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Toward Sustainable Agricultural Systems in the 21st Century 3 Improving Productivity and Environmental Sustainability in U.S. Farming Systems The widespread implementation of management practices that improve productivity and environmental sustainability, along with new environmental policies and regulations in the last 20 years, have been effective in reducing many detrimental effects of agriculture. Research aimed at understanding how these management practices and engineering approaches work continue to provide additional tools for progress toward the sustainability goals outlined in Chapter 1. This chapter briefly discusses some of the management approaches and practices that are relevant to productivity and environmental sustainability and have an impact on agriculture’s natural resource base (goals 1 and 2 in Chapter 1). Table 3-1 illustrates the relationships between the two sustainability goals and subgoals, management activities and specific practices that can be used to reach the goals, and a selection of potential indicators that are or could be used to assess progress toward specific goals. Each section in this chapter discusses how specific practices can contribute to crop or livestock productivity and improve various aspects of environmental sustainability or enhance the quality of a resource. The extent to which the practices are adopted by farmers is discussed if data are available. However, a practice by itself might improve sustainability in relation to one goal but might have a negative effect on another; hence, the disadvantages of each practice are also discussed. A farm is a system that contains multiple interrelated elements, and the interrelationships between environmental conditions, management, and biological processes determine such outcomes as the environmental impact, efficiency, and resilience of the farm (Drinkwater, 2002). Some of the disadvantages of certain practices might be overcome if a complementary practice is used. In other words, the collective outcome of several agricultural practices would be different from simply adding the anticipated outcome of individual practices. Therefore, many in the scientific community have been adopting a “systems” approach, which emphasizes the connectivity and interactions among components and processes and across multiple scales, to understand and harness complex processes. “Systems agriculture” is an approach to agricultural research, technology development, or extension that analyzes agriculture and its component farming
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Toward Sustainable Agricultural Systems in the 21st Century TABLE 3-1 An Illustration of Activities and Practices Used to Achieve Agroecological Sustainability Goals and of Indicators for Evaluating Sustainability Agroecological Sustainability Goals Examples of Indicators Activity Examples of Practices 1. Satisfy human food, fiber, feed, and fuel needs a. Sustain adequate crop production • Yield per unit area, yield per unit resource use (energy, water, and nutrients) • Crop management • Fertility, pest, and water management (see below for specifics). Plant breeding and genetic modification to improve yield and stress tolerance. • Plant breeding • Crops bred for increased resistance to biotic and abiotic stresses, enhanced nutrient use efficiency, and yield stability b. Sustain adequate animal production • Production per unit land, production per animal, production per unit resource use (energy, water, nutrients), mortality, duration of productive life, conversion of feedstuff to human edible products, animal health • Animal husbandry • Use of local feedstuffs, careful use of resources (labor, water, energy), breeding for increasing feed efficiency, animal health and welfare, herd health management (disease prevention), improved housing environments, judicious use of antibiotics, waste management, manure applications to field, and advanced treatment technologies for manure 2. Maintain and enhance environmental quality and resource base a. Maintain or improve soil quality • Soil nutrient levels, nutrient use efficiency • Soil-fertility management • Fertilizer and organic amendment application, use of soil and tissue tests, nutrient budget calculations • Soil organic matter content, microbial and macrofaunal populations and communities • Organic-matter management • Conservation tillage, organic amendments, composts, green manure • Soil physical structure such as bulk density, water-holding capacity, aggregate stability, porosity, water infiltration rate • Organic-matter management • Conservation tillage, organic amendments, compost, green manure b. Maintain or improve water quality • Fertility inputs, field or farm nutrient budget balances, nutrient, pesticide, and pathogen concentrations in water courses, leaching estimates, nutrient or water model outputs • Soil-fertility management • Use of nutrient budgets, use of slow release fertilizers and organic amendments, plant nutrient tissue tests, soil nutrient tests, manure disposal
