THE OESTROUS CYCLE OF THE COW: HORMONAL PARAMETERS AND TIME RELATIONSHIPS CONCERNING OESTRUS, OVULATION, AND ELECTRICAL RESISTANCE OF THE VAGINAL MUCUS

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Abstract
A synopsis is presented of hormonal profiles (peripheral plasma values of LH, FSH, progesterone; urinary oestrogens) during the oestrous cycle of cattle. After ovulation (day one) progesterone levels begin to increase from about day 4, reach a plateau around day 10, and decrease to basal concentrations starting on days 17 to 18. Corresponding with the progesterone decrease, urinary oestrogens show a main increase reaching its maximum one day before ovulation. LH levels are fairly constant from ovulation until day 5, small but significant peaks can be observed between days 6 and 10 in some cows (6 out of 21) and levels then decrease from day 11 to 13. A gradual additional increase begins on day 17, resulting in the preovulatory LH peak 25.7 ± 6.9 h (n = 75) before ovulation. This peak has a duration of 7.4 ± 2.6 h (n = 89) and occurs any time of the day but preferentially around 8 a. m. and 8 p. m. respectively. Likewise related are the two main periods for ovulation. During oestrus, (average duration of 16.9 ± 4.9 h (n = 28) the electrical resistance of the vaginal mucus decreases and the lowest values always correspond with the LHpeak. FSH levels show a wavelike pattern with peaks around days 4, 8, 12–13, 17, 18 and also coinciding with the pre-ovulatory LH peak. The sharp decrease in FSH between the two peaks on days 17 and 18 correlates with the decrease of progesterone during luteolysis. Obviously there is a parallelism between FSH elevations and follicular growth (determined by means of pelviscopic observations).