Parasite Considerations in Times of Drought
By Dave Sparks, DVM
Oklahoma State University
Late summer and early fall are usually time of low impact from internal parasites because drying shortens the life span of the infective larvae. This year, however, due to dry weather the scene may be a little different. Although it is dry enough for anybody, we also have much shorter grass than is usual for this time of year. To review a moment, most round worms pass their eggs in the manure. The eggs are fairly resistant to the environment, but they hatch into larvae that change through developmental stages to reach the L3 stage, which is the stage that infects the animal. These larvae then move up the grass plants where they are ingested as the animals graze. When forage is of normal height for late summer they can’t crawl high enough to be a serious problem. Rarely are they able to move upwards more than 4 to 5 inches and 80% of the larvae are in the lower 2 inches of the forage. If your animals are grazing 1 to 2 ½ inch tall forage, they may be getting large numbers of infective larvae, even though it is dry. You can help to alleviate the problem by watching your animals graze. Try to avoid this danger zone if possible and/or modifying your de-worming program to fit the conditions.
In times of drought, as you well know, financial considerations change too. Mature cattle develop some degree of immunity to parasites so that the worms may not cause disease, but waste nutrition. In normal years when there is plenty of standing forage there may not be enough benefit to pay for the cost of the worming. This year every bite of standing forage is valuable, and many producers are already buying hay, often at inflated prices. Now nutrition that goes to feed the parasite burden has a real financial impact. In order for your cows to get all the feed you are paying for, it may very well save significantly on feed cost to eliminate any worms present.
This year, many producers are planning to early wean their calves. If you will be weaning and keeping your calves or purchasing calves for fall grazing or backgrounding, you might want to rethink your parasite control program for calves. While calves are not big shedders of parasite eggs, their mothers are, and by this time of the summer they should be grazing right along side the cows. Unfortunately, they don’t have their mother’s level of immunity, and an exposure rate that cheats the cow out of nutrition can cause poor performance or even clinical disease in calves and younger yearlings.
Every producer’s situation is different. Discuss your conditions with your local veterinarian to determine what, if any, changes you should make in your parasite control program.
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