The biology of Zea mays L. (maize)

Biology document BIO2020-01: A companion document to Directive 94-08 (Dir94-08), Assessment Criteria for Determining Environmental Safety of Plant with Novel Traits.

This document replaces BIO1994-11 (The Biology of Zea mays (L.) (Maize)).

On this page

• 1. General administrative information
  • 1.1 Background
  • 1.2 Scope
• 2. Identity
  • 2.1 Name(s)
  • 2.2 Family
  • 2.3 Synonym(s)
  • 2.4 Common name(s)
  • 2.5 Taxonomy and genetics
  • 2.6 General description
• 3. Geographical distribution
  • 3.1 Origin and history of introduction
  • 3.2 Native range
  • 3.3 Introduced range
  • 3.4 Potential range in North America
  • 3.5 Habitat
• 4. Biology
  • 4.1 Reproductive biology
  • 4.2 Breeding and seed production
  • 4.3 Cultivation and use as a crop
  • 4.4 Gene flow during commercial seed and biomass production
  • 4.5 Cultivated Zea mays as a volunteer weed
    • 4.5.1 Cultural/mechanical control
    • 4.5.2 Chemical control
    • 4.5.3 Integrated weed management
    • 4.5.4 Biological control
  • 4.6 Means of movement and dispersal
• 5. Related species of Zea mays
  • 5.1 Inter-species/genus hybridization
  • 5.2 Potential for introgression of genetic information from Zea mays into relatives
  • 5.3 Summary of the ecology of relatives of Zea mays
• 6. Potential interaction of Zea mays with other life forms
• 7. References

1. General administrative information

1.1 Background

The Canadian Food Inspection Agency's (CFIA) Plant and Biotechnology Risk Assessment (PBRA) unit is responsible for assessing the potential risk to the environment from the release of plants with novel traits (PNTs) into the Canadian environment.

Risk assessments conducted by the PBRA unit require biological information about the plant species being assessed. Therefore, these assessments can be done in conjunction with species-specific biology documents that provide the necessary biological information. When a PNT is assessed, these biology documents serve as companion documents to Dir94-08: Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits.

1.2 Scope

This document is intended to provide background information on the biology of Zea mays, including

• its identity
• geographical distribution
• reproductive biology
• related species
• the potential for gene introgression from mays into relatives
• details of the life forms with which it has the potential to interact

Such information will be used during risk assessments conducted by the PBRA unit. Specifically, it may be used to characterize the potential risk from the release of the plant into the Canadian environment with regard to

• weediness/invasiveness
• gene flow
• plant pest properties
• impacts on other organisms
• impact on biodiversity

2. Identity

2.1 Name(s)

Zea mays L. ^{Footnote 151}

2.2 Family

Poaceae (Gramineae) family, commonly known as the grass family^{Footnote 151} .

2.3 Synonym(s)

None^{Footnote 87 Footnote 151}.

2.4 Common name(s)

Zea mays is commonly referred to as Indian corn, corn, maize, maïs, blé d'Inde^{Footnote 27 Footnote 151}

2.5 Taxonomy and genetics

The genus Zea is a member of the tribe Andropogoneae of the grass family (Poaceae / Gramineae) ^{Footnote 151}. Andropogoneae comprises approximately 90 genera and over 1000 species, including sugarcane (Saccharum officinarum) and sorghum (Sorghum bicolor) ^{Footnote 151 Footnote 160}. There are 5 recognized species in the genus Zea, namely:

• Zea diploperennis
• Zea luxurians
• Zea nicaraguensis
• Zea perennis
• Zea mays

Zea mays is subdivided into 4 subspecies, including:

• m. subsp. huehuetenangensis
• m. subsp. mexicana
• m. subsp. parviglumis
• m. subsp. mays^{Footnote 113 Footnote 151}

Z. mays is diploid (2n = 20) ^{Footnote 113}.

2.6 General description

Corn is an erect annual grass which typically reaches 2 to 3 meters (m) in height and occasionally grows as tall as 7 m^{Footnote 44 Footnote 54 Footnote 157}. The plant possesses a single main culm with nodes and internodes and, depending upon genetic background and plant population density, will occasionally possess 1 to 2 lateral branches known as tillers in the lower leaf axils^{Footnote 44 Footnote 157}. Nodes gradually taper towards the top of the plant^Footnote 6.

