What is a weed?
The classic definition of a weed is any plant growing in the wrong place at the wrong time.
Anything growing in a crop that is not that crop, including previous crop volunteers, are weeds.
Weeds reduce yield and grain quality by competing with the crop for resources, however, not all weeds compete equally so a better way to look at the problem is weed management rather than weed control.
Weed management requires a greater understanding of weed and crop biology and their interaction.
Why Control Weeds?
- Weeds compete with crops for resources including space, light, moisture, nutrients and carbon dioxide.
- Weeds reduce yield and grain quality.
- Weeds reduce the economic viability of the crop.
- Weeds can act as alternative hosts for crop insect pests or diseases.
Effect on Yield
The effect of weeds on yield varies depending on the species of weed and crop, not all weeds are equally problematic, some points you need to consider are;
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Will the crop become more competitive as it develops?
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Weeds compete for moisture, nutrients, sunlight and space, and their effects are felt greater by the crop when seedlings are small. |
| How much moisture and nutrients will the weeds take from the crop? | Weeds that grow large or are very prolific, are more likely to consume more resources than small or infrequent weeds. |
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Will weeds shade and compete for light?
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Weeds with a low or short growth habit will likely cause fewer problems than tall growing or are climbing weeds, particularly as the crop develops. |
| Is the weed very prolific? | Some weeds produce lots of seed and a small weed population can become a large weed population if not contained early on |
It’s often said that the best way to compete with weeds is to out-compete them by ensuring a rapidly growing, healthy crop.
A vigorous crop is a great start, but also giving the crop a helping hand particularly through that vulnerable stage, from emergence to the crop canopy closing over, is also a good idea.
Farmers essentially have two approaches when considering weed management in sunflower crops, and these are generally referred to as mechanical or chemical weed control.
Mechanical Weed Control
While there is no definitive definition to describe mechanical weed control, it is basically any method other than the use of chemicals.
The following list describes the main mechanical weed control options open to commercial sunflower crop production.
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Manual removal
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Hand hoeing can be effective in row crops but is time-consuming, slow, and requires enough labour to be available, but can be useful in containing localised weed problems. |
| Cultivation | Ploughing and other cultivation methods carried out after harvest and before planting the next crop, can destroy or bury weeds and weed seeds and is very effective, but they can bring dormant weed seeds to the surface continuing the cycle of weed growth. |
| Flame weeders |
Flame weeders, often used in organic farming systems, can be very effective in controlling emerged weeds. The aim is not to totally burn the weed but to expose it to sudden and severe heating which denatures the protein inside the weed leading to death. |
| Inter-row cultivation |
Cultivating the soil between the crop plants works well in row crops such as sunflowers. The main advantages are it will work in windy conditions when it’s not possible to spray, it’s a rapid operation, is comparatively cheap to buy and run, and is easy to operate. The disadvantages are that it needs the right soil conditions to be effective (too wet and seedlings can re-root and start growing again), it doesn’t control weeds in the row, it can knock the seedlings about, and you will lose precious moisture as the soil is disturbed. |
Chemical Weed Control
Herbicides
First off, it’s a good idea to understand some basic herbicide terminology.
| Herbicide | Sometimes referred to as weed killers, they are chemical substances used to control unwanted plants. |
| Trade name | This is the proprietary or brand name given by a pesticide manufacturer and is the most prominent name on product labels and in marketing. |
| Active Ingredient | Often abbreviated to a.i. and is the active substance or substances contained within an herbicide that is toxic to the weed. |
| Selective herbicides | Control specific weed species, while leaving the crop unharmed. |
| Non-selective herbicides | Control all or nearly all vegetation including a crop, often used between crops for total vegetation control. |
| Pre-planting | Pre-planting herbicides are applied to the soil before the crop is planted and may be mechanically incorporated into the soil. |
| Pre-emergence | Pre-emergence herbicides are applied before the weed seedlings emerge through the soil. |
| Post-emergence | Post-emergence herbicides are applied after weed seedlings have emerged through the soil. |
| Soil-applied | Applied to the soil and taken up by the root or shoot of the emerging weed seedlings. |
| Foliar-applied | Generally post-emergence herbicides applied to the above-ground part of the plant and can be trans-located (systemic) throughout the plant or remain at a specific site (contact). |
| Residual herbicides | These remain on or in the soil surface killing weeds over a period of time as they germinate through the herbicide zone, the length of activity depends on a number of factors but is typically reduced by rainfall. |
Economic use of herbicides
Herbicides can provide effective economic control of weeds if used correctly, but for herbicides to work well you need to pay attention to the detail.
| Effective crop agronomy | Includes using adequate seed rates and plant spacing, suitable seedbeds, correct drilling date, correct use of fertilizer, and good pest and disease protection. |
| Soil type | Soil acting herbicides may not be recommended when soil organic matter is more than 10% as organic matter locks up the herbicide. |
| Consolidated seedbeds | This helps good spray distribution and encourages rapid and even weed germination. |
| Moist seedbeds | Encourages weed root growth near the surface which results in early contact between roots and herbicide. |
| Weather |
Rainfall will wash herbicides into the root zone, weeds growing well will be more susceptible to herbicides, however, prolonged rainfall can cause leaching. Humid warm air improves foliar acting herbicides, frost, low temperature, drought or drying winds may affect herbicide efficiency and increase the risk of crop damage. |
| Applying herbicides |
Use correct spray quality, low water volume (less than 200l/ha) produces high work rates but can reduce efficiency if not done correctly. Low volumes will control small weeds with an open crop canopy, high volumes improve weed control in thick crops or dense weed population, high pressure improves weed control in thick crops as it can “push” spray into the crop. Angled nozzles can work well, particularly on rough seedbeds. Change nozzles regularly. |
Sunflower herbicide technology
In addition to the standard pre and post-emergence herbicides, farmers now have access to Clearfield®, Clearfield® Plus and Express® technology.
These hybrids use conventional plant breeding techniques to develop varieties of sunflowers that are tolerant to imidazolinone and sulfonylurea herbicides, while conventional sunflower varieties are susceptible to imidazolinone and sulfonylurea herbicides.
This allows for effective and robust season-long weed control to a broad spectrum of broadleaf and grass weeds including broomrape.
