How much does that horse weigh?

Abstract

Knowledge of body weight can be critical when making management decisions. But data are often difficult to obtain. The use of scales is increasing, but scales are not yet accessible to many horse owners. Milner and Hewitt1 wrote “the confidence of many horsemen in their ability to judge the weight of a horse by eye is very much misplaced.” Johnson et al2 found that horsepersons and veterinarians were likely to significantly underestimate the weight of horses by visual appraisal. What about body measurements? Body measurements have been used to estimate body weight for a wide variety of animals including fish,3 squid,4 ostriches,5 elephants,6 whales,7 and extinct primates.8 Selva and Downing9 have compiled from published literature an impressive list of means and ranges of adult body weights of animals. They obtained data from nearly 60% of the extant species of mammals. They also developed equations for converting body length to body mass for 13 orders of mammals. Body measurements have been used to estimate weight of domestic animals such as cattle,10 water buffalo,11 goats,12 and donkeys.13 Several equations have been developed for horses. The earliest published equation of which I am aware (wt [kg] Girth3 (meters) 80) was published in 1964 by Marcenac and Aublet.14 It is highly likely that other equations were available earlier. Milner and Hewitt1 in 1969 reported that the formula they developed by using 108 horses (wt Girth2 length over 228.1), with weight measured in pounds and girth and length measured in inches, had an average absolute percentage error of only 5.6%. It was concluded that the use of length in addition to heart girth size increased accuracy. Carroll and Huntington15 developed equations for estimates of body weight from body measurements and body condition score. They constructed nomograms for estimation of body weight from condition score and height measurements and from girth and length measurements. It was concluded that “condition scoring can be used as a repeatable and objective method of assessing a horse’s body condition. This is essential for instituting appropriate feeding management and monitoring the effect of different feeding regimes. It can be used with height estimation or measurements to provide a useful weight estimate that is almost as accurate as that derived from the girth and length formula or nomogram.” The body score condition system used by Carroll and Huntington had a range of 1 to 5 rather than 1 to 9 in the system developed by Henneke et al,16 which is widely used in the United States. Ellis and Hollands17 reported that the accuracy of estimation could be improved by also considering height of horses. They recently reported a study with 2000 horses representing 112 breeds and type.18 Each horse was weighed, and heart girth, height, and body condition were measured. Body weight was estimated with 4 tapes that were commercially available in England. Two of the tapes were height specific, and 2 were for general use. Significant differences were found among the tapes. One tape was “unable to provide a reasonable estimate of the weight of horses weighing more than 750 kg and one tape was of little value for horses weighing more than 560 kg.” One tape was more accurate than the others for horses under 14.2 hands. It was concluded that methods for estimating weight can be improved by taking into account variables such as height or breeds. It was also concluded that even though the tapes in which height was considered produced very useful information, further improvement is needed. It was suggested that body condition, breed, and age should be investigated further so that more accurate methods can be established. In summary, heart girth was one of the first body measurements used to estimate body weight of horses. Estimates of body weight were improved by including length and height. Weight tapes have been useful, but it is likely that improvements can be made by taking into consideration body condition score, breed, and perhaps function and age.