PS2-16. Impact of Crude Protein-deficient Diets and Benzoic Acid Supplementation on Nitrogen Utilization in Nursery Pigs

Dietary crude protein (CP) plays a central role in supporting growth and protein accretion in nursery pigs. Protein provides indispensable amino acids (IAA) to meet estimated requirements and nitrogen for the synthesis of dispensable amino acids (DAA). However, high CP diets often exceed estimated requirements, resulting in inefficient nitrogen utilization and greater urinary and fecal nitrogen excretion. Benzoic acid (BA) is an organic acid that enhances growth performance and nutrient digestibility, and it requires glycine (Gly) for elimination as hippuric acid (HA) in urine. This process diverts nitrogen from retention toward excretion which may be particularly relevant in pigs fed low protein diets. This study evaluated the impact of BA supplementation in a CP-deficient diet on nitrogen utilization in nursery pigs. Twenty-four castrated male pigs (initial BW = 13.60 ± 1.14 kg) were individually housed in metabolism crates and assigned to one of four dietary treatments (n = 6): 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 3,728 kcal ME/kg, 15.3% CP, and 1.34% standardized ileal digestible (SID) 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 nitrogen balance, feed intake and refusals were recorded at each meal and urine and feces were collected and pooled daily for nitrogen analysis. Blood samples were collected at the start and end of the study for plasma amino acid and HA analyses. Average daily gain and nitrogen intake were not affected by BA inclusion (P > 0.10; Table 1). There was a linear effect (P < 0.01) for apparent total tract digestibility of nitrogen, which increased from 89.2% to 94.4%, whereas fecal nitrogen excretion decreased from 1.46 to 0.76 g/d (P < 0.01) and urinary nitrogen excretion increased from 1.61 to 2.47 g/d (P < 0.01) with increasing BA inclusion, indicating improved nitrogen digestion and absorption. Plasma HA and urinary nitrogen also increased linearly (P < 0.01), indicating greater HA synthesis from BA and Gly and subsequent excretion. Whole-body nitrogen retention was not affected by increasing BA content (P > 0.10). Among plasma amino acids, there was no overall effect of diet Gly and or Lys concentration (P > 0.10). However, there was a trend for a linear decrease in Gly concentration, and a linear increase in Lys (P = 0.04) and total IAA (P = 0.04), with increasing BA inclusion. Overall, BA effectively enhanced protein digestibility, even in CP-deficient diets. However, increasing BA inclusion increased urinary nitrogen excretion through HA formation.