Rainfall and previous crop effects on crop yields
Afield study was conducted between 1972 and 1982 to compare the effects of previous crop on row crop yields under rainfed conditions in eastern Nebraska. The objectives were to determine the effects of fallow and three previous crops: corn (Lea. maysLJ, soybeans /Glycine max (L.) Mem], and grain sorghum /Sorghum bicolor (L.) Moench], on the growth and grain yield of the same crops. The study was conducted on a Sharpsburg silty clay loam (fine, montmorillonitic, mesicf Typic Argiudoll). Corn grain yield was most variable (C. V. 23.4percent) compared to soybean (C. V. 13.6percent) or grain sorghum (C. V. 9.5 percent) yields. Corn was also the most sensitive crop to previous crop effects. The range of treatment yields for each crop was 47 percent, 22 percent, and 11 percent of the overall means for corn, soybean, and sorghum, respectively. Previous crop affected yields for all crops, but the effects were not consistent across years. All crops produced highest yield following fallow. Yields of corn, soybean, and grain sorghum following fallow were 74, 25, and 10 percent higher than their respective monoculture yields. In years of average precipitation, a corn-soybean sequence produced the greatest yield. In years having above- or below-normal precipitation, a grain sorghum-soybean sequence produced the highest yield.
Continuous Corn and Soybean Yield Penalties across Hundreds of Thousands of Fields
541 In the United States, soybean is the most commonly rotated crop with corn and has been for decades. For corn, a yield benefi t for rotation has been widely reported, with results from 28 fi eld trial studies of crop rotation exhibiting, on average, a 7.8% increase in yield from the practice (Erickson, 2008) and a wide degree of variation across studies. Th e yield benefi t of rotation may derive from improved soil structure (Barber, 1972), decreased disease pressure (Meese et al., 1991), decreased allelopathy between corn residue and growing plants (Martin et al., 1990) and increased N availability for corn in rotated systems (Stanger and Lauer, 2008). Nitrogen availability, however, is thought to be the primary driver behind this benefi t (Gentry et al., 2013). Benefi ts for soybean also have a wide range, reported as 8 to 17% by Crookston et al. (1991), an average of 25% by Edwards et al. (1988), approximately 4% by Nafzinger (2007), and for no-till fi rst-year soybean, 13% by Pedersen and Lauer (2002), with pest pressure from soybean cyst nematode historically cited as a major reason for this benefi t (Dabney et al., 1988). Several studies have also focused on the specifi c circumstances in which rotation provides the greatest increase in crop production. Based on previous year’s moisture, Peterson et al. (1990) found large diff erences in rotation benefi ts in eastcentral Nebraska. Porter et al. (1997) performed a statistical analysis of environment-driven changes in rotation benefi ts, fi nding that for each Mg ha–1 of rainfed corn yield, the benefi t of a corn–soybean rotation over continuous corn decreased by 4.1%, implying that more favorable growing conditions might counter the CCYP. Soybean may benefi t more from rotation in stressed circumstances as well. Porter et al. (1997) found the yield advantage of rotated soybean decreased by 5.8% for every Mg ha–1 that continuous soybean yield increased. Typically, analyses of yield drag from continuous cropping rely on split-plot fi eld trials for data. Such trials have an advantage, in that the systematic testing of crop sequences they entail creates strong claims for causality (or lack thereof). However, they are subject to the weakness that the standard errors for such experiments are dependent on the weather, soil qualities, and management practices at the location(s) they took place. Given this, it can be diffi cult to generalize fi ndings outside of specifi c experimental settings and circumstances. Another advantage of fi eld trials is Continuous Corn and Soybean Yield Penalties across Hundreds of Thousands of Fields
The Determination of Physicochemical Characteristics of Biodiesel and Its Blending Oils
The viscosity,combustion heat,sulfur content,carbon residue,ash,flash point,moisture and cetane value of biodiesel and its blending oils with diesel were determined.The influence of temperature on the viscosity was studied,and the physicochemical characteristics of biodiesel and its blending oils were compared with diesel.The results indicated that biodiesel and its blending oils had some superior performances and good application prospect.