Growth of Thoroughbreds fed a low-protein supplement fortified with lysineand threonine

Growth and protein status were examined in Thoroughbred foals and yearlings offered pasture supplements with different crude protein contents and amin o acid compositions. Both supplements contained 3.0 Mcal/kg DM, 10% corn oi l, 1.4% calcium, and three sources of fiber. The control supplement contain ed 14% CP and 22% soybean meal, whereas the experimental supplement contain ed 9% CP and 3% soybean meal and was fortified with 0.6% lysine and 0.4% th reonine. Mares and foals were fed twice daily (0700 and 1400) and kept on 1 2.14-hectare pastures (mixed grass and ladino clover) until weaning (6 mo). Weanlings continued on specified supplements and pastures for seven additi onal months. Physical measurements and blood samples were taken monthly for a period of 14 mo. Measurements included BW, ADG, body condition (BC), wit her height, hip height, body length, girth, forearm length, front and hind cannon length, and carpus and fetlock circumference. Blood plasma analysis included total protein, albumin, creatinine, and urea nitrogen. Effect of d iet and time were evaluated by analysis of variance with repeated measures. No differences (P greater than or equal to 0.05) were found in physical me asurements between the control and Lys/Thr groups (ADG 0.7 +/- 0.02 kg/d, B C 4.9 +/- 0.05) for the observational period. Blood data also showed no dif ference (P greater than or equal to 0.05) for the period (albumin 2.9 +/- 0 .03 g/dL, total protein 5.7 +/- 0.10 g/dL, creatinine 1.1 +/- 0.02 mg/dL). Differences in ADG between the control and experimental groups were observe d in November (0.73 +/- 0.06 vs 0.91 +/- 0.04 kg/d, P = 0.01) and December (0.56 +/- 0.04 vs 0.67 +/- 0.06 kg/d, P = 0.07) and again in April (0.65 +/ - 0.12 vs 0.86 +/- 0.06 kg/d, P = 0.06). These results suggest that the foa ls offered the Lys/Thr grew at the same or greater rates than foals on the control supplement. When a low-protein diet was fortified with the first tw o limiting amino acids, the protein was utilized more efficiently for growt h and development. This study gives further insight into the role of protei n quality during a foal's Ist yr of growth.