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Toward Sustainable Agricultural Systems in the 21st Century • Ground cover, USLEa, direct measures of nutrient, sediment and pesticide fluxes, area in cover crops or perennial vegetation, soil aggregate stability, water-holding capacity, porosity, water infiltration rate • Crop-vegetation management, nutrient management, and erosion and runoff control • Plant cover crops, use of organic amendments, soil and tissue tests, conservation tillage, mulches, grass waterways, buffer strips, riparian vegetation, treatment wetlands c. Conserve freshwater supply • Crop water use efficiency, water consumption, ground water overdraft, pumping rates • Irrigation management • Drip irrigation, irrigation scheduling based on evapotransporation or soil moisture d. Reduce pesticide use • Pest populations, natural enemy populations, weed biomass, percent weed cover, vegetation diversity, presence of perennial habitat • Management of pest complex • Integrated pest management practices, biological and ecological approaches, soil organic matter management, crop breeding e. Conserve and enhance biodiversity • Biodiversity estimates (for example, number of plant species, number of species within selected animal groups, habitat diversity, landscape complexity, and connectivity) • Habitat management • In-field insectaries, hedgerows, riparian vegetation, habitat corridors, natural habitat fragments aUniversal soil loss equation. systems in a holistic way. Chapter 5 uses a few farming systems to illustrate how systems research is conducted and how the practices can work together to achieve multiple environmental, economic, and social sustainability goals. The following sections, however, focus on a series of activities that constitute crop and animal production, and highlight particular practices that are seen, or have the potential, to enhance sustainability. The emphasis is on developments that have occurred over the last 20 years. SOIL MANAGEMENT Management of soil to improve sustainability is a complex matter that requires a thorough understanding of its physical, chemical, and biological attributes and their interactions. Proper soil management is a key component of sustainable agricultural production practices as it produces crops and animals that are healthier and less susceptible to pests and diseases. It provides a number of important ecosystem services, such as reduced nitrogen runoff and better water-holding capacity (NRC, 1993). Mismanagement of soil can result in physical, chemical, and biological degradation (Lal, 2004b), as discussed in Chapter 2. Soil management is critical to improving environmental sustainability of farming systems. Proper soil management practices aim to:
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Toward Sustainable Agricultural Systems in the 21st Century Maintain or build up soil organic matter. Improve soil structure by increasing soil aggregates. The soil aggregates would in turn enhance water-holding capacity of soil. Minimize erosion. Reduction in wind erosion would improve air quality. Reduction in water and tillage erosion would improve water quality by reducing sediment loading. Enhance soil microbial activities and diversity. Reduce soil-borne pathogens. Conservation Tillage One of the most important changes in U.S. agriculture in the last 20 years has been the movement away from conventional tillage to conservation tillage. Conventional tillage, such as moldboard plowing, results in considerable disturbance of the soil and breaks down its aggregate structure. Because aggregation reduces soil density and helps to maintain a balance of air and water in the soil, disturbance by tillage that breaks aggregates apart can result in soil compaction and reduced oxygen levels. Although conventional tillage contributes to weed and pest control, it also destroys habitats or disrupts the life cycle of some beneficial organisms (for example, earthworms and microorganisms) and reduces soil organic matter in the surface layer. Increased soil erosion as a result of intensive tillage is long recognized (NRC, 1989). Tillage erosion is the downslope displacement of soil through the action of tillage (Lindstrom, 2006) and results in soil loss on hilltops and soil accumulation at the base of slopes. Because water erosion tends to be more important at the base of slopes than at hilltop positions, tillage erosion tends to reinforce water erosion (Government of Manitoba, 2009) and thereby increases sediment runoff and sediment loading into surrounding surface water. Phosphorus, herbicides, and other contaminants that absorb readily to soil particles move with sediment into surface water. Phosphorus from agricultural fertilizers enriches the receiving bodies of water and can cause large blooms of algae. Conservation tillage is an agricultural practice that reduces soil erosion and water runoff, increases soil water retention, and reduces soil degradation. Conservation tillage, including ridge-till, mulch-till, and no-till practices, is any tillage and planting system that leaves 30 percent or more of the soil surface covered by crop residues after planting to reduce soil erosion by water. No-till leaves 50 to 100 percent of the soil surface covered from harvest to planting, depending on the crop residue, because it uses specifically designed seed planters or drills to penetrate all remaining surface residues (Huggins and Reganold, 2008). Comparisons of conventional tillage practices to conservation tillage in corn, soybean, and winter wheat found that systems that use conservation tillage tend to use more herbicides for each crop, but less insecticides (USDA-ERS, 2005). Impact of Conservation Tillage Physical Properties of Soil Soil under no-till management has been shown to have a higher proportion of water stable aggregates (Karlen et al., 1994a; Abid and Lal, 2008), and the aggregates have larger geometric mean diameter and mean weight diameter compared to chisel-plowed soil (Abid and Lal, 2008). The large aggregates contain finer soil textures that assist in retaining more water than small aggregates. Arshad et al. (1999) compiled data collected from two sites in northern British Columbia to ascertain the long-term effects of conventional tillage and
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Toward Sustainable Agricultural Systems in the 21st Century no-till on soil components thought to be important in surface soil structural improvement. They observed that soil water retention was greater under no-till compared with conventional till without dramatically altering bulk density because of redistribution of pore size classes into more small pores and less large pores. No-till and other conservation tillage systems can work in a wide range of climates, soils, and geographic areas. Continuous no-till is also applicable to most crops, with the notable exceptions of wetland rice and root crops, such as potatoes. However, no-till crop production on fine-textured, poorly drained soils can be problematic and often results in decreased yields. Yields of no-till corn, for instance, are often reduced by 5 to 10 percent on those kinds of soils, compared with yields with conventional tillage, particularly in northern regions. Because the crop residue blocks the sun’s rays from warming the earth to the same degree as occurs with conventional tillage, soil temperatures are colder in the spring, which can slow seed germination and curtail the early growth of warm-season crops, such as corn, in northern latitudes (Huggins and Reganold, 2008). Soil Organic Matter The amount of organic matter in soil subject to conventional tillage has been compared to soil subject to conservation tillage or no-till in different locations. Dell et al. (2008) quantified the impacts of no-till and rye (Secale cereale L.) cover crops on soil carbon and physical properties. They found that the no-till fields had 50 percent more carbon particulate and mineral-associated pools in the upper 5 cm compared to conventional tillage. The sizes of the carbon pools below 5 cm in the two fields were similar. The stability of the soil aggregates is proportional to the carbon pool size. Another study by Motta et al. (2007) compared soil organic carbon at different depths of the soil in cotton fields subject to conventional tillage and no-till. They found that the no-till fields had much higher particulate organic carbon within the top 3 cm. Some scientists have questioned if substantial soil carbon sequestration can be accomplished by changing from conventional plowing to conservation tillage. Baker et al. (2007b) argued that soils were sampled to a depth of 30 cm or less in essentially all cases where conservation tillage was found to sequester carbon. In the few studies where sampling extended deeper than 30 cm, conservation tillage has shown no consistent accrual of soil organic carbon. Instead conservation tillage showed a difference in the distribution of soil organic carbon, with higher concentrations near the surface in conservation tillage and higher concentrations in deeper layers under conventional tillage. Blanco-Canqui and Lal (2008) assessed the impacts of long-term no-till and plow-based cropping systems on soil carbon sequestration in the top 60 cm of soils across Kentucky, Ohio, and Pennsylvania. They found that no-till farming increased organic carbon concentrations in the upper layers of some soils, but it did not store more organic carbon than plowed soils for the whole soil profile. In fact, total soil profile organic carbon was significantly higher in plowed-based soils in a number of the areas sampled. In another study, Christopher et al. (2009) found that the soil organic carbon pool in the whole soil profile (0–60 cm) was never greater in no-till than conventionally tilled fields across 12 contrasting but representative soils in the Midwestern United States and was actually lower in the no-till soils in some areas. Soil Microbial Activity and Diversity Bacteria, fungi, and nematodes are important in maintaining the physical structure of soil. In a study of soil quality with data collected following a long-term tillage study on continuous corn, Karlen et al. (1994a) found that plots managed using no-till practices have higher microbial activity and earthworm populations. Motta et al. (2007) also found higher microbial biomass in no-till cotton fields compared to conventional-till ones.
