Black-grass: The magic number

Black-grass populations of more than 30 black-grass plants/m2 are unlikely to be adequately controlled by herbicide programmes alone, Bayer CropScience has warned growers.

Meta-analysis of three years of fully replicated independent trials has revealed the ‘sweet spot’ above which cultural controls must be introduced to maintain a typical population and, crucially, preserve yield.

“Resistance isn’t the main factor here,” notes Miles Taylor, Crop Agronomy Coordinator at Bayer CropScience.

“It’s the sheer number of plants that overwhelms the control mechanism. Once you start to get above 30 plants/m2 the high seed return means that black-grass begins to fight back.”

Mr Taylor points to the results graph and its exponential growth curve. “Growers can quickly find themselves at the black-grass control ‘cliff edge’ - and once there, it’s no surprise that the problem is much harder to control.

“For example, if your herbicide programme controls 97 per cent of a population of 100 plants/m2 you’re left with three plants/m2. If the same programme is applied to a starting population of 1,000 plants/m2, 30 plants will remain. Bearing in mind each single plant can easily bear 10 tillers, by summer there could be 300 tillers all producing seed. That’s above the point at which population growth can be controlled.”

Yield, of course, is also adversely influenced, stresses Mr Taylor. “More than 25 black-grass plants/m2 can reduce yield by as much as 10 per cent.”

He says the key to winning the black-grass battle is a balanced programme that features both cultural controls and effective herbicide treatments.

“Finding the balance is crucial, but it’s important to realise that cultural controls always have a place in grass-weed control programmes.

“If you have less than 30 plants/m2 then I would still advise the use of cultural controls to reduce the population. It’s good practice and helps prevent build up of resistance.

“If you’re seeing more than 30 plants/m2 then it’s even more crucial to use all available tools. Pre-em herbicides such as Liberator (flufenacet + diflufenican) are very effective, but when populations are high, you can’t rely on them alone and will need to consider combining one or more cultural techniques.

“Many people are returning to the plough, while others find repeated stale seedbeds the best option.”