Abstract
BackgroundThe timing of the first postpartum ovulation is an important factor affecting the timing of estrous resumption in dairy cows. The first postpartum ovulation is delayed in cows producing large amounts of milk with an intensive negative energy balance. The antral follicle count (AFC) and serum anti-Müllerian hormone concentrations are known to be indicators of the ovarian reserve, which is the number and quality of follicles left in a pair of ovaries and known as an indicator of female fertility. Cows with higher AFC have been proven to show higher pregnancy rate and shorter calving to conception intervals; however, the relationship between the timing of the first postpartum ovulation and ovarian reserve remains unclear. Therefore, this study examined the relationships between postpartum follicular dynamics, the ovarian cycle, nutritional status, and ovarian reserve.MethodsTransrectal ultrasonography was conducted from calving to 70–120 days in milk (DIM) in 26 cows to monitor AFC, follicular dynamics and the ovarian cycle. Body weight (BW) and milk yield were used as indicators of nutritional status.ResultsThe first postpartum ovulation was significantly later in cows with low AFC (< 25) than in those with high AFC (≥25), while changes in BW from calving to the nadir and milk production were similar in both groups. The present results also suggested that cows with low AFC and a delayed first postpartum ovulation had a shorter first ovarian cycle after the first postpartum ovulation. The mean DIM of the first postpartum artificial insemination (AI) and days open (days from calving to AI with which pregnancy was achieved) were similar in high and low AFC groups.ConclusionsThe first postpartum ovulation was significantly earlier in cows with high AFC than in those with low AFC. The assumed reason for this result was higher sensitivity to luteinizing hormone and larger androstenedione and estradiol production in follicles in high AFC cows. Therefore, cows with high AFC may be more fertile than those with low AFC while their milk production increase and BW decrease; it means they are in negative energy balance.(340/350 words)
Highlights
The timing of the first postpartum ovulation is an important factor affecting the timing of estrous resumption in dairy cows
In the second ovulation after the first ovarian cycle, which is defined as the period from the first to second postpartum ovulation accompanied by corpus luteum (CL) formation [4], the proportion of cows exhibiting estrous symptoms increases to 40% [3]
These findings indicate that the opportunity for first artificial insemination (AI) based on estrus detection is delayed when the time of the first postpartum ovulation becomes later in days in milk (DIM)
Summary
The timing of the first postpartum ovulation is an important factor affecting the timing of estrous resumption in dairy cows. In the second ovulation after the first ovarian cycle, which is defined as the period from the first to second postpartum ovulation accompanied by CL formation [4], the proportion of cows exhibiting estrous symptoms increases to 40% [3]. These findings indicate that the opportunity for first artificial insemination (AI) based on estrus detection is delayed when the time of the first postpartum ovulation becomes later in days in milk (DIM). The resumption of an ovarian cycle is important for reproductive management in cattle
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