Weight estimation in horses and ponies using a commercial Horse weight tape
Summary
Background: Accurate weight estimation is critical when dosing drugs, especially
anthelminitcs and antibiotics due to the rise in resistance against these drugs. Weight is
often assed visually but there are tools, equations and weight tapes to aid owners and
veterinarians.
Aim: The aim of this study is to analyze the accuracy of the Pavo and Virbac Horse Weight
Tape®
Methods: The bodyweight of twenty Horses and twenty ponies (12 >100 cm and 8 <100
cm) was measured using the commercial weight tape and a weighbridge. Horses and ponies
were divided into groups based on their height at the withers. Statistical analysis was
performed using a paired T-test and regression analysis.
Results: Only the pony group (>100 cm) had a significant difference between the weight
tape and weighbridge. On average the horses and ponies (>100 cm) where underestimated
by -0.92 % (± 3.64) and -1.64 % (± 4.64) respectively. The small ponies where overestimated
by 14.30 % (±15.95).
Conclusion: The weight tape is accurate for horses and ponies higher than 100 cm. For small
(Shetland) ponies it is not accurate. Therefore we advise to use correction formulas as given
in this article.
Introduction
There is a large range in sizes and weights of equids, small ponies can weigh less than 100 kg
while large draft horses can weigh over a 1000 kg. Accurately measuring or estimating the
bodyweight is therefore very important when prescribing veterinary drugs. Underestimation
of bodyweight will result in under-dosing of drugs. This might result in lower plasma
concentrations of drugs and therefore less effective treatment. Under-dosing is also a
predisposing factor for the development of resistance against drugs, mainly antibiotics and
anthelmintics. (Eysker et al. 2006; Larsen et al. 2011; Waller 1987; Nielsen et al. 2010)
Especially the resistance against anthelmintics is a growing problem in equids as shown by
several reports concerning this problem. (Kaplan et al. 2004; Larsen et al. 2011; Nielsen et al.
2010; Traversa et al. 2009; Waller 1987)
Unfortunately most veterinarians and horse owners do not have access to a weighbridge in
field conditions. That is probably the reason why most owners and veterinarians rely on
visual estimation (Asquith et al. 1991)(Fritzen et al. 2010; Von Samson-Himmelstjerna et al.
2009). Research by Asquith et al showed that weight is often underestimated.(Asquith et al.
1991) Their research group consisted of 253 horsemen, of which 62 veterinarians, estimating
the weight of 5 horses. On average 80% of the horsemen underestimated the weight by a
mean of 16%. Even more worryingly is the fact that 91% of the 62 vets underestimated the
weight by on average 18%. Overall 20% of the horsemen overestimated the weight by 8%.
There are several companies that have introduced weight tapes to aid horse owners with
estimating weight more accurately. Girth length is the basic parameter used in almost every
weight tape or equation (Ellis and Hollands 2002) and has been combined with multiple
other parameters like body length (Jones et al. 1989; Reavell 1999)(Milner and Hewitt 1969),
height at the withers(Carroll and Huntington 1988), body condition score (Carroll and
Huntington 1988; Kienzle and Schramme 2004), abdominal girth (Jones et al. 1989; Milner
and Hewitt 1969) body contours (Hale et al. 2011). One of the weight tapes distributed in
the Netherlands is the Pavo and Virbac Horse Weight Tape®. This tape uses only the chest
girth to predict the bodyweight. The purpose of this study is to test how accurate this tape is
in horses and whether it is accurately enough to be used on ponies.
Materials and Methods
Horses and ponies
Twenty horses and twenty ponies were selected, all stabled or visiting the department of
Equine Sciences of Utrecht University. The horses and ponies were divided into two groups
based on height at the withers. According to Dutch standards a horse was included in the
horse group if it had a height at the withers of 159 cm or more. Everything smaller than 159
was considered a pony. The ponies were subdivided into two groups, group A were eight
ponies below 100 cm and group B twelve ponies between 100 cm and 159 cm.
Various breeds were used, the horse group consisted of 19 Royal Dutch Sport Horses and
one Friesian. The Friesian was 153 cm high but considering breed and build we decided to
include it in the horse group. The pony Group A consisted of eight shetland ponies. Group B
consisted of 2 Welsh, 2 Haflinger, 2 New forest, 1 Arabian, 1 Icelandic, 1 Fjord, 1 Connemara,
1 Tinker and 1 Appaloosa pony. The mean age, height and weight of all groups are
summarized in Table 1.