Leaves of corn are broad and arranged in two vertical rows on opposite sides of the stalk. Leaf blades possess parallel veins with a prominent mid-rib. The upper surface of the leaf blade is hairy, while the lower surface is hairless. There is considerable variation in the number, size, and orientation of leaves amongst corn varieties, with temperate corn hybrids producing an average of 15 leaves and tropical hybrids producing up to 48 leaves^{Footnote 44 Footnote 54 Footnote 157}. The leaves and stalks of the plant are generally green; however, the accumulation of anthocyanins may result in a purple or reddish-brown colour.

Corn is a monoecious plant with imperfect flowers (that is, separate male and female flowers on the same plant). The male flower is called the tassel and the female flower is called the ear. The ear initial is covered in specialized leaves known as husks. Husks differ in appearance from the leaves on the stalk, with the function of surrounding and protecting the developing ear^{Footnote 141}. The cob consists of a robust rachi, with 4 to 30 rows of florets, each containing a single ovule. From each floret, a style will elongate towards the tip of the cob, forming long strands of tissue known as silks. The silks are covered by short hairs used to capture pollen grains^Footnote 81. Silk colour can vary from green to red, and following mechanical damage, silk tissues may turn brown due to oxidation of phenolic compounds^Footnote 93. Tassels are prominent branch structures located at the top of the corn plant and consist of a central spike and a variable number of lateral branches. Anthers can be yellow, purple, or red. After fertilization, the each ovule will develop into a kernel.

Grain and cob colour often vary between varieties. Cob colour can be white, red, purple, or brown. The pericarp (seed coat) of the kernel can be colourless, red, purple, or brown. The aleurone (outside layer of the endosperm tissue) can be blue, purple, or brown due to anthocyanins, white due to the lack of anthocyanins and carotenoids, or yellow or orange due to the presence of carotenoids^Footnote 93. Unique physicochemical characteristics produce a range of corn types including dent, flint, flour, pop, and sweet corn^{Footnote 137} (see Table: "Characteristics of corn types").

The root system of corn is seasonal with brace roots that start 2 or 3 joints above the ground and main roots that descend up to 2 m vertically and 1.2 m horizontally^{Footnote 44 Footnote 157}. Brace roots range in colour from green to red due to the presence of anthocyanins.

3. Geographical distribution

3.1 Origin and history of introduction

Corn was domesticated from the teosinte Z. mays subsp. parviglumis approximately 9,000 years ago^{Footnote 50 Footnote 107}, likely in the Balsas region of southwest Mexico^{Footnote 129 Footnote 134}. The earliest archaeological evidence of corn was found in the Tehuacan Valley, Mexico, and is estimated to be ~8,700 years old^{Footnote 129}. Following domestication, corn spread throughout the Americas, cultivated by Indigenous peoples, which led to the establishment of open-pollinated ancestral landraces^Footnote 66.

Modern corn in the United States and Canada traces its origins to 2 of these landraces, the Northern Flints and the Southern Dents, which hybridized to form what is now referred to as the Corn Belt Dents^{Footnote 49 Footnote 157}. By the mid to late 1800s, open-pollinated varieties of corn developed from the Corn Belt Dents were cultivated. By the early 1900s, the inbred-hybrid concept had been proposed which led to greater uniformity of the varieties without loss of vigour, and is generally regarded as the beginnings of the hybrid corn era. By the 1940s, most corn acreage was hybrid varieties, rather than open-pollinated varieties^{Footnote 49 Footnote 74 Footnote 127}.

3.2 Native range

North America Mexico (Chihuahua, Durango, Guanajuato, Jalisco, Michoacán)^{Footnote 151}

South America Guatemala^{Footnote 151}

3.3 Introduced range

Corn is grown worldwide. The following 20 countries were the highest corn producing countries in 2017^Footnote 57.

Asia China, India, Indonesia, Philippines

Africa Egypt, Nigeria, South Africa, Ethiopia, Tanzania

North America Canada, Mexico, United States

South America Argentina, Brazil

Europe France, Hungary, Italy, Romania, Russian Federation, Ukraine

3.4 Potential range in North America

Corn possesses a wide potential distribution in Canada that includes all provinces and may continue to expand northward as temperatures increase and new varieties of corn are produced. Currently, field corn is grown in every province in Canada, with Ontario, Quebec, and Manitoba producing approximately 63%, 26%, and 9% of corn yield, respectively^{Footnote 146}. In the United States, corn is cultivated in every state for which there is survey data^{Footnote 153}. Iowa and Illinois account for nearly one-third of the national acreage^{Footnote 152}.

