Fertility is both the most important as well as the most misunderstood trait in cattle production. In terms of importance it is second only to stocking rate in determining ranch profitability. In terms of being misunderstood in cattle circles it is at the top of the list. There are two categories of cow fertility: academic and practical.
Understanding the difference between academic fertility and practical fertility is crucial to understanding the difference between productivity and profitability. It also clarifies the issue of the heritability of fertility.

Thanks to Daniel Suárez for the photos.

The cows pictured above illustrate the difference between practical and academic fertility. They are the same age and in the same herd. The top cow is pregnant for the second time and the bottom one pregnant for the first time.

Fertility is determined by hormonal balance and body condition. Body condition is determined by genetics (inherent body condition) and nutrition (environment; management, feed). Neither cow is hormonally imbalanced. Both cows are under similar management.
Therefore, the difference in fertility is determined by a difference in Inherent Body Condition.
The top cow has a higher Practical Fertility. The bottom cow is Academically Fertile in the sense that she is hormonally balanced, but requires feed in order to have sufficient body condition (fat) to enable conception.

The difference in Inherent Body Condition is the result of differences in:

  • Frame Size
  • Climatic Adaptation
  • Parasite / Disease Resistance
  • Appetite

One of the biggest impediments to inherent body condition is a large frame resulting from inappropriate selection.
Generally speaking, practical fertility can be increased by decreasing frame size.

The above cow was bred by Dean Moser. She produced her 4th calf at the age of 4 years.
This is illustrative of another category of fertility – extreme fertility – resulting from a combination of hormonal balance, inherent body condition (small frame) and nutrition.

I doubt whether cattlemen are ready for such an extreme in terms of productive efficiency and profitability.