I'd get a soil test done - that will tell you what you need to apply by way of lime, phosphate, potassium and micronutrients. It won't tell you how much nitrogen to apply.
Nitrogen is the stuff that makes some grasses, especially the ryegrasses, grow long and lush. Good for cows and sheep, bad news for horses. If you have clover in the sward, it will fix nitrogen from the atmosphere and make it available to other plant species. If you apply bagged nitrogen to a mixed sward - ie clover and mixed plant species, this will favour the ryegrasses, which will grow well and outcompete the other species. A mixed sward is better for horses, and probably all stock, in that it tends to have different species to suit different conditions - drought, wet - and often these species will contain nutrients good for stock.
If it IS meadow grass ie grass that is in a long term ley that has been cut for hay every year, think twice before putting lots of nitrogen on it. As Roxy says, you may come to regret it.