INTERRELATIONSHIPS OF LACTATION AND POSTPARTUM ANOVULATION IN SUCKLEDAND MILKED COWS

Reproduction is a major factor limiting efficiency of beef and dairy c attle production. Onset of lactation and reestablishment of postpartum estrous cycles are energy-competing processes, with lactation having a greater priority for dietary nutrients and body reserves through hom eorhetic controls. Prolonged postpartum anestrus or anovulation limits reproductive efficiency by delaying or preventing conception. Sucklin g or milking frequency (>4x daily) delays first ovulation longer than 2 x milking. However, mere presence of a calf (own or foster) to which the cow (udder-intact or mastectomized) is bonded delays onset of pos tpartum ovulation. Furthermore, anovulation was prolonged when cows ma intained a bond with their own calves while their milk is harvested by an unrelated suckling calf, but not when removed by 2x milking. Altho ugh continuous presence of a nonsuckling calf prolongs anovulation onl y slightly, yield and composition of milk for cows milked 2 x daily ar e greater due to increased synthesis or ejection of milk associated wi th calf presence. When lactation is interrupted 13 to 18 days after ca lving and normal suckling, cows ovulated during the next 4 weeks while neither milked nor suckled, When these cows were reunited with their calves after 4 weeks to provide ad libitum suckling, milk secretion wa s reinitiated successfully. Suckling plus milking increases milk yield beyond milking alone at similar harvest frequencies. Greater milk yie ld in milked cows, associated with increased energy deficits and incre ased milking frequency, inhibits estrual expression to a greater exten t than the onset of postpartum ovulation. Energy balance is the key to the timing of first postpartum ovulation in milked cows. Acute, dynam ic changes in energy balance during early lactation or at weaning, rat her than absolute magnitude of energy balance, provide the cow informa tion about its metabolic status. Cows respond to positive changes in e nergy balance by resuming ovarian cycles during energy deficiency. Ova rian follicular development and resumption of follicular waves in suck led and milked cows do not limit early ovulation but rather the failur e of dominant follicles to ovulate in the face of inadequate LH pulse frequency. Further work is needed to elucidate which metabolic signals serve as effecters of increased pulsatile LH secretion and their pivo tal role in reinitiating first postpartum follicular maturation and ov ulation in cattle. (C) 1997 Elsevier Science B.V.