Spatial Analysis of Forage Grass Trials across Locations, Years, and Harvests

field trials. The use of row-column analysis or neighbor analysis has been shown to increase the precision of a Spatial analyses of yield trials are a powerful method of adjusting large number of grain yield trials (Cullis and Gleeson, treatment means for spatial variation and improving statistical precision of mean estimation. Because yield trials are typically repeated 1989; Cullis and Gleeson, 1991; Kempton et al., 1994). across multiple locations and years, spatial analysis methods must Recent analyses of forage grass cultivar trials of a be adapted for combined analyses across locations and years. The range of cool season forage species have shown that it objective of this study was to evaluate the relative efficiency of nearest is possible to improve the precision of cultivar yield neighbor analysis (NNA) across locations and years for several perenestimates within a location through both optimizing the nial forage grass trials. Three spatial adjustment methods were develnumber of replicates sown, based on the likely differoped: preadjustment based on total forage yield, postadjustment ences between the cultivars under test, and utilizing based on total forage yield, and preadjustment based on forage yield statistical analyses that account for spatial variability in of individual harvests. For cool-season grasses on a multiple-harvest plot yield (Casler, 1999a,b; Smith and Kearney, 2002). management, NNA had relative efficiencies of 105 to 135% across When RCB designs were compared with lattice designs locations, years, and trials. Within trials, there was some consistency across harvests, resulting in greater improvements in precision for and NNA in a comparison of 27 perennial cool-season adjustment based on total yield. Across locations and years, the three grass trials, NNA was shown to provide more precise spatial adjustment methods always ranked the same in relative effiestimates of mean forage yield than either the lattice ciency: preadjustment by harvest preadjustment of total yield or RCB designs (Casler, 1999b). The improvements in postadjustment of total yield. The advantage of the preadjustment precision of entry means were shown to be incremental methods was likely due to fitting heterogeneous slopes (adjustment with an average improvement in precision of 15% due factors) across locations, years, and/or harvests. In contrast, trials to the use of RCB designs, an additional 17% due to with a single-harvest management for biomass production always had the lattice analysis, and a further 22 to 30% due to trend relatively low relative efficiency of NNA. Trial operators should assess analysis or NNA (Casler, 1999b). the relative efficiency of NNA on early harvests from all locations These improvements in the precision of the estimawithin a trial and if the relative efficiencies are large, they should consider the use of NNA across locations and years to adjust entry tion of cultivar herbage yield are of great importance means. given the rapid increase in the number of forage grass cultivars on the market, the relatively low rates of genetic gain for forage yield (0.1–0.5% yr 1; Van Wijk and Reheul, 1991; Casler, 1998; Casler et al., 2000), and P forage grass species are routinely tested the reduction in funds available for cultivar testing in a for improvements in forage yield through the use of number of countries. Nearest neighbor adjustment of replicated plot trials in a number of years and locations. cultivar means for individual trials provides improved These cultivar evaluation programs are essential for precision for cultivar means, but does not provide a making choices between forage grass cultivars and also direct assessment of cultivar environment interactions, for assessing whether new cultivars are broadly suited which require a combined analysis across locations or across a range of environments or possess more specific years. Supplemental analysis of adjusted cultivar means adaptation to certain environmental niches. could provide this information (Cullis et al., 1998), but The majority of forage cultivar evaluation trials are would not provide a test of each cultivar environment sown in a randomized complete block (RCB) design component (location, year, and location year). Thus, (APPEC, 1996). While the RCB design may be an effecthe need still exists to develop techniques to allow for tive way of controlling spatial variation in field trials in analysis of spatially adjusted means across environments one direction, it is ineffective when the spatial variability and years as forage cultivar trials are usually conducted is continuous in two directions, leading to considerable across 2 to 3 yr in a number of locations (Casler, within-block variability (Lin et al., 1993). There has a 1999a,b). been a marked change in the way that multienvironment The objective of this study was to evaluate several trial data from annual grain crop variety testing trials methods to use NNA to account for spatial variability are analyzed with a move toward spatial analysis (Gleein the yields of forage plots from nine separate cultivar son and Cullis, 1987; Cullis and Gleeson, 1989) to better evaluation trials conducted across locations and years. accommodate the plot-to-plot variation observed in The trials cover two distinct classes of forage cultivar evaluations: multiple-harvest hay trials of cool-season K.F. Smith, Agriculture Victoria, CRC for Molecular Plant Breeding, Pastoral and Veterinary Inst., Private Bag 105, Hamilton, VIC 3300, grasses, for which season-total forage yield is the trait Australia; M.D. Casler, USDA-ARS, U.S. Dairy Forage Research of interest, and single-harvest biomass trials of a warmCenter, Madison, WI 53706-1108. Received 19 Feb. 2003. *Correseason grass. sponding author ([email protected]). Abbreviations: LSR, least significant range; NNA, nearest neighbor Published in Crop Sci. 44:56–62 (2004).  Crop Science Society of America analysis; Pre-IH, preadjustment by individual harvests; RCB, randomized complete block. 677 S. Segoe Rd., Madison, WI 53711 USA