Long-term cropping effects on maize:: Crop evapotranspiration and grain yield

Long-term cropping in the Rolling Pampa of Argentina may re duce available sop water for maize (Zea mays L.) production. Grain yield, however, has not decreased possibly because of genetic improvement. Our objectives were to: (i) evaluate root and shoot growth and its relation to crop evapotranspiration (ETa), (ii) analyze interactions among hybrids and length of the cropping period on grain yield and ifs components, and (iii) establish functional relations between grain yield components and their determinant processes. Three hybrids with contrasting production stability coefficients and representing different eras of maize breeding were grown at three farms in 1994 to 1996 in fields with short (S) and long (L) cropping periods on silty clay loan sods (tine, illitic, thermic Typic Argiudolls). Silking took place 5 to 6 d later in L than in S and a significant (P < 0.05) reduction (21-36%) of ETa was observed in L vs. S around this stage, Lower root abundance (reduction of 60%) and canopy size (reduction of 6-13%) in L have probably contributed to less ET, In addition, the ontogenic delay exposed crops grown in L to more stressful meteorological conditions around silking, Kernel number (KN), closely related to grain yield (r(2) > 0.65), was significantly associated with daily ETa around silking, The modern hybrid established a greater KN mm(-1) d of ETa (853 kernels m(-2) mm(-1) d) compared with the older ones (386 kernels m(-2) mm(-1) d). This characteristic may mask the deleterious effects of longterm cropping on maize grain yield.