© 2013 Plant Management Network.
The Long-Term Agroecological Research (LTAR) Experiment Supports Organic Yields, Soil Quality, and Economic Performance in Iowa
Kathleen Delate, Departments of Agronomy and Horticulture, 106 Horticulture Hall, Iowa State University, Ames, IA 50011-1100; Cynthia Cambardella, USDA-ARS, National Laboratory for Agriculture and the Environment, Ames, IA 50011-3120; Craig Chase, Iowa State University Extension, Iowa State University, Ames, IA 50010-2046; Ann Johanns, Iowa State University Extension, Osage, IA 50461-1122; and Robert Turnbull, Iowa State University Extension, Iowa State University, Ames, IA 50011-3141
Corresponding author: Kathleen Delate. email@example.com
Delate, K., Cambardella, C., Chase, C., Johanns, A., and Turnbull, R. 2013. The Long-Term Agroecological Research (LTAR) experiment supports organic yields, soil quality, and economic performance in Iowa. Online. Crop Management doi:10.1094/CM-2013-0429-02-RS.
The Long-Term Agroecological Research (LTAR) experiment, at the Iowa State University (ISU) Neely-Kinyon Farm in Greenfield, IA, was established in 1998 to compare the agronomic, ecological, and economic performance of certified organic cropping systems to conventional counterparts. Cropping systems were designed based on local farmer input and practices. In the second LTAR phase (2002 to 2010), equivalent organic and conventional corn (Zea mays) and soybean (Glycine max) yields were achieved in the organic corn-soybean-oat (Avena sativa)/alfalfa (Medicago sativa) (C–S–O/A) and corn-soybean-oat/alfalfa-alfalfa (C–S–O/A–A) rotations compared to the conventional corn-soybean rotation (C–S). Organic oat and alfalfa yields, at 103 bu/acre and 4.4 tons/acre, respectively, exceeded county averages of 73 bu/acre and 3.3. tons/acre, for the same period. Similar plant protection occurred in organic crops, without the use of petrochemicals, compared to conventional crops maintained with synthetic pesticides. In Fall 2009, soil organic carbon, total nitrogen, and extractable K and Ca were 5.7%, 9.5%, 14.2%, and 10.8% higher in organic soils, respectively. Soil properties related to biologically active organic matter were up to 40% higher in organic soils. Economic returns to land and management in 2010 were $510/acre in the organic C-S-O/A-A rotation compared to $351/acre in the C-S rotation. The LTAR experiment will be continued as a valuable demonstration of the potential for organic crops to achieve comparable yields while increasing carbon sequestration and economic returns compared to conventional corn and soybean rotations.
Driven by consumer demand, organic product sales increased from $3.6 billion in 1997 to $26.6 billion in 2009 (31). In the most recent USDA-ERS survey in 2008, there were 907,916 acres of organic grain crops among the 4.8 million organic acres in the US (45). Several factors propelled the increased consumption of organic foods in the US, including consumer preference for lower pesticide residues in food (2), nutrition and health concerns (43), negative environmental impacts associated with conventional production (48), and consistent federal organic standards (44). Farmers also are interested in producing organic crops that meet the "triple bottom line" of environmental sustainability, economic viability, and social equity. However, organic product supply has lagged market demand owing to the perceived obstacles to successful transition to organic production (15,20).
Adoption of land management strategies fostering soil carbon (C) sequestration will be important in developing new mitigation strategies and technologies reducing C emissions (39). Recommendations fostering C sequestration include reducing tillage intensity and integrating multifunctional cropping rotations including forage legumes, small grains, and animal manure/compost soil amendments. Organic agriculture promotes such practices, as certified organic farming requires "soil-building crop rotations" and soil amendments to optimize production (44). Each of these practices affects soil and water quality, C sequestration, nitrogen (N) cycling, and other functions. Non-point-source contamination is a major water quality concern in the upper Midwest (22) where nutrients, especially nitrate-nitrogen (NO3-N), are susceptible to leaching (16) due to subsurface draining of highly productive but poorly drained soils (29). A potential strategy for improving water quality by reducing NO3-N loss is the adoption of practices such as organically managed cropping systems of extended rotations that include small grains and forage legumes that increase soil organic matter content and may improve water and soil nutrient retention (26). When the 2002 and 2007 US Farm Bills offered incentives supporting transition to certified organic production, research-based recommendations were needed to identify suitable crop rotations providing high yields, marketable grain quality, and adequate soil fertility during and beyond transition. The Long-Term Agroecological Research (LTAR) experiment was established to address these issues.
