Getting on top of weeds in maize

Good weed control is essential to achieve high yields of good quality forage maize. Yield reductions of up to 80 % were recorded in Teagasc trials, where weed infestations were high and where no herbicide was applied compared with the best performing herbicide treatments. In addition, the amount of grain formed in the untreated plots was very low.

As well as the yield effect, the presence of some weeds in maize silage can cause problems when the silage is fed to animals – nightshade is a case in point.

Maize is particularly susceptible to competition from weeds in the early stages of growth, so good early season control is required. Even short periods of early competition between weeds and maize, when the crop is developing its root system, will have detrimental consequences. Therefore, the aim of successful weed control in maize is to eliminate weeds before they begin competing with the crop, which will normally be somewhere between the two- and five-leaf stage of the maize.

Not just herbicides

The first step to good weed control is a good rotation. This is important to minimise the build-up of weeds that are difficult to control in maize. Many of the problems that occur with weed control, particularly with crops sown under plastic, arise where maize is grown repeatedly over a number of years in the same field. This allows any weeds that have not been well controlled in the previous year to multiply and cause an even bigger problem in the following season.

Rotating with grass or another arable crop will help prevent the build-up of these weeds and make control in subsequent crops easier.

Ideally, your herbicide strategy should begin before the ground is even ploughed for maize. Weeds such as scutch grass, thistles and docks are difficult and somewhat costly to control in maize but they can be effectively and economically controlled with glyphosate before the ground is ploughed. Ideally, at least 10 days should be left between spraying and ploughing to allow the glyphosate to translocate to the roots of the weeds. Otherwise, control is likely to be poor.

A large proportion of maize is likely to be grown under plastic again in 2014. Achieving good weed control in crops sown under plastic can be more difficult than in crops grown without plastic.

Normally, the objective is to try to achieve season long weed control from a herbicide applied at sowing and, therefore, you will be relying on the residual activity of the chosen herbicides.

Residual products work best on fine seedbeds, which ensures that there is a complete ‘film’ of herbicide on the soil surface through which any emerging weed will have to pass, thereby coming into contact with the herbicide.

There would be some moisture present to allow the herbicide be in solution at the time of weed emergence. This is generally not a problem under the plastic, where there will normally be plenty of moisture and the soil surface will usually be clod-free.

However, in the areas between the plastic rows the soil is often cloddier and prone to drying out. The greater amount of clods means that it will be more difficult to get a complete film of herbicide and it will be easier for weeds to emerge above the soil surface without coming in contact with sufficient herbicide to control it. This means that weed control in the areas between the plastic rows is often less effective than under the plastic rows and, in some instances, a follow-up treatment will be required after crop emergence to control weeds in the area between the plastic rows.

It is important to remember that not all products are cleared for use under plastic. So, when choosing a product, it is important that it is allowed in plastic-sown crops. Growers should read the product label or view the list of approved products on the PCS website (www. http://www.pcs.agriculture.gov.ie) to check that the product is approved for use in your situation.

Some products, such as Calaris and its generic equivalents, have off-label clearance for use under plastic. Weed control in plastic-sown crops will generally be based around a pendimethalin (PDM) product.

There are a number of straight pendimethalin products on the market with different rates of application (Table 1).

Pendimethalin can be used alone and this strategy can work well, particularly under plastic, where weed infestations are likely to be low. But a follow-up treatment may be needed post crop emergence in the areas not covered by the plastic.

However, in most situations, a partner product will be added to the pendimethalin, with Calaris or a generic equivalent, and Cadou Star being the two main mixing products. A robust rate of pendimethalin mixed with either Cadou Star (0.5kg to 0.75 kg/ha) or Calaris (1.0L to 1.5 L/ha) will generally give good season long control of the majority of weeds. The higher rates are generally used for weedy fields. Calaris (used pre-emergence) is at the grower’s own risk as it is an off-label recommendation.

Wing-P is a new product that will be available in 2014 which contains pendimethalin combined with a second active ingredient, dimethenamid-P. It is approved for use in both open sown and plastic-covered crops. It can give good control without a partner product where high weed numbers are not expected, i.e. fresh ground. However, the addition of either Calaris or Cadou Star can be beneficial on very weedy fields in terms of enhancing the persistency of control.

Nowadays, some growers only put herbicide under the plastic at sowing and treat the area between the plastic sheets after weeds have emerged. This can work well and overcome problems of poor weed control between the plastic sheets when all the herbicide is applied at sowing.

In this case, pendimethalin alone, or Wing-P applied under the plastic at sowing, should give good control of the weeds under the plastic.

A follow-up application of Calaris or Callisto at the four- to six-leaf stage of the crop will control the weeds between the plastic. Using adapters that confine spraying to the area between the plastic in the follow-up treatment will reduce the overall herbicide cost considerably. It must be remembered that only one application of Calaris, or its generic equivalents, is allowed per season, so if you have used Calaris at sowing you cannot use it for a follow-up treatment.

Sown in the open

For crops sown without plastic, there is an option to use similar pre-emergence strategies to those used in plastic sown crops but, in practice, most growers will use a post emergence option. Where a pre-emergence strategy is used, it is important that the herbicide is applied as soon as possible after sowing, ideally within two days of sowing. Pendimethalin can be applied post emergence on open-sown crops but its efficacy is likely to be lower than where it is applied pre-emergence.

While there is a large range of post-emergence herbicide options for open-sown crops, the most common product used will be Calaris or its generic equivalent. The rate of Calaris used will be between 1.0L and 1.5L/ha, with the higher rates used where weed infestations are high. The ideal timing is when the crop has between four and six leaves. At this stage, the majority of weeds will have emerged but will not have started to compete with the crop. Where a follow-up treatment is needed to control weeds that have come through an earlier herbicide application, Bromoxynil products can give cost effective broadspectrum knockdown of weeds, provided the crop has not gone past the nine-leaf stage. These products need to be applied before the weeds get too big.

Difficult weeds such as thistle, scutch, wild oats and volunteer potatoes may need specialist follow-up herbicides such as Accent, Clopyralid, Elumis, Titus or Fluroxypyr for effective control. For scutch or wild oats, Accent, Titus or Elumis will be the choices; they will suppress the scutch and control the wild oats and will also give some control of other broadleaved weeds.

For thistles, Clopyralid will be the main product used. Fluroxypyr can also be used for thistles and will also give reasonable control of volunteer potatoes, but it can be severe on the crop so should only be used where necessary.