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Effects of Photoperiod on Growth, Carcass Composition, Prolactin, Growth Hormone and Cortisol in Prepubertal and Postpubertal Holstein Heifers^1,2,

Michigan State University⁵ , East Lansing 48824–1225

Abstract

Effects of photoperiod on growth, carcass composition and serum concentrations of prolactin, growth hormone and cortisol were determined in prepubertal and postpubertal Holstein heifers. Forty-two prepubertal (avg body wt 84 ± 3.0 kg) and 42 postpubertal (avg body wt 303 ± 7.1 kg) Holstein heifers were utilized. Ten prepubertal and 10 postpubertal heifers were slaughtered before treatment began to obtain initial carcass data. The remaining 32 prepubertal and 32 postpubertal heifers were paired by body weight and randomly assigned to short-day (8 h of light:16 h of dark) or long-day (16 h of light:8 h of dark) photoperiods. After exposure to treatments for an average of 139 d, 10 prepubertal and 10 postpubertal heifers from each photoperiod treatment were slaughtered. In prepubertal heifers, photoperiod did not affect (P>.10) average daily body weight gain, carcass weight, carcass composition, accretion of carcass fat and carcass protein, or serum concentrations of prolactin, growth hormone or cortisol. However, prepubertal heifers exposed to long-day photoperiods had reduced (P<.01) urinary N-methylhistidine excretion compared with heifers given short-day photoperiods. Postpubertal heifers exposed to short-day photoperiods had greater average body weight daily gain than animals exposed to long-day photoperiods. Although there was no effect of photoperiod (P>.10) on carcass or fat depot weights, postpubertal heifers exposed to short days had greater (P=.06) percentages of fat and reduced (P=.07) percentages of protein in the soft tissue of the 9-10-11 rib sections. Fat accretion was greater (P<.05) in carcasses of postpubertal heifers exposed to short days than heifers given long-day photoperiods, but there was no effect (P>.10) of photoperiod on protein accretion. Photoperiod did not affect serum concentrations of growth hormone and cortisol, but serum prolactin tended (P<.10) to be greater in postpubertal heifers exposed to long days. Under the conditions of this experiment, we conclude that exposure to short-day photoperiods stimulated body weight gain and fat accretion in postpubertal but not prepubertal Holstein heifers.

¹ Michigan Agr. Exp. Sta. Journal Article No. 11958. This research was supported in part by USDA grants 901-15-2 and 59-2261 -D-2-072-0 and NIH grant HD 17738.

² The authors gratefully acknowlege the technical assistance of D. Steven Lyth and Tom Forton.

³ On leave from Massey Univ., Palmerston North, New Zealand.

⁴ Present address: Lennoxville Res. Sta., Agriculture Canada, Lennoxville, Quebec.

⁵ Dept. of Anim. Sci.