In some instances rumen capacity is reached before an animal has consumed enough forage to meet energy demands. This phenomenon can be seen when a high fiber hay is fed to cows with high energy demands such as cows nursing calves. While the cow's energy requirement increases her potential intake, her intake becomes limited due to the low digestibility and rate of passage of the hay. The rumen becomes full of hay before enough hay has been consumed to meet energy demands. The result is full cows that lose body condition.
Forages are good sources of vitamins A and E and minerals.
Depends on what your needs are. Dairymen require more precise measurements than a beef cow-calf producer. For the majority adding TDN will be beneficial. Table 1 should help you further understand the quality of your hay once tested. Quality standards for legume, grass, or grass legume hay.