Paraquat promotes soil health in new corn cropping system
In the near future, the food versus fuel dilemma will be addressed by using ‘waste’ crop residues instead of grain as the feedstock for bioethanol. 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 biomass 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.
Research 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-till, 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 grasses, 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 tillage systems can be found here.
Read the facts about paraquat here.
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 Agriculture Network (2007). Managing cover crops profitably. (Available for download at the website of the Midwest Cover Crops Council
The brand name for the leading paraquat product is Gramoxone