In most places in the Midwest, August and early September are the ideal times to plant your cover crop. Many are using aerial application while corn and soybeans are still in the field. Drilling is still preferred in some parts, however, because the seed-to-soil contact is better and, thus, you can put out fewer pounds of seed per acre.
For more information about annual ryegrass planting guidelines, click here.
Also, here is another handy one-page flyer about the different varieties of annual ryegrass available and, most important, the questions to ask before purchase.