A lot of original work for N on grass was done right after WWII. One point that made was that you can extend the grazing season about two weeks in both spring and fall with a timely N app, and you can also get a high # grass for # N payback at this time. I think the soil is too cool for the natural mineralization process to work early and late in the season. Grass is green but it just doesn't grow
So you want to make a small N app when the grass just starts to green up, unless your winter feed is very reasonable. Most people are too late in the spring. More than 40# N/acre will reduce clover content.
I'm just north of I-20 in East Central Georgia. I just had nitrogen applied to my winter grazing (rye, oats, and ryegrass) this past Tuesday. Then got 3/4 of an inch of rain that night. Maple, elm, and pear trees are all putting out and daffodils are blooming like crazy. I think now is the time for fertilizer in the South.