Just after the new year, a rancher who runs a few hundred late-spring cows called me. He was having difficulty getting his weaned calves on background feed and they were just not doing “good.” Although, he dewormed these calves once off-pasture with a commercial pour-on, I told him they might still have stomach worms. So he agreed to treat them again — this time using a drench dewormer containing a different active deworming medication.
I saw the same calves in middle of February and they were eating with vigour and growing — or as I like to say, “Like weeds.” This little story is a good reminder that cattle are very susceptible to stomach worms and there should always be a good program in place to control worms at all times.
In hindsight, whether these cattle worms had resistance against the first dewormer is unknown. It’s only a snapshot of the round stomach worm’s detrimental effects upon cattle performance, such as reduced feed intake, poor feed efficiency and inferior weight gains. It has also been proven that even a modest worm infestation in young stock can compromise their immune system. Mature beef cows are also affected, but have some acquired immunity.
Not always obvious
Except for cases of severe diarrhea and anemia showing up in some animals, many producers may not realize their cow herd is infected with worms. I believe it’s because the hidden nature of a worm problem has much to do with the opportunistic subtlety of the stomach worm’s (Ostertagia ostertagi) life cycle:
- Infected cattle pass microscopic eggs in manure onto the ground.
- Eggs hatch and these first- and second-stage larvae live in the manure patty.
- The infective third-stage larva develops in about a week (infective for months afterwards).
- Third-stage larva migrate to vegetative during moist and moderate weather conditions.
- Cattle ingest third-stage larva on grass, where it migrates to the abomasum mucosa.
- Matures into fourth-stage larva and to final egg-laying adults.
- Cycle repeats itself.
The entire cycle of the stomach worm encompasses a period of about three weeks. It can also spend part of this cycle in hibernation in yearlings and mature cattle. These are fourth-stage larvae that arrest development before becoming adults. This is thought to be a survival technique in which worms evolved to avoid the adverse cold of winter as well as hot and dry conditions in the summer.
Interestingly, the first- and second-stage larvae can also survive outside the cow during tough Canadian winters on pastures. They simply become active once rising temperatures and available moisture make it optimal for their continued development.
The contamination of pastures by stomach worms parallels such activity during the grazing season. It often leads to an initial increase of worms during the first couple of spring months, followed by a dip in midsummer and literally ending in a population explosion as many say, “when the kids go back to school” (early fall).
Meanwhile, new calves are starting to pick up contaminated grass, acquiring worms and depositing them back as shed eggs, which therefore contributes to the majority of late-pasture contamination.
Effective chemical control of worm eggs on pasture (and in drylot) has been around since the early 1980s with the development of a new class of dewormers, known as the anthemintics. This class can be divided into many chemical groups, but the avermectins (ivermectin and dormectin) and benzimidazoles (fenbendazole) are some of the leading dewormers against stomach worms in cattle.
Avermectin dewormers are not only effective against the latter larvae/adult stages of the stomach worm living in cattle, but can eliminate worms hibernating in their abomasum as well as has weeks of residual power. Producers also like the avermectin such as ivermectin, because it controls other external as well as other internal parasites. In comparison, fenbendazole has effective control against stomach worms, but is not effective against other parasites. It also has little lasting control.
To me, this does not mean one dewormer is better than the other. It only gives the producer a choice tailored to the situation. For example, the reasons I recommended that my friend (above) use a fenbendazole drench on his weaned calves are three-fold: (1) we targeted only stomach worms, (2) residual activity was not an issue; these animals were forage/grain-fed calves and (3) there was suspected worm resistance against the ivermectin. Plus I wanted to assure the animals were getting the recommended dose of dewormer and drenching was my choice.
Like me, when producers make their own personal dewormer choice, they often find the accompanied advice often varies as to when and how many times to treat cattle and also how to prevent worm buildup on pasture. Tradition dictates to treat cattle when the highest number of worms is still developing in overwintered beef cows and not yet shed on the ground. This means to me that cow-calf operators should deworm cows just prior to their release on pasture.
In contrast, a new protocol says it is not necessary to treat grazing cattle right away and they can wait up to six weeks on pasture before being dewormed. The idea being: deworm calves after the first few weeks on pasture and then process the mature cows a couple of weeks, afterward.
These worm specialists believe this allows an effective dewormer to clean up the overwintered cattle as well as stop egg shedding for the next four to six weeks. The benefit being that there is not a buildup of worms on pasture at the end of the grazing season.
Regardless of which of these two schools of thought one follows, there are some timeless management plans that should be considered in order to prevent such heavy worm loads. For example, since rotational pastures often have higher worm loads due to higher stocking rates; it is important not to speed animals throughout all available pastures as well as not to overgraze. In addition, some rotational pastures known to carry high worm loads might be rested for a year to clean/reduce worm infestation for the following season.
What does it cost?
Nobody will disagree that a high worm load will eventually cause a potential loss of revenue. However, many producers ask two common questions:
- How much is the cost of deworming treatment, and
- Does it pay to deworm cattle?
Let’s illustrate a general situation – one U.S. gallon of Safeguard (fenbendazole) @ $400 (CDN) is used at the rate of 2.3 ml/100 lbs. bodyweight. It is recommended that cows be treated prior to being released on pasture and cow-calf pairs be treated after six weeks. As a result some Canadian research shows an 18 lb. weaning weight advantage due to deworming. Therefore, my input cost (cow + calf) = $5.84/calf and estimated revenue = 18 lb. x $2/lb. = $36/calf; net return = $30.16/treated calf. For a 350 cow-calf operation that is an extra $10,000 in revenue.
This is good news! Deworming cattle shows a clear economic advantage, which has been realized on many real cattle operations as well as the reduction of sick cows and calves or lowered death loss due to worms. It’s unrealistic to think we can rid our cattle entirely of stomach worms, yet I believe we should take this approach to control them.