Final Report for GNC08-101

Soil fertility and weed management in long-term conventional and organic crop rotations

Project Type: Graduate Student
Funds awarded in 2008: $10,000.00
Projected End Date: 12/31/2009
Grant Recipient: University of Nebraska-Lincoln
Region: North Central
State: Nebraska
Graduate Student:
Faculty Advisor:
Charles Francis
Grain Place Foundation
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Project Information

Summary:

Growing concern for public health, the environment and social issues have stimulated the modern organic agricultural movement. Certified organic production systems eliminate agricultural chemicals and reduce external inputs to minimize environmental impact and increase farm income. The USDA National Organic Program specifies the identity of organic foods and provides consumers with uniform criteria on which to choose food that is produced without synthetic chemicals or transgenic organisms. Organic agriculture is growing rapidly, but many challenges remain.

The threat of reduced crop yield in organic systems has been a primary argument among critics of organic agriculture. The assertion that conventional methods must be used to feed a growing population has led to numerous studies comparing yields in organic and conventional cropping systems. The results have been mixed, but for most crops conventional systems produce five to ten percent higher grain yields than organic systems. If yields are reduced in organic systems, it is important for researchers to close that gap and to identify other potential benefits of organic agriculture.

Two potential benefits of organic agroecosystems are the long-term sustainability of soil fertility and the conservation of biodiversity. In this study we compared long-term soil properties, yields and weed communities among two organic and two conventional crop rotations. Improved soil fertility and weed biodiversity have been observed in organic agroecosystems and both can provide substantial benefits to the crop and surrounding ecosystems.

The results of this study provide valuable production information for both organic and conventional farmers. Comparing yields of organic and conventional grain crops and determining the effects of soil fertility and weed populations on crop yield will assist farmers in making future management decisions. Long-term soil fertility levels also determine the sustainability of fertility building programs in organic and conventional systems. Lastly, the comparison of weed diversity among rotations addresses the possibility and practicality of preserving biodiversity in agroecosystems.

Introduction:

Spurred by concerns about increasing farm size, environmental pollution, reduced biodiversity across the landscape, and increased consumer demand for organic products, there have been several long-term studies investigating the differences between conventional and organic cropping systems (Lockeretz et al. 1981; Posner et al. 2008; Pimentel et al. 2005; Porter et al. 2003; Cavigelli et al. 2008; Kirchmann et al. 2007). Thus far, the literature has been somewhat contradictory. Reduced yields were reported for most crops in organic systems (Lockeretz et al. 1981; Posner et al. 2008; Porter et al. 2003; Cavigelli et al. 2008), while some organic systems produce equal or greater yields relative to conventional (Pimentel et al. 2005).

Organic cropping systems rely on complex crop rotations for weed and pest control, and for soil nutrients based on biological nitrogen fixation and the recycling of nutrients (Gosling and Shepherd 2005). Additionally, organic cropping systems often rely on organic soil amendments such as farmyard manure or compost. These organic amendments have been associated with improved soil properties (e.g. increased soil organic matter and water holding capacity, lower bulk density and enhanced pH stabilization, and increased soil levels of Ca, Mg, K and P). The inclusion of a perennial forage such as alfalfa in rotation may further improve soil structure, soil organic matter and nutrient cycling due to its capacity to biologically fix atmospheric nitrogen (Riley et al. 2008).

Complex organic crop rotations may promote diverse weed communities. When maintained below acceptable economic thresholds, diverse weed communities can provide significant benefits to the crop and surrounding ecosystems. Modern agricultural practices have led to a decline in the diversity of weeds in agroecosystems due to widespread use of herbicides and simplicity of crop rotations (Leeson et al., 2000; Murphy et al., 2006). As a result, many now view organic agriculture as a means of protecting biodiversity within agroecosystems.

If dominant weed species are not managed, organic agriculture may not be a viable solution for the maintenance of biodiversity in agroecosystems. If dominant weed species are managed properly, one could expect the seedbank and aboveground diversity to be greatest in organic systems due to the complexity of crop rotations (Murphy et al., 2006) and least in conventional systems due to the intensive use of herbicides (Mahn, 1984; Wicks et al., 1988; Moreby and Southway, 1999; Menalled et al., 2001). While the relationship between organic cropping systems and weed diversity has been well established in the aboveground weed community, fewer studies have examined this relationship in the seedbank community of long-term crop rotations.

