Select paddocks with as few broadleaf weeds as possible and delay sowing to provide an opportunity to use knock-down herbicides and cultural control.
Pre-emergent weed control
There are two commonly used pre-emergent strategies used on field peas in Western Australia. For both strategies it is important not to sow field peas too shallow; sow at least 5cm deep and where diuron is used, level the seedbed by harrowing and/or rolling before applying the herbicide, otherwise, rain may concentrate the diuron in the furrows.
Apply 1-1.5L/ha diuron (500g a.i./L) and 1-2L/ha trifluralin (400g a.i./L) before sowing when the crop is sown with knife points. The diuron controls many broadleaf weeds and the trifluralin controls grasses and wireweed. In conventionally cultivated systems, which leave the paddock relatively flat, apply diuron post-sowing pre-emergent (PSPE). The trifluralin is still best applied before sowing to ensure that it is well mixed with the soil.
For no-till field peas apply 34g/ha Spinnaker® and 1-1.5L/ha diuron PSPE. Spinnaker® will control wireweed, some grass weeds, and a wide spectrum of broadleaf weeds. It is important to level the seedbed by harrowing and/or rolling before applying this mixture.
Post-emergent weed control
There are many herbicides registered for post-emergent weed control in field peas. These are mostly applied at the 3-6 node stage of the crop. Some of the more popular options for post-emergent control of broadleaf weeds are listed below. Note that not all of these herbicides are compatible with post-emergent grass killers, nor with oil or wetter.
Brodal® and metribuzin mixes: usually 60mL/ha Brodal® (500g a.i./L) and 60mL/ha metribuzin (750g a.i./L) is adequate for cruciferous weeds and capeweed (WA permit applied for). Higher rates of metribuzin will improve doublegee control, but are also more likely to damage the crop. At higher rates Brodal® and metribuzin can be used alone, but they are more likely to damage the crop and are less effective against weeds than when mixed together.
45g/ha Raptor® + 2% BS1000 will control cruciferous weeds, lupins, barley grass, brome grass, and volunteer cereals. It will not control annual ryegrass and cannot be tank mixed with post-emergent grass killers.
700-1000mL/ha MCPA 250 (as the sodium salt) will control late germinating cruciferous weeds and can be applied to field peas from the 6-8 node stage. However, it will retard the crop, and may cause stem distortions for up to two weeks after application. This option should only be used when good growing conditions are expected following application so the crop can recover.
Crop topping aims to prevent seed set of surviving in-crop weeds, hence lower the weed burden in the following crop. The early maturity of field peas makes them ideally suited to crop topping.
Reglone can be used for crop topping, but the most economical chemical is Gramoxone at 600mL/ha for ryegrass, or 800mL/ha for wild radish. Timing is critical to the success of crop topping. Spraying too early will reduce the crop’s yield potential, and spraying too late will have little effect on the weeds. Once the field pea seeds have reached 30% moisture, or when the lower 75% of the pods are brown, with firm seeds and leathery pods, crop topping will not reduce crop yield. The crop can still have green tips at this stage and the operation may improve harvest efficiency by making the crop ripen more evenly.
Glyphosate should only be used when the crop is to be swathed. Glyphosate should be applied at 50-75% brown pods if it is used in crop topping a swathed crop.
Don't use glyphosate to desiccate or crop top a seed crop. Because it can reduce seed viability.