When is the last time you went over your parasite-control program? If it’s been a while, you may need to dust off your plans and give them a tweak or two, since recommendations have probably changed since you last thought about parasites.

Products have changed, too, becoming more potent and easier to use. Plus, using the proper product at the proper time can have a significant positive economic impact on your dairy.

This combination of factors means you need to update your knowledge base. Here are some things you need to consider as you examine your plan.

Negative impacts

Strategic deworming for your herd and individual animals (as needed) must be a foundation practice, says Don Bliss, veterinary parasitologist and owner of MidAmerica Agricultural Research Inc., in Verona, Wis. “Worms suppress animal appetites and disturb immune function, which means you’re not getting a return on the other technologies you use, like vaccines or your nutrition program.”

A good number of dairy farmers have taken this advice to heart. According the National Animal Health Monitoring System’s Dairy 2007 survey, 63.3 percent of operations used dewormers in their cow herd. Meanwhile, 69.4 percent of operations dewormed heifers.

This probably means fewer animals carry heavy worm burdens than years ago, but even moderate levels still carry an economic weight due to immunosuppression, anemia or physical damage to the gastrointestinal tract.

And you can’t always see the problem. Many cows and heifers, especially those on pasture, may have some level of parasitic disease that can’t be diagnosed by observation, explains Sandy Costello, Penn State University extension dairy health educator. “Despite these subclinical signs, the parasites may have a negative impact on feed efficiency, nitrogen balance, weight gain and milk production.”

It is also apparent that as animals become more efficient, it takes fewer parasites to cause economic losses.

A University of Wisconsin study shows that improvement in milk production due to deworming was greatest in the best-managed herds, Bliss notes. “It takes fewer parasites to disturb a cow milking 30,000 pounds of milk than one milking 15,000 pounds. The more efficient an animal is, the greater impact parasites can have on maintaining this efficiency.”

Early-lactation factor

Nowhere is that more obvious than for early-lactation cows. Again, research at the University of Wisconsin indicates that cows that were less than 90 days in milk and were experimentally exposed to infective larvae produced 6.4 pounds less milk per day than their non-infected counterparts. “This is when cows are just beginning their greatest period of lactation stress,” says Bliss. “If parasites are present in the animal, or if she is being exposed to infective larvae during this period, another physiological stress is being added to an already-stressed animal.”

On-farm trials in Georgia show similar results. Those data indicate that proper deworming treatments of cows at calving (with follow-up treatments as necessary) yield up to 250 more pounds of milk during that lactation. At $12 per hundredweight, that equals a gain of $30 per cow. In a 200-cow herd, that’s an extra $6,000 in your milk check, minus treatment expense, of course.

Vulnerable herds

Most people associate parasite-control with grazing operations, and those herds do carry the greatest risk of parasite contamination.

Meanwhile, cows on pasture only while dry still maintain a moderate-to-high exposure, and cows with access to an exercise lot also carry a moderate risk. Cows with access to dirt dry lots and cows on concrete dry lots or cows completely confined face a low risk of parasite contamination.

Keep in mind, though, that low contamination does not mean no contamination.

Parasites are not just a problem in grazing herds. A study of 141 herds in six counties in New York, Pennsylvania, Wisconsin and Minnesota estimated the prevalence of parasites. In each herd, veterinarians collected fecal samples from 10 first-calf heifers which were 60 days in milk or less. Parasites were found in 77 percent of the herds. The study also showed that 50 percent of the animals in confinement (66 herds) were positive for parasites.

That’s because cows and heifers brought into your operation may have spent time on infested pastures or facilities and bring the parasites along when they enter your herd.

“You need to conduct fecal egg counts to know your exposure and contamination level,” argues Bliss, so that you can properly address potential parasite problems, regardless of your management system.

Resist resistance

As with any management tool, you want to gain the most benefit from your deworming program. This means being aware of, and managing against, parasite resistance to products. For the most part, resistance has not appeared with cattle products to the degree that it has appeared in other animal species, like sheep.

But, from a global perspective, dewormer resistance has been detected in all of the economically important gastrointestinal worms of cattle, says William Shulaw, Ohio State University extension veterinarian. The reports have come from Australia, New Zealand, South Africa, Brazil, Argentina, some countries of Europe and the United States.

“This is not to say that all worms of cattle in all countries are resistant to all classes of dewormer or that the level of resistance is complete,” he says. “But, it does indicate that we should be concerned about the sustainability of our current deworming practices.”

High-resistance risk strategies include treating adult animals when there is no identified need, improperly dosing animals and moving newly treated animals onto “clean” pastures.

You can do better. Get fecal egg counts to determine parasite burdens, develop a treatment plan with your veterinarian and follow label directions.  

“Get informed about modern parasite-control strategies and gather information about the parasite status of your farm,” recommends Shulaw. “We can learn from sheep producers who been struggling with resistance issues for years.”