Is it possible to select cattle that are more resistant to flies? Yes, the experts say. But even though the estimated heritability of fly resistance is very high at .58, it is only one of many traits a cattle producer can consider.
Nonetheless, a few people are selecting breeding stock with natural resistance to flies. “You can do this, but it takes time,” says Doug Colwell, livestock parasitologist with Agriculture and Agri-Food Canada, Lethbridge, Alberta. “There are always some individuals in the herd that are more attractive to flies, and some that are more resistant. About 80% of the flies are usually on 20% of the animals. If you pick out the cows that are heavily covered with flies, nine times out of 10 they are the ones that will be heavily affected next year. If you get rid of those cattle, eventually you will have animals with lower populations of flies,” he says.
With selective breeding, a person can utilize those genetically resistant animals and select for that trait along with the other desirable traits we want in our cattle. Most producers are more interested in other traits, however, and fly resistance may be low on their list of priorities.
Dayton Steelman, emeritus professor of veterinary entomology, University of Arkansas-Fayetteville, has worked on several projects over the years looking at fly resistance in cattle. “When I moved to the University of Arkansas in 1983, I had the opportunity to work with a world famous animal breeder, C.J. Brown, in the Animal Science Department. The University had a herd of about 400 Angus cattle. Working through a Southern Regional Research Project, Dr. Brown’s idea was to develop cows that are well-adapted to their environment’s forage capability,” says Steelman.
“Brown was breeding cattle that could thrive in the Arkansas Ozarks where forage is not as lush as some other regions. He had ‘saved’ a herd of Angus that were the breed type of the 1950s. They measured less than 112.5 cm in height at the hip (about 44 inches). Brown used these cattle in various breeding groups, keeping some of them the same and selecting others for larger size. Eventually he had one group of cattle that were still small (112.5 to 117.5 cm), some that were medium height (117.5-120 cm) and some larger-framed cattle that measured 120 to 126 cm. The ‘modern’ Angus by that time were 126 cm tall at the hip,” he says.
Data shows differences
Steelman collected individual horn fly data on more than 400 Angus cows at the University of Arkansas and a USDA research center at Booneville on a weekly basis (for 14 to 16 weeks) for four years, on each cow. The cattle received no insecticide treatments during this time. “We used these data to calculate heritability and repeatability. There was a statistically significant difference in horn fly numbers among the four groups and the ‘modern’ group at Booneville. The data showed that as the Angus cattle were selected for greater frame size and got larger, they had significantly greater numbers of horn flies,” he says.
“C.J. Brown retired and Hayden Brown continued our horn fly studies. He had 100 Charolais, as well as 18 Chianina cows that had been donated to the University. They were interesting to compare to Angus in the fly studies.”
Chianina cattle are white with black skin. Charolais are white with cream-colored skin. “If you compared number of flies on the Chianina cattle with the flies on black cattle, the numbers were always lower on Chianina, but there were also individual differences within the breeds. By that time, we also had Red Poll, Polled and horned Herefords to compare with the Angus. There was a statistically significant difference among the breeds, as well as statistically significant differences among individuals within each of the breeds. We analyzed the data and came up with 0.58 heritability estimate for horn fly resistance,” Steelman says.
“An animal scientist at Lethbridge, Alberta, Canada, had more than 400 Angus cows and conducted a study looking at differences in number of hairs per square inch, and reported significant differences, depending on cattle size. When I reviewed his work, I realized the significance of breeding for larger frame size. When you take small-framed Angus and breed them up until you get a larger frame size, you still have the same number of hairs. According to his data they have the same number of hairs at birth and you’ve just spread them out farther apart on the bigger animal,” explains Steelman.
“So I ran a study and found that purebred Brahman cattle had the highest number of hairs per square inch, of all breeds. The Chianina were right up there, too, and looking at the other breeds, the hair number per square inch was less and less as the animals were selected for larger frame size.
He washed the animals with hexane and determined that whatever was being secreted by the two oil glands and one sweat gland associated with each hair had an effect on fly attraction/repellency; the more hairs the animal had, the more glands in the skin to secrete the various chemicals. “This is where we got the individual differences among breeds and within breeds,” he says.
“I developed some bulls created via embryo transplant, using semen obtained from a 4-year-old Simmental bull at the USDA Clay Center in Nebraska. He was predominantly red with a little white, and I selected him after making repeated fly counts. He was in a pasture that contained 100, 4-year-old bulls of various breeds, and he had the lowest fly counts of any bulls we’d ever observed. He consistently had 50 or less flies, while the other bulls in the same pasture had 550 to 1,000 flies on average, during the entire study. The USDA laboratory let me have several straws of semen collected from that low-fly bull,” says Steelman.
“We put that semen into a Red Poll cow that had the lowest fly numbers all season, over the four years of our study. We collected her embryos and put them into surrogate mothers. From those I developed two full-sibling bulls that we used in further studies.”
In addition, Steelman collected semen from Angus, Hereford, Chianina and Charolais bulls, and using artificial insemination (AI), developed a small group of cows of different breeds that had all been developed through embryo transfer from horn fly resistant cows—identified within each of their respective breeds. “We compared these horn fly resistant cows to herds of Herefords, Angus, crossbred Angus-Brahman and Hereford-Brahman cattle. We discovered we could influence fly counts by using Brahman or Chianina breeding but we were producing a huge animal that in most cases is too big for the environment.”
Today, cattle breeding and selection focuses on many things, but heritability of fly resistance is often overlooked. “All the animal breeders I’ve worked with state that horn fly resistance is a heritable trait in the bull that is expressed in his female offspring. If you keep replacement heifers, keep them from a bull that has as much fly resistance as you can get,” Steelman suggests.
If you are keeping bulls, select them from dams that don’t have many flies. People make selections based on many traits, but in this day and age of trying to reduce inputs as much as possible, this trait becomes important. If you have a high-fly cow and she isn’t producing top calves, cull her and keep one that has more fly resistance.
“As you continue selecting, you can turn the herd slowly toward more fly resistance. You can cull cows that have lots of flies and keep younger cows that don’t have as many,” he explains. Fly resistance depends on several things, including breed, size of the animals, and individual differences. Research continues to identify the specific gene(s) involved with horn fly resistance in cattle.
Heather Smith Thomas is a rancher and freelance writer based in Salmon, Idaho.
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