Do you know your horse’s weight? Callum Blair MRCVS explains the importance of getting it right for worming.

The dose of wormers is determined through extensive research to establish the optimal amount of drug to achieve the desired effect (kill parasites) while avoiding undesirable side effects. Accordingly, if the full recommended does is administered the level of efficacy can be predicted under normal circumstances. Without access to a weighbridge it can be tempting to make an estimate fit in with the weight that one syringe will treat (or half a syringe if you have a pony). This is not universal across all products as syringes vary from sufficient to treat 575kg up to 700kg.

Inaccurate dosing favours the development of resistance to wormer drugs. Do not be tempted to under dose your horse for any reason, especially to make your horse fit in with the weight of one tube/sachet. This is false economy and not only puts your horse at risk but could also create problems for the horse population as a whole.

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 under dosed 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. 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.

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 this is very unreliable. Studies have shown that horse owners and handlers tend to under, rather than over estimate their horse’s bodyweight and most do so by approximately 20%. This means that many horses may be unintentionally under dosed with wormer.


A recent survey has shown that a frightening proportion of horse owners would have inadequately wormed their horses because they estimated their weight. In the same survey, owners’ guesses of their horse’s weight ranged from as low as half to over twice the actual weight. It also showed that owners tended to over estimate the weight of horses under 14.2hh, but under estimate the weight of horses over this height. The bigger the horse the greater the error tended to be so it is advisable that before any horse or pony is treated it should either be weighed or a good estimation of its weight obtained by using a weigh tape, which will also make allowances for body shape and not just base the weight on height alone.


Horse scales or a weighbridge are the most accurate way to weigh your horse. Unfortunately most horse owners do not have access to such equipment. The best alternative to scales is a weight tape which is used around the horse’s girth to give an estimated weight in kilograms. Alternatively you can use the weight estimation formula by taking two measurements - a horse’s girth measurement (measured around the horse’s barrel, directly behind the elbow and passing over the back at the lowest part of the wither) and the horse’s length (measured diagonally from the point of the shoulder to the point of the buttock). The formula is:

WEIGHT (kg) = GIRTH (cm)2 x LENGTH (cm) 11877



•  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

•   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

•  Re-check your horse’s weight every time you worm as things 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

•  Remember to check the total bodyweight your wormer will treat


Under dosing is a known risk factor of resistance development. It is essential to administer the correct dosage.