Dewormers: classes, resistance, and why rotating blindly is old advice

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Research. We have not put a parasite plan through a season here yet. What follows is how these products are categorised, which part of the label actually tells you something, and the questions worth taking to a vet before you spend anything. When we have a year of numbers and a plan built on them, this page gets updated with the date.

There are only a few active ingredients on that shelf, and the brand name on the bottle is the least informative thing about it.

The classes, not the brands

Most livestock dewormers sold here fall into three chemical classes:

• Benzimidazoles — fenbendazole, albendazole, oxfendazole. The "white" group.
• Macrocyclic lactones — ivermectin, eprinomectin, moxidectin, doramectin. The "clear" group.
• Imidazothiazoles and tetrahydropyrimidines — levamisole, morantel, pyrantel. The "yellow" group.

Two bottles with different names, different label art and a price gap of several dollars can hold the same active at the same concentration. The active ingredient is printed on the label, usually in smaller type than the brand. Read that line first, every time.

Species matters as much as class. Products are approved for particular species, and going outside the label is a decision for your vet.

Rotating on a calendar is old advice

The instruction most of us grew up with was to switch classes every year. That predates most of what is now understood about resistance. Rotating blind exposes the same parasite population to every class in turn, and can end with nothing that any of the three still does much to.

Treat the animals that need treating, keep part of the parasite population unexposed — refugia is the term you will see — and find out what still works on your ground before you buy a case of anything.

The test that decides it

A fecal egg count is the answer to "which of these should I buy". A sample goes to a vet or a lab and you get a number. Sample again after treatment and the change in that number is your evidence about your own ground rather than someone else's. Your vet sets the timing. Do not improvise it.

Cost bands worth knowing:

• $8 to $30 per sample through a vet or a mail-in lab.
• $150 to $400 for a microscope, slides and chamber to run them yourself, plus the learning.

The home setup only pays back at volume — realistically twenty or thirty samples a year. Below that, mailing them is cheaper and the results are better. Advice written for a two-hundred-head operation does not scale down here, and rarely admits it.

Pour-on, drench, injectable

Pour-on. Least restraint needed, which is why small places reach for it first. It is also the route with the widest variation in how much actually gets absorbed: rain, a wet or caked coat, and hair length all move it. Cattle groom each other, so a treated animal loses some and an untreated one picks some up. Partial exposure spread across a herd is exactly the condition that favours resistant parasites.

Injectable. Goes where you put it, so it is the most predictable of the three. Needs a chute or genuinely good restraint, and there are injection-site and carcass considerations the label covers.

Oral drench. Delivered to the gut. Needs a drench gun — $20 to $60 — and a head you can hold still. Slowest per animal and least forgiving of a poor handling setup.

Whichever route, the amount is set by body weight and the label states how. Guessing weight by eye is where most small places go wrong, and the error runs low rather than high. A weight tape is $8 to $20. A platform scale starts near $400 and will not earn that back on a handful of animals.

What it will not do

A dewormer is not a diagnosis, and I am not going to tell you what to give or when. That is on the label and in the conversation with your vet, including meat and milk withdrawal periods.

It also does not fix management. Stocking density, pasture rest and whether animals graze the same ground continuously move parasite load more than the bottle does. Resting paddocks properly usually means temporary fence, and temporary fence needs a charger that still works when the grass comes up — there is a page on that.

What we are buying

The count comes before the bottle. That is the order this page argues for and the order we are following, and no sample has gone to a lab yet, so there is no product to name and no class to name it in. Buying fills this section in: the lab we settle on, its price and turnaround, the active ingredient and class we ended up with, and what a dose came to. It does not lift the Research tag. That takes a year of numbers — counts before treatment and counts after, run on this ground — and a plan built on them. The date goes on when that exists.

Before you buy

• Read the active ingredient before the brand name. Two shelves of bottles, three classes.
• Book the fecal egg count before you buy the bottle. Buying first and testing later is the expensive order.
• Take the label to your vet. Species, weight basis and withdrawal periods are their call, not the internet's.

— Chris D., McEwen, Tennessee