the Symposium on Wheats for More Tropical Environments - cimmyt

233water content in <strong>the</strong> surface arablelayer of an oxisol soil (red latosol) when<strong>the</strong> zero tillage system is adopted (7).This practice in <strong>the</strong> soybean-wheatrotation leaves <strong>the</strong> soil surface coveredby <strong>the</strong> crop residues. This cover ormulch reduces water evaporation andstabilizes <strong>the</strong> temperature in <strong>the</strong> arablelayer. By reducing soil disturbancethrough no-tillage. <strong>the</strong> soil structure isimproved. soil compaction is minimizedand roots can penetrate more deeply.resulting in better growth. As aconsequence of more uniform andVigorous growth. <strong>the</strong> wheat crop canbetter tolerate inadequate rainfall. andgrain yields can be increased (Table 3).However. <strong>the</strong>re were negative effects offrost in <strong>the</strong> 1979 season under notillage;damage was more pronounceddue to <strong>the</strong> lower temperature caused by<strong>the</strong> mulching effect on <strong>the</strong> soil surface.Table 2. Rainfall during <strong>the</strong> various wheat growth stages as related to grain yields undernor<strong>the</strong>rn Parana semitropical conditions, Brazil, 1968 to 1978Rainfall (mm)AverageGrowth stage 1968 1972 1974 1968-78Sowing-shooting (M arch/ApriI) 93 157 197 60Shooting-heading (May/June) 68 156 313 108Heading-maturity (July/August) 88 415 80 167Total rainfall 249 728 590 875Wheat yields (kg/ha) (899) (245) (1547)Source: Godoy and Bernardes (3)Table 3. Comparison of wheat yields under inadequate and normal rainfallconditions related to tillage systems, nor<strong>the</strong>rn Parana, Brazil, 1977 to 1980Rainfall/yearWheat yields (kg/ha)according to tillage systemConventional No-tillageInadequate19771978Normal197919804-year average (1977-1980).,g/ Frost damageSource: Vieira (7)60950718541867120996720361281.,g/17991521