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The estrus cycle is governed by hormones (mainly estrogen) in the cow. However, the ambient temperature and nutrition also play an important part in the estrus cycle. Under natural conditions most cows will come on heat towards the end of the main rains. The onset of heat is linked to an improved plane of nutrition. Service should be timed so that the cow will calve 3 or 4 weeks after the main rains start. This will ensure that adequate supplies of forage are available at the time of peak milk production.
Accurate heat detection in the dairy cow is essential. There are a number of signs of heat as shown in the diagrams. The one foolproof sign is when the cow stands to be mounted by another cow and does not move away. The other signs of heat are not always as reliable as this method. They may be a good indication that the cow is coming into heat. On many small farms there may only be one cow with no other bovine animal running with the cow. In other cases, the cow may be tethered and not in contact with other animals. In these situations, heat detection may be difficult, but there are a number of signs as follows:
As well as at milking times, the herd should be observed for at least 10 minutes another 3 times per day. Early in the morning and late in the evening are the best times for accurate heat detection. The cows are likely to be calm and undisturbed at these times. Heat may occur during the night so early morning and late evening observance is absolutely crucial. If nutrition is poor, heat may only last a few hours and may be undetected. Good nutrition based on high quality forage is essential.
Early in the morning and late in the evening are the best times for accurate heat detection. The cows are likely to be calm and undisturbed at these times. Heat may occur during the night so early morning and late evening observance is absolutely crucial. If nutrition is poor, heat may only last a few hours and may be undetected. Good nutrition based on high quality forage is essential.
The cow's ovaries produce ovules or eggs about every 21 days, provided the cow is not pregnant. Estrus (heat period) lasts one or two days. The cow is attracted to the bull at this time. Ovulation occurs at the end of the heat period. If the cow is mated with a bull within 18 to 24 hours of estrus, the egg should be fertilized.