Effects of photoperiod on the development of beef replacement heifers from weaning until first lactation
Glover, Nanette D.
The effects of photoperiod on the development of beef replacement heifers from weaning until first lactation were determined by assigning 144 crossbred heifers on the basis of weaning weight (225 +- 23 kg) and sire breed (British/Continental) to two outdoor housing facilities, with different photoperiod treatments, in a completely randomized design. From December 1, 1998 (day 0) until May 20, 1999, heifers in one facility received supplemental light (423 lux, 1 m above ground) to extend the daily photoperiod (natural + supplemental light) to 16 hours (EP). Heifers in the other facility experienced natural photoperiod only (NP). Measurements of body weight gain, backfat, and concentration of prolactin in blood serum were made every 28 d. Observations for estrus behaviour were made twice daily, in the morning and evening, and were confirmed by serum progesterone concentration in blood samples taken 8-12 d after observed estrus. Two methods of estrous synchronization and timed artificial insemination (AI) were used for first service. Maternal performance as two-year olds was evaluated in terms of calving date (n = 105), milk production and composition during the period of peak lactation (n = 32), and calf weight. (Abstract shortened by UMI.)