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weed seed identification wheat head

Black-grass, Alopecurus myosuroides, can seriously reduce crop yields through competition for nutrients, especially nitrogen. Although on an individual plant basis black-grass is only moderately competitive, very high populations can occur (many hundreds of plants/m2) which can reduce yield by >70% in serious cases. On average, black-grass populations of 12 – 25 plants/m2 cause yield losses of 0.4 – 0.8 t wheat /ha but losses can be much higher in conditions which favour black-grass (e.g. uncompetitive crops).

Since first being found in Oxfordshire in 1982, herbicide-resistant black-grass has now spread to virtually all of the estimated 20,000 farms in 37 counties where herbicides are applied regularly for black-grass control. Populations show resistance to a wide range of different modes of action, with both ACCase and ALS target site resistance (TSR) and non-target site resistance (NTSR), especially enhanced metabolism, now widespread. The frequency of multiple resistance can be demonstrated by results from tests on 132 black-grass samples collected and tested by Rothamsted Research in 2014: 98% showed resistance to at least one herbicide, with 77% resistant to mesosulfuron + iodosulfuron, 60% resistant to cycloxydim, 90% resistant to fenoxaprop and 79% resistant to all three herbicides.

Damage

A competitive grass-weed with distinctive dark coloured seed heads (hence its common name) and now the most important herbicide-resistant weed of arable crops in Western Europe. More of a problem on heavier clay or silt soils, especially if drilled early and where there is minimum tillage.

Black-grass is an annual grass-weed propagated solely by seeds and, if uncontrolled, weed populations can increase by >10 fold/year. For successful long-term control, seed return must be minimised. The following five key characteristics underpin successful non-chemical management strategies:

Herbicide use must be integrated with greater use of non-chemical control methods to reduce the reliance on herbicides. The potential of such methods for black-grass control (from Lutman, Moss, Cook & Welham, 2013) is summarised below. The wide range of control level for ploughing can be explained by considering where in the soil profile the black-grass seed is and this will depend on the cropping history at a location. If it is near the top from recent seed shed ploughing may help to bury it. Conversely, if the black-grass seed is lower down ploughing may make the situation worse (which is why the control range includes negative values). This needs to be considered when deciding whether to plough or not. Delayed autumn drilling can reduce black-grass but if it interferes with establishment of the main crop there may be more holes where the black-grass can come in. Therefore a balance needs to be found and drilling should not be delayed too long to ensure the main crop comes up well. Spring cropping and wider rotations are the most reliable ways of controlling black-grass.

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