Hereford heifers with lower blood urea nitrogen breeding values excrete urine with lower nitrogen concentration

Abstract

The aim of this study was to investigate the effect of blood urea nitrogen (N) breeding value (BUNBV) on the concentration of urinary N (UN) and urinary urea N (UUN) of Hereford heifers. Thirty two heifers with divergent BUNBV: 16 low (BUNBV–1.95 to −0.58, liveweight 208 ± 4.4 kg and age 303 ± 16.7 d; LBUN) and 16 high (BUNBV 0.32–1.91, liveweight 204 ± 4.8 kg and age 302 ± 7.9 d; HBUN) were studied over a 14-day period during which they were fed lucerne silage. Individual urine, faecal, and blood spot samples were collected in the morning (0900 h) and afternoon (1400 h) on four non-consecutive days during the second experimental week. Intakes of dry matter and N, and liveweight were similar across treatments. Positive relationships existed between BUNBV and UN (R²= 0.51; P < 0.001), and BUNBV and UUN (R²= 0.57; P < 0.001). One unit increase in BUNBV increased heifer UN by 0.60 ± 0.254 g/L and UUN by 0.54 ± 0.221 g/L across both sampling times (AM and PM). This suggests that breeding for low BUN may reduce the environmental impact of pastoral beef production systems. However, further research is needed to compare N balance and urination behaviours of beef heifers divergent in BUNBV