Farm Scale Evaluations of herbicide-tolerant crops: assessment of within-field variation and sampling methodology for arable weeds

The Farm Scale Evaluations (FSEs) of genetically modified herbicide-tolerant crops (GMHT) compared the effects of conventional and GMHT crop management regimes on weed densities. Weeds were sampled in quadrats at 5 distances along 12 transects systematically arranged within experimental half-field treatment units. Weed densities declined with increasing distance from field margins. Random variation within transects was about 80% of the total within-half- field variation, i.e. relatively large variation at a small scale. The spatial autocorrelation between counts on the same transect declined as the distance between quadrats increased. Sampling schemes with fewer distances along transects required more transects but fewer quadrats to achieve similar precision as the 5-distance scheme; on average 32 quadrats were sufficient. The frequency of detection of treatment effects (5% level) was little affected by using six or even three transects (30 and 15 quadrats respectively). However, the use of the 12 transects greatly improved the detection of treatment effects at 1 % and 0. 1 % levels. The effectiveness of the reduced schemes for detecting differences between treatments (5% level) with as few as three transects compared with 12, reflects, in part, the relatively large treatment effects (e.g. 1.5-fold or greater), which emerged for many of the sampling occasions. The FSEs provide an unusually extensive data set. Both the data and the methodology developed for their design and analysis are valuable for the planning of future weed sampling schemes.