For an established lawn area where fertilizer has been applied each year, it is appropriate to reduce the amount of fertilizer applied by at least one half.
Research results show that leaving grass clippings on the lawn can reduce nitrogen fertilizer needs by 50%. The rate of nitrogen fertilizers can be safely reduced because grass clippings contain about 4% nitrogen by dry weight. When you remove the clippings from the lawn you remove the nitrogen recently applied as fertilizer (because it is now being taken up into the lawn that you are now mowing!). Please note that many research studies have shown that leaving clippings on the lawn will not contribute to or create thatch.
Most times of the year it is easy to leave clippings on the lawn. Unfortunately, during the rapid growth period in May it is often difficult to mow frequently enough to make the clippings disappear back on the lawn. There are two solutions to excess clippings in May. First, a good mulching mower can oftentimes make the clippings disappear, but sometimes the lawn has to be re-mowed to obtain an acceptable look. Second, the amount of growth in May can be reduced by holding off on spring fertilization until mid May or so.