The stage of maturity at which forages are cut (whether it be 1st, 2nd, or 3rd cutting) has a major influence on the quality of that forage. Forage crops generally decline in nutritive value as they mature.
As forage plants mature, it is typical for an increase in Acid Detergent Fiber (ADF) to occur. ADF is the percentage of highly indigestible plant material in forage comprised of cellulose and lignin. A low ADF value indicates greater digestibility and therefore "better quality" hay.
ADF values are important because they reflect the ability of an animal to digest the forage. As ADF increases, digestibility usually decreases.
Neutral Detergent Fiber (NDF) is the percentage of cell wall material in the hay that is partially available to the animal and is made up of cellulose, hemicellulose, and lignin. As the NDF percentages increase, the dry matter intake will generally decline (meaning the animal will consume less).
NDF is very important because it estimates that fraction of forage that, if it is to be used by the animal, must first be broken down by gastrointestinal microorganisms.
Lignin is a non-carbohydrate substance that is the main factor influencing the digestibility of plant cell wall material. As lignin increases, digestibility, intake and animal performance usually decrease, and the percentage of ADF and NDF increases. Simultaneously there is a decline in the Crude Protein (the total amount of nitrogen in a forage indicative of its ability to meet an animal's protein needs). Thus, relative feed value (RFV), an index that ranks forages relative to the digestible dry matter intake, declines with maturity.
The complex carbohydrates that are in hay include hemicellulose, cellulose, and lignin, forming the cell wall of the plant. This provides fiber in the animal's diet and is important to a healthy gastrointestinal system. The solubles or digestible part of the hay is primarily the cell contents.
As the plant matures, the hemicellulose changes to cellulose and is not as digestible, which leads back to the timing of the cutting being a critical factor in the quality of the hay. It is not so much the cutting (1st, 2nd, 3rd, etc.,) but the maturity of the plant at the time it is harvested. If the hay is cut when relatively immature it is darker green in color, higher in nutrients, more digestible, with a higher dry matter intake potential. But given more growing time that same hay will be more mature with fewer nutrients, larger and coarser in texture, have a higher cellulose content and will be less digestible, and dry matter intake potential will drop significantly; it's all a matter of timing and what Mother Nature allows you to do.
Leaves of both grasses and legumes contain a much greater concentration of digestible nutrients than do stems, as stated above. Therefore, as the proportion of leaves to stems becomes higher with each successive cutting (1st versus 2nd versus 3rd), it is easy to understand how the nutritive quality of the successive cuttings becomes higher. There are some sources out there that are circulating incorrect information that the 1st cutting of any forage is the highest in nutrition, having a higher nutritive value than 2nd, 3rd, or any successive cuttings. This is really not possible if all the variables are equal. The only possible manner in which this could occur would be that the 2nd and 3rd cutting forage being compared were both allowed to become far too mature so as to decrease digestibility, nutrition, and intake, while being compared to a pre-bloom 1st cutting forage of the same type (alfalfa, Orchardgrass, Timothy, Mix, etc.).