Select Sires

When Cows Show Heat
Transcript of Select Sires' Reproductive Moment Program
on DairyLine Radio Which Aired Dec. 16, 2004
With Ray Nebel,
extension specialist and professor of reproductive management,
Virginia Tech

This week’s Reproductive Moment is with Ray Nebel, extension specialist and professor of reproductive managements at Virginia Tech. Ray, one dairy producer noted when he started breeding cows by A.I. that more cows showed heat at night. Is this true and is there any research on this?

Traditionally, we say that more cows show heat at night, early in the morning or late in the afternoon and there is some truth in it. However, biologically, what makes a cow come in heat is the follicular development: The development of the follicle and the maturation of the egg that is going to be ovulated, and the increase in estrogen associated with that growth. Estrogen got its name because it brings the cow into estrus, or what as we commonly call "heat." Estrus is probably easier to detect at night because there are no other disturbances. We are not out feeding or milking. There is more quiet time, you might say, so the cows can interact. Therefore, it is a little bit easier to catch cows in heat at night.

It might be easier, but is there more of it? At one time, the thinking was that more cows showed standing heat at night. However, there has been some work at your university, there at Virginia Tech, where you recorded over 2,600 heat periods and those findings showed otherwise.

Right, we did research electronically recording the onset of estrus, and found that it occurs equally throughout the day because of the hormone estrogen. Once the follicle is developed, it doesn’t stop and say, "Well, it’s not dark yet, so I’m not going to finish out." It goes ahead and develops. The onset of estrus occurs equally throughout the day, and that’s why we miss a lot of cows in heat when we try concentrate our heat detection early in the morning and late at night. Theoretically, to catch the greatest number of cows in heat, we need to spread that out. Therefore, the optimal way to detect cows in heat would be with three heat detection periods: one in the morning, one somewhere around noon and one in the evening.

Can you explain what heat watch is?

Heat watch is an electronic method that we use to detect cows in heat. It’s an electronic patch that is put on the tail head of the cow. When the cow is mounted by a herd mate, it presses a sensor that sends an electronic signal to a receiver, and gives us an exact time and date of when that mounting event occurred. So, no matter when that cow is mounted by a herd mate, that pressure transmits a signal electronically to a computer, and we can record the date and time of every occurrence.

If you have any questions, or for more information, contact your Select Sires Reproductive Solutions™ specialist, they’ll be glad to help. This week’s Reproductive Moment has been with Ray Nebel, extension specialist and professor of reproductive management at Virginia Tech.

™Reproductive Solutions is a trademark of Select Sires Inc.

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