Effects of d-cloprostenol dose and corpus luteum age on ovulation, luteal function, and morphology in nonlactating dairy cows with early corpora lutea

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Luteolysis is a key event in cattle reproduction. A standard dose of exogenous PGF2α will induce full luteolysis in the majority of cows with a matured corpus luteum (CL). However, this will not occur in cows with a CL <5d old. To date, it is not known whether a larger dose will have a more potent luteolytic effect in cows during early diestrus. The objective of this study was to characterize the effect of 2 doses of d-cloprostenol (150 and 300μg) on the progesterone concentration, luteal diameter, and ovulation rate in nonlactating dairy cattle 96 to 132h postovulation. Twenty nonlactating dairy cows were included in the study. Each cow received 2 treatments of d-cloprostenol in 2 consecutive cycles: a standard dose of 150μg and a double dose of 300μg. The cows were allocated randomly to 1 of 4 groups (5 cows in each group) according to the age of the CL at the time of treatment: 96, 108, 120, and 132h. The exact time of ovulation was known within 12h, because of twice per day ultrasound examination. The CL diameter and progesterone concentration were measured before treatment (d 0) and 2 and 4d after treatment. Within each CL age group, the effect of d-cloprostenol dose on luteolysis was determined. More cows treated with double dose tended to have full luteolysis compared with the standard dose (8/10 vs. 4/10, respectively). This effect was only apparent in cows with CL of 120 and 132h but not in earlier CL. The interval from treatment to ovulation was shorter (3.3±0.1d) in cows treated with a double dose than in cows treated with the standard dose (4.5±0.4d).