Yielding of winter wheat was evaluated based on 30-year results of cultivartesting experiment carried out by the Research Centre for Cultivar Testing at Seroczyn (latitude = 52o 00’, longitude = 21o 56’) over 1970-2000. The average yield for all the cultivars tested constituted the basic unit for analysis. The climate observations for a given research period were conducted in the same location as the experiment. The analysis of precipitation and of the energy factor, namely potential plant evapotranspiration, showed their high variability over research years. The variability of winter wheat grain yield was mostly affected by weather extremes, inflicting on stress conditions over specific plant development phases which, consequently, limit their yielding potential. Both the water shortages due to long rainless periods, (of about 20 days) as well as heavy daily precipitation (over 20 mm) or stormy precipitation at the end of the vegetation period are mostly responsible for yield decreases.

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