Management tip: Horn fly control

Q. Can you help me advise a rancher on the value of the use of fly control ear tags in calves?

A. First the horn flies prefer the bigger animals ‐ cows, steers, heifers, especially bulls – and don’t spend much time on the calves. So the basic recommendation is to tag the mature animals not the calves. Calves are usually tagged at branding which is kind of a waste of tags, plus it’s too early in the season. Generally, horn flies, at infestation levels of more than 250 flies per animal, cause loss in weaning weights due to lower milk production in the cows. I’ve done ear tag studies at Ft. Keogh and found fly levels over 1000 per animal in the control (untagged) while the tagged cattle were relatively free of flies. So the tags work pretty good in keeping flies off the cows.

There is a difference in the effectiveness of these tags. One problem is resistance to some of the pyrethroidcontaining ear tags. It is strongly recommended that a rancher not use the same tag two years in a row but alternate tags. There are effective pyrethroid tags that work well and these can be rotated the following year with tags containing an organophosphate insecticide or a chlorinated hydrocarbon. Usually two tags per animal are recommended and tag the animals as late as possible, say early to mid‐June.

As for the economics, there have been studies at the University of Nebraska showing that weaning weights can be reduced by 8% at around 500 flies per cow. The higher the infestation levels the bigger the potential impact. It’s difficult to predict what the fly levels will be like for any summer. Usually, hot, dry weather results in high fly numbers. If there’s over 200 – 300 flies per cow, then it would likely pay to invest in fly control.

Another option would be the use of dust bags. The will provide 75 – 80% control of horn flies. Dust bags are most effective when placed in a forced‐use situation and they do require weekly checking.

Source: Dr. Greg Johnson, Professor of Veterinary Entomology

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