For even and uniform distribution of the spray mixture on the soil surface the seedbed should be relatively fine, firm, weed free and free of big clods. Existing weeds must be eliminated by seedbed preparation to provide pre-emergent herbicides the opportunity to be effective. Most pre-emergent herbicides need to be in the soil at the point when weed seed starts germinating, not later.
When larger clods (rough seedbed) are not disintegrated, weed seeds within the clod are not exposed to the herbicides with subsequent unacceptable results. A fine and firm seedbed is required for optimum germination conditions for the crop as well as the weeds. The germination of weed seeds will also be more even resulting in better control. An excess of plant and crop residues (e.g. stubble) on the soil surface may have a screening or shade effect which could prevent the herbicides from reaching the soil.
Rainfall and availability of moisture
Rainfall before, but especially shortly after application, as well as the available soil moisture plays a major role in the efficacy of soil-applied herbicides. Rain is necessary to carry soil-applied herbicides into the lower soil profile (leaching) so that it could be absorbed by weed seedlings. Soil moisture plays an important role in the absorption of such herbicides because it must be in the soil solution to be adsorbed. Too much rain however, could cause leaching past the seed level. It can also cause stress because of water logging, which results in decreased herbicides absorption and poor weed control.
Soil clay type and content
The clay content as well as type of clay affects behaviour and efficacy of herbicides, but this behaviour is very specific for a particular herbicide. Two main types of clay are found in the soil – swelling type (montmorilonite) and non-swelling type (koalinite). Herbicides are adsorbed stronger on the swelling type of clay than on the non-swelling types. In this way less herbicide is available for plant absorption and application rates need to be adapted to higher application rates.
The level of the soil pH can play an important role in the activity of certain soil-applied herbicides. Various studies, specifically with atrazine, have confirmed this fact. It has been found that atrazine is much more active at higher pH values because less atrazine is absorbed on the clay particles of the soil.
Triazine, which is the main ingredient in atrazine herbicides are, for example, poorly absorbed in soils with a pH value higher than seven. Within normal pH levels found in good maize soil of between 4,5 ph to 6,5 ph, the effect is not significant. The sulfonylureas are slowly hydrolysed in soil with a high pH which may have an influence on the residual action and also on follow-up crops. High soil pH could result in better control over a longer period, but unfortunately will also have a higher risk of crop damage on successive crops.
“The application of pre-emergence soil-applied herbicides has to be as close as possible to seedbed preparation to prevent the germination and development of weed, before getting in contact with herbicide.”
Time of application
The time of application has an important influence on the efficacy of herbicides. The application of pre-emergence soil-applied herbicides has to be as close as possible to seedbed preparation to prevent the germination and development of weed, before getting in contact with herbicide. One should strictly adhere to the recommendations on the label.
The application method plays an important role in the efficacy of herbicides, whether sprayed or applied in granular form.
The herbicide must be evenly applied over the soil surface. As general guidelines it is recommended that at least 200 litres of spray mixture be used for ground application and at least 30 litres of spray mixture per hectare for aerial application (where applicable). To ensure the best results, special attention must be paid to the type and condition of spray equipment.
Depth of germination
The depth of germination of weed seeds also plays an important role in the efficacy of herbicides. The herbicide application will be less effective when the weed germinate deeper or shallower than the herbicide’s concentration zone, for example where rainfall was insufficient to leach the herbicide into the upper soil profile.
Even with soil incorporated herbicides it was sometimes found that weeds germinate deeper than the zone of herbicides activity. Large seeded weeds usually germinate deeper while small seeded weeds germinate closer to the surface.
• Crop protection for the industry by Dr B. Mlathi and Mrs. A.J. Kruger.