Good weather and favorable ground conditions from mid April has allowed good progress with spring barley sowing into warm soil, combined with sufficient moisture should see crops germinate quickly. So what should the strategy be for controlling weeds and what are the options available?
Herbicide resistance - problem chickweed
Herbicide resistance occurs when a weed survives a rate of herbicide that would normally kill it. The most common type of herbicide resistance in Northern Ireland is chickweed resistance to ALS inhibitors. This means over time most populations of chickweed have found a way to survive applications of ALS inhibitor herbicides. ALS inhibitors are commonly used sulfonylurea herbicides and can be identified as such on the product label by their mode of action B. The mode of action can be seen on the product label as a letter or letters within a circle for each active ingredient contained within the product.
Anti-resistance strategies
A varied crop rotation of cereals with break crop such as potatoes, rape, beans or grass leys is the best strategy to stop herbicide resistance developing. The differing sowing dates, growth habits and herbicides used reduces the likelihood of weeds developing resistance.
If the rotation cannot support a wide range of crops it is still possible to minimise the risk of herbicide resistance developing by stacking or sequencing ALS inhibitors with products that have a different mode of action. Stacking is combining two products with different modes of action in the same tank mix. Sequencing breaks the cycle of repeatedly using ALS inhibitors year after year in the same field by substitution with a product with different mode of action.
If weed control is poor investigate to identify if resistance is the underlying cause. Telltale signs of herbicide resistance are:
- live weeds beside dead weeds of the same species
- herbicide not killing one susceptible species but effectively controlling other susceptible weed species
- reduced efficacy of the same product in the same fields over a number of years
Pay particular attention to fields where the same crops are grown year after year and the same herbicides are used. If you suspect a herbicide resistance problem developing contact your local advisor who will give guidance on collection of weed samples for resistance testing.
General principles of herbicide resistance management
- use multiple herbicide sites-of-action with overlapping weed spectrums in rotation, sequences, or mixtures
- use the full recommended herbicide rate and proper application timing for the hardest to control weed species present in the field
- scout fields after herbicide application to ensure control has been achieved. Avoid allowing weeds to reproduce by seed or to proliferate vegetatively
- clean equipment to minimise weed seed spread between fields
- start with a clean field and control weeds early by using a burn down treatment or tillage in combination with a pre-emergence residual herbicide as appropriate
- use cultural practices such as cultivation and crop rotation, where appropriate
- use good agronomic principles that enhance crop competitiveness
For more information on herbicide resistance see herbicide resistance action committee website
Herbicide options for spring barley
As increasing numbers of active ingredients have their approvals revoked more products are withdrawn from market. As the choice of products available to control weeds is reduced the chance of resistance developing is likely to increase if the situation is not managed carefully. There are a broad range of well known and commonly used hormone products (MCPA, Mecoprop-P Dicamba, 2,4-D) and sulfonylurea’s (e.g. Hussar, Ally max, Harmony M) which offer relatively low cost weed control but may not be that effective on all problem weeds such as resistant chickweed, annual meadow grass or wild oats.
One way to reduce the pressure on spring barley herbicides may be to apply a pre-emergence residual herbicide. There are a number of on label approvals as well as winter cereal herbicides which have extensions of minor use for spring barley. Products containing pendimethalin, flufenacet or chlorotoluron will control a wide range of broad leaved weeds including herbicide resistant chickweed as well as effecting grass weeds including annual meadow grass and wild oats. Products containing piclinofen (Picona or Orient) or prosulfocarb (Defy) have off label approvals for controlling annual meadow grass in spring barley. Note of caution any product applied as an off label approval is done so entirely at the growers risk and growers must have a copy of the extension of use in their spray records.
Examples of products which can be applied pre-emergence only are Stomp (full approval) and Orient (EMU 20130386). It may be necessary to follow up these with a post emergence herbicide depending on weed spectrum and may not be necessary if a competitive crop establishes quickly.
Liberator (EMU 20121010) can be applied post emergence at 0.3ltr/ha half the autumn rate, and contains diflufenican which should have good activity on chickweed. If annual meadow grass has been historically problematic in a particular field then Crystal (EMU 20130951) might be a good option as it combines two active ingredients flufenacet and pendimethalin effective on annual meadow grass.
Defy (EMU 20131373) can be applied peri-emergence up until three leaves unfolded. Whilst the extension for use is for annual meadow grass Defy will also be effective on rye-grass. Diflufenican (Hurricane) can be added to control a wider spectrum of weeds.
For growers with chlorotoluron containing products such as Buckler (EMU 20121442) still in store this may be a useful opportunity to use stocks before approvals expire. For the most up to date information on approved products and extensions of minor use see the HSE pesticide register search page
Whilst residual herbicides may not fully control the entire weed spectrum it will make surviving weeds more easily killed by follow up post emergence products. Pre-em application gives flexibility for follow up treatments should weather conditions hinder application at the optimum timing and reduces the chance of herbicide resistance developing in post emergence products. One particularly difficult to control problem is wild oats. Some pre-emergence actives such as flufenacet in Liberator will have some activity on wild oats but may need a follow up treatment with Axial (pinoxaden), the only graminicide approved for wild oat control in spring barley.