Influence of zeranol and cottonseed meal supplementation on performance by lambs fed prairie hay

One 42-day growth trial and one digestion trial (16 lambs per trial; average BW 29±2.3 kg) were conducted to examine interactions between zeranol and supplemental cottonseed meal in lambs given ad libitum access to prairie hay (8% CP). There were no zeranol by cottonseed meal interactions (P>0.10) in either trial. Zeranol increased (P<0.05) average daily gain from d 21 to 42, but not from d 0 to 21, or 0 to 42. On d 21 and 42, zeranol-implanted lambs had reduced (P<0.07) serum urea-nitrogen compared with non-implanted lambs. Zeranol did not influence (P>0.10) DMI, N balance, fiber digestibility, or serum concentrations of FFA, somatotropin, and prolactin. Supplemental cottonseed meal improved (P<0.10) average daily gain from d 0 to 21 and 0 to 42, but not from d 21 to 42. Supplemental cottonseed meal increased serum insulin (P<0.07) and serum urea-nitrogen concentrations (P<0.01) on d 21 and 42. Cottonseed meal decreased (P<0.05) ADF digestibility, but did not influence DM, OM and NDF digestibilities, N balance, DMI, or serum concentrations of FFA, somatotropin and prolactin. Even with supplemental cottonseed meal, the prairie hay fed to lambs supported only low rates of gain (70 to 130 g/d), which probably contributed to the marginal effectiveness of zeranol. We conclude that zeranol implants would be of limited benefit for lambs in similar feeding situations.