Failure to meet the minimum forage requirement of 1.5% of the horse’s bodyweight and the opportunity for foraging for a minimum of 8 h a day (not going without this opportunity longer than four to five consecutive hours) can have both physiological and behavioural consequences. To provide an energy source for horses, rations often include starch rather than fibre. This can result in health issues related to the gastrointestinal tract (GIT) in the horse. In the stomach, the main concern is equine gastric ulcer syndrome (EGUS) and, more specifically, equine squamous gastric disease (ESGD). Ulcerations are caused either by increasing acidity in the stomach (from starch ingestion and reduced saliva production) or splashing of acidic juices caused by a lack of a forage barrier prior to exercise or prolonged periods without fibrous feed intake, which allows the stomach to collapse and spread acidic gastric fluids into the upper squamous regions of the stomach. In the hindgut, starch that has escaped digestion in the small intestine causes microbial instability and increased production of volatile fatty acids (VFA) and lactic acid. This puts horses at great risk for acidosis and subsequent laminitis. Shifts in the hindgut microbiota will also affect a horse’s behaviour via the gut-brain axis, as well as potentially compromise immune function. Reduced fluid intake caused by reduced saliva production can result in colic. Choosing a fibrous alternative for starch in a high-energy diet greatly reduces the risk of EGUS and acidosis and improves digestion, GIT pH, body condition, behaviour, immune functions, and performance. Providing hay can reduce crib-biting, wood-chewing, coprophagia, the consumption of bedding, aggression, and stress, and subsequently increase social bonding and affiliation with conspecifics. Adequate fibre intake is related to reduced clinical signs of EGUS, reduced reactivity, and better adaptation to weaning. Lignophagia (wood chewing) has also been observed in horses that are foraging, and this is thought to reflect low fibre content in the available forage (for example, early vegetative, lush pasture).

Racehorses are often supplemented extra iron with the expectation that the iron will improve overall performance and health. A survey of 120 U.S. Thoroughbred trainers, representing 1978 Thoroughbreds from various regions of the U.S., was conducted to determine the average amount of dietary iron fed to Thoroughbred racehorses per day. Survey results indicated racehorses were fed an average of 3900 mg of iron per day from hay and grain alone. This exceeds the 0.8 mg/kg BW or 400 mg for a 500 kg working horse that the NRC 2007 recommends per day. Supplements increased the daily average intake of iron by an additional 500 mg Fe. Some equine nutritionists propose that excess dietary iron may be a causative factor in insulin resistance (IR). However, the occurrence of IR in Thoroughbred racehorses is very rare. This study did not find one confirmed veterinary diagnosis of IR in any of the surveyed trainers’ Thoroughbred horses, whether racing, on a layoff, or retired. Given the iron content in these diets easily exceeds the NRC minimum daily requirements, it seems unlikely that dietary iron is an independent causative factor in IR.