24 July 1998

Hit rampant weeds hard pre-harvest is message

By Andrew Swallow

WET weather may have hampered early harvest and stimulated unprecedented weed growth in lodged and standing crops alike. But at least the damp conditions mean those weeds can be hit hard, agree growers and experts.

"We have had no sun and plenty of moisture. It is ideal for in-crop Roundup," says Andrew Lewis of Raby Farms, Telford, Shropshire. He plans to take out couch with pre-harvest sprays on about a sixth of the farms 500ha (1200 acres) of cereals this year.

Most weeds will be patch sprayed. But volunteer potatoes in wheat-fields going into sugar beet merit a whole field dose. "We get quite a good effect on potatoes," he notes.

Product choice depends on conditions at time of application. "If it is a good settled time we use a straight glyphosate formulation. But if it is at all catchy we will go with Roundup Biactive because it dries in an hour."

The situation is similar in the east of the country. "Couch has reared its head in a lot of crops this year," says Dick Neale for ag-chem distributor Hutchinsons. "But crops are dying back rapidly while weeds are still green, so it is a good year for couch control."

His advice is to look at pre-harvest weed control in the context of the whole rotation, and beware of cutting costs with cheaper products. "There is proof that two or three years later there is less couch re-growth with the better glyphosate products."

Rates should be matched to weed population. Low populations of 10-20 stems/sq m have a light root mass, so 2.0 litres/ha will give good control. At over 75 stems/sq m 4.0 litres/ha is needed.

In the absence of weed problems a 1.0 litre/ha harvest management application may still be justified, Mr Neale suggests. "Spraying off in blocks can help sequence harvest, keeping combines moving and cutting drying costs. At about £4/ha the cost is minimal when compared to a crop of 10t/ha. An added benefit is that any secondary, late tillers, are killed, preventing green grains spoiling Hagberg numbers."

Timing for harvest conditioning and weed control should be when the grain is at or below 30% moisture. "In practice this is when the neck of the ear starts to turn brown, or when the grain holds a thumbnail mark."