Foxtails

Foxtail images above: Upper left: Giant foxtail inflorescence (Scott Morris, Cornell University). Upper right: Green foxtail inflorescence (Scott Morris, Cornell University). Above: Mature yellow foxtail plant with inflorescence (Randall Prostak, University of Massachusetts).

Identification

Other common names:

• Giant foxtail:  Faber's foxtail, Chinese millet
• Green foxtail:  bottle-grass, pigeon-grass, wild millet, green brittle grass, pussy grass, green bristlegrass, green bottle grass
• Yellow Foxtail:  pigeon-grass, summer-grass, golden foxtail, wild millet, glaucous bristly foxtail, yellow bristle grass

Family:  grass family, Poaceae

Habit:  Erect or sprawling summer annual grasses.

Description:  Seedlings are rolled in the bud, uncurling roughly parallel to the ground.  All ligules of all three species are very short and hairy.  Auricles are absent. 

• Giant foxtail:  The seed leaf is oval, 0.5-1” (1.3-2.5 cm) long by 0.1-0.2” (0.3-0.5 cm) wide.  Young true leaves have short hairs on the sheath margin and upper blade surface. 
• Green foxtail:  The seed leaf is 0.5-0.8" (1.3-2 cm) long by 0.1-0.16" (0.25-0.4 cm) wide.  Young true leaf blades are hairless, rough, and narrow, and light green with red near the collar; sheaths are densely hairy, slightly compressed, and open at the top.
• Yellow foxtail:  The seed leaf is elongated, up to 2” (5 cm) long by 0.13” (0.3 cm wide).  Leaf blade edges are smooth to slightly rough, and the rest of the blade is hairless and flat.  The collar region is green and smooth with a scattering of long, silky hairs.  The sheaths are hairless, ridged, and compressed.  The lower seedling stem may be red. 

Mature plants have hairy, 0.04-0.13” (0.1-0.3 cm) long, ligules.  The root system of these tillering plants is fibrous.

• Giant foxtail:  Stems can reach up to 6.5 ft (2 m), but are usually 2.5-4 ft (0.8-1.2 m) tall.  Stems are upright and hairless.  Sheaths are light green and open, with a hairy edge.  Ligules are densely hairy.  Blades are arching, 12-20” (30-51 cm) long by 0.3-0.8” (0.8-2 cm) wide, and light green, with a white midrib; blades have short rough hairs covering their upper surface.
• Green foxtail:  Thin, round stems are 0.5-3 ft (15-91 cm) tall and radiate vertically from the center of the plant.  Sheaths are concentrated at the stem base, slightly flat, smooth to rough, self-wrapping and hairy edged.  The collar may be reddish.  Light green blades are flat, hairless, and 10-12” (25-30 cm) long by 0.25-0.38” (0.5-1 cm) wide.  
• Yellow foxtail:  Stems can reach 2-4 ft (0.6-1.2 m) in height.  Sheaths are flattened, hairless, split open at the top, red at the base and collar, and keeled about a ridged midvein.  Keeled blades are 10-12” (25-30 cm) long by 0.25-0.5” (0.6-1.3 cm) wide, with 0.13-0.4” (0.3-1 cm) long, silky hairs on the upper surface near the collar.  Plants are sometimes red at the base.

Inflorescences, located at stem tops, are bristly, spike-like panicles.  Spikelets are set in densely packed, short-branched whorls.  Spikelets are one-flowered.  What we refer to as seeds below include a thin, dry tightly adhering layer of fruit tissue.

