Coping with New Challenges in Cereal Weed Control (Section II)

Trend Towards Earlier Sowing and Lower Seed Rates

In recent years there has been a trend towards both earlier sowing and the use of lower seed rates than heretofore. Both these factors can have a significant effect on weed problems in a crop. In general higher seeding rates will allow the crop to achieve ground cover earlier and thereby increase the crop's competitiveness towards weeds. A key method of combating weeds in organic agriculture is to use high seed rates, which make it difficult for weeds to get established. The open nature of a crop of winter wheat sown at low seed rate is ideal for weed establishment and growth. This is likely to be a key factor contributing to the problems being experienced with annual meadow-grass in winter wheat crops in recent times.

Sowing date is also important. With winter wheat, early sowing means that weeds germinate and establish in relatively good growing conditions and can become established more quickly. However, as the cereal's main competitive advantage is for light, which does not come into play until stem extension in the spring, it is at a disadvantage to the weed in early-sown crops. The well-established weeds are then more difficult to control with herbicides.

Adoption of Reduced Tillage Systems

The increase in the acreage devoted to reduced tillage has also lead to increased problems with herbicide weed control. Some of these problems are undoubtedly linked to the use of earlier sowing and low seed rates as outlined above, with consequent effects on crop competitiveness. However, there are other associated problems. This technique relies on getting weed seeds to germinate before sowing and then killing them, as opposed to ploughing them to a depth from which they will have difficulty in germinating. The relatively short interval between cultivation after harvest and sowing, which is often associated with dry soil conditions, probably means that many weed seeds do not germinate until after the crop is sown, meaning that the crop will experience increased weed levels. While the germination period could be extended, this could lead to poor conditions at sowing and is probably not a realistic option. The increase in organic matter levels, which occur as a result of reduced tillage, will also have a deleterious effect on the effectiveness of the herbicides used for weed control in winter cereals.

Occurrence of New Weeds

A number of new weed problems are beginning to emerge. Increasing incidences of sterile brome on intensive winter cereal farms is frustrating the efforts of even top class growers to combat this weed.Canary grass (Phalaris minor) is causing problems in a limited number of fields in Wicklow, Kildare, Cork and North Tipperary.

Herbicide Resistance

Recently a new challenge for weed control has been receiving attention, that of herbicide resistance. Herbicide resistance is the inherited ability of a weed to survive a rate of herbicide that would normally kill it. However not all cases of apparent herbicide failure are attributable to resistance. Herbicides may fail to kill weeds because of inappropriate choice of herbicide, adverse weather conditions at application, weed size, inappropriate dose or faulty application. When such factors have been eliminated resistance can be suspected. In the UK, black grass resistance is extensive and there is growing concern with resistance in wild oats, ryegrass and certain broad-leaved weeds. Dupont have confirmed two incidences of resistance (chickweed and corn marigold) to sulfonylureas in Ireland in 2003. A nation-wide survey by Teagasc tillage advisers identified a further 9 suspected sulfonylurea resistant cases (chickweed and marigold) in counties Carlow, Donegal, Meath, Wexford and Wicklow. We intend to further investigate these and any other problems in the coming year.

Herbicides can be classified into groups according to their site of activity within the plant (Table 9). Most herbicides work by interfering with plant enzymes. Most, but not all, herbicides will bind themselves onto specific enzymes - and render them inactive, killing the weed. This makes them very susceptible to the development of resistance within weed populations to that herbicide, as any change in the structure of the enzyme may prevent the herbicide from acting. Other herbicides are more complex, with a range of mechanisms.

There are three modes of action that are most significant to Irish growers:

ALS inhibitors. ALS inhibitors are so called because they inhibit a single enzyme called acetolactate synthase. Key members of the group are the sulfonylureas (SUs), such as Ally, Cameo, Monitor, Hussar etc. Boxer and Dagger, while not SUs are ALS inhibitors. Herbicides with this mode of action predominate in modern crop production. Unfortunately the number of resistance problems continue to increase internationally.

ACCase inhibitors. - ACCase inhibitors target a single enzyme, also in this case acetyl CoA carboxylase. Herbicides with this mode of action include the fops (Topik, Cheetah, Falcon) and the dims (Grasp and Checkmate).

Photosystem II inhibitors. This group of herbicides act by disrupting the photosynthetic process in the plant. However, herbicides within this group do not all work at exactly the same point in the process. As a result this group is more complicated, because although the ureas and the triazines are both in this group resistance may develop to the ureas while the triazines may be unaffected. For this reason scientists have split this group into two-sub group, one containing the triazines and one containing the ureas.

