Using technology to put pressure on weeds
Herbicide resistant annual ryegrass has been an on-going challenge for grain producers in the Mintaro area of South Australia since the 1980s.
Agricultural consultant Mick Faulkner has worked alongside growers as they tackle the problem and has been impressed with the tenacity of some growers in the area as they adopted a new attitude toward weed control.
“The growers that changed their attitude to a ‘no tolerance’ approach to weeds are the ones that have been able to gain the upper hand with herbicide resistant weeds,” he said. “This approach works because if you have no weeds you have no trouble with herbicide resistance.”
When developing a weed control plan with growers Mick looks for tactics that have at least a 92 per cent control rate. The aim being to bring weed density right down and preventing any survivors from setting seed to keep reducing the weed seed bank.
One grower who has fully embraced this no-tolerance policy is Mark Sandow, and in doing so he has almost eliminated herbicide resistant annual ryegrass, wild oats and wild radish from the 1900 ha of cropping land he owns and leases in the 500–600 mm rainfall zone of South Australia.
“There are paddocks on Mark’s property where annual ryegrass covered 17 per cent of the area to begin with and this is now down to one per cent or less by doing everything possible to stop weed seed set,” said Mick. “Having low density and knowing where those weeds are in a paddock helps keep chemical costs right down as you are treating known problem areas, not the whole paddock.”
Annual ryegrass was the first herbicide-resistant weed on the property and its resistance was confirmed 35 years ago. In the early days Mark used haymaking to manage weedy paddocks. Although he found it difficult to successfully make hay in their high rainfall conditions it did help with the weeds until better options came available.
Over the last five or six years Mark has used GPS capability in his tractors to map and manage weeds. “We started using GPS to mark the location of weed patches and this gave us a better understanding of the scale of the problem,” he said. “It actually showed that the weed density was probably less than we first thought and gave us more confidence that the problem was manageable.”
Since then the Sandows have upgraded to the more accurate RTK GPS system and rather than ‘dropping a flag’ Mark now marks lines from one side of a weedy patch to the other.
“This provides a much better weed map and we have no doubt about where the weeds are in a paddock,” he said. “When we treat these patches we use more expensive herbicides but on a much smaller area. I also check for any misses after a spray and pull out any plants that have survived.”
Mark has found that most apparent ‘survivors’ have been due to reasons other than a chemical failure, but he recognises the need to physically remove older plants to prevent them setting seed.
The Sandows direct drill all crops to save time and avoid erosion on some steeper areas of the farm. While not using a fully controlled traffic system Mark is still seeing benefits of keeping the most frequent traffic—the sprayer and urea spreader—on permanent tracks. “In a weedy patch the elimination of weeds is the highest priority and I am not overly-concerned if removing the weeds in that area also causes some crop losses,” he said. “Overall the crop losses are less now than they were simply because we keep to the permanent tracks and less crop is knocked down during spraying.”
A very significant benefit of low weed density in a paddock is that cropping choices are much wider than if the grower is having to consider weed control as part of the rotation decisions. Mick said that with more choice, growers are able to sow crops that are likely to be the most profitable that season without being bound by weed control concerns.
The Sandow’s farming system includes wheat, faba bean and canola in rotation on 2.5 m wheeltracks and sheep graze the stubble after harvest. The main soil type on the farm is red-brown earth over limestone with smaller areas of black clay. The crop rotation works well across the whole cropping area giving the Sandows more options to rotate chemical groups between cereals and broadleaf crops.
“We lease some cropping land from my cousin and the sheep belong to him,” said Mark. “Having the sheep graze the stubble helps keep weeds under control over summer and provides some extra feed for the sheep. In years that we don’t have sheep on a paddock it is clear to see the increase in weeds over summer. There is no doubt that they are doing an effective job.”
Mark also uses narrow windrow burning in the canola crops as another weed control tactic. If there is a particularly high stubble load after back-to-back wheat Mark may also burn the stubble, although this is fairly rare. “I have tried cutting the straw under these circumstances but have noticed increased disease pressure so find burning is a better option in these paddocks, which will also help reduce the weed seed bank,” he said.
Farming in a high rainfall zone means Mark is faced with several germinations of weeds over summer and autumn. He treats pre-seeding germinations as they occur using a one-off spray to remove weeds such as volunteer cereals, potato weed, salvation Jane and wild radish. “Having a clean field to sow into is essential to conserve moisture and to minimise in-crop weed pressure,” he said. “Our attitude to weeds has always been ‘if you see it, try to fix it’ and I am also willing to sacrifice a small area in a crop if that is the best way to reduce weed numbers.”
After the autumn break Mark does the first spray of the season applying Sakura prior to sowing wheat and following up with Boxer Gold post-emergence in the known problem areas. “This is where the weed maps in the GPS system really show their worth,” he said. “I know that I am applying the herbicide where it is needed while keeping a lid on the cost.”
Mark uses crop competition as another way to combat weeds in-crop. He and Mick choose the most competitive variety and increase the seeding rate to keep the crop density high. Mark sows all crops using a flexicoil bar with narrow point tines. The tines are set at 25 cm (10 inch) to keep the crop rows as narrow as possible while still managing the stubble and minimising the risk of herbicide damage to the crop from pre-emergent herbicides. “We find we can sow in all conditions—wet or dry—and get sufficient trash clearance using this set-up,” he said. “We plant the crops on narrow rows to increase their competitive ability without compromising yield.”
Mark is also very conscious of the potential for glyphosate resistance to evolve in non-crop areas such as along fencelines. He sprays glyphosate initially, follows up with Spray Seed for any misses and then uses a hoe or pulls out any remaining weeds. Mark has removed most of the internal fences and has incorporated the land into the crop area where he keeps a close eye on any weeds that grow where the fences once were.