The most profitable dairy cow in a herd isn”t always the highest producing cow — health and longevity, fertility and calving ease, milk composition and other factors all influence a cow”s profitability. New tools of molecular genetics allow researchers to pinpoint the genes that influence these traits, and use them in selective breeding programs. The University of Wisconsin-Madison dairy herd, where dairy geneticists are developing a line of Holstein X Jersey crosses, will be the focus of one such program.
Holsteins and Jerseys are the most profitable dairy cattle in North America, but the breeds vary widely for many traits, says Kent Weigel, extension dairy genetics specialist at the UW-Madison. For example, Holsteins are prone to fertility and calving problems, lower milk component percentages, and shorter herd life — traits in which Jerseys excel. Conversely, Jerseys are more susceptible to milk fever and mastitis.
Improving fertility, calving ease, and productive life can reduce production costs, while improving milk composition can enhance the profitability of both producers and cheese plants.
Dairy cattle breeders have traditionally relied on within-breed selection for total merit. Molecular genetics offers breeders a whole new approach. Beneficial genes can be traced back to each breed, and marker-assisted selection can be used to transfer desirable genes from one breed to another. The best experimental designs for detecting these genes involve crossbred populations created from distinct breeds or lines, Weigel explains. This approach has been used effectively in swine and beef cattle.
“Our objective is to identify genes that influence calf mortality, growth, calving ease, fertility, somatic cell count, milk volume, fat percentage, protein percentage, and susceptibility to milk fever, using performance records and DNA samples from a crossbred Holstein X Jersey population,” Weigel says. “This information can be used to enhance our understanding of the inheritance of important dairy traits, and it can facilitate the development of ”optimal” composite populations.”
For the next four years, two-thirds of the milking cows in the Madison and Arlington herds will be mated to young Holstein X Jersey crossbred sires from ABS Global, Alta Genetics, and Select Sires. About 100 “backcross” females with the 3/4 Holstein : 1/4 Jersey genotype will be created each year. The resulting backcross population will allow researchers to identify desirable genes that differ in the Jersey and Holstein breeds.
The remaining milking cows will be mated to young Holstein AI sires, creating about 50 pure Holstein control females each year. All virgin Holstein heifers are still being mated to elite proven Holstein sires to maintain some of the herd lineage.
Data regarding immune function, scours, respiratory disease, and growth will be measured in the backcross and control calves, and age at first estrus and conception rate will be measured in the yearling heifers. Body condition score, internal pelvic area, duration of labor, and calving difficulty score will be measured at first calving, and birth weight, condition of the calf, and composition of the dam”s colostrum will be evaluated. Blood calcium, clinical milk fever, days to first heat, conception rate, milk yield, fat percentage, protein percentage, somatic cell count, and clinical mastitis will be measured during first and subsequent lactations.
“We will determine the genotype of each backcross animal at 175 ”marked” locations that span the bovine genome, and we will search for relationships between the health and performance of our crossbred animals and the genes they inherited from the Holstein and Jersey breeds,” Weigel says.
“To increase the power of our experiment, we”ll also identify commercial herds that have mated these crossbred sires to Holstein dams, and we”ll collect DNA samples and performance data from the resulting backcross animals on these farms. Lastly, it”s important to mention that all of the backcross females in the UW-Madison herd will be mated to pure Holstein sires, such that calves in the next generation will be 7/8 Holstein : 1/8 Jersey.”
“Many of the problems in the dairy industry now are less in production and more in reproduction and health, areas where hybrid vigor is likely to play a larger role,” says Lou Armentano, chair of the dairy science department. The new breeding program will help dairy scientists find ways of overcoming those problems. “The way we”ve done genetics in the past, without knowing where these genes are, has been like playing 8-ball pool in the dark – we”d take a shot, then turn on the lights to see what we got,” Armentano says. “Now we”re learning where these valuable genes are, and it”s like turning on the lights beforehand – we can carefully plan each shot, and take a much more effective approach to dairy cattle breeding.”