Effect of soil tillage, crop rotation and nitrogen application rates on grain yield of spring wheat (Triticum aestivum L.) in the Swartland wheat producing area of the Republic of South Africa
Studies on spring wheat yield components were conducted during the 2000 and 2001 growing seasons only, while grain yield responses to tillage, crop rotation and N rates during these years were compared to the mean yield for the years 1992, 1994, 1996 and 1998 at Langgewens Experimental Farm in the Western Cape as part of a long-term tillage and crop rotation trial. Four tillage methods, namely conventional tillage (CT), tine tillage (TT), minimum tillage (MT), and no-tillage (NT) were compared. Crop rotation systems used were continuous wheat (WW) and wheat/lupin/wheat/canola (WLWC). Three rates of N fertilisers (60, 100 and 140 kg N ha-1) were applied. Tillage methods had a significant effect on the number of spikes m-2, spikelets per spike and number of kernels per spike in both the 2000 and 2001 growing seasons. Thousand kernel mass was not significantly affected by different tillage methods in both seasons. Mean grain yield for the years 1992, 1994, 1996 and 1998 did not differ significantly as a result of different tillage methods, but differences due to tillage methods were found in both the 2000 and 2001 growing seasons. Crop rotation had no significant effect on the mentioned yield components. Significantly higher mean yield for the years 1992, 1994, 1996 and 1998 were produced when wheat was rotated with lupins and canola (WLWC) compared to monoculture wheat (WW). Application of different rates of N had no significant effect on the number of spikes m-2, number of spikelets per spike and 1000-kernel mass in both growing seasons. While higher rates of N increased the number of kernels per spike in 2001 growing season, no significant differences were observed between different N rates in the dry season (2000) and the results even indicated a decrease in yield with increasing N rates in 2000. Higher grain yields were obtained by application of higher rates of N fertiliser during the wet season 2001, but significant tillage x crop rotation and tillage x nitrogen interaction for the mean grain yield during the 1992-1998 period indicated that yield responses to tillage were related to soil conditions.