When is a cow in estrus? Clinical and practical aspects.

Good detection of estrus is critically important in dairy husbandry. Incorrect detection of estrus is related to loss of profit due to
extended calving intervals, milk loss, veterinary costs, etc. Detection of estrus remains a major problem despites enormous progress in
the knowledge of reproductive physiology of the cow and in development of estrus detection aids. To achieve good estrus detection,
many factors have to be taken into account. On one hand a cow has to express estrus and on the other hand the farmer has to detect it.
Combined action of several hormones causes physiological changes that lead to ovulation and an environment in the uterus that allows
sperm to fertilize the egg. Besides these internal actions, a number of external changes can be observed. When using visual observations,
time of the day and time spend on observation have a great impact on detection rates. Many devices are available to aid in estrus
detection, such as pedometers, mount devices, temperature, and hormone measurements.
Expression of estrus can be influenced by many factors. Heritability, number of days postpartum, lactation number, milk production,
and health are known to influence estrus expression. Environmental factors like nutrition, season, housing, herd size, etc. also play a role
in estrus expression. To evaluate estrus detection, record keeping is very important; a number of formulas can be used to assess detection
efficiency. Besides the farmer, the veterinarian and inseminator can play an important role in estrus confirmation and good insemination
strategy. In the end, the time of ovulation and the age of the egg at sperm penetration is critical for conception. Therefore, emphasis in
research needs to be on the timing of insemination relative to ovulation, and thus on the detection of ovulation