Integrated weed management in no-till systems

Integrated weed management and no-till are advanced agronomic tools with common aims to improve efficiency and profitabilty, while reducing the environmental impact of crop production. Although advanced in concept, these tools are straightforward and can be adapted for use in all cropping systems, from highly mechanised ones to subsistence farming, all around the world.

Tillage is a well proven means of controlling weeds, so are other methods good enough to use in an integrated approach to weed management in no-till systems? This article examines how farmers can reap the rewards of both techniques together.

Plowing: is there a better way?

Farmers around the world know just how hard it is to control weeds. They tend to come back with a vengance, especially when the many elements causing weed problems have not been appreciated and addressed. Aiming to manage weeds rather than control them is not only more realistic, but if Integrated Weed Management (IWM) is applied properly, it can reduce costs, protect the soil, and support pest and disease control.

No-till systems also provide economic and environmental advantages. However, in no-till, the traditional means of weed management by ploughing to prepare a field for cropping is not used. Plowing, even though it effectively removes weeds by burial, is costly, time consuming, and can cause soil erosion and compaction.

In addition, plowing is now recognised as contributing to problems of energy and climate change by its high fuel requirements and through its action on the soil. Plowing encourages the breakdown of soil organic matter, releasing carbon dioxide. If soils are not tilled they can very effectively lock-up carbon in organic matter. No-till could not have been developed without the innovation of non-selective herbicides, particularly paraquat, later followed by glyphosate.

IWM: A Solution to Weed Shifts and Resistance

Using herbicides greatly simplifies weed control, but relying too heavily on any chemical weedkiller ultimately results in some weed species becoming resistant. Resistance has arisen quickly in some cases, eg to the sulfonylureas, and more slowly, but inevitably, in others, eg glyphosate. Resistance to paraquat has only arisen in isolated cases in a few unimportant species over the course of more than 40 years. This is not least due to the way in which paraquat has generally been used in conjunction with other herbicides and cultural weed control methods. Not only herbicides are at issue. Any single approach to weed control will cause the many different weeds in any field to change, or ‘shift’, in terms of their relative competitive pressure on the crop and new species will invade to fill ecological niches left by those most effectively controlled.

Weed resistance and weed shifts have caused researchers and farmers to develop IWM systems which benefit the whole farm. IWM involves using a total agronomy approach to minimise the overall impact of weeds, and indeed, maximise the benefits which the presence of a non-competitive weed cover can give. In IWM, the judicious use of herbicides is incorporated into a system based on cultural methods. These are designed to suppress weed germination or return of seed to the soil, and to offer opportunities to attack particular groups, eg spring or autumn germinators. Crop rotations, cover crops, varying row widths, mechanical or manual weed removal are all used. There is no blueprint. Farmers must draw on appropriate techniques to suit the crop, location, season, etc.

No-Till: A Challenge for IWM

Plowing is one IWM technique that can not be used in any no-till system. In conservation agriculture systems, where shallow cultivations are used, but the soil is not inverted, stale seedbeds can be a valuable way of reducing weed pressure. With this technique, light cultivations encourage weed seeds to germinate so seedlings can be destroyed by further cultivations or herbicides. However, even without any soil cultivations, many techniques can be used together very successfully to manage weeds in no-till.

Thornapple: a highly competitive weedFarmers recognise that no-till weeds will be different. Annuals which germinate easily on the soil surface (otherwise buried too deep by ploughing to germinate readily) and perennials are much more prolific. Scouting for weeds and knowing how competitive each species is, and, therefore, the relative importance of their removal, is essential.

In Australian cotton, for example, it has been calculated that the threshold weed density for an economic benefit from removal of a large Datura ferox (thornapple) is only 1 plant per 73 metres of cotton row. Apart from the direct competitive effect from shading, use of water and nutrients, thornapple is host to Heliothis, spider mites and Verticillium wilt.

IWM techniques can be divided into:

  • General good farming practices: using seed certified weed-free; cleaning combine harvesters between crops; removing mud from tractor wheels when moving between fields.
  • Preventative approaches: crop rotation and agronomic practices in each crop to discourage the emergence of weeds, eg cover crops, mulches.
  • Curative approaches: using various methods to remove weeds when present, eg herbicides, flaming.

The major non-cultivation weed control techniques suitable for no-till and their benefits are listed in Table 1.

Table 1. Main IWM techniques applicable to no-till and their advantages or issues.
 

Technique Contribution to Weed Control Other Benefits or Issues
Crop rotation Weeds tend to infest crops with similar features and life-cycles, so rotating between cereals and broadleaved crops, or winter and spring sown crops, helps to contain weeds and offers windows for control  Enriches soil nutrient status, improves soil structure, reduces pest and disease problems
Cover crops and mulches Crops such as rye or wheat sown over winter before planting a spring crop shade out weeds and have an allelopathic effect, ie they excrete chemicals which prevent weed seed germination Protect soil against erosion, provide habitats for benefical organisms, may have a fertilizer value if leguminous
Nurse crops and intercropping Two crops are grown together (in various ways) effectively shading out weeds or ensuring early crop establishment to gain a competitive advantage Saves time, saves labor and increases overall productivity
Higher seed rate or narrow crop rows Full ground cover is achieved faster, shading out weeds Can increase yield, but may require more seed
Irrigation management Used to encourage flushes of weed emergence Water can be a scare resource
Herbicides Control weeds as per label Save time and labor.  Must rotate to avoid resistance and weed shifts.  Important not to remove uncompetitive species unnecessarily
Mowing Temporarily removes weeds or cover crop while maintaining ground cover May need several mowings and therefore demanding on time and fuel use
Flame weeding or other thermal methods (boiling water, foam) May give broad spectrum control Requires repeat treatments and only effective at susceptible growth stages and where growing points are exposed.  Fire hazard
Manual weeding Selective removal of escapes or low density weeds Effective if weeds have not been allowed to shed seed, but can be laborious and time consuming with delays to planting