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Toward Sustainable Agricultural Systems in the 21st Century Soil Erosion The greater the percentage of ground cover (residue or mulch), the lower is the soil loss ratio (Figure 3-1) due to water and wind. The soil loss ratio (SLR) is an estimate of the ratio of soil loss under actual conditions to losses experienced under the reference condition of clean-tilled continuous-fallow conditions (the reference condition). Leaving 30 percent of the soil surface covered with residue, as with conservation tillage, reduces erosion by half as compared with bare, fallow soil. Leaving 50 to 100 percent of the surface covered throughout the year, as no-till does, reduces soil erosion dramatically. Montgomery (2007) looked at numerous studies on conventional (n = 448) and conservation (n = 47) agricultural systems and found an average net soil loss of 3.9 mm/yr under conventional agriculture and 0.12 under conservation agriculture that included conservation tillage, no-till methods, and terracing. Montgomery further examined 39 studies involving direct comparisons of soil erosion under conventional and no-till methods representing a wide variety of settings with different erosion rates and showed that no-till practices reduce soil erosion up to 1,000 times, enough to bring agricultural erosion rates into line with rates of soil production. Sediment Loading and Water Quality Agriculture is a major contributor to sediment pollution, primarily because of improper farming practices that increase soil erosion. Farming on steep slopes, excessive heavy tillage, and lack of conservation practices are principal causes. A number of studies document the effectiveness of conservation or no-till on reducing sediment in runoff. Blevins et al. (1990) compared the contributions of no-till, chisel-plow tillage, and conventional tillage systems used in corn production to sediment losses and surface runoff on a Maury silt loam. Over a four-year period, they measured soil losses of 20, 0.71, and 0.55 Mg/ha from conventional, chisel-plow, and no-till systems, respectively. Amounts of nitrate (NO3–), soluble phosphorus, and atrazine leaving the plots in surface runoff were greatest from conventional tillage and about equal from chisel-plow and no-till. Chichester and Richardson (1992) compared the effect of no-till and conventional chisel-till soil management on runoff FIGURE 3-1 Soil loss ratio and percent ground cover. SOURCE: McCarthy et al. (1993). Reprinted with permission from the University of Missouri Extension.
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Toward Sustainable Agricultural Systems in the 21st Century water volumes, sediment loss, and nitrogen and phosphorus loss from small watersheds on a clay soil. They found that runoff volume was not changed by tillage system, but sediment loss and nitrogen and phosphorus losses in runoff were far less, on average, from no-till than from chisel-till. Average annual quantities for sediment and nutrient losses were: 160 kg/ha and 1575 kg/ha for sediment, 3.8 kg/ha and 8.1 kg/ha for nitrogen, and 0.8 kg/ha and 1.5 kg/ha for phosphorus for no-till and chisel-till, respectively. Although erosion remains a significant problem in the United States, conservation and tillage changes have resulted in substantial improvements over the last 30 years. Soil erosion on cropland declined, as a result of changes in tillage practices and land retirement, from 3.1 billion tons per year in 1982 to 1.8 billion tons per year in 2001, while sheet and rill erosion dropped by almost 41 percent, and wind erosion dropped by 43 percent during the same time period (NRI, 2003). Air Quality With the advent of reduced and “zero” tillage in the past few decades made possible through the use of herbicides, releases of carbon dioxide (CO2), nitrous oxide (N2O), and particulate matter from agricultural soil have been reduced (Robertson et al., 2000; Madden et al., 2008). Reduced tillage reversed some of the soil carbon decline in surface soils. The impacts of tillage and different cropping systems on soil carbon (discussed earlier in this chapter) can be translated with reasonable accuracy into changes in CO2 flux over time. When CO2 flux is calculated and N2O and methane (CH4) fluxes are measured, the overall atmospheric impact of production systems can be assessed. Unfortunately, there are few production systems where such gaseous flux measurements have been done over a sufficient time span. One of the best sources of data comes from the Long Term Ecological Research (LTER) sites funded by the National Science Foundation (NSF). The LTER data in Box 3-1 are presented not to represent