Relationship of Neck Dimensions and Height in Different Breeds of Horses: Possible Connection to Equine Recurrent Laryngeal Neuropathy

Abstract

Recurrent laryngeal neuropathy (RLN) is a condition involving dysfunction of the recurrent laryngeal nerve in horses which results in paralysis of the left side of the larynx and obstruction of the upper airway. The condition has been studied extensively and previous studies have shown the prevalence of RLN increases as the height and neck length of the horse increases. It is possible an increase in neck length that outpaces height may explain the higher prevalence of RLN in taller horses. The hypothesis of the current study is that the neck length and circumference of a horse will increase as its height increases, with the relationship for neck length being consistent between breeds of horses. This study included eight different measurements. After 14 weeks of data collection, a total of 114 horses were measured including Arabians, Belgians, and Quarter Horses. Analyses of the coefficient of determination were done to determine which neck measurement had the best linear fit for height across all breeds (length and circumference done separately). As a secondary analysis, the neck length measurement with the best linear fit for height was determined for each breed. These analyses supported the hypothesis that the neck length and circumference increased as the height of the horse increased, but the relationship for neck length was not consistent between breeds of horses. The results also showed that the most reproducible neck length measurement between breeds of horses was the distance from the poll to the withers when the neck was in an upright position.

Introduction

Equine recurrent laryngeal neuropathy (RLN) is a condition involving dysfunction of the recurrent laryngeal nerve which causes paralysis of the left side of the larynx, resulting in upper airway obstruction. The recurrent laryngeal nerve is one of the longest nerves in a horse’s body, running down the neck from the larynx, wrapping around the aorta, then back up to the larynx. Recurrent laryngeal neuropathy is a common cause of poor athletic performance in horses because it causes obstruction of airflow during exercise and “roaring.” Roaring is an abnormal inspiratory noise due the airway blockage. Previous studies have shown the prevalence of RLN increases as the height and neck length of the horse increases (1). There is a possibility that the higher prevalence of RLN in taller horses may be explained by an increase in neck length that outpaces height. Studies of RLN in multiple breeds of horses have included height as a factor (2)(3). However, the previous studies of RLN used different specifications and methods to measure the horses’ neck lengths (4).
It is not clear whether the different methods of measuring the horses’ necks used in the previously published materials affected the results or conclusions of the previous studies. Two areas related to neck and height measurements that are unclear include the following: firstly if horse height is correlated with neck size/length; and secondly if the ratios of height and neck measurements are similar for various breeds of horses. It is possible the breed of horse included in the previous RLN studies may have affected the outcome.
The study’s hypothesis was that the length and circumference of the horse’s neck increase as the height of the horse increases. It was also hypothesized that the relationship for neck length will be consistent across breeds of horses. There were three independent variables involved in this study: breed, height, and sex. The various neck measurements were dependent variables. Each of the independent variables were tested with each of the dependent variables. The primary objective of this study was to examine if there is a direct relationship between height and neck measurements across breeds of horses. The secondary objectives were to determine if the relationship between height and neck length measurement are similar between breeds and to determine the most reproducible neck measurement. The limitation of this study is that it focused on measurements of horses with different characteristics, but it did not directly link the measurements in horses with RLN. To address this limitation, a calculated estimate of the length of the laryngeal nerve was evaluated. The study was conducted over a total of 17 weeks (14 weeks of data collection and 3 weeks of data analysis).

