Cow-Calf Commentary for Iowa Cattleman Magazine
By Randie Culbertson, Cow-Calf Extension Specialist
August 2025
Genetic selection for resistance to ticks and flies.
It has been hard to miss conversations in the news about New World Screwworm (NWS) and the confirmed cases of theleriosis caused by the Asian Longhorned tick here in Iowa. Management strategies can be taken to reduce the risk to your herd, but can you also make genetic selection for resistance?
As with any trait you want to make genetic improvement on, you first have to identify cattle exhibiting the desired trait. Selecting cattle for resistance to ticks or flies involves identifying the breeding animals that naturally exhibit greater tolerance or resistance to these pests. Unlike management strategies that can change from year to year, making genetic selection for a trait has lasting effects for your herd.
Fly Resistance
We all have that one friend who gets eaten alive by mosquitoes while someone sitting right next to them is barely bothered. You may have noticed a similar phenomenon in your cow herd. Some cows will have a higher, more dense number of flies on them compared to others. Research from Arkansas has demonstrated individual variation in fly resistance, with cows classified as susceptible having double the number of flies compared to those classified as resistant. Data has also shown animals classified as fly resistant remained in the same classification year after year. Age does not influence the fly counts, implying that cows demonstrating a resistance to flies as a 2-year-old would remain resistant as they age. The estimation of genetic parameters for fly resistance has been limited. Published heritabilities for fly resistance have been high (>0.50), but the populations used were limited with relatively small numbers of phenotypes. However, these heritabilities indicate a large genetic influence for fly resistance, so rapid genetic improvement could be made.
Quantifying the number of flies on an animal is challenging and has been one reason for limited industry-wide adoption. Research has examined the use of cameras and machine learning to collect phenotypes for fly resistance. Although these technologies show potential for the identification of fly resistance, they are still a long way from industry adoption due to accuracy and cost effectiveness.
Tick Resistance
Roughly 80% of the world's cattle population is at risk of tick-borne diseases, with particular focus in the tropics and subtropics. Tick scoring is used in tropical regions such as Australia and Brazil to determine an animal’s resistance, and is based on the number of ticks that mature on an animal. Rather than counting each tick on an animal, a score allows an animal to be categorized based on the density of tick infestation on the side of an animals including inside the flank and legs. Scores range from 0 to 5, with 0 being a high resistance or very few ticks visible on one side, whereas a score of 5 is a very low resistance, equating to an animal with over 150 ticks visible. Tick scoring should coincide with tick season (spring or start of the wet season) to achieve the greatest variability in tick numbers across animals in a contemporary group. As with identifying animals resistant to flies, the ability to collect tick scores is labor-intensive and somewhat subjective. Advancements in camera technology and artificial intelligence are showing promise for tick scoring.
Several studies have published heritabilities for tick resistance ranging from 0.13 to 0.44, with most studies conducted in Australia and Brazil including both Bos indicus and Bos Taurus cattle exposed to tropical environments. Ongoing research is working to identify genetic markers to identify replacement animals and improve genetic selection.
Breed differences
There is a significant breed difference in fly and tick resistance. Bos indicus cattle, such as Brahman, are known for their resistance to flies and tick, an attribute of the breed for their adaptation to tropical environments. Their resistance is due to breed characteristics such as skin thickness, coat type, color, hair density, and skin secretions. While these advantages make them a logical option for some environments, Bos indicus cattle lack some key breed characteristics such as meat quality and production, and ability to withstand cold temperatures for production here in Iowa. In comparison, European cattle (i.e., Angus, Hereford, Simmental, etc.) often lack the characteristics that contribute to resistance.
Although breed can be a key component for resistance, research has demonstrated that within-breed variation exists. This variation allows for the potential to make genetic improvement within breeds that are not Bos indicus, and better suited for Iowa’s environment and production system.
Genetic Selection in the United States.
Most research for genetic improvement on tick and fly resistance has been conducted in Australia and Brazil, countries that manage cattle in tropical environments where exposure to diseases transmitted by flies and ticks is high. In the United States, the research on these traits has been limited to the Southeastern region.
For producers in the U.S. who want to make genetic improvement for fly or tick resistance, the major challenge is the lack of genetic tools for selection, such as expected progeny differences (EPD) or genomic testing. Currently, the Beef Improvement Federation (BIF), an organization that establishes and standardizes guidelines for the collection of records and phenotypes for use in making genetic improvement and national cattle evaluations, has no guidelines for the collection or analysis of tick and fly resistance data in U.S. beef cattle. Without this data, breed associations are unable to develop EPDs for these traits.
What does this mean for Iowa producers? Thankfully, in Iowa, we have winters that can provide relief from ticks and flies. Without genetic tools for selection for tick and fly resistance, producers can still use phenotypic selection to choose replacements. Management strategies to minimize exposure are still your best bet. Although heritabilities are high for these traits, environmental influence still plays a significant role, so utilizing the management tools at your disposal is key. If you have concerns about flies and ticks in your herd, your first step should be to contact your veterinarian to develop a strategy for maintaining a health herd.
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