Growth and Performance of Growing-Finishing Lambs Exposed to Long or Short Photoperiods1

The effects of photoperiod (8 hr light: 16 hr dark [8L:16D] or 16L:8D) on performance of growing-finishing lambs were evaluated in an experiment with 24 rams and 24 wethers. All lambs were caged in groups of two in a controlled environment and fed a pelleted diet ad libitum for the duration of the study (10 to 22 weeks of age). Performance and carcass data showed that both photoperiod and sex of lamb affected growth rate (P less than .01) and feed efficiency (P less than .05) but that only sex of lamb affected carcass quality (P less than .05) and yield (P less than .10). A sex by photoperiod interaction not observed for any of the traits measured. Average daily gain (grams/per day) and feed efficiency (feed/gain) for lambs in the four treatment groups tested were: rams (16L:8D), 410 and 4.3; rams (8L:16D), 340 and 4.5; wethers (16L:8D), 345 and 4.6, and wethers (8L:16D(, 300 and 4.8 respectively. Carcass weight, like slaughter weight, was affected by photoperiod (P less than .01) and sex of the lamb (P less than .05); both rams and wethers exposed to long photoperiods had the heaviest carcasses. Ram carcasses were leaner, had better yield grades and were heavier than wether carcasses, yet both ran and wether carcasses graded Choice. Testosterone concentrations in serum were undetectable (less than .2 ng/ml) at 22 weeks of age in wethers but were elevated (P less than .01) in ram lambs exposed to short (3.4 +/- 1.4 ng/ml) or long (.8 +/- .2 ng/ml) photoperiods. Prolactin concentrations, on the other hand, were not influenced by sex of lamb but were influenced (P less than .01) by photoperiod; i.e., prolactin concentrations were high in rams (190 +/- 34 ng/ml) and wethers (178 +/- 28 ng/ml) exposed to long photoperiods and low in rams (43 +/- 9 ng/ml) and wethers (27 +/- 11 ng/ml) exposed to short photoperiods. These results suggest that testosterone and prolactin may independently affect growth and performance of growing-finishing lambs.