Table 1: Mean age, height and weight of all groups
Mean age
Mean height
Mean weight
Horses
10.3 years
(± SD 4.7)
163 cm
(± SD 5)
566 kg
(± SD 67)
Ponies overall
13.1 years
(± SD 5.5)
112 cm
(± SD 28)
331 kg
(± SD 153)
Ponies group A
12.5 years
(± SD 3.7)
91 cm
(± SD 9)
173 kg
(± SD 44)
Ponies group B
13.42 years
(± SD 6.6)
143 cm
(± SD 11)
437 kg
(± SD 95)
Table 1: Mean values and standard deviations of age, height and actual weight of all four groups of
horses and ponies.
Protocol
All horses and ponies were weighed using the commercial weight tape 1. Using an aluminum
horse height measuring stick2 the height at the withers was determined to the nearest
centimeter. Finally the horses were weighed to the nearest kilogram on the weighbridge3.
Statistics
All data was collected into Microsoft Excel 20034 and the statistical analysis was done using
SPSS 16.05. The relative difference between the tape and the weighbridge was calculated.
This is the difference between the estimated and actual weight divided over the actual
weight. A paired T-test was used to determine if there was a significant difference between
the actual and estimated weights. A regression analysis was preformed between the actual
and estimated weight to calculate correction formulas. Also the correlation between the
height at the withers and the relative error of the tape was calculated
Results
There was no significant difference between the estimates from the Horse Weight Tape®
and the weighbridge in horses, ponies overall and ponies group B (>100 cm). There was a
significant difference in the ponies group A (Shetland ponies <100 cm). The ponies group B
and horses were on average underestimated by -1.64 % (± 4.64) and -0.92 % (± 3.64)
respectively. As opposed to the ponies group A which were overestimated by on average
14.30% (± 15.95). All the data is summarized in Table 2. The correlations between the real
and estimated weights are displayed in Figure 2.
Horses
Actual weights ranged from 497 kg to 738 kg, the mean actual weight was 566 kg (± SD 67).
There was no significant difference between the estimated and actual weights. The
calculated correction formula for horses is: Real weight = 0.963 x estimated weight + 26.3.
Ponies overall
The actual weights ranged from 101 kg to 560 kg, the actual mean weight was 331 kg (± SD
153 kg). There was no significant difference between the estimated and actual weights. The
calculated correction formula for the ponies overall is: real weight = 1.099 x estimated
weight -36.470.
Ponies group A
The actual weights ranged from 101 kg to 235 kg, the actual mean weight was 173 kg (± SD
44 kg). There was a significant difference (P<0,05) between the estimated and the actual
weights. The calculated correction formula for the group A ponies is: real weight = 0.843 x
estimated weight + 9.103.
Ponies group B
The actual weights ranged from 198 kg to 560 kg, the mean being 437 kg (± SD 95 kg). There
was no significant difference between the estimated and actual weights. The correction
formula for the group B ponies is: real weight = 1.048 x estimated weight -11.426
Table 2: Results of all four groups of horses and ponies
Horses
Ponies overall
Percentage underestimated
60 %
50 %
Mean underestimation
Percentage overestimated
Mean overestimation
Average difference between
estimate and real weight
Average relative difference
Pearson correlation between
estimate and real weight
Group A
25 %
Group B
66.7 %
-3.08 %
(± SD 1.93)
30 %
-3.68 %
(± SD 3.30)
50 %
-2.25%
(± SD 2.51)
75 %
-4,04 %
(± SD 3.51)
33,3 %
3.08 %
(± SD 3.38)
-5.50 kg
(± SD 20.02)
-0.92 %
(± SD 3.64 )
0.955
(P<0.01)
13.15 %
(± SD 13.84)
3.20 kg
(± SD 25.85)
4.73 %
(± SD 13.05)
0.990
(P<0.01)
19.81 %
(± SD 14.45)
21.50 kg
(± SD 21.71)
14.30 %
(± SD 15.95)
0.884
(P<0.01)
3.15 %
(± SD 2.04)
-9.00 kg
(± SD 21.18)
-1.64 %
(± SD 4.64)
0.976
(P<0.01)
Table 2: Results of all groups summarized, all mean values are presented with their standard
deviations. The significance for the pearson correlation was set at P<0.01).
Correlation between height and the estimate
The correlation between the height at the withers and relative difference between the
weight tape and weighbridge was 0.786 (P<0.05). This correlation is represented in figure 1.
Figure 1: Relation between the height at the withers and the relative difference between the
estimated weight and the real weight.
