Influence of dietary methionine concentration on growth and nitrogen balance in weanling Quarter Horses

Twenty-four Quarter Horse weanlings (120 ± 10 d) were blocked by age into 4 groups (n=6) for a 56-d trial to evaluate the influence of dietary Met concentration on growth and N balance. Weanling horses were housed by block and individually fed concentrates twice daily at 1.75% BW (as-fed basis). Weanling horses were randomly assigned to 1 of 4 concentrate treatments: basal (0.20 Met), basal + 0.03% Met (0.23 Met), basal + 0.07% Met (0.27 Met), and basal + 0.11% Met (0.31 Met). Diets were formulated to be isonitrogenous and isocaloric and contained equal amounts of Lys and Thr. Coastal bermudagrass hay (Cynodon dactylon) was individually fed at 0.75% BW (as-fed basis). Growth measurements, BW, rump fat, and plasma were obtained every 7 d. The final 4 d consisted of total collection of urine and feces. Feed, fecal, and urine samples were analyzed for N content, and N balance was calculated. Urine was analyzed for urea and ammonia concentrations. Plasma was analyzed for urea concentration. Grain, hay, and fecal samples were analyzed for nutrient composition. Data were analyzed using the PROC MIXED procedure of SAS. Linear, quadratic, and cubic effects were tested in the form of contrasts. There was no influence (P>0.25) of treatment on growth measurements, N balance, or urinary urea or ammonia. Intake of Lys and Thr did not differ (P>0.08) among treatments. Methionine intake increased as expected with increasing Met inclusion in the diet (P<0.01). Plasma urea N responded quadratically (P=0.04) to treatment. An unexpected peak in PUN was observed with 0.27 Met. The results indicate future studies that can ensure Met as the only dietary variable are necessary to better explain the Met requirements of weanling horses.