Course: Weed control | Last Updates: 10th October 2015
What are wild oats?
There are two species of wild oats that are weeds in UK arable systems, the common wild oat and the winter wild oat. However, the two species can cross to form hybrids and they can also cross with cultivated oats.
The common wild oat (Avena fatua) is an important weed in all parts of the UK and occurs on most types of soil. Unlike most grassweeds it is a problem in winter and spring crops.
The winter wild oat (Avena sterilis ssp ludoviciana) has a more local distribution and, in contrast to the common wild oat, is generally only a problem in winter crops. In a survey of weeds in conventional cereals in central southern England in 1982, winter wild oat was found in 7% of winter wheat crops, 3% of winter barley and 1% of spring barley.
Why are wild oats a problem?
They are very competitive weeds because the plants are similar to cereal plants in growth and development and additionally, they can grow to a larger size and shade the crop during grain fill. Per plant, wild oats are the most competitive annual grassweed in cereals, and, in some experiments, one plant/sq m has reduced winter wheat yields by more than 1%.
Wild oats are also a host of cereal cyst eelworm (Heteroderma avenae) and cereal stem eelworm (Ditylenchus dipsaci). They can be infected with barley yellow dwarf virus and are susceptible to attack by several insects and fungi that afflict cereals. The frit fly (Oscinella frit) damages wild and cultivated oats.
Wild oat germination
Unlike most grassweeds, wild oats are normally self-pollinated, although some out-crossing can occur. This results in seed from different habitats and from within the same habitat having different biology.
For example, different field stocks of common wild oats can germinate at different times, creating a variation of emergence patterns between and within fields. As an approximate guide, about one-third of plants germinate in the autumn and the remainder in spring.
The small autumn flush is from September until early November and the main flush is from January to early May.
Germination is spread over several weeks and is initiated when the soil temperature rises to 6C, providing sufficient moisture is present. The most competitive wild oat plants are those that emerge with or within three weeks of the crop. These also produce the most seeds per plant.
Winter wild oats germinate predominantly in winter or early spring. The most favourable temperatures for germination are between 7C and 13C, in contrast to the common wild oat, which germinates better at higher temperatures. The main period of emergence is from October to March, with a peak around November and December.
Factors affecting wild oats and control
A lot is made of the longevity of the viability of the seed in the soil; the most extreme claim is that seed found in Tutankhamun’s tomb was still viable. In fact, when common wild oat seeds are left on the soil surface their viability declines in a few months because of germination, predation and fungal attack.
Therefore, leaving stubble uncultivated for as long as possible is an effective cultural control measure for freshly-shed seed.
However, freshly-shed seed incorporated into the soil can remain dormant but viable for up to six years. If the soil is cultivated regularly, the majority of seeds will survive for only two to three years.
Extreme cold weather can encourage a huge germination of seed in the spring, but the last time the required conditions occurred for an emergence on such a scale was in the mid-1980s. Hot and dry conditions during seed maturation shorten the dormancy of the seed.
The seeds of wild oats are larger than other grassweeds and they can emerge from depths of up to 200mm (8in). This means that long-term population trends are little affected by cultivation method, so levels of control required with non-inversion tillage are only slighter higher.
Overall, control levels of about 85% of viable seed will contain populations at their current levels. However, higher levels of control will be required where populations are competitive. Such populations can occur in patches, which, where no control measures are adopted, can expand by 1m to 3m in the direction of harvesting and cultivation.
Perhaps due to the level of self-pollination and the resulting variation in the genetic structure of different field stocks, the weed has generally not developed resistance to herbicides as rapidly as blackgrass and Italian ryegrass.
However, it has been proven in Canada that the variation between field stocks resulted in some being resistant to herbicides before herbicides were introduced.
There are a number of field stocks in the UK that contain the enhanced metabolism mechanism of resistance. This mechanism affects the efficacy of a range of modes of action, although the efficacy of some active ingredients are unaffected, including tri-allate, isoproturon and cycloxydim.
There are also field stocks that contain target-site resistance to the fops but generally not to the dims.
Moderate to low populations of wild oats will not reduce the yield of winter cereals if they are treated by the first to second node stage of the crop.
However, high populations need to be treated with herbicides earlier. Wild oats that have enhanced metabolism mechanism of resistance have to be treated when they are small if good control is to be achieved. Wild oats in winter cereals are more difficult to control with herbicides in the spring when the soil is dry.
A single hand-roguing removes about 85% of the plants, which is usually enough to contain populations at a low level. The immature plants must be pulled up completely otherwise the remaining tillers will be encouraged to produce further panicles.
Unripe seeds of wild oats are viable and non-dormant, therefore, hand-pulled wild oats – even with green panicles – must be disposed of carefully.
The two species can be distinguished only towards maturity. The common wild oat has two or three awned seeds per spikelet and at maturity they naturally break apart.
The winter wild oat rarely has three seeds per spikelet, and where this does occur, the smallest seed does not have an awn. And the first and second seed in the spikelet have to be broken apart at maturity.
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