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Use of ¹⁵N abundance in tail hair to predict feed efficiency and response to a urea supplement in young cattle fed a tropical grass forage

Brandon Fraser, Karen EYRE , John Gaughan, Gene Wijffels, Luis Felipe Silva

Abstract

Context: Beef cattle performance varies substantially in environments with low protein forage. Nitrogen use efficiency (NUE) explains a large portion of this variation. The natural abundance of the ¹⁵N isotope (δ¹⁵N) in biological tissues can be used as a proxy for NUE. Aims: This study aimed to use tail hair δ¹⁵N from a medium-quality forage-only diet to assist in predicting the performance and nitrogen metabolism response of Brahman steers to a urea supplemented diet. Methods: A complete randomised block design with two distinct periods (diets) was conducted using 24 Brahman steers with an initial liveweight of 238 ± 29 kg (mean ± SD) and aged 10-12 months. The steers were fed a medium-quality hay diet (baseline diet with crude protein (CP) 76 g/kg DM) for 56 days, followed by an N-supplemented diet (hay plus molasses and urea, 92 g CP/kg DM) for another 56 days. Each feeding period concluded with a 7-day evaluation in metabolic crates to assess nitrogen metabolism parameters. Key Findings: Steer rankings based on δ¹⁵N levels in tail hair were consistent across both diets. Significant individual variation was observed in average daily gain (ADG) on the baseline diet (-125 to 181 g/day) and in response to the urea supplemented diet (-11 g/day to 480 g/day). Markedly, δ¹⁵N from the baseline diet was significantly correlated with ADG, NUE, and feed conversion efficiency (FCE) during the urea supplemented diet. Conclusions: Tail hair δ¹⁵N on the baseline diet was able to assist in predicting ADG, NUE, and FCE during the urea supplemented diet. Implications: Tail hair δ¹⁵N may be used to identify more productive and efficient cattle.

AN24210  Accepted 23 March 2025

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