• Giant foxtail:  Panicles are nodding, 1.5-8” (4-20 cm) long by up to 1.25” (3.2 cm) wide, green-purple, and are the largest of the three species.  Each 0.13” (0.3 cm) long spikelet has 3-6 yellow green, 0.2-0.4” (0.5-1 cm) long bristles.  Seeds are 0.1” (0.3 cm) long by 0.06” (0.15 cm) wide and wrinkled.
• Green foxtail:  Panicles are erect, 1.25-3.25” (3-8 cm) long by 0.5-1” (1.3-2.5 cm) wide, and greener and broader than yellow foxtail.  Each 0.06-0.08” (0.15-0.2 cm) long spikelet has 1-3 green-purple, slightly barbed, 0.25-0.5” (0.6-1.3 cm) long bristles.  There are several pale green to purple spikelets per branch.   Seeds are 0.09” (0.23 cm) long by 0.05” (0.12 cm) wide and slightly cross-wrinkled with one flat side and one round side.   
• Yellow foxtail:  Panicles are erect, 0.75-6” (2-15 cm) long by 0.5” (1.3 cm) wide, yellowish, and hairy along the central stem.  Each 0.08-0.1” (0.2-0.25 cm) long spikelet has 5 or more yellowish-brown, 0.16-0.35” (0.4-0.9 cm) long bristles.  There is one spikelet per branch.  Seeds are 0.13” (0.3 cm) long by 0.08’ (0.2 cm) wide, horizontally ridged, dull gray-yellow, pointy-tipped, and ellipse shaped.  Seeds are the largest of the three species.

Similar species:  Bristly foxtail (Setaria verticillata (L.) P. Beauv.) is a weak-stemmed, lodging plant with blue-green leaves.  Its spikelets are widely spaced and cling to objects.  Fall panicum (Panicum dichotomiflorum Michx.) and the crabgrasses (Digitaria spp.) both have dense hairs on the sheaths and blades at the seedling stage.  Mature fall panicum has swollen, angled stem joints, no hairs, and a white midvein.  Crabgrass tillers heavily and has a translucent, jagged ligule.

Giant foxtail images above: Upper left: Inflorescence (Scott Morris, Cornell University). Upper right: Leaf surface (Scott Morris, Cornell University). Bottom: Stand of mature plants (Antonio DiTommaso, Cornell University).

Green foxtail images above: Upper left: Seedling  (Scott Morris, Cornell University).  Upper right:Mature plant and inflorescence (Scott Morris, Cornell University). Above: Green foxtail collar  (Scott Morris, Cornell University).

Yellow foxtail images above: Upper left: Seedling (Jack Clark, University of California). Upper right: Mature plant with inflorescence (Randall Prostak, University of Massachusetts). Bottom: Yellow foxtail hairs at the base of a leaf blade (Scott Morris, Cornell University).

Management

The key to managing the foxtails lies in the timing and depth of tillage.  Since the seeds (particularly of yellow foxtail) are relatively large and highly palatable, leaving them on the soil surface over winter will encourage seed predation by birds, rodents and insects and thereby decrease the population.  Shallow cultivation in late spring to incorporate the seeds followed by a short fallow will cause a flush of emergence.  The seedlings can then be killed by tillage for seedbed preparation.  Moldboard plowing prior to seedbed preparation will place seeds from the previous year too deep for emergence.  Since few foxtail seeds persist for more than two years, most of the seeds buried by plowing will die before they return to emergence depth.  Planting short cycle vegetable crops in mid spring will similarly decrease the population.  Summer fallows are relatively ineffective against foxtail because the seeds enter secondary dormancy in hot weather.  Avoid moldboard plowing in late summer and fall after seed-drop when most seeds will be dormant. 

Foxtails form a thin thread of tissue that feeds energy from the seed into the base of the shoot.  This can be easily broken by a rotary hoe or tine weeder just before or shortly after emergence.  Once crown roots form, however, plants are much more difficult to kill.  Because green and yellow foxtail elongate rapidly, even young plants are difficult to bury during cultivation, so inter-row cultivation should throw soil into the row as soon as the crop can tolerate it.  Foxtails re-root readily if the soil is moist. 

Giant foxtail is well-adapted to no-tillage cropping systems because of its capacity to germinate at the soil surface, particularly in the presence of crop residue which can provide a moist microclimate for establishment (Buhler and Mester 1991, Mester and Buhler 1991).  This species is particularly adapted to germination and establishment in organic roll-kill systems (Teasdale and Mirsky 2015), so maintaining a low giant foxtail seedbank is important for the success of these systems.  Organic mulch is relatively ineffective for suppressing of yellow and green foxtail because of these species' ability to elongate and grow vertically when shaded while depending on substantial food reserves in the seed.  Giant foxtail is more susceptible to mulching, but can thrive where gaps in the mulch or sparse soil coverage occur.  Even if emergence is not suppressed by mulch, growth of foxtail species can be delayed and reduced by mulches, thereby giving crops a competitive advantage (Williams et al. 1998).  

