When to inseminate the cow? Insemination, ovulation and fertilization in dairy cattle

Keywords: dairy cattle; oestrus; behaviour; pedometer; reproductive hormones; ovulation time; insemination strategyIn dairy practice, calving rates after first insemination are often less than 50%. Part of this low percentage might be explained by wrongly timed inseminations. The aim was to establish the relationship between various oestrus characteristics and ovulation time in order to investigate whether these oestrus characteristics could predict ovulation time and to study the consequences of variation in the interval between insemination and ovulation on the success of fertilization and embryonic characteristics. The ultimate goal of the project was to come to an optimal insemination strategy that can be used in practice. The relationship between behavioural oestrous signs, changes in activity (measured by pedometers) and progesterone profiles and time of ovulation were. It was shown that monitoring progesterone profiles was not suitable to predict time of ovulation because of the large variation found in the decrease of progesterone relative to ovulation. Although the prediction of ovation time using behavioural oestrus signs was quite accurate, it seems not suitable for practice because of the high labour requirements. The increase in the number of steps measured by pedometers seems useful to predict time of ovulation accurately and could be easily implemented in practice. Effects of the interval between insemination and ovulation on success of fertilization and embryonic characteristics were also studied. It was shown that the insemination-ovulation interval in which high fertilization rates were observed was quite long (insemination from 36 to 12h before ovulation), while the interval in which the majority of fertilized ovum developed into a good quality embryo was considerably shorter (insemination from 24 to 12h before ovulation); the interval between insemination and ovulation did not affect the sex ratio of the embryos.In conclusion, the best chance to increase calving rates seems to be to use the insemination strategy, in which cows are inseminated between 5 to 17h after the first increase in the number of steps.