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Weeds and weed control

Wild oats infesting a barley cropWild oats infesting a barley crop Paraquat is used to control a huge range of weeds worldwide, but to control weeds effectively and sustainably it is important to understand them.

Why does a plant become a weed? How can different types of weeds be described? What are the features of weeds and the way they grow which can be targeted by herbicides for successful control? Why is paraquat such a useful tool for farmers?

What are weeds?

Weeds are usually described as unwanted plants. Weeds grow on arable land which is waiting to be planted and then a new flush of weed seedlings emerge with the crop. In perennial crops like fruit, vines, rubber and oil palm, weeds grow continuously with new growth prompted by the weather and changing seasons.

Weeds are unwanted for many reasons:

  • They compete with crop plants for sunlight, water and soil nutrients, reducing yields and quality.
  • They may provide a habitat for pests and diseases from which these can attack the crop.
  • Large, climbing or spiny weeds can make it difficult to get into the crop for pest and disease control, fertilizer application, harvesting and other operations.

Crabgrass: a warm season annual grass (Digitaria spp.)Crabgrass: a warm season annual grass (Digitaria spp.)

Weeds are often unattractive, but they are not always a problem. They can play an important role in reducing soil erosion and provide habitats for beneficial insects and wildlife, increasing biodiversity.  However, it is not only the effects on the current crop that count and weeds must be managed.  “One year’s seeding means seven years’ weeding” the saying goes. Weeds become a problem when they reach a critical size or number, and these will depend how aggressive a particular species is.  Weed management is part of any farmer’s job and paraquat is a very economic, environmentally sound and flexible tool.

Weed types

Weeds are classified on the basis of leaf shape, on their life-cycle, and on their climate or seasonal preference.

  • Broadleaves or grasses? Weeds have leaves in a huge variety of shapes, but the grasses with long narrow leaves readily stand-apart and almost all the rest are grouped as broad leaves. Broadleaved weeds have seeds with a pair of storage organs which after germination become the first ‘leaves’, actually the cotyledons – hence the other name often used:  dicotyledons, or dicots.
  • Grasses are monocotyledonous (monocots). There are a few exceptions in that the odd monocot can have broad leaves, like the important tropical weeds in the Commelina genus. Another grass-like class with relatively few members is the sedges. These are important because they are difficult to control. In fact, purple nutsedge (Cyperus rotundus) has been called the “world’s worst weed”.

Purple nutsedge: a perennial sedge (Cyperus rotundus)Purple nutsedge: a perennial sedge (Cyperus rotundus)

  • Annuals or perennials? Annuals germinate, flower and set seed in a single season. Perennials have underground storage organs, often rhizomes, which enable them to grow for many years. They can reproduce both from seed and by extending their rhizomes from which daughter plants grow. A third type germinates in one season and flowers in another.  These are biennials. Passing through winter prompts them to ‘bolt’ by elongating a tall flowering stem.
  • Cool season or warm season, etc.? Weeds have evolved to grow best in particular temperature and daylengths. These terms tend to define the crops they are found in and the time at which they germinate, e.g. winter annuals or summer annuals. Also, in tropical climates with dry seasons and rainy seasons, some species tend to be more prevalent in one season than the other.

Features of weeds

Weeds are vulnerable to herbicides if their internal biochemical processes can be accessed. Once inside a plant cell an effective herbicide will disrupt normal functioning leading to death. However, to kill the plant itself, all the various growing points – at shoot and root tips, and the buds on stems and rhizomes – must be killed too.

Herbicides enter plants by two main routes:

  1. Directly into shoots
  2. Through the soil into seeds, roots or rhizomes

Post-emergence herbicides enter shoots and some also act via the soil. Pre-emergence herbicides affect germinating seeds and have some degree of persistence in the soil to give a residual effect preventing further flushes of germination.

