Many dairy producers practice some crossbreeding, and the numbers increase every year. Motivating factors include a desire to improve fertility, survival, milk components, and calving ease. Some producers want cows smaller than mature Holsteins. Several large, long-term dairy crossbreeding experiments have been conducted in the United States in the past. Cows involved in previous projects were not the result of intensive selection programs for type and production that produced today’s purebred populations.

The Merit indexes are dairy sire selection tools published by USDA that combine genetic evaluations for production, health, fitness, and fertility traits. The indexes are designed to improve the lifetime economic performance of future dairy cows. Periodic revisions include new traits and adjusted economic weights. This document describes the indexes as revised for August 2006. Future changes are inevitable, thus the title “2006 Version.” 

The mating of related individuals is called inbreeding. New dairy animals created by AI or natural service inherit a random sampling of the genetic makeup of each parent. If the parents are related, some of the genes transmitted to offspring by each parent will be copies of the same genes found in the common ancestor(s) which caused the parents to be related. As the genetic relationship between parents increases, the likelihood that pairs of genes in offspring are copies of a single gene in an ancestor generations back increases. Such genes are said to be "identical by descent."

Dairy producers have selected for higher milk production for many years. Genetic improvement causes an average Holstein cow born in 2003 to produce over 7,000 pounds more milk in one lactation than her ancestor born in 1960 produced. Type traits, particularly udders and feet and legs, have also improved because of intensive selection. However, the health and fertility of dairy cows cannot be included among these success stories. Genetic trend was responsible for half of a 9-point decline in pregnancy rate in Holsteins between 1960 and its low point in 1995. Dairy-cattle breeders responded by developing national genetic evaluation programs for a number of fitness traits in recent years. 

The Merit indexes are dairy sire selection tools published by USDA that combine genetic evaluations for production, health, fitness, and fertility traits. The indexes are designed to improve the lifetime economic performance of future dairy cows. Periodic revisions include new traits and adjusted economic weights. This document describes the indexes as revised for August 2006. Future changes are inevitable, thus the title “2006 Version.” 

Culling decisions affect the profitability of the dairy herd. Feed resources and management skills used to maintain unproductive cows would generate more income if applied to productive cows. DHI records contain important information to help guide culling decisions. This guideline describes three systems for rating cows for production traits. Producers recognize that information about production must be combined with reproductive and health status, age, and other factors to make profitable culling decisions. Suggestions for combining information to make good culling decisions are offered.

Concepts surrounding the word "Heritability" (frequently represented by the symbol h²) are among the most important that a breeder of dairy cattle should understand. Heritability applies to a single trait measured on animals in a specific population at a given point in time. Estimates of heritability for a trait can differ between breeds of dairy cattle and may change slowly over time. Heritability is estimated from performance records on animals and pedigree information used to establish genetic relationships between those animals. Heritability helps explain the degree to which genes control expression of a trait. Heritability is used to calculate genetic evaluations, to predict response to selection, and to help producers decide if it is more efficient to improve traits through management or through selection. This guideline highlights definitions and uses of heritability and lists estimates of heritability for several important traits in dairy cattle breeding.