The National Variety Trials (NVT) program is used by plant breeding companies to evaluate the yield potential of new crop varieties independently across a large range of Australian growing conditions. By comparison with the remaining NVT crops, grower decisions are further complicated in canola because of its vulnerability to the infestation of weeds. A measure historically used by farmers for the management of weeds is the application of a herbicide (chemistry) treatment. The choice of chemistry is important as it restricts variety selection to those bred with the specific tolerance. The set of varieties currently evaluated in NVT are tolerant to one of three chemistries, namely imidazolinone (I; but marketed as Clearfield), glyphosate (Roundup Ready; R) or triazine (T), or have no specific tolerance (i.e. conventional canola; C). Consequently, canola has a more complex testing regime than the remaining NVT crops as each trial has a nested treatment structure involving both chemistries and varieties.
Canola trials are conducted in locations across the Australian grain belt and reflect best farmer practice for each district. Every site is partitioned into several field blocks and plots are allocated to the treatments according to orthogonal block designs. A spray boom is used to administer each chemistry but is pragmatic in the sense that large areas are treated simultaneously. This precludes the application of different sprays to plots in the same block. Randomisation is therefore restricted so varieties in a single block are tolerant to the same chemistry. However, as the number of chemistries and blocks are exactly equal, there is no information to estimate the experimental error variation and both are statistically confounded. Consequently, growers are limited to evaluating varieties with the same tolerance as comparisons across chemistries are invalid. This also has important implications on the statistical analysis, which is discussed in this talk.