LATE-NIGHT SUCKLING INHIBITS ONSET OF POSTPARTUM ESTROUS ACTIVITY IN BEEF-COWS

To determine whether suckling of calves late at night would prolong la ctation anoestrus, 51 Hereford-type cows (21 - 29 days postpartum) wer e divided into three treatment groups. In treatments 1 (night suckling ) and 2 (day suckling) cows suckled their calves five times every 24 h ours; the calves in treatment 1 did not suckle between 09:00 and 17:00 and those in treatment 2 between 21:00 and 05:00. Cows in treatment 3 suckled only once daily at 17:00. By 80 days after calving a signific antly greater proportion (P < 0.001) of the night-suckled cows had not returned to oestrus than those from the other treatments. The mean in terval to first oestrus was significantly (P < 0.001) longer in treatm ent 1 (67.0 +/- 1.5 days) than in treatments 2 (42.2 +/- 2.5) and 3 (4 5.7 +/- 3.2). Lactation anoestrus does not appear to be dependent on t he number of sucklings per 24 hours, but rather on the time of night w hen suckling occurs.