There has been a big campaign by lots of feed companies and people against starch in the past decade and I’m not entirely convinced the rationale is correct, let alone the science.
I’ve recently seen some very misleading statements about starch and energy from feed and supplement companies. These statements are aimed at “scaring” horse owners away from starch and selling low-starch “safer” products.
Starch is high in cereals such as oats, barley, wheat, maize, rice and low in forages (most) and zero in linseed, soyabean and sunflower seeds. It may be a surprise that it is also zero in molasses!
And yes, feeding a lot of starch is not good or necessary for a horse at maintenance or in light work. For horses in moderate to hard work, it is not good to feed starch in a few, large meals. It is not good to feed starch on an empty stomach; you should always try to feed after forage. This is because forage slows the passage of starch-based feeds along the small intestine and reduces the chance of undigested starch reaching the large intestine where fermentation can lead to increased acidity and hindgut disturbance, possibly even leading to colic and/or laminitis. We also know that meals with a high starch volume can contribute to gastric ulceration.
Also, please note I said “a lot” of starch. I’m referring to the amount. It is not the concentration of starch (grams of starch per kg of feed) that is the issue but the absolute amount (grams) fed per day.
As an example, if you have a feed that is 40% starch (40g per 100g of feed), for example oats, and feed 100g in total of that feed you are feeding your horse 40/100*100 = 40g of starch. If you feed something else which is 2.5% starch but feed 10kg, for example grass hay, then you are feeding 2.5/100*10000 = 250g of starch. Very obvious, I’m sure but often misunderstood or forgotten!
Horses store energy in two ways, either as fat or as carbohydrate (sugar units chemically joined together). Sugar units linked together in plants is called starch and sugar units linked together in animals (including horses) is called glycogen.
Around 90% of glycogen is stored in the muscles and 10% in the liver. Around 10% of fat is stored in the muscles and 90% in other sites around the body.
Horses rely on glycogen as their main energy source for all types of exercise, whether that is jumping, dressage or running short and fast (e.g. racehorses) or longer and slower (e.g. endurance horses). Performance in all these sports is dependent on muscle glycogen.
We have many different pieces of evidence that indicate that horses in moderate to hard work require starch to maximise muscle glycogen stores and for optimal performance. For example, ask any racehorse trainer if he can train racehorses on a high fibre and high fat, low starch diet. Those that have tried it soon revert to starch! Whilst a high forage low starch diet alters their metabolic response to exercise it also results in lower muscle glycogen stores before exercise.
A Swedish study published in 2012 (Jansson and Lindberg, 2012; https://pubmed.ncbi.nlm.nih.gov/22717208/) showed that feeding a low starch and high fibre diet altered the metabolic response to exercise and may have some health benefits (compared with a traditional high starch diet) but the downside was a lower muscle glycogen level before exercise. Everything we know points to a relationship between muscle glycogen and performance – low muscle glycogen, low performance and vice versa.
Several studies have demonstrated that horses on low starch diets have lower muscle glycogen than those on diets containing a moderate to high amount of starch. And another study showed that diets low in NSC (non-structural carbohydrate) can slow the rate of muscle glycogen repletion (restoration) after exercise (Mesquita et al. 2014; https://beva.onlinelibrary.wiley.com/toc/20423306/2014/46/S46)
So starch in the diet may carry risks, but eliminating it totally for horses in more than moderate work may also carry a risk … of reduced performance. And this effect would be highest in the sports that use most glycogen: short explosive and fast sport such as racing, eventing, show-jumping, carriage driving and longer duration sports such as endurance.