 Less widespread methods not listed in Table 1 include

  • Brush weeding, where fast rotating brushes uproot weeds; minimising herbicide use by, eg band spraying or using impregrated rope wicks to smear a systemic herbicide on weeds taller than the crop
  • Varying crop sowing dates to attack weeds tending to emerge at different times
  • Using animals such as geese to selectively remove palatable weeds

Some other biological control methods have been used. Often, however, these have generally been applied in non-agricultural situations. The weevil Hylobius transversovittatus has been introduced to control purple loosestrife (Lythrum salicaria) in wetlands of many American states. Although common problem field weeds such as pigweeds have been targeted, biological control methods are usually highly selective for single weed species.

Using any individual method in IWM has consequences for the rest of the system. For instance, moving between crops in the rotation will mean selecting from a different range of herbicides and choosing one herbicide will mean that another with a different mode of action should be selected next time. This rotation of herbicides as well as crops will significantly reduce weed shifts and the development of resistance.   Computer-based decision tools to assist making complex decisions about the compatibility and effectiveness of various IWM methods are now available. One example is the RIM model developed in Western Australia to select the best ways of controlling herbicide resistant annual ryegrass (Lolium rigidum).

Role of Paraquat: Economic, Social and Environmental Benefits

When it comes to using herbicides in IWM paraquat is an ideal choice, especially in no-till systems.  As a broad spectrum, non-selective herbicide, paraquat removes almost all weed growth. Its fast and reliable action in all weathers, including when there are only short intervals to spray between rain showers and when it is cold, means that it has huge flexibility to fit in to IWM systems. It is particularly useful in situations varying from burning down weeds or cover crops in cold, wet springs, to quickly removing weeds in between crops in double cropping systems in much warmer climates.

In developing countries, some of the techniques used in IWM are less well-suited to crop production on small-holdings, often in difficult terrain.

The time-consuming drudgery of weedingFor instance, any needing machinery may be too sophisticated or costly. Access may be a problem too. Manual labor tends to be heavily relied upon. Here, paraquat is especially useful. Relatively large crop areas can be treated quickly and paraquat’s speed of action means that it is easy to see which weeds have already been sprayed. With as little as 15 minutes needed for paraquat to become rainfast, spraying can continue between tropical showers.

Many hours otherwise spent hand weeding can be used much more profitably to improve the economic and social well-being of rural communities.  Saving time is often the key to success in these situations. Less time weeding means more time in school or more time producing goods to trade. Removing the need to cultivate and adopting no-till systems using paraquat can also mean growing more crops each year. The next crop can follow almost immediately after a harvest.

In irrigated crops, especially rice-wheat rotations such as across northern parts of the Indian sub-continent, no-till can also save significant amounts of water. Cultivating the soil after the rice crop has been harvested normally results in heavy water loss through evaporation. Without plowing, and using paraquat for weed management, wheat can be planted earlier, avoiding rapid growth at times of the year when rainfall is low.

In addition to its economic and social benefits, using paraquat has low environmental impact. Central to this is the fact that paraquat is inactivated immediately on contact with soil. Its extremely tight binding to soil means that it can not leach and has no residual effects. Not ony does this give full flexibility in choosing crops in a rotation, but it means that less aggressive weeds can be allowed to grow back, so protecting soil from erosion and providing habitats for beneficial insects and other wildlife.

IWM in no-till farming requires an intelligent approach. Using paraquat to remove weeds prior to planting can provide the flexibility to utilise a diverse range of other tools for successful integrated weed management and give many other benefits to farmers, their communities and the environment.

References

General information on IWM:
Buhler, DD (2002). Challenges and opportunities for integrated weed management. Weed Science, 50, (3), 273–280
Cotton Catchment Communities, Australia
National Sustainable Agriculture Information Service (ATTRA), USA:
Saskatchewan Soil Conservation Association

Herbicide resistance:
International Survey of Herbicide Resistant Weeds

Biological control:
Burki, HM, et al (1997). Biological control of pigweeds (Amaranthus retroflexus, L. A. Powellii, S. Watson and A. Bouchonii Thell.) with phytophagous insects, fungal pathogens and crop management. Integrated Pest Management Reviews, 2, (2), 51-59.

Expert systems:
RIM decision tool

Less developed agriculture:
Humphreys, E, et al (2006). Increasing field scale water productivity of rice-wheat systems in the Indo-Gangetic Basin. International Rice Congress, 9-13 October 2006, New Delhi.
Rice Wheat Consortium for the Indo-Gangetic plains