Control weeds early to maximise sunflower yields

October 2021

With the increase in sunflower yields, the sunflower price increase and the need to apply crop rotation in especially maize monoculture, sunflower cultivation has become more appealing. It is also a good method to improve soil health.

A problem with the cultivation of the sunflower crop has always been broadleaf weed control, especially if the producer does not want to cultivate too much as to retain soil moisture. 

BROADLEAF WEED CONTROL
The first step in the fight against broadleaf weeds was the registration of Alachlor, which is a pre-emergence grass herbicide capable to control a reasonable broadleaf spectrum weeds. The herbicide could however not control the more serious and difficult to control broadleaf weeds. Alachlor herbicide control also decreased towards the end of the season, so the producer had to cultivate once or twice to keep the fields clean.

The next step for broadleaf weed control in sunflowers was the registration of flurochloridone (Racer CS/Radical 250 EC/Rapid 250 EC and Sun-Down 250 CS). These herbicides must be applied directly after planting before the sunflowers emerge. 

The planting depth of the sunflower seed is also important. If planted too shallow, the sunflower seedlings will be harmed by the herbicide. There are several alachlor formulation on the market that are registered along with the various flurochloridone formulations, for example Alachlor 480 CS, or Alanex 384 EC or Alanex 480 CS. Flurochloridone residues in the soil can also harm certain succession crops.

The next breakthrough in broadleaf weed control in sunflowers is the Clearfield^® programme. In the beginning Clearfield^®sunflower varieties were treated with Euro-Lightning^® herbicide, but it caused yellowing and stunting of the Clearfield^® sunflower varieties from time to time. This led to development of Clearfield^® Plus sunflower varieties which are more resistant to the active ingredients of Euro-Lightning^®. Euro-Lightning^® Plus was then released which contains softer adjuvants and solvents.

If the Clearfield^® Plus herbicide is applied to weeds between the two and six leaf stage within 32 days after planting, incredible weed control is acquired. This herbicide mainly controls broadleaf weeds, but under optimal (warm moist) conditions grasses will also be controlled – especially if the grasses are very small during application. 

Several herbicides similar to the regular Euro-Lightning^® have since been registered, namely Captora/Mistic and Imimox. Technically these herbicides can be used on Clearfield^® Plus cultivars because these cultivars are more resistant to the active ingredients in the herbicides, but they are not registered as such.

SUMMARY
In summary, the following applies to weed control in sunflowers:

• Always consult the relevant product labels for recommendations.
• High density of sedges and clay percentage > 16%: Apply Eradicate Plus (for example) ten days before planting, incorporated into the soil. Seal the soil surface to keep the EPTC gas in the soil as long as possible.
• For a high density of grasses: Apply Alachlor, metolachlor 960 or S-metolachlor 960 directly after planting. Use on all clay percentage soils but apply according to the clay percentage. It mainly controls grasses but some broadleaf weeds are also controlled. Yellow nutsedge can be excellently controlled to variable controlled depen­ding on the environmental conditions.
• High density of broadleaf weeds: Flurochloridone to be applied strictly pre-emergence. Apply according to clay percentage from 11% clay.
• High density of broadleaf and grass weeds in conventional sunflower varieties: Flurochloridone plus alachlor from 5% clay may be applied. Strictly pre-emergence application.
• High density of broadleaf and grass weeds in Clearfield^® Plus sunflower cultivars: Metolachlor to be applied on all clay percentages – applied according to clay percentages with plant, followed by Euro-Lightning^® Plus at 2 litres/ha on the two to six leaf stage of the weeds as well as sunflowers. Application must be done before 32 days after planting.

Publication: October 2021

Section: Pula/Imvula