3.5 Habitat

Corn is a C4 grass best grown in warm, sub-humid climates with high rainfall (600 to 1500 millimeters) and long photoperiods^{Footnote 6 Footnote 44 Footnote 112 Footnote 132}. Corn can be cultivated in tropical and warm temperate zones and is mostly grown between 40 degrees south in Chile and Argentina and 58 degrees north in Canada and Russia, at varying altitudes^{Footnote 6 Footnote 122}.

Corn is adapted to a wide variety of soil types ranging from sand to heavy clay, though corn performs best on well-structured soils with intermediate texture (sandy loams to clay loams)^{Footnote 112}. Typically, corn is grown on slightly acidic to neutral soil (pH 5.5-7)^{Footnote 6 Footnote 44}. Soil temperatures of at least 10 degrees Celsius (°C) are required for germination and emergence, while field temperatures of at least 18°C are optimal for germination^{Footnote 44 Footnote 86 Footnote 95}. Higher temperatures (30 to 33°C) are required for maximum plant growth^Footnote 14.

Corn is sensitive to drought stress, particularly at the flowering stage; however, it can temporarily tolerate dry conditions^{Footnote 13 Footnote 40 Footnote 112}. Frost can result in the loss of the entire crop ^{Footnote 44 Footnote 163}.

Generally, corn is incapable of sustained reproduction outside of domestic cultivation and is non-invasive of natural habitats, although it can volunteer in cultivated crops or grow in unmanaged fields or disturbed habitats (such as roadsides, areas around grain elevators, and fallow fields)^{Footnote 68 Footnote 113 Footnote 141}.

4. Biology

4.1 Reproductive biology

Corn is a monoecious plant that reproduces naturally by seed. It is predominantly cross-pollinated by wind. Self-pollination is limited, occurring at a rate of approximately 5%, and influenced by the tassel size and environmental conditions at the time of pollen release^{Footnote 9 Footnote 39 Footnote 131}. Corn does not require insect mediated pollination for fertilization and the importance of insects for corn pollination is negligible^{Footnote 162}.

The growth stages of corn consist of 2 phases: vegetative and reproductive^{Footnote 135 Footnote 141}. In the vegetative phase, leaves, roots, stems, and early reproductive structures develop^{Footnote 141}. The above-ground portion of the corn plant develops from the shoot apical meristem^Footnote 88. Leaves are initiated one at a time, with consecutive leaves arising from opposite sides of the shoot apical meristem, giving rise to an alternating leaf pattern. As each leaf is initiated, an axillary meristem is formed at the base of the leaf axil.

Once floral transition occurs (that is, switch from vegetative to reproductive growth), the shoot apical meristem transitions to an inflorescence apical meristem that will generate the tassel^{Footnote 41 Footnote 88}. Tassel development starts with the inflorescence apical meristem producing a series of lateral meristems; the first few are called branch meristems and are responsible for forming the tassel's major branches^Footnote 88. The subsequent lateral meristems (further removed from the vegetative shoot) form spikelet pair meristems, which can also form on branch meristems^{Footnote 88 Footnote 112}. Each spikelet pair meristem produces a short branch with two spikelet meristems, with each spikelet meristem producing a glume primordial and forming 2 floral meristems (upper and lower). Each floral meristem produces the floral organs (palea, 2 lodicules, 3 stamens, 1 pistil) and is supported by the lemma^Footnote 88. The pistil development is arrested and the imperfect male inflorescence results^Footnote 88.

Most axillary meristems will remain dormant; however, some of the lower ones may give rise to tillers and some of the upper ones will give rise to ear initials^{Footnote 88 Footnote 141}. Ears arise from axillary meristems that have transitioned into inflorescence meristems and then developed into ear initials through a reiterative process of meristem initiation and organ differentiation^{Footnote 109 Footnote 159}. Each spikelet pair meristem will initiate a spikelet meristem and itself become a spikelet meristem. Each spikelet meristem will then initiate a floral meristem and itself become a floral meristem. The duration and rate of the reiterative process of spikelet pair meristem initiation determine the number of florets on the mature ear initial, with some evidence that later flowering genotypes develop more florets per row^Footnote 17. Each inflorescence is surrounded by modified sheaths (called husks) that act as a protective covering^{Footnote 44 Footnote 141}. Ears initiate the development of silk from their ovules. Silk development begins near the base of the ear and proceeds towards the tip of the ear over several days^Footnote 81.