Crop rotation, weed density management and nutrient inputs may also have an effect on seedbank diversity and relative quantity of grass and broadleaf species within a system. Grass and broadleaf weed populations are often related to the growth habit, phenology and morphology of the different crops in a rotation (Liebman and Dyck, 1993; Barberi et al., 1997; Menalled et al., 2001). Understanding the factors that influence weed communities in long-term crop rotations will be especially useful to organic farmers, who consistently rank weed management as the most important research priority (Walz, 1999; MNDA, 2007).

Project Objectives:
Objectives and Hypotheses

In 1975, the Long-Term Rotation Experiment at the Agricultural Research and Development Center near Mead, NE was initiated to determine the effects of crop rotation and animal manure on grain yield. The first four cycles of the rotation were reported by Lesoing (1992). The experiment was redesigned in 1996 to compare forage- and animal manure-based organic rotations with conventional crop rotations. The objectives of the current study were to:
1) determine the effects of management system on soil chemical and physical properties and how these factors contribute to grain yield;
2) and to evaluate seedbank density, species richness, evenness and diversity along with aboveground grass and broadleaf weed abundance and broadleaf weed diversity within and among organic and conventional crop rotations.

We hypothesized that the application of bovine manure will:
(i) increase grain yields in the organic animal manure rotation compared to the organic green manure rotation;
(ii) increase concentration of soil nutrients, including P, Ca, Mg, K, Zn, as well as increase soil pH values; and
(iii) increase levels of soil organic matter among all rotations.

We also hypothesized:
(i) that the conventional rotations (CR and DIR) will have greater grain yields than both organic rotations (OAM and OGM) in all crops;
(ii) grain yields will be affected by soil P and organic matter levels; and
(iii) in years of less than average rainfall, yields will be greatest in rotations with high levels of organic matter content.

Lastly, we hypothesized that weed density, species richness, evenness, diversity, and aboveground weed biomass will each be greatest in the organic crop rotations. The results of this study will help to document the competitiveness and long-term sustainability of organic and conventional crop rotations; results will also help to determine whether or not organic cropping systems provide a solution for increasing biodiversity in agroecosystems and generate insight useful for designing appropriate weed management strategies.

Cooperators

Click linked name(s) to expand
  • Sam Wortman

Research

Materials and methods:

A long-term crop rotation experiment was conducted at the University of Nebraska Agricultural Research and Development Center near Mead, Nebraska. It was initiated in 1975 and redesigned in 1996 to evaluate the productivity of organic and conventional rotations that differed in crop rotational diversity, weed management and nutrient inputs.

Treatments included four management treatments: conventional rotation (CR), diversified conventional rotation (DIR), organic animal manure rotation (OAM) and organic green manure rotation (OGM). These four management treatments were established in 1996. From 1975 to 1995 the treatments included continuous corn with synthetic fertilizer and herbicides (HFI CC), a four-year rotation with synthetic fertilizer and herbicide inputs (HF), a four-year rotation with manure only (ORG), and a four-year rotation with only synthetic fertilizer inputs (FO).

In 1996, the HFI CC treatment was converted to CR, the HF converted to DIR, the ORG was renamed the OAM treatment (but did not change in practice), and the FO was converted to OGM. The CR treatment was maintained in a corn – soybean – sorghum – soybean (sequence 1) or sorghum – soybean – corn – soybean (sequence 2) rotation with synthetic fertilizer, chemical herbicides and mechanical tillage. The DIR treatment was in a corn – sorghum – soybean – wheat rotation (sequence 1) or a sorghum – corn – soybean – wheat rotation (sequence 2), and also was managed with synthetic fertilizer, chemical herbicides and mechanical tillage. The OAM treatment included soybean – corn – soybean – wheat (sequence 1) or soybean – sorghum – soybean – wheat (sequence 2) and was managed with frequent mechanical tillage for weed control and bovine manure applications before the corn or sorghum and wheat crops. Lastly, the OGM treatment included an alfalfa – alfalfa – corn – wheat rotation (sequence 1) or an alfalfa – alfalfa – sorghum – wheat rotation (sequence 2). Management of the OGM treatment was limited to one bovine manure application before corn or sorghum every eight years to alleviate potential phosphorus deficiency and utilized frequent mechanical tillage for weed control.

