Forty-four percent of questioned horse owners did not weigh their horses before worming, a survey has found.
A total of 559 horse owners completed the online British Riding Clubs horse health survey in February 2014.
It was commissioned by animal health company Zoetis and contained 21 questions on general horse health, care and management.
Zoetis is warning owners that they could be under-dosing.
This in turn can cause worms to develop resistance to wormers and impact on how infestations are managed in the future.
One case study found a 13.1hh Connemara weighed 60kg more than the owners expected.
Another showed that a full tube of wormer would not be enough to treat one competition-fit 16.3hh warmblood involved in the study.
The company warned that resistance to wormers is a growing problem
Resistance happens when parasites are no longer susceptible to a drug used to control them.
These then multiply and, if the process continues, only resistant worms remain.
Wendy Talbot, Zoetis vet, advised that horses should be weighed at least once a year using a weighbridge.
“Our case studies give a very real perspective on how easy it is to under-estimate the weight of our horses,” she said.
“Weigh tapes, although less accurate [than weighbridges], are a great way to keep tabs on weight once you have the accuracy of the weighbridge weight to refer to and have factored in any discrepancies.
“Remember that many horses will change in weight throughout the year.”
She added some feed manufactures offer a portable weighbridge service.
Knowing the current weight of the horses that you wish to treat before purchasing your wormers can help your vet or SQP to ensure that you buy the correct dose in each case.