Panicum miliaceum L.

Identification

Other common names: proso millet, proso, millet, hog millet, broom corn millet, broom corn, panic millet 

Family: Grass family, Poaceae

Habit: Tall, upright annual grass

Description: Seedlings are upright. The leaves are rolled in the bud, lack auricles and are densely covered in short, stiff hairs. The ligule is a fringe of hairs 0.08–0.16 inch long that are fused at the base. The seed remains attached to the seedling at the root. Mature plants reach 1–4.25 feet tall. Stems are upright or occasionally nodding. The collar is pale green or white and partially encloses the stem. The sheath is open and densely hairy. The ligule is similar to that of the seedling. Leaves are 4–12 inches long by 0.25–1 inch wide, with a conspicuous, strongly ridged pale green or white midvein. The leaves are sparsely to densely covered in long hairs (or occasionally are hairless). The roots are fibrous. The inflorescence is a 3–12 inches long, terminal, upright to nodding, branching panicle that remains partially enclosed in the leaf. Panicle branches are smooth to rough. Spikelets occur singly at the end of branches and are 0.18–0.21 inch long, smooth and have prominent green veins. Each spikelet produces a single seed, which is shiny, smooth, white to reddish-brown or black and 0.13 inch long by 0.1 inch wide. The apparent seed includes a thin, tight covering of fruit tissue.

Similar species: Witchgrass (Panicum capillare L.) and fall panicum (Panicum dichotomiflorum Michx.) have similar habits and inflorescences to wild-proso millet. Witchgrass is shorter (8–36 inches) and has smaller spikelets (0.08–0.13 inch long) than wild-proso millet. Fall panicum is hairless at maturity, and the seedling leaves have hair only on the underside.

Note: Wild-proso millet populations vary greatly in the degree to which they depart from the domesticated crop and behave as weeds. In general, black seeded populations tend to be weedier than populations with light colored seeds.

Management

Crop rotation is an important component of wild-proso millet management because seed banks of this species are rapidly depleted without substantial annual input of seeds to the soil. Any plants that emerge in alfalfa will be mowed before they can produce seeds, and most of the relatively short-lived seeds will die during the sod phase of the rotation. For example, a Wisconsin study found that four years of alfalfa reduced seedlings emerging in a following corn crop by 80%. Winter wheat is very competitive against wild-proso millet and will usually be harvested before the weed can go to seed. Spring cereals also suppress this weed well since they become well established by the time it emerges, and, in principle, a large proportion of the seeds produced can be captured or destroyed during combine harvest. In contrast, corn, soybeans and dry beans are poor competitors with wild-proso millet. Late spring planting of summer crops, however, can reduce wild-proso millet density relative to earlier planting dates. Crop-like biotypes are more susceptible to competitive stress than are dark-seeded biotypes.

Avoid fall tillage if possible: The relatively large seeds are highly attractive to a wide range of seed predators and are also more susceptible to other forms of mortality when near the soil surface. Even the soil disturbance associated with no-till drilling of cover crops may be sufficient to protect the seeds. Deep tillage in the spring, however, will reduce seedling emergence following a season with heavy seed rain, and since the annual death rate of seeds is high even deep in the soil, relatively few seeds will return to within emergence depth following tillage in subsequent years.

Rotary hoeing can kill a substantial proportion of the first flush of seedlings. Time the hoeing for just before the seedlings emerge, about seven to 10 days after last tillage. Some seedlings will emerge from below the depth of hoeing. Configure tine weeders to bury these just after emergence. The shoot will resprout if broken above the seed, so burying seedlings is preferable to breaking them. Inter-row cultivation of row crops often catches the young wild-proso millet after most have emerged and can provide up to 95% control. Completely covering seedlings with 0.8 inch of soil kills them, so, if possible, hill crop rows while seedlings are still small.

Wild-proso millet is still spreading to new fields in most of its range and preventing new infestations, particularly of the black seeded biotypes, is essential. Till and harvest infested fields last since tillage equipment, combines and forage harvesters are major means of spread. Similarly, clean machinery before moving from an infested to a clean field. Pull up new outbreaks, and if the plants have flowered, remove them from the field.

Ecology

Origin and distribution: The species is native to Eurasia and was one of the first domesticated grain crops. The first black seeded biotype in North America was found in Minnesota and Wisconsin in 1970 and was probably introduced from Europe or Asia. Additional, more crop-like weedy biotypes appear to arise spontaneously by mutation from domesticated proso millet. Wild-proso millet now occurs in most of the United States and southern Canada but is most problematic from southern Ontario through the Midwest to the Northwest. In addition to temperate North America, it occurs in Mediterranean Europe and the Middle East, south Asia, Japan, Australia, New Zealand and South Africa.

Seed weight: Population mean seed weight ranges from 4–7 mg. 

