Hay should be cut in the boot stage, which is when the seed head is inside the stalk. A good way to time a cutting is when the first seed heads start to emerge in the field. Later harvests actually can yield less, because lower leaves are dropping from the plants, which in turn also reduces forage quality and palatability. Weed pressures also can increase during drought years. Many weeds, especially perennials, have deep roots and more access to moisture than pasture grasses.
Weeds get an additional advantage when grasses are grazed and become less competitive.
When those weeds drop their seeds they will find perfect conditions to establish in weakened or denuded pastures. To reduce future weed problems, mow the pasture before weeds produce seed. For many weeds the best time to mow is near the time of flowering. The plant has already expended energy to produce the flower stalk and is less likely to have the resources to send up a second stalk. Poisonous plants also can be a bigger danger during drought.