overall U.S. agricultural fluxes, but rather to show comparisons for the predominant gases between natural and managed systems, and the contribution of key management practices. The net greenhouse-gas emissions from agriculture in the United States were estimated to be about 50 g of CO2 equivalent/m2 per year (West and Marland, 2002). That estimate is comparable to the data presented in Table 3-2 in Box 3-1. A large database, several models for soil carbon accumulation, and ongoing research at the Natural Resources Ecology Laboratory in Colorado are focusing on the carbon accumulation potential of soils under different management (Easter et al., 2007). Data from other long-term organic comparisons in California, Wisconsin, and Pennsylvania give similar effects for carbon sequestration. Studies conducted on finer-textured soils (most other than the Michigan trial) show higher levels of carbon sequestration and hence could be expected to show greater global warming mitigation potential. None of those studies monitored greenhouse gas over the long term. The bottom line is that agricultural systems can be designed and managed to compare favorably with natural ecosystems if moldboard plowing is eliminated in favor of either reduced or zero tillage. Zero-till can also reduce emissions of particulate matter, especially if the practice is used with mulching or cover cropping. For example, in the dryland areas of wheat production on the eastern side of the Cascades and in portions of the Great Plains, clean fallow for moisture conservation has long been practiced. The fallowing leads to occasional high wind-blown soil erosion with increasingly unacceptable air quality problems (Sharratt and Lauer, 2006). New programs for crop rotation, weed and disease control, and reduced-till planting in the dryland wheat-growing area of eastern Washington are promising (Schillinger et al., 2007). Barley grown every other year seems to reduce rhizoctonia bare-patch area in wheat. Risk due to uncertain rainfall appears higher in crop rotation
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Toward Sustainable Agricultural Systems in the 21st Century BOX 3-1 Tillage and Rotation Effects on Climate Change Greenhouse-Gas Emissions and Energy Use The Kellogg Biological Station in southwestern Michigan is the only Long Term Ecological Research (LTER) site devoted to agricultural systems comparisons. It includes comparisons between agricultural and natural ecosystems in various stages of disturbance, making it unique. Greenhouse-gas flux measurements provide comparative data for agricultural systems and natural ecosystems at varying maturity stages (Robertson et al., 2000). Four corn–wheat–soybean rotations were replicated: (1) conventional chemical inputs and conventional (moldboard) plowing, (2) conventional inputs and zero tillage, (3) reduced chemical inputs, and (4) organic with no chemical inputs. Systems 3 and 4 included a winter legume cover crop following the corn and the wheat portions of the rotation. The trials were carried out on a silt loam soil that had been in continuous cultivation since the mid-1800s. Data are from the first eight years of the trial from 1991 to 1999. The net greenhouse warming potentials for the several agricultural systems and for comparison natural ecosystems in various stages of maturity are shown in Table 3-2. The annual crop rotations produced surface soil carbon changes consistent with those of other long-term trials in the U.S. corn belt. Conventional tillage with rotation had no change, indicating the long-term soil carbon equilibrium under conventional management with moldboard plowing that existed at the start of the experiment. No-till had the highest added carbon in soil and had negative CO2 release (–110 g/m2 per year of CO2 equivalent). The low-input and organic systems were next. Perennial crops (alfalfa and poplar) had significantly higher carbon sequestration than annual crops. Natural communities added soil carbon depending on their length of time in development. The early succession treatment was kept in grasses and other herbaceous plants by annual mowing. That treatment was thus similar to a standard set-aside common to many farms enrolled in the U.S. Department of Agriculture (USDA) National Resource Conservation Service (NRCS) conservation programs. Such treatment has large carbon sequestration in the early decades following implementation, and therefore a significant greenhouse warming mitigation effect. Inputs calculated as CO2 equivalents differed according to cropping system. Nitrogen fixation by legumes and from denitrificaton emits N2O as a byproduct. It was roughly the same for the annual