Final answer:
The stalk of the corn plant is the least digestible to beef cows grazing residues during winter due to its high fiber and structural carbohydrates content.
Step-by-step explanation:
The least digestible part of a corn plant to beef cows grazing residues during winter is d. the stalk.
Corn residue is a common feed for beef cows during the winter, and it typically consists of the leftover parts of corn plants after grain harvest. These parts include the grain, husk, leaf, and stalk. While the grain is the most nutritious part and highly digestible, the stalk is made up of fiber and structural carbohydrates such as cellulose and lignin which are harder for cows to digest. Animals can digest the husk and leaf portions better, but the stalk provides the lowest digestibility due to its tough and woody structure. Digestibility is crucial for livestock, especially in the winter when they need to maintain body heat and condition. Therefore, managing the grazing of corn residues is important to ensure that cows receive adequate nutrition.