Name the primary dietary source of protein for ruminants.

Alfalfa hay is recognized as a primary dietary source of protein for ruminants due to its high protein content and palatability. It is a legume forage that not only provides essential proteins but also offers other nutrients, including fiber, vitamins, and minerals, which are critical for the overall health and productivity of ruminant animals such as cows, sheep, and goats. The protein in alfalfa is readily digestible, making it an efficient feed source for supporting the metabolic needs of these animals, especially during lactation and growth phases.

Additionally, while other options contain protein, they typically do not match the protein quality and quantity found in alfalfa hay. Corn silage is more focused on providing energy, barley is a cereal grain with lower protein levels than alfalfa, and wheat bran also contains protein but in lesser amounts and may not be as efficiently utilized by ruminants compared to alfalfa. Therefore, alfalfa hay stands out as a superior choice for protein supplementation in ruminant diets.