Interrelationships of lactation and postpartum anovulation in suckled and milked cows

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Abstract Reproduction is a major factor limiting efficiency of beef and dairy cattle 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 homeorhetic controls. Prolonged postpartum anestrus or anovulation limits reproductive efficiency by delaying or preventing conception. Suckling or milking frequency (> 4 × daily) delays first ovulation longer than 2 × milking. However, mere presence of a calf (own or foster) to which the cow (udder-intact or mastectomized) is bonded delays onset of postpartum ovulation. Furthermore, anovulation was prolonged when cows maintained a bond with their own calves while their milk is harvested by an unrelated suckling calf, but not when removed by 2 × milking. Although continuous presence of a nonsuckling calf prolongs anovulation only slightly, yield and composition of milk for cows milked 2 × daily are greater due to increased synthesis or ejection of milk associated with calf presence. When lactation is interrupted 13 to 18 days after calving 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 was reinitiated successfully. Suckling plus milking increases milk yield beyond milking alone at similar harvest frequencies. Greater milk yield in milked cows, associated with increased energy deficits and increased milking frequency, inhibits estrual expression to a greater extent than the onset of postpartum ovulation. Energy balance is the key to the timing of first postpartum ovulation in milked cows. Acute, dynamic changes in energy balance during early lactation or at weaning, rather than absolute magnitude of energy balance, provide the cow information about its metabolic status. Cows respond to positive changes in energy balance by resuming ovarian cycles during energy deficiency. Ovarian follicular development and resumption of follicular waves in suckled and milked cows do not limit early ovulation but rather the failure of dominant follicles to ovulate in the face of inadequate LH pulse frequency. Further work is needed to elucidate which metabolic signals serve as effectors of increased pulsatile LH secretion and their pivotal role in reinitiating first postpartum follicular maturation and ovulation in cattle.