As the production and health of your cows is related to what they eat and that needs to be grazing from your pastures, it is best to have them in the best of health you can.
Assuming that you are not in drought, or flood, or it is not posiable to drive a machine accross your grazing then soil analysis is essential to find out what i in your soil.
Clover can give your grass all the N it needs, but your pasture also needs phosphate, potash, sulphur, manganise, magnesium, and a full range of micro nutrients. Just as in a cow if one isshort or there is an inbalance and that is the limiting factor.
Just because grass looks OK to use does not mean very much.