Latin America is an important contributor to the worldwide beef business, but in general, there are limited studies considering strategies to reduce nitrogen contamination in their production systems. The study's goal was to assess the effect of two nutritional strategies to balance energy and protein supply in fattening heifers on performance, ruminal metabolism, and carcass characteristics. A total of 24 crossbred heifers (initial body weight 'BW 'of 372 ± 36 kg) were used to create two blocks (based on live weight) of two pens each, that were equipped with individual feeders. Within each block, half of the animals were assigned to a diet based on tabular Crude Protein (CP) requirements denominated Crude Protein Diet 'CPD' but without a ruminal degradable protein balance. The other half received a diet denominated Metabolizable Protein Diet 'MPD', formulated with the metabolizable protein system, balanced for the ruminal degradable protein. Both diets had the same ingredients and as well as similar synchrony indexes (0.80 and 0.83, respectively). For nitrogen concentration in feces and urine as well as microbial crude protein synthesis, a total of 12 heifers (three per pen) were randomly selected to collect samples. The dataset was analyzed as a randomized complete block design with a 5% significance. No diet × time interaction was observed for Average Daily Gain 'ADG' (p = 0.89), but there was an effect of the time on ADG (p ≤ 0.001). No differences were observed neither for final weight, dry matter intake 'DMI', and feed conversion rate (p > 0.05). Heifers fed with CPD showed greater cold carcass weight (p = 0.041), but without differences in ribeye area, backfat thickness, pH, dressing %, and marbling (p > 0.05). Differences between diets were observed for the in vitro parameters as well as for the Total Volatile Fatty Acids 'VFA' and NH3 (p < 0.05). Total N concentrations (urine + feces) of heifers fed with MDP was lower than in those fed with the CPD (p < 0.01), but no differences were observed in microbial protein, purine derivatives, and creatinine (p > 0.05). We conclude that the combination of synchrony and the metabolizable protein system achieve greater efficiency in the use of nitrogen, without negatively affecting animals' performance or the quality of the carcass.
Keywords: Metabolizable protein system; Nitrogen efficiency; Synchrony index.