Relations between estrus expression, follicle size and time of ovulation in grazing dairy cows

Abstract

To maximize pregnancy rates in dairy cattle it is important to synchronize the moment of ovulation with insemination time and the latter can be predicted by the occurrence of standing heat. However, less tan half of the cows show standing heat at estrus. Furthermore, the interval between standing heat and ovulation varies widely. To be able to predict the moment of ovulation of an individual cow more precisely, this study examines the relations between the interval from estrus to ovulation with the intensity of estrous behaviour and diameter of dominant follicle. The various signs of estrous behaviour of 44 cows were recorded during three daily observation periods of 30 minutes each. From the moment a cow was considered to be in estrus, transrectal ultrasonography was carried out twice daily to measure the diameter of the dominant follicle and to determine the moment of ovulation. Negative correlations were observed between the estrus score and the interval estrus to ovulation (r=-0.43; P=0.004), the follicle diameter and the interval estrus to ovulation (r=-0.32; P=0.036) and a positive correlation between the estrus score and the follicle diameter (r=0.53; P<0.001). These correlations imply that cows with a more intense estrous behaviour have larger dominant follicles and are closer to ovulation. On the basis of these results it is possible to give a reliable prediction on the momento of ovulation using the estrous behavior score or the diameter of the dominant follicle.