Grass weeds in cereals are an increasing worry for three main reasons.

  • If they are not controlled, they will expand and rob yield potential.
  • Chemical control costs money and the cost is increasing because the challenges are increasing over time.
  • Weeds have a considerable potential to produce resistance to families of herbicide actives.
  • One of the benefits of farming fresh ground after a long ley is the likelihood of few or no grass weeds. In this scenario, the cost of production is lower and the risk to a self-contained unit is zero. But this has been a rare experience in practice and it is worth thinking about why grass weeds have expanded so much.

    There are now many weeds to contend with. In the early days of winter cereals, the main challenge was rough-stalk meadowgrass. Annual meadow grass was not seen as a problem back then because it seemed easier to get the crop to compete early in the year due to the fresh ground. But perhaps we have a different and more aggressive strain in our fields now, which makes control essential.

    Back then scutch was the major challenge; now it’s rare. But this might change again if things do not go well for glyphosate registration. Wild oats was the main annual weed challenge and even though we were given better control products over time, the problem just expanded due to a general indifference. Now we have one case of wild oat resistance to Axial – this should sound a warning bell.

    Then came sterile brome, which took advantage of the barren ground where Roundup was used to kill the existing vegetation on hedges. It began there, but then gradually moved out into fields. However, it is still well controlled by good burial, so if you have it in ploughed ground, look at your plough. Other brome grasses have since sneaked in and are a potential problem.

    Blackgrass has been in the country since the early 1980s but never managed to become a serious problem until now. This was partly because of active management by the growers, but now early drilling, which is regarded as the most favourable husbandry activity for its propagation, has become widespread.

    Many of these problems mainly exploit winter crops, but spring crops have not escaped either. Wild oats are now almost ubiquitous in spring cereals and canary grasses have become a very serious problem in places. Some spring cereal fields also have a very serious problem with annual meadowgrass.

    The list will not end here either. There have been others and now some fields have a problem with rat’s tail fescue. Many growers have had a problem with different ryegrasses and creeping softgrass was a huge problem in the recent past.

    Where did they come from?

    Have you ever wondered why or how these grasses have come to be problems in our tillage fields? If you haven’t, now is a good time to start. We can ill afford to be handing out money to control more and more problems when the end game will inevitably be resistance.

    Perhaps the best place to start is to ask where these problems come from. The answer is likely to differ depending on the species but there is likely to be one common fact – they all start from just a handful of plants. And these were brought on to your farm/field if they did not already exist.

    Methods of field contamination

    There are many ways that alien seeds can be brought into a clean field. The seeds we plant could have some unwanted seeds present. Irish certified seed operates very high standards in this regard but one must continue to be suspicious that imported seed can be a source of unwanted seeds. This has been one route for contamination for decades, but herbicide resistance now adds considerably to this concern, especially for blackgrass.

    Importation of machinery from abroad can also pose a risk due to seeds carried in on the machine. In Australia, machinery is heavily inspected for seed contamination when it travels from state to state. For us, imported combines are the worst offender and many growers have witnessed what grows in the spot where an imported machine was first run. If you don’t stop such plants from setting seed, you can quickly turn a single plant into a forest of weed.

    Imported balers also pose a big risk, but imported straw must be much greater if it ends up on farms. No one seems to know if grass weed seeds can survive composting to contaminate land after the spreading of spent mushroom compost. Weeds may differ in this regard and the likelihood of seeds being caught up in straw will vary with the species. But this is a subject that requires some clarification as we promote the use of mushroom compost to help improve soil health.

    Another possibility is the fact that you could have brought in these seeds yourself. Many weed seeds can cling to cloths or footwear, and growers visit many farms, home and away, where different weed problems exist. We are very lax on biosecurity in this regard.

    That said, it is likely that the greatest amount of spread occurs from field to field locally. Most of the above mechanisms still apply, but combines and balers have to be the biggest risk. In this regard, contracted machinery services must be regarded as a risk, which also exists with contracted sprayers later in the season. But you can also get spread within your own land for the same reasons. And then you have groups walking crops and you can also get some transfer from birds and other wildlife.

    It is not easy to control the risk associated with seeds that can come in on combines and balers. This could be your own machines out on conacre or it could be a contractor’s machine. But if you have land that is currently clean of most or all grass weeds, it is very much in your best interest to keep new weed problems out. An annual €10/ac cost for a spray to control a grass weed is a bill for €3,000 on 300 acres and that’s a recurring annual cost. And many grass weeds continue to cost more than this for successful control, assuming you can control them.

