Volunteer cereals arise from seed shed at or before crop harvest. In barley the whole ear may break off while in wheat individual grains tend to fall from the spikelet. Some cultivars are more resistant to shedding and lose 5% of grains compared with an average of 16%. Cereal seedlings can emerge from straw used as mulch or as animal bedding. Seed can also be carried on and dispersed by farm machinery. Seedlings that reach maturity will produce a further generation of potential volunteer seedlings.
If present in large numbers in subsequent cereal crops, volunteer cereals will result in too high a plant population leading to yield depression and increasing the risk of disease problems. Volunteer cereals can also compromise varietal purity. A survey of weeds in conventional cereals in central southern England in 1982, found volunteer cereals in 4.2, 4.0 and 2.0% of winter wheat, winter barley and spring barley fields respectively. In another survey the mean level of varietal impurity in samples of wheat and barley was around 3%. Barley, and in particular winter barley, was more contaminated than wheat by other cereal species.
It has been suggested that 28% of sugar beet fields and 88% of oilseed rape fields are infested with volunteer cereals. Volunteer barley can have a severe effect in the autumn on the growth of winter oilseed rape although the effect on yield may not be severe. Volunteer cereals can be a particular problem in autumn sown or planted vegetable crops grown within an arable rotation. Volunteer cereals occur in maiden fruit crops when soft fruit crops follow cereals in the rotation. Straw used as mulch in strawberries and in raspberries can be an ongoing source of cereal seeds. Volunteer cereals can make up 40% of the ground cover in the first year of set-aside but give way to other species in later years.
In crops grown as a break from cereals the presence of cereal volunteers negates any pest and disease control benefits. They can act as a green bridge that perpetuates soil-borne and foliar diseases, and carries cereal pests through to the next drilled cereal crop.
Dormancy in cereal grains is normally a desirable characteristic that prevents pre-harvest sprouting but in freshly shed cereal seed it can limit early germination and restricts post-shedding control measures. Seed dormancy varies between cultivars but may be influenced by temperatures during seed ripening. Seeds that ripen at higher temperatures and in dry conditions are less dormant. A cool wet period at or around harvest may encourage the retention of dormancy. Even if non-dormant, shed cereal grains will not germinate until they absorb sufficient moisture. In a dry autumn this may be after a succeeding crop has been sown. Straw residues in soil can also inhibit germination and delay seedling emergence.
Persistence and Spread
Some reports suggest that cereal grains have a lifespan of up to 5 years in soil but lengthy persistence is thought to be unlikely even if seed is buried deeply in soil. After 5 years in dry storage wheat and oats gave 85 and 99% germination respectively.
Greater losses of seeds are likely from the soil surface than following burial by cultivation. Ploughing and other cultivations soon after harvest are likely to encourage retention of dormancy. Increased numbers of volunteer cereals may occur in crops grown under reduced tillage because of the retention of seeds near the soil surface. Winter barley seed broadcast over the soil and ploughed or tine cultivated emerged in the greatest numbers following the tine cultivations. Between 60 and 90% of potential seedling emerged in the year after sowing following tine cultivation. Around 14% of possible seedlings emerged in the year after ploughing.
Seed contamination has been a major source of volunteer cereals. In tests of cereal seed purity by the Official Seed Testing Station in the period 1978-1981, wheat was found in 25-28% of barley samples and barley was found in 25-28% of wheat seed samples. The highest number of seeds recorded in a 125 g sample was 300. Wheat seeds were found in up to 20% of certified barley seed samples and up to 26% of oat samples tested in the period 1986 to 1997. Oat seeds were found in up to 2% of barley and 1% of barley seed samples tested. Barley seeds were found in up to 50% of oat and 37% of wheat seed samples tested.
Careful choice of varieties with some resistance to shedding can help to reduce the volunteer problem. Shallow post-harvest stubble cultivations usually encourage germination of shed cereal grains. Emerging volunteers can then be controlled by subsequent cultivations. However, the timing may conflict with the need to control wild oats (Avena fatua).
In Scotland, ploughing down stubbles to bury barley seed deeply is considered the best management option. However, some cultivars have somewhat higher dormancy levels and seed may persist and germinate after ploughing again in the future.
Early removal of volunteer winter barley in peas and spring beans sown in March prevented yield loss if carried out when peas and beans were at the 2-3 and 2-4 node stage respectively. Allowing the cereal to remain for a further 4 weeks before removal resulted in some yield loss in beans and in peas the loss was as great as that of the unweeded crop.
In Canada, the effect of different densities of volunteer barley on the yield of spring-sown rapeseed was assessed to determine a threshold value at which it was economic to control the cereal. The cost of weed control set against the cash returns from the improved crop yield indicated that a density of 15 or more barley seedlings was needed to justify control measures economically.
Updated November 2007.