To ingest is to take a substance into the body by swallowing it through the mouth. In horses, ingestion is the primary route by which feed, water, oral medications, supplements, and environmental toxins enter the digestive system. Once a substance is ingested, it passes through the intestinal absorption downstream for absorption or excretion. The rate and completeness of ingestion influence drug bioavailability, caloric uptake, and toxic load.
Horses that ingest sand or soil while grazing on sparse sand and toxin ingestion from pasture accumulate abrasive material in the ventral colon, a leading cause of sand colic. Voluntary feed ingestion slows during illness, dental pain (see dental pain slowing feed intake), or high ambient temperature; monitoring daily dry-matter intake is therefore a core husbandry practice. Involuntary ingestion of toxic plants is a common emergency scenario requiring immediate veterinary assessment.
Further Reading
The biological process of ingestion is explained in the ingestion article on Wikipedia. How ingestion relates to equine digestive health is addressed in the Merck Veterinary Manual section on Digestive Disorders of Horses.