Timing of ovulation in relation to onset of estrus and LH peak in yak (Poephagus grunniens L.)

The objective of the study was to determine the timing of ovulation in relation to onset of estrus and the preovulatory LH peak in yaks. For this purpose, a sensitive LH enzymeimmunoassay previously established in buffaloes was successfully validated for measuring the hormone in yak plasma. Plasma LH and progesterone were estimated from blood samples collected from eight non-lactating cycling yaks at 2 h intervals after estrus onset until 6 h after ovulation (ovulation was confirmed by palpation of ovaries per rectum). The mean ± S.E.M. preovulatory plasma LH peak was 10.11 ± 0.35 ng/ml with the values ranging from 8.75 to 11.51 ng/ml in individual yaks. The mean ± S.E.M. duration of the LH surge was 7.25 ± 0.55 h with a range of 6-10 h. Onset of LH surge (mean ± S.E.M.) occurred 3.0 ± 0.65 h after the onset of estrus. Mean plasma progesterone stayed low (<0.25 ng/ml) during the entire duration of sampling. Ovulation occurred 30.5 ± 0.82 h (range, 28-34 h) after the onset of estrus and 20.25 ± 1.03 h after the end of LH surge. The occurrence of the LH peaks within a narrow time frame of 4-8 h post estrus onset in yaks could have contributed to the animals ovulating within a narrow time interval.