Keeping up with dairy genetic change


LEXINGTON, Ky. – Throughout the past 50 years, the U.S. dairy population has been undergoing genetic change. Thanks to records kept by the Dairy Herd Improvement program and the use of pedigrees of dairy bulls and cows, that change has been documented.
Jack McAllister, University of Kentucky dairy specialist, said recent summaries of production data have been interesting.
More than double. “Tracking milk production of cows in 1957 through to cows born in 2003 has shown that the average milk production per lactation of Holstein cows born in 1957 was 12,949 pounds,” he said. “That number rose to 26,331 in 2003.”
Lactation is defined as a period of time when a cow produces milk. Cows can be in lactation during seven months of their 9-month gestation period, and after calves are born they are in lactation once again. This cycle can go on for many years, and cows sometimes produce as much as 100 pounds of milk per day.
Milk production has more than doubled in the past 50 years, and records lead experts such as McAllister to conclude that it is largely due to better genetics.
Genetics. Genetic merit for milk production has increased 7,383 pounds, or about 55 percent of the total change, he said.
“This is a remarkable achievement,” he added. “It has come about due to the selection of higher-quality cows, which has been practiced across the eight or so generations of cattle represented in those years.
“Producers chose parents of young heifer calves in dairy herds and young bulls to be progeny tested through artificial insemination.”
Because genetic change is always happening, McAllister said the genetic evaluation system for dairy cows and bulls must take that into account.
Base chosen. “Producers can establish a genetic base for milk yield and they can compare that against the genetic merit of cows and bulls of a specific dairy breed. The genetic base chosen is of cows born in a particular year,” McAllister explained.
“The base is updated every five years, and it was previously updated in February 2000. During that five-year period, genetic merit for milk yield in Holsteins increased by 592 pounds, or almost 120 pounds per year.”
He emphasized that dairy producers should consider the rise in genetic merit when choosing bulls for artificial insemination.
“This can be done most effectively by choosing bulls that have predicted transmitting abilities (PTA) for milk,” he said.
Use percentiles. “Instead of using an absolute measure of what is good transmitting ability, such as pounds, we should be using percentiles. If we use percentiles, such as the 80th percentile, we’ll always be using bulls in the top 20 percentile regardless of what an individual bull’s numerical value is for PTA,” McAllister said.
“It’s important for Kentucky dairy producers to keep up with genetic change and be aware of what’s going on in the industry to improve genetics.”


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