Your horse escaped, found, opened and emptied that grain bin but he still looks fine; is this an emergency? It very well may be a critical and time sensitive, life-threatening emergency. Possible consequences of this grain overload are diarrhea, colic, colitis, endotoxemia, metabolic acidosis and laminitis (founder).
There are several factors that will influence the progression of events after such an overeating incident. The amount of concentrated feed consumed, the type of concentrated feed consumed, and the horse's individual metabolism all play a part in the outcome. Large quantities of high grain diets without conditioning of the horse's metabolism can lead to an overload of soluble carbohydrates. Although, as mentioned, there are no hard and fast rules for how much grain will trigger these unwanted consequences, I usually become concerned if a horse consumes more than twice their normal feeding of a high carbohydrate diet. The high carbohydrate load in these cases "overwhelm" the small intestines ability to process and absorb and result in a high percentage of carbohydrates entering the large intestine.
Carbohydrates entering the large intestine effect the normal microbial flora of the large intestine, rapid fermentation of the carbohydrates by certain bacteria lead to high levels of lactic acid in the "fermentation vat". This increased level of lactic acid has many consequences including a change in osmotic gradient, change in ph and multiple subsequent metabolic changes. Major changes in the proportion of bacterial flora also occur. Increase in the growth rate of the lactic acid producing bacteria and a dying off of gram-negative bacteria due to the overwhelming acid environment is typically seen. Intestinal inflammation and even ulceration can occur as these events progress.
As the events of a grain overload proceed systemic endotoxemia (toxicity), caused by the dying off of gram-negative bacteria and their release of toxins, and sepsis (infection) may overpower all of the horses normal digestive metabolism. These horses can become very ill, very quickly and in some cases even heroic efforts to save the horses life are futile.
With any case of carbohydrate overload, early treatment and intervention is key to a successful outcome. In mild cases the only treatment that might be needed is to hold the patient off of all feed for some period of time and then to exclude grains from the diet for several days. In other cases treatment with oil or charcoal administered via a nasogastric tube and anti- inflammatory are indicated. In more advanced cases, intravenous fluids, bicarbonate and calcium therapy, oral cathartics, broad-spectrum antibiotics may be added to the treatment regimen.
Many times we are faced with a recent invasion of the grain room or grain bin and it is unclear exactly how much grain was ingested and at exactly what time. As you can imagine this makes it very difficult to recommend the most conservative of treatment, which is to hold off of feeds, since we never know which animal may react poorly to the carbohydrates and progress with clinical signs. So as a rule if your horse or horses have had a grain "party" I typically recommend at least a mineral oil dessert.