Low-quality hay is even less green, with larger stems and fewer leaves. Research has shown that horses prefer hays with fine leaves and stems. So you have more waste with mature hay that has larger, coarser stems and fewer leaves. And the energy value decreases, too. So you'll have to feed more hay and grain for an adequate energy intake.
Protein also is limited in such hays.
You may feed lower- quality grass hay to mature, idle horses and pleasure horses ridden or worked little in winter. Yearlings also need at least a medium-quality hay. As hay quality decreases, horses may eat less. So you may have to feed grain. In other words, match hay quality to the type of horses when you have limited hay of variable quality.