Black-grass

Black-grass
Other names: 

Black twitch, hungerweed, rat-tail grass, slender foxtail

Latin names: 

Alopecurus myosuroides Huds (Alopecurus agrestis L.)

Weed Type: 
Occurrence: 

Black-grass is a native annual grass weed that occurs throughout the UK but is found mainly in the cereal growing areas of southern and eastern England. It rarely occurs outside of cultivated land and is most abundant in winter crops. Black-grass is often found spreading into arable fields from the field margins. It is chiefly confined to heavy land, occurring only occasionally on sandy or gravely soil but has begun to appear on chalk.

Some black-grass populations have developed resistance to some widely used graminicides and this has contributed to an increase of the weed on conventional farms.

In addition to its effect on cereal yield, black-grass suffers from ergot (Claviceps purpurea) and this can result in contamination of the grain at harvest. The fungus is the same strain that infects wheat.

Biology: 

Black-grass flowers from May to August, sometimes into October. The flower heads appear above the cereal crop in May and June. Additional flower heads may develop on side branches. The flowers are wind pollinated but are self-fertile. Seed production ranges from just 50 up to 6,000 seeds per plant. Seeds ripen quickly and many are shed before cereal harvest. Those that remain on the plant are likely to contaminate the harvested grain.

Some seeds are able to germinate soon after shedding, others remain dormant for a few months. Light and fluctuating temperatures appear to increase seed germination. If conditions are favourable most seeds germinate in the autumn, mainly in late October and early November. There is a smaller flush of seedlings in spring, which is greater if autumn germination has been prevented by weather or soil conditions. Dormancy is induced when seeds are present in waterlogged soil. Around 60-70% of the seedlings in a flush are derived from seeds less than a year old. Seedlings that emerge from depths of 2 to 5 cm in a ploughed soil are unlikely to be from freshly shed seeds. Black-grass seeds normally germinate near the soil surface, even shallow burial will result in some degree of dormancy. Emergence of black-grass seedlings decreases with increasing depth of burial in soil.

Persistence and Spread: 

The decline of seed in soil is greatest in the first year but appreciable numbers can remain after 4 years in both cultivated and undisturbed soil. If seeds are ploughed down deeply they may retain viability for 11 years. Black-grass seed does not persist for long in peat soils. Seed sown in the field and followed over a 5-year period in winter or spring cereals had an annual decline rate of 80%. Emerged seedlings represented 15% of the seedbank. Seeds in dry storage will still germinate after 13 years but viability begins to decline after 8 years. Seeds stored under granary conditions gave 34% germination after 1 year and 7% after 4 years.

The position of black-grass patches remains relatively stable in arable fields. The grass exhibits little ability to spread rapidly into other areas but persists where it has become established unless control measures are taken.

Black-grass seed has been found as a contaminant in cereal, grass and clover seed samples. Contamination was greater in home-saved seed.

Seed is not considered to survive passage through the digestive systems of birds or animals but it has been found in cattle droppings.

Management: 

In the past, straw and stubble burning after cereal harvest killed a significant proportion of the freshly shed black-grass seeds but this is no longer an option.

Early drilling of winter cereals leads to severe infestations because it coincides with peak black-grass emergence and the weed has time to tiller before winter. Sowing cereals before 25 October has been shown to increase black-grass infestations, sowing after 5 November has led to a decrease. However, there may be a substantial loss in yield if winter wheat is drilled after mid-November. Infestations are often worse on badly drained, heavy soils where there is a high proportion of winter cropping. In cereals, narrow crop rows (18 cm) suppress black-grass growth more than rows wider apart (35 or 53 cm).

Where black-grass seed has been shed in the previous crop, 80-90% of black-grass plants in a direct-drilled winter wheat crop will be from the fresh seeds. Where there is only a small reserve of seeds in the soil, ploughing generally reduces the level of infestation from fresh seeds while tine cultivations leave most seeds ready to germinate in the surface layers of soil. However, on a field that has suffered black-grass infestations for many years viable seeds will be present throughout the soil profile and ploughing will unearth the buried seeds ready to germinate in large numbers. A population model used to determine the effect of soil tillage on the level of black-grass infestations in winter cereals suggests that to prevent a likely build-up of black-grass under minimum tillage systems the land should be ploughed every 5 years.

Weather conditions during the maturation of black-grass seed can affect the level of seed dormancy. Seed can be more dormant following a cool damp summer and ploughing may be more appropriate to avoid a prolonged period of seedling emergence. After a hot summer, the seed is less dormant and will germinate readily if conditions are favourable after shedding, negating the need for ploughing.

In spring cereals, early sowings suffer worse black-grass problems but delaying drilling can reduce the potential yield of the crop. Black-grass is less competitive in a spring crop but it can be a significant source of fresh seed.

Black-grass seedlings are very susceptible to mechanical injury and do not tolerate trampling or soil disturbance. Shallow ploughing and surface tillage will encourage seeds to germinate freely. The seedlings can then be destroyed by further cultivations or ploughing. In root crops, thorough cultivations will destroy the young plants. In cereals, harrowing when the soil is dry kills small seedlings. In grass seed crops, mowing or grazing in the first year may be required to prevent the black-grass seeding and reduce infestations in year 2.

Fallowing can reduce seed numbers in soil dramatically and an autumn fallow is ideal for this autumn germinating weed. However, seed numbers will build up again rapidly if seed shedding occurs in subsequent crops.

Strategies developed to deal with herbicide resistant black-grass may also be relevant in organic systems. They consist of ploughing at least once every 4-5 years, including a higher proportion of spring crops in the rotation and delaying the sowing of autumn cereals.

Updated October 2007.

Fully referenced review: