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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

Why it is almost impossible to catch cows in heat!!!

Dairy Pipeline: September 1996

by Ray L. Nebel
Extension Scientist , Reproduction
Virginia Tech

How many times has your Vet or consultant said that cows are cycling but no one is "catching" them in heat? In fact, many times I have written that the heat detection efficiency for DHI herds in Virginia is less than 50%, which means that less than half of the eligible heats are being detected. Since early in 1991, Virginia Tech has been developing, evaluating and using what is now being marketed as the HeatWatch® electronic heat detection system. This system provides continuous 24 hour observation of cows for mounting activity that is associated with heat. We now have the best information ever for the best indicator of estrus behavior, "standing to be mounted." Cows at the Virginia Tech Dairy Cattle Center averaged 11 mounts per heat period. Milking Holsteins averaged 10 mounts and Jerseys averaged 14.5 mounts per heat period. Holstein cows that came in heat during the summer (July-August-September) averaged 5 mounts per heat period compared to Holstein cows in the fall that averaged 11 mounts. It was very interesting to observe that from summer to winter the total number of mounts per heat period differed, but the duration of time of mounting activity did not change. The average heat period for cows at the Virginia Tech Dairy Cattle Center was 8_ hours; however, there was a tremendous variation (as with other characteristics I have mentioned) in the duration or amount of time that cows were mounted by herdmates (ranged from a few seconds to over 30 hours). When we divided the cows into four production groups based on milk yield, the lowest producing cows exhibited a greater number of standing events. The intensity of expression of heat was much less for higher producing cows. I have stated that these finding are for cows at the Virginia Tech Dairy Cattle Center because there is a difference in estrus behavior or "mounting activity" across farms. We have recently completed a field study with 17 herds that used the HeatWatch® system and differences did exist from farm to farm. What does this mean for a farm that has not purchased this new heat detection system: 1) At least three observation periods are necessary - average duration of heat is only 8_ hours, 2) Other activities (scraping lots, filling freestalls, etc) must not be performed during the time allotted for observing cows - the average cow has only 11 mounts, and 3) All of the heat periods will never be detected without observing cows 24 hours a day, because too many cows show very little activity - less than 3 mounts for only a few seconds.

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