The water should be not an issue if from a reservoir. Webfoot is right you need to fertilize and also right you need to have your soil tested. While good results might be obtained with just nitrogen better results might be obtained with a triple blend, but that blend depends on your soil's need. It is too bad you don't have a county agent. Also, do others with good results harrow their hayfields either before or after fertilizing?
The thing with a hayfield is that the natural process has been interrupted when you cut the hay. You don't have animals eating the grass and leaving manure and some grass left to fertilize and reseed the field for the next season. I still think a light plowing, harrowing, chiseling, discing, raking, or whatever to work up a few inches of topsoil is well worth the expense. If this hayfield has never been reseeded, consider that when you hay you have disturbed the natural process of reseeding and you are depending solely on the roots and whatever blows in which is usually weeds or undesirable grass. A good example of declining mountain pastures is overgrazing where natural reseeding is not happening. Even with all the extra manure, the grass will decline over the years.
A good example is our bodies need a certain ratio of protein, carbs, and fats to function their best. A heavy meat eater might need to add more carbs, where a nonmeat eater needs to pay attention to proteins. Maybe both need to cut back on fats. The same with plants needing nitrogen, phosphorus, and potassium (potash). Your hayfield is no doubt depleted of all these, but what ratios you need to add back requires a soil test. Nitrogen just like protein may show more immediate and impressive results, but plants need a balance just like our bodies for long-lasting results and health.