crop systems and for alfalfa in those systems. The biological pathways for nitrogen fixation and “leakage” and those for commercial fertilizer bioconversion differ, but the net effects on N2O evolution are similar. Denitrification is higher when soils high in nitrogen experience waterlogging, producing low oxygen levels when soil temperatures are high. In LTER, the systems without fertilizer and with fewer legumes showed lower N2O evolution. All systems oxidized CH4 (removed it from the atmosphere), with the natural communities oxidizing slightly more than the commercial systems because of the canopy composition. than in the traditional wheat-summer fallow. Modified tillage implements that undercut the root zone have promise. Some farmers in the area used no-till planting with rotations with success. The results are not ready for widespread adoption; continued research is essential. This research program appears to be a flagship program for low-rainfall cropping systems in the Pacific Northwest. Energy Use In 2006, no-till was practiced on 62.4 million acres of cropland in the United States and resulted in an annual savings of 243 million gallons of fuel for tillage (Table 3-3). The energy saving was estimated solely on the basis of reduced requirements for direct energy inputs for tillage. That estimate did not include the additional efficiencies gained from increased productivity as a result of increased soil quality as described above for enhancement of ecosystem services. When calculated for a 2100-acre Michigan corn–oats–soybean–wheat rotation farm, diesel fuel savings over conventional tillage would have been 28 percent for mulch-till, 27 percent for ridge-till, and 52 percent for no-till (USDA-NRCS, 2008a). In drier areas such as the western Corn Belt, returns are uncertain because of high variability in
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Toward Sustainable Agricultural Systems in the 21st Century TABLE 3-2 Relative Radiative Forcing Potential for Different Management Systems Based on Soil Carbon Sequestration, Agronomic Inputs, and Trace Gas Fluxesa CO2 Equivalents of Change (g/m2 per year)b Ecosystem Management Soil C N fertilizer Lime Fuel N2O CH4 Net Global Warming Period Annual crops (corn–soybean–wheat rotation)c Conventional tillage 0 27 23 16 52 –4d 114 No-till –110 27 34 12 56 –5 14 Low input with legume cover –40 9 19 20 60 –5 63 Organic with legume cover –29 0 0 19 56 –5 41 Perennial crops Alfalfa –161 0 80 8 59 –6 –20 Poplar –117 5 0 2 10 –5 –105 Successional communities Early successional –220 0 0 0 15 –6 –211 Mid-successional (historically tilled) –32 0 0 0 16 –15 –31 Mid-successional (never tilled) 0 0 0 0 18 –17 1 Late-successional forest 0 0 0 0 21 –25 –4 aData source: Robertson et al. (2000). bResults based on eight years of data (1991–1999), using IPCC (1996) conversion factors. cSix replications of each for annual and perennial crops. Successional communities were nearby on similar soil types. Conventional and no-till treatments had full herbicide and fertilizer use. Low-input treatment used banded herbicides at low rates and low levels of nitrogen. Organic treatment had occasional lime input only, but no herbicides or fertilizer. dNegative values represent a net CO2 equivalent uptake, or a net reduction in greenhouse gases and a reduction in atmospheric radiative forcing. Comparison of net effect on greenhouse-gas emissions showed that no-till had the least greenhouse-gas impact among the annual cropping systems. Conventional tillage and chemical use had the highest greenhouse-gas emission impact. The low-input system had lower greenhouse-gas impact than conventional tillage, but its yields were lower. The organic system had yields close to those of zero-till, followed by low input. Perennial systems and early succession communities had the most positive effects on reducing greenhouse-gas emissions. rainfall. The accounting of long-term effects, including impact of increased surface organic matter and changed fertilizer requirements during the transition period, complicates total energy balance considerably. In U.S. studies, outputs are most often calculated as energy content of the harvested product. That measure of output complicates comparisons because TABLE 3-3 Energy Savings and Production Potential from Conservation Practices and Measures in the United States Conservation Practice Conservation Measurement Resource Savings Energy Costs Reduction On-farm (per acre) Total Million $ Crop residue management 62.4 million acres of no-till (CTIC) $11.70 243 million gallons 730 Conversion of additional 50 million acres to no-till $11.70 195 million gallons 585 SOURCE: USDA-NRCS (2006).
