EFFECTS OF ZERANOL AND 2 DIETARY LEVELS OF CALCIUM AND PHOSPHORUS ON PERFORMANCE, CARCASS AND BONE CHARACTERISTICS, AND CALCIUM STATUS IN GROWING LAMBS

Seventy-two crossbred wether lambs (average initial weight, 25.1 kg) were used to determine the interaction between zeranol treatment and two dietary levels of Ca and P (.8 and.6% vs .4 and .3% Ca and P, respectively) in a 2 x 2 factorial arrangement on performance, carcass and bone characteristics, and serum concentrations of parathyroid hormone (PTH) and Ca. Lambs were implanted on d 0 and 56 with 12 mg of zeranol. Lambs had ad libitum access to feed for 105 d. On d 99, blood samples were collected. Implanted lambs had 12% greater (P < .01) daily feed intake, 26% greater (P < .10) ADG, and a 12% improvement in (P < .10) feed efficiency compared with nonimplanted lambs. Zeranol-treated lambs had increased (P < .05) bone cortical area, breaking load, and width of the metacarpal compared with nonimplanted lambs. Lambs fed the .8% Ca and .6% P diet had a higher (P < .05) percentage of bone ash than lambs fed the .4 % Ca and .3% P diet. However, there were no differences (P > .05) in the percentage of Ca, P, Mg, or Zn in metacarpal bones due either to higher dietary Ca and P or to implant treatments. Serum concentration of PTH was greater (P < .10) in lambs fed .8 % Ca and.6% P than in those receiving .4% Ca and .3 % P. Serum concentrations of PTH and Ca pooled across treatments were greater (P < .05) before feeding than at 1 h after feeding. Results from this study indicated that zeranol improved feed efficiency and ADG and increased daily feed intake in feedlot lambs. There were structural changes in metacarpal bones from implanted lambs, but these changes were not a result of changes in Ca, P, Mg, and Zn concentrations.