Ideal hay drying weather would be a couple of days of sunny breezy weather, but Mother Nature doesn't work that way. Farmers have to constantly watch the weather and be their own weathermen to decide when are the best conditions to bale hay. Sometimes, due to the weather, there is not choice but to let the hay mature more before it is cut. Growing hay is not an exact science but a gamble and an art. Uncontrollable factors sometimes change all the best laid plans.
More protein is stored in the leaves than the stem.
This is dependent on the stage of maturity. The fiber makeup of the plant is also changed with maturity. The leaves of the plant contain the most digestible portion. As the plant gets more mature more lignin is deposited in the stem. The most important thing is to keep your rabbit eating grass hay.