Effects of niacin supplementation in a high-concentrate diet on growth performance, nutrient digestibility, and microbial protein synthesis of finishing lambs

Long-term feeding of high-concentrate diets will lead to rumen metabolic dysfunction, oxidative stress and inflammation of ruminants, and affect production performance. Niacin (NA) has been shown to mitigate these adverse effects by regulating the rumen environment and alleviating damage to the gastrointestinal barrier caused by high-concentrate diets. The present study was conducted to investigate the effects of niacin supplementation on growth performance, nutrient digestibility, nitrogen balance, and microbial protein (MCP) synthesis in lambs fed a high-concentrate diet. A total of forty-eight 1/2 Dorper × 1/2 thin-tailed Han crossbred male lambs (means body weight (BW) of 22.8 ± 1.86 kg) were divided into four levels (0, 400, 800, and 1200 mg/kg diet dry matter) of NA supplementation in lamb diet, respectively control (CON), 400NA, 800NA, and 1200NA. After the adaptation period, the feeding experiment lasted for 60 days, and then six lambs in each group were randomly selected for the digestion and metabolism trial. The results showed that average daily gain (ADG) during 30–60 days and the whole feeding period increased linearly with the increase of niacin supplementation (P < 0.05), and the dry matter intake (DMI) tended to increase linearly (P = 0.098). Dietary NA supplementation significantly increased rumen pH value (P < 0.05), with the 800NA group exhibiting significantly higher pH value compared to the CON group at 1, 3, 4, 6, 8, 10, and 12 hours post-feeding (P < 0.05). In particular, ruminal pH values showed a positive quadratic (P < 0.05) response with incremental levels of NA at 8, 10, and 12 hours post-feeding, and the highest pH value was observed in the 800NA group. As the dietary supplementation with niacin increased, the digestibility of dry matter (DM), organic matter (OM), crude protein (CP), and neutral detergent fiber (NDF) linearly increased (P < 0.05), and gross energy and nitrogen intake (N intake) also increased linearly (P < 0.05). The energy loss in feces, urine, and methane (CH₄) was not affected by niacin supplementation (P > 0.05). Niacin supplementation linearly decreased the feces and urinary nitrogen (g/kg nitrogen-intake) (P < 0.05), thus, the retained nitrogen linearly increased (P < 0.05). In addition, the total urine purine derivatives (PDs) and the estimated MCP increased linearly with increasing niacin levels (P < 0.05). Overall, the administration of niacin to finishing lambs fed a high-concentrate diet resulted in enhanced nutrient utilization and MCP synthesis, thereby promoting superior growth performance.