Dormancy and germination: Seed dormancy at maturity varies with biotype. In contrast with domesticated proso millet, which had 100% germination at maturity, three wild biotypes had 51–87% germination even though seeds had higher than 99% viability. In another study, only 22% of seeds were capable of germination 10 weeks after appearance of the inflorescence. Biotypes with dark colored seeds take up water more slowly and germinate more slowly than those with white or golden colored seeds. Domesticated proso millet germinates at temperatures of 50–108°F, with the optimum being 68–86°F. The lower threshold for wild-proso millet germination is 45°F. Under field conditions, seedlings begin emerging the week after day/night temperatures exceed 77/49°F. Dormancy is overcome by moist, cold temperatures of 41°F and the absence of light during germination.

Seed longevity: Seeds from light seeded, crop-like populations averaged only 4% survival over the winter, though seeds of one population had 13–40% survival depending on burial depth. None of the crop-like seeds survived a second winter. In contrast, black seeded populations had greater than 70% overwinter survival at 2 inches and most had greater than 90% survival at 8 inches. In Canada, survival of black seeded populations buried at 8 inches for 11 months averaged 40%, and very few seeds survived for 42 months. Another study found an average annual seed mortality of 39% over a 54 month period. This figure is close to the 42% mortality rate from a one-year study. Seeds survived better in a moderately drained soil than in a well-drained sand or a poorly drained clay loam. Seed predators rapidly remove seeds from the soil surface. Although deep burial at 6–8 inches promotes seed survival relative to more shallow burial at 2 inches, the increase in survival is small relative to many other weed species.

Season of emergence: In Ontario, Wisconsin and Colorado, most seedlings emerge in late May and June, with a few continuing to emerge later in the summer. 

Emergence depth: The average depth of emergence is 1–2 inches, and the maximum depth from which seedlings can emerge is 5.4 inches. Emergence from the soil surface is minimal.

Photosynthetic pathway: C4

Sensitivity to frost: The species tolerates cold better than most C4 plants but will not tolerate frost.

Drought tolerance: Domesticated proso millet is possibly the most drought tolerant of all cereals, despite a shallow root system, and wild-proso millet is similarly drought tolerant. Young wild-proso millet can tolerate at least two weeks of drought, and many plants survive such droughts even when raked half out of the soil. 

Mycorrhiza: No reports are available, but it probably is mycorrhizal based on the mycorrhizal status of other Panicum species.

Response to fertility: No information is available on the response of weed biotypes to nutrient applications. Domestic proso millet is less responsive to fertility than most crops. The recommended N application of 36 pounds per acre only resulted in a 6–40% increase in dry weight. The species does best on soils with a pH of 5.8–6.8.

Soil physical requirements: The species is well adapted to medium and fine textured soils but does poorly on course textured soils. It does not tolerate anaerobic or saline soils.

Response to shade: Experiments in Washington and Illinois found a negative correlation between wild-proso millet seed production and the percentage of light intercepted by sweet corn canopies. Nevertheless, wild-proso millet is moderately shade tolerant. Dry weight was reduced only about 25% with 47% shade. With 90% shade, however, dry weight was reduced about 80%, but this reduction is less than for many other weeds. Plants grew substantially taller but thinner when shaded, which potentially allows them to grow through crop canopies. Even with 90% shade, plants still produced seeds.

Sensitivity to disturbance: Wild-proso millet is highly resistant to physical damage. Crushing young plants with tractor tires or repeated mowing had little effect on survival or subsequent growth. Many plants even survived rotary tilling or disking. In controlled experiments, even complete removal of either roots (below the seed) or shoots had little effect on survival of seedlings at the three- or six-leaf stage, and even at the one- and two-leaf stages, many plants recovered from severe damage. Only a very few plants, however, recovered from complete burial under 0.8 inch of soil. 

Time from emergence to reproduction: Maturation is more rapid as day length shortens. Flowering occurs in 2.5–4 weeks with a 10- to 12-hour day length, whereas flowering takes 7.5 weeks with a 16-hour day length. Some seeds will germinate as early as four weeks after appearance of the inflorescence, but even after 10 weeks nearly 80% of seeds were still dormant. Dark seeded biotypes mature in six to seven weeks after emergence, whereas many light seeded biotypes take nine weeks.

Pollination: Wild-proso millet is primarily self-pollinating, with out-crossing rates of less than 10%.

Reproduction: Wild-proso millet plants grown without competition produced 69,000–94,000 seeds per plant. Under competitive conditions, seed production of 420–620 seeds per plant were observed. 

Dispersal: Seeds are spread by birds, mammals and farm machinery. Combine harvesters spread the seeds substantial distances within fields and probably between fields as well.

Common natural enemies: Sphacelotheca destruens (head smut) and Ustilago crameri (kernel smut) can substantially damage plants. Birds, rodents and insects eat many seeds.

Palatability: Cultivated proso millet is harvested as grain for human and animal consumption or as fodder, and at least the crop-like, light-seed-color weedy biotypes are similarly palatable. Wild-proso millet can, however, cause poisoning of young sheep and goats.

