Resumption of ovarian cyclicity in post-partum beef and dairy cows.

There is a variable anoestrous period following parturition in the cow. Follicular growth generally resumes within 7-10 days in the majority of cows associated with a transient follicle-stimulating hormone (FSH) rise that occurs within 3-5 days of parturition. Dairy cows that are not nutritionally stressed generally ovulate their first post-partum dominant follicle (approximately 15 days), whereas beef suckler cows in good body condition normally have a mean of 3.2 +/- 0.2 dominant follicles (approximately 30 days) to first ovulation; and beef cows in poor body condition have a mean of 10.6 +/- 1.2 dominant follicles (approximately 70-100 days) to first ovulation. The lack of ovulation of dominant follicles during the post-partum period is associated with infrequent luteinizing hormone (LH) pulses, with both suckling and low level of nutrition being implicated in the prolonged suppression of LH pulses in the absence of progesterone. In dairy cows, the normal pattern of early resumption of ovulation may be delayed in high-yielding Holstein-type cows generally because of the effects of severe negative energy balance, dystocia, retained placental membranes and uterine infections. First ovulation in both dairy and beef cows is generally silent (i.e., no behavioural oestrus) and is generally (>70%) followed by a short cycle. The key to optimizing resumption of ovulation in both beef and dairy cows is appropriate pre-calving nutrition and management so that cows calve down in optimal body condition (body condition score; BCS; 2.75-3.0) with post-partum body condition loss restricted to <0.5 BCS units.