That option usually depends upon their future. Yearlings that will be placed in the breeding herd or in performance at a later date can graze pasture during the summer without being fed grain or concentrates. Owners often ask what are the best protein feeds for yearlings. Alfalfa hay is fed to yearlings in place of grass hays because of its higher protein and lysine content. Lysine is the most limiting amino acid in growing horses.
Soybean meal is a common protein source in horse rations, especially for young, growing horses.
It is a high-quality protein and a cheaper protein source that linseed or cottonseed meals, especially when protein quality is considered. Half the fillies were fed bermudagrass hay and a corn-based concentrate supplemented with soybean meal. The other fillies received a similar corn-concentrate feed without soybean meal, but with alfalfa hay. Total protein, lysine, and energy content of both rations was similar. There were no differences between the two groups in their weight gain or growth in wither height, hip height, length, heart girth, forearm, gaskin, or cannon bone.