Showing Heat at Night
Transcript of Select Sires' Reproductive Moment Program
on DairyLine Radio Which Aired Dec. 21, 2006
With Ray Nebel,
Senior Reproduction and Herd Management Specialist, Select Sires Inc

Ray, one dairy producers asked when he began breeding cows by AI, do more cows show heat at night? Is this true and has there been any research on this?

Traditionally we say more cows show heat at night, early in the morning or late in the afternoon. There is some truth in it; however, biologically, what makes a cow come in heat is the follicular development. The development of that follicle the maturation of that egg that’s going to be ovulated. As it grows it increases in estrogen and the hormone estrogen got its name because it causes the cow to come into estrus, or what we commonly call heat. That occurs equally throughout the day, but cows do express estrus and are easier to detect at night because there are no other disturbances because we’re not out there 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 there is more of it? At one time the thinking was that more cows showed standing heat at night; however, there has been some work completed at Virginia Tech where you recorded over 2,600 heat periods and those findings showed otherwise.

Right. We did some work electronically recording the onset of estrus and it equally occurred throughout the day. With the hormone estrogen the follicle, once it develops, doesn’t stop and say “well, it’s not dark yet so I’m not going to finish out.” It goes ahead and develops so the onset of estrus occurs equally throughout the day. That is why we miss a lot of cows in heat when we try to concentrate our heat detection in early morning or late at night. Theoretically to catch the most number of cows in heat we need to spread that out, three heat detection periods: one in the morning, somewhere around noon; then one in the afternoon, and one in the evening would be the optimum way to detect cows in heat.

Can you explain what heat-watch is?

Heat watch is the electronic method that we use to detect cows in heat. It is an electronic patch put on the tailhead of the cow that when a cow is mounted from a herd mate it suppresses the sensor that sends an electronic signal to a receiver and gives us an exact time and date of when that event occurred. So if it occurred at midnight or two in the morning or eight or noon; whenever that cow is mounted by a herd mate that pressure is sent and transmitted electronically to a  computer so we can record every mount and when that occurrence was.

Thanks, Ray. That’s Ray Nebel, senior reproduction and dairy herd management specialist for Select Sires.