Fig. 1. Effect of within-row space available on grain yield of individual corn plants. Data are from an unpublished study conducted at Urbana, Illinois by the author in 2002. Row spacing was 30 inches, and the hybrid used was Pioneer 33P67. The correlation was not significant for 25,000 plants per acre, but was significant (P < 0.01, r = 0.22, n = 196) for 30,000 plants per acre, where yield increased 2.37 g per ear for each inch of additional space.

Image from Crop Management article:
Inter- and Intraplant Competition in Corn