Best practice cover crop destruction

Taken from Farmers Guardian, February 2017, written by Abby Kellett

Cover crop destruction is next on the agenda for many growers, with some looking to spray off cover crops as soon as conditions allow.

But deciding how and when to destroy cover crops can be challenging and depends on a number of factors, including soil type, cover crop characteristics and the method used for establishing a spring crop.

Given recent cold temperatures, growers are reminded the efficacy of herbicides, including glyphosate, is likely to be impaired.

Careful

Seed company Kings technical adviser Paul Brown says: ""Growers have to be careful when applying glyphosate-like products at this time of year, as it works much slower when temperatures are cold.

""Some cover crops are quite thick this year, so growers should be using a high water volume to get the best plant coverage and give products the best opportunity to work.""

Mr Brown says some of his customers graze cover crops to reduce the size of the cover crop canopy.

""I find lots of people, probably about 25-30 percent of my customers, have animals grazing the cover crop, which makes it easier to kill and can sometimes eliminate the need to use a herbicide altogether.""

However, Mr Brown says there is a risk of some regrowth where grazing is used as the 'stand-alone' destruction strategy, so care should be taken to ensure weeds can be removed in the following crop.

He says: ""Many are following a cover crop with spring barley, so it is easy to kill weeds with a standard herbicide application, even if there is some cover crop regrowth.

""But you can get problems where spring beans or sugar beet precede a cover crop, so it is crucial the cover crop is dead and grazing may not be the best way to do that.

""However, it may be an idea to graze it, leave it for a couple of weeks, then potentially apply a herbicide if there is any subsequent regrowth.""

Similarly, where cereals follow the cover crop, Mr Brown suggests growers may want to look at using alternative 'desiccant-type' products, such as Shark (carfentra­ zone-ethyl), to reduce reliance on glyphosate.

If growers choose to use alternative products, he recommends checking the product label to ensure the interval needed between spraying and subsequent crop drilling is appropriate.

Another option to aid cover crop destruction is the use of crimper rollers, which damage the crop, all depends on what you are trying to achieve.""

DSV sales manager Michael Farr says: ""When you roll plants, you snap the stalk off at the base, which weakens the plant's ability to recover.

""Depending on what is left, you may need to apply a total herbicide after that to ensure the crop is completely destroyed, but in some instances you may not need it.""

Ultimately, the method growers choose to remove their cover crop depends largely on the objective of the cover crop, whether it be black-grass control, soil structure improvement or nutrient capture.

Debate

Mr Farr says: ""If you are looking to do a biofumigation job, you can plough it straight in and not spray it at all. It all depends on what you are trying to achieve.""

However, according to Chris Martin, of Agrovista, glyphosate is the 'only option'.

He says: ""For me, ploughing defeats the point of the cover crop which is used to improve soil structure, water and nutrient retention and organic matter levels. By ploughing, you are simply undoing all this good work.""

When to destroy

Having a clear idea of the purpose of the cover crop is important in deciding when it will be destroyed.

Mr Martin says: ""If you are looking for grass-weed control, I would get the cover crop burnt off early to allow the soil surface to dry out and allow for minimum disturbance at drilling.

""If you are not looking for weed control and you are going to be moving soil and trash about, you can go later.

""If maximum N fixation is your goal, you may want to leave the cover crop in longer, although most of the fixation will be done earlier on in the season.""

However, creating good soil conditions ahead of spring drilling is often a key driver in determining the timing of cover crop destruction.

Typically, on heavier land, cover crops are removed earlier to allow time for land to dry out.

However, Mr Farr says there are two potential approaches.

He says: ""You can either have the crop so it is actively growing and drying out the soil or, if you are in a wet situation, you can go in earlier to allow evaporation to dry out the soil.""

Priority

For those working with heavier land, Mr Brown recommends burning off the crop six to eight weeks before establishment, with large crops taking priority.

Where soils are lighter, he says growers can leave spraying until immediately before drilling and can potentially drill directly into the cover crop.

Midlands agronomist Luke Wheeler says he has seen improved results where crops have been sprayed off early.

He says: ""Spraying off cover crops at least 30 days prior to drilling the next crop can give a significant yield advantage to the spring crop compared with destruction shortly before drilling.

""I have seen yield data to suggest the difference could be as much as one tonne/hectare. This is likely a result of nitrogen being removed from the soil supply to feed the decaying cover crop.""

Earlier nitrogen availability is thought to be another advantage associated with earlier destruction.

Mr Farr says: ""If you destroy the cover crop earlier, chances are you will get a nitrogen bleed back earlier the following season.""

But subsequent N availability can vary depending on the C:N ratio of the cover crop, according to Mr Martin.