A TECHNIQUE FOR MINIMIZING CONFOUNDING OF GENOTYPE X YEAR AND GENOTYPE X CROP TYPE EFFECTS IN SUGARCANE

Genotype x year (G x Y) and genotype x crop (G x C) interactions may b e confounded in sugar cane when data is obtained from the plant crop a nd/or from ratoon crops in successive years. A technique to minimise t he effects of confounding of the G x C interaction with the G x Y inte raction is presented. The mean yield of cane from neighbouring farms w as used as an indicator of the year (environment) effect. It was used to re-analyse yields from three experiments comparing six genotypes gr own over a plant crop and three ratoons. Two experiments were grown un der rainfed conditions and one experiment was irrigated. The confounde d interactions were partitioned into a linear contrast on the farm yie lds (a year effect) and a crop effect that was the remainder. When the yields adjusted for the farm yield effects were compared with origina l yields of cane, yields of plant and first ratoon were reduced and th ose of third ratoon were increased. There were only minor changes in t he ranking of genotypes on yield. It was concluded that the re-analysi s using farm yield showed that the confounding effect of years on the interpretation of genotype x crop effects was small.