Methodology

Data from three different breeds of horses (Quarter Horses, Arabians, and Belgians) was collected. The same measurement methods were used for all breeds. The gender, approximate age, height, and eight measurements of the neck or shoulder for each horse were taken (Figures 1a and 1b). Horse height is measured in hands (one hand is 4 inches or approximately 10 cm). The height measurement was taken from the ground to the highest point of the horse’s withers using a height measuring stick specific for horses. A tape measure was used to measure the shoulder and the neck at different positions (to the nearest centimetre).
The first measurement of neck length was from the horse’s withers to the poll when the neck was in an upright position which was a comfortable head-level for the horse. The second measurement of neck length was from the horse’s withers to the poll when the neck was in a downward position (nose touching the ground). This was achieved by placing grain or hay on the ground in front of the horse. The third measurement of neck length was the neck from the throat latch to the centre of the breastbone when the neck was in an upright position. The fourth measurement of neck length was from the throat latch to the centre of the breastbone when the neck was in a downward position. The fifth measurement was of the shoulder and was taken from a point midway between the pectoral muscles to the point midway between the withers and the elbow (Figure 1a). An estimate of the length of the laryngeal nerve was calculated by multiplying the sum of the third and fifth neck measurements by two. This calculation approximates the path of the laryngeal nerve which runs down the horse’s neck from the larynx to the aorta. It then wraps around the aorta and returns up the neck to the larynx.
The first measurement of neck circumference was the circumference of the neck at the throat latch. This was measured at the area of the neck closest to the head. The second measurement of neck circumference was the circumference of the centre of the neck. The third measurement of neck circumference was the circumference of the lower end of the neck. This measurement was located where the neck connects to the chest. The three measurements of neck circumference were used as an indicator of the size of the horse (muscle or fat mass). Since these measurements are not reflective of the length of the laryngeal nerve, they were not considered when determining the most reproducible neck measurement.
Data was collected from several horse farms to ensure a variety of breeds of horses were included in the study. The goal was to collect data from approximately 100 horses. The owner of each horse signed a consent form providing permission to collect measurements of their horse. During the first week of the study a spreadsheet was created to use for data collection and the measuring devices were prepared. A time frame of 14 weeks was set aside to collect data from 11 different horse farms. A total of 114 horses were measured: 43 Quarter Horses, 24 Arabians, and 47 Belgians.
After collecting the data, three weeks were set aside to analyse the data. These analyses included calculation of the coefficient of determination (R²) for the various neck measurements to the height of the horse for the whole population (All Breeds). The coefficient of determination is the square of the correlation coefficient between the x and y values (r). It provides a measure of the amount of variance in the dependent variable (neck and shoulder measurements) explained by the independent variables (height, breed, sex). Both r and R² were calculated for the data provided in the supporting Google Sheets spreadsheet using the appropriate formulas. The coefficient of determination was also analysed by breed (Results and Appendix 1 for additional graphs) and gender (Appendix 2). The coefficient of determination indicates how closely the data fits a regression model. In this study, linear models were used.

Figure 1a: Neck and Shoulder Measurements taken in the Upward Neck Position

Figure 1b: Neck Measurements taken in the Downward Position

Results

Neck Length and Shoulder Measurements

During the collection of data and before conducting the correlation analyses, the variability of the various measurements was reviewed with Dr. Edward Robinson (veterinary advisor and retired Emeritus Professor from the MSU College of Veterinary Medicine). It was determined that Length #4 (throat latch to the centre of the breastbone in a downward position) was not accurate because not all the horses cooperated to lower their head to the necessary level. This resulted in having a shorter measurement than if these horses would have fully lowered their heads. Therefore, Length #4 was not used in the analyses.
Table 1 and Figure 2 below provide the mean and standard error for Lengths #1 (poll to withers with neck in upright position), #2 (poll to withers with neck in downward position), #3 (throat latch to the centre of the breastbone with the neck in the upward position), #5 (a point midway between the pectoral muscles to the point midway between the withers and the elbow), and the wither height.
Table 1: Mean Neck Length Measurements and Wither Height

Measurement	Number of Horses	Mean ±SE (cm)
Neck Length #1	114	96.19±0.81
Neck Length #2	107	124.97±1.31
Neck Length #3	114	53.48±0.37
Neck Length #5	114	49.43±0.58
Wither Height	114	204.63±0.57

Figure 2:

As expected, Length #2 was longer than Length #1 since it was measured when the horse’s head and neck were in the downward position. As mentioned above for Length #4, measurements taken in the downward neck position are variable because horses of different heights have to stretch their necks to different lengths to touch the ground and some horses were not cooperative. Despite the variability in Length #2, analyses were still completed for Length #2 to evaluate the difference in the coefficients of determination between the measurements of the neck in the upward position compared to the downward position.
Table 2a provides the sum of squares for the total and residual values as well as the coefficient of determination for Length #1, Length #2, Length #3, and Length #5 versus wither height for All Breeds together (n=114). Table 2b provides the coefficient of determination and the comparisons of measurements for Quarter Horses (n=43), Arabians (n=24), and Belgians (n=47) individually.
Table 2a: Sum of Squares Total, Sum of Squares Residual, and Coefficient of Determination (R²) for Neck or Shoulder Measurements vs. Wither Height—All Breeds