Weed Characteristics Summary Table

Common groundsel
Growth habitSeed weight (mg)Seed dormancy at sheddingFactors breaking dormancyOptimum temperature for germination (F)Seed mortality in untilled soil (%/year)Seed mortality in tilled soil (%/year)Typical emergence seasonOptimum emergence depth (inches)
tall3.8–7.2Variablecms68–8610–60 (black seed)
60–96 (light seed)
nalate spring1–2
Photosynthesis typeFrost toleranceDrought toleranceMycorrhizaResponse to nutrientsEmergence to flowering (weeks)Flowering to viable seed (weeks)Pollination Typical & high seed production (seeds per plant)
C4lowhighprobablymoderate2–44–5self, can cross500 & 90,000

Table Key

General: The designation “–” signifies that data is not available or the category is not applicable.

Growth habit: A two-word description; the first word indicates relative height (tall, medium, short, prostrate) and second word indicates degree of branching (erect, branching, vining).

Seed weight: Range of reported values in units of “mg per seed.”

Seed dormancy at shedding: “Yes” if most seeds are dormant when shed, “Variable” if dormancy is highly variable, “No” if most seeds are not dormant.

Factors breaking dormancy: The principle factors that are reported to break dormancy and facilitate germination. The order of listing does not imply order of importance. Abbreviations are:

scd = seed coat deterioration

cms = a period subjected to cold, moist soil conditions

wst = warm soil temperatures

li = light

at = alternating day-night temperatures

ni = nitrates

Optimum temperature range for germination: Temperature (Fahrenheit) range that provides for optimum germination of non-dormant seeds. Germination at lower percentages can occur outside of this range. The dash refers to temperature range, and the slash refers to alternating day/night temperature amplitudes.

Seed mortality in untilled soil: Range of mortality estimates (percentage of seed mortality in one year) for buried seeds in untilled soil. Values were chosen where possible for seeds placed at depths below the emergence depth for the species and left undisturbed until assessment. Mortality primarily represents seed deterioration in soil.

Seed mortality in tilled soil: Range of mortality estimates (percentage of seed mortality in one year) for seeds in tilled soil. Values were chosen for seeds placed within the tillage depth and subjected to at least annual tillage events. Seed losses are the result of dormancy-breaking cues induced by tillage, germination and deterioration of un-germinated seeds.

Typical emergence season: Time of year when most emergence occurs in the typical regions of occurrence for each weed. Some emergence may occur outside of this range.

Optimum emergence depth: Soil depths (in inches below the soil surface) from which most seedlings emerge. Lower rates of emergence usually will occur at depths just above or just below this range.

Photosynthesis type: Codes “C3” or “C4” refer to the metabolic pathway for fixing carbon dioxide during photosynthesis. Generally, C3 plants function better in cooler seasons or environments and C4 plants function better in warmer seasons or environments.

Frost tolerance: Relative tolerance of plants to freezing temperatures (high, moderate, low).

Drought tolerance: Relative tolerance of plants to drought (high, moderate, low).

Mycorrhiza: Presence of mycorrhizal fungi. “Yes” if present; “no” if documented not to be present, “unclear” if there are reports of both presence and absence; “variable” if the weed can function either with or without, depending on the soil environment.

Response to nutrients: Relative plant growth response to the nutrient content of soil, primarily N, P, K (high, moderate, low).

Emergence to flowering: Length of time (weeks) after emergence for plants to begin flowering given typical emergence in the region of occurrence. For species emerging in fall, “emergence to flowering” means time from resumption of growth in spring to first flowering.

Flowering to viable seed: Length of time (weeks) after flowering for seeds to become viable.

Pollination: “Self” refers to species that exclusively self-pollinate, “cross” refers to species that exclusively cross-pollinate, “self, can cross” refer to species that primarily self-pollinate, but also cross-pollinate at a low rate, and “both” refers to species that both self-pollinate and cross-pollinate at relatively similar rates.

Typical and high seed production potential: The first value is seed production (seeds per plant) under typical conditions with crop and weed competition. The second value, high seed production, refers to conditions of low density without crop competition. Numbers are rounded off to a magnitude that is representative of often highly variable reported values.

Further Reading

Bough, M., J.C. Colosi and P.B. Cavers. 1986. The major weedy biotypes of proso millet (Panicum miliaceum) in Canada. Canadian Journal of Botany 64: 1188–1198.

Carpenter, J.L., and H.L. Hopen. 1985. A comparison of the biology of wild and cultivated proso millet (Panicum miliaceum). Weed Science 33: 795–799.

Cavers, P.B., and M. Kane. 1990. Responses of proso millet (Panicum miliaceum) seedlings to mechanical damage and/or drought treatments. Weed Technology 4: 425–432.

McCanny, S.J., and P.B. Cavers. 1988. Spread of proso millet (Panicum miliaceum L.) in Ontario, Canada. II. Dispersal by combines. Weed Research 28: 67–72.

Patterson, D.T., A.E. Russell, D.A. Mortensen, R.D. Coffin and E.P. Flint. 1986. Effects of temperature and photoperiod on Texas panicum (Panicum texanum) and wild-proso millet (Panicum miliaceum). Weed Science 34: 876–882.