Top tips for destroying a cover crop this season – Farmers Guardian
Farmers who have established over-winter cover crops ahead of planned spring cereals need to think carefully about how and when to terminate the cover ...
Getting timing right for cover crop destruction can mean the difference between a yield gain or a yield penalty.
By November time, the function of cover crops is normally much lower than earlier in the season, so for growers on soils in excess of 25per cent clay, it is usually better to terminate early and let water dissipate back through the profile and weather over during winter.
Early termination is particularly important for mixes containing cereals, especially oats or rye, sown ahead of spring barley, oats or wheat, as it helps avoid any allelopathic effects on establishment of the commercial crop and allows soil moisture to readjust, says Hutchinsons agronomist, Alice Cannon.
“Cover crops regulate water movement. While alive, they bring water up to the surface – cereals in particular hold water near the soil surface where the majority of their root mass is, so terminating in good time means water has time to dissipate back down the soil pro-file, optimising the seedbed for drilling at the earliest possible opportunity.
“Of all the cereal options, oats are most likely to keep the upper soil wettest for longest.”
However, the situation is more complicated where cover crop drilling was delayed due to lack of rain.
Miss Cannon says: “You might want to try and leave it a bit longer, maybe even until after Christmas, depending on what is in the mix and what crop you are following it with.
“If you leave it longer before spraying-off cover containing a cereal element, remember you still need to allow at least six to eight weeks before drilling a cereal crop, due to the potential allelopathy. The highest risk is post-oats or rye within covers.
“This is why species choice is such a big part of cover crop management. The method and timing of termination is almost as important as picking the right cover crop.”
Grazing is becoming more widely used, providing useful additional income, breaking down the organic matter into readily-available forms, and building soil biology. However, Miss Cannon says care is needed, especially grazing on clay-rich ground prone to waterlogging and compaction.
“You need a good shepherd who will move sheep regularly and remove them from the field before surface compaction occurs, or you can soon go backwards.
“Leaving sheep grazing when soil is wet risks causing compaction right in the seeding zone, which will end up in a yield loss, or mean you have to cultivate it, again running the risk of moisture loss in the spring.”
It is also important that shepherds understand cover should not be grazed right to the ground, as a small amount of green material should be left so that cover and any regrowth can take up glyphosate applied before drilling.
It is therefore important to plan grazing mixtures for a ‘fast feed ‘where soil improvement is a key priority.
Stubble turnips give some of the best performance as sheep feed, but as a monocrop, sheep have to remain on the field to consume the roots, and soil surface damage is inevitable where clay content is high, she says.
Rolling is an effective termination option in some countries, but experience in the UK has been less successful.
Miss Cannon says: “We do not get the hard frosts required. You need frosts of around -4degC, where ground is solid for at least a couple of days, for rolling to be effective.”
Crimper rolling does not require frost, but the roller must be a specific design, able to crimp the plant stems in several places, so they only work effectively on taller cover crop stands and are not very effective on younger grass-weeds or cereals.
Early termination is less important for growers planning to follow cover crops with other spring-sown crops, such as sugar beet.
Miss Cannon says: “You can wait until February to spray it off because you have got plenty of time before you are drilling the sugar beet.
However, the type of sugar beet drill is a key consideration here; double disc drills are far more able to cope with surface residue than older shoe-type drills.”
Where growers are looking to use glyphosate to spray off cover containing radishes, phacelia and broad-leaved weeds, such as thistle and cranesbill, then higher doses are needed to achieve full kill.
Switching to a 2, 4-D + glyphosate product may be a more effective alternative in these situations, however growers need to be aware of post-application sowing intervals, which might be up to 28 days, even 60 days in a few cases.
If there are large amounts of woody material still to breakdown in the spring, then more nitrogen may be used by soil bacteria during this process, potentially reducing the amount available to establishing crops.
Cover crops, with the exception of legumes, are nutrient cyclers rather than creators of nutrients, Miss Cannon says.
Their presence in soil feeds the nutrient cycle process, but the cycle must be started and maintained. The first cover crop in the cycle removes nutrients and they may not cycle back intime for the following cash crop.
To counter this, extra attention must be given to seedbed preparation, sowing the correct seed rate, consolidating seedbeds well, and getting early nutrition right, either with nutrient-boosting seed treatments, placed fertilisers and/or applying a higher dose of nitrogen earlier.
If the cover does not contain a cereal, growers could drill straight into the green, and drills can cope with doing so, Miss Cannon says. “But, if you have got machinery that cannot cope with lots of biomass, such as Suffolk coulters, or tined machines, that can get ‘knotted up’, you may still need to spray it off early, or consider flailing, or even ploughing cover in.
“Ploughing is not recommended as it undoes all the good work from the cover crop. Moving soil in any significant way runs the risk of moisture loss into the spring, as has happened in recent years.”
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