Benzoic acid decreases biological value of nitrogen without affecting nitrogen retention in nursery pigs fed crude protein-limited diets.

Benzoic acid (BA) is an organic acid feed additive that increases protein digestibility and growth performance in nursery pigs. However, its post-absorptive metabolism to hippuric acid (HA) requires nitrogen from Gly. The objective of this study was to evaluate the effect of increasing BA intake in crude protein (CP)-limited diets on nitrogen utilization in nursery pigs. Thirty-six castrated male pigs (initial body weight = 14.25 ± 1.22 kg) were housed individually in metabolism crates and assigned to one of four treatment diets (n = 9): 1) BA-0 (BA, 0 g/kg); 2) BA-0.5 (BA, 5 g/kg); 3) BA-1 (BA, 10 g/kg); or 4) BA-1.5 (BA, 15 g/kg); all diets contained 3,577 kcal metabolizable energy (ME)/kg, 15.3% CP, and 1.34% standardized ileal digestible Lys. Pigs were adapted to treatment diets for 7 d and received three equal meals at 08:00, 12:00, and 16:00 at 2.1 × ME requirements for maintenance; water was provided freely with each meal. During the 7-d experimental period, feed intake and refusals were recorded at each meal; fresh fecal samples were collected daily and pooled; and total urine output was collected daily, subsampled, and pooled. Diet, feces, and urine were analyzed for nitrogen content. Blood was collected at the start and end of the experimental period for plasma amino acid, BA, and HA analysis. Nitrogen intake was not affected by BA inclusion. Increasing BA intake increased apparent total tract digestibility of nitrogen from 89.6% to 94.5% (linear, P < 0.01), decreased fecal nitrogen output from 1.45 to 0.77 g/d (linear, P < 0.01), and increased urine nitrogen output from 1.60 to 2.52 g/d (linear, P < 0.01). However, whole-body nitrogen retention was not different among treatments. While nitrogen retention efficiency was likewise not affected by BA inclusion, the biological value of nitrogen decreased from 87.1% to 80.8% with increasing BA intake (linear, P < 0.01). Plasma and urine HA concentration and urine HA output increased, whereas plasma Gly concentration decreased, with increasing BA intake (linear, P < 0.01). Collectively, BA effectively improved protein digestibility, even in CP-limited diets, but greater nitrogen output in urine counteracted this effect, leading to no overall differences in nitrogen retention. Since the biological value of nitrogen was reduced with greater BA intakes, both dietary CP and Gly content should be considered when formulating nursery diets with added BA.