By Blake Jackson
While commercial dewormers have improved animal health and productivity, their overuse has led to parasite resistance.
“As dewormers lose effectiveness, producers need to develop a broader strategy for managing internal parasites in their livestock,” said Eric Meusch, University of Missouri Extension livestock specialist.
Resistance on the Rise
A recent Missouri study funded by Merck Animal Health found resistance to macrocyclic lactone dewormers (commonly used pour-on and injectable products) in beef herds across the state. The benzimidazole class ("white" dewormer) remained effective. This trend aligns with data from Merck Animal Health.
"The popularity and frequency of use of these products has likely led to resistance," explains Meusch.
Understanding the Enemy
Effective parasite control requires understanding their life cycle. Stomach worms require stages inside cattle and on pastures. Cattle become infected by grazing contaminated pastures, which in turn become infected from adult worms in cattle.
Resistant worms survive dewormer treatments, lay eggs in pastures, and increase the resistant parasite population over time.
Strategic Management for Healthy Herds
Better grazing practices can limit parasite exposure. Rotational grazing reduces the time cattle spend on pastures with high larvae concentration. Leaving a higher residual when grazing (avoiding close grazing) can also minimize exposure.
Strategic deworming disrupts the parasite life cycle. When temperatures drop below freezing for a few nights (typically late fall), deworming becomes effective as reinfection from pastures is unlikely.
This ensures cattle enter winter free of stomach worms and benefit most from supplemental feed. Additionally, it prevents further egg deposition on pastures.
Springtime parasite activity resumes when temperatures rise above 50 degrees Fahrenheit. Deworming six weeks after these temperatures are consistently met targets reinfected cattle before they start laying eggs, further reducing pasture contamination.
While visually identifying heavily infected cows is challenging, culling animals with chronic symptoms can improve overall herd health.
“By managing grazing and strategically deworming cattle to avoid further pasture contamination, producers can keep their animals healthy and productive, as well as help ensure the long-term efficacy of deworming products,” said Meusch.
For more information, contact your local MU Extension center or email Eric Meusch at meusche@missouri.edu.
Photo Credit: gettyimages-pamwalker68
Categories: Missouri, Livestock, Beef Cattle