379C.A. Hudson, et al. THE EFFECTS OF INTAKE LEVEL ON THE DIGESTIBILITY AND RETENTION OF COPPER, ZINC AND MANGANESE IN SEDENTARY HORSES

Few studies have directly evaluated the effects of intake level on the true digestibility and retention of copper (Cu), zinc (Zn) and manganese (Mn). Furthermore, the manganese requirements of horses have not been established (NRC, 1989). To address these questions, four mature Thoroughbred horses (mean age [ ± SD] 13 ± 2.2 yr; weight 559 ± 47 kg) were studied in a 16-week longitudinal experiment that consisted of 4 periods, each with a 23-day adaptation period followed by a 5-day complete collection digestion trial. In Period 1, horses were fed an unfortified diet (basal Cu, Zn and Mn intake) consisting of 6.8 kg of alfalfa cubes and 3 kg of sweet feed (45 % cracked corn, 45 % whole oats, 10% molasses). In Periods 2, 3 and 4, respectively, the diet was fortified with a supple-ment providing 50%, 100 % and 200 % of the National Research Council [NRC] (1989) requirements for Cu, Zn and Mn. During each adaptation period, the horses were housed in box stalls at night and given paddock turnout during the day. Muzzles were worn to prevent grazing. During the 5-day digestion trial, the horses were fitted with harnesses that permitted the complete and separate collection o