By: , and Cargill Animal Nutrition

A horse’s topline — the muscles that support the spine, from neck to hindquarters — plays an important role in how a horse performs, looks and feels.

Awareness about this topic, however, is limited. Those who do understand the need for a healthy topline have likely heard a lot of lore and conflicting information about how to achieve it. Exercise, saddle fit, genetics and age are most frequently blamed for a poor topline. Nutrition plays the most critical role, and is often overlooked as a solution to build and maintain the topline. This paper defines equine topline, outlines a method to accurately evaluate it, and provides nutritional recommendations for topline improvement.

What is Topline?

Topline, simply put, is the muscle groups that run along a horse’s spine. The topline of a horse stretches along the vertebral column (spine) from the end of the neck at the wither area, down the back and loin, and over the top of the hip into the croup region. Three main muscle groups surround each side of this boney column. The longissimus dorsi is attached to the pelvis, the entire thoracic vertebrae and the last four cervical vertebrae. The latissimus dorsi attaches the upper and mid back vertebrae to the lower lumbar vertebrae. The thoracic trapezius attaches the neck and mid back vertebrae to the shoulder blade. Topline can be affected to some extent by conformation, specifically as it pertains to the angle of the hip and shoulder.

Topline’s Impact on Performance

Because topline is a muscling topic, its connection to performance is directly linked to how a horse builds and utilizes muscle. Muscle makes up 40-55 percent of a horse’s mature body weight1. Protein, which is comprised of amino acid (AA) chains, makes up more than 90 percent of tendon, hair, hoof, and skin tissues, approximately 73 percent of muscle tissue, and 30 percent of bone on a dry matter basis. The bottom line is that protein—or more specifically amino acids—are integral to the structure and function of tissues throughout the body.

Muscle tissue is largely responsible for movement and locomotion. Larger muscles allow for higher, powerful output potential2. Muscle also tends to be one of the most adaptable tissue types in the body. Age, training type and intensity, and nutrition all have a significant impact on the properties of muscle fibers. The horse’s topline, along with the abdominal muscles, should function similarly to the human core (abdominal and lower back musculature). Especially when performing work and under saddle, these muscle groups have to support weight while providing forward propulsion and collection.

Making sure these muscle groups are fully developed with ideal nutrition and a proper training and conditioning program helps ensure the horse can perform to their genetic potential.

Seven out of 10 AAEP veterinarians believe topline is key to the horse’s well-being.

A research trial conducted in partnership with Cargill and the University of Minnesota (unpublished data) demonstrated the influence of nutrition on percent body fat and lean muscle mass in 12 mature, sedentary, light breed horses. Horses were transitioned from a lower plane of nutrition to a pelleted concentrate containing optimal calories balanced with a superior amino acid model, mineral and vitamin nutrition. By altering only the concentrate portion of the horse’s diet, an average 7.5 percent increase in loin muscle depth and an average 10.2 percent increase in estimated percent body fat from day 1 to day 96 of the trial was observed. Ultrasonography was used to quantify the depth of the loin (longissimus dorsi) muscle, for estimation of percent body fat3.

Exercise will condition (impact) muscle that is present, but work by itself does not build or maintain muscle. To fuel, repair, build and recover muscle, equine diets must optimally contain a superior amino acid profile. Urschel and colleagues demonstrated that dietary amino acids are the main drivers of protein synthesis in horses. Following a fasting period, horses either continued to have feed withheld, or received a high protein meal at time 0 and then 30 minutes later. Horses that were fed demonstrated increased markers of protein synthesis in blood and muscle biopsy samples; no work was required to initiate this response4.

Veterinarians Agree

Topline also is a key indicator of overall horse health, according to equine veterinarians. A survey of American Association of Equine Practitioners (AAEP) found seven out of 105 believe adequate muscling surrounding/supporting the horse’s spine – the topline – is key to the horse’s well-being. Of those same vets, 62 percent believe a healthy topline is key to fewer injuries.

Awareness of Topline

While awareness of topline is moderately high among consumers, there is room for more education. The majority of all respondents in a recent survey6 understood topline has ramifications on a horse’s health and appearance, and believe topline can be addressed through proper diet and exercise. The research also shows many horse owners do not realize nutrition is the single most important factor in achieving a healthy topline.

