Grass weeds pose an increasing challenge

Grass weeds were the focus of the afternoon session of the recent Irish Tillage and Land Use Society (ITLUS) conference. One of the most notable grass weed experts, Stephen Moss (recently retired from Rothamsted in the UK), gave an outline of the main problems and the increasing challenges of control.

Given the history of the development of herbicide resistance in blackgrass, Stephen stated frequently in his presentation that one should not, and cannot, rely on chemicals as the sole control mechanism. While the problem is worse in blackgrass, because it out-crosses at flowering, the risk of resistance is real for all grass weeds and chemical resistance has already been found in most weeds.

Brome grasses

Stephen started by talking about brome grasses because they are increasingly important in Irish fields. There are five main species, most of which we know and have. Sterile brome is most common and its sparse head and sprawling growth habit are easily recognisable. However, Giant brome is a bit taller.

The other three species look quite different as they have a much tighter head structure, especially a tighter spikelet structure. The three in this category are soft brome, which is very common in Irish grassland, meadow brome and rye brome. All look somewhat similar and so proper identification is important.

Stephen said that a useful identification guide can be found on www.rothamsted.ac.uk/weeds-and-herbicide-resistance/identification-brome-grasses-2015

Understand the weed

Understanding the biology of any weed is important to help plan control. This is now known as agro ecology.

Sterile brome is a very aggressive weed, with five plants/m2 capable of a 5% yield reduction in winter wheat. Seed germination is encouraged by burial, so autumn cultivation helps to get seeds to germinate.

It is not very persistent in the soil and suffers an 85% to 90% annual population decline. So you can quickly grow down numbers with good management.

Other grass weeds are more persistent, with blackgrass and ryegrass more likely to be back to a 70% to 80% annual decline. This weed tends to be very common in hedgerows and bare areas and spreads into fields from there.

“Herbicide is only one control option,” Stephen emphasised. Shallow cultivation early post-harvest helps germinate sterile and soft brome, as well as volunteer cereals. However, other species are better left on the surface in the presence of light for a time before cultivation.

But Stephen acknowledged that all husbandry decisions are a compromise that need to be based on knowledge.

Ploughing is very useful to control sterile brome, but good inversion and thrash burial is essential. Stephen emphasised again that early autumn sowing is a huge driver in the increasing significance of grass weeds, especially blackgrass and sterile brome.

The use of autumn stale seedbed cultivation helps against brome because it encourages seed germination. Most growers use glyphosate between cultivations to kill volunteers, but Stephen warned against this practice.

There is already some evidence of glyphosate resistance in brome and the more often it is sprayed, the greater the risk that resistance will evolve.

The advice is to spray before planting, but to let cultivation do the intervening control. Alternatives to herbicides remain important because chemicals are not equally effective on all brome species.

Where a grower is actively trying to get on top of a weed, it is important to spray off infested patches in the growing crop as a means of preventing seed return.

Rogueing brome is very difficult, so getting out into your crop with a knapsack and Roundup can be a huge help.

Planting an infested field down to grass for a few years can also be hugely beneficial to reduce grass weed seed numbers. As a guide, this should be at least two years for brome and three years for blackgrass. Alternating the plough and non-inversion systems can also help.

Other grass problems

Wild oats is a virtually ubiquitous weed in Ireland and almost every field must be treated. “We take the continued activity of these herbicides for granted at our cost,” Stephen warned.

There are already reports of resistance in wild oats in the UK to Axial and Pacifica. This specific incidence meant that the field had to be sown down to grass due to the forest of wild oats.

Hand rogueing must remain part of a control strategy in order to prevent seed return of what might be a resistant plant. If a resistant plant, or a single plant of a new species is rogued, it can prevent a whole new problem from appearing.

Italian ryegrass is another potentially serious grass weed and this has shown resistance to a range of herbicides. Resistance has been most common where resistant blackgrass is also present.

Rat’s tail fescue is another potentially serious weed that is present in some fields in this country. An unusual feature of this fine-bladed species is that it can get very bad very quickly, as if it has come out of nowhere, and fade again quickly also.

“The fops and dims give no control,” Stephen said, “as it is naturally resistant to them, but flufenacet works well on this species.” Ploughing is very useful here also.

Increasing reports of blackgrass adds it to our list of serious problems. This is a highly adaptive weed and Stephen advised growers to do all they can to prevent its entry into a field. And if it gets in, do all that is necessary to get rid of it, he emphasised. This should be viewed as a zero tolerance weed.

“The critical thing is to use a mix of control methods,” Stephen concluded. The fact that we do not have chemical control concerns yet makes these problems more manageable, but dependence on chemicals will only lead to one outcome – resistance.

A grower’s experience

Garrett Headon runs a tillage and contracting business in north Kildare/south Dublin with his father, Eugene. The farming business has used different establishment methods over the past decade or so.

His main grass weed challenges have been wild oats, annual meadow grass and sterile brome. Scutch used to be a problem, he said, and he never wants to see blackgrass.

The approach to grass weeds is five-pronged:

Ploughing

The plough is the traditional crop establishment system and is a useful tool to help keep weed problems at bay, especially grass weeds.

It works on brome, which has been a problem, but most of its advantages depend on good clean inversion and burial. The problem is output and cost and the need for bigger output has resulted in wider bodies which must dig deeper, leaving more soil to firm. He had gone away from the plough for a number of years, but is returning to it to help get on top of grass weeds.

Min-till

Min-till or non-inversion targets cultivation in the top 7cm to 10cm, but soil loosening often takes place down to 30cm to 35cm.

It is a good system to get over ground and to get acres sown, but grass weed control can be very difficult.

Garrett asked if the system might be more suited to spring cropping in our climate.

Strip till

Brome has also been a problem with strip-till and Garrett commented that what you save on establishment you can spend on grass weed control. And slugs are an added challenge.

Looking to the future

“We are now trying to use the strengths of each system to best advantage,” Garrett said. “We continue to change our farming system to help the soil. We are now using organic manures, rotations and cover cropping. We have even reverted to some spring cropping.

“In future, we intend to use a mix of spring and winter crops to help reduce dependence on chemicals. Absolutism does not work in farming,” Garrett stated.

ITLUS medal

Cathal McCabe was named as the ITLUS medal winner for 2015 at the recent AGM. The award is made to the person in final year agriculture in UCD who receives the highest marks in crop husbandry and who goes on to do post-graduate research. Cathal is the son of well-known UCD lecturer Dr Tom McCabe.