Introduction

Gastrointestinal parasites, also known as helminths, nematodes or stongyles, deprive cattle of nutrients and can reduce milk production and daily weight gain. Effective parasite management includes pasture management and strategically deworming cattle at specific times of the year to minimize parasite populations in both cattle and on the pasture. Deworming is beneficial for beef and dairy operations, provided it is done correctly with effective products. However, due to anthelmintic resistance, relying solely on dewormers is insufficient.

Which cattle should be dewormed?

Infective helminth larvae infest growing forages, exposing cattle during grazing. Young and high-producing cattle grazing pasture benefit the most from controlling their parasitism. Cattle develop resistance to gastrointestinal parasites as they age, and with good nutrition, it is possible to overcome parasite levels. Most third lactation and older cattle do not need deworming, while first and second lactation animals may benefit from pre-calving or post-calving deworming. Monitor their body condition, health, and production, and use fecal egg counts (FEC) to determine when deworming is warranted.

Cattle housed in dry lots typically remain parasite-free. Test purchased cattle or those with unknown histories using FEC to measure parasite levels. If needed, deworm them before placing them in a dry lot and use FEC-reduction tests (FECR) to monitor anthelmintic effectiveness and determine if further deworming is required based on body condition scores (BCS) and production measures.

Which cattle Should NOT be dewormed?

To maintain a population of parasites that are susceptible to anthelmintics, ensure that some animals in your operation remain untreated. Untreated animals, known as refugia, host the production of anthelmintic-susceptible eggs on pasture. These eggs eventually develop into anthelmintic-susceptible adult parasites. Mating with other susceptible parasites within the animals results in offspring that can be effectively controlled by anthelmintics.

Parasites not killed by anthelmintics likely have resistant genetics to the product used, which they pass to their offspring. Mating anthelmintic-resistant with susceptible parasites provided by refugia produces a mixed population of offspring. This mixture helps maintain some efficacy of the deworming agent and slows the progression of complete resistance to the product in use.

Maintain refugia in third lactation and older beef and dairy animals who are most able to handle a parasite population. Use FEC to measure parasitism and determine the need for deworming as dairy calves leave the hutch or during their first co-mingling event. For those raised on dry lots, follow the earlier-stated deworming recommendations.

Do not deworm pastured, suckling beef calves that weigh less than 200 lbs. since they receive most nutrients from their mothers, not the pasture. Wait to deworm heavier beef calves before weaning to reduce their parasite burden during the stress of weaning. Depending on how much nutrition they received from sucking, calves at weaning may have low helminth populations. However, the more weaned calves graze, the more exposure to parasites they will have. Wean them on dry lots or pasture that has not recently been grazed or has regrown following hay making. Base your deworming decision on FEC, the calves’ BCS, and rate of gain.

As FEC results warrant, reduce internal parasite levels from the previous grazing season by strategically deworming in the fall and before spring pasture turnout. Ensure that the next grazing animals following recently dewormed animals are older, more resistant animals.

Pastured stockers/backgrounders or post-weaned youngstock raised to pre-finish or breeding weight have the highest risk of disease and production losses due to parasite burdens. Right before grazing begins, measure their FECs to determine parasite burdens and use FECR to know what survives following treatment. Deworm calves before their first grazing event, then four weeks after, and again four weeks after that. Known as the “0-4-8” thumb rule, this strategic deworming will reduce pasture contamination throughout the grazing season. Alternatively, with managed grazing, use BCS and FECs to guide deworming decisions; it is less labor intense and economical to pick up feces and test, then to work cattle for blanket treatment that may not be needed.

Use FEC to guide decisions

Strategically manage parasitism with FEC results and the animals’ BCS and production measures along with pasture condition. Measure FECs of newly purchased individuals or groups of animals to understand their parasite levels and the need for deworming. FECs only count the number of eggs; this test does not identify them. You need to request different testing for species identification.

Keep seasonality and the parasite’s life cycle in mind as you interpret FEC results:

• Fall FECs indicate worm burdens from the previous grazing period and this information helps with deworming decisions to reduce internal overwintering parasite stages. Low FEC indicates exposure, which is good because cattle will develop immunity to internal parasites. Continue to feed a balanced ration over winter while monitoring BCS and rate of gain. High FEC levels indicate a need for deworming, especially in young cattle, and to improve feed efficiency over winter. Deworming based on high FEC levels also reduces the source of eggs on next spring’s pastures.
• FECs measured in the winter indicate worm burdens from the previous grazing period and if previous deworming was successful. Winter counts may be falsely low because of inhibited larvae and adults that are not shedding eggs. High counts decrease feed efficiency and add stress on the cow if winter calving occurs.
• Spring FECs indicate worm burdens from the previous grazing period and if previous deworming was successful. Time testing with the spring rise in temperatures and before grass turnout. The lower the FEC at the beginning of spring, the lower the pasture contamination will be during the following summer and fall season if pastures are managed.
• Measured after spring turnout, FECs may indicate levels of both overwintered worms inside the animal and new infestation from contaminated pasture as the dormant stages on pasture resume their lifecycle and infectivity with moisture and warmer temperatures.
• Measured in the summer, low or negative FECs indicate successful treatment or successful managed grazing. FEC results can include misleading high counts if pasture conditions have deteriorated, causing low fecal output which can inflate FECs.

Testing Cattle

Stuttgen, Mills-Lloyd. Deworming: Relationships, Resistance, Refugia. 2015

(Figure 2) Fecal egg count and fecal egg count reduction tests. From Parasite Patrol: Mastering Fecal egg Counts for Healthier Herds and Flocks.

Use influences efficacy

Read the anthelmintic label for proper storage, dosing and withdrawal time. Understand the label’s effectiveness for the life cycle stage you are treating. Weigh animals and correctly administer the correct dose. Both overdosing and underdosing lead to the development of anthelmintic-resistant parasites.

Injectable and orally drenched products that are correctly administered are more likely to provide an effective dose compared to feed through or pour-on products. If you use a pour-on, then administer it correctly to all cattle in that group. Cattle will groom their neighbor and lick off the product; therefore, untreated cattle cannot remain with the treated ones; the whole group becomes underdosed. If you choose to use a feed-through product, make sure there is adequate bunk space, so all cattle get their dose. Use FECR to determine the product’s and/or your administration’s effectiveness.