A spring application of nitrogen (N) will help promote pasture growth, particularly if you find yourself lacking in hay reserves. With cooler temperatures remaining, applications of granular or liquid fertilizers are recommended over manures, because soil microbes are needed to release N from manure; microbes prefer warmer temperatures.
As your pastures greenup with warmer weather, you may be anxious to let your animals out to graze. But be patient. Walk pastures and observe greenup and grass height prior to any fertilizer applications or releasing animals to graze. There are potential animal health issues associated with excessive spring growth due to N applications. For example, grass tetany often occurs in the spring, where exacerbated levels of K in forages may cause low Mg uptake. Also, make sure your grasses are at least 4 inches tall before grazing to ensure adequate growth during and after grazing.