To determine whether suckling of calves late at night would prolong lactation anoestrus, 51 Hereford-type cows (21 â€“ 29 days postpartum) were divided into three treatment groups. In treatments 1 (night suckling) and 2 (day suckling) cows suckled their calves five times every 24 hours; 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 significantly greater proportion (P < 0.001) of the night-suckled cows had not returned to oestrus than those from the other treatments. The mean interval to first oestrus was significantly (P < 0.001) longer in treatment 1 (67.0 Â± 1.5 days) than in treatments 2 (42.2 Â± 2.5) and 3 (45.7 Â± 3.2). Lactation anoestrus does not appear to be dependent on the number of sucklings per 24 hours, but rather on the time of night when suckling occurs.