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Failure of Photoperiod to Alter Body Growth and Carcass Composition in Beef Steers^1,2,

Michigan State University⁴, East Lansing 48824-1225

Abstract

In each of two experiments, 70 crossbred steers were blocked by BW and assigned to initial slaughter groups or to treatments in a 2 x 2 design. In Exp. 1, treatments were 168 d of photoperiod (8 h of light [L]:16 h of dark [D] or 16L:8D) and plane of nutrition (high energy [HPN] or low energy [LPN]). On d –22, 67 and 155, blood was sampled every 20 min for 8 h. Relative to LPN, HPN increased (P < .01) ADG by 28%, carcass weight by 26% and accretion of carcass fat by 109% and carcass protein by 20%. On d 155, compared with LPN, HPN increased (P < .01) serum insulin (INS; 1.09 vs .64 ng/ml) and lowered (P < .05) growth hormone (GH; 2.14 vs 3.70 ng/ml), but prolactin was not affected. Photoperiod did not affect BW gains, carcass composition or serum hormones. In Exp. 2, treatments were 113 d of photoperiod (8L:16D or 16L:8D) and Synovex-S implant (presence [IMP] or absence [NONIMP]). On d 93, blood was sampled every 30 min for 10 h. Relative to NONIMP, IMP increased (P < .01) ADG by 12% and accretion of carcass protein by 16%. Implants did not affect carcass weight or accretion of fat. Compared with NONIMP, IMP increased (P < .05) GH (3.16 vs 2.39 ng/ml) and INS (.68 vs .46 ng/ml) but did not affect PRL. Photoperiod did not affect BW gain, carcass composition or serum hormones. We conclude that photoperiod fails to influence growth and carcass composition of steers.

¹ Michigan Agric. Exp. Sta. Journal Article No. 12705. This research was supported in part by USDA grants 84-CRSR-2-2340 and 87-CRCR-1-2302.

² The authors gratefully acknowledge the technical assistance of Tom Forton and Elaine Fink.

³ Present address: The Agric. Inst., Grange Res. Centre, Grange, Dunsany, Co. Meath, Ireland.

⁴ Dept. of Anim. Sci.