December 2017
Farm & Field

Principles of Parasite Prevention

Fall deworming is even more important after a wet summer.

  Self-fed Safe-Guard 20% Protein Dewormer Blocks offer a deworming option that doesn’t involve running cattle through chutes. This could be an ideal method for heavily pregnant cows or producers without access to chutes or working facilities.

Internal parasites create an economic burden on cattle producers. Internal parasites in cattle reduce feed intake, reduce average daily gain and alter the animal’s immune system. Expensive nutrients fed to sustain cattle are diverted to sustain the parasitic organisms instead.

Parasitism is classified as either clinical or subclinical. Clinical parasitism means the body is overwhelmed to the point that disease symptoms are present (anemia, edema, diarrhea, rough coat, etc.). These symptoms are easier to see and treat. However, if one waits until clinical symptoms appear, the damage has already been done and the animal has been inefficient for quite some time. Subclinical parasite infections do not show outward disease symptoms, but the animals are less productive (lowered milk production, reduced weight gain, altered carcass composition, reduced conception rates, etc.). The time to deworm is in the subclinical stage before major damage has been done and money has been lost due to poor productivity.

In general, younger animals and animals under stress are more likely to be negatively affected by parasites. Mature cattle acquire some degree of immunity. However, mature cows near calving time are very susceptible to parasites as their own immunity is suppressed due to the pregnancy. Also, bulls are more susceptible to internal parasites than cows.


The Life cycle

Figure 1. Basic Life Cycle of Common Gastrointestinal Nematodes of Cattle.  

In order to control internal parasites, one must understand their lifecycle and how they are transmitted to cattle. Luckily, most of the economically important internal parasites (roundworms, stomach worms, barber pole worms, etc.) have similar life cycles (Figure 1). The adult nematodes mate and produce eggs within the host. These eggs pass out of the gut in the feces. The eggs hatch, and the larvae go through several stages before they reach the infective stage. This infective stage migrates from the manure pat onto moist grass (Figure 2) where it is consumed by the host. The larvae mature into adults, completing the life cycle. It is important to note that these larvae need moisture (from rain or dew) and soil temperatures of 55-85 degrees to swim up the blades of grass. As the grass dries, the larvae move back down into the soil. Cattle do not pick up larvae from dry pastures. Thus, the warm, wet conditions we’ve experienced this summer have been ideal for internal parasite transmission.




Control of internal parasites is a never-ending battle entailing a combination of pasture management and strategic use of anthelmintics. Control should be aimed at reducing exposure to infective larvae and disrupting the life cycle. Control practices include the following:

  Figure 2. Infective larvae found in a drop of dew.
  • Move susceptible cattle to clean pastures. Clean pastures are those that haven’t been grazed by other cattle for at least 12 months as larvae can survive for up to a year in pastures. Rotating pastures with other species (horses, sheep or goats) also acts to clean pastures as these parasites are species specific. When the larvae are ingested by anything other than cattle, they will die harmlessly in the gut.

  • Do not overstock pastures. Overstocking pastures forces cattle to graze closer to the ground and thus pick up more larvae.

  • If forced to use pastures that aren’t clean, refrain from turning cattle out on new grass until the dew dries.

  • Strategic use of dewormers, especially before calving and/or moving to new pastures.

  • Periodically have fecal egg counts conducted to assess if your current deworming program is effective.

There are many anthelmintic products on the market to choose from. When choosing a dewormer, consider the following:

  • Production status of animals to be treated (cow vs. calf, beef vs. dairy)

  • Product efficacy for the desired parasites

  • Ease of use

  • Slaughter/milk withdrawals

  • Cost

With all of this in mind, consider use of the Safe-Guard En-Pro-Al Molasses Dewormer Blocks or the Safe-Guard 20% Protein Dewormer Blocks. Both utilize the active drug ingredient fenbendazole, labeled for the removal and control of lungworm, stomach worms, barber pole worms, brown stomach worms, small stomach worms, intestinal worms, hookworms, thread-necked intestinal worms, small intestinal worms, bankrupt worms and nodular worms. These are the only self-fed dewormer blocks on the market.

Self-fed delivery of dewormer has many advantages over other delivery methods. First of all, drenches, injections and pour-ons require cattle be worked in a chute. Not every producer can afford to purchase expensive catch gates and chutes. Also, there are inherent stresses placed on both cattle and humans when cattle are worked, especially when cows are in late pregnancy and/or temperatures and humidity are high. Lastly, there is the safety issue of working cattle in close quarters, especially when utilizing inexperienced help.


To learn more about these two self-fed dewormer options, visit to view the product label or ask for them by name at your local Quality Co-op.

In summary, the warm, wet summer we have just experienced has provided ideal conditions for the transmission of internal parasites. Control of these internal parasites will depend upon a combination of pasture management and strategic use of dewormer products. No matter which anthelmintic product you choose, deworming cattle this fall will be critically important to maintain productivity over the winter. Consult with your local veterinarian to determine the anthelmintic program that is best for your situation.


Safe-Guard is a registered trademark of Merck Animal Health.

En-Pro-Al is a registered trademark of Ridley Block Operations.


Jackie Nix is an animal nutritionist with Ridley Block Operations ( You can contact her at This email address is being protected from spambots. You need JavaScript enabled to view it. or 1-800-325-1486 for questions or to learn more about SWEETLIX mineral and protein supplements for cattle, goats, horses, sheep and wildlife. References available upon request.