Herbicides critical in crop production
Word from the market with AMA
THE soil has a bank of weed seeds and the rains received during the past weeks have led to their germination. The frequency of the rains has made it difficult for farmers to come in with daily operations such as weeding and sometimes spraying of herbicides will be difficult, hence has led to competition by weeds for sunlight, space, water and nutrients.
If weeds are not controlled in maize during the first five weeks after crop emergence, this can lead to a yield reduction of approximately 40 percent.
In cotton, weed presence during the first eight weeks reduces yield by 2 percent per day. Considering these challenges farmers must resort to the use of herbicides for the control of weeds.
However, out of misunderstanding some farmers complain that herbicides are not effective, and they damage the soil. Taking a closer look there are key factors to consider when selecting herbicides.
The farmer has to consider the chemical formulation of herbicides. Herbicides can be in different forms such as flowable, wettable powder, granules, emulsifiable concentrate among others.
The chemical formulation will determine whether the herbicide is broad spectrum, controlling many weed species or narrow spectrum that kills few weed species.
The equipment of dispersion should be taken into consideration whether the farmer is going to use a boom sprayer or a knapsack. Some herbicides can be hormonal and using a boom sprayer can lead to crop damage after a certain growth stage.
Whereas at times weeds might not be widespread in the field and spot applications using a knapsack can increase efficacy of the herbicide.
Seed maize growers should always pay particular attention to their choice of herbicides as these have productivity implications.
Herbicides can be classified as pre-emergence which are applied before the crop and weeds have emerged or post emergence which are applied when both the crop and weeds have emerged.
Another category is the pre-plant clean-ups that are applied to clear off emerged weeds. Since we are in the planting window, sugar bean farmers who have not planted can use pre-emergence herbicides such as S-Metolachlor and pre-plant clean ups such as Glyphosate and Diquat.
Farmers can come in with post emergence herbicides such as Stellar star, Dicamba and Nicosulfron depending on the weed species in maize.
The type of weed species referred to as the weed spectrum plays a critical role. Weed identification at seedling stage contributes to the effectiveness of the herbicide applied.
Weeds can be classified as grasses, broadleaves or sedges. On the other hand, herbicides may also be classified as grass herbicides, broadleaf herbicides or herbicides that kill both grasses and broadleaves.
Fluazifop-p-butyl controls grasses in broadleaved crops such as sugar bean, tomatoes, cotton, brassicas and soyabean. Dicamba controls of broadleaves in maize. Halosulfron controls yellow nutsedge in maize. Stellar star controls both grasses and broadleaves in maize.
The knowledge of the follow up crop is critical when selecting herbicides. Herbicides have different residual periods which refers to the time of herbicide activity in the soil. For example a cereal crop grower could have applied Atrazine for the control of broadleaves.
However, Atrazine has a residual effect of up to eighteen months. This means broadleaved crops such as tomatoes, sugarbean and soyabean planted during this period will be affected. Soil type is a major determinant when selecting a pre-emergence herbicide.
Clay soils will require higher rates as compared to sandy soils. For post emergence herbicides the crop stage is important. Taking a closer look at traditional grains such as Sorghum, Atrazine has to be applied when the crop is at 5 leaf stage to avoid crop damage.
Environmental conditions are important when selecting herbicides, for example rainfall being experienced. Herbicides are prone to be washed off leaf surfaces if rain fastness periods are not observed, but herbicides with inbuilt or added adjuvants would provide better efficacy as loss due to washing away is minimised.
For the best weed control measures and herbicide selection farmers should consult their Agronomists for assistance. Farming as a business requires no guess work.
The use of herbicides will assist the farmer in effective weed control and maximum yield returns.
◆ This article was written by Murimisi Justice Chembela. Word from the market is a column produced by the Agricultural Marketing Authority. Feedback mmlambo@ama.co.zw or gmashiri@ama.co.zw or chembelaj@gmail.com; chembelaj@zfc.co.zw