Can Egg Follicle Size Indicate Cow Fertility?

Research to determine why cows that are artificially inseminated after induced ovulation seem less fertile than cows inseminated after a natural heat shows size matters. Research by Tom Geary and George Perry provides interesting results.

As part of his graduate research studies, George Perry wanted to figure out why a decrease in fertility occurs for cows bred from synchronized heats. "We hypothesized that the synchronized cows would ovulate a wide range of follicle sizes. This could affect estrogen and progesterone levels—and fertility" says Perry.

To test this hypothesis, the researchers studied 45 cows at the University of Missouri in 2001. 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. On a larger scale research trial on 273 cows at the Fort Keogh lab, 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 (slightly less than half an inch).

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

It appears that if the follicle is capable of ovulating naturally, it doesn't matter what size it is. Follicle size seems to affect fertility only if cows are induced to ovulate. "Our follicle research will help us focus on where problems are occurring with current estrus synchronization and artificial insemination protocols," Perry summarizes.

The Basics of a Follicle

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.

Adapted from an article by Amy Spillman, formerly with USDA’s Agricultural Reseach Service
Tom Geary is a reproductive physiologist at the Fort Keogh USDA Livestock and Range Research Laboratory in Miles City, Montana. George Perry is a Graduate Student, University of Missouri-Columbia.

Click here for a printable version of this article (PDF)

For more herd management articles click here

Posted Oct 7th

<< Back to Top