Soil Sampling and Grain Yield:
Since the beginning of the current rotations in 1996 soil samples were taken at the beginning of each 4-year rotation cycle. Therefore, samples were taken in the early spring of 1996, 2000, 2004 and 2008. All samples were analyzed for soil pH, organic matter content (OMC), phosphorus, potassium, calcium, magnesium, sulfate and zinc. Growing season precipitation was determined using the sum of daily precipitation measurements from the High Plains Regional Climate Center station located on the University of Nebraska Turf Farm near Mead, NE (Lat.(deg)= 41.17 Long.(deg)= 96.47 Elev.(m)= 366), approximately one km from the rotation experiment.

Grain crop yield was measured by harvesting the middle 4.6 m of each experimental unit, transferring the combine contents into a weigh wagon and measuring grain water content in the lab. Corn grain yields were adjusted to 0.155, sorghum to 0.140 and soybean and wheat to 0.130 g kg-1 water content.

Weed Community Sampling:
Aboveground weed sampling was coupled with seedbank analysis to quantify differences among treatments in grass and broadleaf weed pressure and weed species diversity. Two methods were used to measure the weed seedbank: elutriation of soil seedbank samples and weed emergence from soil samples placed in greenhouse conditions. Weed seedbanks were sampled in late fall (post-harvest) following the 2007 and 2008 cropping seasons.

Data Analysis:
Grain yield and soil properties, estimates of species diversity, evenness and richness for the seedbank, along with seedbank density of grass and broadleaf species, aboveground grass and broadleaf biomass, were then compared among management treatments using PROC MIXED (SAS Version 9.1, SAS Inst., Cary, N.C.).

Research results and discussion:

Soil Nutrient Levels:
In all sampling years between 1979 and 1990, pH, organic matter, phosphorus and potassium were greatest in the organic animal manure rotation, as reported by Lesoing (1992).
Between 1996-2008, many of the soil chemical and physical properties were affected by soil amendments and time. Phosphorus concentrations in soils with manure amendments (OAM and OGM) were greater than soils with synthetic fertilizers in 2004 and 2008. Soils in the OAM treatment, which have received biennial manure amendments since 1975, had greater phosphorus concentrations than all other treatments between 1996-2008, confirming the trends established in the first four cycles of the experiment (Lesoing, 1992). Phosphorus concentrations have remained stable in CR, DIR, and OGM treatments, whereas concentrations have increased in the OAM treatment between 1996 and 2008. Similarly, organic matter content was greater in soils with animal (OAM) and green manure (OGM) amendments. While organic matter is relatively stable in the CR, DIR, and OAM treatments, the OGM treatment experienced an increase in organic matter from 1996 to 2008. Across all years, pH was greatest in the OAM treatment, followed by OGM, DIR and CR. Over time, soils amended with synthetic fertilizers (CR and DIR) have become more acidic, while soils amended with manure have become more alkaline (OAM) or remained stable (OGM). Soil levels of calcium, potassium, magnesium, and zinc were generally greater in treatments amended with manure between 1996 and 2008. Soil levels of sodium and sulfate were inconsistent among treatments and over time.

In this long-term rotation experiment, differences in soil properties were largely due to differences in the soil amendments among treatments. Phosphorus, organic matter content (OMC), pH, calcium, potassium, magnesium and zinc were all greater in soils amended with manure compared to soil amended with synthetic fertilizers. Greater concentrations of calcium, potassium, magnesium, OMC and phosphorus in organically amended soils compared to synthetically fertilized soils has been observed in several studies (Bulluck III et al. 2002; Clark et al. 1998; Ekeberg and Riley 1995; Delate and Cambardella 2004). While many of these nutrients are rarely applied by farmers (excluding phosphorus), the accrual of these nutrients may serve as additional insurance against potential yield limiting factors (Bulluck III et al. 2002).

