Influence of Methionine on Growth and Nitrogen Balance in Weanling Quarter Horses

Twenty-four Quarter horse weanlings (120 d +/- 10 d) were blocked by age into 4 groups (n = 6) for a 56 d trial to evaluate the influence of methionine on growth and nitrogen retention. Weanlings were housed by block and individually fed 1 of 4 concentrate diets twice daily at 1.5% BW (as fed). Weanlings were randomly assigned to 1 of 4 treatments: basal (0.20 MET), basal + 0.03% methionine (0.23 MET), basal + 0.07% methionine (0.27 MET), and basal + 0.11% methionine (0.31 MET). Diets were formulated to be isonitrogenous, isocaloric, and contain equal amounts of LYS and THR. Coastal bermudagrass hay was individually fed at 0.75% BW (as fed). Growth measurements, body weight, rump fat, and plasma were obtained every 7 d. The final 4 days consisted of total collection of urine and feces. Feed, fecal, and urine samples were analyzed for nitrogen content and nitrogen balance was calculated. Urine was analyzed for urea and ammonia concentration. Plasma was analyzed for urea concentration. Grain, hay, and fecal samples were analyzed for nutrient composition. Data were analyzed using the PROC MIX procedure of SAS. Linear, quadratic, and cubic effects were tested in the form of contrasts. There was no influence of treatment on growth measurements, nitrogen balance, or urinary urea or ammonia. Intake of LYS and THR (g/d) did not differ among treatments (P = 0.78 and P = 0.38 respectively). Plasma urea nitrogen (PUN) was influenced by treatment (P = 0.005) exhibiting quadratic (P = 0.04) and cubic (P = 0.002) effects. An unexpected peak in PUN was observed with 0.27 MET. Upon analysis, 0.20 MET contained more lysine than formulated, and 0.27 MET contained the least lysine. Treatments 0.20 MET and 0.31 MET contained more threonine compared to formulations. These differences may explain unexpected values of PUN concentration. Results suggest future studies that more closely isolate methionine as the only dietary variable are necessary to better explain the methionine requirements of weanling horses.