Grass Weed Control In Cereals

Certain grass weeds are becoming a very big problem for UK farmers in arable rotations.  This is due to a combination of reasons which is providing the ‘perfect storm’ for their proliferation.

Most farmers are experiencing this to some degree, although certain farming systems and soil types create the correct conditions for grass weeds to multiply.

The most common species to cause problems for the arable farmer are black grass, bromes, ryegrass and wild oats.  The problem is getting worse and so I am going to discuss the probable reasons for this and what we are doing about it on our farm.

 

Reasons For Grass Weed Proliferation

On many farms the rotation hasn’t changed for a number of years, but the problem has got much worse over the past few years.  So what has changed?

Certain key herbicides have had their licences revoked.  This has led to a reliance on fewer herbicides which may not be as effective or this may lead to resistance and so worse control.  Resistance to Atlantis is certainly evident, which used to give good post emergence control of blackgrass in wheat.

There has been a trend towards non-inversion tillage by farmers in an effort to cut crop establishment costs.  This can bring certain benefits to the soil, but does usually result in a greater number of grass weed seeds germinating and growing in the crop.

We have seen some wetter weather over recent years and many farm drainage systems have come to the end of their lives creating wetter soils which create conditions conducive to grass weed growth.

 

Situation On Our Farm

Approximatey 60 percent of our farm is of a medium to heavy soil type which we farm with autumn sown crops.  The reason for this is that crop establishment and yields are more consistent than spring cropping on this type of land.

However, we aren’t too bad for grass weeds.  We have one particularly bad patch of black grass that is about the size of a tennis court.  For the past 4 or 5 years we have worked at erradicating this patch. The black grass population has reduced in this patch, but we haven’t got rid of it as yet.  We have sprayed tank mixes of pre-emergence herbicides in the autumn with reasonable effect, but far from total control.  We have used Atalantis, but this is becoming less effective.  We have sprayed remaining plants off with glyphosate in the crop just as the seed heads have formed. We have hand rogued any remaining plants on the outer edges of the patch.  This strategy is working, but we are still not rid of this patch and it looks like it may take a few years yet to erradicate it altogether.

When beans or oilseed rape are grown in the field there are some different herbicides available, but we are finding that they are not as effective as they once were.  We pay close attention to the correct water volumes, nozzle choice and weather conditions when applying agrochemicals so they are applied in the most effective way.

The rest of the farm is relatively free of black grass, although it is in low populations across all the winter cropping soils.  I walk every field each year and hand rogue any plants.  This is hard and tedious work.  In a typical 15 acre field I would expect to pull about 20 plants that the herbicides haven’t contolled.  I think this is time well spent, as the populations can quickly increase and we could soon get into the situation like we did with the small tennis court sized patch.

If I see a black grass plant in the crop when I am travelling through with the crop sprayer I stop, get out of the tractor and pull the offending plant and remove it from the field.  When ever I pull a blackgrass plant I carry it off the field just in case any of the seeds have set.

 

Why Black Grass

You will notice that I have been talking about black grass, and haven’t really touched on the bromes, ryegrass, wild oats, annual meadow grass or any others.  These are all problematic weeds to the arable farmers, but black grass is potentially by far the worst.

Black grass plants are extremely prolific in their seed setting and shedding, typically shedding 1000 – 2000 seeds per plant.  The seeds are also usually shed before harvest, so no seeds are removed by the harvesting procedure.  This is not the case for wild oats or brome, where a certain proportion of the seed is harvested by the combine and removed from the field and many less seeds are produced per plant.

The heavier the soil type the more moisture retentive it tends to be, which is not only the best conditions for grass weeds to grow, but also the least conducive to spring cropping (spring cropping can be  good for black grass reduction as the weed doesn’t tend to germinate in spring.  Heavier soils are also more difficult to create a fine seed bed, this can lead to a lower germination rate of the crop and hence less crop competion against the weeds.  Any large clods can weather down with rain and then weed seeds can germinate with no chemical layer in place to kill them.

As our farm is currently 250 acres it is possible for me to walk the crops and rogue the few plants that are there.  Obviously this isn’t so easy on much larger farms, as there is only a few weeks to walk the crops and remove the weeds.  As soon as the populations start to get too high then hand weeding isn’t an option anyway.

 

Precision Control

One future control measure that some larger farms may be able to employ is precision mapping of the weed patches via GPS.  Once identified it would be possible to adjust herbicide concentrations in these areas, patch spray glyphosate and patch cultivate to reduce the populations.

These are weeds which start in patches before spreading across the field, so precision farming methods would work well.  Systems are already in development to identify and map the weeds.

 

Conclusion

We hope to eradicate the single patch of black grass within the next few years by continually working on it.  This season the wheat in that field was sprayed pre-emergence, but we still had a few plants.  We decided not to spray with Atlantis, as we thought we could pull them by hand and didn’t want to add to the resistance problem.  There were approximately 100 plants when I pulled them in mid May.  I will keep going back to see if any more plants become visible through late may and June.

It will be an ongoing practice to hand rogue all the fields.

We will continue to use a plough based cultivation stategy in most situations.  We believe there may be a place for some non-inversion crop establishment as an aid to keeping burried seeds down for an extra season.  Normally this would be when oilseed rape is established.

Non-inversion establishment on our soils later in the autumn have given variable results so we don’t see it as been useful on our farm for the wheat or barley drilling.  It can give better seed to soil contact than ploughing and retain moisture and germination rate in a dry time, but it has also tended to increase the chances of winter waterlogging compared to ploughing.

We have grown spring beans for the past two seasons which will help with control.  We have also introduced grass silage leys into the arabe rotation.  Any grass weeds are mown and harvested in the silage before they seed.

Ourselves and other farmers need to be aware of the spread of grass weeds from farm to farm through combine harvesters.  There is a tendancy for farmers to rely more on contractors or to share combine harvesters, so this is a path for spreading the weed seeds.  We own our own combine, but we do do some contracting work with it.  If we were to go to another farm that had a grass weed problem we would cut the problem areas first and then finish off in the cleaner part of the crop so that the inside of the combine would be relatively clean before returning back to our farm.  We would refuse any contracting work in fields with bad black grass populations. We are also cautious to not import any manures as we would not know if the bedding straw had been full of grass weed seeds.

These all seem like quite fastidious management decisions, but we believe they are worthwhile.  There are few, if any, new herbicides currently in development and we could loose some more of the actives that we have available.  Therefore, the situation could get more difficult.