Field Design Factors Affecting the Precision of Ryegrass Forage Yield Estimation

Field-based agronomic and genetic research relies heavily on data generated from field trials. Therefore, it is imperative to optimize the precision of yield estimates in cultivar evaluation trials. Experimental error in yield trials is sensitive to several factors, some of which are classified as experimental design factors (i.e., plot size, block size, and number of replicates). The objective of this study was to conduct a retrospective case-study analysis of long-term perennial ryegrass (Lolium perenne L.), Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot], and hybrid ryegrass (Lolium boucheanum) forage yield trials to quantify the effects of block size, block shape, plot size, and number of replicates on trial precision. A total of 142 forage yield trials sown 2001 to 2011 across five locations by the Department of Agriculture, Food and the Marine (DAFM), Ireland, were considered. The contribution of row and column sum of squares (SS) to total SS decreased with increasing block size from 6 (71% of total SS) to 33 cultivars per block (51% of total SS). Empirical estimates of LSD decreased with increasing number of replicates only for square blocks, but not for rectangular blocks. Across different block sizes from 6 to 42 plots per block, row-column designs were 36% more efficient than complete blocks. Increasing plot size was consistently ineffective. Use of incomplete block designs would significantly improve precision for the DAFM trials. This case study provides a template by which any research with long-term data can be used to design more efficient field trials.