The process of genetic selection in dairy cattle involves more than simply selecting for higher milk yield. While milk production is crucial, dairy farmers prioritize a combination of traits that influence both the economic viability and sustainability of their operations.

Key Points of Genetic Selection:

1. Economic-Driven Decisions: Most genetic selection decisions are influenced by economic considerations. Dairy farming is expensive, and farms must generate more revenue than they spend. The primary revenue sources from dairy cows include milk components (fat and protein), progeny, and beef production.
2. Lifetime Net Merit Index: To determine which bulls will produce the most economically efficient daughters, traits are assigned economic values. The USDA produces a selection index called the Lifetime Net Merit Index, which represents the expected revenue that a sire’s daughters will generate compared to an average bull’s daughters. A negative value indicates a below-average bull.
3. Traits with Positive Economic Value:
   • Protein Yield: Protein yield is highly valued, especially in regions where milk is primarily used for cheese production, such as the southwest United States. The value of a pound of protein is projected to be $2.48, with a net revenue of $1.49 after accounting for feed and health expenses.
   • Fat Yield: Fat is another valuable component of milk, particularly for products like ice cream.
   • Productive Life: Longer productive life reduces costs associated with heifer raising and contributes to overall profitability.
   • Udder Conformation and Sound Feet/Legs: These traits are essential for cow longevity and productivity.
   • Fertility and Calving Ability: Fertile cows with easy calving abilities reduce breeding and health costs.
4. Traits with Negative Economic Value:
   • Milk Yield: Surprisingly, milk yield has a slightly negative economic value. This is because after accounting for fat and protein, milk is mostly water. Farmers are charged for transporting this water from the farm to the processing plant. The negative weight on milk yield encourages the selection of milk with higher concentrations of fat and protein.
   • Somatic Cell Score: High levels of somatic cells degrade cheese quality and shorten milk’s shelf life. High somatic cell scores are also associated with udder infections, leading to higher health costs.
   • Body Size: Larger cows require more feed, increasing expenses without a proportional increase in beef revenue.
5. Relative Emphasis on Traits:
   • About 44% of the emphasis is placed on the yield of fat and protein.
   • Approximately 19% is placed on productive life, promoting longer herd life.
   • Around 15% of the emphasis is on mastitis resistance (combining somatic cell score and udder health).
   • Successful reproduction, including fertility and calving ability, also receives significant weighting (15%).
6. Economic Merit Indexes: The use of indexes simplifies the selection process by consolidating multiple traits into a single economic value. This helps farmers rank bulls effectively without having to weigh each trait individually. Various organizations have their own indexes that align with the specific needs and values of their members, leading to differences in trait emphasis, such as protein yield or conformation.

Overall, genetic selection in dairy farming involves a complex balancing act between maximizing economic returns and ensuring the health, longevity, and productivity of dairy cattle.