Robust pre- and post-em programme key for maize success

The most effective way to achieve weed-free maize is via a pre- and post-emergence herbicide programme.

The timing of post-emergence weed control treatments is critical and should be targeted when the first cotyledon weeds show. Once the crop has reached the 4–6 leaf stage, the maize seedlings can usually withstand and out-compete most weeds; however, there is still a need – up to the 6-8 leaf stage of the crop – to clean up competitors such as black-grass, ryegrass, potatoes, thistles and black nightshade, to minimise potential problems for the following crop.

Listed below are the current post-emergence options available:

Nicosulfuron e.g. Milagro: For use where grass weeds, particularly ryegrass, is a problem. The herbicide does have a very limited range of control for broad-leaved weeds. For the common weeds of orache, nightshade and fat hen, some control can be achieved at higher rates of use – however, success will depend on the weather, size of weeds etc.

Mesotrione e.g. Callisto: On its own, this herbicide controls two of the problem weeds – black nightshade and fat hen. Control of orache is relatively weak.

Nicosulfuron + mesotrione e.g. Elumis: This mixture either comes as a ready-made formulated mix or can be made up as a tank mix. This combination does give the widest range of control of broadleaved weeds of any post-emergence product and is the mix of choice to follow a pre-emergence application The weakness of the mix is its relatively poor control of orache. The best control is achieved when the weeds are at their smallest.

Mesotrione + S-metalochlor e.g. Camix: A pre-emergence herbicide that can be used as a post-emergence spray if required. Compared to mesotrione alone, it does not bring much to the party in the post-emergence slot.

Foramsulfuron + iodosulfuron + isoxadifen e.g. MaisTer: Whilst the susceptible list for MaisTer is relatively small, and one of the main three, orache, is missing, these chemicals comprise the only herbicide that will have a good effect on black-grass and, to a lesser extent, cranesbill.

Prosulfuron e.g. Peak: On some farms, the three main problem weeds (orache, fat hen and black nightshade) are not present, and it is more the polygonums that are a problem. In this case, Peak presents a good alternative – however, Peak is not an option where rimsulfuron, nicosulfuron or foramsulfuron + iIodosulfuron (due to only one sulphonyl urea herbicide per season) have been used, so is of limited use.

Pyridate e.g. Diva: Pyridate is now the only herbicide that controls the three main weeds in maize: fat hen, orache and nightshade. Control of all three can be achieved up to the 4-leaf stage of the weeds, so whilst not a ‘fire engine treatment’, pyridate can be applied later if needs be. It must be said though that if weeds have already reached the 4-leaf stage, then significant yield losses have already occurred.

The best control from Pyridate is as a two-spray programme using 0.75-litres/ha followed by another treatment seven days later.

Fluroxypyr e.g. Starane: The main use for Fluroxypyr is as a follow-up spray for the control of black bindweed, knotgrass, or fumitory. The herbicide does achieve good control of the above if the weeds are not too big. Fluroxypyr is also used by some as part of a programme
for the control of volunteer potatoes.

Clopyralid e.g. Dow Shield: Clopyralid is again mainly used as a follow-up spray where mayweeds or thistles are present. Clopyralid is also used as part of a programme for the control of volunteer potatoes.

Fluroxypyr + clopyralid e.g. Leystar: The mix of Fluroxypyr and Clopyralid obviously does give an additional small amount of weed control but using either core ingredient alone will give a more targeted effect.

Dicamba e.g. Oceal: A limited weed spectrum but good on fat hen if it is a problem weed. Would normally use this as a follow-up spray where required.

Rimsulfuron e.g. Titus: Although not on the label, Rimsulfuron does have an effect on grass weeds as well, with ryegrass being well controlled sometimes. However, it can only be applied to certain maize varieties (which need to be checked with your supplier before use), so not for everyone.

Weed control where undersowing grass

The most important objective of weed control is that the maize crop remains weed-free until the 4-6 leaf stage of the crop as it is this early competition, between weeds and maize, that has the biggest negative effect on final crop yield.

Where there are no grass weeds present, the option to use a broad-leaved only herbicide should be considered. Whilst all the broad-leaved herbicides are reasonably safe provided that there is a gap between the application of them and the undersowing (normally herbicide application is at the 3–4 leaf stage and the undersowing is at the 6-leaf stage), herbicides with no residual activity – i.e. fluroxypyr, clopyralid, dicamba, pyridate – can/should be used where there is a shorter time gap between herbicide application and undersowing.

Where grass weeds are a problem, then the grassweed herbicides – nicosulfuron, rimsulfuron or Iodosulfuron + foramsulfuron – will still have to be used at the earliest post-emergence timing.

Cultural weed control

The MGA ran a trial last year looking at growing maize with reduced herbicides, the rationale being can we use fewer active ingredients per hectare to still grow a commercial crop.

The cultural options available to growers are hoeing, harrowing and band spraying. Organic farmers are 100% reliant on hoeing, which does give good results – although you’d probably have to go two or even three times in order to get an acceptable weed control level. Advice to non-organic farmers would be to use a fairly robust pre-emergence herbicide, followed by perhaps one hoeing pass, depending on the weed burden – it also depends where the weeds are, as conventional hoes only work between the row, rather than in-row between each plant. Although there are hoeing options which can navigate between the rows, these are much more expensive so are an investment which would only really make sense to commercial growers with vegetables in the rotation rather than the average maize grower.

Band spraying is quite an old-fashioned technique, which focuses on weeds between the rows rather than those nestled beside the crops, but some farmers do use this with glyphosate as a follow-up to an initial pre-em programme.

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