Effects of Flies on Dairy Cattle Welfare and Productivity

Brad Heins, Associate Professor, Organic Dairy Management

April 2017

Fly control is always a hot topic with dairy producers because there are not a lot of viable options to alleviate fly pressure.  Nuisance flies are typically most active from May to October in northern regions, and are active year round in warmer climates. These flies feed on cattle, whether housed indoors or on pasture. Cows are irritated by fly feeding behavior and can become very stressed under excessive fly populations. Cows also become restless and spend less time lying down when under heavy fly pressure. Prolonged exposure to fly irritation can lead to a decrease in production.  Although flies cannot realistically be eliminated from a farm, producers benefit from fly management with more comfortable animals and people. Proper management can keep fly populations in check to minimize negative effects. 

Three important blood sucking pest flies on dairy cattle in the Upper Midwest are the stable fly, horn fly, and face fly.  Stable flies develop as maggots in a wide array of decomposing organic matter, including soiled animal bedding and soiled feed debris that accumulates wherever cattle are confined.  Dairy farm surveys indicate calf hutch bedding is a prominent source of stable flies around dairies, and choice of bedding material can minimize (pine shavings and sawdust contained fewer flies than straw) stable fly production. More recently, it has also become apparent that feed debris and manure that accumulate during winter are also important sources of stable flies, especially where overwintered debris piles remain intact into the following summer. 

Adult stable flies closely resemble house flies, but are easily distinguished by piercing mouthparts that protrude from under the head. These flies are often found on the legs of cows.  Bites from stable flies are painful. Frequent bites stress cattle, leading to decreased weight gain and milk production. Control of adult stable flies is difficult. Only a small percentage of total population are found on cows at any given time and most chemical sprays are rubbed or washed off of cows before achieving satisfactory results. Reduction or elimination of breeding sites is one of the most common methods to manage populations. Any effort to manage breeding sites must often be coordinated with neighboring farms to prevent dispersal. 

Horn flies are small, biting flies that are primarily pests of cattle. The horn fly develops in fresh cattle dung pats and nowhere else.   These flies spend almost all their time on a cow, often along the back or sides, where they feed several times per day. Cows can temporarily dislodge horn flies with head throws or tail flicks, but flies will quickly settle on the same or a nearby cow once the cow stills. Several factors may influence horn fly attraction to a particular cow, including color, breed of cow, time of day and innate, heritable resistance.  Great effort is spent on controlling this fly. Chemical, mechanical and biological control methods have been developed as ways to manage horn fly populations. 

Face flies resemble house flies and feed on bodily secretions, usually around the eyes and mouth of cows. Face flies spend relatively little time on their host. These flies are most active during the day and are typically a problem to pastured cattle, as they seldom enter barns or animal shelters.  Although feeding habits of face flies are annoying to cattle, there is little evidence of negatively affected growth or milk production. 

Fly presence encourages cows to move more frequently to newer areas and to alter feeding bouts. Cows annoyed by flies exhaust energy once directed toward production in an attempt to dislodge flies. Intensity of attacks varies with time of day and weather conditions. Flies are particularly active when winds are low and temperatures are high. Under intense attack, cows often abandon eating and bunch close together. Bunching is a herd response to fly activity where cows attempt to limit surface area exposed to attack. Oftentimes, cows will gather in a tight circle with heads in the center. However, cows in such close proximity have increased risk of heat stress and weight loss. 

At the University of Minnesota West Central Research and Outreach Center dairy, we have been evaluating a unique method (Spalding Cow-Vac™) for controlling pasture flies without the use of chemicals.  The Cow-Vac are compatible with grazing dairying, because a trap can be positioned at the entrance to a milking parlor, where cows come and go twice per day. 

During the summer of 2015, we evaluated the efficacy of the Cow-Vac in on-farm dairy production systems to control horn flies, stable flies, and face flies. The study partnered with eight grazing dairy farms in Minnesota, and herds ranged from 30 to 350 cows in size. 

Fly tableThe results of fly counts and milk production for the presence or absence of the Cow-Vac on farms are in the accompanying table (Table 1).  Horn fly numbers on cows were reduced by 44% on farm in the presence of a Cow-Vac compared to the absence of a Cow-Vac.  Stable fly and face fly numbers were similar on farm whether the Cow-Vac was present or absent on farms.

Milk production was similar for farms with the Cow-Vac compared to without the CowVac.  In summary, these results indicate the Cow-Vac was effective in reducing horn fly numbers on cows and reduced horn fly growth rates during the pasture season in dairy production systems. 

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