- Traditional methods to determine
when to inseminate beef cows depend on accurate detection of heat.
A common practice used to inseminate cows is the AM/PM. rule. With this
method, cows detected in estrus in the morning are inseminated in the evening,
and cows first exhibiting estrus in the evening are inseminated the following
morning. Twice daily visual observation of cows and tail marking do not
allow precise detection of the onset of estrus. OSU researchers have
examined the length of heat and number of mounts that occur in beef cattle
using an electronic heat detection system called HeatWatchTM.
Cows were in heat 17 - 18 hours in the summer time and about 14 hours in
the winter. They were mounted about 70 times in winter but only 44
times in the summer. There is tremendous variation among cows around
these average numbers.
Dairy cattle may have a shorter duration of estrus and less mounts than beef cows. Duration of estrus in dairy cattle averaged 9.5 hours with 10.1 mounts, and season of the year did not influence behavior (Walker and co-workers, 1996). Dransfield and coworkers (1998) found an average duration of estrus of 7.1 hours with 8.5 mounts in dairy cows. Less mounts and a shorter duration of estrus in dairy cows than in beef cows may be influenced by environmental factors that differ between production practices for the breeds. Confinement, milking, contact with humans, stress, and other factors may influence estrous behavior of cattle.
The timing of insemination in relation to the start or ending of standing heat has been studied for over 50 years. More recently researchers have studied the time from the onset of standing heat to actual ovulation or the release of the egg from the follicle. With newer technology this can be accomplished more accurately. Although there was considerable variation among the dairy cows studied, they concluded that the average number of hours from the onset of standing heat to ovulation was 27.6 hours. This time is considerably shorter than what is being found by Oklahoma State University beef researchers who have concluded that ovulation in beef cows occurs much later (31.5 hours after first mount). This new knowledge about the differences in dairy and beef cows may be useful to both kinds of producers. The old AM/PM rule basically splits the difference between the two kinds of cattle and was a good generalization for all cattle. However, breeding slightly earlier than 12 hours after first heat may have a small advantage for the dairyman who has electronic heat detection or a very watchful eye. In fact research from Virginia and New York has indicated that the optimum AI conception rates occurred at 4 - 12 hours after the start of standing heat in dairy cows (Dransfield and coworkers). In beef cattle data, there seems to be an indication that the optimum time of insemination is later. Researchers from Ohio State University reported on a small study in beef cattle comparing the time of start of standing heat to the time of artificial insemination and the results are in figure 1.
They found that cows inseminated between 10 and 18 hours after onset
of standing heat had significantly greater conception rates than cows that
were inseminated earlier than 10 hours or later than 18 hours.