Decisions that producers make for their crops are often based upon a fixed calendar date. In perennial ryegrass this may include fertilizer, herbicide, fungicide, insecticide, and plant growth regulator applications. Typically, producers begin monitoring their fields to decide when to apply their inputs, based on this fixed date. Due to variations in growing conditions from year to year, producers can miss the ideal timing. The use of growing degree days (GDD) is a far more consistent method for determining plant development. In perennial ryegrass, GDD are determined by adding the maximum and minimum daily temperatures, and then dividing by two.