Flies are much more than a nuisance

Valley Vet
While cattle are fighting flies, they are not grazing and gaining the weight that they could be.

Flies are much more than a nuisance. Their economic impact can reach deep into a cattleman’s pockets.  

Flies: What's the harm?

“Excessive fly numbers affect cattle production in different ways – none of them profitably,” explains Arnold Nagely, DVM, co-founder of Valley Vet Supply. “While cattle are fighting flies, they are not grazing and gaining the weight that they could be. Starting control measures before fly populations build usually yield better results. As with many challenges, prevention or early intervention is prudent. Timely spraying, back rubs, and insecticide ear tags are among several methods utilized to stay ahead of major infestations.” 

What's recommended to keep flies off cattle?

Fly control options have evolved over the years to offer producers with better protection for their cattle operations. Dr. Nagely shared two of the greatest advancements he has seen throughout his veterinary career, and before co-founding Valley Vet Supply in 1985, alongside fellow veterinarian Dr. Ray Shultz.  

Two of the greatest fly control advancements include:  

  1. Insecticide application methods, which have evolved over the decades. Mist sprayers on ATVs are a popular insecticide delivery tool. Also, commonplace today are Fly Killer Kovers for mineral feeders, cattle rubs and even CO2-powered applicators for insecticide capsules, which offer a quick and convenient method to treat small- to medium-sized herds. 
  2. New insecticide chemicals have become available, particularly as ingredients in insecticide ear tags and in the capsules for CO2-powered applicators. 

Warm weather provides the perfect environment for a rapid increase in fly populations. Of the different species of flies affecting cattle operations from herd health to operational profitability, there are a number of fly control methods available to help control them and dramatically limit profit loss. Let’s review the different fly species and their economic impact, according to data from Elanco Animal Health, along with recommended action to take for effective control.  

Face flies can cause reduced grazing and weight loss. In fact, it takes only 12 flies on a cow’s face to potentially reduce grazing time by as much as an hour a day.

Which fly species are the greatest risk to cattle?

Face flies can cause reduced grazing and weight loss. In fact, it takes only 12 flies on a cow’s face to potentially reduce grazing time by as much as an hour a day. Face flies also transmit the pinkeye-causing bacteria known as Moraxella bovis, plummeting profit by as much as $12 per hundredweight, compared to healthy calves sold without pinkeye.  

“Pinkeye is a costly problem in cattle, to say nothing of the nuisance and inconvenience of continually treating new cases, just when the operator needs to be in the field or attending to other responsibilities,” said Dr. Nagely. “We know the causative agent for pinkeye is likely a Moraxella bacteria, and that tall seed heads can scratch the animal’s eyes making them vulnerable to bacterial invasion. But it’s the dern face flies that spread the disease from animal-to-animal, and sometimes herd-to-herd across the fence.”  

Horn flies cost U.S. livestock producers $1 billion annually, due to decreased feed intake, weight loss and diminished milk production. Even more, horn flies also are linked to summer mastitis outbreaks. A single horn fly can take a blood meal from a calf up to 30 times a day, and the impact on rate of gain for yearlings is colossal, reducing yearling weights by 18%. Compared to yearling cattle experiencing heavy horn fly infestations, those with horn fly protection had anywhere from 15 to 50 pounds greater weight-gain advantage. Additionally, calf weaning weights were 10 to 15 pounds higher when horn flies were controlled on mother cows.  

House flies cause aggravation in cattle, pen avoidance and reduced feed intake. Like the horn fly, the common housefly also transmits mastitis-causing bacteria.  

Stable flies cost producers $2.2 million every year, stable flies can cause blood loss, reduced milk production by up to 40%, and decreased weight gain by as much as .48 pounds per day.  

“A common tell-tale sign of stable fly invasion, is grass cattle huddled in the corner of the pasture, rather than out grazing. Or they may take refuge in the pond to keep those flies off their legs and bellies,” cautioned Dr. Nagely. In addition to reaping havoc with pasture cattle, stable flies also can be a real menace to show calves around the barn. 

What are other best practices to control flies?

In addition to fly control methods mentioned above, try not to underestimate the value of regular cleaning around your barn and property. With some quick cleaning every day or so, you can minimize the fly egg production cycle by removing common fly breeding areas (flies are most drawn to moist areas, such as manure and excess plant material, like old, unused silage.) Keep these tips in mind:   

Tidy barns and lots, as well as feed bunks.  

  • Clean barns and lots often, weekly if possible, spreading the manure so it dries.  
  • Remove unused or wet spoiled feed. 
  • Spread out or remove uneaten hay to dry quickly. 
  • Use fly predators, which are cocoon-conquering bugs that kill immature flies. 

Prohibit the fly population through pasture management. 

  • Help manage moist, wet areas of pastures through drainage where practical. 
  • Keep overburden of plant residue at a minimum by controlling and cutting back excessive weeds and plant growth.  
  • Use larvicide feed-throughs in the mineral, preferably beginning before turn-out. 

It is crucial for producers to leverage effective insecticides and fly control methods to help keep cattle healthy. Begin methods of fly control early, and practice insecticide rotation, for the best results against emerging fly populations. Visit to learn more.