Extension Spotlight

Parasites can have a significant impact on cattle. During summer, flies can directly affect the growth of younger cattle as well as cows nursing calves. Thus, revenues are affected by flies as well due to decreased gains in young cattle and decreased recovery of cows. Controlling flies is relatively easy and has a positive return on the expense.

The impact of flies on beef cattle has been studied multiple times over the past 50 years. As fly numbers increase on individual animals, weight gains become reduced compared to animals treated with fly controls. These decreased weight gains equate financially to reduced revenue as animals are lighter weight at sale time. It also affects breeding animals including growing bred heifers and nursing cows trying to maintain or increase body condition.

Overall, it is estimated flies account for $1.75 billion in losses to the beef industry. The main concerns are two blood feeding flies, horn flies and stable flies. Horn flies are small black flies that tend to be on the belly, sides and back of cattle. These flies have a short life span of 10 to 20 days. During their adult stage, horn flies feed on 24 to 38 blood meals per day. Females leave the host animal long enough to go they their eggs in fresh manure and then return to feed. In studies horn flies have been shown to reduce weight gains as much as 15 percent in growing calves and up to 18 percent in replacement heifers. These reduced weight gains could equate to $90 to $125 less income on 600 pound calves in the current market. Nursing cows left untreated for horn flies may weigh 50 to 60 pounds lighter at the end of the summer than cows treated for flies. This is equal to half of a body condition score.

Stable flies are another blood feeder. They prefer the legs of cattle. Research found cattle left untreated for stable flies had daily weight gains reduced on average by .44 pounds per animal per day. During a 90 period, this can create a financial reduction of up to $100 per head at current prices for 600 pound calves. The research further concluded 30 percent of the reduced gain was caused directly by the blood feeding, but 70 percent came from cattle bunching together and creating heat stress on those animals.

Other fly species can also irritate cattle and negatively affect them. Heel flies do not bite, but lay eggs on cattle. When the eggs hatch, the larvae enter the body under the skin and migrate through the body eventually locating on the back. The larvae make a hole in the skin to breathe and then will finally emerge from the hole as mature adults. These open sores attract other fly species to the open wounds.

Face flies do not bite or blood feed, but instead feed on fluids and secretions from the animals’ body. They are found around the mouth and eyes on the face. Face flies can act as a vector for the pinkeye virus and therefore can create a negative impact on cattle.

Controlling flies is the best way to prevent these adverse effects. Flies cannot be completely eliminated but should be kept below what is considered the Economic Injury Level. For horn flies the EIL is less than 200 flies on an individual and stable flies should be less than five flies per leg per animal.

Using insecticides is the primary method of fly control. The goal is to treat cattle prior to fly levels reaching the EIL and to break the life cycle of the fly species. Insecticides can be applied as a liquid spray, powder, oil or an impregnated ear tags (fly tags). Many general parasiticides for cattle include an anthelmintic to control parasitic worms along with an insecticide and can be applied as a liquid pour on product, injectable solution or slow release bolus. Insect growth regulators (IGRs) are products meant to prevent flies from reproducing, breaking the fly life cycle. IGRs can be given as a fed supplement or pour-on product.

In a recent survey, beef cattle producers estimated they spend from $9.50 to $12.40 per head on fly control depending on how long flies were present in their area. When compared to the possible negative impact of $100 less per head due to reduced gains, fly controls are cost effective.

Other management practices can also help to limit flies. Rotation of insecticides helps prevent resistance to the products over generations of flies. Proper management of manure, compost piles and other materials such as decomposing feeds limit places where flies can lay eggs and larvae can live.

Whether it is reduced weight gains or the possibility of disease, flies are not only an annoyance to cattle but have a negative impact financially. Proper fly control counters those negatives and has a positive impact on the bottom line of the beef enterprise. If you have questions on which are the most effective products, consult with your veterinarian.