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Toward Sustainable Agricultural Systems in the 21st Century different crops in the rotation produce considerably different energy amounts, and their relative yields change dramatically over years; hence, long-term studies are needed for meaningful comparisons, which partly explains the paucity of such comparisons. Disadvantages of Conservation Tillage Potential problems with conservation tillage include weed control, soil crusting and compaction, flooding or poor drainage, delays in planting because fields are too wet or too cold, carryover of diseases or pests in crop residue, fewer options to work fertilizers and pesticides into the soil, new machinery requirements, increased risk of shifting weed populations that are resistant to specific herbicides, and the need for above-average farm management skills (Peigne et al., 2007; Huggins and Reganold, 2008). Because conservation tillage increases the size and prevalence of macropores in soil, there has been concern about increased leaching of pesticides to ground water in particular during heavy rainfall (Shipitalo et al., 2000). In some cases, tillage residues such as rye can have allelopathic effects on seed germination in other crops, especially when seeds are planted directly into recently killed rye residues or some mow-killed mulches (Mitchell et al., 2000). High carbon-to-nitrogen ratios in crop residues can also cause problems such as reduced nitrogen availability (Gebhardt et al., 1985; Troeh and Thompson, 2005; Baker et al., 2007). Some of the problems mentioned above might be more prevalent in vegetable production systems than in field crops. Successful vegetable production with conservation tillage depends on careful crop selection. Crops that germinate quickly and grow rapidly in the first few weeks after planting are more competitive with weeds than crops that initially grow slowly. Cool-season vegetables perform better in spring no-till plantings than warm-season crops (Hoyt and Konsler, 1988). The availability of specialized equipment for planting horticultural crops in no-till systems can be a limitation, but large-seeded vegetables such as sweet corn, snap beans, and squash have been successfully planted with no-till planters designed for field corn or soybean, and no-till planters for planting cabbage, broccoli, and other vegetable transplants in no-till soils have been developed (Hoyt, 1999; Peet, 2008). The impact of reduced tillage and no-till on rates of chemical use and on nutrient leaching has been mixed because it depends on whether herbicide and pesticide uses are increased as a result of reduced tillage and how nutrients and agricultural chemicals are applied (Lal, 1991; Daverede et al., 2003). There is, however, evidence that pesticide leaching and NO3– in drainage water is higher under no-till conditions because of movement through intact macropores (Isensee and Sadeghi, 1996; Stoddard et al., 2005). In addition, higher average concentration and load of soluble phosphorus have been found in runoff water of no-till systems compared to other tillage systems (McIsaac et al., 1995). Moldboard plowing has been shown to reduce nitrogen and phosphorus runoff by redistributing the nutrients into the soil profile (Gilley et al., 2007). Similarly, Garcia et al. (2007) and Quinke et al. (2007) proposed and demonstrated a promising strategy of tilling one-time only with a moldboard plow to reduce phosphorus in runoff, followed by no-till management. They observed a significant reduction in soluble phosphorus accumulation in runoff with no negative effects on soil quality or crop yield. Further research is needed on management of no-till systems to reduce negative water quality effects. In organic farming systems, reduced tillage raises specific challenges because the use of herbicides to kill the preceding crop is prohibited. Nonetheless, the sparse research on reduced tillage methods (strip till, ridge till, or shallow tillage) has shown promising results (Schonbeck, 2009). The choice of crop rotation, cover crop, and cover crop management is
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Toward Sustainable Agricultural Systems in the 21st Century critical. Winter-hardy cover crops that are amenable to no-till, no-herbicide management can be killed by mowing or rolling in early summer. Non-winter-hardy crops planted two to three months prior to the anticipated frost-kill date can be used to form in situ mulch and suppress winter and early spring weeds. Even with the use of managed cover crops, continuous no-till does not yet appear feasible under organic systems and more research is needed in this area. A high standard of management is required to successfully implement conservation tillage practices in organic systems, and the practices need to be tailored to local soil and site conditions (Kuepper, 2001; Peigne et al., 2007). Adoption of Conservation Tillage The passage of the Food Security Act by Congress in 1985 tied soil conservation practices to farmer eligibility for government-sponsored crop deficiency payments, crop loans, storage payments, federal crop insurance, and disaster payments. The overall purpose of the act was to remove incentives to produce crops on highly erodible land, and the program affected more than 125 million acres nationwide. In 1990, 26 percent of planted crop acreage was under conservation tillage practices; that number rose to 41 percent in 2004 (CTIC, 2004). Among the conservation tillage practices, no-till has been used on an increasing proportion of land (from 17 million acres in 1990 to 61 million acres in 2004; Figure 3-2). Although weed control with conventional herbicides was successfully used on millions of acres of no-till (Derksen et al., 2002) before genetically engineered (GE) crop varieties with herbicide tolerance (HT) were introduced, GE corn, soybean, and crop varieties with HT might have further encouraged the adoption of conservation tillage practices, because FIGURE 3-2 Area of cropland in the United States managed by different tillage systems from 1990 to 2004. SOURCE: USDA-ERS (Sandretto and Payne, 2006).
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