Dr Nerida Richards
Cereal grains like corn are an important part of a racing thoroughbred’s diet. Cereal grains contain starch, which is digested and absorbed by the horse as glucose. Glucose is the muscle fuel of choice for fast sprinting and is essential to allow a horse to build and replenish muscle glycogen supplies. If muscles run out of glycogen they fatigue quickly, so providing glucose from the diet is critical to maintain performance.
Corn is something you will find in nearly every racing stable. In fact, a survey of racing thoroughbred feeding practices (Richards et al 2006) found that over 70% of trainers fed corn with an average of 1.7 kg being fed per horse/day. Corn is an excellent source of energy and a rich source of starch for horses.
The ‘Dark Side’ of corn
While corn contains more starch than other grains (corn is 70% starch on average, while barley is 60% and oats just 40% starch), the starch in corn is not well digested by horses. Less than 30% of the starch contained in whole or cracked corn is digested by enzymes in the small intestine and absorbed as glucose. The rest travels undigested to the hindgut where it is rapidly fermented by bacteria. The volatile fatty acids (VFA) produced during this fermentation process are absorbed by the horse and used for energy, so the horse does still gain some benefit, however the fermentation of starch in the hindgut has many downsides, including:
- Lactic acid and VFA accumulation – rapid starch fermentation favours bacteria that produce lactic acid and large amounts of VFAs. These acids accumulate in the hindgut and cause the gut environment to become acidic in a condition known as ‘hindgut acidosis’. Horses with hindgut acidosis will often have soft droppings that have an acidic or pungent smell.
- Loss of appetite – the starch fermenting bacteria produce an enzyme called thiaminase that destroys vitamin B1. Vitamin B1 is crucial for maintaining appetite. Horses with hindgut acidosis will often have poor appetites and experience weight loss.
- Reduced fibre fermentation – the acidic hindgut environment will either kill off or inactivate the bacteria that ferment fibre from hay and chaff. Hay and chaff form an important source of energy for racing horses. Loss of the ability to digest fibre leads to weight loss and may also further exacerbate vitamin B deficiencies.
- Laminitis – hindgut acidosis is well known to cause laminitis.
- Behavioural changes – horses with hindgut acidosis will also exhibit changes in behaviour including chewing timber and eating bedding. It is possible that acidosis also causes nervy behaviour in some horses.
The Australian thoroughbred industry survey showed that more than 90% of corn fed to racing thoroughbreds was fed either cracked or whole. 27% of horses had a faecal pH of less than 6.2, indicating hindgut acidosis was occurring in more than a quarter of horses surveyed.
The only feed related factor that could be attributed to the low faecal pH in these horses was the amount of corn fed to that horse each day. The more corn a horse was fed, the lower its faecal pH dropped. In other words, corn was causing hindgut acidosis.
Is corn off the menu?
Not at all! Corn is and will always remain a valuable feed ingredient for racing horses. The trick to feeding it is to make sure it is fed cooked. Cooking corn rearranges the starch to make it much more digestible in the horse’s small intestine. Cooking corn and changing its main site of digestion to the small intestine instead of the hindgut will provide horses with more glucose to fuel muscles and removes the negative impact uncooked corn has on the hindgut.
Corn can be cooked by extrusion or micronising processes. Both cooking methods use heat to change the structure of the starch to make it easy to digest. While both methods of cooking provide a far superior feed when compared to cracked or whole corn, extruded corn is the most digestible.
In combination with heat, extrusion also uses moisture and pressure to cook the corn, resulting in more digestible starch. Studies (Richards 2003) have shown that more than 70% of the starch from extruded corn is digested by small intestinal enzymes in 15 minutes compared to just under 35% of starch from micronised corn and less than 15% from cracked corn.
Starch Digestion Rates
What does this all mean?
Cereal grains are an important part of a racing thoroughbred’s diet, providing energy in the form of glucose to fuel muscles, especially during fast work. However, grains like corn are not well digested in a horse’s small intestine so feeding them uncooked ultimately leads to the rapid fermentation of starch in the hindgut, hindgut acidosis and its related problems including loss of appetite and weight loss. Cooking corn changes the structure of the starch it contains making it almost 5 times more digestible than uncooked corn starch. The end result, racing horses will get more energy from the corn with less chance of any negative impact. So if you are going to feed corn, feed it cooked!
The Pryde’s EasiFeed range of racing feeds contain fully extruded corn and barley for superior digestibility, together with other high quality extruded ingredients like full fat soybean and lupins, blended with cold pressed canola oil and honey. With feeds and supplements to suit any preferred method of feeding, contact Pryde’s EasiFeed for a full assessment of your race feed regime. Call 1300 732 267 or email info@prydes.com.au