Weed and Feed application
If you are referring to a crabgrass preemergent look for a preemergent that contains no fertilizer. Look at our Turfgrass Maintenance Calendar for more information and see Spring Maintenance http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG112_Turfgrass_Main...
In general weed and feed products are not recommended because of the need to treat the entire yard and can contribute to overfertilizing your lawn.
If you do use a ‘weed and feed’, the fertilizer contained in the product needs to be included in the total amount of nitrogen that you apply to your lawn for the year and follow label directions. The best time to fertilize is in the fall not the spring. Excessive nitrogen applications in spring and summer increases disease problems and reduces stress tolerance.
Identify and spot treat broadleaf weeds with liquid broadleaf weed herbicides when they are actively growing. Follow label directions. http://extension.umd.edu/hgic/lawns/lawn-weed-identification