Ad libitum suckling by a foster calf in the presence or absence of Ad libitum suckling by a foster calf in the presence or absence of the cow's own calf prolongs postpartum anestrus to first ovarian the cow's own calf prolongs postpartum anestrus to first ovarian cycle cycle

Five treatments were initiated approximately 15 days after calving: 1) calf was weaned from its dam (CW); 2) calf was present continually with its own dam (CP-O); 3) calf was present continually with its own dam but contact with the udder was restricted (CR); 4) foster calf was pre sent continually but the cow's own calf was absent (CP-F); and 5) foster calf was present con tinually, and the dam's own calf was present but restricted (CR+F).


Summary
Five treatments were initiated approximately 15 days after calving: 1) calf was weaned from its dam (CW); 2) calf was present continually with its own dam (CP-O); 3) calf was present continually with its own dam but contac t with the udder was restricted (CR); 4) foster calf was pre sent continually but the cow's own calf was absent (CP-F); and 5) foster calf was present con t inually, and the dam's own calf was present but restricted (CR+F).Cows weane d at 15 days (CW) cycled in about 2 weeks, whereas cows in the CR treatment cycled 1 week later, and cows in the CP-O treatmen t did not cycle for about 5 weeks.Cows fostering calves in the presence (CR+F) or absence (CP-F) of their own calves had extende d anestrus periods similar to those in cows nursing their own calve s (CP-O).If a cow bonds with a foster calf (as in the CP-F treatment), then the duration of anestrus is lengthened.We conclude that anestrus is prolonged only when milk is removed by a calf (her own or a foster calf) to which the cow is bonded.

Introduction
The length of gestation in cows limits producers to one calf crop per year.Suckling lengthen s the period from parturition to first estrus, which may lengthen the calving interval beyond the ideal 1 year.Reproductive efficiency of cows could be increased by reducing the interval to first estrus.
The cow-calf suckling interaction is a critical component in lengthening anestrus.Cows suckled continuall y have longer intervals to first estrus than cows whose calves have been weaned .Anestrus is longer in cows that are nursed more frequently.Presence of nonsucklin g (muzzled or nose-plated to prevent suckling ) calves lengthen s anestrus as much as when calves can suckle, because the perception of suckling or milk removal is maintained with continued calf presence.
When maintained with their own calves, udder-intac t cows and cows with denervated udders had similar intervals to postpartum estrus.Mastectomized cows maintained with their calves remained anestrus longer than weane d mastectomized cows, suggesting that presence of the ca l f but not the mammary gland was essential for prolonging anestrus.Both mastectomize d cows and udder-intact cows, whose calves were restricted s o they could not suckle , cycled about a week later than cows whose calves were not present.
Cows that limit-nursed foster c a lves (limited to four suckling bouts per day for 10 minutes) had shorter intervals to first estrus than cows nursing their ow n calves.This suggests that the cow must recognize her own calf to be nursing in order to prolong anestrus.The present experim ent was designed to determine whether nursing a foster calf in the presence or absence of the cow's own calf wo uld alter the interval to the first ovulation.

Experimental Procedures
Thirty-on e multiparous, crossbred (Angus × Hereford) cow-calf pairs were assigned randomly to five treatments at 15 days after birth: 1) calves were weaned permanently from their dams (calf weaned; CW); 2) calves were placed in a p en within the dam's individual pen, where th e calf could make tactile contact to the dam's head and neck but could have no contact with the mammary gland (calf restricted; CR); 3) cows had unrestricted access to their own calves (own calf present; CP-O); 4) cows had unrestrict ed contact from a foster calves (foster calf present; CP-F); 5) cows had restricted contact with their own calves (CR treatment) plus unrestricted contact from foster calves (CR+F).
Cows were fed individually to meet NRC recommendations , and intakes were adjusted weekly according to i ndividual body weight and condi tion.The CW and CR cows were fed as dry second-trimester, pregnant, beef cows and the CP-O, CP-F and CR+F cows were fed as superio r milk producers.Restricted calves in the CR+F and CR treatments were fed milk replacer twice daily.
Daily blo od samples were taken from cows to determine the onset of the first progesterone rise after treatment initiation.
Ovulation generall y occurs 1 to 2 days before serum progesteron e exceeds .5 ng/ml for at least 2 days.

Results and Discussion
The postpartum interval to first rise in progesterone (Table 1) was shorter (P<.05) in the CW (13.6 ± 4.4 d) and CR (22.0 ± 4.4 d) treatments than in the or CR+F (35.7 ± 4.4 d) treatments.Although no significant difference occu rred between the CW and CR treatments, there was an app arent delay in estrus of about 8 days.This supports an earlier report that maintained cow-c alf recognition even in the absence of suckling prolongs anovulation (1994 Cattleman' s Day; KAES Report of Progress 704:111).The current data s uggest that cows in the CP-F treatment formed a new bond with their foster calves , and that bonding prolonged the postpartum interval to first estrus, because cows in the CR-F treatment had prolonged anestrus equal to CP-O.Apparently, both milk remova l and a cow-calf bond (with own calf or with a foster calf) are essential for prolonged anestrus .Thus, we can conclude that a cow does not require her own" calf to suckle to prolong anestrus, be cause the cow-calf suckling interaction can be newly formed with a foster calf.Means with uncommon superscript letters differ (P < .05). x,y

Table 1 . Average Intervals to First Postpartum Rise in Serum Concentration of Progesterone
Progesterone rise indi cates ovulation occurred and the first postpartum estrous cycle was initiated. a