Relative difference between weight tape and
weighbridge VS height at the withers
Relative difference
between actual and
estimated weight (%)
50
40
30
20
10
0
-10 50
100
150
200
-20
Height at the withers (cm)
Horses and ponies
Trendline
Figure 1: The vertical axis is the relative difference between the actual and real weights. The
horizontal axis is the height at the withers. The trendline shows a quadratic relation with a
correlation coefficient of 0.786 (P<0.05).
Figure 2: The actual weights versus the estimated weights
Actual weight (kg)
Actual weight VS estimated weight
800
700
600
500
400
300
200
100
0
0
200
400
600
800
Estimated weight (kg)
Horses
Ponies group A
Ponies group B
Figure 2: The actual weight as measured by the weight bridge plotted against the estimated weight.
The trendlines represent linear relations between the actual and estimated weight in the different
groups.
Discussion
From the presented data it can be concluded that the weight tape will be more accurate in
horses than in ponies. This conclusion can be drawn from the lower average relative
difference between the weight tape and weighbridge in the horses when compared with all
the ponies. But it appears that this difference was mainly caused by the small Shetland
ponies. That is why the pony group was divided into two subgroups. This revealed that group
A is the only group in which there was a significant difference between the estimates and
actual weights. The small ponies from group A were mostly overestimated by the weight
tape, on average by almost 15%. This as opposed to the ponies in Group B in which the tape
was almost as accurate as it was for horses. This phenomenon can be seen in figure 2 where
the ponies group A trendline is lower and has a slightly different slope compared with the
other two groups. What causes this inaccuracy in small ponies is unknown. Height is most
likely to have some effect upon the accuracy of the weight tape, since there is a cubic
relation (R= 0.786) between height and the average deviation. But this correlation does not
explain all of the differences found. Breed and build might also have an effect on the
accuracy of the weight tape. Unfortunately there were no other small breeds (< 100 cm)
available than Shetland ponies. It might be interesting to repeat this setup with a larger
number of ponies of varying breeds en heights.
It was unexpected that the correlation between the real and estimated weight was lower in
horses than within the ponies overall and group B. It was opposed to all other data that
indicated that the weight tape was most accurate in horses. This high correlation in ponies
overall and group B might have been caused by the higher range in bodyweight which makes
outliers appear smaller in group B.
It should be noted that the weight tape has the tendency to underestimate the weight in
horses slightly as demonstrated by the average relative difference, the percentage of
underestimation. The ponies higher than 100 cm at the withers seem to be slightly
underestimated as well. This is opposed to the ponies in group A in which the weight tape
has the tendency to overestimate the bodyweight. But the four different calculated
correction formulas should take this into account. The correction formulas already separate
horses and ponies from different sizes and should correct the under- or overestimation of
the Horse Weight Tape®.
As covered in the introduction, underestimation leads to underdosing of drugs. This
underdosing is a predisposing factor for resistance against anthelmintics and antibiotics. This
growing problem can benefit from more accurate weight estimations. Bearing in mind the
results from Asquith et al. (Asquith et al. 1991) it is advisable for veterinarians to critically
asses their own weight estimation accuracy. This can be done by estimating the bodyweight
of horses at the practice and compare this to a weighbridge or the Pavo and Virbac weight
tape applying the correction formulas presented in this study. Furthermore it is advisable for
veterinarians to promote the use of the Horse Weight Tape® and its correction formulas by
owners. Nielsen et al. advised to add 15% to the estimated weight, using a weight tape,
when dosing anthelmintics (Nielsen et al. 2010). But results from our study indicate that this
could lead to overdosing, particularly in small ponies. Therefore we would like to advise
veterinarians and owners to use the correction formulas presented in this study. These
correction formulas will increase the accuracy of the estimate from the Horse Weight Tape®.
For veterinarians in the field, unable to use the corrections formulas it is advisable to bear in
mind that the weight tape is accurate in horses and ponies higher than 100 cm. But it has the
tendency to underestimate the bodyweight in horses and ponies higher than 100 cm, as
shown in table 2. For small (Shetland) ponies the weight tape is not accurate and has the
tendency to overestimate the bodyweight. Whether this is due the height or breed is
unknown. Therefore we advise veterinarians and in particular owners to use the weight tape
and apply the correction formulas or as a rule of thumb add 10% to the estimate when
dosing anthelmintics for horses or ponies higher than 100 cm. When dosing anthelmintics
for small (Shetland) ponies, smaller than 100 cm, it is advisable as a rule of thumb to
decrease the estimate from the weight tape by 10% to prevent overdosing.
