Fly Spray, Deworming, and Fertility in Bulls

– Dr. Les Anderson, Beef Extension Specialist, University of Kentucky

The impact of insecticides that contain pyrethroids and commercial dewormers on fertility in bulls has been the subject of a great deal of conversation lately. A thorough review of the science seems necessary since bull turn-out is upon us. Our discussion will start with commercial dewormers since more is known about the use of dewormers on bull fertility.

About 7 years ago, several beef producers and large animal veterinarians were convinced that use of commercial dewormers like Cydectin, Eprinex, and Dectomax reduced sperm quality and the ability of a bull to pass the breeding soundness exam (BSE). The impact of these dewormers on bull fertility was examined as part of FDA approval. None of these studies reported any detrimental effects on sperm motility, morphology, concentration, or semen output. As a follow-up, Dr. Phil Prater at Morehead State University and I conducted a small trial just to verify the previously reported data. A BSE was obtained on bulls and 32 bulls that passed were randomly assigned (8 per treatment) to be dewormed with Dectomax (Phizer Animal Health), Cydectin (Fort Dodge Animal Health), Eprinex (Merial), or they were not dewormed. Each bull was subjected to a BSE at 1, 2, 4, and 8 weeks after deworming. No differences were observed in sperm motility, morphology, the bull’s ability to pass the BSE. So, based upon these and the previously published data, the use of Dectomax, Cydectin, or Eprinex does not impact bull fertility. One word of caution; we have not tested any of the new generic brands of ivermectins available so these data are only valid for the brand names listed.

The impact of insecticides that contain either pyrethroid or bifenthrin is much more difficult to discern. Insecticides that contain these products have been used for nearly 40 years to control flies. Early reports by the World Health Organization and many others indicated no adverse effects of pyrethroids on male fertility. This data seems to correlate with the experience of many beef producers that put two fly tags in right before they turn their bulls out and have no fertility issues in pasture situations.

Later work in laboratory animals did show a detrimental impact on semen quality, quantity, and libido and a study by Cornell in 2003 demonstrated that bifenthrin reduced semen quality in bulls but semen quality returned to normal within 60 days. Dr. Volkmann at the University of Missouri has shared clinic trials in which he has attributed reduced sperm motility and overall semen quality to exposure of bulls to bifenthrin and pyrethroids. Biologically, these compounds have been shown to disrupt enzymes necessary for the production of testosterone which would have profoundly negative impacts on the male to produce sperm. The key to this puzzle is most likely dose. According to Dr. Jeff Scott, toxicologist at Cornell University, the doses used in most of these trials are not doses most beef cattle would be exposed to in a normal production situation.

To summarize, it is quite clear that deworming bulls with Eprinex, Dectomax, or Cydectin will not reduce bull fertility. Exposure of bulls to high levels of pyrethroids or bifenthrin may temporarily reduce sperm motility, morphology, and semen quality. Perhaps the best management strategy would be to simply use one form of fly control (fly tags, fly spray, and fly rubs) in bulls.