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Best seed, best establishment, fewer weeds

Crop competition is an important strategy in the Weedsmart Big 6 to manage herbicide resistance – but it’s not all about row width and seeding rate – the size of the seed sown makes a difference too.

Rohan Brill, Research and Development agronomist, NSW DPI based in Wagga Wagga, along with colleagues at Trangie and Tamworth, has been teasing apart whether the improved early performance of canola hybrids over open-pollinated (OP) cultivars comes from the generally-larger seed size of hybrids or from hybrid vigour.

Rohan Brill, Research and Development agronomist, NSW DPI based in Wagga Wagga, says most of the improved early growth in canola hybrids lies in the larger seed size, with heterosis providing an additional benefit. Sowing large canola seed, regardless of the cultivar, is key to strong early crop growth and the crop’s ability to compete with weeds. (Photo: Gregor Heard)

“It seems that most of the improved early growth in hybrids lies in the larger seed size, with heterosis providing an additional benefit,” he says. “Our study showed that sowing large canola seed, regardless of the cultivar, is key to strong early crop growth and the crop’s ability to compete with weeds.”

As a reliable rule of thumb, Mr Brill recommends cleaning and grading all farmer-retained OP canola seed to collect planting seed that is 2 mm in diameter or larger. In both glasshouse and field experiments following this rule led to improved open-pollinated TT canola emergence and early biomass production. Seed size had a greater effect on early biomass production than did cultivar type (hybrid vs OP).

“A 10 per cent difference in seed diameter produces a 33 per cent difference in seed volume, so it is very important not to skimp on the 2 mm diameter rule,” he says. “There was a stark contrast between plots sown with 2 mm seed compared to 1.8 mm seed. It is very hard to tell the difference visually between these two seed sizes but the visual difference in early vigour and biomass production was very clear.”

A 10 per cent difference in seed diameter produces a 33 per cent difference in seed volume, so it is very important not to skimp on the 2 mm diameter rule. There was a stark contrast between plots sown with 2 mm seed (left) compared to 1.8 mm seed (right).

“This trial has shown that grading planting seed to extract the largest seeds is a far better way to increase crop competition and yield than increasing the seeding rate of smaller seed,” he says. “For example, a grower might take 30 tonne of grain from a clean paddock and screen it hard at harvest to take out 2 tonne of the best seed and sell the rest. Then store the 2 tonne of seed in a cool place before grading it (including using a gravity table) to 2 mm and above prior to planting. With the larger but still relatively cheap seed, a sowing rate of at least 3 kg/ha is recommended, resulting in a plant stand of 30 plants/m2 even at an establishment rate of 50 per cent. There is little penalty from sowing at even higher sowing rates, as DAFWA research found that too few plants has a greater economic penalty than too many plants.”

The DAFWA canola seed size ready-reckoner is an excellent resource for growers to use and is available on the Department’s website. Simply line up retained OP canola seed along a 10 cm section of a ruler and count the number of seeds. To achieve best results there should be 50 seeds or less to take up the 10 cm length. Use this information to calculate seed rate.

“Sowing larger seed is essential if planting early, in early-mid April rather than mid-May, to maximise grain yield potential in canola,” he says. “The larger seed can be safely sown slightly deeper than optimal for each soil type and still achieve reliable crop establishment.”

Sowing in early-mid April into residual soil moisture gives the crop a head-start on weed germinations and the strong crop growth provides valuable support for the pre-emergent herbicides. The only catch is to make sure that the cultivar selected for sowing early will flower in the optimal window for your district.

Larger seeds possess more energy resources than smaller seeds, enabling the seedling to push through more soil and stubble and to grow past insect damage and outcompete weeds. While planting larger OP cultivar seed will help close the gap in early vigour between OP and hybrid cultivars, the herbicide tolerant traits of hybrids continue to provide other benefits.

Mr Brill says the extra effort to grade seed to 2 mm will see an improvement in yield provided attention is also given to good soil nutrition, timing and harvester speed. “Driving the header too fast has been shown to cause large yield losses,” he says. “This represents a direct loss of income and the volunteer canola plants suck moisture from the soil, potentially compromising the early sowing opportunity for the following season.”

Crop competition is the cheapest form of weed control and it usually comes hand in hand with improved yield; a genuine win all-round.

Sown side-by-side, one hour apart, these Gem canola plots were sown at 2.6 kg/ha with 80 kg/ha MAP. The only difference is that the farmer retained seed on the left was graded to 2 mm while the plot on the right was sown with purchased seed, straight from the bag. (Photo: Warwick and Di Holding, Pontara Grain, Yerong Creek)

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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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