Rethinking strategic deworming in beef cattle

Vet Advice with Dr. Ron Clarke

Concepts around deworming the beef cow have moved from “not necessary” to “routine” in many progressive cow-calf operations. The evolution of internal parasite control as an integral component of health management shadowed development of highly effective chemical families that revolutionized parasite control. Reasons for the shift include:

  • Economic studies that clearly demonstrated deworming pays.
  • Evidence that current approaches to deworming were unsustainable due to development of parasite resistance. Resistance was an inevitable consequence of shoddy deworming practices.
  • Realization that haphazard, unstructured deworming programs grow increasingly unacceptable to consumers and society.
  • The link between parasitism and the need to actively manage nutrition programs.
  • Critical evaluation of all factors related to the play between reproduction and parasitism in beef herds.
  • A fuller understanding of all factors affecting immunity and health.

Strategically dewormed cattle have shown to produce more milk, show improved feed efficiency, improved reproductive performance, maintain better body condition scores, and possess stronger immune systems. Waiting until animals appear parasitized means parasite damage has already occurred.

Internal parasites exist as complex communities in the gut. Modern technology has provided better diagnostic approaches to assessing parasite load and an improved understanding of parasite response to therapy. The concept of Nemabiome testing has helped unravel the mystery of what species of parasite are involved in these communities and quantify the proportion of each species. A wide variation in management systems means cattle producers should work with a veterinarian to tailor individual parasite control programs, plus develop ways of monitoring the progress of parasite control within herds.

The number of approved products presently available has made the process of deworming easier. Although there is a range of limitations for pour-ons, injectables, drenches, and feed additives the different formulations have greatly expanded options for inclusion in strategic deworming programs.

Many strategic deworming concepts have been developed for calves and replacement heifers, but have fallen short for the cow herd, the real economic engine of commercial cattle operations where body condition, immunity, colostrum quality, milk production and reproductive capacity can be negatively influenced by a heavy parasite load.

The brown stomach worm (Ostertagia ostertagi) is the primary nematode causing production losses in mature cows. Eggs are passed in manure and hatch on pasture. Cattle ingest Ostertagia larva while grazing. Larva penetrate gastric glands lining the abomasum or fourth stomach where they complete their lifecycle and emerge as adult worms. Larva can live in the glands for months and their presence while developing into adults damages the abomasa lining, temporarily disrupting acid production, causing hemorrhage and upsetting normal digestion.

Recent studies have shown that invading gastrointestinal parasites can redirect the host’s immune system. Ostertagia, in particular, has been shown to alter the immune response to vaccines.

A second form of Ostertagiasis, Type II Ostertagiasis, occurs when worms become less active and encyst in the lining of the abomasum during cold or dry seasons, to re-emerge later damaging the gut lining and causing diarrhea, inappetence and unthriftiness.

Clinical disease caused by Ostertagia is often subclinical. For example, appetite suppression and failure to maintain condition through the last trimester of pregnancy often goes unnoticed by producers. Although subclinical effects are real and significant, they are difficult to quantify.

Low-quality forages amplify the effects of parasitism. Because parasite load is not uniformly spread across a cow herd, individual animals can often suffer exaggerated weight loss compared to a herdmate. It’s been shown that 20 per cent of cows within a herd may harbour 80 per cent of the parasites. There’s an old idiom that you can feed a cow out of a worm problem, but can’t worm a cow out of a feed problem. Nutrition remains a fundamental precept for sustaining brood cows under any condition.

Internal parasites cost the North American beef industry in excess of US$2 billion annually. In a major study conducted by John D. Lawrence and Maro Ibarburu, Iowa State University looking at the economic analysis of pharmaceutical technologies in modern beef production the authors evaluated the impact on costs and resulting beef supplies and prices if existing pharmaceutical technologies were no longer used in beef production. They examined five key technologies: growth promotants, ionophores, antimicrobials, dewormers and fly control. Their model predicted non-users of the five technologies would see more than a US$582/head increase in cost of production over the lifetime of an animal or $151/cwt. Of that amount over $200 or 34.6 per cent was attributable to use of dewormers.

The goal is to prevent economic loss and reduce parasite contamination by eliminating parasite egg shedding for a period of time at least equal to the life cycle of the parasite removed.

Timing of deworming needs to be considered in relation to season of the year, type of grazing program, and management practices. If the dewormer fails to eliminate egg shedding following treatment, the accrued benefits of parasite control are greatly reduced.

There are two important considerations: make sure cattle are parasite free during the winter and at the beginning of the grazing season. An important step in strategic deworming occurs after spring grazing begins. Spring deworming should be given after cows have had a chance to graze, but before invading larva have had a chance to mature into adult parasites (approximately six weeks after the first larva are ingested). Even with continued grazing and reinfection, another six weeks will be required for the second round of fecal egg shedding by adult worms. This strategic spring treatment in adult cows prevents egg shedding for 12 weeks or three months into the grazing season. Effective deworming in late November or early December then again six weeks after grazing commences prevents parasite contamination of the environment for approximately six months.

About the author


Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).


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