Growth and Nitrogen Utilization of Nursery Pigs Fed Reduced Crude Protein Diets with Benzoic Acid Supplementation

Abstract

Reducing dietary crude protein (CP) can lower feed costs and nitrogen waste in swine production but may limit nitrogen available for dispensable amino acid synthesis. This is especially relevant when benzoic acid (BA) is used as a feed additive since its excretion in urine requires conjugation to glycine, forming hippuric acid (HA). Two experiments evaluated the effects of BA supplementation in CP-deficient diets (CP levels below the estimated animal requirement) on growth performance and nitrogen utilization in nursery pigs. In the first growth performance experiment, at 28 d of age, a total of 175 pigs (initial BW = 9.50 ± 0.93 kg) were weaned and placed into mixed-sex nursery pens blocked by body weight (5 pigs/pen). They received a commercial nursery diet for 4 d. At 32 d, pigs were weighed and pens were assigned to one of three diets for four weeks: 1) control crude protein (CON; 19.8% CP; n = 12); 2) low crude protein (LCP; 15.8% CP; n = 11); or 3) LCP + benzoic acid (15.8% CP with 9 g/kg BA; n = 12). All indispensable amino acids were provided according to the animal requirements. Pigs fed CP-deficient diets with BA showed reduced growth performance compared to CP-deficient diets without BA (P < 0.05). This corresponded to lower plasma glycine and greater plasma indispensable amino acid and HA concentrations (P < 0.05). In the second nitrogen balance experiment, a total of 24 castrated male pigs (initial BW = 13.60 ± 1.14 kg) were individually housed in metabolism crates and assigned to one of four CP-deficient diets (15.3% CP): 1) BA-0 (0 g/kg; n = 6); 2) BA-0.5 (5 g/kg; n = 6); 3) BA-1 (10 g/kg; n = 6); or 4) BA-1.5 (15 g/kg; n = 6). All indispensable amino acids were provided according to the animal requirements. Pigs were adapted to their diets for 7 d and fed three equal meals daily at 08:00, 12:00, and 16:00 at 2.1 × ME for maintenance, with water provided at each meal. Increasing BA inclusion increased urinary nitrogen excretion in a dose-dependent manner (Linear, P < 0.05); however, increasing BA inclusion also decreased fecal nitrogen excretion in a dose-dependent manner (Linear, P < 0.05), resulting in no subsequent difference in whole body nitrogen retention among groups. Both plasma and urine HA concentrations increased with increasing BA inclusion (Linear, P < 0.05) and corresponded to a trend for a linear decrease in plasma glycine concentration (P = 0.10) and a linear increase in plasma indispensable amino acid concentrations (P < 0.05). Overall, our findings demonstrate that BA reduces growth performance in nursery pigs fed CPdeficient diets. Although urinary nitrogen excretion increased with increasing BA intake, the corresponding reduction in fecal nitrogen excretion offset this, leading to no differences in whole body nitrogen retention. Since glycine was used for HA synthesis, both CP and glycine supply should be considered when using BA as a feed additive in low CP nursery diets.