Moisture loss from mown forage is initially quite rapid. It occurs primarily through the stomata (microscopic pores) that are concentrated on the leaves and, to a lesser extent, the stems.
Most of the water loss from both grasses and legumes is from the leaves, although some moisture (up to 30% in grasses) is drawn from the stems and evaporates through the leaves.
After the forage is cut, the stomata usually close to conserve moisture. This is a plant survival mechanism and occurs more quickly on a hot, drying day than a cooler, overcast day.
The delay in closing of the stomata will depend on plant moisture content and the humidity within the swath, but usually occurs between 30 minutes and two hours after cutting.
For most species, this stomatal closure occurs before 30% of the initial moisture has been lost. When the stomata are fully closed, water vapour can still move through the epidermis or cuticle (outer skin) of the leaves, leaf sheaths and stems, although the rate of moisture loss is reduced to about 10% of that of open stomata.
A young, vegetative crop or pasture contains significantly more leaf than stem; as plants mature, the proportion of leaf declines. Typically, lucerne contains 55-60% leaf at the vegetative stage of growth, declining to 35-40% when in the full bloom to early pod stages.
In perennial ryegrass, the percentage of leaf falls from 85% at the early vegetative stage to 20% when fully in head. As plants mature, the proportion of soluble cell contents in the stems also falls as more structural fibre is produced.
These changes explain the more rapid wilt achieved with leafier material compared to more stemmy material.