Fly resistance: Cattle genetics target fliesWritten by Heather Smith Thomas
Horn flies are the most common summertime irritant for cattle, but there are ways to reduce these costly pests besides toxic chemicals.
Dayton Steelman, emeritus professor of Veterinary Entomology at the University of Arkansas-Fayetteville, Ark., thinks the use of insecticides lost some benefit when fly ear tags were developed in the 1970s.
“One of the reasons flies have developed resistance to insecticides used in those tags is cattle producers leaving tags on cattle instead of taking them out at the end of the horn fly season,” he says.
After potency starts to wane, the tags no longer kill all the flies. The ones that survive start the selection for resistant flies because they are the ones that are left to reproduce.
“It got to the point where ear tags being marketed were changing every year, and producers only had a one- or two-year period of effectiveness for any particular tag. They had to keep changing tags and were losing ground with the flies,” says Steelman.
Steelman taught at Louisiana State University for 18 years and recognized the value of Brahman for their insect resistance, realizing that some cattle are resistant to horn flies.
“When I moved to the University of Arkansas in 1983, I worked with a famous breeder, Dr. C.J. Brown, in the Animal Science Department. He felt that cattle ought to be able to perform with very little input from humans and was breeding cattle that could thrive on grass,” Steelman says. “He had a herd of Angus cattle that were the breed type of the 1950s.”
He continues, “They measured less than 112.5 centimeters in height at the hip, or about 44 inches. He divided these cattle into breeding groups, keeping some of them the same, and selecting others for larger size. He eventually had groups that were still small – 11.5 to 117.5 centimeters, some that were medium height – from 117.5 to 120 centimeters – and some larger-framed cattle that measured 120 to 126 cm. The ‘modern’ Angus by that time were 126 centimeters tall at the hip. I worked with Dr. Brown on these projects.”
Data and research
“I collected horn fly data on more than 400 Angus cows at University of Arkansas and also at the USDA Center at Booneville on a weekly basis for 14 to 16 weeks in summer, for four years, on each cow,” Steelman explains. “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 cattle were selected for larger frame size, they had significantly greater numbers of horn flies,” he says.
“We also had Red Poll, polled and horned Herefords, to compare with the Angus. There was a statistically significant difference among breeds, as well as among individuals within each of the breeds. We analyzed the data we collected over the four years and came up with 0.58 or 58 percent heritability estimate for horn fly resistance,” Steelman says.
“An animal scientist at Lethbridge, Alberta in Canada had a large herd of Angus and conducted a study looking at differences in the number of hairs per square inch on more than 400 Angus and reported significant differences among the cattle, depending on their size,” Steelman continues. “I ran a study and found that purebred Brahman cattle had the highest number of hairs per square inch, of all breeds.”
He also notes that Chianina had high hair counts and also good resistance. In the other breeds the hair number per square inch was less and less as the animals were selected for larger frame size.
“I 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 or repellency, and the more hairs the animal had, the more glands in the skin to secrete these various chemicals. This is where we get individual differences,” he says.
“I was lucky to be working with people trained in molecular biology. They helped me conduct research to identify genes that were heritable. I developed some bulls created via embryo transplant, using semen obtained from a four-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,” Steelman comments.
The bull was in a pasture that contained 100 four-year-old bulls of various breeds, and he had the lowest fly counts of any bulls the team had ever observed, with consistently 50 or less flies, while the other bulls in the same pasture had 550 to 1,000 flies on average, during the study.
“The USDA lab let me have several straws of semen from that low-fly bull,” says Steelman.
“We put that semen into a Red Poll cow that had the lowest fly numbers over the four years of our study. We collected her embryos and from those I developed two full-sibling bulls that we used in further studies,” Steelman says. “We also collected semen from Angus, Hereford, Chianina and Charolais bulls and, with 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.”
The cattle were then compared to herds of Herefords, Angus, crossbred Angus-Brahman and Hereford-Brahman cattle, he says.
Today cattle breeding and selection focuses on many things, but the heritability of fly resistance is often overlooked.
“As we go through our herd, make note of which cattle have the most flies. If a high-producing cow has a really good calf every year, there’s no need to sell, but we can help her out by treating her individually. Instead of treating the whole herd, just treat those that we know have high fly numbers every year. The flies that are attracted to those cows will get killed,” he says.
“All the animal breeders I’ve worked with say that horn fly resistance is a heritable trait in the bull that is expressed in his female offspring. If we are keeping replacement heifers, keep them from a bull that has as much fly resistance as we can get,” Steelman continues.
However, if a producer is keeping bulls, he recommends that they select from dams that don’t have many flies.
“If we have a cow that’s a high fly cow and she isn’t producing top calves, cull her and keep a young one that has more fly resistance. As we continue selecting, we can turn the herd toward more fly resistance,” he explains.