| As part of his doctoral research,
Perry wanted to figure out why this decrease in fertility occurs. Earlier
studies had shown that when dairy cows are induced to ovulate, they exhibit
a wide range of follicle sizes. Perry says, "We hypothesized that
the synchronized beef cows would also ovulate a wide range of follicle
sizes. This could affect estrogen and progesterone levelsand fertility."
To test this hypothesis, the researchers studied 45 cows at the University
of Missouri in 2001. Perry measured their ovulating follicles when they
were induced to ovulate, but before artificial insemination. Using an
ultrasound machine, he classified each cow's ovulatory follicle as small
(less than 12 millimeters), medium (12.516 mm), or large (more
than16 mm). About 4 weeks after the cows were inseminated, he used ultrasound
to determine whether they were pregnant.
The scientists found that 25 days after breeding, only about 57 percent
of the cows with small follicles were pregnant, and by day 60, only
29 percent were still pregnant. Of the cows with large follicles, however,
67 percent were pregnant at day 25, and they all remained pregnant through
day 60. Even more impressively, 78 percent of the cows with medium follicles
were pregnant at day 25, and 71 percent were still pregnant at day 60.
These results convinced the scientists to repeat the study on a larger
scale. In 2002, they began research on 273 cows at the Fort Keogh lab.
Although the study had grown considerably, they noticed the same general
trend of small-follicle cows having lower fertility.
Because there were more animals, the researchers could compare more
specific follicle size groups. All embryonic deaths occurred in the
synchronized cows with ovulatory follicles less than or equal to 11
Though their data reiterated the importance of follicle size, the scientists
determined something else during the Fort Keogh study. Cows that expressed
a natural heat also had ovulatory follicles of varying sizes. But regardless
of whether the follicles were small, medium, or large, the cows' fertility
rates were similar.
Natural Heat vs. Induced Ovulation
Cows in heat allow other cows to mount them, showing producers when
to breed them. However, heat detection is unnecessary in cows that are
induced to ovulate at a specific time.
Although the cows in the Fort Keogh study were all supposed to ovulate
at the same time, not all did. To detect those that came into heat naturally,
the researchers outfitted the entire herd with electronic transmitters.
Geary explains, "The transmitters are glued to the tailhead of
the cows and allow us to continuously monitor for mount activity. A
computer records the time and duration of each mount."
Adds Perry, "We considered a cow to be in standing estrus, or
heat, when she received three mounts lasting 2 seconds or longer within
a 4-hour period." At that point, the researchers measured her follicle
The follicle sizes of the cows that exhibited a natural heat varied
just as much as those of cows induced to ovulate. However, it appears
that if the follicle is capable of ovulating naturally, it doesn't matter
what size it is. A cow in standing estrus with an 11-mm follicle has
the same chance of becoming pregnant and maintaining the pregnancy as
a cow with a 16-mm follicle. Follicle size seems to affect fertility
only if cows are induced to ovulate.
The Bigger Picture
Although follicle size isn't quite the fertility indicator the researchers
thought, the information they gathered may still benefit U.S. beef producers
and help them manage herds more efficiently.
Geary explains, "Currently, less than 6 percent of U.S. beef cows
are artificially inseminated because of the cost associated with detecting
when a cow is in heat. But scientists in industry and academia continue
to look for more effective ways of inducing ovulation, so producers
won't need to dedicate valuable resources to heat detection."
"Our follicle research will help us focus on where problems are
occurring with current estrus synchronization and artificial insemination
protocols," adds Perry.
Ultimately, such protocols will allow cattle producers to inseminate
a large number of animals at once with genetically superior germplasm
and simplify the management of pregnant animals.By Amy
Spillman, formerly with ARS.
This research is part of Food Animal Production, an ARS National
Program (#101) described on the World Wide Web at www.nps.ars.usda.gov.
Thomas W. Geary is with
the USDA-ARS Fort
Keogh Livestock and Range Research Laboratory, 243 Fort Keogh Rd.,
Miles City, MT 59301; phone (406) 232-8215, fax (406) 232-8209.
"Can Egg Follicle Size Indicate Cow Fertility?" was
published in the April
2003 issue of Agricultural Research magazine.