This trial is based on the use of a winter crop to reduce the need for herbicidal weed control within the following maize crop. This year several winter crops were grown (faba beans, ryegrass, oats and three clovers) with the maize planted after different post-winter crop management practices:

• Spray out winter crop three weeks prior to planting maize (leaving crop residue);
• Spray out winter crop three weeks prior to planting maize, plus mow to reduce winter crop residue;
• Spray out winter crop three weeks prior to planting maize, plus mow to reduce winter crop residue, then residue incorporation with power-harrow.

Current herbicide availability allows for a range of weed control options and in most cases these herbicides are so effective that growers tend to rely on them exclusively to control weeds. However, with an increasing number of weeds developing resistance to herbicides this total reliance on herbicides may not be in the grower’s best interest.

Approaches to weed control – Questions & Answers

Remember when using herbicides to rotate regularly used chemicals with herbicides with a different mode of action to reduce the evolution of resistance.

Do you know the history of the field and what weeds are usually present?

No...Play safe and use a pre-emergence herbicide combination, follow up with a post-emergence herbicide if required.

Do you have a weed or weeds that are particularly difficult to manage?

Yes...Seek professional guidance on how to deal with your particular problem.

Do you want to use cultural methods to reduce dependence on herbicides?

Yes...Go to the cultural weed control section below.

Are the expected weeds mostly broadleaf weeds or are they mostly grass weeds?

Broadleaf weeds

Is triazine (atrazine)-resistant fathen present?

Yes...It may be advisable to use a pre-emergence herbicide to target this weed (acetochlor usually gives adequate control) as most post-emergence herbicides have to be applied when the fathen is very small to be effective. This often means that a second application is required to control late germinating weeds.

No...If you expect a high population of broadleaf weeds then it may be best to play safe and use a pre-emergence herbicide combination, as the post-emergence herbicide should be applied within two weeks of emergence when many weeds are present. A follow-up herbicide may then be required to control late germinating weeds.

If you expect a low population of broadleaf weeds then these could be targeted with a single post-emergence herbicide application about 3-4 weeks after emergence.

Grass weeds

It is difficult to control a high population grass weeds, especially summer grass, using post-emergence herbicides only as the application timing has to be very precise and more than one application is usually required. Therefore, it may be best to use a pre-emergence herbicide to target the grass weeds with a follow�up post-emergence herbicide to control late germinating grass weeds. Note also that the post-emergence herbicides available differ in their efficacy on specific grasses.

Low populations of grass weeds may be controlled with only post-emergence applications but more than one application will probably be required and the first application must be applied when the grass weeds are very small, usually within two weeks of emergence.

Cultural weed control

Cultural weed control has traditionally been limited to two main areas, pre-plant seedbed preparation and post-emergence inter-row cultivation. Very few growers are set up for inter-row cultivation, which leaves pre-plant seedbed preparation as the only cultural method available to most growers. This is best used in cultivated sites but may be useful in some minimum-till situations.

The practice of a stale seedbed involves cultivating the site to trigger the germination of weeds and then to kill these weeds with either a further very shallow cultivation or with a knock-down herbicide. It is largely ineffective in direct-drilled sites as here the weeds usually emerge following a staggered germination. Note also with direct drilling the staggered germination of weeds makes it more difficult to time post-emergence applications for effective weed control.

Other important considerations

Do you expect to get adequate rainfall to active the pre-emergence herbicide?

No...Then either incorporate the pre-emergence herbicide soon after application or switch to all post-emergence weed control.

Do you expect to be able to apply post-emergence herbicides in the narrow application window recommended?

No...Play safe and use a pre-emergence herbicide combination, follow up with a post-emergence herbicide if required.

Source: Trevor James and Mike Trolove, AgResearch; Sam McDougall, FAR