Comparisons of Measurements	Sum of Squares–Total	Sum of Squares–Residual	R2
Length 1 vs. Height	1063262	4332.58	0.48
Length 2 vs. Height	1690442	6642.69	0.66
Length 3 vs. Height	327889	1512.72	0.16
Length 5 vs. Height	282721	1538.52	0.63

Values calculated with appropriate formulas in supporting Google Sheets Spreadsheet.
Table 2b: Coefficient of Determination (R²) for Neck or Shoulder Measurements vs. Wither Height—by Breed

Breed, n	L1 vs. Height	L2 vs. Height	L3 vs. Height	L5 vs. Height
Quarter Horses, 43	0.34	0.59	0.16	0.0006
Arabians, 24	0.01	0.003	0.39	0.0004
Belgians, 47	0.20	0.29	0.30	0.48

Values calculated with appropriate formulas in supporting Google Sheets Spreadsheet.
For All Breeds together and for Quarter Horses, Length #2 had the best coefficient of determination versus height (r² = 0.66, 0.59, respectively) compared to the other measurements. Length #3 had the best coefficient of determination for Arabians (r² = 0.39) and Belgians (r² = 0.30). Length #5 was a measurement of the shoulder, not the neck, and was used to calculate the length of the recurrent laryngeal nerve (Appendix 3). Therefore, Length #5 was not considered when selecting the measurement with the best coefficient of determination that would later be used to identify the most reproducible neck measurement.
The most reproducible measurement for All Breeds of horses together was Length #1 (poll to withers in an upward position). This data showed that as a horse’s height increased the length of their neck did as well. A graphical presentation of Length #1 versus wither height is presented below for All Breeds (Figure 3a), Quarter Horses (Figure 4a), Arabians (Figure 5a), and Belgians (Figure 6a).
Figure 3a:

Figure 4a:

Figure 5a:

Figure 6a:

In Appendix 1, graphs of the coefficient of determination for Lengths #2 through #5 versus height are provided for All Breeds and each breed individually. Graphs of the coefficient of determination for Lengths #1through #5 versus height by gender are provided in Appendix 2. In Appendix 3, the coefficient of determination for the calculated nerve length (using Lengths #3 and #5) versus height for All Breeds, each breed individually, and gender are provided.

Neck Circumference

Table 3 provides the mean, standard error of the mean, and coefficient of determination for Circumference #1, #2, and #3 versus wither height. The data is presented for All Breeds together (n=114).
Table 3: Coefficient of Determination (r²) for Neck Circumference Measurements vs. Wither Height—All Breeds

All Breeds, n=114	Circumference #1	Circumference #2	Circumference #3
Mean ± standard error (cm)	86.93 ± 0.94	110.24 ± 1.58	146.89 ± 2.03
Coefficient of Determination
	C1 vs. Height	C2 vs. Height	C3 vs. Height
	0.69	0.55	0.57

As was observed for the neck length measurements, the horse’s height increased as the circumference of the neck did as well.

