The horn flies get all the attention in June because you can see them. Internal parasites do their work where you can’t. A load of stomach worms won’t drop a cow in the corral, but it’ll take the bloom off her, pull her body condition down going into breeding, and leave yearlings gaining half what they should on good green feed. By the time you notice rough hair coats and gut-empty calves, the money’s already gone.
Most Montana outfits deworm once, sometime around turnout or spring working, and call it done. That’s not a bad start. But the way parasites pick up on summer pasture, and the way the old products are starting to lose their punch, are both reasons to think a little harder about the how and the when instead of just the whether.
Where the worms come from
Cattle don’t carry a big worm load through a Montana winter the way they might in a wetter, warmer country. Cold knocks the pasture larvae back hard. What happens instead is a slow build through the grazing season. Cows shed eggs in the manure, the larvae crawl up wet grass in the morning, cattle graze them back in, and the load climbs through July and August. The animals eating closest to the ground and grazing the same paddocks longest pick up the most.
That’s why the young stock gets hit worst. Calves and yearlings haven’t built much immunity, they graze tight to the dirt, and they’re the ones you’re counting on to gain. A mature cow can carry a fair worm burden and still look decent, but she’s spending groceries fighting it that ought to be going into milk and getting bred back.
Two things matter for timing. One, the pasture larvae come off manure pats a few weeks after cattle turn out, so deworming right at turnout cleans the animal but does nothing about the reinfection waiting in the grass. Two, wet years grow more larvae than dry ones. If June came in green and soggy in your country, the parasite pressure will be heavier than in a droughty summer when the larvae bake off the grass.
Time it so it counts
A treatment at spring turnout lowers the load cows carry onto clean spring grass and cuts down egg shedding early. That’s worth doing. But on younger cattle especially, a second treatment in mid-to-late summer often pays, because that’s when the pasture load has climbed and the animals are carrying the most worms of the year. You’re catching the burden at its peak instead of before it built.
Match it to a day you’ve already got cattle gathered. If you’re moving pairs to summer country, sorting to breed, or bringing yearlings through the chute for anything else, that’s your window. Handling cattle in the July heat just to worm them isn’t worth the shrink and the risk. Piggyback it on work you’re doing anyway.
Watch the product too. A pour-on is easy and it hits lice and grubs along with the gut worms, which is why so many people reach for it. An oral drench or an injectable puts a more reliable dose into the animal for the internal parasites. Whatever you use, dose to the actual weight of the biggest animals in the group, not a guess on the low side. Underdosing does more than waste the product.
The resistance problem is real
The dewormers that have carried the industry for decades are wearing out in a lot of places. Worms that survive a light dose pass on the ability to survive the next one, and treating the whole herd on the same day every year, year after year, selects hard for the survivors. It’s the same story that played out with the horn flies and their fly tags.
You don’t fix that overnight, but a few habits slow it down. Dose to weight so you’re actually killing worms instead of trimming the easy ones. Don’t turn freshly treated cattle straight onto a clean, ungrazed pasture, because the only worms left to seed that ground are the resistant ones. And leave your most productive mature cows out of a summer treatment if they’re carrying condition and doing their job. Refusing to treat every last animal keeps a population of worms around that never saw the product, and that dilutes the resistant ones.
The cheap way to know whether any of this is working is a fecal egg count. Your vet can run one before you treat and again a couple weeks after on the same animals. If the count barely moved, the product you’ve been buying isn’t earning its keep, and you’d rather find that out from a manure sample than from a set of light calves in the fall.
