NIAB - National Institute of Agricultural Botany

Orson's Oracle

Confused over blackgrass

Posted on 22/08/2013 by Jim Orson

A number of farmers have confessed to me that they are confused over the best approach to take to control black-grass. After reading many articles in the press over the past couple of weeks, I can see why. Conflicting views have been expressed and some are, in my opinion, plainly wrong.

Let’s take the simple issue of the impact of primary cultivation. What’s the impact on black-grass populations if the land is ploughed every year compared to the long- term adoption of shallow non-plough tillage, deeper non-plough tillage or direct drilling? 

The answer is clearly that the lowest populations occur with ploughing. However, ploughing is painfully slow, an awful lot more equipment is needed to maintain timeliness and there are other arguments against the plough from a weed control point of view. Ploughing can result in poorer seedbeds, with less black-grass germination and emergence prior to sowing, their roots establishing at greater depths compared to very shallow tillage or direct drilling and a reduced level of control with pre-emergence herbicides.Small blackgrass

Despite these negative features, every LONG-TERM experiment, both in northern Europe as well as in the UK, has demonstrated that, although this position must be weakened by the increased reliance on pre-emergence herbicides, the plough is best.

However, in the SHORT-TERM it may not be best. 

Typically, the plough can result in higher numbers of black-grass where seed shed has been low in the year of ploughing but high in the year before. Higher numbers of viable seed can then be brought to the surface when compared to non-plough tillage.

This is why there is always some experimental data around to show that an alternative approach may be better. It also shows that, in general, farmers should not slavishly follow one particular cultivation approach, because herbicide resistance results in us all having to be more tactical nowadays.

There has also been some remarkable advice in one or two recent press articles saying that the pre-emergence herbicides should be applied straight after drilling regardless of soil conditions. The advice is that even if it is bone-dry the herbicides should still be applied; the argument put forward is that they are true pre-emergence herbicides and the risk of a delay in application cannot be taken. 

There are lots of things to say about this.

Firstly, these herbicides enter the plant through the developing roots and/or shoots. If it is bone-dry the weeds will not germinate and so the herbicides will be degrading on the soil surface whilst not contributing at all to weed control. All pre-emergence herbicides apart from tri-allate are far more efficacious when applied to a moist soil surface and they also rely on soil moisture to be taken up by the plant. 

The dread scenario is that which occurred in the autumn of 2011 when the soil surface was dry but there was just sufficient moisture below the soil surface for some black-grass to germinate and emerge. This is the situation we should fear the most. If it is so dry that the black-grass will not germinate - do not apply pre-emergence residual herbicides including tri-allate.

Secondly, on the subject of whether or not pre-emergence herbicides should be applied as soon as possible - peri-emergence application is just as effective as pre-emergence. In fact, one of the more widely used herbicides is consistently more effective when applied peri-emergence. Seedbeds are more likely to be moist and settled peri-emergence. When they are dry pre-emergence, but moist peri-emergence, the advantage of peri-emergence application is very significant. Although saying this, I recognise that the peri-emergence window of application is vanishingly small, but please remember the principles. 

I have to say it … in order to avoid confusion over the various and often conflicting statements in the press, consider subscribing to an organisation such as NIAB TAG which retains both a scientific and practical perspective on technical issues. 

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