AD-LIBITUM SUCKLING BY AN UNRELATED CALF IN THE PRESENCE OR ABSENCE OF A COWS OWN CALF PROLONGS POSTPARTUM ANOVULATION

Our objective was to determine whether onset of first postpartum ovula tion would be altered in suckled cows nursing an unrelated calf in the presence or absence of their own nonsuckling calf. In a 2-yr study, A ngus x Hereford cows were assigned randomly to five treatments between d 13 and 18 postpartum for 4 wk: 1) own calf was weaned (OCW, n = 9); 2) own calf was present continuously (OCP, n = 12); 3) own calf was p resent continuously but restricted from udder contact (OCR, n = 9); 4) unrelated calf was present continuously after own calf was removed (U CP, n = 10); and 5) unrelated calf was present continuously and own ca lf was present but restricted as in the OCR treatment (OCR+UCP, n = 10 ). Interval to the first increase in progesterone (ovulation) was less (P < .05) in OCW (14.7 +/- 3.4 d) and OCR (19.9 +/- 3.4 d) than in th e OCP (35.0 +/- 2.9 d), UCP (38.0 +/- 3.4 d), and OCR+UCP (37.6 +/- 3. 4 d) treatments. The OCW cows showed no maternal bond with their own c alves after 4 wk of treatment, whereas OCR, OCP, and OCR+UCP cows were bonded to their own natural calves. Cows in the UCP treatment, suckle d by unrelated calves, formed new maternal bonds with those calves, re sulting in prolonged anovulation. Further, OCR+UCP cows had prolonged anovulation because maternal bonds were maintained with their own rest ricted calves while milk was removed by unrelated calves. We conclude that milk removal preceded by a continuously reinforced cow-calf bond (original or reestablished) is essential to prolong anovulation in bee f cows.