Lizzie Drury answers:
Pica is the desire to eat unusual substances that possess little or no nutritional value, such as bedding, dirt, hair and faeces.
Mineral or vitamin imbalances are often cited as the cause of pica, but this is not always the case. Horses have been found to have a true appetite for only three things – energy, salt and water.
Horses do not typically express nutritional imbalances as pica, and so they do not seek ways to rectify nutritional imbalances.
Wet or dry?
Coprophagy (eating manure) is more often seen in young horses, particularly foals, but if it is observed in older animals, fibre and roughage intake is usually the issue.
Although you say that your horse is eating endless amounts of haylage, does it have a good dry-matter level or is it a fairly wet haylage?
Haylage that has a high water content will have a low fibre content, which will mean that the horse will ‘crave’ more roughage, which is essential for a healthy, functioning digestive tract.
I would firstly recommend that you weigh your haylage, so that you know exactly how much of it he is eating. If he has any left over in the morning, then weigh that, too. You could try supplementing his haylage intake with a good-quality chaff, which will increase his dry-matter intake.
Does he get plenty of turnout? If he has restricted turnout, then it may just be that he is bored in the stable. Therefore, regular turnout and socialisation with other horses may help alleviate the current problem.
It would also be a good idea to check his concentrate diet with respect to feeding levels. You mention that you are feeding Weight Gain with some other feeds. What are these other feeds?
It could be that you are feeding too much of the wrong types of feed and causing undigested starch to reach the hindgut. This could lead to a pH reduction caused by lactic acid build-up, so perhaps your horse is just feeling a little uncomfortable.
If your horse’s diet has been carefully designed to provide adequate nutrients and if he is allowed maximal time in the field with peers, the likelihood of him being simply inquisitive or bored far outweighs the notion that something is amiss in his diet.