Manufactures adresses’
1a: Pavo / Virbac Horse Weight Tape®: Stimulan/Pavo, Boxmeer, the Netherlands
1b: Pavo / Virbac Horse Weight Tape® : Virbac, Barneveld, The Netherlands
2: Aluminum horse height measuring sticks (<100cm and >100cm): Master Cheng Enterprise
Co., Ltd, Taipei, Taiwan
3: Grupo Epelsa BCN 100M: Grupo Epelsa, Barcelona, Spain
4: Microsoft Excel 2003: Microsoft corporation, Redmond (WA), USA
5: SPSS 16.0, IBM, Armonk (NY), USA
References
Asquith, R. L., Johnson, E. L., Kivipelto, J., and Depew, C. (1991) Erroneous weight estimation
of horses. Proceedings of the Annual Convention of the American Association of Equine
Practitioners. 599-607
Carroll, C. L., Huntington, P. J. (1988) Body condition scoring and weight estimation of
horses. Equine Vet. J. 20, 41-45
Ellis, J. M., Hollands, T. (2002) Use of height-specific weigh tapes to estimate the bodyweight
of horses. Vet. Rec. 150, 632-634
Eysker, M., Van Doorn, D. C. K., Lems, S. N., Weteling, A., and Ploeger, H. W. (2006) Too
often deworming in horses is mostly ineffective and leads to anthelmintic resistance.
Tijdschr. Diergeneeskd. 131, 524-530
Fritzen, B., Rohn, K., Schnieder, T., and von Samson-Himmelstjerna, G. (2010) Endoparasite
control management on horse farms - lessons from worm prevalence and questionnaire
data. Equine Vet. J. 42, 79-83
Hale, C. E., Hemmings, A. J., and Harber, C. (2011) The accuracy of weight estimation
equations for horses using two methods of body measurement. 145
Jones, R. S., Lawrence, T. L., Veevers, A., Cleave, N., and Hall, J. (1989) Accuracy of prediction
of the liveweight of horses from body measurements. Vet. Rec. 125, 549-553
Kaplan, R. M., Klei, T. R., Lyons, E. T., Lester, G., Courtney, C. H., French, D. D., Tolliver, S. C.,
Vidyashankar, A. N., and Zhao, Y. (2004) Prevalence of anthelmintic resistant
cyathostomes on horse farms. J. Am. Vet. Med. Assoc. 225, 903-910
Kienzle, E., Schramme, S. C. (2004) Body condition scoring and prediction of body weight in
adult warm blooded horses. Pferdeheilkunde. 20, 517-524
Larsen, M. L., Ritz, C., Petersen, S. L., and Nielsen, M. K. (2011) Determination of ivermectin
efficacy against cyathostomins and Parascaris equorum on horse farms using selective
therapy. Veterinary Journal. 188, 44-47
Milner, J., Hewitt, D. (1969) Weight of horses: improved estimates based on girth and length.
Canadian Veterinary Journal. 10, 314-316
Nielsen, M. K., Fritzen, B., Duncan, J. L., Guillot, J., Eysker, M., Dorchies, P., Laugier, C.,
Beugnet, F., Meana, A., Lussot-Kervern, I., and von Samson-Himmelstjerna, G. (2010)
Practical aspects of equine parasite control: A review based upon a workshop discussion
consensus. Equine Vet. J. 42, 460-468
Reavell, D. G. (1999) Measuring and estimating the weight of horses with tapes, formulae
and by visual assessment. Equine Veterinary Education. 11, 314-317
Traversa, D., Von Samson-Himmelstjerna, G., Demeler, J., Milillo, P., Schürmann, S., Barnes,
H., Otranto, D., Perrucci, S., Di Regalbono, A. F., Beraldo, P., Boeckh, A., and Cobb, R.
(2009) Anthelmintic resistance in cyathostomin populations from horse yards in Italy,
United Kingdom and Germany. Parasites and Vectors. 2,
Von Samson-Himmelstjerna, G., Traversa, D., Demeler, J., Rohn, K., Milillo, P., Schurmann, S.,
Lia, R., Perrucci, S., Di Regalbono, A. F., Beraldo, P., Barnes, H., Cobb, R., and Boeckh, A.
(2009) Effects of worm control practices examined by a combined faecal egg count and
questionnaire survey on horse farms in Germany, Italy and the UK. Parasites and
Vectors. 2,
Waller, P. J. (1987) Anthelminthic resistance and the future for roundworm control. Vet.
Parasitol. 25, 177-191