While it may not be possible to draw definitive comparisons among treatments due to varying management practices between conventional and organic systems (e.g. manure application in the organic system), it is important to note that organic crop rotations with manure amendments tend to increase soil nutrients and OMC. Despite intense and frequent tillage, organic systems tend to retain greater amounts of soil carbon (Marriott and Wander 2006). The demonstrated ability of legume-based organic systems to build soil OMC combined with the fertility benefits of animal manure application holds great potential for the long-term sustainability of organic farming.

Grain Yield:
Averaged between 1996 and 2007, corn yield was greatest in the DIR treatment (7651 kg ha-1), followed by CR (7379 kg ha-1), OAM (6558 kg ha-1) and OGM (5045 kg ha-1). Both precipitation (measured between April 1 and September 31 each season) and treatment affected corn yield, but there was no precipitation x treatment interaction. Neither phosphorus nor organic matter content affected corn yield, but there was a phosphorus x treatment interaction.

Averaged between 1996 and 2007, sorghum yield was greatest in the CR treatment (6364 kg ha-1), followed by DIR (6138 kg ha-1), OAM (5246 kg ha-1) and OGM (4593 kg ha-1). Both precipitation and treatment affected sorghum yield, but there was no precipitation x treatment interaction. Neither phosphorus nor organic matter content affected sorghum yields.

Averaged between 1996 and 2007, soybean yield was greatest in the CR treatment (2597 kg ha-1), followed by DIR (2369 kg ha-1) and OAM (1990 kg ha-1). Both precipitation and treatment affected soybean yield, but there was no precipitation x treatment interaction. Organic matter did affect soybean yield, however this was likely not a causal relationship. Phosphorus had no effect on soybean yield. There are no soybean yields in the OGM treatment since this crop is not included in that treatment.

Averaged between 1996 and 2007, wheat yield was greatest in the OAM treatment (3066 kg ha-1), followed by DIR (2809 kg ha-1) and OGM (2392 kg ha-1). Treatment did affect wheat yield, but there was no effect of precipitation (measured between October 1 and June 15 each season) on yield and there was no precipitation x treatment interaction. Phosphorus levels did affect wheat yields. Wheat yields were greatest in the OAM treatment, which also had the greatest phosphorus levels due to the application of bovine manure every other year. There are no wheat yields in the CR treatment because wheat was not included in that treatment.

Alfalfa Yield:
Averaged between 1996 and 2007, alfalfa forage yields were 7413 kg ha-1 in the OGM treatment. This is the only treatment that included alfalfa, therefore yields were compared with the 12-year county average for conventional alfalfa forage production. The Saunders County, NE average during this time for non-irrigated alfalfa hay was 7635 kg ha-1, which is not substantially greater than the 12-year yield mean in the OGM treatment (USDA 2009).

Many of the yield reductions in the organic rotations of this study seem to be caused by increased weed cover and biomass. Posner et al. (2008) and Porter et al. (2003) reported that in years when mechanical weed control was effective, organic grain yields were equivalent to conventional. These results suggest that weed pressure is the major limiting factor in organic corn and soybean production. In 2007 and 2008, we observed greater broadleaved and grass weed biomass in corn and soybean plots of the organic rotations compared to the conventional rotations (Wortman et al., 2009, unpublished data). In the absence of nutrient deficiencies, the increased weed biomass is likely responsible for the yield loss observed within the organic rotations of this study. While grain yields were generally reduced in the organic cropping systems compared to the conventional systems, lower production costs may increase the economic competitiveness of the organic system without considering current organic price premiums (Porter et al. 2003; Pimentel et al. 2005).

Overall, there was no positive effect of organic matter or phosphorus on grain yield in any crop except wheat. Phosphorus was maintained near sufficiency levels for corn, sorghum and soybean in all treatments, so we did not expect an increase in yield among these crops due to greater phosphorus levels. Our results suggest that wheat is the most responsive crop to animal manure applications and phosphorus, further increasing its usefulness in organic cropping systems.

