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To deworm or not to deworm... that is the question!

Dairy Pipeline: June 1999

Ernest Hovingh
Extension Veterinarian,
Virginia-Maryland Regional College of Veterinary Medicine
Virginia Tech
(540) 231-5234

There continues to be much debate about the value of using anthelmintics ('dewormers') to treat internal parasites in adult dairy cattle. Over the years numerous studies have attempted to settle this debate 'once and for all.' However, many of the studies conflict in the method and timing of the treatment and, most importantly, in their conclusions about the benefit of deworming mature cattle. While more than half of the studies indicate an improvement in performance (milk yield) in treated cattle, the range of responses is quite large. This means that even if treatment improves performance it may not be economically justifiable to use anthelmintics in this group of cattle. The use of strategic spring and summer deworming programs (rather than treating at dry-off or freshening, for example) may result in a more predictable response to treatment. Plans are underway to investigate this hypothesis. Given the unpredictable response to anthelmintic treatment (some studies actually showed a drop in milk production after treatment!), one recommendation would be to work with your veterinarian to assess the level of internal parasitism in your mature herd, especially if they are on pasture during the spring and summer. If fecal egg counts indicate the presence of a large number of 'worms' in the animals, a deworming program could be put in place. Although this approach could not guarantee that treatment would be economically justifiable, it would increase the chances of observing a positive and worthwhile response in milk production.

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