A.W. Becker, M. Ruffo, and F.E. Below 1
While the Corn Belt has relied heavily on a corn-soybean rotation for the better part of 60 years, recent events like the discovery of soybean rust in the U.S. and an increase in demand for ethanol have facilitated a shift in production to more continuous corn. Unfortunately, numerous studies have shown that corn grown in rotation consistently yields better than continuous corn (Pederson and Lauer, 2003; Peterson and Varvel, 1989) for a number of reasons. Increased yields from rotated corn have been linked to an increase in soil tilth, a reduction in insect, weed, or disease pressure, and to more readily available N to the corn plant.
Although not completely clear how, most of the detriment from continuous corn apprears to be due to the corn residue. Yakle and Cruse (1984) suggest that corn residue is allelopathic to itself, while Crookston et al. (1988) concluded that some other detriment of the residue exists in the soil where continuous corn is grown. Some have suggested that soybean nodules contribute N to the soil (Brophy and Heichel, 1989), while others have shown that minimal N for corn in rotation comes from soybeans (Bergerou et al., 2004). One possibility is that corn residue immobilizes fertilizer N (Gentry et al., 2001), thereby making it unavailable to the plant early in development. Regardless of the mechanism, some inhibitory aspect exists in soil where corn was the previous crop.
A logical place to start in trying to understand the plant response to continuous corn is the root system, since this is the part of the plant in contact with the corn residue. We found one previous study (using a minirhizotron) showing that continuous corn produced fewer roots below the plow layer, but had more above 12.5 cm compared to corn in rotation with soybean (Nickel et al., 1995). Our earlier findings from this project also showed more roots in continuous corn during vegetative growth throughout a 30 cm soil profile. Our objective for this year’s research was to examine early root and shoot growth and how these parameters respond to fertilizer N and previous crop residue.
Our 2006 trial was carried out on a second Management Evaluation Nursery (MEN), located at the Crop Sciences Research and Education Center, Champaign, IL. We had to establish two separate fields in order to have a statistically valid comparison of corn and soybean as the previous crop in the same field for two consecutive years. Corn and soybean were grown in 2005 to establish the site, and the corn was fertilized with 100 lbs of N per acre. The soil type at the MEN2 is a Flanagan/Drummer silty clay loam with a high productivity index, but also is responsive to N. This series is slightly more productive than the Catlin/Flanagan silt loam that comprises the MEN 1. The MEN 1 from last year’s study has been set up to support a similar experiment in 2007.
Treatments consisted of two hybrids, the previous crop (soybean and 3rd year corn), and the N rates (0, 60, 120, 180, and 240 lbs/acre). The experimental design was a split-split-plot arrangement of treatments in a randomized complete block design with 4 replications. Previous crop was the main plot, N rate was the sub-plot, and hybrid was the sub-sub plot. An experimental unit was a 4 row plot (rows 17.5’ long, spaced 30” apart) where three rows were used for destructive sampling and one inner row for yield determination. The field was overplanted on April 26th and plots were thinned to a 31,000 plants/acre stand with emergence counts taken prior to thinning. The N treatments were applied as granular ammonium sulfate at the V3 growth stage and incorporated with field cultivation.
Roots were sampled at two growth stages, V8 and V12, when rapid plant N uptake is just beginning and when N is accumulating at a linear rate. To sample the roots, a 2” split-core sampler was driven into the ground at two distances; 4” perpendicular to the plant at V8 and 6” perpendicular to the plant at V12. The cores were taken to a depth of 12”, then subdivided into three equal depths. Roots were extracted from the soil, cleaned, and analyzed by WinRhizo, a computer scanning program. In conjunction with the root samplings, whole shoots were sampled for N analysis and biomass accumulation. Four plant aggregate samples were obtained, oven dried, weighed, and ground for N analysis (and other nutrients at V12). Plots were hand harvested for grain yield on September 10th.In contrast to last year, 2006 provided nearly ideal growing conditions. Temperatures were 3 degrees (°F) above the average for April, but May and June were slightly cooler than average (~1° F), and precipitation was near average for those two months (data not shown). This is in stark contrast to the same time period in 2005, where May and June were much hotter and drier than normal. July and August were both slightly warmer than normal in 2006, but precipitation was also slightly above the average, so the crops in East Central Illinois experienced good weather conditions during most of the growing season. As a result, this year’s growth and yield measurements were consistently better when compared to the 2005 trial. The first evidence of this came at VE, where emergence numbers were much higher than those of the previous year. Plants grown in rotation emerged at an 85% average in 2005 and 96% in 2006, compared to 73% in 2005 and 90% in 2006 for continuous corn.
