Hoary alyssum is classified as a noxious invasive weed. You may have had a few plants in the field in the past that produced seed that was dormant in the soil prior to you planting the pasture. In a year like 2018 with very hot, dry conditions in certain parts of the state, this weed will grow when the desirable plants struggle to have any growth at all. Soils that favor hoary alyssum are usually coarse textured sandy and gravelly soils.

The plant can be an annual, biennial (it starts in the fall and matures the next year), or a short-lived perennial that spreads only by seed.

Since the plants can be toxic to horses, it's important to get control of the weed. When it's during the heat of the summer, this weed will be very hard to control chemically. There are no herbicides labeled for control of broadleaf weeds in a forage mixture. Since you have a combination of alfalfa, brome, orchard, and timothy in your hay field, you will not be able to use a herbicide without killing either the grass or the alfalfa. The sprays recommended will be for either a grass, or for a legume, but not both.

The best way to control this weed will be to provide a good fertility program for the pasture. To encourage good growth of the forages make sure to not over-graze. If you have not had a soil test in the past three years, I would suggest getting a sample to have a nutrient analysis of the soil.

Mowing may be your best method of doing some short-term control of the weed.

I wish I had better news for you on how to control hoary alyssum. It is a very tough weed to control.