Plant yield is strongly influenced by the amount of water used by it. In this sense, yield response factor (K-y) allows evaluating the decrease in yield as a function of the evapotranspiration that occurs, which may be an important tool for harvest forecasting. Therefore, this study aimed to determine K-y for winter corn crop grown in the region of Dourados, MS, Brazil. The yield function analysis based on the water factor, which requires actual (Y-a) and maximum (Y-m) yield data, as well as actual (ETa) and maximum (ETm) evapotranspiration, was used to adjust K-y. Y-m was assumed to be 8,820 kg ha(-1), while Y-a was obtained for each season from the database SIDRA from IBGE. ETm and ETa were estimated by sequential daily water balance, considering four different sowing dates (all in the off-season). In total, seasons from 2001 to 2013 were analyzed. The high K-y value obtained (1.22) for winter corn crop showed its high sensitivity to water deficit. Its yield is strongly influenced by water availability so that for every 1 reduction in evapotranspiration, there is a 1.22 yield reduction.
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