TECHNICAL
Everything about pre-emergence herbicides Pre-emergence herbicides act on weed seedlings at early stages of development before they become established, but have no impacts on dormant (non-germinated) seeds. Ardi Ash : AgFirst Nelson research associate They arrest the growth of the shoot and/or root thorough a specific mechanism of action (MOA) which kills the weed. This ranges from interruption of key physiological processes such as photosynthesis, to inhibition of the biosynthesis of vital amino acids, by blocking the active site of the respective enzyme (Fig. 1 ). Therefore, timing of herbicide application is very important, and for efficient and safe application, the life cycles of the target weed species need to be well considered. Due to impacts of environmental variables on seedling development, this is not always an easy estimation and may require modelling when you are dealing with a few weed species on a large scale. WeedSOFT is one of the softwares, developed by the University of Nebraska, which can support growers in making decisions about herbicides based on their efficacy, cost effectiveness, environmental safety as well as the specific weed species, their density and emergence time. Due to the long-lasting activity of pre-emergents, they are classified as ’residual’ herbicides (as opposed to ’knock-down’ herbicides with a one-off action). This is due to the fact that pre-emergence herbicides once in the soil usually require activation by rain or irrigation. Activated pre-emergence herbicides in the soil create a barrier layer against weed seed germination (Fig. 2a ). On the other hand, weed seedlings can still grow on soils containing inactive pre-emergent at the topsoil (Fig. 2b ). Under dry conditions, non-activated herbicides can be carried over from season to season, posing a risk of damage to non-target plants and jeopardising growers’ herbicide calculations. Pre-emergence herbicides stay in the soil for some eight to 12 weeks and prevent weed growth until hydrolysed or degraded (by soil microorganisms). Generally, preemergence herbicides persist longer in alkaline soils, as chemical hydrolysis is slow at high pH levels and microbial decomposition is the primary breakdown pathway. Although reapplication before the active ingredient loses effectiveness is possible, it can be tricky and the product guidelines as well as environmental factors must be carefully considered.
Enzyme A
Enzyme A
Herbicide A Amino acid synthesis
acid ino Am
Synth esis
Figure 1 . Schematic inhibition of amino-acid synthesis through blockage of the enzyme’s active site by the herbicide (Luke Gerlach)
Activated pre-emergence barrier
Inactive pre-emergence granules
Control (no pre-emergent)
Figure 2 . Roles of pre-emergence herbicide in weed control: a) preemergence barrier, activated by rain or irrigation arrests germination of weed seeds; b) non-activated (but incorporated) pre-emergence herbicide (granules) have no impacts on weed growth; c) weed growth in herbicide-free zone
Particularly in case of herbicides controlling broadleaf weeds, more caution is needed. Excessive application can potentially injure non-target plants including annual crops. More than one application of pre-emergent per season is not recommended. Moreover, similar to any other pesticides, there are potential risks of unwanted movement of pre-emergent through run-off, volatilisation or leaching prior to or after incorporation into the soil (Fig. 3 ). With no two seasons ever the same, the observed performance of different herbicides is not only attributed to their chemical properties, but is also correlated to their interactions with soil, environment, and the particular weed species and weed burden they are placed under. To choose a right pre-emergence herbicide for your orchard or farm, pay careful attention to the technical information on the product label. The ORCHARDIST : JULY 2022
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