Benzoic acid supplementation in crude protein-deficient diets reduces nursery pig growth performance

The objective of this study was to evaluate the interaction between benzoic acid, which is excreted in urine as its Gly conjugate hippuric acid, and low crude protein (CP) diets on nursery pig growth performance. At 28 d age, pigs were weaned and divided into nursery room pens according to body weight and sex (5 mixed-sex pigs per pen). Pigs were fed a commercial starter diet for 4 d; at 32 d age, pigs were weighed (initial body weight, 9.50 ± 0.93 kg) and pens were assigned to one of three dietary treatments: 1) control (CON; 19.8% CP; n = 12 pens); 2) low crude protein (LCP; 15.8% CP; n = 11 pens) and 3) low crude protein + benzoic acid (LCP + BA; 15.8% CP; 0.9% benzoic acid; n = 12 pens). Pigs were fed for 4 wk and pig body weights and feed disappearance were measured weekly to calculate average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G:F; gain-to-feed ratio). On days 0, 14, and 28, blood was collected from one pig per pen by jugular venipuncture. Serum was analyzed for hippuric acid and amino acid concentrations. The ADG of CON group (580 ± 11 g/d) was greater than LCP (544 ± 11 g/d) and LCP + BA (503 ± 11 g/d; P < 0.05); ADG of LCP was also greater than LCP + BA (P < 0.05). There was no difference in ADFI among groups (P > 0.05). However, G:F of LCP + BA (0.536 ± 0.006 g/g) was lower than both CON (0.579 ± 0.006 g/g) and LCP (0.576 ± 0.006 g/g; P < 0.05); there was no difference in G:F between CON and LCP (P > 0.10). Serum hippuric acid concentration was greater in the LCP + BA group compared to either the CON and LCP groups on day 14 and day 28 (P < 0.05). Serum Gly concentration in LCP + BA (513 ± 33 µmol/L) was lower than LCP (630 ± 33 µmol/L; P < 0.05), whereas Gly of CON (578 ± 33 µmol/L) was intermediate. Serum Lys concentration was lowest in CON (105 ± 9 µmol/L), intermediate in LCP (138 ± 8 µmol/L), and greatest in LCP + BA (167 ± 8 µmol/L; P < 0.01). The addition of benzoic acid to CP-deficient diets reduces nursery pig growth performance to a greater extent than CP-deficient diets alone. Greater serum Lys in pigs fed with benzoic acid suggests that Lys was not used as efficiently for growth. Collectively, supplementing benzoic acid in CP-deficient nursery diets could be problematic by decreasing pig growth performance.