Systems Approach To Combat Grass Weeds In Min-Till

July, 2004 - Minimum tillage must be treated as a whole new system rather than just a new cultivation regime if growing grass weed problems are to be avoided without dramatic increases in the agrochemical bill.

This is the experience of Masstock agronomist, Andrew Richards following extensive studies on a number of farms over several years as part of the company’s SMART farming programme.

"Minimum tillage has obvious attractions for growers keen to save on cultivation costs and make the most of increasingly scarce labour," he agreed. "But it has equally obvious technical limitations where grass weed control is concerned.

"Abandoning the plough means you have to cope with large populations of viable annual black-grass and brome seeds on or near the soil surface each season instead of just a small proportion of viable seeds ploughed back-up. It also means a rapid flush of weed establishment in the seedbed, rather than a slower emergence from seeds germinating at different depths in the soil profile.

"This presents few problems in theory if you can get in a good stale seedbed to decimate annual weed seedlings ahead of drilling. But it all breaks down in practice if, as so many growers have found to their cost in recent years, there isn’t the time to achieve good pre-planting weed control."

Andrew Richards explains that, in reality, the timing of autumn cultivations is invariably driven by the condition of the soil and demands of the rotation, not the need for weed control. In the past few years, both wet and dry autumn conditions have conspired against effective stale seedbeds for many, he points out. And the pressure always seems to be on to get drilled-up as early as possible.

Add to this the growing trend towards lower seed rates, and he expresses no surprise whatsoever that grass weed problems have become a very costly problem for a growing number of min-tillers.

"Comparative black-grass control trials under different wheat cultivation regimes at the Brackley SMART Farm have, for instance, clearly shown the extent of the problem," he observed. "In the first year of our trials in wheat sown after OSR, we recorded noticeably higher weed populations in the untreated plots under min-till than under the plough. However, with the aid of Roundup pre-planting on the min-till side, our full pre-em/ post-em programme ensured equally good control. "In the second year, though, when we couldn’t achieve a decent stale seedbed and didn’t use a pre-em, the min-till black-grass populations exploded to such an extent that even a robust post-em programme couldn’t control them. In contrast, the same spraying regime under the plough left us with a fraction of the number of black-grass heads – just 34/m2 compared to 924/m2 under min-till.

"Our SMART farming experience with brome has been very similar," added Andrew Richards. "Get a decent stale seedbed and you can keep on top of the threat in minimum tillage. But fail to achieve good stale seedbed control where the weed is present and you will have problems; especially so, with the available pre-ems giving decidedly intermediate control of the weed."

So what can be done to keep the twin threats of black-grass and brome reliably in check under today’s high pressure minimum tillage regimes ? Well, Andrew Richards identifies a number of practical options firmly based upon a whole system approach.

First and foremost, he stresses it is essential to maximise the control achieved over these weeds in the cereal break. As well as a robust autumn grass-weed herbicide programme, he advocates pre-harvest OSR treatment with Roundup to control any weeds that do get through, in addition to giving good crop desiccation.

"If you can’t achieve tip-top control in the break, you really need to avoid growing second wheats where the grass-weed threat is particularly serious," he insisted. "Because that’s where the problem will really manifest itself.

"You should seriously consider going back to a break again. Or why not sow a spring crop to give you an extended autumn stale seedbed opportunity. After all, our latest Cleanacres MAP Group costings show a significant gross margin advantage for spring over winter barley. And that’s before any account is taken of potential environmental payments for over-wintered stubbles.

"In fact, I recommend a spring crop in all minimum tillage rotations as a way of taking the pressure off expensive selective grass weed chemistry."

Andrew Richards also advises min-tillers to keep their wheat sowing rates up wherever the grass weed threat is significant, pointing that strong crop competition can make all the difference in reducing the extent to which grass weed populations develop.

"Our SMART farm trials have underlined the value of higher seed rates under min-till regimes," he explained. "While the same number of black-grass plants established at the outset, only 63 heads/m2 developed at a 300 seeds/m2 sowing rate compared to 183 heads/m2 at a 200 seeds/m2 rate.

"Both black-grass and brome thrive wherever they have space early enough in the season. Denying them this opportunity is vital. So alongside a competitive crop you need a robust autumn herbicide programme. Critical in this respect is a combination of a good pre-em and early post-em treatment to combat the very much more rapid emergence and establishment of grass weeds in min-till.

"If you have brome, it’s also vital to avoid giving this weed extra space by using an autumn post-em programme which selectively controls black-grass," added Andrew Richards. "S/U sequence restrictions further demand careful planning in the autumn to allow the use of either of the available spring brome killers – Monitor (sulfosulfuron) or Ethos (propoxycarbazone-sodium).

"Where brome is a problem it invariably tends to develop from the headlands. So, why not consider managing them separately from a herbicide point-of-view. Use a standard S/U-based programme on the bulk of the crop, but go for a pendimethalin mix on the headlands. Then you can safely use Monitor or Ethos to hit the brome problem in the spring. And perhaps increase your seed rates on the headlands too.

"It’s early days yet, but the introduction of Atlantis is likely to give further options," he added," particularly as far as black-grass and ryegrass control are concerned."

Whatever action is taken, Andrew Richards is adamant that growers cannot afford to rely on chemistry alone to overcome the grass weed control limitations of minimum tillage. Those who develop their entire systems intelligently as part of a tillage change can profit from substantial savings in cultivation costs without an upsurge in black-grass and brome problems. But those who don’t will all too often see their cultivation cost savings seriously eroded by escalating herbicide costs and/or yield losses from these weeds.