Vet Column 6-29-09 |

Vet Column 6-29-09

David L. Morris, DVM, Ph.D.
Fort Collins, Colo.

Based upon the USDA National Animal Health Monitoring System Beef 2007-08 study, 3.9 percent of beef heifers and 1.1 percent of beef cows are artificially inseminated only to achieve pregnancy, and an additional 12.4 percent of beef heifers and 4.1 percent of beef cows are exposed to both artificial insemination and natural service. Stated differently, 79.2 percent of beef heifers and 94.2 percent of beef cows are only exposed to bulls via natural service. Only 7.6 percent of beef operations in 2006 used artificial insemination.

Comparatively, and based upon the USDA National Animal Health Monitoring System Dairy 2007 study, 72.5 percent of dairy pregnancies occur via artificial insemination, with 88.4 percent of dairy operations achieving pregnancies via artificial insemination.

Estrous, or heat, behavior differs for beef and dairy cows. Multiple studies have indicated that synchronized beef heifers were in heat for eight to 15 hours and received 27 to 50 mounts per estrus when monitored continuously with the HeatWatch system. Beef cows were in heat for 16 hours and received 47 mounts.

On the other hand, dairy cows were in heat for seven to 10 hours and only received eight to 14 mounts. Additional studies have indicated that several visual observations for estrus throughout the day detected 68 to 90 percent of cows in estrus. It was also noted that efficiency of estrous detection increased when the number of observation periods increased. One author found that 27 percent of heats in beef heifers were not detected with twice daily visual observation.

Authors from Oklahoma State University recently published data concerning the effect of number of cows in heat and confinement area on the estrous behavior of beef cows. In this study, 32 non-lactating mature (3-5 years-of-age) Angus and Angus X Hereford cows were used. They were placed in either a 196 feet by 328 feet drylot or in a 30 acre pasture. Comparisons were made of all cows synchronized to be in heat, and so that 1, 2-3, 4-6, and 7 or more cows were in heat at the same time to observe behavior. Duration of heat and the number of mounts were recorded by using the HeatWatchÆ system.

Results in this study indicated that HeatWatch recorded all heats that were detected with constant visual observation of cows during a 28-hour period of time. HeatWatch records mounts in which the pressure sensor is activated for two or more seconds. Various authors have found that HeatWatch has an efficiency of 89.5 to 100 percent and accuracy of 78 to 100 percent.

Cows in drylot were in estrus sooner (62 hours) after the prostaglandin administration compared with cows on pasture (73 hours). Since it is known that social order influences heat behavior, the shorter intervals to heat after prostaglandin in the drylot may be related to the closer association among cows in drylot than on pasture.

When only one cow was in heat at a time, an average of 11 mounts occurred during heat. When seven or more cows were in heat at any one time, an average of 50 mounts per cow occurred during estrus. Cows in drylot were in heat longer (16.4 hours) than cows on pasture (14.2 hours).

The authors in this study concluded that increasing the number of beef cows in heat at the same time will increase the number of times a cow is mounted and the length of time in behavioral estrus. This study confirms what many cattlemen know. The chances of catching a beef cow in heat increases with having more cows in heat at the same time.

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