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Australian Herbicide Resistance Initiative (AHRI)

Focus

Grass

Australian golfer Adam Scott keeps his eye on the ball. His ability to concentrate and focus amongst the chaos of a major golf tournament is outstanding. We would all love to focus on one task and do it well, but the reality of life is that we are rarely afforded this luxury. For grain growers, managing resistant weeds is just another thing to fit into the complexity that is farming.

Growers that afford themselves the time and motivation to focus on managing resistant weeds are having a win. They declare war on the weed seed bank and have a ‘take no prisoners’ attitude. One grower commented that he “would never declare a cease fire on the war on weeds”. He means business!

The results of a long term ‘Focus Paddock’ study are proof that when growers’ put their minds to it, they can beat resistant weeds. The growers that use a diverse range of tools are winning the numbers game and reducing their weed seed bank.

The Focus Paddock study was a GRDC funded, Department of Agriculture and Food of WA project conducted by Peter Newman over the past 13 years. Peter is now the leader of communications with AHRI and is continuing to monitor these focus paddocks. This study started with 31 focus paddocks, 27 of which are continuing to be monitored. Four paddocks have dropped off due to changes in ownership / land use (i.e.one paddock was planted to trees). Ryegrass was monitored by quadrat counts each spring to determine the number of surviving ryegrass in crop or pasture.

97%

Growers in this study have successfully eroded their ryegrass numbers in crop by 97%. In the spring of 2001 there were 187 ryegrass plants/m2 on average in crop or pasture. That number was just 5 ryegrass plants /m2 in the spring of 2013.

Graph average surviving ryegrass in august

How did they do it?

Firstly, they universally used pre-emergent herbicides at full label rates. As ryegrass evolved higher levels of resistance to post-emergent grass-selective herbicides in the early 2000’s, growers began to make the switch to using more grass-selective pre-emergent herbicides, in particular trifluralin. In 2001, only 50% of the paddocks were sprayed with grass-selective pre-emergent herbicides. This increased to 95% of paddocks by 2010. These herbicides typically achieve 75% to 90% ryegrass control. While there is no resistance to these herbicides in these paddocks at present, there are always surviving ryegrass due to the efficacy that is achievable with pre-ems.

Graph showing pre-sowing herbicide

But pre-emergent herbicides alone are not the answer

Given that pre-emergent herbicides give 75 to 90% control of ryegrass there are obviously always weeds that set seed. It would seem crazy to spread these weed seeds back out over the paddock. A low number of surviving weeds in crop each year is not acceptable as they contribute to the future seed bank. This is bad news for two reasons; 1) there are weeds in the following crop, and 2) these weeds are sprayed with more herbicide and resistance is selected for.

The graph below demonstrates the benefits of using harvest weed seed control (HWSC). In the “plus HWSC” group, 10 growers used windrow burning and 2 growers used chaff carts.

Graph showing benefits of using harvest weed seed control

The plus HWSC group

  • 12 growers in this group who have used HWSC, on average, in 38% of crop years.
  • The selection criteria for this group was that they must have used HWSC at least four times over the 13 year period and/or used HWSC for the past three years in a row. This group is larger than has been reported in the past because a number of growers have adopted HWSC in the past three years.
  • Average cropping intensity was 91% over 13 years
  • Ryegrass density of this group has been 1 plant/m2 or fewer over the past six years.

The minus HWSC group

  • 14 growers in this group who have used HWSC, on average, in 12% of crop years
  • Average cropping intensity was 88% over 13 years
  • Average ryegrass density of 6.5 plants/m2 over the past six years. There were 8.8 ryegrass /m2 in 2013 for the minus HWSC group compared to 0.5 ryegrass /m2 for the plus HWSC group.
  • This group includes one grower who towed a chaff cart for eight years but has not done so for the past three years and has seen an increase in ryegrass numbers.

Nine or ten ryegrass per square metre may not sound like much, but if they are healthy plants with a lot of tillers they can make a big mess and set a lot of seed. The photograph below is of a lupin paddock in 2013 with 11 ryegrass/m2 (see image below). This paddock was harvested with a chaff cart from 2002 to 2009. There were zero ryegrass in this paddock from 2008 to 2010. The grower purchased a new harvester for the 2010 harvest and did not attach the chaff cart due to the difficulties of performing this job. Three harvests without a chaff cart has seen the ryegrass numbers increase.

Wild radish

Wild Radish

Of the 27 paddocks counted in 2013, only five paddocks had surviving wild radish in spring. Four of these paddocks were from growers in the minus HWSC group and one was from a grower who practices HWSC who had some wild radish surviving in a lupin crop.

What else are they doing?

The success that growers in this study have achieved is not only due to harvest weed seed control. The main factor is the change in attitude. Growers who are successfully managing resistant weeds have very high standards of weed control and simply do not accept survivors. If they have surviving weeds in crop they do something to minimise the number of seeds entering the seed bank. Some of the weed management tools that have been successful are:

  • Changing crop rotation. Some of the growers in the focus paddock study were previously in a long term lupin : wheat rotation. Many changed their rotation such that lupins were grown less often (every 4 to 6 six years instead of every second year).
  • Crop topping has become a common practice in lupin crops. Paraquat is applied at or after 80% leaf drop of the lupin when ryegrass is at the flowering to soft dough stage.
  • Applying lime to correct sub-soil acidity is mentioned by many of the growers. They generally believe that if their soil is healthy they have a greater chance of killing weeds with herbicides and growing a competitive crop.
  • High quality spray application is also a focus of many of these growers. They go ‘killing weeds’, they don’t just go ‘spraying’.
  • The mouldboard plough has seen a resurgence in the area. None of the focus paddocks have been ploughed but some of these growers own a plough. The mouldboard plough is used to correct non-wetting soil, bury lime to correct sub-soil acidity, and bury weed seeds before returning to no till.
  • Chemical fallow has become an inclusion in the rotation for many growers, particularly in the lower rainfall areas. This has come about due to a reduction in the area of lupins and many livestock left the region after droughts in 2006 and 2007. Chemical fallow is a high risk situation for glyphosate resistance.
  • Crop competition is a focus of many of the growers through increased seeding rates, and in some cases, narrow row spacing. Many growers are now using a paired row or ribbon seeding boot to effectively reduce their crop row spacing.
  • Livestock (mainly sheep) in the rotation are still a key tool for a few of the focus paddock growers. These growers generally crop for several years and then drop the paddock into pasture for a year or two to get weed numbers down again.

What is the key to weed control and reducing the weed seed bank? Focus!

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