Animals : an Open Access Journal from MDPI · 3 authors, 3 centres
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This study analyzed 1716 horses and found that commercial weigh tapes underestimate equine body weight by an average of 5.68%, with greater underestimation in heavier horses. Body condition score, breed, and bone density significantly influenced weigh tape accuracy, while height and muscle top-line score did not. Clinically, these findings highlight that weigh tapes are unreliable for precise dosing of medications and anthelmintics, particularly in larger or heavier horses.
**Background:** Accurate measurement of equine body weight is critical for calculating medication dosages, anthelmintic doses, and feed quantities. Underdosing anthelmintics can promote resistant parasites, and incorrect medication dosing can have serious health consequences. A previous study found 67.2% of horses received inadequate antimicrobial dosing before colic surgery due to incorrect weight estimates. Equine weighbridges are the gold standard but are expensive and not widely available. Weigh tapes offer a practical alternative but are known to be less accurate. This study aimed to identify which horse-based variables (height, body condition score, breed, muscle top-line score, bone type) affect weigh tape readings.
**Methods:** A retrospective analysis was conducted on anonymized data from Baileys Horse Feeds nutrition consultations collected between January 2017 and August 2019. Of 2675 records, 1716 complete records were analyzed after excluding 959 with missing data. All horses were over two years old. Each horse had a weigh tape reading (Shires Equestrian Horse & Pony Weighband, PVC, max 206 cm) taken behind the point of the elbow, vertically over the withers, following exhalation on a level surface. True body weight was measured on a calibrated Newmarket Wireless Portable 3 Piece Horse Weigher (max 2000 kg, accurate to 1 kg). Breed was categorized into four groups: small ponies <350 kg (n=237, 13.8%), large ponies >350 kg but <14.2 hh/147 cm (n=322, 18.8%), lighter horses such as thoroughbreds (n=701, 40.9%), and stockier breeds such as drafts (n=456, 26.6%). Body condition score was assessed using the 1–9 Henneke system. Muscle top-line score was assigned using a company-developed scoring system. Bone type was subjectively classified as 'heavy weight', 'medium weight', or 'light weight'. Linear and quadratic regression models were compared using likelihood ratio tests, with a critical p-value of 0.05.
**Key Results:** The overall mean weigh tape reading was 32.82 kg lower than the mean actual weight (range −151 to 64 kg). The average error was −5.68% (95% CI [−6.01%, −5.34%]). A quadratic regression model fit significantly better than a linear model (p < 0.001), confirming a non-linear relationship. Height did not improve model fit (p = 0.417). Body condition score significantly improved prediction: each 0.5 unit increase in BCS added 1.24 kg to the weigh tape reading for horses of the same actual body weight (p < 0.001). The majority of horses (72%) had BCS between 4 and 6. Breed groupings improved model fit (p < 0.001), with up to 1.87 kg differences between breed groups. Stockier horse breeds (SH) tended to be underestimated, while small ponies (SP) were slightly overestimated and more accurate. Muscle top-line score did not improve prediction (p = 0.442). Bone type significantly improved model fit (p = 0.00039), with heavy and medium weight-boned horses having lower weigh tape error compared to light weight-boned horses.
**Clinical Implications:** Weigh tapes cannot be relied upon for accurate body weight measurement, particularly in heavier horses where underestimation is greatest. This has direct clinical relevance for medication and anthelmintic dosing—underestimation of weight could lead to underdosing, contributing to anthelmintic resistance and therapeutic failure. The average error of −5.68% means that for a 500 kg horse, the weigh tape would underestimate weight by approximately 28 kg. Clinicians and owners should be aware that breed and body condition influence accuracy: the tape is more accurate in pony breeds and may be more accurate in overweight horses (BCS ≥6). Height and muscle development do not appear to affect readings. Until breed- or type-specific tapes are developed, weigh tapes should be used cautiously, and weighbridge measurements should be used whenever accurate dosing is critical.