A small number of plants in any weed population are likely to be naturally resistant to a given herbicide. If that herbicide is not applied these plants are not selected for and remain at very low levels within the population. Where repeated application of that herbicide occurs it will allow these plants to survive and set seed, while killing the plants that are not resistant. As a result the resistant plants will multiply until they dominate the population.

The two main resistance mechanisms are target site and enhanced metabolism. The mechanism present will influence the pattern of resistance, particularly to the cross-resistance profile and the dose response.

Target site resistance blocks the site of activity specific to the herbicide's mode of action. This usually results in complete resistance to herbicides acting on that specific site but not to herbicides acting on different targets. It usually occurs with herbicides that have a very specific mode of action, e.g. affect a particular enzyme. Two types have been identified in the UK: one affects 'fop' and 'dim' grass weed herbicides (ACCase inhibitors); the other affects sulfonylurea broad-leaved weed herbicides (ALS inhibitors). Resistant populations can develop rapidly when target site resistance occurs.

Enhanced metabolism results in herbicide detoxification. Resistance tends to be partial, with plant growth being stunted rather than a complete loss of efficacy. With this type of resistance plants may be cross-resistant to herbicides with different modes of action. Currently, this is the most common resistance mechanism in grass weeds in the UK.

Weeds are relatively immobile compared to diseases or pests. Herbicide resistance usually develops on individual farms due to the weed control programmes used. Herbicide resistance can be more easily prevented on a specific farm, compared to fungicide or insecticide resistance. Preventing resistance occurring is an easier and cheaper option than managing a confirmed resistance situation. Factors that influence the occurrence of resistance are outlined in Table 10.

Cultural Control

The first step in any weed control strategy should be to maximise the use of cultural control. This will reduce the need for herbicides, thereby reducing cost and will also reduce the risk of resistance developing. Many options are available, although some may conflict with advice for control of pests and diseases or for reducing nitrate leaching. The main options include manipulating crop competitiveness, crop rotation and using techniques which prevent the spread of weeds.

Establishing a competitive crop is a key method of cultural control in cereals. Cereal species vary in terms of their competitive ability, with oats being more competitive than barley, which in turn is more competitive than wheat. Therefore where serious weed problems are continually encountered, using a more competitive crop can be considered. However, in most instances factors other than weed competitiveness dictate crop choice. Therefore, the aim must be to make the chosen crop as competitive as possible. As outlined earlier, sowing date and seeding rate are key factors which affect the competitiveness of a crop.

Adopting as diverse a rotation as possible using autumn and spring-sown crops, including non-cereals, will help to reduce the dominance of most annual grass weeds. Overall herbicide use may be reduced and the choice of herbicide modes of action extended.

Hand rogueing of weeds is feasible at very low weed populations or for patches of tall weeds, e.g. wild oats. This will prevent seed return. Cutting tall weeds is also an option. Avoiding the spread of seeds and plants will also be an aid to weed control. This can occur through contaminated seed, combine harvesters, cultivation equipment, straw or manure. All field equipment should be cleaned between fields.

Other options which reduce the need for selective herbicides include spraying stubbleor seedbeds with a non-selective herbicide before sowing, to ensure that all weed seedlings which have emerged are destroyed. Using set-aside to reduce populations of troublesome weeds should also be considered. The use of mechanical weed control, e.g. harrowing, can also be considered to reduce the necessity for selective herbicides.

The stale seedbed technique used in reduced tillage can be usefully used to assist control of annual weeds. Unploughed land is cultivated with a tined or disc harrow to 5-8 cm (2-3.5 in) and rolled immediately to conserve moisture. A glyphosate based product, e.g. Roundup, Gallup, or Sting, is applied 2-3 weeks later to kill emerged weeds. Sowing can proceed 6 hours after application of Sting and 5-6 days following glyphosate. Where non-inversion tillage is being used, delayed drilling winter cereals will allow a high proportion of weed seedlings to emerge and be controlled before sowing. Black-grass and Italian ryegrass infestations can be effectively reduced if there is sufficient soil moisture for germination. Badly infested fields should be sown last.

Ploughing reduces weed numbers, particularly of species that are relatively non-persistent in the soil, e.g. black-grass and Italian ryegrass. Consider rotational ploughing every two to five years, if annual ploughing is not feasible.

Herbicide Strategies