STRESS INDUCTION AFFECTS COPPER AND ZINC BALANCE IN CALVES FED ORGANIC AND INORGANIC COPPER AND ZINC SOURCES

This study determined whether Cu and Zn balance was affected by feedin g either Zn methionine (ZnMet) + Cu lysine (CuLys) or Zn sulfate (ZnSO 4) + CU sulfate (CuSO4) before and after stressing calves. Eight Charo lais crossbred steer calves weighing 167 +/- 5 kg were randomly assign ed to two treatments in a crossover experimental design. The millet ha y and soybean meal diet when supplemented with the inorganic salts pro vided 9.2 ppm of Cu and 36.6 ppm of Zn or when fortified with the meta l complexes contained 10.5 ppm of Cu and 36.6 ppm of Zn. Gentled calve s were fed their respective diets for 28 d before an 18-d mineral bala nce trial was conducted. Collection consisted of five periods: 1) a 5- d baseline period, 2) 3 d of no Cu and Zn supplement, 3) 3 d of stress consisting of feed and water restriction and ACTH (80 IU) injections i.m. every 8 h, 4) 3 d of refeeding with no Cu and Zn supplement, and 5) 4 d of Cu and Zn repletion. Calves fed CuLys had 53% greater appare nt Cu absorption and increased Cu retention (P < .05) during repletion compared with calves fed CuSO4. The 18-d mean retention of Cu from Cu Lys was greater (P < .05) than that from CuSO4. No differences (P > .0 5) in apparent absorption or retention of Zn were found between Zn sou rces, although during the 18-d trial mean retention was 58% higher whe n ZnMet was fed. Urinary Cu and Zn excretion decreased (P < .01) durin g stress. Compared with the baseline period, Cu retention became negat ive (P < .001) during the 9 d when no supplemental mineral was fed and returned to positive balance at repletion. Relative to the baseline p eriod, Zn retention decreased (P < .10) when no added mineral was fed, became negative during stress (P < .001), and returned to positive ba lance with repletion. The results indicated that Cu was better retaine d from CuLys than froM CuSO4 and that significant changes occurred in Cu and Zn balance with supplementation and stress.