Foxtails are susceptible to crop competition and especially shade.  Winter grains suppress all three species well due to an early head start in the spring.  Oats and spring barley will establish at lower temperatures than any of the foxtails, so early planting gives these crops a head start and a competitive advantage.

When competing with yellow foxtail, corn yield losses tend to decrease as nitrogen fertility improves, even though the foxtail becomes more vigorous.  This apparently occurs because N stimulates deep penetration of corn roots which allows corn to access water unavailable to the foxtail during late season dry periods (Staniforth 1957).  Giant foxtail, however, continues to increase in size at compost rates above those that maximize corn yield (Little et al. 2015).

Because the foxtails usually do not set seed until after harvest of both winter and spring sown grain crops, clean-up of fields after harvest of these crops can prevent seed set and interrupt the species' life cycle (Kegode et al. 1999).  Clean up after early harvested vegetables is similarly useful.  Consequently, crops that allow mid-summer harvest should be planned into the rotation if foxtails are a problem.  Green foxtail is considered a suitable species for management with weed seed harvesting equipment in the Canadian prairies (Burton et al. 2016, 2017). Electrocution with a Weed ZapperTM controlled most giant foxtail plants that were reproductive and growing above a crop canopy, while it reduced viability of the majority of seeds present
during treatment (Schreier et al. 2022).

Ecology

Origin and distribution:  Giant foxtail originated in eastern Asia and now occupies most of the eastern two-thirds of North America.  It is particularly a problem in the corn belt.  Green foxtail is native to Europe and is now widespread in the temperate regions of the world.  It is most troublesome in the northern Midwest.  Yellow foxtail is native to Eurasia, and occurs throughout Asia, Europe, North America, and the wetter parts of Australia.  It also occurs in southern Africa, the Caribbean, and the Andean countries of South America (Holm et al. 1997). 

Seed weight:  Giant foxtail, 1.5 mg (Mohler et al. 2016), 1.6 mg (Harbur and Owen 2004, Teasdale unpublished), 1.8 mg (Shergill et al. 2020); green foxtail, 0.6-1.5 mg (Dawson and Bruns 1962, Gaba et al. 2019, Mohler unpublished, Stevens 1932, Zhao et al. 2011); yellow foxtail, 1.9-4.2 mg (Dawson and Bruns 1962, Steel et al. 1983, Stoller and Wax 1973, Zhao et al. 2011).

Dormancy and germination:  Newly dispersed seeds are usually completely dormant (Hu et al. 2018, Steel et al. 1983), though some varieties of green foxtail apparently produce non-dormant seeds (Douglas et al. 1985).  Seeds after-ripen and lose dormancy most quickly when exposed to cool, moist conditions (Douglas et al. 1985, Hu et al. 2018, Steel et al. 1983).   Consequently, many are ready to germinate when weather warms in the spring.  Significant germination begins at temperatures in the range of 50-59 °F (10-15 °C).  The range of temperatures suitable for germination widens as the seeds after-ripen.  Thus, fully after-ripened seeds may germinate well at 68-95 °F (20-35 °C), but such temperatures will often maintain dormancy in seeds that have not fully after-ripened (Douglas et al. 1985).  High temperatures greater than 86 °F (30 °C) frequently induce secondary dormancy (Dekker 2003).  Thus, foxtail seeds commonly cycle out of dormancy in winter and into dormancy in the summer (Masin et al. 2006).  Day-night alternation in temperature promotes germination (Douglas et al. 1985, Steel et al. 1983), but light usually does not (Dekker 2003, Steel et al. 1983).  