Site Description and Data Collection
Supported by the Leopold Center for Sustainable Agriculture at Iowa State University (ISU), the 17-acre LTAR experiment was established in 1998 at the ISU Neely-Kinyon Farm in Greenfield, IA, as a long-term cropping systems experiment examining the agronomic, soil quality, and economic outcomes from conventional and organic cropping systems typical of the Midwest (10). The LTAR is a systems experiment where treatments consisting of farmer-developed practices (e.g., soil amendments, tillage, crop selection/rotation) form the management strategy (11). The predominant soil at the LTAR site is a moderately well-drained Macksburg silty clay loam (fine, smectitic, mesic Aquic Argiudoll). The experimental site is located on a ridge top with a uniform slope of 0 to 2%. Research plots are 140 × 70 ft. A 2-year conventional corn-soybean (C–S) system is compared to 3-year and 4-year organic systems of corn-soybean-oats/alfalfa (C–S–O/A) and corn-soybean-oats/alfalfa-alfalfa (C–S–O/A–A), using identical crop varieties in each system. The design is completely randomized, due to uniform soil type and slope, with four replications of each cropping system treatment. All crops in each rotation treatment are planted every year of the experiment. A hay crop [alfalfa, red fescue (Festuca rubra) and oats] was planted in 30-ft strips around each plot to maintain the required buffer between conventional and organic plots (44).
During the first LTAR phase (1998-2001), organic corn and soybean yields were equivalent to conventional yields (11). In the fourth year, organic corn yield in the 4-year rotation was 131 bu/acre, compared to the 112-bu/acre conventional yield, and organic soybean yields (45 bu/acre) exceeded conventional yields of 40 bu/acre. Few soil differences were observed between systems, with soil fertility in organic plots sufficient to support high yields, due to manure applications and crop rotations. Grass and broadleaf weed populations varied between the organic and conventional systems each year, but the impact on yield was negligible. Corn borer (Ostrinia nubilalis) and bean leaf beetle (Ceratoma trifurcata) populations were indistinguishable between systems, with no effect on yield. Our objectives in the second phase (2002-2010) of the LTAR experiment were to: (i) compare yields, soil quality, and pests of organic systems relying on locally derived inputs with the conventional C-S system requiring greater levels of external, fossil-fuel based inputs; and (ii) conduct an economic analysis of the C–S–O/A organic system compared with the C–S system.
Field operations from 2002-2010 are outlined in Table 1. Crop varieties were changed annually as improved varieties for yield or pest resistance became available (12). Identical crop varieties and planting dates were used in all treatments each year to minimize initial differences between systems. Grain and forage varieties were sourced as certified organic seed whenever available. Oats and alfalfa (O/A) were drilled together, with the alfalfa in the 3-year-rotation plots remaining uncut to supply N to the succeeding corn crop. Alfalfa plots in the 4-year-rotation plots remained from spring of the establishment year to spring of the third year, before disking prior to corn planting. Conventional corn plots were injected with 28% urea ammonium nitrate (UAN) at a rate providing 140 lb N/acre. Hoop-house swine compost was applied to organic corn and oat plots to provide 140 lb N/acre and 60 lb N/acre, respectively. Average nutrient content of the compost was 0.78, 0.96, and 1.37% N, P, and K, respectively, over the course of the experiment. Conventional pesticide applications were based on ISU recommendations. All organic corn and soybean plots were rotary hoed and row-cultivated an average of three times each per season.
