Black-grass – an apology

23 Jul 2015

A few weeks back I wrote a blog (Black-grass allelopathy nonsense - posted on 30th April) on my analysis of the very limited and indirect evidence as to whether or not multiple applications of glyphosate to seedling black-grass in ‘false seedbeds’ were beneficial, in terms of reducing numbers in succeeding crops. I came to the conclusion that there was no compelling evidence to support multiple applications.

However, I have now been supplied with evidence from one well conducted trial that specifically investigated this subject and which suggests that multiple applications may well be beneficial and so I have to retract what I said a few weeks back. However, I remain of the view that the possible explanation cannot be allelopathy. Allelopathic exudates from black-grass roots have never been identified.  The most likely explanation is that emerged black-grass plants shade the soil surface restricting further black-grass germination.

There are two further comments to add. Firstly, should a winter crop be sown if there are sufficient black-grass plants emerging in a ‘false seedbed’ to provide enough shade to reduce further germination? Numbers in the crop may well be too high to control adequately with herbicides. Secondly, is the threat of black-grass resistance to glyphosate sufficiently high to avoid multiple applications and to rely more on shallow cultivations to kill some or all of the emerged plants? Nobody knows the answer to that conundrum but black-grass has shown it is a worthy foe and has developed resistance to a range of modes of action.

One method of running down the seedbank of viable black-grass seed is to spring crop. However, in some areas this year, there are horrific numbers of the weed emerged in spring sown crops. I have noted that these instances tend to be in the areas that received the worst of the deluge in 2012 and the soils in that summer would have been waterlogged. This has led me to the conclusion that these fields are not infested with a new spring emerging strain of black-grass, as some suggest, but are continuing to suffer from the impact of exceptional levels of dormancy in the seed set in 2012. I realise the some may think that I am off my head and eventually will have to offer another apology for this observation! However, the HGCA Project Report 498 on dormancy shows that with typical levels of high dormancy, the majority of seed will germinate at least 12 months after shedding. It is only a small step to say that with exceptional levels of dormancy, the germination of a significant proportion of the seed could be delayed for a couple of years.

The other explanation is that there is always a small proportion of black-grass that will germinate in spring crops and so a high spring emergence is a reflection of an exceptionally high soil seedbank. I do not quite buy that one in the cases that I have experienced this year. I have only witnessed such high populations in spring sown crops since the summer of 2012 and I have been around a long time. The inevitable conclusion is that the weather and soil conditions during seed maturation that year had a profound impact on future germination patterns.

I initially got it in the neck from one colleague this spring because he tried a half-field comparison of cultivating/drilling or direct drilling a spring sown cereal. The experience in the industry is that ‘true’ direct drilling will reduce black-grass emergence in spring sown crops because of the lack of soil disturbance. In turns out that in this case there was no difference in black-grass emergence, which was extremely high, between the two cultivation approaches tested. However, where the land was direct drilled the soil soon cracked in the dry conditions and a very high proportion of the black-grass was emerging from the cracks. It shows why agronomists often have to mention a few caveats when they give advice!