Warm-season grasses provide good pasture and hay and use soil nutrients efficiently. If you can benefit from extra growth, fertilize now, in late May, for higher yields. 

Where you have moisture, these grasses will begin growing rapidly as soil and air temperature increase.  Adding nitrogen now will help these grasses produce extra hay or grazing.

The amount of nitrogen to apply depends on your situation. For starters, will you effectively use the extra grass to graze more cattle or increase hay yields?  It's surprising how often we apply fertilizer and then waste extra growth by grazing poorly.  This may seem obvious, but unless you economically harvest extra growth as hay or with livestock, don't fertilize.

Also consider the ability of your warm-season grass to yield more.  Taller warm-season grasses like big bluestem, switchgrass, and indiangrass will give a larger response to fertilizer than will shorter grasses like little bluestem, sideoats grama, or blue grama.

And finally, adjust fertilizer for moisture conditions. In eastern Nebraska, 50 to 60 lb of nitrogen per acre works well for average or better moisture conditions.  In central and western Nebraska only subirrigated meadows normally have enough growth potential to respond to added nitrogen; there about 40 lb per acre will do.  Then graze or cut the hay on a timely basis for profitable returns.

Many of these fields may also need a little phosphorus so follow soil test recommendations to determine the amount needed.