As early as V8, plants grown under continuous corn had noticeably smaller shoots (aboveground portion) than rotated plants (Figure 1). This effect was consistent over the two diverse years, and occurred well before there were any shoot growth differences attributed to N supply. In neither year could increases in the N supply overcome the growth differences from continuous corn.
The amount of roots in continuous corn varied between the two years, while roots of rotated plants were remarkably similar across the environments (Table 1). In the poorer year of 2005, continuous corn seedlings produced longer roots that had a greater surface area than plants grown in rotation. For the better year of 2006, there was no difference between the length or the surface area of roots in both previous crops. This finding is contrary to the shoot growth reduction from continuous corn and suggests a difference in assimilate partitioning. To test partitioning, a shoot:root ratio was calculated using shoot dry weight divided by root surface area (Table 1). In both years, the shoot:root ratio was substantially higher for rotated plants compared to those grown in continuous corn. Thus, the increase in root growth from continuous corn was at the expense of shoot growth, at a time in the plant’s life cycle when a large portion of the reproductive potential is being determined.
Plants were sampled again at V12 and by this stage a growth response to N supply was clearly evident (data not shown). The growth difference from previous crop was still present (between 25-30% larger shoots in rotated plants), and like at V8, this difference could not be overcome with additional N. Similar to V8, rotated plants also had a higher shoot:root ratio at V12 than plants grown in continuous corn.
The V12 shoots were also analyzed for mineral nutrient concentration and the macronutrient data is shown in Figure 2. For each of the six macronutrients, the tissue nutrient status was either greater (P, K, S) or similar in continuous corn compared to rotated plants. Similar results were obtained in 2005 and 2006, except that K levels were higher and N levels lower in 2006. These findings, and the lower shoot:root ratio of continuous corn plants, suggest that nutrient deficiencies are not the cause of the poorer shoot growth under continuous corn compared to rotated plants.
The early effects of continuous corn on shoot growth were also observed in the final grain yield, as continuous corn yielded about 25 bu/A less than rotated corn in both years (Figure 3). This difference was similar in spite of the 90 bu/A yield difference between years, and in neither year could the yield decrease from continuous corn be ameliorated with additional N.
In contrast to fertilizer recommendation systems that normally prescribe more N for continuous corn, our findings showed that similar amounts of N were required for both management practices. The N requirement, however, was highly influenced by the year, with 70-75 lbs/A of N optimizing yield in 2005 compared to 140-145 lbs/A in 2006.
For two very different years, the previous crop had an early and profound impact on aboveground vegetative growth that was independent of the amount of N supplied. Seedlings grown in continuous corn had lower shoot growth, but similar or higher mineral nutrient concentration, and similar or greater root growth. Although increasing the N supply enhanced growth and grain yield, it did not overcome the detriment associated with continuous corn. These data suggest that corn residue is the major detriment in continuous corn production. In order to devise management strategies to help alleviate the productivity decrease from continuous corn, a better understanding of how corn residue alters plant growth and partitioning is needed.
1 A. W. Becker is a Research Assistant, M. Ruffo is a Postdoctoral Research Associate, and F.E. Below is a Professor, Dept. of Crop Sciences, University of Illinois, Urbana IL
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