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Proper dosage is critical for efficacy, economics and cattle health

By: Heather Smith Thomas

When treating cattle with antibiotics, dewormers and other medications, it is important to administer proper dosage—which is generally determined by weight of the animal. You need to know the weight, rather than guessing. Under-dosing may not give desired results, and overdosing in some instances can be harmful. With dewormers, under-dosing won’t kill the parasites and may lead to drug resistance.

Dr. Steve Hendrick runs a feedlot, dairy and cow-calf veterinary practice in Coaldale, Alberta, and says vaccines are not an issue. These are usually dosed at 2 milliliters or 5 milliliters per head, depending on product; the purpose is to provide antigen to stimulate an immune response; it’s not weight-specific and will be the same for a calf or a cow.

Antibiotics and dewormers are a different story regarding size of the animal, but for any injection each animal needs to be given the dose specified on the label, administered at the proper site and by proper route, listed on the label.

Dr. Doug Hilbig, Senior Technical Service Veterinarian, Zoetis, says it’s important to read the label to find the dose. “If you have a 550-pound calf, multiply the dosage, such as 1.1cc per hundred pounds: 550 x 1.1 = 6 cc of product.”

Even a slight estimation in the dosage or weight of the calf could make a difference in the correct amount given.

“If you guess 500 pounds, and estimate the dose at 1cc, a 500 pound calf would get 5cc of product. But if the calf actually weighs 530, and the dosage is supposed to be 1.1cc, that calf should have 6cc, and that’s 20% under-dosed,” he says.

When giving antibiotics there are several important considerations, including withdrawal times, says Dr. Nathan Erickson, Assistant Professor, Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan. The withdrawal time established for that antibiotic is based on giving the correct dose, for the correct duration and by the correct method. If you over-dose or give a certain antibiotic more days than recommended, or by incorrect route, it may take longer for residues to be eliminated from the animal’s body.

Under or overdosing can also affect how the antibiotic works. Hilbig says that when you under-dose, the antibiotic isn’t going to work properly.

“Treating a calf will usually cost $12 to $20 per head, so it doesn’t take very many cattle to justify a set of scales. Often people are treating according what these cattle are averaging, but may be treating the lower end of the average, or sometimes the bigger end of the average. We’re wasting money by over-dosing, and if we under-dose we might get upset because the antibiotic doesn’t seem to be working. If I under-dose 20%, then I might have to retreat. So that 20% could cost me another treatment,” says Hilbig.

Not only does under-dosing run the risk of not being effective, it also provides a chance for some of the more resistant parasites or pathogens to survive. “There is a push today to avoid development of resistant microbes or parasites,” says Hendrick. Producers perpetuate this problem if they continuously under-dose.

“That doesn’t mean we should err on the side of overdosing. We have to be smart about it because there are also disadvantages when overdosing.” Not only will it be expensive, but there may be adverse side effects for the animal if you give too much of a certain drug. Overuse of antibiotics in some situations may kill off the “good bugs” in the digestive tract and lead to other problems.

When treating sick calves that might be dehydrated, overdosing with certain antibiotics and anti-inflammatories can be dangerous. “If the calf is severely dehydrated (from scours) some drugs are very hard on the kidneys and other organs,” says Hendrick.

“A calf’s body generally contains a higher water content (percentage of weight) than an adult and they also dehydrate more readily,” he says. They would be more likely to suffer kidney damage with overdose of certain drugs. It is always wise to try to use proper dosage.

Dosing cows and bulls are another issue.

“Producers may think their cows weigh about 1200 pounds, when in reality they have some that weigh 1600 to 1800 pounds. When trying to estimate weight, people can easily be off by 200 pounds or more,” says Hendrick.

Some people are good at estimating weight, but it can be deceptive comparing animals that are short and stocky versus tall and leggy, or long-bodied versus short-backed and thick. The herd average is what producers go by, for delousing/deworming treatments, setting the dose gun for a certain dose. “The problem is that there’s often a swing of 100 to 200 pounds either way in a group. You will be overdosing some and under-dosing others. You need a scale at your squeeze chute, so you could dose each animal correctly,” he says.

Some people think they can’t afford a scale, but a plain set of scales is not very expensive, according to Hilbig. “When you think about the cost of cattle, cost of the chute, and the cost of medicine, if you buy $1,000 worth of medicine and overdose 20%, that’s $200 you wasted. If you are treating many cattle, it quickly pays to own scales.

“It’s also nice to know when treating cattle if they are gaining weight or losing weight, and know where your cattle are on weight. This can determine what you want to do with those cattle regarding marketing,” says Hilbig.

Hendrick says proper restraint is important when medicating. This enables you to use proper technique, whether giving oral medication or an injection. When giving a subcutaneous injection you want the animal restrained so you can make sure the entire dose gets deposited under the skin and doesn’t leak out. You might think you are giving proper dose, but if it doesn’t get there, you are under-dosing.

Erickson sums it up simply: Correct dosage is important for multiple reasons that include economics, food safety, minimizing development of resistant pathogens and parasites, and efficacy—for health of the animal.

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