A Farm Journal article spells it out plainly and simply. Here’s a few paragraphs from that article. You can read the whole article by clicking here.

One of the cover crops that does double duty to achieve all of the above benefits is annual ryegrass. It comes with a caveat: using a cover crop such as annual ryegrass “involves a learning curve,” says Dan Towery, agronomy consultant with Ag Conservation Solutions in Lafayette, Ind. With an extensive and fibrous root structure, unlike other cover crops, annual ryegrass can be a challenge if you don’t do your homework and adjust to adverse weather conditions, he cautions. “You need to know the growth pattern of annual ryegrass and use the proper system to kill it.”

The biggest challenge in getting a good ryegrass kill is working with the weather, according to Mike Plumer, farmer and consultant with Conservation Agriculture near Creal Springs, Ill. Pay attention to the maturity stage of ryegrass and the ambient air temperature, Plumer advises.

Both consultants say annual ryegrass is ideally sprayed with glyphosate once the temperature gets above 50°F (when annual ryegrass actively starts growing) and before it reaches the first joint stage, or 7″ to 10″ tall. Ryegrass is easier to kill young, but there will be less mulch left on the ground. Larger plants (less than 16″) can be controlled with warmer expected temperatures, but wait too long to spray and the plants will be entering the boot stage, with the threat of seed production. Also, the plants are producing lignin in the stem at this stage, tying up nitrogen and making it unavailable to young plants, and residue decomposition is slowed.