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
- 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).