The preliminary estimation of expected yields and the accuracy of this evaluation provide information for decisions related to the harvest. The quantification of predictions makes it possible to estimate the accuracy of the prognosis. The yields that can be expected at the end of the vegetation season depend on the intensity of the photosynthetic activity. Numerous devices are now available to measure the quantity of photosynthetically active pigments in leaves, including the instrument GreenSeekerTM used in the present experiments, which records the value of normalized difference vegetation index. The present work attempted to answer the question of whether the yield could be predicted by means of multiple measurements during the vegetation period. Other questions raised were which phenophase was the most suitable for predicting yield, how values recorded at different times correlated with the yield, whether the strength of this correlation increased or decreased as harvest approached, and whether yield could be estimated at flowering, or in even earlier phenophases.