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How current weed control affects your planning for next year

June 2019

Dr Johann Strauss, Senior Scientist,
Western Cape Department of Agriculture. Send an email
to johannst@elsenburg.com

Weed control is a high priority when it comes to crop and pasture production. Weeds can cause serious yield losses in cereal, oil crops and pastures from year to year if not properly controlled. As a farmer you have to know the weed status of each and every camp on your farm. Knowledge is power.

Proper weed control is dependent on various factors. It is important to follow the label instructions of each and every herbicide you wish to apply. Half and double doses instead of the recommended rate can hasten the onset of herbicide resistant weeds on your farm. If this occurs, it might cause the farmer to make drastic steps to rid fields of herbicide resistant weeds. 

Applying the herbicide on the field requires the correct water volume as well. If you apply the herbicide with too little water, you will not get enough herbicide coverage of the culprits you want to eliminate. If the water volume is too high the herbicide can run off the plants and again result in poor control. The quality of the water can also affect the effectiveness of the applied herbicide. When spraying mixes, make sure that the products you wish to apply can be blended, otherwise it could cause serious damage to your crop or pasture. Also make sure when applying post emergent herbicides that it is done at the correct growth stage of the crop.

The farmer must realise that the breakdown period of products differ and that these breakdown periods can be shorter or longer depending on the climate and the biological life in the soil. Certain herbicides can also have a detrimental effect on soil life, suppressing the amounts of micro-organisms in the soil. Why is it important to know the length of these breakdown periods? If you plant a crop that is sensitive to the specific herbicide you have used this year, it can cause serious problems with germination and eventual plant density of a camp. If you lose a large number of plants and the crop stand is sparse, it opens the door to competition from weeds. 

Even if you adhere to the rules on planting a crop following the use of a certain herbicide, the climate can still cause an effect in the following season. This can happen very easily if herbicides were applied in a very dry year and not all residues of the herbicides have been broken down. This has happened a few times in the last two or three dry years experienced in the Western Cape. It is therefore very important to read the label and know the length of these periods of with-holding because certain herbicides need not only a certain period of time, but also a certain amount of moisture to be effectively broken down.

To prevent herbicide resistance, it is important to rotate crops and herbicide modes of action (thus from different groups). The ideal is to rotate broadleaf crops and cereal or grass crops to ease the control of certain types of herbicides. The idea is to manage your grassweeds in the broadleaf crop so that there is low, or no grass weed pressure in the following cereal crop and vice versa with broadleaf weeds in the cereal crop.

If you find yourself in a scenario where your weed control was not as effective as you have hoped for, it might be necessary to rethink your planning for the following season. Let’s say you have planted wheat in 2018 and you struggled to control broadleaf weeds during the season, thus adding a large number of weed seeds to the soil seedbank, and your planning was to plant a broadleaf crop in 2019 it might be worthwhile to change your plans. 

Rather plant another cereal crop to have another season to take out the broadleaf crops. It is not ideal, but in the long run it is more beneficial to lowering the weed seedbank. You can therefore decide to plant wheat again (but it is only advisable if you are not struggling with grassweeds as well) or you can plant oats as a hay crop in order to be able to manage the broadleaf weeds and possible grassweeds that might occur. By cutting the oats as hay you will prevent the broadleaf and grassweeds of setting seed. If this is successful you can go back to your rotation. 

The same applies to a broadleaf crop where you struggle to manage the grassweeds. Say you planted canola and the grass control was poor, rather plant another year of broadleaf, like lupine or peas, to get another shot at controlling the weeds before going back to your original rotation. Here you also have the option to plant a fodder crop or a mixed cover crop and allow the weeds to germinate and bale the material before the weeds set seed. 

It is also important to manage the weeds during the off season. Weeds use water which could have been stored in the soil for the next season. So, if you plant crops in the winter rainfall areas, control the summer weeds and the other way around if you plant crops in the summer rainfall area. 

Remember the importance of keeping the weed seed numbers low to prevent competition with your crop. Read the labels of the products you want to use and plan a crop and herbicide rotation for your farm along with your advisor.

Publication: June 2019

Section: Pula/Imvula