Topline Myths

Exercise:
One of the most common misperceptions about topline is that it can be improved through exercise alone. Research6 shows that horse owners are more likely to believe changes in exercise – rather than diet – positively influence topline. Poor topline development and definition are often blamed on lack of exercise – or incorrect exercise.

While exercise alters existing muscles, the nutritional building blocks of muscle (essential amino acids) must be present in sufficient quantities and balanced with adequate calories to rebuild or augment muscle tissue. In fact, if a horse is worked hard but lacks sufficient amino acids and calories in its diet, existing muscle mass can shrink.

Just like human athletes, athletic equine partners need more essential amino acids than sedentary horses to maximize the effects of training and allow the horse to look and feel its best. Both a deficient amino acid pool and an increase in strenuous work have been shown to increase muscle turnover in horses7 and humans8. Evidence in the scientific literature, reviewed by Zeyner, 20109, indicates young-mature and aged horses in light work, supplemented with higher intakes of essential amino acids, had bigger increases in muscle circumference10 and/or maintained muscle mass better than un-supplemented control groups. These horses also demonstrated a simultaneous decrease in body condition score11 (fat cover).

Genetics:
Another common misperception is that genetics contribute to a poor topline. Many owners believe that poor topline is inherited, and thus inherent, so they see no options for improvement. While genetics can play a small role in a horse’s topline, it is minor compared to the impact of nutrition. Genetics do not limit an individual horse from reaching its full, healthy topline potential. Individual horses will express muscle based on their genetic potential, but every horse can develop adequate muscle to support their topline.

Body Condition (Fat Cover):
A major misconception is that the topline is made up of fat, when in fact, it is mostly muscle. Loss of topline is often referred to as a weight loss; however it is important to identify whether the loss of body weight is due to inadequate caloric intake (determined via BCS), inadequate intake of essential amino acids (determined via Topline Evaluation System), or both.

A lack of condition along the withers, back, loin and croup, indicative of a poor topline, is the result of little to no fat deposition (inadequate caloric intake), poor muscling (amino acid deficiency or imbalance), and, in severe cases, muscle atrophy. Several factors can contribute to muscle loss along the topline including age, workload (overtraining), injury and, most importantly, the horse’s diet. Inadequate amino acid intake, provision of a poorly digestible source of amino acids, and/or imbalance of calories and essential amino acids can all contribute to a poor topline. It is important to note that muscle loss and fat cover must be evaluated separately. There are instances when horses are over-conditioned but malnourished. In other words, horses are overweight but lack muscle. Understanding the horse’s caloric status and amino acid status will tell a much more complete story of what needs to change to achieve optimal body condition and topline.

Evaluating Topline

The Topline Evaluation Score (TES) allows horse owners to easily grade their horse’s topline. This evaluation system assigns a score to help determine the stages of topline development.

TES breaks the topline into three sections:

  1. Withers and mid back
  2. Loin
  3. Croup area

Begin by visually examining the horse in these three areas. If any areas appear sunken in on the sides of the spine, improvement is needed.

Horse owners should also observe whether musculature along the spine is adequate. An ideal topline can be described as well-muscled, displaying a full and rounded athletic appearance, lacking concave or sunken-in areas, providing ability for sustained self-carriage. This region of the horse is a good visual indicator of the whole body amino acid status. Concave, or topline areas that appear “sunken-in,” are never acceptable. Flat areas may be acceptable based on breed and/or genetics, and some breeds/genetic traits may exhibit a bulging muscle around the spine.

The topline of the horse is predominately muscle. However, once a horse gets to a BCS of 8 and above (considered obese), the subcutaneous fat layer over the topline musculature becomes visible. The goal is to maximize the topline musculature without adding fat.

Hands-On Identification

While a visual examination is a good tool to evaluate topline, the addition of a hands-on evaluation is recommended. Visual examinations alone can be misleading, especially with winter hair coats.

Follow these steps to conduct a hands-on evaluation.

