HAVING a crop germinate as close as possible to the targeted time for that variety often helps to optimise yield potential, however for larger operations the need to sow big areas in a short space of time to meet the correct germination window poses logistical challenges.
As a result, sowing some paddocks dry before the expected break to the season is a tactic used by many growers to spread the planting operation and address these issues.
But that means when the season does break, weeds typically emerge with, or sometimes ahead of the crop.
As a result, it's important to have an effective pre-emergent herbicide in place to manage the first weed flush and allow the crop to establish with as little weed competition as possible.
There are several important points to consider when using pre-emergent herbicides and dry sowing.
Knowing your weed seed bank and where the weed seeds are
The size of the expected seed bank is also a very important consideration, as there is no opportunity for a pre-seeding knockdown herbicide when dry sowing.
Independent Consultants Australia Network (ICAN) senior consultant Mark Congreve said a large weed germination emerging with the crop would place extreme pressure on any pre-emergent herbicide.
"No pre-emergent should be expected to achieve 100 per cent control, especially when applied dry, so even the best performing pre-emergent herbicides can still look dirty after the season break if the starting population was several hundred weed seeds per metre," Mr Congreve said.
"Therefore, only select paddocks for dry sowing that are known to have low weed seedbanks and leave sowing of higher weed pressure paddocks until after a robust knockdown program can be implemented on the initial flush after the break."
In a long term zero-till paddock, most of the weed seeds should be close to the soil surface and in this situation, a low-mobile herbicide which stays relatively shallow in the soil profile is likely to be preferred.
However, if the paddock has recently been cultivated, then some of the weed seedbank will be deeper in the profile and therefore a more mobile herbicide may be needed to reach deeper germinating weeds.
Herbicide loss to the environment
Mr Congreve said some herbicides could be broken down by ultraviolet (UV) light or lost to volatilisation if left exposed to the environment.
"For these herbicides, the rate of loss generally increases with warmer temperatures and higher levels of sunlight - which often occurs with dry sown paddocks that are generally planted early," he said.
"If the herbicide being used is subject to these losses, then it will be important to ensure that it is correctly incorporated soon after application."
A common mistake which can occur, especially on very large seeding operations which tend to utilise dry sowing for logistical reasons, is the speed of pre-emergent application is often significantly faster than the speed of sowing.
The sprayer gets further and further ahead of the planter, and hence time to incorporation gets progressively longer.
Ideally, for a volatile herbicide such as trifluralin, growers should not spray more than what can be sown within the following four to six hours.
"Some herbicides will also bind to stubble, especially trifluralin which binds extremely tightly to organic matter," Mr Congreve said.
"For other herbicides, the relative strength of binding provides an indication of the rainfall volume/intensity that will be required to move herbicide off the stubble onto the soil."
Seeder type and set-up
A well set-up knife point and press wheel seeding system balances row spacing, speed of travel and soil characteristics, to remove herbicide treated soil from above the seeding trench and throw it into the inter row area to cover it, but without soil thrown into adjoining seed trenches.
Such a setup will generally reduce volatility and UV light losses, while the mechanical incorporation from this process will also commence some soil binding as long as there is some soil moisture.
Mr Congreve said in low rainfall environments it may be tempting to set up the tyned seeder to leave a distinct planting furrow to capture and concentrate any small rainfall events directly over the seed row in order to assist crop germination.
"Be aware that this practice may also concentrate residual herbicide directly over the seed and lead to crop damage," he said.
"A low disturbance disc seeder provides minimal soil throw and incorporation, leaving surface applied herbicides more exposed to the environment and for some herbicides, this may result in considerable environmental loss.
"Additionally, with many disc seeders, the seed slot may be left open which may result in herbicide easily and rapidly moving into the planting slot following the first rainfall, which can often result in excessive crop injury."
Generally low disturbance disc seeders are not recommended for dry sowing with pre-emergent herbicides, and especially not with volatile herbicides which are subject to significant losses if not adequately incorporated.
Herbicide degradation after incorporation
Typically pre-emergent herbicides will be incorporated when dry sowing and if sowing is early then the soil may still be warm.
All pre-emergent herbicides are degraded by soil microbes with the primary requirements for soil microbial activity being moisture and temperature.
"If the zone where the herbicide has been incorporated is totally dry, then there is unlikely to be any significant degradation occurring, until the opening rainfall," Mr Congreve said.
"However, where there is some moisture in the soil at application, or there are small rainfall events that are not enough to initiate crop germination, then there is risk of some microbial herbicide degradation occurring prior to the true break.
"This will be more significant if sowing early when the soil temperature is still warm."
What happens when the season breaks?
If the herbicide has not been incorporated then opening rainfall will be required to wash the herbicide into the soil.
Even when using an incorporation by sowing (IBS) system using knife points and press wheels, there will still be some herbicide remaining on the soil surface, or on stubble, which will be washed into the soil following rainfall, with the exception of products such as trifluralin which bind very strongly to stubble.
Mr Congreve said the amount and intensity of rainfall along with herbicide properties of water solubility and how strongly they bind to organic matter and soil, would influence the degree to which this incorporation occurs with the first rainfall.
"When starting with a dry soil, the first rainfall is likely to move rapidly down the profile, especially in sandy soils that have much larger air pores," he said.
"Herbicide moving with this initial rainfall will move further than applications where the same herbicide was applied to a full profile at application, where subsequent rainfall will not penetrate as far.
"This occurs with all herbicides; however individual herbicide differences also need to be understood."
The ability of a herbicide to wash off stubble and how far it will move with and after the initial rainfall, depends on both the herbicide solubility and its propensity to bind to soil and organic matter.
Both are important.
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