The present invention relates to a method of labelling soya varieties as to their suitability as a subsequent crop to maize when weed control in that maize crop has been carried out using HPPD inhibitor herbicides, wherein: a) in a first step, a reference soya variety is selected which, when grown as a subsequent crop to maize wherein weed control has been carried out using HPPD inhibitor herbicides, exhibits a visually determinable phytotoxicity with respect to those HPPD inhibitor herbicides; b) in a second step, the phytotoxicity of soya varieties when grown as a subsequent crop to maize wherein weed control has been carried out using HPPD inhibitor herbicides is determined in relation to that reference soya variety in the form of a factor; and c) in a third step, said soya varieties are labelled with said factor.