The three principles of experimental design—randomization, replication, and blocking—have been followed routinely in field experiments. This paper addresses issues of blocking in conducting cultivar performance trials. Most agronomists design (block) their trials by considering the slope of the field at most. However, the direction of field variation might not always follow the slope of the field. Blocking in the direction of field operations is advocated. The objective of this study was to compare precision between three years of horizontal blocking with three years of vertical blocking in North Carolina Official Cotton Variety Trials. No significant difference in precision was found between the two blocking arrangements. However, the standard analysis of variance of a randomized complete block design revealed that vertical blocking produced a larger standard error than when the tests were blocked horizontally. Although vertical blocking results in increased spatial distance, the use of a spatial analysis package should mitigate this problem.
