The effect of interplot competition on yield assessment in triticale trials

Summary A series of experiments was carried out to investigate the effects of interplot competition on grain yield among hexaploid triticale cultivars of widely differing heights grown in 1.5 m wide plots. Three experiments in 1982 tested four triticale cultivars, ranging in final height from 63 to 104 cm, with four wheat standards, 62 to 82 cm in height, using a design balanced for nearest neighbours. Two further experiments in 1983 with the same design included eight triticale cultivars ranging in final height from 92 to 183 cm. In four of the five experiments, plot yields of both wheat and triticale were decreased by between one and two grammes per square metre for every centimetre by which the plot was exceeded by the mean height of its two neighbours. Plots which were taller than their neighbours showed a similar increase in yield. Such systematic biases in cultivar means are not reduced by randomisation and increased replication. Hence, in breeding trials, every effort should be made to group genotypes according to their final height so that competition effects between plots are reduced. For the final evaluation of varieties differing widely in height, where an estimate of error is required for variety comparisons and varietal differences are likely to be small, guard plots should be grown on either side of each experimental plot or harvesting should be restricted to the centre of larger plots.