Nitrate (NO3-) is the form mostly taken up by crops. This N form is absorbed quickly but can also leach out in wet weather or volatilise in very wet weather. Fertilising with N (nitrate or ammonium) N will often provide a big boost to soil life. By fertilising, you are encouraging soil life to break down organic matter.
Nitrate is chemical compound and can be used as a measure of fertilizer output. Nitrate levels are easy to determine. Improper fertilization, i.e., too shortly before mowing, will produce a high nitrate fraction. A high proportion of nitrate can pose a health hazard to cows but will primarily cause less palatable silage in the first place.