Seed longevity:  When deeply buried in undisturbed soil, a few seeds will last for 10-39 years (yellow – Dawson and Bruns 1975, Kivilaan and Bandurski 1973, Toole and Brown 1946; green – Burnside et al. 1981, Dawson and Bruns 1975, Thomas et al. 1986, Toole and Brown 1946).  In agricultural situations, however, the seeds die rapidly.  In Saskatchewan, the great majority of green foxtail seeds died or emerged during the first one to two years (Chepil 1946, Thomas et al. 1986), with a similar result for giant foxtail in a multi-state experiment in the mid-western U.S. (Davis et al. 2005).  In a cropped field with simulated tillage, very few giant foxtail seeds survived two years and none survived for three (Buhler and Hartzler 2001).  Annual seed mortality during the first year in these experiments ranged from 83 to 93%.  In New York, annual mortality rates of giant foxtail seeds buried 6” (15 cm) ranged from 72 to 86% (Mohler et al. 2018).  No green or yellow foxtail seeds buried at 1.7” (4.3 cm) under turf grass survived longer than three years (Masin et al. 2006).

Season of emergence:  Most emergence occurs mid-spring to early summer, but a few plants continue to emerge throughout the growing season (Chepil 1946, Stoller and Wax 1973, Steel et al. 1983, Douglas et al. 1985, Doll 2002, Myers et al. 2004).  A novel phenological calendar developed in Ohio indicated that giant foxtail was 25% emerged when red chokeberry was in first bloom and 80% emerged when multiflora rose was in full bloom; this calendar performed better than emergence models based on accumulated temperature units (Cardina et al. 2007). 

Emergence depth:  Considering the difference in seed size between the three species, their ability to emerge from various depths in the soil is remarkably similar.  Emergence tends to be best with shallow burial (0.5”), but emergence is still high down to 2” (5 cm).  Emergence from deeper in the soil is poor and is negligible from depths > 4” (10 cm).   (Giant – Mester and Buhler 1991; Fausey and Renner 1997; green – Dawson and Bruns 1962; yellow – Dawson and Bruns 1962, Stoller and Wax 1973, Gregg 1973).

 Photosynthetic pathway:  All three species have the C4 pathway (Elmore and Paul 1983).

Sensitivity to frost:  Seedlings are frost sensitive; when exposed to mild frosts, they die or show substantial damage from which they may recover (Gregg 1973).  Most adult plants are already dying by the first frost in autumn.

Drought tolerance:  Yellow foxtail recovers rapidly from drought periods of over one month, even when the plants are young (Gregg 1973).

Mycorrhiza:  Green foxtail roots have been observed with mycorrhizae (Harley and Harley 1987, Pendleton and Smith 1983).  Giant and yellow foxtail are considered weak hosts to mycorrhizal fungi (Vatovec et al. 2005).

Response to fertility:  Yellow foxtail tolerates infertile soil, but responds to fertility with profuse growth and high seed production (Gregg 1973).  It grows on soils with a pH from 6.1-8.0.  Giant foxtail tends to perform more poorly than other foxtails on nitrogen deficient soil (Schreiber 1977).  In contrast, its growth continues to increase up to compost rates that supply 320 lb N/A and 250 lb P2O5/A (360 kg N/A and 280 kg P2O5/A) (Little et al. 2015).   Giant foxtail tends to out-compete yellow foxtail on highly fertile soils (Gregg 1973).  Shoot growth of green foxtail increases greatly up to application rates of 71 lb N/A (80 kg N/ha) and increases only slightly more with higher rates, whereas root growth increases steadily up to application rates of 437 lb N/A (480 kg/ha) (Blackshaw et al. 2003).  In contrast, shoot growth of green foxtail responds greatly to P, but response of roots to P is slight (Blackshaw et al. 2004).

Soil physical requirements:  All the foxtails will grow on a wide range of soil textures, but green and giant do best on sandy to loamy soils (Douglas et al. 1985, CDFA).  Yellow foxtail grows on soil types ranging from clay to river gravel and can tolerate poor drainage (Steel 1983).  In a compaction study, giant foxtail produced a dense, vigorous stand of plants on heavily compacted soil (Gregg 1973).  In the same study, yellow foxtail emerged poorly on the compacted soil, but plants that did establish grew well.  In another study, green foxtail emerged better from compacted soil in one of two years (Boyd and Van Acker 2004)

Response to shade:  Foxtail growth and seed production declines with increased shading by neighboring plants (Dekker 2003, Knake 1972), and giant foxtail is completely suppressed by 98% shade, the amount produced by a high corn population (Knake 1972).  Moderate shading, however, causes plants to grow taller, though with less tillering.  This helps them penetrate into and through the crop leaf canopy.

