Parasite control should be important to all livestock producers. Controlling parasites can aid in production of a superior product, increase herd longevity and improve animal performance, all of which will provide economic benefit to your operation.While there are many methods to control parasites in cattle, this article will focus on utilizing grazing strategies to combat internal parasite levels.

In order to understand how following a grazing plan can aid in parasite control, we must first comprehend parasites and their life cycles. The region and climate will influence the species of parasites we are working to limit; however, we will concentrate on roundworms and liver flukes since they are common throughout the U.S.

Roundworms begin their life cycle as eggs that are passed on through the manure of infected cattle. The eggs spend roughly five days in the feces before they molt into the first larval stage. First-stage and second-stage larvae remain in the manure and then transform in the third larval stage and migrate from the manure onto the moist base of pasture grasses.

This larval phase remains on the pasture grass for approximately three weeks, where it is freely ingested by grazing cattle. Larvae in stages four and five are now inside the gastrointestinal tract, where they begin to mature into adulthood and lay eggs in the digestive tract, causing an overall lack of thriftiness in the animal. The roundworm life cycle is completed in approximately 21 days.

Roundworms can be economically devastating and manifest themselves as subclinical symptoms including: weight loss, reduced milk production, reduced conception rates and altered carcass composition. When subclinical symptoms are present is the time to utilize internal parasite control methods, specifically for roundworms.

Routine observation of animals will aid in the determination if any subclinical symptoms are present, or if any ongoing health issues are present.

The clinical symptoms of roundworms become evident as rough coat, anemia and diarrhea. If these symptoms are visible, a veterinary consultation is recommended to formulate a treatment plan. Clinical level symptoms are beyond the reach of over-the-counter solutions and will require veterinary treatment.

Flukes are the result of a symbiotic relationship between cattle and snails. They are most common in wet areas with alkaline soils. Flukes begin life as an egg passed through an infected animal’s feces onto the wet soil.

Snails ingest the eggs, then several maturation steps later, the larva have aged and multiplied. They are then transported from the snail to wet pasture grasses via insects to form cysts, which contain the next life stage, onto grass blades which are in turn ingested by cattle.

This life stage is called metacercariae. Once consumed, the metacercariae develop into egg-laying adults, beginning the life cycle anew. The fluke life cycle takes 21 days during prime reproductive conditions.

Roundworms and flukes are common internal parasites and prefer to grow in wet and humid microclimates, such as those within the first 3 to 4 inches of pasture grasses. Producers who utilize irrigated pastures or ranch in a humid climate are most susceptible to internal parasite problems.

Those that follow a sound preventative herd health program and provide excellent nutrition will increase the effectiveness of the parasite control methods used.

Grazing as control method

Another method producers should consider to reduce parasite loads and subsequent loss of performance is the use of grazing concepts. There are several grazing concepts that can fit into a multi-level parasite control plan.

A theory to consider is the graze-rest-graze idea, which can also be referred to as grazing on a “clean pasture.” Animals are completely removed from the pasture, and no grazing takes place to ensure a low level of parasite population.

The key to this concept is to know what parasite you are dealing with and its life cycle. The rest interval between grazing periods should take into consideration the total reproductive cycle of the parasite in question and be at least doubled.

This is due to the varied life cycle stage parasites may be at; resting the pasture for two life cycle periods severely decreases the parasite levels present in the pasture. For example, if roundworms are an issue, you would rest the pasture for a minimum of 42 days.

Another idea is to only graze a pasture once per year. This allows for the parasite life cycle to be completed and any eggs or larva to be left in the pasture, not able to complete their life cycle, potentially eliminating all parasite species present in the pasture.

This method works best with a range-type grazing system. If using this method, care should be taken to ensure each rotation into a new pasture has allowed for an adequate rest period.

Grazing a pasture two times annually is an alternative idea to consider when formulating a grazing plan to aid in parasite control. Rotating cattle through a pasture every six months ensures a low level of parasites present in the pasture. Again, care should be taken to guarantee an appropriate rest period has been assigned.

Overgrazing limits

An additional parasite control method through grazing is to avoid overcrowding and overgrazing of pastures. Too many animals in a designated pasture or paddock without appropriate forage levels for feeding and grazing management causes these issues and exponentially increases parasite levels in the pasture.

Figure 1 for an example formula to calculate the numbers of animals per paddock.

When you overcrowd animals into a pasture or paddock, you are increasing the likelihood of spreading parasite infection as cattle compete for shorter and shorter pasture grasses with higher and higher parasite levels present. The shorter the grass, the closer the animal is grazing to the preferred microclimate of parasites.

Evaluating the amount of available forage is important when endeavoring to avoid overgrazing. Walking your pasture with a “pasture” or “grazing” stick is a simple technique that can be used to complete the accompanying formula present on the stick to estimate the amount of forage available for use.

The simplest way to monitor grazing is to always leave a minimum residual of 4 inches. Easy access of the first 3 to 4 inches of pasture grass increases the chance of internal parasite infection in all livestock. Multiple pastures or paddocks allow for a continuous grazing season while permitting some to be at rest, some to increase available forage levels and some to be openly grazed.

Choosing your methods

Knowing which parasite you are trying to control and its life cycle are crucial when utilizing managed grazing as a method of parasite control. This knowledge will influence the timing of your rotations from rested pastures to those open for grazing.

Working with your veterinarian to conduct fecal parasite monitoring can save a lot of time and money when it comes to choosing a parasite control method. These tests are taken randomly from a percentage of animals in the herd. Your veterinarian will then be able to asses which internal parasite species are present and the level of infection.

You can then work toward finding a parasite control method, or combination of methods, to reduce overall internal parasite levels. Continued monitoring after treatment and other control methods, such as a chemical worming program, can aid in evaluating the effectiveness of your parasite control methods.

Life cycles and parasite egg viability are highly dependent on parasite species, its acclimation to the local climate and the overall weather patterns. Consulting with your veterinarian to incorporate several methods for parasite control, including management-intensive grazing practices, into your herd health program can be extremely beneficial.  

For more information on where to obtain a “grazing” or “pasture” stick, contact your local NRCS office, the American Forage and Grassland Council to find your state affiliate.

To learn more about workshops involving intensive-grazing management principles, visit Owyhee County Extension Office and check out the Lost Rivers Grazing Academy link.

• Samantha Graf

• Extension Educator
• University of Idaho
• Email Samantha Graf