Roots are obviously adapted to absorb water, so soil-acting soluble herbicides have an easy way in.  Plant shoots have a thick waxy cuticle to help retain moisture. Foliar herbicides have to cross this barrier to enter. Leaves have pores called stomata through which carbon dioxide, oxygen and water vapour diffuse, but these are generally too small for spray droplets to penetrate.

Once inside, some herbicides move extensively throughout the weed. This systemic movement is either in the transpiration stream of water as it is drawn from the roots to evaporate from leaf stomata, or with sugars produced by photosynthesis in the leaves translocating to the growing points.

Using paraquat for weed control

After paraquat’s properties as a weedkiller were discovered it took a while for these to be fully appreciated. This is because its activity seemed quite limited. Although it impressed with its very fast burndown of most broadleaved and grass weeds, it also killed all crops and had no soil activity, so could not control new flushes of germination like residual herbicides do. Also, it had no systemic activity, so could not move to growing points, particularly to control perennial weeds. However, far-sighted and practical agricultural scientists’ realised that these characteristics could be unique advantages.

Paraquat is inactivated immediately on reaching the ground by being extremely tightly bound to soil particles. So, it cannot affect any crops by root uptake. This means that it can be used selectively either before arable crops are planted or between the rows in orchards, plantations and vegetables. Neither can it leach into groundwater nor affect any beneficial soil organisms.

Having only contact activity means that paraquat quickly removes the immediate problem of weeds by destroying shoots, but allows regrowth from below ground buds or seeds. Soils are protected from erosion because the roots remain to provide anchorage and structure and the shoots regrow to cover the soil and disperse the impact of rain droplets which causes erosion. New flushes of weeds can be swiftly removed by a further paraquat spray.

Although some ‘soft’ perennial grasses are only suppressed by paraquat, these can be useful in controlling erosion and providing habitats for beneficial insects. However, there are aggressive perennial weeds which require the use of the systemic non-selective herbicide glyphosate to control them.

Glyphosate is a very effective herbicide. Like paraquat it has no soil activity, but it is not a panacea and paraquat and glyphosate can be used in complementary ways to achieve optimum weed control. Although glyphosate controls aggressive perennial weeds, it is slow acting and not rainfast.  In sunny weather paraquat will burndown annuals and soft perennials in a few hours, and if storms threaten, paraquat is rainfast in only 15-30 minutes. Unlike glyphosate, paraquat also works well in cold weather when weeds are growing slowly.

Paraquat can be used selectively, avoiding crop damage, simply by ensuring that spray does not substantially contact crop leaves. Weeds can, therefore, be controlled along the strips between crop rows with paraquat, whereas the risk of damage from the systemic glyphosate is too great. Paraquat is also safe to trees and shrubs because it cannot penetrate bark.

Herbicide resistant weeds

Conyza canadensis (horseweed): a broadleaved weed with resistance to glyphosateConyza canadensis (horseweed): a broadleaved weed with resistance to glyphosate In many parts of the world some important weeds are no longer controlled by formerly effective herbicides.

Each case of weed resistance began with the odd individual weed which had some rare mutation enabling it to escape injury by a particular herbicide. The progeny of such individuals have come to dominate certain populations. Weeds which have become resistant to glyphosate are especially worrying.

For 20 years after its introduction, weed resistance to glyphosate had been considered to be highly unlikely.

However, when glyphosate resistant GM crops began to be grown, the amount of glyphosate sprayed increased dramatically – as did the chance of finding weeds with resistance mutations. Over recent years, populations of a number of very significant weeds have become resistant to glyphosate.

Weed resistance to paraquat is known, but the instances are rare and insignificant in comparison to other herbicides such as the sulfonylureas and ACC-ase inhibitors. In fact, paraquat has a vital role to play in combating weed resistance. Paraquat can be used as an alternative non-selective herbicide. The best option is to use paraquat for burndown sprays to clear land of weeds before planting glyphosate resistant crops, reserving glyphosate for post-emergence weed control. There is no viable substitute for glyphosate to control perennial weeds, so using paraquat in this way will protect the use of this invaluable herbicide.