Spatial dependence and block designs in spaced plant herbage trials

Abstract

Data from three ryegrass spaced plant variety trials conducted at Crossnacreevy, Co. Down in each of 1989, 1990 and 1991 were investigated for evidence of spatial dependence. Spatial dependence was most evident in variates measuring the overall dimensions of the plants, especially late season variates. However, the ‘date of ear emergence’ variate, which identifies most of the variety distinctnesses that the trials are intended to detect, was not spatially dependent. On average, 45% of the residual variation in the plot means of the spatially dependent variates was estimated to be of a spatial nature. Consequently, it is likely that the efficiency of analysis of these variates would be improved by using spatial analysis instead of non-spatial analysis or by using alpha (incomplete block) designs instead of complete block designs. The spatial information was used to determine the optimal incomplete block sizes and the likely increase in efficiency due to using alpha designs instead of complete block designs. The optimal incomplete block size for a trial with 10 plants per plot was compared with the optimal incomplete block size for the same trial with single plant plots at the same spacing and was found to be smaller. The gains in efficiency were larger in the trial with 10 plants per plot than in the trial with single plant plots.