Exact confidence limits for covariate-dependent risk in cultivar trials

Yield risk is an important criterion for crop cultivar choice. Two kinds of risk may be of interest, (1) the probability that the yield of a cultivar falls short of some critical level and (2) the probability of a cultivar being outperformed by another cultivar. These risks are governed by the average cultivar performances and by the variability of performances across diverse environments. As a result of among-environment variation, yield in a new environment is, to a large degree, unpredictable. Predictability may be improved by taking into account covariate information on the environments. The present paper proposes methods of risk assessment that exploit covariate information for making specific recommendations. Exact confidence limits for estimated risks are provided. The methods are exemplified using data from a multienvironment trial on triticale (Triticosecale WITTMARK).