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Why consider sowing east-west?

In winter, when the sun is travelling at a lower angle the additional shading of the inter-row that occurs when crops are sown in rows running east-west can help suppress weeds growing in-crop. This effect is most noticeable in southern latitudes.

In Western Australia, Department of Agriculture and Food researcher Dr Catherine Borger demonstrated in six trials that east-west sowing can halve annual ryegrass weed seed set, even though weed biomass may not be reduced. This is a rare opportunity for free weed control that could be worth implementing in suitable paddocks.

Weeds researcher Dr Catherine Borger measured the difference in light penetration through the crop and weed growth when the crop was sown east-west compared to north-south. Photo: DAFWA

cent weed biomass reduction when crop rows ran east–west compared to north–south. At Trangie the barley varieties Hindmarsh and Granger had higher yield and were more competitive against weeds when sown east–west. In these trials weed biomass was measured rather than weed seed production, however both are good indicators of the weed response to the treatments.

NSW DPI research and development agronomist, Greg Brooke says that in many cases there are other, more practical, ways to suppress weeds but that row orientation is well worth considering and implementing if it can be easily done.

“The more non-herbicide tactics a grower can use to support their herbicide program the better,” he says. “Crop yield is not compromised under east–west sowing.”

Researchers in NSW also noticed a stark difference in the growth of the weed fumitory, with the weed being prolific in the north–south rows and absent in the east–west rows.

Growing crops in rows at right-angles to the sun decreases the amount of solar radiation that the weed plants can intercept and use for photosynthesis. This translates into lower biomass production and potentially decreased seed production.

Importance of early weed control
A weed seedling can impact crop yield potential well before the weeds are large enough to take moisture, light and nutrients away from the crop.

Canadian researchers Dr Clarence Swanton and Dr Jessica Gal observed this effect in both corn and soybean. The weeds emerging alongside or soon after the crop seedlings reduced the crop yield potential, with the effect occurring at the seedling stage, long before the weeds could be competing directly for resources such as moisture, nutrients and light. The differences in light wavelengths bouncing off bare soil compared to growing weeds triggers a reaction in the crop plants to put a disproportionate amount of resources into growing taller, with more leaf area—resulting in a comparatively smaller root system.

This response can pre-condition the crop to a lower yield, particularly in years where the crop may encounter moisture stress and have limited ability to seek deeper stored moisture.

DAFWA researcher Dr Catherine Borger says that crop plants adopt this ‘shade avoidance growth characteristic’ to allow them to more effectively shade weeds as the plants mature and begin directly competing for light.

“Since crop plants in an east-west orientation physically shade weeds at an earlier growth stage, weeds have less opportunity to trigger the shade avoidance growth characteristics,” she says.

The interrow area is shaded earlier in the season when the crop is sown east-west, suppressing weed germination and growth early.

Choosing competitive crops and varieties
“The row orientation is one part of increasing crop competition,” says Mr Brooke. “The other factors to consider are row width, seeding rate and crop and variety selection.”

“The difference in competitiveness between crop types is well known with general rules of thumb such as triticale, barley and rye being more competitive than wheat; and cereals are generally more competitive than pulses and canola,” he says. “What is less well understood are the differences between varieties. In one trial Skipper barley achieved a 30–40 per cent greater weed biomass reduction compared to Hindmarsh barley, however the competitive ability of different varieties varies widely between sites and seasons.”

Generally, varieties that exhibit rapid early growth and early canopy closure will provide better weed suppression than less vigorous varieties of the same crop type. Seeding rate and row spacing also play a part in increasing the crop’s competitiveness.

Choosing a highly competitive cultivar in one year of a three to five year program can have a significant and lasting effect on the weed seed bank. If this is coupled with harvest weed seed control, and possibly east–west sowing, the result could be impressive and achieved at a very low cost.

“Introducing more non-herbicide tactics into the farming program gives crops the advantage over weeds and reduces the pressure on the herbicide,” says Mr Brooke. “Farmers have started using tactics like double-sowing known weed areas such as along irrigation channels. With GPS, double pass seeding automatically gives narrow rows, doubles the seeding rate and makes those key management areas much more competitive.”

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Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations. Full rates when mixing herbicides too! When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality. Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL. Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.   Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice. Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.  

Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills. Prevent spray-drift The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum. Bill Gordon’s 10 Tips for Reducing Spray Drift Choose all products in the tank mix carefully. Understand the product mode of action and coverage requirements. Select (and check) the coarsest spray quality that will provide effective control. Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY. Use weather forecasts to inform your spray decisions. Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas. Set the boom height to achieve a double overlap of the spray patterns. Avoid higher spraying speeds. Leave buffers unsprayed if necessary and come back. Continue to monitor conditions, particularly wind speed, at the site during the spray operation High water rates don’t have to slow you down Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job. To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can: Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point. Use a larger pump, e.g. 2.5 inch, to make re-filling quicker. Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution. For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high Water quality and mixing order Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results. Don’t start mixing until the water quality is right Podcast – Mixing herbicides Adjuvants Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives. When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.

Clean borders – avoid evolving resistance on the fence line

About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm. Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed. Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems. In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed. Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak. Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands. If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks. Other resources It’s time for a glyphosate intervention Farm hygiene cottons on – Cleave Rogan, St George What’s new in management of herbicide resistant weeds on fencelines? Keeping the farm clean – Graham Clapham, Norwin Don’t jeopardise glyphosate for clean fencelines Keeping fencelines clean Resistance risk to knock-down herbicides on irrigated cotton farms

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