Wild oat

Images above: Upper left: Wild oat stand (Jack Clark, University of California). Upper right: Wild oat inflorescence (Joseph DiTomaso, University of California, Davis). Bottom: Wild oat young plant (Joseph DiTomaso, University of California, Davis).

Images above: Left: Wild oat spikelet (Jack Clark, University of California). Right: Wild oat ligule (Jack Clark, University of California).

Identification

Other common names:  wheat oats, oat-grass, flax-grass, poor oats, drake, haver-corn, hever, black oats, oats

Family:  grass family, Poaceae

Habit:  Summer annual grass in most of its range, winter annual in California (Jain 1969). 

Description:  The seed leaf is oriented vertically and round tipped.  True leaves are rolled in the bud, usually emerging with a counter-clockwise twist that is retained to maturity.  The ligule is membranous, relatively big, roundly pointed; auricles are absent.  Hairs are sometimes present on the dark green blades; when present, small hairs are evenly distributed and large hairs are scattered.  The upright, sturdy mature plants have 3-5 tillers, and can reach 1-3.5 ft (30-107 cm) in height.  Flattened sheaths have transparent edges and scattered hairs; they are split open near the collar region.  The mature ligule is 0.1-0.25” (0.25-0.6 cm) long and pointed, with an uneven, slightly toothed edge.  Blades are linear, flat, 2.75-15.75” (7-40 cm) long by 0.25-0.75” (0.6-1.9 cm) wide, pointy tipped, and rough, with sparse, long hairs present on leaf edges at the base.  An extensive fibrous root system is present.  The inflorescence is an open, pyramidal, 4-15” (10-38 cm) long by 8” (20 cm) or less wide, panicle.  The widely spaced panicles each have nodding spikelets of 2-3 flowers per branch node.  Each spikelet is covered by papery, 1” (2.5 cm) long glumes and each flower is encased by a 0.75” (1.9 cm) long, hairy lemma.  Lemma hairs are white to brown, bristly, and scattered.  Each lemma has a distinctive, 1.25-1.5” (3-4 cm) long, dark brown to black, awn that is bent at a 90° angle and twists when in contact with water. Lemma coated seeds fall off the panicle at maturity, leaving a round scar where they were attached to the stem; this scar is sometimes surrounded by short, bristly hairs.  Seeds are 0.4” (1 cm) long, range in color from yellow-brown to rust to nearly black.

Similar species:  Cultivated oats (Avena sativa L.) are most easily differentiated from wild oats when the seedhead is present.  Cultivated oats have a narrower, more compact panicle than wild oats, and the non-shattering seed has an unbent lemma bristle that is short enough to sometimes appear absent.  Vegetatively, the two are nearly impossible to tell apart, especially when the plants are young. 

Management

Although wild oat can be a problem weed in many crops, typically it is a weed of cereal grain dominated cropping systems. It is particularly problematic in systems in which spring cereal crops are rotated with other drilled crops like canola and flax. Rotation of drilled crops with row crops that can be cultivated between rows like dry beans, potato and sugar beet will help decrease wild oat infestations. This will be especially true if precision cultivation tools are used to work close to the crop row. Rotating spring-planted crops with perennial forage crops or fall-planted crops also can help control this weed (Benaragama et al. 2022).

The tillage regimen greatly affects wild oat management.  Wild oat seeds survive poorly on the soil surface.  Consequently, unless other weed species are about to go to seed, avoid summer tillage after grain harvest.  Even a few weeks of exposure on the surface will decrease the number of viable seeds present.  Very shallow harrowing of less than 2” (5 cm) in the fall will stimulate up to half of the current crop of seeds to germinate and subsequently die over the winter in cold climates.  In the spring, tillage options depend on the amount of seed produced in the previous crop; if seed production was high, then plowing in the spring will likely bury more seeds than it brings to the surface, whereas, if seed production was low, then shallow spring tillage may be preferable.  In any case, delaying tillage and planting of either a fall or spring sown crop will reduce the density of wild oat by allowing more seeds to germinate before seedbed preparation. Note that rotary hoeing or tine weeding after crop emergence is not very effective because a large proportion of wild oat seedlings emerge from below the operating depth of these implements. Nevertheless, some control can be achieved using a tine weeder with stiff tines to bury young wild oat seedlings.  Not planting every third row and then cultivating the skipped rows has achieved substantial control in European studies.

