Knowing the species and relative population levels of the parasites threatening your herd can help you proactively plan a parasite management program. Properly timing parasite testing and deworming, however, varies by geography, environment, calving season, and more.
“The ideal time to deworm cattle is dictated more by the parasite’s life cycle than when cattle handling takes place,” says Dr. M. Wayne Ayers, senior beef technical consultant for Elanco Animal Health.
To determine the optimal time for both parasite testing and deworming on your ranching operation, Ayers suggests asking these questions.
When are you calving?
Are you calving in spring or fall?
Spring calves experience their environment quite differently from fall calves. The spring calf encounters an increasing number of parasites emerging on the pasture as the grass greens up and is actively growing. In contrast, the fall calf is exposed to a declining parasite population as grass matures and winter approaches. Timing should be guided by the parasite’s life cycle.
“Better understanding ideal timing can help producers adjust cattle handling to coincide with that ideal,” Ayers says. “For example, if cattle usually are branded or put on pasture the first of May but parasite populations aren’t building until June, shifting deworming later to coincide with parasite exposure helps producers do what’s right by cattle and increase the ROI of deworming and handling cattle.”
How many larvae are available on the pasture?
Parasite load on pasture varies based on environmental conditions, increasing when temperatures warm and grass is actively growing, and decreasing when heat or cold extremes slow or halt larval development and survival.
“The risk of parasites rises when ambient daytime temperatures reach the mid-50s and there is adequate moisture. A good indicator of increasing larval load on pasture is rapidly growing green grass. If conditions favor grass growth, they also favor parasite larval development,” Ayers says.
Ideally, producers should deworm cattle a few weeks prior to when larvae populations are at their peak. This helps limit the larval load on pastures by reducing the number of egg-laying females in the animal and reducing survivability of the larvae being consumed from the pasture, thereby reducing the total parasite population that can develop into egg-laying adults.
Where are you located?
Parasite population levels and timing can differ depending on geography, with larvae becoming dormant during periods of extreme heat or cold.
For example, populations in the South may be lower in summer due to extreme heat and decreased rainfall, and higher in spring and fall when temperatures are moderate and adequate moisture supports larval development and survival. In contrast, summer temperatures in northern climates are generally milder, promoting parasite activity.
“Larvae are less active in both hot and cold extremes. If larvae sense conditions are not suitable for survival, they burrow into the ground, and then, when conditions are right, they emerge,” Ayers says.
What is the ideal time to test for parasites?
Parasite testing using fecal egg counts (FEC) as an indicator of parasite load should be performed when adult pest populations reach peak activity, which varies depending on geography and environmental conditions.
In northern areas of the country, fecal egg counts typically rise in late June and July, making mid-summer an ideal time for parasite testing. In more southern areas, the optimum time to test is more likely in spring or fall.
Fecal egg counts have limitations, as they provide a rough estimate of parasite load but do not accurately indicate the related risk of economic loss. The eggs of the four most common worms responsible for the greatest economic impact look identical under a microscope. Additionally, female worms have different egg-laying capacities. There is also variation among species regarding the potential economic loss they can cause.
To better understand the potential risk of economic loss, Ayers recommends performing a coproculture. This involves hatching eggs in the laboratory and identifying the species present by examining the larvae under a microscope or using PCR to determine both the species and their proportions. This provides a more comprehensive understanding of which species are present and their percentage of the total population, thereby assessing potential economic loss. At Elanco, they refer to this holistic method as Species Specific Quantitative Analysis (SSQA).
Using a Species Specific Quantitative Analysis with pre-weaned or weaned calves is a good first step to better understanding parasites on a given ranch.
“Calves get everything that’s available, and they will be shedding eggs at pre-weaning or weaning time, providing producers with a good idea of what those calves were exposed to during the summer months,” Ayers says.
For screening purposes, composite fecal samples can be used. This involves submitting 20 individual fecal samples to the lab, where a specific amount of each sample is blended into a composite for SSQA testing. This method has been shown to statistically represent the “average” of the group.
While composite testing helps determine the parasite population numbers present in a group of cattle on a given pasture, a fecal egg count reduction test (FECRT) helps evaluate the effectiveness of current deworming products. This process involves performing individual fecal egg counts (FEC) on 20 animals within a group of cattle at the time of treatment and then repeating the FEC on the same animals 14-21 days later. This allows us to compare the before-and-after treatment results as a percentage reduction in eggs per gram of feces.
A FECR of 90% or higher indicates the treatment was effective. If the FECR is less than 90%, it suggests some level of resistance could be developing in the parasite population. Performing composite coprocultures on the pre- and post-treatment samples provides additional insight into which species may be showing reduced efficacy to the dewormer used.
Tips to improve parasite testing include:
- Fecal samples should be the size of a chicken egg.
- Samples should be put in individual sealable plastic bags labeled with proper animal identification.
- Be sure to use a new glove to take each fecal sample to avoid cross-contamination.
- Immediately put samples in a Styrofoam cooler with an ice pack. Wrap ice packs in paper to avoid direct contact with the feces, insulating and protecting the eggs from freezing.
- If being kept overnight, store the samples in a refrigerator. Do not freeze fecal samples.
- Ensure samples arrive at the testing lab within 48-72 hours of collection, maintaining refrigerated conditions.
- Separate composite testing by herd units as follows:
- Mature cows
- Three-year-old cows
- Two-year-old cows
- Yearling replacement heifers
- Suckling calves
To learn more about parasite testing or deworming recommendations, contact your local Elanco representative or visit Elanco.us.
Elanco and the diagonal bar logo are trademarks of Elanco or its affiliates. ©2025 Elanco. EM-US-25-0266
