, and the phenotypic one was -0.40 ± 0.04. The negative correlation can be explained by the contrasting isotope fractionation in the urine of cattle, as urine has less δ15N and more δ13C than the diet. The findings of this study have shown for the first time that both δ15N and δ13C in the tail hair of cattle are heritable traits. These results support the feasibility of utilizing tail hair isotopes as a promising approach for identifying cattle with greater nitrogen use efficiency, which is a key trait in low-protein diets. Moreover, genetic selection for nitrogen-efficient cattle can reduce the environmental impact of beef production.