Understanding the challenge of blackgrass

Is there a grower who does not appreciate the potential consequences of allowing blackgrass on to his/her farm?

Unfortunately the answer is yes as too many growers are paying little or no heed to this menace which can close down a tillage business and put real pressure on neighbours.

Some growers have been tackling blackgrass for over two decades with good success. Most should be aware that this weed has become resistant to virtually every available chemical class and that many British growers must now resort to alternative control measures.

Blackgrass is an out-crossing grass and so it is always generating completely new versions of itself. This has been a significant contributor to its continuous development of resistance.

Blackgrass has been on this island since the late 1970s but it failed to become a problem until relatively recently. Experts tell me that early sowing is the significant factor whcih favours the weed and we have done much more of that in the past decade. It is predominantly autumn-germinating, with early autumn being more favourable.

Biology

The earlier you plant, the more seeds will germinate and the better they will grow. It is common in crops to have 20+ tillers on a single plant and most or all of these can produce heads. And with each head capable of producing 200 seeds on average, a single germinating seed could theoretically produce up to 4,000 seeds (20 heads x 200 seeds/head). A single plant with 41 tillers could produce 8,200 seeds and this can infest a huge area.

Research has found that greater than 95% control is essential to prevent the weed population from increasing. Control depends on recoginising a problem and then having effective herbicides.

Some of the infestation in Ireland is still susceptible to most herbicide classes while more of it is resistant to almost everything. Either way, every effort should be made to use cultural controls to help grow out the seed bank.

Research has estimated that the average seed decline in the soil is 74% per year. So if one ploughed down 1,000 seeds per square metre, only one seed might survive five years of no cultivation.

Ploughing down should be part of a control procedure but ploughed-down seeds should be left ploughed down for a number of years. Ploughing itself is a useful control tool but one should always aim to stubble cultivate to continue to grow out the shallow seed bank.

Integrated control

Cultivate shallow and often and this will also affect many other undesirable grass and broadleaved weeds. UK research showed that from an initial population of 1,000 seeds/m2, good ploughing can drop the population to 310/m2 or a drop of 69%. Delayed drilling in the second half of October further decreased the population by 31% to 214/m2. Higher seed rates helped pull the weed population down to 158/m2 and the use of more competitive varieties took out another 22%, to take population down to 123/m2.

Then if you have a herbicide-sensitive population, one might expect 90% control to take the weed number down to 12 plants/m2. But even this number will generate a yield loss and enable substantial seed multiplication for the following year. But having fewer plants present reduces the risk of resistance development in sensitive populations. For anyone with a totally herbicide-resistant population, ploughing down and leaving them buried (grass) is your best bet.

A move to some spring planting would further help. However, it is suggested that this weed could actually adapt to thrive in spring crops also so this should only be part of a control procedure.

The challenge is to stop seed return. Most people who discover the problem post-heading are being rightly advised to take off the infested area but this must be removed before seeds are set on the grass to prevent them being viable. Roundup would also work but it should really be applied earlier as the seeds may fill very quickly. Areas where infestation is very low might be rogued but this will need to be done many times (at least four) as heads keep coming up.

One should also aim to minimise the number of viable seeds in the ground by trying to grow them out using cultivation. This needs to be done as quickly as possible post-harvest as this is when they prefer to germinate. If you get a green hue, cultivate again to encourage another batch. Good cultivation should kill the previous batch. The ultimate goal is zero tolerance and if you can’t rogue it, kill it.

Source

Many ask how these initial seeds are entering into clean fields. Obviously they could come in from a baler or combine, or even a seed drill that was operating previously in infested land. You cannot be too careful in this regard.

It can also come in on your own clothes or boots if you were in an infested area, knowingly or not – Ireland or foreign. Seed, especially imported seed, is a huge risk and the importation of grain of new varieties presents even greater risk.

Machinery imports, especially combines and balers but also cultivation equipment and even tractors, can also pose a risk unless they are thoroughly cleaned down. Hired machinery – combines, balers or cultivation machinery – can all bring in seeds, so do you know where such machines have been before?

There must also be a question mark over compost made from straw containing the grass. Imported straw is a major risk but can we be sure that native straw is clean anymore. If you are using farmyard manure, do you know its source? And even slurry from animals fed a rolled ration of imported grain containing blackgrass seeds poses a risk. And when your neighbour’s field has a lot of this stuff, birds can carry it over the fence.

Given the challenges, the only viable farmer response is zero tolerance.