Ground Cover Supplement : GC Supplement - Spray application technology
GROUND COVER SPRAY APPLICATION TECHNOLOGY 4 The ABC of herbicides Herbicide groups tell the user more than just the potential for resistance By Linda Hall BY UNDERSTANDING THE basics of how herbicides are taken up and moved within a weed, how they inhibit the weed and how they are broken down in the soil or by the crop, growers can optimise herbicide rates and performance. Without this knowledge the potential for poor efficacy and off-site impacts are increased. Herbicides are grouped by site of action for the purpose of resistance management, but the groups also reveal how to enhance product performance. The herbicide group defines how the herbicide enters the plant, how it moves within the plant, how quickly it is degraded by the crops, if surfactants are required, whether efficacy is reduced by mixing products and how all of these factors are affected by environment (Table 1). Knowledge of each herbicide group allows growers to fine-tune product choices, rate structures and product mixes. Herbicides are not easy to introduce into plants. Think about trying to administer a drug to someone by using a brief shower! Some products, such as the Group As, are not very water soluble. They tend to become stuck in the leaf's waxy cuticle. Drought and weed growth stage alter the cuticle and make it more difficult for herbicides to move to where they need to go. Under dry conditions, use the highest recommended rate, a greater water volume and moderate-size droplets to increase the herbicide footprint. Surfactants are critical for some products, such as the Dims (Group A) because these products degrade in sunlight as they sit on the leaf surface. These products need to be assisted into the plant to avoid product loss. Soil-applied and soil-active herbicides fall into two camps. The Group Ds are not water soluble and usually do not move in soil water. They must be in immediate contact with the growing weed seedling, therefore placing them in the layer of soil where the weeds germinate is essential. Some of the water soluble Group Cs can move in soil water and enter the plant from the soil or, in some conditions, leach out of reach of the weed's roots. Some herbicides move throughout the plant in both directions using both phloem, the sugar transport system, and xylem, the water transport system. These include Groups B, I and M. These have activity on perennial weeds and move to meristems, where their sites of action are located. Herbicides that translocate are usually very reliable and are less influenced by weather or spraying parameters. However, they have greater potential to cause damage if allowed to drift. Others, such as most of the Group Cs, move only with the xylem, the water system. These herbicides flush to the edge of the leaf and can fail to kill the growing points of weeds. Spray coverage is critical for these herbicides and they are much more effective on small weeds than large ones. Where the herbicide's target site is located influences how products can be optimised. Most targets are enzymes, located in the growing points of the plants. This is particularly true of herbicides in CHEMICAL CHOICE Read the label: pesticide labels contain information that is valuable when making decisions to minimise the risk of spray drift.
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