Cropping frequency, wheat classes and flexible rotations: Effects on production, nitrogen economy, and water use in a Brown Chernozem

Producers in the semiarid Canadian prairies rely on frequent summerfallowing (F) to conserve water, control weed infestations, and maximize soil mineral N reserves, but this practice often results in soil degradation. A crop rotation experiment was initiated in 1987 on a medium-textured, Orthic Brown Chernozem at Swift Current, Saskatchewan, to determine the most ideal cropping frequency for wheat in this region and whether a fixed rotation such as fallow-wheat (Triticum aestivum L.) - wheat (F-W-W) or F-W-W-W would be more effective than flexible rotations in which fallowing is decided each spring based on criteria such as available soil water (if water), or the need to control perennial weed infestations (if weeds). The study also compared the production of traditional Canada Western Red Spring (CWRS) wheat class with the newer higher-yielding (Hy), Canada Prairie Spring (CPS) wheat class. We analyzed results of six rotations over the first 12 yr of the study. The rotations included F-W-W, F-W-W-W, F-Hy-Hy, Continuous wheat (Cont W), Cont W (if weeds), and Cont W (if water). Reduced tillage management was used and stubble was cut tall to enhance snowtrap. Fertilizer N was applied based on soil tests and fertilizer P was applied based on the general recommendations for the region. Over the 1988–1999 period, weather conditions were generally favourable and yields were above average for this region. Canada Prairie Spring wheat outyielded CWRS by 32% when grown on fallow and by 17% when grown on stubble; however, straw yields of the two wheat classes were similar on fallow and CPS was 11% less than CWRS on stubble. Harvest index (HI) averaged 44% for CPS and 37% for CWRS wheat. Water use efficiency for CWRS wheat grown on fallow averaged 7.2 kg ha-1 mm-1 and for CPS 9.4; when grown on stubble the respective values were 6.3 and 7.5 kg ha-1 mm-1. Grain N concentration for CWRS was slightly higher for wheat grown on fallow (25.7 g kg-1) than on stubble (24.5 g kg-1), but was similar for CPS wheat on grown on fallow and stubble (21.9 g kg-1). Straw N concentration averaged 3.8 g kg-1 for CWRS and 4.4 g kg-1 for CPS. Nitrogen yield for grain from CPS was 9% greater than from CWRS when grown on fallow, but there was no effect of wheat class when grown on stubble. Nitrogen yield of CPS straw was 15% greater than for CWRS when grown on fallow, but on stubble N yield was generally not affected by wheat class. Nitrogen harvest index (NHI) averaged about 80% for both wheat classes, whether grown on fallow or stubble. On a rotation basis, grain produced with F-W-W was 1502 kg ha-1 yr-1. The F-W-W-W and Cont W (if weeds) rotations produced 9% more grain than F-W-W, while Cont W (if water) produced 24% more, F-Hy-Hy produced 26% more, and Cont W produced 30% more than F-W-W. Nitrogen production in the grain, straw and aboveground plant material was lowest in F-W-W, highest in Cont W, and intermediate for other rotations. Although the economic and soil quality assessments have yet to be completed, a preliminary conclusion based on crop production characteristics alone suggests that a flexible cropping system in which available soil water in spring is used as the determining criterion is superior to a fixed F-W-W or F-W-W-W rotation. Key words: Yield, N concentration, N yield, water deficit, wheat classes, regressions