Ovarian size matters

Editor’s note: Heather Dann is a research scientist with the W.H. Miner Agricultural Research Institute in Chazy, N.Y.

As an undergrad, I helped with a few bull tests and sales. I recall long nights of helping wash bulls and measure scrotal circumferences. After dealing with a couple of less than willing participants, I asked about the usefulness of the measurement. I was told “size matters!” The scrotal circumference is a moderately to highly heritable trait and is positively correlated with semen volume and quality.

Since that time, I have wondered if size of the ovaries matter in heifers and cows.

Thanks to a recent journal paper (Ireland et al., 2011), I now know that the high variation in ovary size and the number of follicles and high quality oocytes in ovaries (referred to as ovarian reserve) impacts ovarian function and fertility.

Based on once or twice a day ovarian ultrasonography of cattle, the peak number of follicles growing during different follicular waves of the estrous cycle (referred to as antral follicle count; AFC) varied seven-fold among animals but was very repeatable within animal regardless of age. Thus, AFC can be used to reliably phenotype cattle and predict fertility.

Cattle with low AFC had:

• Smaller ovaries.
• Lower number of morphologically healthy follicles and oocytes (smaller ovarian
• reserve).
• Reduced responsiveness to superovulation and less number of transferrable embryos.
• Chronically heightened gonadotropin secretion but lower progesterone concentration during estrous cycles.
• Reduced endometrial thickness.
• Higher amounts of cumulus cell markers for poorer oocyte quality.

 

These findings indicate that cattle with lower AFC will have suboptimal fertility. As an aside, the AFC in humans is used to predict in vitro fertilization outcomes. A lower AFC results in fewer live births.

The routine use of intensive serial ovarian ultrasonography in cattle to determine AFC is not practical on farm. However, a collection of a single blood sample during the estrous cycle and measurement of serum anti-Müllerian hormone (AMH) may be an option.

Serum AMH is produced by granulosa cells of healthy growing follicles, varies minimally during the estrous cycle, and is highly correlated with AFC.

It was suggested that the ability to identify cattle with inferior or superior fertility based on serum

AMH could change culling practices for heifers (for example, potentially culling inferior fertility heifers before breeding) or breeding programs (like the. use of sexed semen from high-genetic merit bulls in superior fertility heifers or cows).

Why is there so much variation in ovary size, ovarian reserve, and AFC?

There is evidence suggesting that maternal nutrition or disease may contribute to high variation. In a beef heifer study, follicular development of the fetus was inhibited when the dam was fed 60 percent of her energy requirement shortly before conception and during the first trimester. The first trimester is when the peak in number of follicles and oocytes in the fetal ovary.

The offspring from control dams and restricted dams had similar birth weights but the offspring from the restricted dams had 60 percent lower AFC.

In a study on a commercial dairy farm, Holstein cows with a high number of somatic cell count (SCC) measurements greater than 200,000 produced daughters with lower serum AMH, smaller ovaries, and fewer high quality oocytes, and reduced ovarian function than cows with lower SCC measurements.

The role of maternal nutrition and disease on offspring fertility should be a hot topic for research and lead to the development of novel prenatal therapeutic methods and management programs.

* Reference: Ireland, J. J., G. W. Smith, D. Scheetz, F. Jimenez-Krassel, J. K. Folger, J. L. H. Ireland, F. Mossa, P. Lonergan, and A. C. O. Evans. 2011. Reproduction, Fertility and Development 23:1-14.

Source: W.H. Miner Agricultural Research Institute’s Farm Report, August edition

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