Since options for mechanical management of wild oat in grain cropping systems are limited, crop competition is critical for management of this weed.  In general, winter grains are more competitive against this spring germinating species than spring grains.  Among spring drilled crops, the ranking of competitiveness against wild oat is barley > canola > wheat > flax (Sharma and Vanden Born 1978).  Usually, long strawed varieties and species (for example, spelt and rye) are more competitive than semi-dwarf varieties (Harker et al. 2009).  Narrow row spacing and increased seeding density will likely improve yield if wild oat is a problem, and can be guaranteed to decrease seed production of the weed (Harker et al. 2009).  Clipping plants when wild oat panicles first emerge above the crop may reduce shed of viable seeds, particularly in shorter crops such as lentil (Tidemann et al. 2020).

Wet soils favor wild oat, so tile drainage and practices that avoid compaction have long term benefits for managing this weed. 

Inspect seed grain carefully before you buy it!  Wild oat is a common contaminant of most grain crop seed, and even a low percentage contamination can result in planting thousands of wild oat seeds per acre.  Although less than 1% of wild oat mill screenings fed to cattle pass unharmed into the manure, this can still be an enormous number of seeds.  Consequently, grinding wild oat grain before feeding and/or composting manure before spreading is advised.

Ecology

Origin and distribution:  Wild oat probably originated in central Asia, and it presently occurs throughout Europe and Asia except mainland southeast Asia.  It has been introduced into Australia, New Zealand, East Africa, North America and most of South America (Holm et al. 1977).  In North America, wild oat occurs in Alaska, across southern Canada and the U.S.A. except for most of the southeast and the southern great plains.  It is particularly problematic in the northern prairie region (Sharma and Vanden Born 1978).

Seed weight:  14.1 mg (EFBI), 18.6 mg (Gardarin et al. 2010), 22.2-24.4 mg (Chauhan et al. 2006).

Dormancy and germination:  Usually, wild oat seeds are dormant when shed from the parent plant, but lose dormancy over time when exposed to warm, dry conditions (Foley 1994).  The duration of after-ripening is dependent on complex interactions between wild oat genotype and temperatures during both seed development and after-ripening (Sawhney and Naylor 1979).  Generally, warm, dry conditions hasten after-ripening and cool, moist conditions delay it (Sharma and Vanden Born 1978).  Immersion in water, or alternate wetting and drying induces secondary dormancy in previously germinable seeds (Sharma and Vanden Born 1978).  Once seeds become non-dormant, optimum temperatures for germination range from 59-82 °F (15-27.5 °C) (Sharma and Vanden Born 1978, Fernandez-Quinantilla 1990, Guillemin et al. 2013).  Nitrate promotes germination of dormant seeds (Adkins et al. 1984), and applications of N fertilizer have been shown to increase emergence of wild oat in the field.  Light (Sharma and Vanden Born 1978) or a low oxygen atmosphere (Symons et al. 1986) inhibits germination.

Seed longevity: The annual mortality rate of wild oat seeds is high, typically in the range of 70 to 90% per year, but a small fraction of seeds remain viable even after 5 to 9 years (Banting 1962, Barralis et al. 1988, Conn et al. 2006, Mickelson and Grey 2006, Miller and Nalewaja 1990, Thurston 1966).  In Australia, seed losses through germination and decay reached 50% after, on average, 5 months, and 90% after 16 months (Mahajan and Chauhan 2021a).  In Montana, wild oat seed mortality between spring and fall increased linearly, on average, from 36% to 69% as soil moisture content increased from 6% to 24% (Mickelson and Grey 2006).  Wild oat seeds survive poorly on the soil surface, but survival increases when seeds were buried deeper than 4” (10 cm) below the soil surface (Banting 1966, Conn et al. 2006, Miller and Nalewaja 1990).  In Colorado, no wild oat seeds survived burial for two years, leading to speculation that seeds of Colorado biotypes had less intense dormancy and that, consequently, wild oat was a less serious weed problem south of the 43rd parallel (Zorner et al. 1984).

Season of emergence: Most emergence occurs in the spring and early fall (Froud-Williams et al. 1984), but some seedlings can emerge throughout the growing season.  In Minnesota and North Dakota, wild oat emergence was concentrated primarily in a 28 to 42 day period after initial emergence began in spring (Martinson et al. 2007).  In Montana, emergence was concentrated in April, with little emergence occurring in late spring and summer despite varied irrigation treatments during this period (Mickelson and Grey 2006).  In Canada, emergence occurred earlier in the spring under conservation tillage than under conventional tillage (Bullied et al. 2003).

Emergence depth:  Optimum recruitment depth is 0.8 to 2.8” (2 to 7 cm) (Chauhan, et al.  2006, Froud-Williams 1984, Mahajan and Chauhan 2021a), but emergence can occur from seeds as deep as 6 to 9” (16 to 23 cm) (Froud-Williams et al. 1984, Thurston 1951).  Few seedlings emerge from seeds on the soil surface (Froud-Williams et al. 1984, Mahajan and Chauhan 2021a).

