Eradicate weeds and reap the rewards (From Gazette & Herald)

Eradicate weeds and reap the rewards

This week, Dr STEVE ELLIS, of ADAS based at High Mowthorpe, explains how dealing with weed problems early will reap rewards later in the season.

HAD Bill and Ben been a bit more forward thinking they might have resorted to a herbicide to eradicate their annoying 'little weed', thereby increasing their share of the television fee and avoiding all sorts of future problems when she eventually set seed.

Although difficult to understand at the best of times, I understand that they regretted this oversight in later life and were keen for others not to follow their example.

Indeed, they were keen to stress that dealing with weed problems early will reap rewards later in the season.

Control of grass weeds this season has been generally good although there is always going to be some areas where it has been less effective. This may be due to delayed spraying or simply not applying enough chemical. Whatever the cause, missed or surviving grass weeds need attention in the next couple of weeks.

The cause of poor control can be obvious, for example, missed strips or an even spread of surviving weeds where reduced rates were used.

However, if this is not the case then further investigation is needed. Enhanced metabolism resistance gives patchy uneven weed control, whilst symptoms of target-site resistance are of occasional individual surviving plants. Also, a number of weed populations now show both types of resistance.

Further information on herbicide resistance is available at www.pesticides.gov.uk/rags.asp?id=714.

If enhanced metabolism resistance is the problem, it may be possible to reduce weed numbers, by changing the type of herbicide and increasing the rates, but this is likely to be just a short-term solution.

Target-site resistant weeds will only respond to herbicides that hit a different target within the plant and use a completely different mode of action. Again, a follow-up herbicide will only partly solve the problem. Changes to pre-harvest treatments, cultivations and time of drilling may be needed to give more reliable weed control in the long-term.

Brome is an increasing problem that may need specific action. Monitor (sulfosulfuron) and Pacifica (mesosulfuron + iodosulfuron) give control of various brome species and may be worth using where spring grass control is targeted at this grass weed. Check previous herbicide use as ALS/sulfonyl-urea sequence restrictions may apply.

Broadleaved weed growth has also picked up noticeably this week. Speedwell, mayweeds and increasing numbers of polygonums are noticeable in many crops, but cleavers although checked to varying degrees by the cold weather, remain the major problem.

With rising soil temperatures, adequate moisture and weed germination held back by the early cold weather, we can expect plenty of new weeds to emerge over the next few weeks where there is no residual herbicide present. Wild oats are also likely to start appearing in greater numbers.

Avoid problems later in the season by inspecting open crops and spring-sown seedbeds for signs of new weed growth. Spray while most weeds are at the seedling stage.

For those crops now reaching GS 30, the main period of canopy growth will start in two to three weeks. After this, the crop will suppress small weeds, but established ones below the crop canopy will be difficult to hit.

As canopy size increases, it will be difficult to obtain good spray coverage of wild oats, and thick crops with a full canopy from last autumn will need full spray volumes to get good penetration. Wild oat sizes could be more uniform than after some of the more recent mild winters. Later drilled or thinner crops may give spraying opportunities into May, but judge each crop and weed, individually.

Updated: 12:52 Wednesday, April 05, 2006