Wild-oat
Other names
Oatgrass
Latin names
Avena fatua L.
Weed Type
Annual Grass Weeds
Occurrence
Wild-oat is an introduced annual grass weed of arable, waste and rough ground. In Britain, wild-oat was a more serious weed after the Second World War than in the pre-war years. According to a 1951 survey, wild-oat was a problem weed in all parts of England where wheat and barley were grown. It was considered to be even more abundant in the 1990s than the 1960s. Wild-oat is ranked as an important weed in winter and spring cereals through much of Europe.
Wild-oat occurs on most types of soil but is particularly troublesome in cereals on heavy land. It grows over a wide climate range but prefers cool, temperate conditions.
Seed from different habitats and even from within the same habitat may have different genotypes. Populations have developed with resistance to some widely used graminicides. Phytotoxic chemicals that leach from wild-oat straw can inhibit the germination and growth of other plants.
Wild-oat is a host of cereal cyst eelworm (Heteroderma avenae) and cereal stem eelworm (Ditylenchus dipsaci). It can also be infected with barley yellow dwarf virus and is attacked by several insects and fungi that afflict cereals too. The frit fly (Oscinella frit) damages both wild-oats and cultivated oats.
Biology
Wild-oat flowers from June to October. The first panicles become visible above the cereal crop in early June but flowering and panicle production can continue up until crop harvest. The flowers are normally self-pollinated but some out-crossing can occur. Seeds become viable around 10 days after fertilization. In the absence of competition a single well-tillered plant can produce up to 2,000 seeds. However, in a cereal crop the average seed production is 60 seeds per plant. In a less competitive crop, a plant may produce 200 seeds. Wild-oat seeds are shed as they ripen and this occurs over an extended period. The later a crop is harvested, the more seed is shed onto the ground leaving less to contaminate the cereal grain and straw. The seeds possess a long, hygroscopic awn that twists and straightens with changes in humidity pushing seeds into crevices in the soil.
Fresh seeds are dormant or rapidly become dormant but the level of dormancy varies between populations. Temperature and moisture levels during seed development can also affect the level of dormancy. Seed from the smaller secondary floret is usually more dormant than the primary seed. In the autumn the level of dormancy declines and is induced again in late-spring. Light may inhibit seed germination. Damage to the seedcoat can relieve dormancy at anytime by allowing oxygen to reach the seed. Applications of nitrogen-containing fertilizers may break the dormancy of seeds exposed to high light levels.
Wild-oat has two main periods for germination but odd seedlings may appear at anytime. There is a small autumn flush from September until early-November but the main flush is from January to early-May. Germination is spread over several weeks and is initiated when the soil temperature rises to 6°C and there is sufficient moisture present. Wild-oat germinates at higher temperatures than the winter wild-oat (A. sterilis L. ssp. ludoviciana).
Most seedlings originate from seeds in the top 80-100 mm of soil but some emerge from a depth of 150-240 mm. Elongation of the first internode, or mesocotyl, allows wild-oat seedlings to emerge from relatively deep soil layers. Seedlings from deeper in the soil take longer to emerge and are weaker initially. Early growth is slow but then accelerates. Seedlings are frost sensitive until they reach the 3-leaf stage.
Persistence and Spread
When seeds are left on the soil surface viability declines within a few months due to germination, predation and fungal attack. Freshly shed seed incorporated into the soil can remain dormant but viable for up to six years. Land that has been down to pasture for 5-10 years has been found to be infested with wild-oat when ploughed up. If the soil is cultivated regularly the majority of seeds will only survive for 2-3 years. The shallower seeds will germinate and emerge, deeper ones have dormancy enforced on them and cannot germinate while those at intermediate depths may germinate but fail to emerge. Under granary conditions, seed exhibited 50% germination after 1 year and 8% after 4 years.
In cereals, up to 66% of wild-oat seeds are shed before harvest and 20% are lost during the harvesting process. Around 7.5% remain in the harvested grain and the rest are lost in the chaff and straw. Wild-oat was probably introduced originally as a contaminant in cereal seed and the drilling of contaminated grain allows that spread to continue. Farmer saved seed carries a higher risk of contamination than commercial seed. Cereal straw can also carry quantities of wild-oat seed, as can farmyard manure containing contaminated straw used for bedding. The seed can be spread on or by farm machinery especially combine harvesters and baling machines.
Mapping of wild oat infestations has shown that patches remain relatively stable from year to year but expand by 1-3 m in the direction of harvesting and cultivation. Without control a wild-oat population can increase annually by a factor of 3 but there is too much variation to reliably predict the size of the future population based on the extent of the current one.
Rumen digestion generally kills wild-oat seeds after 8-24 hours but diet can affect this and some viable seeds may survive. Seeds that do pass through unharmed may remain viable in manure for several weeks depending on the temperature of the heap. Around 1% of seeds may still be viable after 2 weeks windrow composting but after 4 weeks all are killed. The seed does not generally survive in silage especially after its passage through the digestive system of cattle.
Management
Conditions that favour cereals benefit wild-oats too. They compete for the same resources but the wild-oat is better able to compete in the root zone. The most competitive wild-oat plants are those that emerge before or within 3 weeks of the crop. These are also the ones that produce the majority of seeds. Preparing the seedbed and drilling a cereal within days of harvesting the previous one increases the wild oat problem. The use of combine harvesters has also exacerbated the situation.
It is important to sow clean crop seed. Wild-oat control is improved by shallow cultivations in autumn to induce a proportion of seed to germinate and deeper working in spring to kill the emerged seedlings. Preparing the seedbed around 3 weeks in advance of drilling will allow the earliest wild-oat seedlings to be killed by cultivation. Harrowing may remove later emerging seedlings as will hoeing in root crops. Seedlings are susceptible to cultivation but tillered plants that are broken up may re-root. Seedlings of wild-oat are smaller than those of cereals initially but soon catch up and exceed the crop. The cereal needs to be able to build on its early advantage with more rapid growth and establishment. In winter wheat increasing crop density limits seed production by wild-oat. Narrower row spacing can also help.
Hand roguing of cereals is possible with weed populations of 400-500 wild-oat plants per ha but the weeds must exceed the height of the crop to make this easier. The plants must be pulled up completely otherwise the remaining tillers will be encouraged to produce further panicles. Unripe seeds of wild-oat are viable and non-dormant, therefore, hand-pulled wild-oats even with green panicles must be disposed of carefully.
Combines and other harvesting machinery should be cleaned to remove wild-oat seed. Seeds collected during combining or seed cleaning should be burned and not fed to stock or tipped on manure heaps.
If large numbers of seeds remain on the soil surface after cereal harvest early cultivation leads to more seeds surviving than if cultivations are delayed for 2-3 months. Delaying cultivation until December increases seed losses considerably. Natural deterioration accounts for most of the losses. Some farmers turn hens onto the stubble to eat the wild oat seeds on the surface and scratch up and eat some buried seeds too. Pigeons also eat large quantities of wild oat seeds.
A summer fallow will have little effect on wild-oat seeds in the soil as germination is unlikely to occur at high temperatures. For the same reason crops sown after May have few wild-oats in them. Putting land down to grass leys for 5 years has been recommended to reduce wild-oat seeds in the soil seedbank but although numbers decline initially viable seeds are still likely to be present after 5 years. A period of 8-9 years has been suggested as the period needed to eliminate wild-oat seed in uncultivated soil.