The Topline Score (TLS) came out in other animals, primarily meat animals (hogs, cattle), to determine how much meat was on the hoof. An ultrasound machine scans the back of hogs measuring the back fat and determining the depth of meat (muscle). Few dare do this in horses as this would ruin the horse feeding industry.
Horse owners like to look at a horse with a smooth topline without the bones and ribs showing. To do this, they layer enough fat over the topline to cover it, just like we wear clothes to cover our un-muscled bodies. While this looks good, we also want our horses to be athletic. However, feeding them grain to “look good” causes muscle loss. You can get the details about protein loss in that topic.
The topline is muscle, and muscle is protein, specifically the three amino acids leucine, isoleucine and valine (the branched-chain amino acids or BCAs). Feeding sugar in the form of grains, high starch pasture and hay will never make muscle. Nor will feeding fat . These will only add fat to cover up the loss of muscle.
Topline loss starts at the withers and, over time, moves along the back to the tail head. A topline without loss is labeled “A,” and as the loss progresses towards the tail, it is labeled “B,” “C,” and “D.” Other places to look for muscle loss are the muscles between the ribs, the abdominal muscles (“hay belly”) and the cheek muscles (masseter muscles of the jaw). Muscle loss occurs first in the most active muscles (abdominal, shoulders). When feeding to improve the topline, the reverse order appears. The hay belly is the first to resolve, and the withers and cheek muscles are last to fill.
I like combining the TLS with the BCS. With practice, you can see a low TLS through a high BCS. However, once a horse’s nutrition is corrected, and the BCS reduced, the poor TLS reveals itself. The loss of body fat covering the lost muscling tests the courage and conviction of the horse owner because sometimes the muscle loss is startling and returns slowly. Thoroughly understanding the process and the reasons behind what you see will help owners work through the visual shock and achieve the goal of good BCS and a good TLS.
Below are the four grades of the Topline Score (TLS) A through D
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TLS A – Muscling throughout the top line from withers to the croup.
It is hard to find a TLS of “A” other than to look at foals and race horses. These are the athletes. Older horses competing well usually also have fat added to cover up irregularities. If you see a good-looking top line with no muscle loss at the withers, look to see that the abdomen is also tight. A topline that “looks good” while having a “hay belly” is impossible.
TLS B – muscling is lost at the withers only while the rest of the top line has muscle.
Photographing these “B” horses is difficult as this is the transition phase. If a horse starts to lose muscle at the withers, added grain covers up this loss with fat. The horse is usually young but may have access to as much food as they want. Starvation cases will skip this phase and are not noticed until there is more severe loss of fat and muscle.
TLS C – the muscle loss extends from the withers to the loin.
This loss is common but is usually not alarming to most owners. Resolving this is done by adding grain and calling the saddle fitter. Commonly seen in “hard keepers,” more grain, supplements or exercise are added, thus continuing or increasing the gut inflammation. Many accept this muscle loss as just a lack of exercise (usually from being laid up due to a lameness) when in reality, the cause is a lack of high-quality protein in the diet (specifically leucine, isoleucine and valine) and protein loss from gluconeogenesis (excess glucose in the diet).
TLS D – the muscle loss extends from the withers to the croup.
The muscle loss over the whole top line also has an extended abdomen (“hay belly”), poor hooves and other signs of protein loss. A quick look at the diet shows no high-quality protein in the food or pasture, overwork, starvation, excessive grain in the diet, or any combination of these.