Photosynthetic pathway:  C3.

Sensitivity to frost:  Seedlings are frost sensitive (EFBI), but grow well at cool temperatures.  Fall emerging plants in the northern prairies are killed by hard frost before they mature, but they can survive the winter in California (Sharma and Vanden Born 1978) and Australia (Mahajan and Chauhan 2021b).

Drought tolerance:  Wild oat is relatively sensitive to drought.  Even within a single field, it tends to be more common in wet depressions than on dry knolls (Sharma and Vanden Born 1978).

Mycorrhiza:  Wild oat is mycorrhizal (Harley and Harley 1987).

Response to fertility:  Growth of wild oat is highly responsive to N; in particular, it is more responsive than wheat, and similar in responsiveness to canola (Blackshaw et al. 2003).  Its above-ground growth is moderately responsive to P, being more responsive than wheat, but less responsive than canola (Blackshaw et al. 2004).  Wild oat tolerates soil pH down to 4.5.

Soil physical requirements:  Wild oat tolerates a wide range of soils, but is most common on clay and loam soils (Halvorson and Guertin 2003, Sharma and Vanden Born 1978).

Response to shade:  Wild oat does not grow in shady environments.  In response to shade such as from crop competition, plants grow to the same height, but the number of tillers and seed production are greatly reduced (Sharma and Vanden Born 1978). 

Sensitivity to disturbance:  Wild oat does not reproduce well from vegetative parts and can be killed easily by cultivation.  However, damaged seedlings can regrow in the top 1” (2.5 cm) of soil (Sharma and Vanden Born 1978), presumably if the seed and roots remain intact allowing access to large seed reserves and soil moisture.

Time from emergence to reproduction:  Heading of wild oat plants begins 7 to 8 weeks after emergence and is controlled more by temperature than by day length (Dai et al. 2012).  Wild oat plants mature seeds 3 to 4 weeks after heading with seeds at the tip of the main axis of the panicle ripening and shedding first (Sharma and Vanden Born 1978).  Very few seeds are viable when panicles first emerge above the crop canopy, but seed viability increases rapidly thereafter and, by 6 to 7 weeks after elongation, 95% of the seeds are viable (Tidemann et al. 2020).  The capacity of wild oat to mimic the development of the grain crops that it infests (Cudney 1989, Shirtliffe et al. 2000), and to shatter mature seeds before grain harvest, ensure its success in these crops (Burton et al. 2016, Halvorson and Guertin 2003, Sharma and Vanden Born 1978, Shirtliffe et al. 2000, Tidemann et al. 2017). 

Pollination:  Wild oat mostly self-pollinates but 1-2% of seeds are produced through cross pollination by wind.

Reproduction:  Wild oat produces up to 500 seeds per plant, but 100-150 seeds per plant is more typical (Sharma and Vanden Born 1978).  Plants emerging early produced 120 seeds per plant whereas plants emerging three weeks later produced 61 seeds per plant in the Minnesota Red River Valley (Dai et al. 2012).  Wild oat plants emerging before emergence of a barley crop produced 68 to 81 seeds per plant, whereas plants that emerged after barley produced 2 to 58 seeds per plant, depending on the barley growth stage at which the wild oats emerged (Peters and Wilson 1983).  Plants subjected to intense competition from a vigorous barley crop produced 20-30 seeds/plant (Harker et al. 2009).  The majority of wild oat seeds shatter before harvest of grain crops (Burton et al. 2016, Halvorson and Guertin 2003, Mahajan and Chauhan 2021b, Shirtliffe et al. 2000, Tidemann et al. 2017).  Seeds that mature in the crop understory are 10 to 30% less viable than seeds that mature above the crop leaf canopy (Lehnhoff et al. 2013).

Dispersal:  Seeds fall on the ground without any natural means of dispersal.  For thousands of years, the main mode of spread has been in contaminated seed grain (Sharma and Vanden Born 1978).  The species also commonly moves about on combines.  Only a low percentage of wild oat seeds pass through cattle in a viable condition. 

Common natural enemies:  The natural enemies of wild oat are similar to those of grain crops.

Palatability:  The seeds are edible.  Wild oat mill screenings are commonly sold as a feed grain and have about 90% of the nutritional value of domestic oat.  Wild oat is palatable forage, and extensive tracts of annual grasslands dominated by wild oat are harvested for hay in California (Sharma and Vanden Born 1978).

References:

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