Since alfalfa and timothy are two totally different plants I'll just address the timothy. The first cutting Timothy is allowed to head out and therefore is a much more mature hay. ADF fiber is harder to digest because it contains a higher portion of lignin. Lignin is a part of the cell was that makes up the stem. But that's just my thoughts.
Second cutting is put up in cool weather.
The regrowth is leafier with the stems being finer. Now IF the second cutting had time to mature it would get stemmier. However it barely has time to grow a foot before cool weather sets in and retards growth. Second cutting has a lower stem to leaf ratio. This is the fiber that goes into the cecum.