Step 1. Place the palm of your hand on the side of the horse’s withers. Does it fall inward? If so, some muscle is gone. If it remains flat, depending on the breed/horse, the amount of muscle may be adequate or can still use improvement. If your hand flexes outward there is adequate muscling in that area, unless the horse is obese. When palpating, note the presence of muscle or fat (muscle will feel firm, while fat is spongy).

Step 2. Place your fingertips on the horse’s backbone with your palm facing downward, toward the ribs. Use the same assessment above to evaluate the muscles.

Step 3. Follow the same process for the horse’s loin and croup.

Assessing Your Horse’s Topline

EVALUATION CRITERIA

To evaluate a horse’s topline, refer to the visual descriptions in the chart below. Then assign a grade for each area. Add up the number of areas that are adequate-to-good to determine your horse’s TES grade.

All 3 areas adequate to good = TES score of A
2 of 3 areas = B
1 of 3 areas = C
0 of 3 areas = D

Topline Score: A - Ideal

This Horse Has Ideal Muscle Development:

  • The topline muscles are well developed in all three areas, the spinal processes cannot be seen, and the muscles blend smoothly into the ribs
  • The wither/back and loin of the horse is full and well rounded
  • The croup and hip are full and the stifle muscle is well defined


Topline Score: B

The Sides of the Wither are Concave, as is the Back Between the Vertebrae and the Top of the Ribs:

  • The loin muscles are well developed and are the same height as the spinal process
  • The croup and the hip muscling is adequate; pelvis to point of hip is rounded


Topline Score: C

The Wither/Back and Loin Areas, Between the Vertebrae and the Ribs, are Concave:

  • The ‘spinal process’ in the loin area is higher than the muscles beside it and can easily be seen and palpated
  • Muscles over the croup and hindquarters are well developed and rounded


Topline Score: D

The Entire Topline, Including the Wither/Back, Loin, and Croup Areas, are Concave:

  • The croup appears pointed at the top since the vertebrae and hip bones are higher than the concave muscles in between them
  • In a severely affected horse, the width of its stifle is narrower than the width of the point of hip

Improving Topline

One of the key ways to impact topline is with the right nutrition comprised of quality protein. Since the topline is comprised of muscle, any nutrition that influences muscle will influence the topline.

Every protein has a unique amino acid sequence that determines its structure and function. Due to the prevalence and importance of protein in the horse’s body, it is critical to ensure a horse’s diet contains enough energy (calories) and trace minerals, and provides the right quantity of high-quality protein and essential amino acids to support optimal health and performance. Furthermore, the amino acids must be readily digestible so they are available to the horse for incorporation into tissues and used in biochemical processes.

Indigestible sources of amino acids, and/or inadequate intakes of the essential amino acids, will eventually result in a deficiency that becomes visibly evident. It is important to remember that not all dietary proteins are created equal. Simply consuming a higher crude protein feed or hay may have limited influence on a horse’s topline. It is the quality of the crude protein that is important.

Twenty-two natural amino acids have been identified, and 10 of these are essential to the horse’s diet. Absorption of amino acids occurs in the small intestine, therefore, the essential AA must be consumed in feed, in adequate amounts and specific ratios, to ensure availability and proper utilization. The 10 essential amino acids are:

  • Phenylalanine
  • Valine
  • Threonine
  • Tryptophan
  • Isoleucine
  • Methionine
  • Histidine
  • Arginine
  • Leucine
  • Lysine


Typically, a full list of amino acids is not included on feed tags, so it can be hard for horse owners to know if the essential amino acids are included, much less if the ratio and amount is in balance. Choosing a high quality feed that shows information on amino acid inclusion is a good first step, however it must be noted that bioavailabilty is not noted on the tag. Consumers can look for the most frequently listed amino acids – lysine, methionine, threonine and tryptophan – as a starting point in identifying a feed that will provide a high-quality source of protein. If feed lacks balanced amounts of any of the essential amino acids, topline development and other tissues in the body may be compromised.

