Page 130 - September 2022

VETERINARY VIEWS
MUSCLING THE TOPLINE
by Nancy S. Loving, DVM
Typically, horses are scored in their body condition on a scale of 1-9 by how much
flesh covers ribs, withers, and buttocks.
Body condition score (BCS) looks at the effect of calories on fat coverage. Yet, another area of the body – the topline – deserves similar attention and a topline evaluation system complements the body condition score by evaluating muscle development.
Two tissues cover the topline – fat and/or muscle. Before a horse’s body lays fat along the topline, he must reach a BCS score of 7, which
is considered overweight. In contrast, muscle development of withers, back, loins, croup and hindquarters form the visible topline of horses that aren’t riddled with fat. Often there is performance loss before visual changes are recognized within the topline. It is noteworthy that a horse may
have an ideal BCS yet is deficient in the topline muscles, often due to protein deficiency.
Nutritional technology as relates to muscle development has the potential to provide proactive support to maximize a horse’s performance. Topline evaluation is a tool with a grading system:
• Grade A = ideal
• Grade B = sunken in the back area between
the vertebrae, and the top of ribs is concave
• Grade C = sunken in the back and loin area
• Grade D = sunken in the back, loins, and croup
Atrophy of topline muscles begins in the withers, then the back and gradually extends through the loins and croup and down
into the hip and stifle. Rebuilding muscle occurs in reverse order. Exercise activates muscle conditioning while nutrition provides building blocks in the form of amino acids. To demonstrate the importance of proteins (amino acids), once all water is removed
from a muscle sample, it is comprised of 73% protein and 22% fat.
To build strong muscle, a horse needs all 10 essential amino acids supplied in the diet. The first three in this list are the limiting amino acids that aren’t always provided
in sufficient amount in the diet and can’t
be synthesized by the body, thus limiting muscle growth.
• Lysine – for growth and is the first limiting
amino acid for young horse growth
• Threonine – for older horse repair and
maturation
• Methionine – for hoof and hair
• Valine, Leucine, and Isoleucine – branch-
chain amino acids (BCAA) important for
muscle recovery
• Phenylalanine and Tryptophan – a building
block for proteins as well as being a
precursor for neurotransmitters • Histidine and Arginine – used in
biosynthesis of proteins
The rate of muscle protein synthesis is fastest during growth, and this rate declines as the horse ages. If there is an inadequate amount of any of these essential amino acids in a horse’s diet, protein synthesis occurs only to the level of the limiting amino acid. For example, if a diet contains 125% lysine, 110% methionine, 101% threonine, and 80% tryptophan, then the horse will synthesize all the amino acids into protein at 80% of its potential.
As an example of one essential amino acid important for growth, to meet the daily lysine requirement for a horse, he would have to be fed 34 pounds of corn, 15-32 pounds of grass hay, 2.2 pounds of soybean meal, or one pound of whey protein. Percentage of protein on a feed tag does not give the full picture because it doesn’t break down the individual amino acid content on the label. It is also important to look at practical consumption rates as well as exercise level – intense training doubles protein needs above maintenance. Forage is not just filler, and the best benefits are realized by feeding a high-quality product that provides some proportion of protein.
While exercise conditions muscle, it does not make muscle. Muscle synthesis and repair requires the essential amino acids as well as replacement of nutrients a horse uses while working. A well-balanced diet includes added amino acids for muscle support and these are best administered within 45 minutes after
Atrophy of topline muscles begins in the withers, then the back and gradually extends through the loins and croup and down into the hip and stifle. Rebuilding muscle occurs in reverse order.
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