Sensitivity to disturbance:  Yellow foxtail cut at 2” (5 cm) form many new tillers and short seed stalks, but close grazing of grain stubble by sheep can prevent seed production (Steel et al. 1983).  Uprooting or burial readily kills young foxtail seedlings (Mohler et al. 2016).  Larger plants are also susceptible to hoeing or cultivation, especially shallow cultivation that severs the roots from the shoot (Mohler, personal observation).  New shoots cannot sprout from the roots alone.  However, giant and yellow foxtail shoots and even single tillers will readily root into moist soil (Dekker 2004). 

Time from emergence to reproduction:  Plants emerging in the spring usually begin flowering in July as day length decreases.  Since flowering is triggered by shortening day length, the time from emergence to flowering varies from a few weeks for late emerging plants to three months for early emerging plants (Dekker 2003, Doll 2002).  The first seeds mature about two weeks after flowering (Douglas et al. 1985), but seeds on a head continue to mature and disperse over a 2 to 3 week period (Gregg 1973).

Pollination:  Foxtails normally self-pollinate, but occasionally cross-pollinate by wind (Dekker 2003).

Reproduction:  Flowering usually begins mid-summer for spring germinating plants and plants often continue to flower and set seed until the weather becomes cold.  Very stressed plants may produce only a single seed (Dekker 2003).  In corn and soybean crops in Minnesota, single large inflorescences of giant, green, and yellow foxtail produced about 1,500, 1,500, and 250 seeds each, but averages for the three species were 246, 242, and 52 each (Forcella et al. 2000).  In corn and soybeans, giant foxtail production ranged from 100 to 2,500 seeds per plant depending on experimental treatments (Davis and Liebman 2003, Fausey et al. 1997), while production of 4,000 and 16,000 seeds per plant were observed in soybeans in favorable years (Heggenstaller and Liebman 2006, Hill et al. 2016).  Typically, only 5% or less of green foxtail seeds shattered by the time of field pea, spring wheat, and canola harvest in Saskatchewan (Burton et al. 2016, 2017).  However, at seven site-years in the northcentral and mid-Atlantic U.S.A., over 50% of giant foxtail seeds shattered by soybean harvest at most sites (Schwartz-Lazaro et al. 2021).

Dispersal:  The foxtails spread in contaminated seed grain (Holm et al. 1997, Dewey et al. 1985).  All three species occur in cattle manure (Mt. Pleasant and Schlather 1994).  The ability to survive unharmed in the ruminant gut implies that the seeds can move with the animals when they are sold.  Birds also disperse foxtails (Dekker 2003).  All three species disperse in irrigation water (Kelly and Bruns 1975, Wilson 1980).  Seeds also probably disperse in soil adhering to tires and machinery.

Common natural enemies:  Field crickets were apparently responsible for high consumption rates of giant foxtail seeds (58%/day) in wheat underseeded with red clover in Iowa (Davis and Liebman 2003).  Rodents caused significant over-winter losses of giant foxtail seeds on the soil surface, ranging from 31 to 91% (Williams et al. 2009).

Palatability:  People occasionally gather and eat seeds of green and yellow foxtail.  Both species have been domesticated as grain crops (green – foxtail millet; yellow – korali) (Dekker 2003, DeWet 1995), though foxtail millet (Setaria italica) is no longer fully interfertile with green foxtail (Li et al. 1945).  Young foxtail plants have good forage value, but plants become unpalatable to grazers as they mature due to increased fiber and decreased protein content (Steel et al. 1983).  Yellow and green foxtail were found to be palatable, but giant foxtail unpalatable to sheep (Marten and Anderson 1975).  The bristles of mature seed heads damage the mouths of cattle and horses (Steel et al. 1983, Turnquist et al. 2001).

Notes:  Yellow foxtail apparently produces allelopathic toxins: water extracts of the species inhibit germination of alfalfa, cabbage, radish and soybean but not several other crops (Steel et al. 1983).

References:

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