Reduction of the Nitrogen Excretion and Ammonia Volatilization from Manure of Fattening Bulls during the Finishing Phase By Reducing the Concentrate Crude Protein Concentration

The aim of this study was to evaluate the effects of crude protein (CP) on nitrogen (N) excretion (Study 1), performance (Study 2), and manure ammonia volatilization (Study 3) in Holstein beef animals fed with high-concentrate diets during the finishing phase. In Study 1, 24 Holstein heifers (310 ± 5.3 kg of BW and 251 ± 1.4 d of age) were allocated in individual pens and distributed randomly depending on initial BW to 3 dietary treatments (N1: 14.5% CP and 0.6% urea, N2: 12.5% CP and 0.3% urea, N3: 10.5% CP without urea. Concentrate and straw were recorded daily and BW was recorded every two weeks. At days 30 and 58, a N balance was performed. In Study 2, 150 Holstein bulls (344 ± 2.6 kg BW and 252 ± 0.9 days of age) were allocated in 8 pens (18-19 animals per pen). The evaluated treatments were N2 and N3. Concentrate and straw were offered ad libitum, pen intake was recorded daily. The BW was recorded every 2 wks. After d 70, animals were transported to the slaughterhouse where carcass weight and quality was recorded. For Study 3, a fecal and urine sample form each heifer were collected at d 58 of Study 1, mixed at a ratio in 1:1 (wet weight) and pooled by treatment for ammonia volatilization assays under aerobic and anaerobic conditions. Gas samples collected for the analyses of ammonia concentration at day 1, 3, 7 and weekly during 150 days. Data were analyzed with an analysis of variance. In Study 1, urinary N excretion was greater (P ˂ 0.001) in N1 (91.5 ± 5.72 g/d) compared with N2 (63.1 ± 5.72 g/d) and N3 (50.4 ± 5.72 g/d). No differences in fecal N excretion among treatments were observed. In Study 2, no significant differences were observed between treatments in average dayily gain (ADG, 1.67 ± 0.041 kg/d), concentrate intake (8.1 ± 0.06 kg/d as feed), carcass weight (257 ± 1.7 kg), or dressing percentage (52.3 ± 0.3%). Nitrogen loss from manure estimated from aerobic ammonia volatilization changed significantly during storage (treatment x time interaction, P < 0.01); volatilization of N3 was always lower than N1, being N2 an intermediate case. The reduction of CP from 14.5 to 12.5 and 10.5% is an efficient strategy to reduce urinary N excretion by a 20% and manure ammonia volatilization without impairing performance and carcass quality.