Conservation tillage may collapse under resistant weeds threat

Thursday, 10 December 2009

No-till seeding in US Mid-WestConservation 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. techniques have rapidly become popular ways of preparing fields for cropping because of their many environmental and economic benefits. Non-selectiveDescription A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds). Authoritative On-line References and Resources http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University. herbicides like paraquat are essential components of conservation tillage because fields are not plowed to bury weeds, and desiccated vegetation, stover and stubble provide a protective cover to the soil. This helps to minimise erosion, provides habitats for beneficial insects and other wildlife, and undisturbed soil builds higher levels of organic matter, key to good soil structure and fertility.

Why do farmers adopt conservation tillage?

Benefits of conservation tillage

  • Less soil erosionDescription Displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement. Authoritative On-line References and Resources http://soilerosion.net/ This site brings together reliable information on soil erosion from a wide range of disciplines and sources. It aims to be the definitive internet source for those wishing to find out more about soil loss and soil conservation.
  • Better soil structure & fertility
  • More biodiversityDescription The variety of life in all its forms, levels and combinations. Includes ecosystem diversity, species diversity, and genetic diversity (IUCN, UNEP and WWF, 1991). Authoritative On-line References and Resources http://earthtrends.wri.org/ EarthTrends is a comprehensive online database, maintained by the World Resources Institute, that focuses on environmental, social, and economic trends. Statistics on biodiversity indicators are available.
  • Quick crop establishment
  • Less labor
  • Less machinery required
  • Less fuel used
  • Reduced greenhouse gas emissions
  • Lower costs
  • Cheaper food

 In the US, cropland under conservation tillage has continued to increase year on year as farmers see the value in putting away their plows. The Conservation Technology Information Center (CITC) estimates that over 40% of all crops in the US are now under conservation tillage practices. US conservation tillage area keeps growing

The gold standard is no-till in which there are no soil cultivations at all. Other forms of conservation tillage are shown in the box below.
 

Soil tillage systems1

Conventional Tillage

Any intensive tillage system, usually plowing and subsequent cultivations, in which less than 30% plant residue cover is left after planting. Weed control before planting is mainly or solely by burial in cultivation.

Conservation tillage

Reduced or zero tillage systems which leave at least 30% of field covered by vegetation or plant remains throughout the year. Weed control is mainly or only by herbicides. Conservation tillage systems include:

No-till

No soil cultivations and remains of previous crop are spread over field providing at least 30% coverage throughout the year. Seed is planted in a slot cut through the soil. (If narrow seedbed strips are created this is called strip-till.) Weed control is only by herbicides.

Ridge-till

No soil cultivations apart from making ridges. Seed is planted along ridges and remains of previous crop are left between ridges.

Mulch-till

A shallow cultivations system similar to min-tillDescription Least possible soil disturbance for preparing a seedbed by reducing cultivation and avoiding the use of a plow. It can involve direct drilling, broadcasting into existing stubbles or adopting a strategy of reduced tillage. The main benefits are to reduce tillage energy consumption, to conserve moisture and soil structure, to reduce disturbance of soil organisms and to retain plant cover to minimize erosion. Authoritative On-line References and Resources The European Conservation Agriculture Federation (ECAF) promotes best soil management practices based on minimal soil disturbance by reduced tillage. which leaves more than 30% of the field covered by straw, stover, stubble or cover cropDescription 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. remains.

1 As defined by CTIC

 

The crop which has the greatest amount of no-till and other conservation tillage practices is soybeans. Latest estimates show that in US full season soybeans, 40% of acres are no-tilled. In double crop soybeans, this area rises to more than 70%. This is because when soybeans follow an earlier harvested crop like wheat in the same season, time is of the essence. A fast turn around between harvest and sowing is essential for good yields of the later crop. No-till and cleaning up any weeds with fast-acting paraquat lets farmers get the soybean crop established in good time. 

Proportion of US cropland under various tillage regimes (CTIC, 2007)

 

Weeds fight back

However, a fight back by weeds could jeopardize all the benefits gained in the past couple of decades from the move to conservation tillage. Glyphosate, although much slower acting than paraquat, has become the most widely used non-selective herbicide, particularly since the introduction of GM glyphosate tolerant crops. Since the mid 1990s weed species have, one by one, started to develop resistance to glyphosate. The southern US states have been especially hard hit. Over the past couple of years the focus has been on glyphosate resistant Palmer amaranth. This stubborn weed grows up to ten feet (3 metres) tall and may produce 100,000 seeds from one plant. Reports are of 750,000 acres infested in Tennessee alone in 2009, and of crews having to hack out these monsters which have survived four times the usual rate of glyphosate.

University of Arkansas weed specialist Ken Smith believes that the option of using glyphosate in-crop as well as before planting has lead to bad habits: “plant it dirty and clean it up with glyphosate” was all too often the attitude.

This year, wet conditions in early summer delayed planting in many areas meaning open crop canopies much later than usual. This combined with hot and continued wet weather, meant that weed seed continued to germinate, creating huge pressure on crops from weed competition. Where weed shiftsDescription A change in the weed community within a field i.e. relative abundance or type of weeds. This can be the result of a management practice like herbicide use or any other phenomenon that brings about a change in weed species composition. Species or biotypes adapted to current weed management practices increase, whereas weeds susceptible to those practices decrease. Authoritative On-line References and Resources http://www.weeds.iastate.edu/mgmt/qtr00-1/popdyn.htm A classic article on weed population dynamics on the Iowa State University Weed Science website. had caused harder-to-control species to predominate, and in all too many fields where resistant weeds like Palmer amaranth were present, glyphosate was in trouble.

Extension agents in Arkansas and Tennessee worry when they hear farmers who have come to rely too heavily on glyphosate talking about abandoning conservation tillage. They fear that with such acute weed problems a return to plowing fields before planting could be the only way to maintain productivity.

Aside from the environmental benefits of conservation tillage which would be lost, the economic costs would also be huge. Under conservation tillage a farmer might use only one tractor to cover 1000 acres (400 hectares), but a return to intensive tillage could mean the need to purchase another two tractors to achieve the same output. These high horsepower tractors may cost $200,000 each.

Mix modes of action to keep conservation tillage benefits

Conservation tillage acres will keep growing if glyphosate is not over-used. Spraying herbicides with different modes of action is the way to ward-off the development of resistant weeds. Unfortunately, finding new modes of action has proved to be very difficult for the crop protection industry in recent years. No herbicide with a new mode of action has been introduced for nearly 20 years. However, paraquat controls a very broad spectrum of weeds and works in a completely different way to glyphosate. In conservation tillage systems with glyphosate tolerant crops, a good option to fight the potential for glyphosate resistant weeds to appear is to use paraquat for pre-plant burndown, saving a single application of glyphosate to control weeds in the growing crop.