Final answer:
The milk a calf drinks is channeled directly to the abomasum, bypassing the rumen, via the esophageal groove, which closes during suckling.
Step-by-step explanation:
When a calf is nursing, we want the milk he is drinking to bypass the rumen and go straight into the abomasum. In ruminant animals like calves, there is a structure called the esophageal groove that functions to allow milk to bypass the rumen and reticulum, which are part of the complex stomach system designed for digesting fibrous plant materials.
The esophageal groove closes upon suckling, directing the milk directly to the abomasum, where it can be digested more efficiently without being fermented by the microorganisms present in the rumen.