Paraquat promotes soil health in new corn cropping system

Corn: can it provide food and fuel sustainably?US crop scientists are excited about the potential to protect soil in fields where corn is grown for biofuel production by the use of perennial cover cropsDescription Cover crops are primarily planted not to be harvested for food but to reduce soil erosion, control weeds and improve soil quality. They are usually plowed or tilled under before the next food crop is planted, in which cases the "cover crop" is used as a soil amendment and is synonymous with "green manure crop." Authoritative On-line References and Resources http://attra.ncat.org/attra-pub/covercrop.html ATTRA is the US National Centre for Appropriate Technology's Sustainable Agriculture Information Centre. suppressed by paraquat.

In the near future, the food versus fuel dilemma will be addressed by using ‘waste’ crop residues instead of grain as the feedstock for bioethanolDescription Bioethanol is ethanol of biological origin. Crops containing sugar or starch grown for energy use include sugar beet, sugar cane, maize and wheat. "2nd generation" bioethanol will be made from cellulose from, e.g. waste straw and stover, willow and popular trees, sawdust, reed canary grass, switchgrass, Miscanthus. Authoritative On-line References and Resources http://www.eere.energy.gov/afdc/fuels/ethanol.html The US Department of Energy's Office of Energy Efficiency and Renewable Energy has am Alternative Fuels and Advanced Vehicles Data Center containing key information on all biofuels.. However, the concern is that removal of stover and chaff normally returned to the soil will cause serious erosion and fertility problems.

Iowa State University has conducted a three-year project that has demonstrated how carefully managed perennial cover crops can more than substitute for the biomassDescription Mass of organic matter of non-fossil biological origin which can be exploited for energy purposes. Authoritative On-line References and Resources http://www1.eere.energy.gov/biomass The US Department of Energy's Office of Energy Efficiency and Renewable Energy has a Biomass Program working with industry, academia and US National Laboratories on research into biomass feedstocks and conversion technologies. The goal is cost competitive, high performance biofuels, bioproducts, and biopower. harvested for ethanol production1. The key to success is the use of paraquat and its fast, contact-only action. Paraquat is used to burn down the cover crop in spring, but because it is not translocated and is adsorbed by soil, the cover crop recovers to protect soil after harvest and over winter.

Second generation biofuels

The competition between food and fuel uses of corn and other cereals and oilseeds has been cited as one factor influencing the recent sharp increases in the cost of basic food commodities. In 2010, almost 40% of the 311 million tonnes of corn harvested in the US went to produce ‘first generation’ ethanol from grain (although 25% was ultimately fed to animals in the form of distillers grain)2.

However, ‘second generation’ or ‘cellulosic’ ethanol will soon be made on a commercial scale from straw, stover and chaff3. Despite relieving the pressure on the supply of grain for food, this raises serious concerns for soil. Crop residues protect soil from wind and rain, and ultimately maintain organic matter levels, ensuring fertility and sequestering carbon.

Sustainable harvesting

Figure 1. Effect of crop residues on soil erosion. 1000 lb/acre = 892 kg/haResearch has shown that crop residues left on the soil surface are much more effective at reducing soil loss than if they are incorporated, but also that the ‘breakpoint’ is very sharp – remove too much and the consequences for the soil will be severe4 (Fig.1).

Sustainable crop residue removal rates will vary with factors such as field slope and soil type, tillage system and crop yield. Conservation practices such as no-tillDescription Also known as conservation tillage or zero tillage is a way of growing crops from year to year without disturbing the soil through tillage ie cultivating the soil usually with tractor-drawn implements. Authoritative On-line References and Resources No-till.com:  A portal for on-line information about no-till farming., contour cropping and cover crops must be used to compensate.

Cover crops and paraquat

Cover crops sown after harvest can provide many environmental benefits over winter before being removed when the new crop is established5,6. Various species are used including cereals and other annual grassesDescription The leaves are "narrow" as opposed to the "broad" leaves of broadleaved weeds. Also called 'monocots' having one seed leaf opposed to 'dicots' having two seed leaves. Authoritative On-line References and Resources The International Weed Science Society represents individual associations around the world. , brassicas and legumes. In a new approach, researchers at Iowa State University have been experimenting with perennial cover crops growing between rows of successive corn crops in a strip-till system1. This means that there is no need to sow a new cover crop after harvest, saving time and costs of re-seeding.

Benefits of cover crops

  • Improved soil quality – more organic matter leads to better root growth, greater water retention, better drainage and encourages beneficial soil organisms such as earthworms

  • Less erosion – soil protected from wind and rain over winter

  • Weed control – weeds suppressed by cover crop

  • Insect control – provides a habitat for beneficial predators of insect pests

  • Legume cover crops add nitrogen - reduces need for fertilizer

Key components in this new growing system are the type of cover crop, the corn hybrid and the tillage practices used. After screening 36 potential cover crops, Kentucky bluegrass (Poa pratensis) was chosen. Seed is readily available and it is easily suppressed, yet recovers well. Some corn hybrids were found to perform better than others in this system, indicating scope for plant breeders. The optimum tillage practice was found to be strip-tillage. A four inch (10 cm) wide band is tilled in the autumn into which corn is planted the following spring.

In spring, paraquat is sprayed before the newly sown corn crop emerges, quickly desiccating the Kentucky bluegrass. This then slowly regrows beneath the corn canopy ready to protect the soil again after harvest through to spring. Glyphosate tolerant corn hybrids are grown so that weeds can be controlled by glyphosate along a 10 inch wide band including the planted strip.

The Kentucky bluegrass cover crop ensures that there is at least 85% ground cover throughout the year, leaving less than 15% of the soil exposed and susceptible to erosion. Reseeding of the cover crop may be needed every three to five years.

It is estimated that the organic matter added to the soil by the cover crop is equivalent to that which would have been added by the corn stover and chaff removed as biomass feedstock for cellulosic ethanol. So long as the cover crop is supressed by paraquat before it starts to compete with the corn crop, then the research at Iowa State shows that high grain yields of 200 bushels per acre (12.6 tonnes/ha) are still possible.

More information on biofuels and the importance of no-till and other conservation tillageDescription Any tillage and planting system that covers 30 percent or more of the soil surface with crop residue after planting to reduce soil erosion by water. Authoritative On-line References and Resources Purdue University-based Conservation Technology Information Centre. systems can be found here.

Read the facts about paraquat here.

References

Iowa State University (2011).
National Corn Growers Association. (2011). 2011 World of Corn
US Department of Energy (2011). US Billion-Ton Update.
 Andrews, S S (2006). Crop residue removal for biomass energy production: effects on soils and recommendations. USDA National Resources Conservation Service.
Midwest Cover Crops Council
Sustainable AgricultureDescription Management and conservation of the natural resource base and the use of technological and organizational change in a manner that ensures continued agricultural production from the land for present and future generations. Such practices conserve land, water, and plant and animal genetic resources. They are environmentally non-degrading, technically appropriate, economically viable, and socially acceptable. Sustainability rests on the principle that we must meet the needs of the present without compromising the ability of future generations to meet their own needs. Therefore, stewardship of both natural and human resources is of prime importance. Authoritative On-line References and Resources www.nal.usda.gov Information from the USDA's Alternative Agricultural Systems Information Center. Network (2007). Managing cover crops profitably. (Available for download at the website of the Midwest Cover Crops Council

Notes

The brand name for the leading paraquat product is Gramoxone