Grain was harvested using a field-scale combine, while alfalfa was mowed and raked to dry to proper storage moisture (less than 20%) when sufficient biomass accumulated. A field-scale baler was used and bales individually weighed. Oat straw was baled when dry (averaging 12% moisture) and removed from the plots each year.
Corn and soybean stands were enumerated annually after initial cultivations in organic plots, approximately 3 weeks after planting, by counting the number of plants per 17.3 ft of row from three random areas in each plot. Weed populations (grasses and broadleaves) were enumerated in corn and soybean plots at this time within square-foot quadrats at three randomly selected areas within plots. Soybean plots were sampled for insects by sweeping plots 20 times with a 15-inch-diameter net, placing contents in a Ziplock bag, and freezing until identification. The whorl of three corn plants per plot was examined each July for corn borer larvae damage. Soil was sampled at 6-inch depth next to 12-inch corn and analyzed for late-spring nitrate content by the ISU Soil and Plant Analysis Laboratory, Ames, IA. Corn stalk nitrate samples were collected 3 weeks before harvest from three randomly selected corn plants per plot, and analyzed at the same lab (3). Soybean cyst nematode (SCN) (Heterodera glycines) sampling was completed before harvest each year by removing four 6-inch-deep soil cores per plot and SCN eggs counted at the ISU Plant Disease Clinic (Ames, IA). Three random 8-oz corn and soybean grain samples were collected at harvest from each plot. Compositional analysis was conducted at the ISU Grain Quality Laboratory, Ames, IA, using a Foss Infratec Model 1229 analyzer (Eden Prairie, MN) to determine grain quality.
Soil analysis. Five 1.5-inch-diameter soil cores (from each plot quadrant and the plot center) were collected at a 6-inch depth from all plots each fall after harvest, prior to any tillage/planting, and mixed together to form one composite soil sample. Samples were stored in Ziplock bags and kept cool during transport to the laboratory. Bulk density was calculated using the volume of soil associated with the 5 core samples, the dry weight of soil, and the water content measurements (1,4). Field-moist samples were pushed through a 0.03-inch-diameter sieve. A sub-sample of the 0.03-inch-sieved soil was air-dried and another sub-sample was pushed through a 0.008-inch-diameter sieve. All air-dried samples were stored at room temperature prior to analysis. Soil water content, inorganic N, and microbial biomass C (MBC) were quantified using field-moist 0.03-inch-sieved soil. Soil water content was determined gravimetrically after oven drying overnight at 105°C. After extraction with 2 M KCl, inorganic N [(NO3 +NO2) and NH4] in the filtrate was determined using flow injection technology (Lachat Instruments, Milwaukee, WI). Microbial biomass C (40) was measured with standard soil fumigation (23) and chemical extraction (41,47). Nitrogen mineralization potential was measured by aerobic incubation using a modification of Keeney and Bremners (24) method in Drinkwater et al. (17). Total organic C (TOC), soil inorganic C (SIC), soil organic C (SOC), total N, N mineralization potential, and particulate organic matter (POM) C and N were determined for the air-dried 0.008-inch-sieved samples. Soil organic C (SOC) and total N (TN) were measured using dry combustion methods. Particulate organic matter (POM) fractions were isolated using a modification of the method of Cambardella and Elliott (5) and POM fraction C and N quantified using dry combustion. Macroaggregation was assessed for 0.03-inch-sieved, air-dried samples (6), and calculated as percentage of total soil >250 μm in diameter (macroaggregates). Electrical conductivity (EC), pH, Bray P, and extractable K, Ca, and Mg were quantified for air-dried 0.008-inch-sieved samples. Phosphorous soil test levels (Bray-P) (19) were measured colorimetrically using ascorbic acid-ammonium molybdate reagents. Exchangeable K, Ca, and Mg were extracted with 1M ammonium acetate (49) and measured using atomic absorption spectrophotometry. Soil pH (50) and EC (51) were measured using a 1:2 soil-to-water ratio. All results were expressed on a dry weight basis. Data for plant growth, yields, pest populations, grain quality and soil quality were subjected to analysis of variance and mean separation using Fishers Protected LSD at P ≤ 0.05.
