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Crops notes May 2007

POTATOES

Herbicide choice

The good weather in April has meant that our potato planting, for the first time in years, has been completed relatively early. However, more thought may need to be given to weed control as both grass and broad-leaved weeds will have a longer germination period. When choosing a herbicide we should consider the following points:
  • Stage of crop growth - There are few herbicides that can be used safely once the crop is fully emerged but a number of treatments can be applied at 10 to 40 percent emergence or before the crop is 15 cm (6 inches) tall.
  • Variety grown - Some treatments should not be used on specific varieties, whilst other treatments have restricted use with some varieties. Growers should carefully check the product information before using a herbicide.
  • Seed or ware crops - Herbicides which affect the crop foliage may render a crop liable to be rejected for seed classification. If a seed crop is to be treated make sure the product has approval for use on seed crops.
  • Soil type - Lighter soils usually require lower doses of residual herbicides. Over-dosing can result in damage to the crop. Conversely, on soils with high organic matter levels some residual herbicides are ineffective or a higher dose rate may be recommended.
  • Soil condition - Residual herbicides require moist soils to work well. Heavy rainfall following application on light or stony soils can move some herbicides down to the seed and may adversely affect the following crop. Residual herbicides usually perform poorly on cloddy tilths - often a problem on heavier soils.

Prevent blight from getting an early start

Dumps of last season’s potatoes are a major source of blight infection and their management should be incorporated into the potato production cycle.
We should examine potato dumps regularly for signs of sprout growth and take action to destroy all green material, which could carry blight infection.
To do this, dumps should be covered with plastic and treated with a residual herbicide e.g. dichlobenil granules. As a residual herbicide, dichlobenil granules need a layer of soil to bind on to. Dumps should therefore be levelled and soil added to cover exposed tubers if necessary prior to application.
Residual granules unlike translocated herbicides such as glyphosate only require one treatment applied before any shoots begin to grow. With translocated herbicides, a green leaf area is required to take up the chemical and that foilage could already be infected with blight.

SPRING CEREALS

Time your nitrogen for optimum yields

From three years of benchmarking results across Northern Ireland we have found consistently that the most profitable growers time their nitrogen top dressing at the two-three leaf stage (GS 12 to 13). Later applications than this may green the crop but add little yield. We should also avoid excessive applications of nitrogen, and keep the right side of the law, by fully taking into account any organic manure applications or nitrogen contribution from previous cropping.

Keep weed control simple and cost effective

Aim to apply a mixture of at least two broad-spectrum herbicides whenever most of the weeds are at the two-four leaf stages. This will ensure that weed competition is removed at an early stage of crop development, and may allow a reduced rate of product to be used. I believe it is also good practice to prevent disease becoming established at this stage by tank-mixing a fungicide with your herbicide.

WINTER CEREALS

All crops should have received their main nitrogen top dressing – applying this up to second node detectable (GS 32) is likely to result in the highest yields. Ensure that all wheat crops have received their T1 fungicide application by GS 32. Target the T2 fungicide application for winter barley at early awn emergence (GS 49). However, monitor crops carefully as the presence of active rhynchosporium lesions may require an earlier timing. A triazole plus strobilurin and chlorothalonil mixture is likely to be the most appropriate fungicide choice with the addition of a morpholine for mildew control if necessary.