Timing is key when killing off cover crops with one expert suggesting a herbicide needs to be used some three to four weeks before a spring crop is drilled.

Some species in a cover crop mix can be killed easily, but more persistent ones, such as fodder radish, may need up to a month to be controlled by a weedkiller such as glyphosate.

Cover crops are becoming more popular across many farms to prevent nitrogen leaching into water courses in the autumn and winter, and to help improve soil structure and fertility to allow a possible cut in fertiliser costs.

See also: Top tips from two US no-till and cover crop veterans

Barrie Hunt, technical development manager at Monsanto, which makes the glyphosate product Roundup, puts forward a five-point plan to achieve good destruction of cover crops with the first being the early timing of a herbicide spray.

Watch the four videos – general advice on destroying cover crops and three focusing on mustard, fodder radish and vetch and read the full report below.

“Timing is absolutely critical, growers need to allow time to kill off all the weeds and make sure there are no problem weeds low in the canopy,” he tells Farmers Weekly.

Some species can be killed off by a hard frost such as spring cereals and phacelia, but others such as vetch and fodder radish are more difficult to kill. Mr Hunt’s five-point plan is:

1. Spray off early enough

Glyphosate  needs a minimum of two weeks to work, and to control more difficult weeds, Mr Hunt suggests three to four weeks as a good benchmark.

Make sure you kill everything, and examine the bottom of the canopy carefully in case there is any blackgrass present that might have been shaded from the herbicide spray.

2. Use a rate to match the species

By February, spring cereals, phacelia and mustard will have largely been killed off by some hard frosts, but the tough ones such as vetch and fodder radish will be more difficult to kill.

He suggest a rate of 3 litres/ha of Roundup (360g/litre) is typical, but growers may have to go to 4 litres/ha for tough weeds. This is likely to cost £15-£20/ha.

3. Use best spraying practice

Most of the problems with poor destruction of cover crops come with poor application, says Mr Hunt. So use enough water to give a 150 litres/ha spray covering if needed and make sure speed, nozzles and pressures are appropriate. In a multispecies crop with multilayers, good spray application is essential.

4. Formulation

When looking to spray in late January/early February, if it is not raining then it is probably about to rain. So Mr Hunt suggests using a good formulation which is rainfast.

Glyphosate moves within the sugars of the plant leaves so in cold weather there will be limited movement so a good product is needed to kill the weeds when they are not growing strongly.

5. Control survivors

In thick cover crops, a total plant kill might not be possible so consider using some light cultivation before drilling to control stubborn weeds and/or use an approved glyphosate product with a pre-emergence herbicide on a following spring crop.

Dorset view

Cover crops are becoming more important in the intensive dairying region of north Dorset, usually grown ahead of spring barley or maize.

Regional seed group Pearce Seeds has extensive cover crop trials on a farm just south of Sturminster Newton on a stone brash soil over clay.

“Cover crops are becoming more and more important because of the issue of nitrogen leaching in the winter,” says Tom Diment, the group’s trial manager

In the trial, cover crop plots were drilled 10 August after winter barley, and they established well despite the dry autumn as there was enough moisture after a wet summer.