Although agroforestry alley cropping practices have been shown to improve environmental quality, it could affect the crop yields adjacent to the perennial vegetative buffers due to competition for resources.  The objective of this study was to examine temporal and spatial variation in no-till cultivated corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields in watersheds with agroforestry (tree+grass; AGF) and contour grass (CGS) buffers.  The watershed study was initiated in 1991 and buffers with trees and grass were established in 1997.  Grass buffers consist of redtop (Agrostis gigantean Roth), brome grass (Bromus spp.), and birdsfoot trefoil (Lotus corniculatus L.) were planted at 35-m spacing on contours on the watersheds and pin oak (Quercus palustris Muenchh.), swamp white oak (Q. bicolor Willd.), and bur oak (Q. macrocarpa Michx.) trees were planted alternately at three meter distance at the center of grass buffers on the AGF watershed.  Crop yields were determined by a yield monitor and translated to geo-referenced yield data maps using yield editor software.  The yield data of corn for 2004, 2006 and 2008 and soybean for 2005, 2007, and 2009 were analyzed.  To study the proximity effects of buffers, yields at 0-5 m, 5-10 m, and 0-15 m were compared with yields at 15-20 m from selected agroforestry and grass buffers using ArcGIS 9.3 software.  Corn yield was reduced by 22 to 49% in the AGF and 15 to 32% in the CGS at the 0-5 m compared to 15-20 m from the buffers.  This interference at the buffer-crop interface was enhanced during moisture stress periods and when late planting was done.  Soybean yields were significantly increased at 0-5 m distance from both AGF and CGS buffers compared to 5-10 m from the buffers indicating proximity to buffers was favorable for soybean.  The effect of buffers on soybean yield was minimal during drought (2005) or excessive rainfall (2009) years when the yield potential (< 3.5 Mg ha^-1) was low.  Long-term environmental benefits need to be balanced with yield losses in corn by introducing economically beneficial plant species in buffers.