“When you do that, the cattle that are naturally resistant keep reproducing,” Pharo says. “The cattle that have no resistance basically stop.”

The seedstock producer started scoring his bulls on fly resistance about 10 years ago. Since then, consumer interest in finding bulls naturally resistant to flies and other pests has been rising, he says.

Pharo’s herds are dotted across the country in states with a diversity of climates and forage regimes – Texas, Missouri, North Dakota, Oklahoma and more.

“When you look at any of these herds, they’re going to be more resistant just because they haven’t been treated,” he says. “Whenever you start using chemicals to treat lice or worms or flies, you’re covering up the genetics.”

Pharo is among a growing cohort of livestock producers who are increasingly looking toward their livestock’s inherent resistance to flies and other parasites, instead of treatment programs.

He remembers a time when producers thought they beat flies for good.

“A long time ago we started using fly tags. Everyone thought that was the greatest thing, including me,” he remembers. “Until four years later, and the fly tags weren’t working that well.”

“A chemical is only going to last so long before an organism develops some resistance to it,” Pharo says.

There’s a cycle in the cattle industry you could liken to a game of cat-and-mouse. The livestock pharmaceutical industry is constantly evolving its treatments to keep up with pest resistance, Pharo says. That’s why he has turned his attention toward natural resistance.

“If you stop treating for anything, you’ll improve your herd. As long as you’re treating lice or worming your cattle, no genetic improvement is going to happen.”

Selection games

As a seedstock producer, Pharo says most his clients aren’t coming to him specifically for fly-resistant bulls – they typically have bigger problems in their herd by the time they seek help. But pest resistance is becoming more important to his customers who already have genetically sound cattle.

Selecting for fly resistance plays into Pharo’s philosophy of “choosing the best, most adaptable and survivable animals.”

He says only a handful of bulls out of a hundred will make a “5+” fly-resistance rating. Pharo sends DNA samples from all those bulls and their progeny to John Keele, a molecular biologist at the USDA Agricultural Resource Service in Nebraska.

Measuring fly resistance on a scale of 1 to 5, with 5 being very resistant and 1 being not resistant, is a rating method entomologists use in fieldwork when they want to quickly size up how pestered by flies a particular animal is.

While entomologists are one group of scientists studying the heritability of fly resistance, geneticists like Keele are also paying attention.

“I’ve always been interested in the genetics of how things affect cows, like disease or parasites,” Keele says.

Keele studies a myriad of heritable traits – things like growth rate, carcass data and susceptibility to respiratory disease.

Fly resistance is one of the tougher traits to work on, he says.

Under the microscope

In an ideal research setting, you’d want to observe cattle without any treatment (fly tags or sprays). That would help a scientist elucidate whether an individual’s fly resistance is coming from the animal itself versus the treatment.

But when it comes to securing research funding, animal welfare considerations come into play more than ever before and letting cattle get eaten on by flies in the name of science is a tough sell.

“Every time we get research approved, now we have to show how we’re going to treat the animals kindly,” Keele explains. “Nobody wants to see a bunch of cows being overly pestered by horn flies.”

If you flip through the peer-reviewed science available on the heritability of fly resistance, you’ll see a big variation in results, Keele says.

“What I see in the literature is a broad range of estimates,” Keele says. Some studies peg the heritability of fly resistance as low as 5 to 10 percent. Others find it’s as high as 50 to 75 percent.

Keele says the discrepancy boils down to a difference in scientific method. The studies with smaller sample sizes are generally able to do fly counts more regularly, logging more observations per cow, and they tend to show a higher heritability of genetic fly resistance than larger studies where scientists may not be able to observe individual animals as frequently.

However, it’s still a relatively new field of research.

Lord of the fly resistance

Keele points to one new vein of study he finds interesting, a study suggesting certain cows are more resistant to certain horn flies.

He explains it like this: The horn fly secretes an anticoagulant when it feeds on a cow. (An anticoagulant stymies the blood’s natural clotting process, helping the fly feed easier.) Among horn flies, different individuals carry different strains of anticoagulants.

Keele says new research suggests some cows are more affected by certain anticoagulants strains in horn flies. In other words, some flies feed really well on certain cows, while others don’t.

That finding jives with what retired University of Arkansas entomologist Dayton Steelman learned about horn fly behavior throughout his career.

“Horn flies stay on a cow. The male hardly ever gets off. The female gets off to lay eggs,” Steelman explains, “but they generally go back to the same animal. They’ll even hop animal to animal until they find the one they’ve been on.”

Steelman was one of the early researchers investigating the heritability of pest resistance in the ’80s. He says the horn fly occupied a large part of his focus because it was such a prevalent and damaging pest.

“That has a constant impact on the cow,” Steelman says. “Milk production goes down. Weaning weight goes down.”

Over his career, Steelman’s work contributed to the body of research that suggests fly resistance is, indeed, heritable.

Awaiting more answers

But science doesn’t yet provide a clear-cut answer on how producers can improve the fly resistance of their herd through deliberate genetic selection. However, cattlemen and women have been doing it on a practical level for decades.

Like Pharo, Nebraska rancher John Maddux is striving to improve his herd through genetic selection. His goal is to develop the all-around best animal for his cow-calf operation, and he’s developed his own composite breed in the process.

Maddux’s cows are moderately sized, and he makes sure they thrive without hay or other inputs a rancher might spend money on, like fly treatment.

While Maddux doesn’t select for fly resistance in particular, he also chooses not to treat his herd regularly for flies. By doing so, he hopes the herd is engendering its own resistance to flies and other pests over time.  

PHOTO: At Pharo Cattle Company, a team of evaluators looks over bulls for desirable traits, including innate fly resistance. Photo courtesy of Pharo Cattle Company.

Monica Gokey is a freelance writer based in Idaho. Email Monica Gokey.