Statistical methods to address spatial variation in pasture evaluation trials

Abstract

The primary breeding objective for white clover improvement in eastern Australia is more reliable persistence under summer moisture-stress while maintaining herbage production (especially winter growth). To identify elite germplasm, evaluation of candidate lines is done in situ, under field conditions for which the prospective cultivar is targeted. These conditions (e.g. moisture stress, nutrient infertility, grass competition) may impose spatial influences within the field site that as artefacts mask or alter the uniform expression of response to environment. A significant component of artefact variation which affects interpretation of the data is the spatial variation due to plot position. Failure to correct for this may give unreliable comparisons as some lines inadvertently (through randomisation) are allocated to favourable or unfavourable plots. Our methodology was developed at two sites (Glen Innes , Armidale in northern New South Wales, Australia) in an experiment which sought to identify superior lines among 20 candidate entries. Cultivars of reputed stability and spanning the expected performance range, are replicated at regular intervals through the field site. The difference amongst replicates within cultivar (that is a reduced G X E) is used to estimate spatial trends. Elite lines are identified from temporal profiles of plant performance related to the breeding objectives. We describe (i) design and analysis which allows estimation of the spatial effects, and (ii) the temporal profiling of plant performance. These statistical procedures are seminal in identifying superior breeding lines that subsequently proceed to cultivar development.