Answer: Ryegrass (Lolium) is often used as a cover crop over the winter to protect the soil and build organic matter and is then tilled under the following spring a few weeks before planting. This is a method of "green manuring" which does supply some nitrogen, however its main contribution is that of massive quantities of organic matter in situ, both under and above ground.
Annual rye would freeze out over the winter resulting in a mulch of sorts over the area where it was planted, whereas the perennial or winter rye would survive. It also requires plowing under in the spring. If the grass is heavy you may need to chop it first before you till it in. (A strong rotary mower or bush hog would do although it may require multiple passes.)
In any case, if you planned to cut the grass and then use it in another location you will have to treat it a bit differently: either cutting it green and transporting it to the garden to till in or else cutting the tall mature grass and then allowing the material to dry in the summer air and sun and produce, in effect, hay to use for surface mulch. Generally gardeners prefer to use straw as a surface mulch rather than hay since hay tends to be full of seeds which will sprout and cause a problem in the garden.
Good luck with your project!
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