Let's say for the sake of comparison, that you are feeding your animal a portion of a 2nd cutting hay that was cut when the plant was relatively immature, the nutrition and digestibility are high, and the cellulose content is low. Your animal would be able to digest more of the nutrients out of this hay and would achieve maintenance or weight gain depending upon the size of the servings and the dynamics of this particular animal and his environment/work load.
If this same field of hay had been allowed to grow for another week, the indigestible portion of the plant would have increased, making the hay less nutritious and less palatable, and the amount of digestible energy that the animal is able to extract from that hay is decreased. It would now become necessary to feed a larger size portion of this more mature hay to achieve the same maintenance or weight gain.
If the field was allowed to mature too long, it may not be possible for the animal to gain weight on this particular type of hay due to the fact that the amount of digestible energy he is able to absorb from the hay has decreased significantly, and his dry matter intake (amount he can physically consume) is not sufficient to allow for enough hay to be consumed for gain.
This digestibility factor is the determining factor as to how much "good" your animal will get out of a particular type of hay. You can feed a larger volume of hay, that is low on the digestibility scale, and keep the animal at his ideal body weight (or even experience weight loss); but if feeding a highly digestible, high in nutrients hay, one would obviously need to feed a smaller amount to maintain the animal at his ideal body weight.
Within the confines of each cutting (1st, 2nd, 3rd) it is possible to have varying percentages of "digestibility" depending upon the stage of maturity of the plant at the time of harvesting. Therefore, we take issue with those that out-of-hand discount a 1st cutting of hay stating it will always be too "stemmy or coarse".
In our opinion, nothing could be further from the truth - in fact, it is our belief that many animal owners could benefit from feeding a good quality, relatively immature 1st cutting hay. The nutritional and digestibility level is usually more consistent with the needs of the typical pet animal, and the added benefit to the animal's gastrointestinal and digestive tract of this higher fiber percentage can be invaluable.
If you have fed a beautiful, dark green, leafy 2nd or 3rd cutting hay and your pet has experienced diarrhea for more than just a few days until he adjusted to the hay, there is a good chance your pet could benefit from the binding qualities of the higher fiber content of the 1st cutting hay, as long as it was cut before it became too mature.