Successful strategic deworming is all about timing: planning treatments to kill the most parasites possible. Spring may be the most critical time in a strategic program for the northern United States because adult worms in cattle and infective larvae on pastures are emerging from winter inhibition and both populations are peaking. It's also when calves are beginning to graze and acquire these parasites.
That's why the whole herd should be treated with an endectocide in the spring. Even older cows with some level of immunity remain vulnerable to the effects of subclinical parasitism, and they will also continue to contaminate pastures. In a North Dakota study, cows treated with ivermectin weaned calves a mean of 15.5 pounds heavier than controls, suggesting increased milk production.
Many parasitologists agree that in the northern United States the most important treatment for internal parasite control is in the spring. Producers often assume that harsh winters will stop transmission and kill off parasite larvae on pasture, but this is not true. The period of rapid growth for cool-season forages is the time of year when cattle are exposed to the greatest populations of internal parasites – especially Ostertagia. In fact, the same environmental conditions that cause pasture growth also increase parasite levels on pastures. The greatest returns for spring treatment are achieved when the cow and her calf are both treated at or near the time of turn-out.
After spring treatment, producers should look to summer endectocide treatments. Summer is the time of the year when ectoparasites, including horn flies, become more costly. It is therefore usually optimal to apply a second treatment about eight weeks after the initial treatment, or mid-summer.
Fall is the most convenient time of the year to treat against parasites, and it is high on the producer's priority list because of parasites like lice and liver flukes. In the northern United States, it is also an ideal time to clean cattle of Ostertagia before winter.
In the big picture of parasite control, strategic deworming can help lower parasite loads throughout the year, helping producers better reap the benefits of a planned program year-round.
Source: Merial Animal Health
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