Photoperiod effects on the development of beef heifers

Small, J. A., Glover, N. D., Kennedy, A. D., McCaughey, W. P. and Ward, D. R. 2003. Photoperiod effects on the development of beef heifers. Can. J. Anim. Sci. 83: 721-730. Crossbred beef heifers (n = 144) were assigned at weaning (187 +- 14 d of age) by body weight (225 +- 23 kg) and sire breed (British/Continental) to one of two photoperiod treatments from 21 December 1998 (0 wk) until 10 May 1999 (20 wk): natural photoperiod (NP) that gradually increased from 8.1 h (0 wk) to 15.2 h (20 wk) and, extended photoperiod (EP) that consisted of natural + supplemental light (400 lx, 1 m above ground) to extend photoperiod to 16 h. Rations were formulated for two-steps of body weight gain (0.6 and 1.2 kg d-) to achieve 60% of mature weight at 18 wk. Visual observations of estrus behavior were made twice daily and confirmed by serum progesterone. Body weight, backfat and serum prolactin data were determined for each 4-wk period. Ambient temperatures averaged -12.2 +- 6(degree)C in winter (0 to 12 wk) and 4.2 +- 5(degree)C in spring (12 to 20 wk). Gain in body weight was greater (P < 0.05) and backfat lower (P < 0.05) for EP than NP treatments from -2 to 6 wk and only 1% of heifers had attained puberty during this period. However, as yearlings at similar (P > 0.05) body weight and backfat, more (P < 0.05) EP than NP heifers had attained puberty (84.7% vs. 69.4%). Prolactin was greater (P < 0.05) for EP than NP treatments from 2 to 6 wk (10.3 vs 5.5 +- 1.2 ng mL^-1). Management of photoperiod influences attainment of puberty and prolactin secretion in beef heifers housed in an outdoor environment.