While precipitation positively affected grain yields across all years and crops (excluding wheat), there was not a significant treatment by precipitation interaction. This was surprising given the varying levels of soil organic matter among treatments. Increased organic matter content is associated with greater soil water holding capacity. The lack of variation in wheat yield due to varying precipitation levels demonstrates the drought tolerance of this crop. Adding drought tolerant crops such as wheat to the crop rotation adds stability to the cropping system in regions with variable weather patterns.

Seedbank Diversity

Elutriation Method:
Shannon index for diversity (H’) of the seedbank ranged from 0.37 to 1.12, depending on management treatment and year. Seedbank diversity was generally greater in 2008 than 2007, and greatest in the OGM treatment in both years. In 2007, diversity did not differ between the CR and OAM treatments or between the DIR and OAM treatments, but diversity was greater in the DIR treatment compared to the CR treatment. In 2008, diversity in the OGM treatment was greater than diversity in the OAM and DIR treatments, but not the CR treatment (p = 0.064). Diversity did not differ between the CR, DIR or OAM treatments.
The evenness (J) of species within the seedbank ranged from 0.31 to 0.61, depending on management treatment and year. As with diversity, evenness was generally greater in 2008 than 2007, and greatest in the OGM treatment in 2007. Evenness was not different between the CR and DIR treatments or between the CR and OAM treatments, but evenness was greater in the DIR treatment compared to the OAM treatment in 2007. In 2008, evenness of the seedbank was not different among the OGM, OAM and CR treatments but evenness was greater in the OGM treatment compared to the DIR treatment.

Richness (S) within the seedbank ranged from 2 to 6 species, depending on management treatment and year. In 2007, seedbank richness was greatest in the OGM treatment, followed by the OAM treatment, the DIR treatment, and the CR treatment. In 2008, seedbank richness was again greatest in the OGM treatment, but richness was not different among the remaining treatments. Species richness in the seedbank was greater in 2008 than 2007.

Greenhouse Emergence Method:
The greenhouse emergence method was only utilized in 2008 and resulted in greater estimates of diversity relative to the seedbank elutriation method (1.07 to 1.38 compared to 0.84 to 1.12) because the seedbank elutriation method may exclude small-seeded species due to the size of sieves used in the elutriation process. The sample size used in the greenhouse method is also larger (800 g of soil compared to 200 g elutriated). Thus, the probability of collecting a low density species is greater in the greenhouse emergence method. Despite the benefits of the greenhouse emergence method, the elutriation method is necessary to account for dormant seeds.

Seedbank diversity (H’) was greatest in the OAM and OGM treatments in 2008. Diversity did not differ between the CR and DIR treatments. Seedbank evenness (J) did not differ among any treatments, whereas seedbank richness (S) was greatest in the OGM treatment, followed by the OAM treatment, and richness did not differ between the conventional treatments.

Aboveground Broadleaf Diversity:
Aboveground broadleaf weed diversity was only measured in 2008 and was much smaller than the seedbank species diversity. Aboveground broadleaf weed diversity (H’) was greatest in the OGM treatment in 2008, but diversity did not differ among the remaining treatments. Species evenness (J) did not differ between the OGM and CR treatments but evenness was greater in the OGM treatment compared to the DIR and OAM treatments. Species richness (S) of the OGM treatment was greater than the CR and DIR treatments, but not the OAM treatment. Richness of the OAM treatment was greater than the CR treatment, but not the DIR treatment. There was no difference in species richness between the conventional treatments.

Overall Diversity:
Due to the greater level of crop richness and diversity of crop phenology present in the DIR treatment compared with the CR treatment, we expected greater levels of weed diversity (H’) in the DIR treatment (Liebman and Dyck, 1993; Palmer and Maurer, 1997). We observed this in 2007; seedbank diversity using the elutriation method was greater in the DIR treatment compared to the CR treatment. Diversity did not differ among the two conventional treatments using any method in 2008. While weed diversity was greater in the DIR treatment in 2007, the inconsistent relationship between crop diversity and weed diversity across years suggests that the use of herbicide inhibits any weed diversity that may have been favored by increased crop diversity.

