Insecticides and Their Effect on Bull Fertility

— Written By N.C. Cooperative Extension

As cattle producers embark on another breeding season, the outlook is grand. Pasture is thriving from ample rainfall, and cows emerged from a mild winter in good body condition, and what about those prices, wow! As spring turns into summer, we as producers must deal with both parasites and flies, as both cost us dearly in terms of weight gain, thriftiness and efficiency. Combating the effects of horn and face flies can give us generous returns on our investment. 

Many of our best strategies in dealing with these pests involve the use of various pyrethroid insecticides in the form of sprays, pour-on’s, rubs, or ear tags. Recently, an article in BEEF Magazine, proclaimed the harmful effects to bull fertility when using pyrethroid insecticides near the breeding season, and several producers asked the question, is this really true? . Reviews from various University beef cattle specialists suggest caution is advised when using these products, but do not remove them from the arsenal just yet. 

In summary, the article suggests that pyrethroids may inhibit production of an enzyme utilized in the conversion of testosterone to dihydrotestosterone (DHT), which is necessary for the proper function of many sex accessory glands. In several cases, reports indicate that sperm quality and motility greatly decreased after the bulls were exposed to a pyrethroid compound. After exposure, it generally took 2 – 4 weeks for the performance of these bulls to return to normal. In these instances, it was noted that bifenthrin (a synthetic derivative of native pyrethrin) was the active ingredient in the spray.

Researchers at various universities are giving this issue further examination, by looking at a multitude of data. Cornell University reported in 2003 that exposure to bifenthrin reduced semen quality in bulls, however they recovered to normal around 60 days later. Other reports also indicate that exposure to pyrethroids can temporarily alter sperm motility, and other aspects of fertility, as well. There were other reports indicating that semen quality in rats, rabbits and humans was affected after exposure to pyrethroids.

How do these findings affect producers? First of all, pyrethroids and their effect on male fertility are creating a lot of smoke, and where there is smoke, there is generally fire. However, reliable research based information from controlled, repeated studies is needed, and such is not the case within the recent BEEF article. At present, there is minimal scientific data to support claims that pyrethroids will lower sperm motility and fertility. However, producers should use caution when using pyrethroids. 

Dr. Les Anderson, University of Kentucky Beef Cattle Extension Specialist, suggests that producers only use one method of insect control when using pyrethroids. Utilize either sprays, ear-tags, backrubbers, OR pour-ons. Avoid using ear-tags and sprays, or pour-ons and ear-tags, for example. Producers should administer pyrethroid insecticides by only one method, don’t double up. Further, producers should follow label directions carefully, and accurately mix dilutions so that dosages are according to label. Mixing a pyrethroid spray at stronger than label recommendations can have greater side effects and possibly a greater impact on fertility. Individual animals could also be more sensitive to these products so there can be an additional factor in determining the true effects of these products on fertility. Older more seasoned bulls may be able to overcome or combat the effects of pyrethroid insecticides better than younger, less mature bulls, but we just don’t know. 

Presently, this is the information available, and we should know more as continued research yields more conclusive data. If you would like to know more, please contact the Burke County Cooperative Extension Service at 439-4460.

Damon Pollard
Extension Agent