Economic analysis. An enterprise budget, including an estimate of the costs and returns for certified organic crops, was developed for this study, following methods of Chase et al. (9). Enterprise budgets allocate costs for land, labor, and capital to the most appropriate use, as prioritized by the producer. The economic analysis was a two-part analysis, which included fixed and variable costs for inputs (e.g., machinery operations, equipment depreciation, fuel, seeds, crop fertility sources, pesticides), and a second analysis which included all inputs, labor costs, subsequent yields, and selling price of conventional and organic crops (13). Revenue was determined by multiplying conventional and certified organic prices received for the crop as an organic feed crop by the average yield obtained in the study for each system.
Although high temperatures during some months of the second LTAR phase exceeded 30-year averages, the overall average growing-season temperature was identical to the 30-year average (Table 2). Two flood years, 2008 and 2010, brought June rainfall exceeding 30-year averages by 8 and 7 inches, respectively. Five of the nine years of the study had above-average annual rainfall.
Plant Growth and Yields
From 2002-2010, average corn yields were statistically equivalent in the conventional C-S rotation and in the organic C-S-O/A and C-S-O/A-A rotations (Table 3). There was a trend towards higher corn yields in the 4-year rotation (178 bu/acre), following two years of alfalfa, compared to the 3-year rotation (166 bu/acre). Due to high variability, there were no statistical differences between systems in late-spring soil nitrate and corn stalk nitrate levels at seasons end (Table 3). However, the use of slow-release organic amendments in the organic system may have been associated with nutrient cycling differences (21,35) and more variable soil nitrate levels, which averaged 13 ppm, compared to the conventional system at 23 ppm. Despite lower nitrate levels in organic plots in the first and second LTAR phases, organic yellow dent corn yields were equivalent or greater than conventional counterparts during transition (11), and organic corn yields steadily increased from the first-phase average yield of 130 bu/acre to the second-phase average yield of 172 bu/acre across the C-S-O/A and C-S-O/A-A rotations.
Corn plant populations were similar between systems, averaging 25,568 plants/acre across all rotations (Table 3). These results mirrored those observed in the fourth year of the LTAR experiment, when rotary hoeing operations were balanced by higher seeding rates, and organic plant populations were equivalent to conventional for the first time in that phase (11). Despite a trend towards lower stands in organic soybean plots, plant populations were statistically equivalent in organic and conventional systems, averaging 106,708 plants/acre.
Second-phase LTAR organic oat yields averaged 103 bu/acre, with no difference between the 3- and 4-year rotations (Table 3), contrasting with the 68-bu/acre average oat yield in the first phase (11). Increased soil fertility from additional years of composted manure and alfalfa may have led to increased yields, along with more favorable weather. Organic alfalfa yields averaged 4.4 tons/acre over 9 years, compared to 2.9 tons/acre in the first four years (11), presumably due to higher rainfall in the second phase. Organic oat and alfalfa yields met or exceeded county averages, with 9-year average yields of 73.4 bu/acre and 3.3 tons/acre, respectively (46).
Weed, Insect, and Nematode Pest Status
From 2002 to 2010, grass and broadleaf weeds most likely to reduce corn and soybean yield potentials following tillage operations did not differ between systems (Table 4). While there was a trend towards lower weed populations in the conventional plots, rotary hoeing and row- cultivation operations in organic corn plots were successful in maintaining corn grass and broadleaf weeds under 5 and 7 plants/ft², respectively. Grass and broadleaf weeds in soybeans were less than 4 and 5 plants/ft², respectively, across all rotations, with no statistical differences between conventional and organic plots. These results contrasted with first-phase weed populations, which were greater overall in organic plots. Improved management and rotational effects may have led to decreased weeds in organic plots.