Another collaborative trial between Cargill and University of Minnesota showed proper topline levels can be maintained while a horse’s body weight decreases. The study evaluated muscle mass (longissimus dorsi depth) in eight Quarter Horses undergoing a weight loss regimen. Horses averaged a BCS of 7 at the beginning of the trial, and were fed a cool-season grass hay that supplied 60 percent of their digestible energy (calorie) requirement. The forage was paired with a pelleted ration balancer that provided superior amino acid profile and balanced mineral and vitamin nutrition, fed at manufacturer’s recommendations. All horses safely lost body weight (P<0.0001) over the 21-day period, however longissimus dorsi depth, determined via ultrasonography, and subjective TES scores were maintained (P≥0.25) throughout the trial despite weight loss13.

Availability of Amino Acids

Not all crude protein is created equal. Research estimates small intestine digestibility of protein from forage is 20 to 37 percent14 and digestibility of protein from cereal grains and soybean meal is 50 to 75 percent15. Another trial demonstrated that despite similar intakes of crude protein, differences in the sources of protein and ultimately the quality of protein resulted in less than adequate amino acid intake16. This has also been demonstrated in growing horses17.

Certain ingredients supply essential amino acids better than others. For example, intake of only 3⁄4 of a pound of a milk protein source, or 1-1/2 pounds of soybean meal, or 5 pounds of a typical alfalfa hay separately will meet the same daily lysine requirement of an average maintenance horse. However, it will take over 20 pounds of average quality grass hay or 22 pounds of corn to supply the same lysine intake to meet the maintenance requirement for that same horse. Poor quality grass hays containing a lot of indigestible fiber bulk cannot meet a horse’s lysine need because the animal physically cannot eat enough of it in a day.

Poor quality grass hays containing a lot of indigestible fiber bulk cannot meet a horse’s lysine need because the animal physically cannot eat enough of it in a day.

A horse that is in any stage of work, reproduction or growth will require more lysine and other amino acids than the maintenance horse previously discussed. Therefore, the horse in work will require higher levels of these ingredients, to the point that most grass hays and/ or cereal grains cannot come close to meeting the required nutrient intake to support optimal growth, development and performance, ultimately leading to a loss of topline and compromising tissue integrity and performance potential.

To improve musculature and equine topline, in many cases a horse needs higher-quality feedstuff sources like alfalfa, soy or milk-based ingredients in the diet. These high-quality protein sources provide essential amino acids in reasonable feeding levels to allow for proper muscle development and maintenance, particularly when under stress.

Identifying Feeds with Topline in Mind

With the multitude of horse feeds on the market today, it is challenging to select the right one for developing and/or maintaining the equine topline. Although good quality forage is a critical component of any horse’s diet, depending on forages alone is not a good practice to ensure proper amino acid supply in the diet, particularly for performance, aged, young and growing horses.

A commercial ration should be added to the diet and fed according to tag specifications to ensure that the requirements of horses are met, depending on body weight and work level. Horse owners can look for the levels of limiting amino acids guaranteed on the feed tag (in guaranteed analysis) to ensure that the horse’s needs are met.

Conclusion

Equine topline as an indicator of overall horse health should not be overlooked – nor should the key role that balanced nutrition, specifically quality proteins, play. Adequate intake of feeds that contain sufficient levels of the building blocks of proteins – amino acids – are essential to build and maintain the muscles that make up the equine topline. Topline assessments, paired with body condition scoring, should be done regularly by horse owners to help identify any potential loss of topline and overall condition, so a feeding program can be implemented to help correct any deficits.

A good start to improving topline is to feed a product, according to the tag directions, that guarantees amino acids in the right amounts – and then let the horse’s improved topline development verify that it’s working.


1 Gunn, 1987
2 Hettinger and Muller, 1953; Kearns et al., 2002
3 Westervelt et al., 1976
4 Urschel et al., 2011
5 AAEP equine veterinary study commissioned by Nutrena, 2016
6 PadillaCRT, 2015
7 Gallagher et al., 1999
8 Blomstrand and Saltin, 1999
9 Zeyner, 2010
10 Koslowski and Liebert, 2009; Koslowski et al., 2009
11 Graham-Thiers and Kronfeld, 2005
12 CVET MAY/JUNE 2016
13 Glunk et al., 2015
14 Gibbs et al., 1988
15 Gibbs et al., 1996
16 Graham-Thiers and Bowen, 2011 17Ott et al., 1979