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What Makes Flossie Fertile?
Can Egg Follicle Size
Indicate Cow Fertility?

Reproductive physiologist prepares to take a blood sample from a cow: Click here for full photo caption.
Reproductive physiologist Tom
Geary prepares to take a blood
sample from a cow to measure
her hormones. An ultrasound
monitor on the right indicates
this cow is 45 days pregnant.

According to Tom Geary—a reproductive physiologist at ARS' Fort Keogh Livestock and Range Research Laboratory in Miles City, Montana—only two out of three beef cows that are bred actually give birth. Identifying the most fertile cows could help producers achieve higher pregnancy rates in their herds and make their operations more efficient.

For several years, Geary has been collaborating with professor Michael F. Smith, a reproductive physiologist at the University of Missouri-Columbia. They have been looking for different physiological indicators of fertility in beef cows.

Now, George Perry—a graduate student of Smith's who's been working with Geary—may have discovered a new indicator of beef cow fertility, at least in some situations: the size of egg-bearing structures called follicles. Their studies may ultimately benefit producers who would like to artificially inseminate all their beef cows at the same time and rapidly improve the genetics of their herds.

Technician conducts a radioimmunoassay: Click here for full photo caption.
Technician George Perry
conducts a radioimmunoassay
to measure progesterone
concentration in serum samples
from cows that ovulated
different-sized follicles.

Inside the Cow's Reproductive System

A beef cow has a 21-day estrous cycle, the period from one estrus, or heat, to the next. The estrous cycle is largely controlled by structures called follicles on the cow's ovaries and the hormones these structures produce.

As a cow approaches estrus, a follicle containing an oocyte (egg) appears like a blister on an ovary. The follicle contains cells that produce estrogen. As the follicle grows, the estrogen eventually reaches a threshold level that causes the cow to be in heat. Shortly thereafter, the cow ovulates: the follicle ruptures and releases the egg into the uterus. Later, another hormone—progesterone—is produced. It prepares the uterus for pregnancy and helps maintain the pregnancy.

Geary says, "Many research projects have been conducted in which natural hormones are administered to beef cows to synchronize ovulation. But cows that are artificially inseminated after induced ovulation seem less fertile than cows inseminated after a natural heat."

Three-year-old cow with newborn calf: Click here for full photo caption.
Three-year-old cow with
newborn calf. Natural or
induced ovulation of a
large follicle results in
normal fertility and the
birth of a live calf.
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 levels—and 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 (12.5–16 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 mm.

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 size.

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

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.



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