Corn insect pest populations were similar among the three rotations (Table 4). Damage from corn borers averaged 2%, similar to corn borer damage in the first phase (11), and did not reach the economic threshold level (5% damaged plants) when synthetic or biological insecticides would be required to avoid yield loss. Soybean cyst nematode populations, averaging 31 eggs per 100 cc of soil across all rotations (Table 5), were in the low range reported throughout Iowa, and less than first-phase average population levels of 149 eggs per 100 cc of soil. Bean leaf beetle populations, averaging 16 beetles per 20 sweeps (Table 5), percentage of stained soybeans, total pest insects, and beneficial insect populations were equivalent in conventional and organic rotations over this period, similar to first-phase results (11).
Corn protein content was equivalent between the conventional and organic rotations during the LTAR second phase, averaging 9.8% (Table 6), which contrasted with first-phase results, where conventional corn protein levels exceeded organic in two of the first four years. Corn oil (3.4%) and starch (61%) content were similar among the three rotations over the second phase. Soybean protein, oil, and carbohydrate content also were equivalent across all rotations in the second phase, averaging 36%, 23% and 18%, respectively. In 1 of 4 years of the first phase, soybean protein levels were lower in the C-S rotation compared to the organic rotations, but overall protein differences were indistinguishable, averaging 37% (11). The 1% decrease in protein levels can be explained by the change from food-grade, tofu-type soybeans in the first LTAR phase to feed-grade soybeans in the second phase.
Soil Quality Effects
In Fall 2009, at the last soil-sampling period in the second phase, soil quality indicators were consistently higher in the organic rotations relative to the C-S rotation (Table 7). Organic soils had more SOC and total TN, lower soil acidity, and higher extractable P, K, Ca, and Mg concentrations than conventional soils, although the comparison was not statistically significant for Mg. Soil quality parameters related to biologically-active organic matter, including MBC, particulate organic matter POMC, and potentially mineralizable N (PMN), were up to 40% greater in organic than conventional soils. The 3-year organic rotation had more inorganic P and K than the 4-year organic rotation, reflecting the greater manure application intensity in the 3-year rotation. Potentially mineralizable N was significantly greater in the 4-year organic rotation compared to the 3-year rotation, reflecting the greater organic N inputs from the additional year of alfalfa. Soil quality enhancement was particularly evident for biologically-active forms of organic matter, which are critical for maintenance of N fertility in organic systems, and for basic cation concentrations, which control nutrient availability through the relationship with cation exchange capacity (CEC).
Since 1998, the LTAR organic crop rotations have displayed greater economic performance than the conventional rotation (13,14). This economic advantage persisted in the second LTAR phase despite large price swings for conventional and organic crops and rising production costs. In 2010, for example, total input production costs, including machinery, seed, pesticides, fertilizer, crop insurance, and interest, for the organic C-S-O/A-A rotation averaged $120/acre less than the conventional C-S rotation, based on 2010 input costs (18) (Table 8). The overall lower costs in the organic rotation were due to the absence of petroleum-based fertilizers and pesticides. Manure, obtainable at no cost other than the cost of application, was applied on organic plots for fertilization.
Additionally, economic returns for LTAR organic crops have been consistently greater than conventional crops (8) despite an average doubling of labor for field operations. In 2010, returns to land and management averaged across all crops in the organic C-S-O/A-A rotation (including all labor charges) were $510/acre, compared to $351/acre for the conventional C-S rotation, based on 2010 prices for all crops (Table 9). The return to land and management is the net income that is available to pay land and management costs after deducting production costs. Land charges would be equivalent to local land rental rates and would be identical across every crop in each system, varying by year only. Because the LTAR is conducted at an Experiment Station, management charges would be identical across both systems. In the first 4-year analysis of LTAR returns, organic corn increased average revenues by a factor of 3.54 over conventional corn, while organic soybean returns were 4.84 times greater than conventional soybean revenues (14). Even during the first two transition years, returns were greater in the organic system because of lower costs of production (8, 13).
