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

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 Brahman steers fed a medium-quality forage-only diet to assist in predicting their performance and nitrogen metabolism response to a ureasupplemented 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 ± s.d.) 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 results: 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 from the baseline diet assisted 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.