Weed species diversity was generally greatest in the OGM treatment. Seedbank diversity was greatest in the OGM treatment in 2007 using the elutriation method, and in 2008 when measuring aboveground broadleaf diversity. The OGM treatment shared the greatest diversity with the CR treatment using the seedbank elutriation method and the OAM treatment using the greenhouse emergence method in 2008. The greater diversity observed in the OGM treatment may be the result of several factors. Similar to the DIR treatment, the crop rotation in the OGM treatment is diverse in the morphology and phenology of crops, but there appear to be other factors contributing to the varying levels of diversity in this study.

In addition to the diversity of crops in rotation, the OGM treatment required a diversity of management practices. The inclusion of perennial forage in the OGM treatment required an entirely different management scheme. For 2 years of the 4-year cycle, mechanical cultivation and rotary hoeing were no longer required and weeds were managed as a byproduct of the hay cuttings that occurred 3 or 4 times through the growing season. Furthermore, at the conclusion of the 2-year alfalfa stage, the treatment was moldboard plowed to destroy the forage crop. These two additional management measures increased the diversity of management within the OGM treatment, presumably resulting in increased weed species diversity.

Previous studies have shown that species evenness (J) is greater in monoculture systems coupled with the intensive use of herbicides compared to diverse rotations coupled with low inputs (Squire et al., 2000; Hyvonen and Salonen, 2003; Legere et al., 2005). These results were not observed in our study. There was rarely a consistent difference among treatments, and when there was a difference, the OGM treatment exhibited the greatest levels of species evenness.

Seedbank richness (S) was also greatest in the OGM treatment, followed by the OAM and conventional treatments. These results are consistent with previous studies, in that the use of herbicides may eliminate small populations of susceptible species (Hume, 1987) resulting in relatively greater species richness in low-input cropping systems (Legere et al., 2005). The greater level of species richness in the OGM treatment relative to the OAM treatment may be due to the niche that perennial forage in the rotation provides to weed species adapted to multiple cuttings (Clay and Aguilar, 1998; Cardina et al., 2002; Teasdale et al., 2004).

Seedbank Composition:

The density of broadleaf species in the tillage layer of the CR, DIR, OAM and OGM treatments was dominated by Amaranthus/Chenopodium species seeds in 2007 (87, 93, 93, and 86%, respectively). Overall, broadleaf seedbank density was greatest in the OAM treatment, followed by the OGM treatment and the conventional treatments. The Amaranthus/Chenopodium species seeds also dominated the broadleaf seedbank in the plow layer of the CR, DIR, OAM and OGM treatments (92, 97, 94 and 89%, respectively). However, in the plow layer the broadleaf seedbank density was greatest in the OGM treatment followed by the OAM treatment and the conventional treatments.

Dominant grass weed species in the tillage layer included Setaria and Digitaria spp. Grass weed species seedbank density in the tillage layer was greatest in the OGM treatment, while grass weed species seedbank densities did not differ among the OAM, CR and DIR treatments in 2007. Similar to the tillage layer, grass weed species seedbank densities in the plow layer were greatest in the OGM treatment, and did not differ among the OAM, CR and DIR treatments.

Seedbank density was only measured in the tillage layer (0-10 cm) in 2008. Similar to 2007, Amaranthus/Chenopodium spp. dominated the broadleaf species community in the tillage layer of the seedbank in the CR, DIR, OAM and OGM treatments (86, 87, 85 and 73%, respectively). Broadleaf weed species seedbank density was greatest in the organic treatments and did not differ between the conventional treatments. Also similar to 2007, the grass weed species seedbank community was dominated by Setaria and Digitaria spp. Grass weed species seedbank density was greatest in the OAM treatment, followed by the OGM treatment and did not differ between conventional treatments. Overall, weed seedbank density was greater in 2007 than in 2008.
The greater seedbank density observed in the OAM treatment relative to the OGM treatment was expected. Several studies have shown that the inclusion of a perennial forage crop in a rotation can reduce broadleaf weed populations in low-input or organic systems (Clay and Aguilar, 1998; Kegode et al., 1999; Sjursen, 2001; Porter et al., 2003).

