18/05/2025
Why underdosing your horse with wormers is such a problem and how to calculate your horses weight - strap in for a long post.
Its almost time to start our late autumn/early winter worming and one of the biggest problems we see is people guessing the weight of their horses which then leads to underdosing. This is a problem because wormers contain a specific concentration of active ingredient that is required to be administered to kill the parasites in your horse. When the dosage is too low parasites survive the worming and continue to breed, increasing the population of resistant parasites. Therefore it is very important to determine your horses weight correctly and give the required dose.
Before we go into how to determine your horses weight, Yes we are aware that overdosing can occur and is particularly dangerous for foals, horses with health conditions, low body fat horses and miniatures/ponies. This is again why it's important to select the correct wormer, know your horses weight and read the product information sheet and/or seek veterinary advice.
Recent studies have shown that 85% of owners tend to underestimate their horses weight rather than over estimate, and were out by between 68 to 83 kg!!!! The study also found there was no correlation between the accuracy of estimates and the persons years of experience. This means no matter how long you have been in the horse industry you are still underestimating your horses weight.
There are several ways to determine your horses weight:
Scales - the most accurate method. Most vet clinics have equine scales and several mobile vets in the region have potable scales. There are also some equine professionals such as Integral Equine Nutrition who visit the area and have portable scales.
Public Weigh Bridges - You can take your horse and float over the public weigh bridge, get the amount and then deduct the float weight.
Body Estimation Formula - This is the preferred method when using a tape measure. You need 2 measurements. Girth - place the tape over the horses back at the base/lowest part of wither and around directly behind the elbow or where the girth sits. Note this may mean the tape is at an angle / this is ok. The second measurement is Length - which is from the point of the shoulder (NOT the middle of the chest) diagonally to the point of the buttock. Keep the tape taut and when reading the hind end measurement stand at the horse back leg and read it without peering around the horses body. The calculation is then Girth (cm) x Girth (cm) x Length (cm) / 11877 and that will give body weight in kg.
Weight Tape - this is tape you buy from most horse shops or feed stores and has the horse weights marked on it. The tape is placed in the same position as the Girth measurement above - behind withers and where the girth sits and then you read the horse weight of the tape with no additional math. This method is considered the least accurate of all methods and is more suitable for monitoring horse condition than actual weight.
General advice for either of the tape methods is be consistent - make sure the horse is standing square on a flat surface, done before a meal, use the same tape and have the same person do the measuring.
What happens if my horse is between the markers on the worming tube - the advice is to round up to the nearest 50kg, never round down.
Our challenge to you is to get prepared for worming and determine your horses weight now.