Discussion

This research was intended to determine if the height of the horse is correlated to the length and circumference of the neck. The hypothesis of this study was that the length and circumference of the neck would increase as the height of the horse increased. The relationship for neck length was hypothesized to be consistent across breeds of horses. Overall, the data supported the hypothesis that the length and circumference of the neck increased as the height of the horse increased. However, the relationship for neck length to height was not consistent between breeds. Every measurement of neck length except Length #2 for Arabians had a linear increase as the height increased (Appendix 1, Figure 5b). This may have been due to the fact that there was relatively small variation in the height of the Arabians. In addition, Arabians are fine-breed horses that have more curvature to their neck than those of stock breeds (Quarter Horses) or draft horses (Belgians).
Previously published studies have shown that as the height and neck length of the horse increases the prevalence of equine RLN also increases (1). The previous studies used different methods to measure the horses’ neck lengths (4). Height has been used as a factor in multiple studies of RLN in different breeds (2, 3). The data from this study supported that Length #1 (Figure 1a) was the most reproducible measurement when compared to height across breeds of horses (Table 1). The coefficient of determination for Length #1 was the highest, meaning it had the best linear fit in relationship to height.
Measurements of neck length taken in the downward position (#2 and #4) seem to be less accurate for a couple of reasons. Different heights of horses have to stretch their necks to different lengths to be able to touch the ground. In addition, some horses were uncooperative when trying to get their neck in the downward position. This resulted in the downward neck measurements (Figure 1b) being more variable than the measurements taken in the upward position. Therefore, the upward neck measurements (Length #1 and Length #3) had better linear relationships for neck length and height increasing together.
In the current study, analyses of calculated nerve length versus height (Appendix 3, Table 5a-b, Figures 10-12c) also supported the findings from previous studies. The estimated nerve length had a strong linear relationship with height (Figures 10-12c). The estimated length of the laryngeal nerve was calculated by multiplying the sum of Length #3 and Length #5 by two. The recurrent laryngeal nerve runs in a similar path as these measurements. This supported the hypothesis that the laryngeal nerve length increases as height increases. Recurrent laryngeal neuropathy has been found in previous studies to be more likely to occur in taller horses (4).
There were limitations to this study. One of these limitations was some horses were uncooperative while being measured in the downward position. In general, the Arabians were unable to stretch their necks all the way to the ground. This caused the data for Length #2 and Length #4 (Figure 1a-b) to not be an accurate representation of different breeds of horses’ necks in a fully outstretched downward position. Another limitation of this study was that it focused on measurements of horses with different characteristics, but it did not directly link the measurements in horses with RLN.
The findings from this study can be used to help future researchers of RLN. This research can inform future studies on how to measure the necks of the horses used to examine the laryngeal nerve. Length #1 was shown to be the most reproducible measurement between the three breeds of horses measured in this study. This knowledge can be applied by using this measurement method to consistently measure neck length in studies of RLN.

Conclusion

Equine recurrent laryngeal neuropathy (RLN), or paralysis of the left side of the larynx, is a common cause of airway obstruction and poor athletic performance in horses. This study’s hypothesis was that the length and circumference of the horse’s neck would increase as the height of the horse increased. It was also hypothesized that the relationship for neck length would be consistent across breeds of horses. In addition, the study sought to determine which neck measurement was the most reproducible in horses. After measuring 114 horses (24 Arabians, 43 Quarter Horses, and 47 Belgians), it was found that the length and circumference of the neck increased as the height of the horses increased. The lack of variability of height in Arabians may have contributed to the linear relationship of neck length and height not being consistent with the findings for Belgians and Quarter Horses. This may also have been due to the increased curvature of the neck in Arabians. The most reproducible measurement was from the poll to the withers when the neck was in an upward position (Length #1). These findings should help further the research of RLN studies by providing a reproducible measurement method to use in different breeds of horses.

Acknowledgements

Data was collected from horses at Baldwin Quarter Horses (Quarter Horses), Beckey’s Place (Quarter Horses), Northfork Farms (Quarter Horses and Arabians), Perniciaro Farms (Quarter Horses), and Tom Robertson Quarter Horses (Quarter Horses). Dr. Edward Robinson, a veterinary advisor and retired Emeritus Professor from the MSU College of Veterinary Medicine, arranged for the collection of measurements from Arabians at the Michigan State University Horse Teaching and Research Centre and Belgians from five Amish farms in Camden, Michigan.