Another possible contribution to the density of broadleaf weed species seeds in the tillage layer of the OAM treatment was the application of non-composted bovine manure every other year. Once in the seedbank, these broadleaf species may experience a competitive advantage in the nutrient-rich environment created in the OAM treatment.

Seedbank density and distribution is mainly influenced by tillage system and weed management (Barberi and Lo Cascio, 2001). In this experiment in 2007, broadleaf weed species seedbank abundance in the plow layer (10-20 cm depth) was greatest in the OGM treatment, followed by the OAM treatment. This difference was likely due to the moldboard plow utilized to terminate the alfalfa stand every fourth year in the OGM treatment.

In 2007, grass weed seedbank density of the tillage and plow layer was greatest in the OGM treatment, which was expected due to the competitive advantage gained by grass species in the perennial forage phase of the rotation (Clay and Aguilar, 1998; Cardina et al., 2002; Teasdale et al., 2004). However, in 2008 the density of grass weed species was greatest in the OAM treatment. The lack of herbicide management and timely cultivations in 2008, along with the nutrient-rich soil environment may have contributed to the greater grass density in the OAM treatment (Tilman, 1987; Aerts et al., 1990; Wedin and Tilman, 1993; Moreby and Southway, 1999). Our results suggest that weed community shifts are driven by a combination of environmental and management factors.

Aboveground Weed Biomass:
Broadleaf weed biomass was greater in the OAM corn treatment than the CR and DIR corn treatments in 2007 and 2008. Broadleaf weed biomass in the OGM corn treatment was not different from any other treatment. In soybean, broadleaf weed biomass in the OAM treatment was greater than biomass in the CR and DIR treatments. There were no differences in broadleaf weed biomass among any treatments in sorghum or wheat.

Grass weed biomass in corn was greatest in the OGM treatment. Grass weed biomass in the OAM treatment was greater than biomass in the DIR treatment but not the CR treatment. In sorghum, grass biomass was greatest in the OGM treatment followed by the OAM and conventional treatments. In soybean, grass weed biomass was greatest in the OAM treatment and did not differ between conventional treatments. In wheat, grass weed biomass was greatest in the OGM treatment and did not differ between the DIR and OAM treatments.
These results may suggest that sorghum and wheat are more competitive with summer annual broadleaf weeds, making these crops more competitive options in organic cropping systems with greater weed pressure.

Grass weed biomass was generally greatest in the OGM treatment followed by the OAM treatment and the conventional treatments. This was expected due to the lack of herbicide management in the organic treatments and the greater grass weed seed production during the alfalfa stage of the OGM rotation (Clay and Aguilar, 1998; Cardina et al., 2002; Teasdale et al., 2004).

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:
Outreach

The results of this study have been presented at several Extension workshops for organic farmers throughout the North Central Region (most often in Nebraska, but also in North Dakota and South Dakota).

We recently submitted a manuscript reporting the weed community dynamics in this study for peer-reviewed publication. A second manuscript reporting the differences in yield and soil properties is underway.

Project Outcomes

Project outcomes:
Impact of Study

As a result of this study we are now working with several organic farmers in Nebraska to improve the sustainability of crop rotations (e.g. maximizing nutrient cycling and improving weed control). To this end, we have begun a study to analyze the impact of increasing the diversity of spring-sown cover crop mixtures to maximize weed suppression, soil nutrient cycling and soil moisture (via mulches).

Economic Analysis

Economic analysis of the long-term crop rotations was not included in this project proposal, but we are currently preparing a manuscript that addresses the economic sustainability of each rotation when considering the risks, returns and stability of each system.

Recommendations:

Areas needing additional study

Future Studies

This study demonstrated the benefits of including perennial forages in rotation, and also the difficulty of maintaining sufficient soil fertility levels without animal manure in the system. Many farms are moving away from integrated crop/livestock systems. Therefore, we need to address methods of maximizing soil fertility in the absence of livestock and/or creating integrated crop/livestock systems at the landscape/regional level.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.