Does a reduction in dietary crude protein content affect performance, nutrient requirements, nitrogen losses, and methane emissions in finishing Nellore bulls?

• Reducing crude protein level in diet did not affect intake and nutrient digestibility.
• Carcass characteristics were not affected when increasing crude protein levels in diets.
• The reduction of crude protein levels in diet did not affect methane emission.
• 10% crude protein-based diet reduced urinary N losses in finishing bulls.

An experiment was conducted to evaluate whether a reduction in dietary crude protein (CP) content affects animal performance, energy and protein requirements, N losses, and enteric methane emission in finishing Nellore bulls. Twenty-six animals, with an average age of 20 ± 1.0 months and initial body weight (BW) of 296 ± 8.1 kg were used in this experiment. Four animals were used as baseline reference animals and were slaughtered at the beginning of the experiment. Four animals were fed at maintenance level (MAIN), whereas 18 bulls were divided into 3 groups (n = 6 in each group) and were randomly assigned to the treatments consisting of three levels of CP in the diets: 10, 12, and 14% of CP. At the end of the experiment, all animals were slaughtered to evaluate their chemical body composition, energy and protein requirements, and carcass characteristics. A linear effect was observed for dietary CP level on CP intake and digestibility, while greater values were obtained for animals that were fed 14% CP. Nitrogen metabolism was affected by CP levels, where animals that were fed 12 and 14% CP had greater urinary N losses than those that were fed 10% CP. There was no effect of CP level on retained N, animal performance, and carcass characteristics among diets, and there was no effect of CP level on microbial efficiency and CH4 emissions. Thus, this study showed that for finishing bulls, the level of dietary CP did not interfere with muscle deposition and greenhouse gas emissions. The reduction of CP content in diets does not affect DM intake, animal performance, and carcass characteristics, thereby suggesting that the use of 10% of CP in diets for finishing bulls reduces their environmental impact due to a lower urinary N excretion than 12 and 14% CP-based diets. Animals that were fed 10, 12, and 14% CP diets had emissions equivalent to 3893; 3755; and 4255 g d−1 of CO2, respectively, and no difference was observed among diets. Furthermore, methane emission is not affected by CP levels ranging between 10 to 14% which, on average, is 16.3 g kg−1 of DM intake. Our study found that a decreased CP level did not influence animal performance, but it did decrease N losses in manure without affecting methane emissions. However, it is important to highlight that more studies are necessary to confirm these results.