It is a widely held opinion that horses "need" grain.Horses have been fed grain from at least the time of the Roman Empire, probably for as long as people have eaten grains. Grain is a more concentrated source of calories than hay or pasture. It is much, much higher in starch than hay or pasture. That starch can be digested in the horse's small intestine but there is a limit to how much they can handle. In a horse that is not accustomed to eating grain, as little as 1 g of starch/kg of body weight, the equivalent of 2.75 pounds of plain oats for an 1100 lb horse, will result in spill over into the large intestine. Horses can adapt to eating high starch foods over time. In adapted horses, the recommended safe upper limit would be 5.5 lbs plain oats per meal for an 1100 lb horse. However, a recent study has shown as little as 1.3 g of starch per kg of body weight can cause inflammation and the appearance of bacterial lipopolysaccharide and bacterial lactate in the blood.
To read more on this study click here.Those changes could be prevented by feeding the horse a time-release intestinal buffer. Since the evidence of inflammation (inflammatory cytokine level) occurred only one hour after feeding, it must have originated in the stomach and/or small intestine from fermenting organisms in those areas causing pH to drop. As undesirable as inflammation and bacterial products in the blood sound, these events reverse themselves. Obviously, horses are not getting ill from this. However, until more is known about the significance or long-term consequences you should think twice about heavy grain feeding. Grain is completely out of the question for horses with metabolic syndrome. It should be minimized or eliminated for growing horses with metabolic syndrome. It should be minimized or eliminated for growing horses because of the connection with developmental orthopedic disease. Inactive and pleasure horses can also do quite well with no grain and will maintain a much healthier weight - Ideally a body condition score no higher than 5
To read more on body scoring click here.