385 Feed efficiency of beef cattle in low-protein diets is driven by nitrogen use efficiency

Abstract

Abstract Performance of cattle in rangeland systems is driven by the ability to efficiently use nutrients during periods of restricted availability. Thirty Bos indicus steers (398 ± 24 kg BW) were used to evaluate the relationship between feed efficiency (FE) and nitrogen use efficiency (NUE). The hypothesis was that FE would be related to NUE in protein restricted diets, but not in high-protein diets. Steers used in a completely randomized block design were classified by residual gain in low (LFE), medium (MFE) and highly feed efficient (HFE), after being fed for periods of 70 days with diets supplying either 70% (LP) or 100% (HP) of their rumen degradable protein requirements. After each 70-day period, animals were adapted to metabolism crates for two days, and NUE was measured for five days. About 10% of daily faecal and urine output of each animal was collected for N analysis. Results of N intake from LFE, MFE and HFE in the LP diet were 21.6, 21.2 and 22.2 g N/100 kg BW (P = 0.63), while the total N excretion was 20.3, 18.0 and 19.0 g N/100 kg BW (P = 0.45) for LFE, MFE and HFE, respectively. NUE values were 11.1, 26.9 and 28.0 g retained N/100 g of digested N (P = 0.04). In the HP diet, N intake was 31.6, 30.8 and 26.8 g/100 kg BW (P = 0.12) for LFE, MFE, and HFE, respectively. Total N excretion was 26.9, 28.8 and 27.5 g N/100 kg BW (P = 0.83), respectively. NUE in the HP diet was 21.0, 5.2 and -6.3 g retained N/100 g of digested N (P = 0.04). These results support the hypothesis that FE is dependent on NUE in protein restricted diets; whereas when evaluated in protein abundant diets, FE is not related to NUE.