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2u1 internal parasites
The control of internal parasites in a cost-effective manner isan important issue in feedlot production. Scientific evidenceindicates that it can be cost-effective to control internalparasites in Canadian feedlots. However, there are severalfactors involved in determining the most appropriate methodof internal parasite control. The following article discusses thepertinent information with respect to internal parasite controlin feedlot cattle.
The economic importance of internal parasites, such asgastrointestinal worms and lungworms, in feedlot cattle hasbeen a controversial topic. It is known that internal parasitismcan decrease an animal’s productivity. However, the decisionwhether/which anthelminthics (dewormers) to use in feedlotcattle requires that the economic value of productionimprovements is greater than the cost of treatment.
Virtually all feedlot cattle have some level of internalparasitism ( The primary parasites involved aregastrointestinal worms, such as Ostertagia ostertagi
(brownstomach worm), Cooperia spp.
(small intestinal worm or cattlebankrupt worm), Trichostrongylus
(hair worms), andNematodirus helvetianus
(thread-necked or thin-necked worm).
In certain areas, lungworms (Dictyocaulus viviparus)
may alsobe present. Levels of parasitism are greatest in youngeranimals and in animals coming off pasture ( Younger cattleare most susceptible to parasites, because they lack theprevious exposure and immune system development to keepparasite burdens under control as compared to older cattle.
Cattle coming off pasture tend to have higher parasite burdensbecause worms are transmitted and spread on pasture. Theadult worms live and reproduce in the animal’s gastrointestinaltract. They lay eggs that are passed out in the manure. These
eggs hatch on the pasture and develop into infective stagelarvae. These larvae migrate up blades of grass and areconsumed by the cattle as they graze. The larvae continuetheir maturation in the gut, thus completing the life-cycle.
Historically, gastrointestinal worms have not been considereda significant production problem in Canada, because harshwinter conditions keep parasite populations from reaching thehigher levels seen in warmer climates. Additionally, cattle withclinical signs of parasitism (diarrhea, weight loss, rough haircoat, etc.) are rarely observed in Canada; therefore, it wasassumed that internal parasites were not causing muchdamage. It is now known that subclinical (no obvious signs ofinfection) parasitism can cause great losses in terms ofdecreased productivity. In feedlot animals, subclinicalparasitism can cause inferior rates of gain and feed conversion.
It is assumed that losses occur as a result of a number of factors,including the diversion of nutrients to parasite growth andreproduction, interference with nutrient absorption byreducing available surface area and direct damage to the gutlining, and interference with digestion by damaging enzyme-producing cells necessary for the breakdown of feed. The trueextent of subclinical parasitism is unknown and it is difficult toestimate because the effects are not visible. However, the)demonstrate the potential for economic losses. With this studyas a guideline and knowing that approximately 2.6 millioncattle were fed in Canada in 1995, subclinical parasitism couldbe costing the feedlot industry upwards of 17 million dollars
A review of the pertinent literature reveals diverse opinionson the cost-benefit of deworming feedlot cattle ( Early,small scale studies conducted in research facilities at varioustimes of the year and utilizing cattle of mixed ages wereinconclusive regarding the benefits of deworming feedlotanimals. Research data obtained from large scale commercialfield trials in western Canada demonstrate that dewormingfall placed calves on arrival at the feedlot results in significantimprovements in average daily gain (ADG) and feedconversion (by 6.5% over cattle treated for external parasites only. The feedto gain ratio (F:G) was improved by 3.0%. These improvementsin rate of gain and feed conversion resulted in a net economicbenefit of approximately $7.00 per animal in one study (
should be noted that these data show a cost-benefit to usingthe specific deworming programs tested in each study(ivermectin and oxfendazole). It may not be appropriate toextrapolate these results to other deworming strategies.
In yearling cattle, parasite levels are generally not as high as incalves because older cattle have started to develop immunityto parasites. As a result, conventional wisdom held thatdeworming yearling cattle was not a cost-effectivemanagement decision. However, fall placed yearling feedlotcattle (usually coming off pasture) have higher parasite levelsthan yearling cattle placed at other times of the year.
Moreover, recent research data obtained from a large scalecommercial field trial in western Canada demonstrated thatdeworming fall placed yearling cattle on arrival at the feedlotresulted in significant improvements in rate of gain and feedconversion. In this study, cattle treated with ivermectin had a3.5% improvement in ADG over controls treated with anexternal parasite control product only. In the same study,treated cattle also had an improvement in F:G of 2.7%. Thus,the use of anthelminthics in fall-placed yearling feedlot cattlecan be a cost-effective management decision.
In animals arriving at the feedlot at times of the year otherthan fall, a strategic deworming approach may be appropriate.
This strategy involves assessment of each group of cattlearriving at the feedlot. Factors to consider during theassessment should include: physical appearance, geographicalorigin, age, body weight, length of feeding period, anddeworming history. In general, younger, lighter cattle fromareas with high rainfall and densely stocked pastures are morelikely to be heavily parasitized and more likely to benefit fromdeworming.
Research data from western Canada indicates that dewormingfall placed cattle (calves and yearlings) is a cost-effectivemanagement strategy. In animals arriving at the feedlot attimes of the year other than fall, a strategic dewormingprogram should be used. The most appropriate product to use) depends on a number of factors and should bediscussed with a feedlot veterinarian.
GI = Most gastrointestinal wormsL = LungwormsIL4 = Inhibited L4 larvae of Ostertagia
Ex = External parasites* = subcutaneous injection** =withdrawal times vary depending on formulation
1. Williams, J.C. 1983. Ecology and control of gastrointestinal nematodes of beef
cattle. Vet Clin N Amer: Lg An Prac 5:183-205.
2. Schultz, R.H. 1994. Thoughts on current anthelmintic needs and usage. Bovine
3. Gibbs, H,C. 1992. The effects of subclinical disease of bovine gastrointestinal
nematodiasis. Compendium 14: 669-677.
4. Bauck, S.W., Jim, G.K., Guichon, P.T., Newcomb, K.M., Cox, J.L., Barrick,
R.A. 1989. Comparative cost-effectiveness of ivermectin versus topicalorganophosphate in feedlot calves. Can Vet J 30: 161-164.
5. Jim, G.K., Booker, C.W., Guichon, P.T. 1992. Comparison of a combination of
oxfendazole and fenthion versus ivermectin in feedlot calves. Can Vet J 33:599-604.
6. Reinmeyer, C.R. 1994. Parasitisms of dairy and beef cattle in the United
7. Myers, G.H., Grant, R.J. Effects of fenbendazole and ivermectin on performance
of feedlot cattle. Agri-Practice 9: 39-42.
8. Stockdale, P.H.G., Harries, W.N. 1979. Treatment of feedlot cattle in Alberta for
gastrointestinal nematodes. Can Vet J 20:223-226.
9. Bennett, K. 1995. Compendium of Veterinary Products. Fourth Edition.
Hensall, Ontario: North American Compendiums Ltd.
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