Evaluating the impact of crop sequence in complex crop rotations

Understanding crop sequences is necessary in determining the productivity and sustainability of a long-term rotation. When a sequence is strategically organized, it potentially benefits the environment, the farmer and the community as a whole. Well sequenced crops may improve soil quality and crop production hence evaluation is paramount to determining the sustainability of any rotation. This thesis sought to identify crop rotation options that may enhance wheat and corn production in South Dakota. Evaluation was carried out in two parts: First, we assessed the effect of preceding legume or oat crops on subsequent corn and winter wheat grain yields and soil microbial communities. This two year study was carried out within established long-term no-till crop rotation experiment. The preceding crop residue and subsequent corn and winter wheat grain were sampled and analyzed for yield, nutrient content and fiber content. Microbial mineralization activity in the soil and with added substrates and nutrients was assessed using the BDoxy 96-well plate method just prior to corn and wheat crop planting. Corn grain yield following soybeans (4773 and 5794 kg/ha) and field pea (5294 and 7863 kg/ha) in (2007 and 2008) respectively and grain quality was not significantly different in both years. Soybeans yielded significantly (P < 0.05) more residue biomass, had a higher lignin content and a lower C/N ratio than field pea. Microbial mineralization was significantly higher in soybean-harvested soil than in field pea-harvested soil when different substrates and nutrients were added. Wheat yield and quality following oat and field pea was not significantly different in both years. No fertilizer-N was applied in the subsequent wheat following field pea. Microbial mineralization in field pea and oat-harvested soil was not significantly different but the addition of n significantly increased activity. Field pea root extract was a preferred substrate by the microbes. From these studies, microbial activity analysis helped explain preceding crop contribution in the rotation. It is also shown that legume contribution differs with legume specie.;Secondly, we evaluated the effect of early season crop canopy development on weed emergence. Weed germination rates were quantified in corn, soybeans, spring wheat, field pea, alfalfa and fallow. Emerged green foxtail, pigweed species, and common lambsquarters seedlings were counted and removed by hand at each counting. Crop leaf area growth daily soil moisture and temperature were measured for each crop species. The time (days after tillage - DAT) required for each weed species to reach 50 or 90% emergence was similar (P > 0.05) among crop species except for foxtail which took significantly less DAT days to reach 50% under the wheat (13 DAT) and alfalfa (14 DAT) canopies in 2007. Wheat canopy also took significantly less DAT to reach 90% emergence in 2007. Weed emergence was done on average by 38 DAT. However, leaf area index differed (P < 0.05) among crops with wheat and alfalfa having the most soil cover (LAI > 1.5) and corn having the least canopy cover. The differences in the wheat canopy show that weed emergence might be affected by crop canopy presence but full canopy cover (LAI = 1) is achieved later when weeds are done emerging, at about 38 DAT.