Works Cited

1. Mcgivney, C. L., K. F. Gough, B. A. Mcgivney, G. Farries, E. W. Hill, and L. M. Katz. “Exploratory Factor Analysis of Signalment and Conformational Measurements in Thoroughbred Horses with and without Recurrent Laryngeal Neuropathy.” Equine Veterinary Journal 51, no. 2 (2018): 179–84. https://doi.org/10.1111/evj.12984.
2. Boyko, Adam R, Samantha A Brooks, Ashley Behan-Braman, Marta Castelhano, Elizabeth Corey, Kyle C Oliveira, June E Swinburne, et al. “Genomic Analysis Establishes Correlation between Growth and Laryngeal Neuropathy in Thoroughbreds.” BMC Genomics 15, no. 1 (2014): 259. https://doi.org/10.1186/1471-2164-15-259.
3. Brakenhoff, Jeffrey E., Susan J. Holcombe, Joe G. Hauptman, Holly K. Smith, Frank A. Nickels, and John P. Caron. “The Prevalence of Laryngeal Disease in a Large Population of Competition Draft Horses.” Veterinary Surgery 35, no. 6 (2006): 579–83. https://doi.org/10.1111/j.1532-950x.2006.00192.x.
4. Brooks, S. A., S. Makvandi-Nejad, E. Chu, J. J. Allen, C. Streeter, E. Gu, B. Mccleery, B. A. Murphy, R. Bellone, and N. B. Sutter. “Morphological Variation in the Horse: Defining Complex Traits of Body Size and Shape.” Animal Genetics 41 (2010): 159–65. https://doi.org/10.1111/j.1365-2052.2010.02127.x

Appendix 1

Graphs of the Coefficient of Determination in Results
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Appendix 2

Coefficient of Determination by Gender
Table 4: Coefficient of Determination (r²) for Neck Lengths vs. Height– by Gender

Gender, n	L1 vs. Height	L2 vs. Height	L3 vs. Height	L5 vs. Height
Mares, 71	0.45	0.63	0.13	0.65
Geldings, 37	0.55	0.73	0.20	0.54
Stallions, 6	0.55	0.76	0.46	0.99

This table shows that Length #2 has the best coefficient of determination for the geldings (r² = 0.73), and Length #5 has the best coefficient of determination for the mares (r² = 0.65) and the stallions (r² = 0.99). However, Length #2 was taken in the downward position, so it was not considered when determining the most reproducible measurement. Length #5 was a measurement of the shoulder and was only measured to be able to calculate the length of the laryngeal nerve. Therefore, the most reproducible measurement across genders was Length #1 (poll to withers in an upward position). This data showed that for all three genders, as a horse’s height increased, the length of their neck did as well.

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Appendix 3

Calculation of Estimated Laryngeal Nerve Length
Figure 10 provides the mean height and estimated laryngeal nerve length for mares, geldings, and stallions.
Figure 10:

The data shows the mean height and estimated length of the laryngeal nerve (sum of Length #3 and Length #5 multiplied by two) was the greatest in stallions and was the smallest in mares. Overall, male horses were taller and had a longer laryngeal nerve than female horses.
Table 5a: Coefficient of Determination (r²) for Estimated Laryngeal Nerve Length vs. Height–All Breed and by Breed

Breed, n	Estimated Laryngeal Nerve Length vs. Height
All Breeds, 114	0.56
Quarter Horse, 43	0.26
Arabian, 24	0.22
Belgian, 47	0.49

The coefficient of determination for estimated laryngeal nerve length was the highest for All Breeds (r² = 0.56) compared to the breeds individually.
Table 5b: Coefficient of Determination (r²) for Nerve Length vs. Height– by Gender

Sex, n	Nerve Length vs. Height
Mare, 71	0.58
Gelding, 37	0.57
Stallion, 6	0.85

Stallions had the highest coefficient of determination (r² = 0.85) for estimated laryngeal nerve length compared to mares and geldings.
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Appendix 4

Pictures of Taking Measurements

Measuring the height of a Quarter Horse with the stick at Northfork Farms

Measuring Length #1 on a Quarter Horse at Northfork Farms

Measuring Circumference #1 on an Arabian at the Michigan State University Horse Teaching and Research Centre

Measuring Length #5 on an Arabian at the Michigan State University Horse Teaching and Research Centre

Measuring the height of a Belgian at an Amish farm in Camden, Michigan

Measuring Circumference #3 on a Quarter Horse at Tom Robertson Quarter Horses

About the Authors

Katherine and Elizabeth West are 17-year-old Seniors at Williamston High School in Williamston Michigan. Both are members of their school’s Math and Science Academy, Mu Alpha Theta club, and Student Government. Elizabeth plays Varsity Basketball and Katherine plays Varsity Tennis. Katherine and Elizabeth show Quarter Horses and are the Michigan Quarter Horse Youth President and Vice President.