Knowing your Horse's Weight

For wormers to treat internal parasite burdens effectively, horses must be dosed with the appropriate amount of wormer for their bodyweight. The dose rates for wormers are determined through extensive research to establish the optimal amount of drug to achieve the desired effect (kill parasites) whilst avoiding undesirable side effects. If horses are under-dosed, either as a result of poor weight estimation or if some of the wormer is spat out instead of being swallowed, the internal parasites are exposed to a sub-lethal amount of the drug; this has two very serious implications:

  1. The horse may be left at risk of worm related diseases.
  2. The risk of wormer (anthelmintic) resistance is increased.

Don’t be tempted to try to make the weight of your horse conveniently fit in with what “one syringe of wormer will treat” or “half a syringe” in the case of ponies. A recent survey of leisure horses showed that in reality horse weights rarely comply with this approach, not least because “a syringe” is not a standard size and can be designed to treat 575 kg, 600 kg or 700 kg depending on the product.

The best way to get an accurate weight is to use a weighbridge but not everyone has access to one of these so more often than not weights tend to be based on visual estimation or by using height based charts as a guide. These are notoriously inaccurate and studies have shown that guesses are on average 10 – 20% below the actual weight. If using these methods it is important to take into account the body type, condition and work level of the horse or pony, failure to do so could result in potentially harmful inaccuracies.

Weigh tapes can provide a useful guide to a horse’s weight but vary considerably in accuracy so you should check what research has been done to validate the accuracy of the tape you use.

This leaves weight estimation formulae as one of the most reliable (and cheapest) methods for equine weight estimation. By including a variety of measurements these formulae can take into account the body shape or type of the horse. The following formula is frequently used and has been shown to be one of the most accurate ways of estimating the weight of a horse or pony:

No matter what method is used to determine the weight of a horse it should be remembered that its weight is unlikely to be constant throughout the year and will vary according to the level of work it is doing.