Over the 13 years of the LTAR experiment, organic corn and soybean yields have been equivalent or greater than conventional counterparts in the initial four years (11) and in the second nine years reported here. Results from the LTAR experiment have been similar to other comparisons of organic and conventional systems across the US, although LTAR organic yields have often exceeded those reported in the literature. During the first LTAR phase, corn yields in the organic system (C-S-O/A and C-S-O/A-A rotations) were 91.8% of conventional corn yields in the C-S rotation, while organic soybean yields were 99.6% of conventional soybean yield (11). In the second phase, the advantage of the longer organic rotation was demonstrated by the numerically higher corn yields in the C-S-O/A-A rotation, which averaged 99% of the average conventional corn yield, compared to 92% in the C-S-O/A rotation. Organic soybean yields over the 9-year period were 5% and 4% greater in the C-S-O/A and the C-S-O/A-A rotation, respectively, than conventional soybean yields. These results compare to those obtained by Posner et al. (34), who found, over a combined 13- and 8-year trial at two sites in Wisconsin, organic corn, soybean, and winter wheat yields were 90% of conventional counterparts, and organic forage crop yields and quality were equivalent or greater than conventional counterparts. For 34% of site-years, organic yields were lower, due to inadequate weed management. Similar results were obtained in Minnesota when weeds affected organic yields and organic soybean yields were 16 to 19% lower than conventional yields (33). Organic corn yields, however, were only 7 to 9% lower than the conventional system.
Soil quality, as evidenced by higher soil microbial diversity, has been shown to be greater in organic relative to conventional systems (27). In Maryland, despite the use of tillage in organic systems, soil combustible C and N were higher after 9 years in an organic system that included cover crops compared with three conventional no-till systems, two of which included cover crops, suggesting that organic practices can potentially provide greater long-term soil benefits than conventional no-till (42). In a 4-year Montana study, PMN was 23% higher in the organic system, with equal or greater winter wheat yields and superior grain quality than the conventional no-till system (30). Increases in soil organic matter pools also were reported in a transitional organic system in Michigan, as measured by increases in total and labile C and N (37). With the development of carbon markets and payments for carbon sequestration, additional revenue could accrue to organic producers who increase their soil quality (38). Research has shown significantly lower environmental costs, lower energy requirements, and higher carbon sequestration rates in organic systems relative to conventional counterparts in other long-term comparisons (25,28,32,36).
While conventional corn and soybean prices and revenues have been historically high since 2008, fertilizer, pesticide, seed, and fuel costs also have increased dramatically, reducing net economic returns to conventional production. Organic production is portrayed as a classic risk/reward situation in which producers assume crop production and marketing risk to receive higher average returns. An economic comparison of conventional and organic crops in a long-term trial in Maryland showed that production costs were greatest in conventional tilled systems, while the organic systems costs were similar to the conventional no-till system (7). Returns from the organic systems were greater than conventional systems, similar to LTAR results, and the lowest variability of returns occurred in the longer, more diverse organic rotation. Producers using organic practices that lead to higher rates of soil carbon sequestration and greenhouse gas (GHG) emission reduction will be better positioned for carbon-offset markets and potentially derive more profit by selling their emission credits. Thus, economic and environmental benefits from GHG reduction and enhanced soil quality could incentivize increased conversion from conventional to organic production. In the next LTAR phase, we will examine more closely the combined ecological and economic benefits of organic systems based on crop productivity, carbon sequestration, nutrient balance, reduction of nitrate leaching, and GHG mitigation.
We would like to thank the Leopold Center for Sustainable Agriculture for their support of the LTAR experiment; the Iowa State University Neely-Kinyon Farm managers, Bob Burcham, Bernie Havlovic, Jeff Butler, Randy Breach and Kirk Schwarte; Jody Ohmacht and undergraduate research assistants at the National Laboratory for Agriculture and the Environment; and Diego Viteri, Andrea McKern, and all the undergraduate and graduate students in the Organic Agriculture Program for their assistance in data collection and analysis over the nine years of this study. We also thank Catherine Greene of the USDA-ERS for organizing the National Organic Conference where these results were presented.
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