Winter crop post-harvest clean-up

Posted in Agronomy alert on Nov 20, 2015

Large weeds that escaped herbicide control this season will be hard to kill using traditional fallow herbicide programs. Other tactics might be full or patch cultivation, grazing, fire or optical spraying with robust herbicide rates.

Winter crop post-harvest clean-up

by Paul McIntosh, Pulse Australia, Industry Development Manager (Northern) Email: Paul McIntosh P: 0429 566 198

Just prior to harvest there were many chickpea paddocks with large weeds towering above the crop that had germinated and grown through the life of the crop.

With very little rain received after the application of herbicides like Balance® 750 at planting time, much of the isoxaflutole product was left on the soil surface, stubble layers and stalks. The small amount of rain that did fall however was just enough to germinate the weed seed but did not sufficiently incorporate the residual chemical.

As the winter progressed, weeds like milk thistle, prickly lettuce, turnip and buckwheat grew well and by the time significant rainfall came to incorporate the herbicide, the weeds were large and unruly. Given the circumstances, Balance still did an incredible job in many instances to suppress weed seeding ability and any extra growth after the late incorporating rain.

Then came the very dry finish with many chickpea crops dying, rather than progressing to a natural physiological maturity, so there was little need to apply a pre-harvest spray of a glyphosate product, with or without a metsulfuron product. The fact that big chickpea bushes had generally spread out into wheel track areas, increasing the risk of breaking off branches with many pods attached, influenced many growers in their decision not to spray pre-harvest.

The end result is that after harvesting these areas with no pre-harvest spray applied, there are going to be some very large tough weeds with well-developed root systems but severely reduced leaf area index from the harvest operation, which will limit their ability to absorb any fallow herbicide.

Fallow weed control on these blocks will be more difficult than usual and growers will need to consider different options.

The easiest approach is to implement some mechanical operation that really targets these large, old tap-rooted plants. It will take more than the disc harrows for this task—a more robust mechanical operation such as a chisel plough with sweeps that will remove these large plants and give you the opportunity to implement the remaining summer fallow weed control efforts.

Of course this will destroy stubble, so there needs to be a judgement call regarding the relative priorities of weed control and stubble cover. If the soil surface is in a reasonably soft condition then using a bladeplough will give solid weed control without disturbing the stubble too much.

If herbicides are used rather than mechanical control, be prepared to use high rates of product and repeated sprays to kill these old weed survivors. The spray tank mixes to control these plants will be varied and unusual, requiring commitment and focus to be successful.

Other tactics that can be implemented against these hard-to-kill weeds include grazing with cattle or sheep, fire or green sensor sprayers. Even strategic or pocket cultivation techniques can give you some weed control relief. In the face of the ever-increasing problem of herbicide resistance, the main aim has to be stopping seed set in suspect paddocks.

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