Weed management in sunflower crops
Maximise crop competition
It is important to select paddocks with low weed populations, particularly broadleaf weeds, as registered herbicide options for use in-crop are limited.
Weed competition studies have shown control is critical in the first seven weeks following sunflower emergence, when sunflower plants are not competitive against weeds. Low to moderate levels of weed competition can reduce early crop biomass by 39% and final yield by 16%.
Uniform emergence in clean paddocks will support the plant structure to quickly provide inter-row shading (full canopy closure) and maximise crop competition.
Higher plant populations (e.g. > 30,000 plants/ha) and narrower row spacing (e.g. < 1.0 m) improve crop competition but are only suitable in high rainfall areas.
Summer-growing grass weed seedlings can be controlled with a post-emergent grass selective herbicide. Growing sunflowers can be a valuable part of an integrated weed management (IWM)strategy for controlling summer grasses and feather top Rhodes grass. Sunflower production allows the use of herbicides from the A, C, D, J, K, L mode of action (MOA) groups for high risk summer grasses, and removes some of the reliance on glyphosate for total summer grass control.
Unfortunately, all selective grass herbicides use the Group A mode-of-action and are at a high risk of developing resistance if IWM programs are not implemented. Resistant grasses can be controlled using a combination of crop rotation, pre-emergent herbicide and inter-row spraying or inter-row cultivation.
Perennial weeds pose a particular problem as they are very difficult to control once established. Often the herbicides most effective on perennial species have long plantback periods for sunflowers.
Integrated weed management
The aim of developing an integrated weed management (IWM) plan is to achieve and maintain low weed numbers, while remaining profitable. To lower weed numbers, aim to reduce the weed seed-bank, and in the case of perennials, the tubers, corms, bulbs or rhizomes (vegetative propagules). This relies on a ‘zero weed seed set’ policy, as opposed to short term economic weed control, where a few weeds are left to survive and to set seed.
A ‘zero weed seed set’ policy involves using a number of weed control tactics to target the same species, or group of species, within the one season. The ‘double knock’ technique, which involves the use of two herbicide applications from different mode of action (MOA) groups in close succession, e.g. within 7–10 days of each other, is a useful technique to stop seed set in hard to kill weeds.
As there are few in-crop weed control options for sunflowers, weed seed-banks must be targeted in the fallow and in other phases of the crop rotation.
Desiccation of the sunflower crop as an aid to harvest also provides an opportunity for non-selective knockdown control of weeds present at harvest.