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Fall rains brought relief to many drought-stricken ranchers. With winter grasses growing and water supplies refilling, the last thing many cattlemen have on their minds is a drought-related parasite problem. But they should. Conventional wisdom leads us to believe that internal parasites could not survive months of drought coupled with extremely high temperatures. Thus cattle would be grazing clean pastures. However, twenty year-old research in Australia challenges those ideas. From February 1980 through April 1981, a drought affected the CSIRO Pastrol Research Laboratory in Armidale, New South Wales, Australia. Researchers observed a 50 percent reduction in rainfall, increased evaporation rates and temperatures exceeding normal averages. During this drought, scientists conducted trials to determine pasture contamination rates. The forage was monitored through herbage analysis for nematode larvae, with the levels found in the grass so low, the trial was nearly abandoned. However, once the drought was over, larvae counts on the grass greatly exceed levels recorded in years of normal rainfall. Larvae were even recovered from forage for as long as 18 months after the last calf had grazed the pasture. The scientists concluded that, surprisingly, the nematode eggs expelled by cattle through their feces hatched, and the larvae remained in the feces throughout the drought, instead of migrating to forage. Once rains resumed in May 1981, the increased number of larvae in the feces migrated to the forage, resulting in higher than normal pasture contamination rates. Therefore, cattle grazing the area were exposed to the larvae during the first two months of normal rainfall after the drought. Due to the large number of surviving larvae, the cattle were contaminated at higher levels than traditional parasite challenges. It is critical to advise producers that a drought does not eliminate parasite concerns, but actually can be a process which concentrates the parasite threat to animals, according to a Veterinary Bulletin by Donald W. Briskey, D.V.M. with Merial. He believes that the Australian research challenges conventional wisdom that extreme climactic conditions can completely destroy parasites. Briskey suggests producers consider the possibility of increased parasite loads in their forage when planning for parasite treatment. |
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