Shaping the future of animal health

Weight awareness

Why do I need to know the weight of my horse?

For wormers to effectively treat internal parasite burdens, every horse must be dosed with the appropriate amount of wormer according to their bodyweight. If horses are underdosed with wormer, the internal parasites are exposed to a sub-lethal amount of the drug. This may leave a population of worms that are able to tolerate the wormer.

Resistance to the “BZ” class of wormer is already widespread throughout Australia. Surveys have shown it to be a significant problem on most properties throughout Australia.1

It is important to protect the wormers that remain effective as best we can. One of the most important ways of doing this is to ensure that animals are dosed to the correct bodyweight, preferable erring on the side of a slight overdose.*

What’s wrong with guessing the weight of my horse?

Prior to worming, very few horses are accurately weighed on a weighbridge or horse scales. More often than not, the dose of wormer a horse receives is based on a visual estimation of its weight. Unfortunately, visual interpretation of a horse’s weight is very unreliable. Studies have shown that horse owners and handlers tend to under, rather than overestimate their horse’s bodyweight.2,3

Most people underestimate weight by approximately 20%. This means that many horses may be unintentionally under-dosed with wormer simply because a more accurate weight estimation method has not been used.3

Does my horse weigh more than I think it does?

horse weightMost wormers on the market in Australia only treat a horse up to 600kgs some as little as 575kg or even 450kg, however many horses can weigh more than the amount a single syringe of wormer is designed to treat. All Virbac wormers such as Equimax®, Equimax® LV and Strategy-T® are now available in doses large enough to treat all horses weighing up to 700kg.

A survey of Australian horses was conducted with regards to their size and weight to get an accurate picture of the “average” Australian horse. This survey sampled many horses from a different variety of breeds and uses and produced some interesting findings. One thing that was very clear from the survey was that horses in Australia are generally heavier than it was thought.

It is interesting to note that this survey was conducted during drought conditions, and therefore the majority of horses were of average body condition. There was only one horse in the survey larger than 17 hands, so large animals were not actively sought. This survey indicated that a significant proportion of horses will be underdosed if only one syringe of wormer is administered.

Virbac has taken into account this evolution in bodyweight by modifying its syringes to accommodate the treatment of horses that weigh up to 700kgs in a single dose.

How can I get an accurate estimation?

Horse scales or a weighbridge are the most accurate way to weigh your horse. Unfortunately most horse owners do not have access to a set of scales suitable to weigh their horses on.

The best alternative to scales is a weight tape or using the weight estimation formula. This is done by:

  1. Weight Tape – The tape is placed around the horse’s girth and the estimated weight is read from the tape in kilograms.
  2. The weight can be calculated by using a weight estimation formula.  

Girth - the tape (weight tape or conventional measuring tape) should be positioned snugly around the horse’s barrel, directly behind the elbow and passing over the back at the lowest part of the withers. The weight is then either read in kgs, if using a weight tape or the girth circumference in cms used in combination with the length to calculate the horses weight.

Length – Place one end of the tape on the point of the shoulder, keep the tape taut and run it diagonally to the point of the buttock. Stand by the horse’s back leg and read the length in cm’s you can see without having to peer around the horse’s body.

Click here for weight calculator

Note: The weight survey indicated that some commercially available weight tapes tend to significantly underestimate the weight of horses when compared to scales. Virbac’s Equimax® branded tape was shown to be a more reliable method of weight estimation for worming. If using a weight tape other than the Equimax® branded one it is best to use the formula and the tape initially to check the accuracy of your tape.

Weight awarenes tips

  • Always use scales, a weight tape or the weight estimation formula prior to worming your horse. This is especially important for larger animals that may require more than a standard 600kg syringe.
  • Virbac’s wormers are designed to treat horses up to 700kg which covers the majority of horses.
  • It is important to accurately estimate the weight of ponies or foals if you are intending to share one syringe of wormer between two animals. Often ponies are heavier than you think. Equimax® Elevation and Strategy-T® will reliably treat 2 ponies of average weight.
  • Re-check your horse’s weight every time you worm as a horse’s weight can change considerably with work, age and life stage (e.g. pregnancy, retirement).
  • Round up the weight estimation to the nearest 50kg calibration on the syringe, never round down.*6
  • Remember to check the total bodyweight your wormer will treat. Eventhough Virbac wormers treat horses up to 700kg, most other wormers treat less.
  • Underdosing is a known risk factor of resistance development. It is essential to administer the correct dosage regimen.8
  • The risk of underdosing with products treating 600kgs or less should be avoided.9

*No adverse reactions have been witnessed with 5 x the recommended dose of Equimax® and Equimax® LV and 4 x the recommended dose of Strategy-T®.


  1. Results of national efficacy survey 2007 (data on file) revealed BZ resistance on greater than 90% of properties surveyed.
  2. Asquith, R. Johnson, E. Kivipelto, J and Depew, C. (1990). Erroneous weight estimation of horses. Proceedings of the annual convention of the American Association of equine practitioners.599-607.
  3. Ellis, J.M. Hollands, T. 1998. Accuracy of the different methods of estimating the weight of horses. Veterinary Record 143 335-336
  4. Results of national weight survey 2007 (data on file).
  5. Carroll, C and Huntigton, P(1988). Body condition scoring and weight estimation of horses. Equine veterinary journal 20(1) 21-45
  6. Data on file.
  7. Results of multi-drench efficacy trial 2006 & 2007 (data on file).
  8. Bousquet, E et al, Ease of Adminsitration and the Risk of Underdosing Endectocide Oral Paste in Horses, 21st International Conference of WAAVP, Belgium 2007
  9. Sangster, NC, Pharmacology of Anthelmintics resistance in cyathostomes: will it occur with the avermectin/milbemycins? Vet. Parasitol., 1999; 85: 189-204.
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