Rotating crops and herbicide MOA
Cropping history dictates the crop rotation decisions on Grant and Kate Wilson’s Hopetoun farm in Victoria’s southern mallee region. The Wilson’s use a conservative rotation with up to one-third of the farm under vetch or fallow at any one time.
Their no-till cropping rotation, which includes wheat, lentils, canola, barley, lupins and field pea, is governed by disease and weed management considerations, proving that integrated weed and disease management can go hand in hand.
“There was a time when we just ignored summer weeds, but not now,” says Grant. “We are very conscious of weed management all year round and the potential for herbicide resistance to really limit our options if it gets out of hand.”
Following a ‘no cereal after cereal’ policy, Grant usually tries for a two year break between cereal crops. “We also rotate between different wheat cultivars to make the most of their disease management traits,” he says.
“Following a cereal we would be looking to plant a legume, usually a lentil crop, but if we were faced with a broader weed problem we would choose an ‘imi’ tolerant lentil or possibly decide to grow field pea instead of lentils to take advantage of the wider range of selective herbicides registered in field pea.”
Being in a lower rainfall zone (325 mm or 13 inches), the Wilsons find a conservative rotation is safer in the long run and gives them more options to manage annual ryegrass.
Pulse crops offer the best combination of tactics to tackle ryegrass in-crop using grass selective herbicides and desiccation. Recently Grant started using pre-emergent herbicides such as Boxer Gold® and Sakura® to reduce their reliance on trifluralin in wheat. “So far we have had mixed results, especially in dry weather, when there was insufficient moisture to properly activate the herbicide, on top of poor crop competition due to the seasonal conditions,” he says. “It is an expensive option but when it works it provides good control across the paddock.”
Grant prefers to use Boxer Gold® before sowing with knife-points and press wheels on the seeder.
The heavier soils are in a fallow rotation to conserve soil moisture. Most fallow paddocks will usually be sprayed once or twice, depending on rainfall, over the summer.
The Wilsons grow vetch as a green manure crop on their lighter soils to improve soil health and control weeds. When the vetch reaches maximum biomass Grant sprays it out to gain maximum benefit from the high biomass production. All other crops are left as standing stubble and this year they will be moving into inter-row sowing.
“We now have RTK guidance fitted to the seeder and expect to see some real improvements in crop establishment,” he says. “With a more even sowing depth we should get more uniform germination and that will increase the crop’s ability to out-compete weeds.”
The soil type across the Wilson’s farm ranges from sand to loam and varies in pH. Kate, an independent agronomist, takes production-limiting factors such as soil pH and boron levels into account when planning the rotation, particularly with lupins being more sensitive to higher pH and lentils being sensitive to boron levels. The potential for herbicide residues to still be present after a dry summer is also a consideration, particularly on higher pH soils.
The Wilsons crop between 4000 and 5000 ha a year and generally do not have livestock, however they do fatten lambs on stubble as the opportunity arises.
Annual ryegrass has some resistance to Group A ‘fops’ but so far ‘dim’ herbicides are still effective. Kate customises their herbicide mixes to preserve the effectiveness of the dim herbicides by avoiding unnecessary usage.
They have also resisted a move into glyphosate tolerant varieties because they are concerned about the potential over-use of glyphosate that may lead to glyphosate resistance in weeds. However, they do grow some herbicide tolerant crops that utilise different herbicide modes of action.
The Wilsons choose not to grow imi-tolerant cereals because they believe this would lead to an over-use of Group B chemistry, which is known to lead to herbicide resistance in weeds such as brome grass.
They use imi-tolerant Clearfield canola to a limited degree as another option to control annual ryegrass using imazapic/imazapyr (Group B) products and rotate herbicides as best they can in an attempt to stave off herbicide resistance in grass weeds.
“We use crop desiccation to stop weed seed set to avoid the need for narrow windrow burning to destroy weed seeds after harvest,” says Grant. “Desiccation seems most effective in legumes, particularly lentils. In cereals we have also used herbicides to croptop weeds according to product label instructions.”
“Fleabane is a new weed in this area having become noticeable for the first time during the wet summer in 2010,” says Grant. “A double knock treatment has been recommended for fleabane control and we will also introduce cultivation if required in some situations.”
The Wilson’s tread a fine line between maximising crop competition and conserving soil moisture. “Plant health is our main interest and we use a range of tactics to reduce weed pressure in-crop,” says Grant. “With legumes in the rotation we avoid applying much starter-N fertiliser and we make an effort over summer to get the paddocks as clean as possible before seeding.”
The Wilsons plant their crops as narrow as possible using a standard cereal seeding rate of 60 kg/ha on a 250 mm row spacing. They find this row spacing narrow enough to provide early crop competition and not be too rough, while still being able to inter-row sow.
Taking a thoughtful and long-term view of herbicide use within their cropping system has kept a lid on herbicide resistance on the Wilson’s farms so far and preserved a wide range of herbicide chemistry. Herbicide use is supported with non-herbicide tactics and the rotation of modes of action within and between crop types.