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Internal Cattle Parasites in 2012

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Traditional internal parasite control in cow herds has often been in conjunction with other trips through the chute, such as first calf-working in the early summer and at weaning in the fall. In recent years, however, producers have trended away from the routine of convenient deworming in favor of a more deliberate, strategic approach.

Strategic control requires knowing the cycle of parasite infestation. Conditions specific to each ranch along with year-to-year variability in factors affecting this cycle is why a one-size-fits-all approach is not the most efficient strategy. Producers should work with their veterinarian to customize a strategic control program tailored to their resources, management and objectives.

Since we can never completely rid a herd of parasites, the goal of strategic control is preventing heavy infestations rather than waiting until they exist. The basic objective is to stop adult parasites in the cow from shedding eggs for as long as possible, especially during the early part of the grazing season. Here are the basic steps for assessing and designing a strategic deworming program.

Start with fresh manure containing parasite eggs. When conditions are ideal, between 60 and 90 degrees with moisture, the eggs hatch and the larvae reach infective stage in 14 days or less. Infective larvae must have moisture, from rain or heavy dew, to leave the manure and swim up the grass. The larvae don't go far; most are found within 4-5 inches of the manure and less than 2 inches up the grass leaf. This is why close grazing makes the problem worse. When the infective larvae begin to migrate out of the manure, they have 60 to 90 days to be swallowed by a cow. Once ingested, they will develop into adults in a few days and begin shedding new eggs in four to six weeks.

The process slows in winter, but the eggs or larvae can survive very well and become infective when it warms up. Numbers build rapidly in the spring with these surviving larvae. During droughts like that of 2011 and in the typical hot and dry conditions of July and August, the process is slowed or suspended without moisture.

A very general strategic program for mature cows begins with treatment at the beginning of winter. Deworming at weaning in mid-fall may leave time for the cows to become re-infested. Treating at the beginning of cold weather gives the herd a better chance of staying parasite-free through the winter.

The next treatment is determined by when grazing begins. Cows will naturally ingest over-wintered larvae in contaminated pastures. This re-infestation is acceptable if the next treatment occurs before new shedding of eggs in four to six weeks. Even with continued ingestion, it will be another four to six weeks before new eggs are shed. Hopefully by then, we are into the hot and dry part of the summer.

So where are we in May 2012? This past winter has been very friendly to internal parasites: average highs in the upper 50s and lows above freezing through February. March averages were 20 degrees higher than usual and rainfall was more than adequate. It could be argued that our grazing season began with the rains last October, with no interruption since. If that is true, internal parasites are taking their toll. Many other variables affect this cycle. Review the pieces of the puzzle with your veterinarian and plan your strategy.

Clay Wright
Former Technical Consultation Manager