Cattle Today

Cattle Today



by: Heather Smith Thomas

Internal parasites (stomach and intestinal worms) rob cattle of nutrition, reducing growth and weight gains in young cattle and hindering optimum production in all classes of cattle. Heavy infestations can also create health issues; worms can be a stress, making the host animal more vulnerable to disease. Deworming cattle at the proper time in the life cycle of the target parasite to eliminate egg-laying adults in the tract, or at a time of year to most effectively minimize re-contamination of pasture with worm eggs, can keep re-infection at a low level.

Dr. Patty Scharko, DVM, MPH, ACVPM (Extension Ruminant Veterinarian, University of Kentucky) says there are a number of good deworming products available for producers, including Ivomec and Ivomec Plus (Merial), Dectomax and Valbazon (Pfizer), Cydectin (Fort Dodge), Panacur/Safeguard (Intervet) and some generic products. Some of these are pour-ons, some are injectable, and a few are given orally. The white paste dewormers (the benzimadozoles) are given orally and are effective for short term deworming, whereas the pour-ons and injectables tend to be longer acting. Some of these are endectocides—effective against both internal and external parasites.

There are several classes of deworming drugs. The macrolides (long-acting, since they kill larval stages anywhere in the body as well as mature worms in the gut) are also endectocides, and include Cydectin, Dectomax, Ivomec Eprinex, Ivomec and some generic equivalents. Many of these come in pour-on or injectable form. The oral dewormers (white paste) are shorter acting because they mainly kill the adult worms in the gut. These products include Panacur/Safeguard, Valbazon, Benzelmin, etc.

“Not only does a producer need to figure out when to worm, but also what to use,” explains Scharko. The dewormers don't all work the same way and some may be more effective than others, especially for different worms, and in different situations.

The best time of year to worm can also vary a bit, depending on your climate and seasons. “Here in Kentucky the best time is usually late June/early July. Many producers have a problem with this however, because this isn't a time of year they are working their cattle. In that case, I recommend doing it sometime in July—like when they round up the cows to take the bull out. A few small studies show that early summer is a good time to deworm, in areas that get dry in summer. The parasites don't survive as well in the pasture when it's dry, and since 90 percent of the worms are in the pasture rather than in the cattle, this is a good time to take advantage of that,” she says.

Best control of worms is achieved by trying to work with the seasonal cycle. At a recent meeting she attended, two of the three pharmaceutical veterinarians recommended early summer deworming, and then deworming again in the fall—using a product in the fall that will not only get the worms but also give lice/grub control.

Regarding use of pour-ons versus injectable products, Scharko states that whenever they've done trials on worms they've had to use injectables, but she actually prefers the pour ons, for several reasons. “One is beef quality assurance—no needles. Another reason is that in summer the pour-on also gives a little bit of fly control as well as worm control, and gives lice control in winter,” she says.

Your choice may also be partly determined by weather. If it's going to rain or the animals are already wet, you should use an injectable, or choose a pour-on that is not affected by a wet hair coat. Some are only effective when the animal is dry (and must be dry for several hours afterward). “We did a trial here in Kentucky a few years ago, looking to see if there was any worm resistance to ivermectin, and I didn't check the weather and two hours after we worked the cattle it poured rain. You need at least four hours after treatment with the animal dry,” she explains.

The stockman needs to be aware that some products will not be very effective if the animal is wet or soon will be wet, and also needs to realize there are withdrawal times (before slaughter) for dewormers. Some have no withdrawal period while others have a lengthy time factor. “The generic ivermectin, for instance, which many stockmen use because it's cheaper, has a 48 day meat withdrawal. It is also not rain-fast. The little bit of research on this product showed that it also doesn't last as long in effectiveness. But it's still economical—even if you have to do it twice. At 1/3 the price of the other products it pencils out.” It is important, however, to read labels and know how best to use the various products. The Food and Drug Administration is monitoring drug residues in meat, so dewormer withdrawal times should be heeded.

Since calves are most adversely affected by worms, it often pays to do fecal egg counts—to know what kind of parasite load the calves might have, and whether they need dewormed or not. “Egg counts may vary with the time of year, and the animal, but often if it's 500 eggs per gram or more, the calves need to be dewormed,” she says. Work with your local veterinarian to determine the extent of parasite load, and then create an appropriate deworming program—regarding time of year for most effectiveness and selection of products.

“We did a small trial here in Kentucky on weaned calves and found there are some parasites that did not respond to Ivomec but did respond to the white dewormers such as Safeguard. So there are some resistance issues emerging. There are some serious resistance problems regarding dewormers in goats and people thought it would never happen in cattle, but it may be starting to occur. The brown stomach worm is the major offender, causing the most problems in cattle, and so far, at least, it does not appear to be resistant yet, at least in the two herds we tested here in Kentucky.”


Worms have developed amazing ways to survive and infect the host. The hatched larvae need moisture in order to move from manure onto adjacent forage plants (to be eaten), and can become dormant during dry seasons, waiting until environmental conditions improve. Some species go into an arrested, dormant state within the host (encysted in the wall of the gut) during winter or dry seasons, waiting to emerge and lay eggs when environmental conditions for pasture infection improve.      

Pasture management is as important as deworming, to minimize conditions in which worms can be readily spread. Overgrazing tends to increase worm load because the larvae are generally on the lowest part of the plant (the first four inches), and overgrazed pastures are eaten closer to the ground. Even well managed intensive grazing systems (rotational grazing or mob grazing) tend to increase worm infestation because there are more animals per acre (covering more area with manure), animals are forced to graze plants adjacent to manure, and may also graze the grasses closer, where the most worm larvae are located. Cattle in rotational grazing systems may need diligent deworming because they have more exposure to these parasites, explains Scharko.

Another way to deal with this problem is to break the life cycle by alternating species, such as sheep or goats. The worms are host specific and can't complete their life cycle in a different species. “Some people also use pastured poultry, to break up the manure pats. The larvae can't survive as well,” she says. If you are doing rotational grazing, she recommends taking fecal egg counts to monitor the actual parasite load—to know whether or not you need to deworm those cattle more often.

Irrigated and humid climate pastures provide ideal opportunities for larval migration onto plants; infestation is generally less in dry regions on dryland pastures. Hay removal during summer opens up the plants to light and heat, which can reduce larval numbers. Harrowing of pastures should always be done in hot, dry weather when the manure pats can be exposed to heat and drying. There should be no animals present in the pasture when it's harrowed (spreading manure over the clean grasses) to pick up worm larvae before they dry out and die.


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