    The ease with which we have committed to this recurring annual cost is part of the reason why our production costs have crept up in recent years. There was a time when only some fields had wild oats and now meadowgrass is an annual target. It now seems highly likely that we will lose IPU in the near future, so we must begin to think differently about all these problems.

    Zero tolerance preference

    Increasing production cost has been one of the main challenges to profitability and increasing chemical bills have contributed to this. In the case of chemicals, our increasing dependence on chemical control options has been a significant contributor to this increased cost. When a new problem occurs, the response has mainly been to look for another chemical solution (cost).

    For many years, those solutions were available and affordable and they provided a realistic solution across a big acreage. Now many tools are either being withdrawn from the market or resistance is a concern and we just cannot afford them. Either way, it is time to think about alternative control methods which either reduce the bank of seeds in the ground or prevent germinated seeds from setting seed, and preferably both.

    Given the many challenges which already face us, the zero tolerance option may appear to have passed. However, this must remain an objective in profitable tillage farming. Creeping softgrass was a real scourge a few years ago but it was heavily targeted, with the help of set-aside, to eliminate it completely from most fields.

    All individual weeds, grass or otherwise, need a coordinated approach to their control. When even one target weed survives, it will shed seeds to continue the problem and the cost. But if this plant survived because it was resistant, then you have a whole new problem developing, which may not have a chemical solution. If we do not help our herbicides with other control techniques, this is inevitable.

  • Rogueing:
  • This is not a pleasant task but it is the best way to help ensure that seeds are not being returned to the ground to prolong or build the problem or to allow resistance to build. Rogueing is time-consuming and a big job on big acres, especially after the plants have set viable seeds (post flowering), when they must be physically removed. But it is important to remember that rogueing is as important where you are considering home-saved seed as it is with certified seed – if you have undesirables in your seed, then you just spread them around your farm.

    The challenge of rogueing is influenced by the weed targets. It has long been a useful technique for wild oats as the strong stems can be used to pull the full plant and its tillers from the ground to help prevent tiller recovery. In contrast, sterile brome plants have numerous sprawling tillers which break off easily when pulled, leaving the others to continue the growth cycle.

    Many other grasses are also very difficult to rogue because the full plants will not pull easily from the ground. Rough-stalk meadowgrass would be another example, and canary grasses, blackgrass and rat’s tail fescue might also be very difficult or impossible to rogue successfully. And rogueing would be a futile exercise with scutch, which will propagate from the surviving rhizomes in the soil.

  • Spot spraying:
    • Many fields now have patches of a specific weed that are so bad that they cannot be rogued. These could be very small patches of weeds such as sterile brome or bigger areas of 1-2 acres of any of the grass weeds and even blackgrass. Firstly, such areas should not be planted to a cereal crop or, secondly, if they were planted, these areas should be burned off during the growing season before they can set viable seeds. This is a job for a knapsack sprayer or possibly a section of the boom. Taking the time to take out small pockets can be very worthwhile.

    • Cultivations:
    The path to zero tolerance has to be helped by stubble cultivation. Helping to actively grow out a proportion of the weed bank in the soil is critical to helping to get numbers down. If you can get numbers low enough to rogue 20% of your area, chemical cost will be reduced by 20%. There is always a compromise with cultivation timing for specific weeds but, in general, cultivate as quickly as possible post harvest as weeds prefer to germinate earlier.

    Think about your specific weed problem. In a ploughing situation, sterile brome is worst at hedges and ins-and-outs because burial is not complete. If you leave 3-4m unplanted by the offending hedges and cultivate these for just one season, you can grow out most of the seeds in that ground. And moving the ins-and-outs 3-4m further from the hedge will help get better burial where the brome seeds exist. But if any patches do come through, bring out the knapsack and burn out the patch.

    Other specific weeds need their own approach, but you can never win the battle if you allow more seeds back into the soil than germinated initially. Reducing seedbank numbers is not optional. You may not have the tools to kill them in the future so you need to move to decrease weed number in the direction of zero tolerance as quickly as possible. You can start by not planting badly infested areas and using cultivation, and possibly catch cropping, to help reduce the numbers in the seed bank.

    Thinking differently is no longer optional!

    To read the full Crop Protection Focus Supplement, click here.