By mid-June you’ve probably already pulled a calf or two off to the side with a watery, squinting eye. That early stage is easy to miss while you’re still finishing branding and worrying about hay. By the time you notice the cloudy, ulcerated eye that won’t open, the bacteria has had a head start and the rest of the herd is likely already exposed.
Pinkeye, or infectious bovine keratoconjunctivitis, runs on three things in Montana summers: face flies, irritation to the eye, and the Moraxella bacteria that does the actual damage. Knock down any one of those and you slow the whole thing. Hit all three and you can usually keep an outbreak from turning into a wreck.
Flies move it, dust and seedheads start it
Face flies are the delivery system. They feed on the moisture around the eyes, scratch the surface, and carry bacteria from one animal to the next. A pasture full of cattle bunched in the shade swatting flies is a pasture spreading pinkeye. Fly tags help if you put them in at the right time — too early in the season and they’re losing potency by the time fly numbers peak in July and August. A lot of producers wait until flies actually show up rather than tagging at turnout. Rotate the chemical class year to year so you’re not breeding resistant flies on your own place.
Tags alone rarely do the whole job. Dust bags hung where cattle have to walk under them to reach water or mineral, pour-ons, and oilers all add up. The point is constant, low-level pressure on the fly population, not one big treatment.
Then there’s the mechanical irritation. Tall, mature grass with stiff seedheads pokes eyes when cattle graze headfirst into it. Same with cheatgrass and foxtail in dry years. Dust off a dirt road or a heavily used trail does it too. UV glare on light-faced cattle adds insult. You can’t eliminate all of it, but clipping rank pasture or moving cattle off the worst of the seedheads during the danger window helps more than people expect.
Catch it early, treat it right
The first sign is usually excess tearing and a calf holding one eye partly shut, blinking against the light. At that stage a single dose of the right long-acting antibiotic, given early, often clears it before the eye ulcerates. Wait until the eye is cloudy white with a crater in the middle and you’re looking at a longer recovery, possible permanent scarring, and a calf that’s lost real weight from not being able to see and graze well.
Talk to your vet about which antibiotic and what label requirements apply — products and withdrawal times change, and you want it in writing for your own records. A few practical things that hold up regardless of the drug:
- Treat early and treat the whole affected eye, not just half-heartedly.
- Patch or otherwise protect a badly ulcerated eye from sun and flies if your vet recommends it.
- Separate the worst cases if you can, since they’re shedding the most bacteria and drawing the most flies.
- Handle pinkeye cattle last when you’re working them, and don’t share the same dusty squeeze chute traffic without thinking about spread.
Vaccines exist, but results are mixed because there are multiple strains and the timing has to be right to build immunity before fly season. Some operations swear by them, others see little. If you’ve had bad outbreaks year after year, it’s worth a conversation with your vet about an autogenous or commercial option matched to your situation.
Nutrition and grazing play into it
Cattle short on copper, selenium, or vitamin A heal slower and fight infection worse. Montana ground is variable on trace minerals, and a free-choice mineral that actually gets consumed matters more than the bag it came in. Check that cattle are eating it — empty mineral feeders tell you something.
Grazing management ties back in here too. Rotating cattle through paddocks keeps grass from going fully rank and stemmy, which cuts the eye-poking and keeps better feed in front of them. It also breaks up the manure where flies breed. None of this is a silver bullet, but a herd on managed grass, eating mineral, with flies under control, simply gets less pinkeye than one left to fend for itself on overgrazed, fly-blown summer pasture.
The math is straightforward. A calf that loses sight in an eye for a few weeks in the heart of the grazing season comes up light at weaning, and a scarred eye can dock you at the sale. The work to prevent it is cheap by comparison, and most of it you’d be doing anyway in June.
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