Yield comparisons in onion variety trials

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Abstract
Valid yield comparisons in maincrop onion trials are made difficult by differences in plant density between varieties. In an attempt to overcome this, replicates of a control variety, sown at a range of densities arranged in a systematic design, were grown adjacent to the main yield trials during the period 1962–65. The model w-θ =α + β ρ ø (where w = wt. per plant, ρ = density and α, β, ø and θ are parameters) was fitted to the data from these replicates. The fitted curves obtained were used to compare yields of test varieties with those of a standard control at the same densities. This procedure was compared with a method of adjusting yields to a common density, using covariance analysis. As a result, modifications to designs for onion yield trials are suggested, incorporating a sufficient range of densities to enable curves of the above form to be fitted, and thereby improve yield comparisons between test varieties and controls.

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