Usually, synchrony occurs between pollen shed and silking resulting in high kernel set. However, severe plant stress such as drought can delay silking and reduce pollen grain production^{Footnote 32 Footnote 73 Footnote 112 Footnote 141}. The interval between pollen shed and silk emergence is referred to as the anthesis-silking interval^{Footnote 16 Footnote 141}. Pollen is shed continuously for a week or more, releasing approximately 10 million pollen grains from each corn tassel^{Footnote 9 Footnote 93 Footnote 157}. Pollen can be released easily (for example, in response to vibration or low winds), and disperse short distances of up to 30 meters^Footnote 7. Once released from the tassel, pollen viability declines^Footnote 39. In some cases, horizontal winds allow pollen to disperse several hundred meters^{Footnote 7 Footnote 9}. Pollen grains fall from the tassels (anthesis) onto the silks of the ears, which are coated with sticky hairs that capture the pollen grains^Footnote 81). Pollen grains germinate immediately, extending a pollen tube into the silk strand, transferring genetic material to the female ovule for fertilization to occur.

4.2 Breeding and seed production

Corn breeding has evolved over the past century. Initially, growers used population improvement approaches to improve locally adapted open-pollinated varieties and often saved seed for planting the next year^{Footnote 130}. Modern corn breeding is almost exclusively focused on producing broadly adapted hybrids to be sold to growers. Corn exhibits heterosis or hybrid vigour, meaning that heterozygous hybrid progeny outperform their homozygous parents. Hybrid corn breeding is highly specialized and does not lend itself to saving seed for planting^{Footnote 110 Footnote 165}. Hybrid-based breeding was first used in the early 1900s by breeders in the United States^{Footnote 99 Footnote 127}. Single crosses in hybrid breeding are achieved by crossing a male and a female inbred line (Line A × Line B = A × B). To increase vigour and yield, single crosses were substituted with double crosses, where 4 unrelated inbred lines are used in the production of 2 parent lines, which are then crossed resulting in the double cross hybrid seed (that is, (A × B) × (C × D))^{Footnote 141}.

Conventional breeding has been used to improve traits including yield, tolerance to high plant densities, canopy architecture, stalk and root strength, pest and disease resistance, tolerance to biotic and abiotic stresses, maturity, kernel composition, and nutrient content. Specialty corn varieties include food-grade, sweet corn, popcorn, and silage^{Footnote 6 Footnote 141}. Corn hybrids with superior yield potential have been continuously introduced in Canada over the last 40 years, resulting in annual yield increases of roughly 1.5%^{Footnote 117 Footnote 141}. Several important mutations in corn arose spontaneously:

• the 2 major genes for use in sweet corn, su1 and sh1
• the reduced lignin mutation bm3 for use in silage
• the wx1 mutation that results in >95% amylopectin starch in the kernel
• the ae1 mutation that results in >65% amylose starch in the kernel for use in speciality starches^Footnote 39.

Growers regularly update their hybrid selection to remain competitive. To select hybrids, growers may use results from annual hybrid performance trials^{Footnote 121}, corn hybrid growing strips on their farms, and consideration of the crop heat units required for maturity^{Footnote 117}.

Many methods can be used to prevent self-pollination of the female parent line and to prevent cross-pollination with an undesired male parent line. These methods include manual emasculation (detasseling), application of male-specific gametocides, and the use of cytoplasmic or nuclear-encoded male sterility^{Footnote 128}. Cytoplasmic male-sterility is controlled by genes from both mitochondrial and nuclear genomes, whereas nuclear-encoded male sterility is controlled by nuclear encoded genes alone ^{Footnote 147 Footnote 158 Footnote 168}. Detasseling is the predominant method in commercial hybrid seed production in corn^{Footnote 140}. In breeding nurseries, receptive ear shoots are protected from unwanted pollination by ear shoot bags that cover the silks. Pollen produced from tassels is collected in bags that cover the tassels, often placed on the day before pollination^{Footnote 141}. Controlled hand pollinations are then made by exposing the ear shoot on the selected female parent and covering it with the bag containing pollen from the selected male parent.

Modern biotechnology has been used to introduce novel traits into corn including tolerance to herbicides and insect pests, increased ear biomass, dry-grind corn processing characteristics, improved digestibility, elevated levels of free lysine, and male sterility^Footnote 36. Often, multiple novel traits are combined in the same corn plant to convey tolerance to multiple pests and herbicides.

A male-sterile corn line that does not produce pollen was authorized by CFIA in 1996^Footnote 36. The introduced DNA includes the barnase gene which codes for the production of barnase ribonuclease in the tapetum cell layer of the anther at a specific stage in anther development. The barnase ribonuclease results in male-sterile plants by affecting RNA production, disrupting cell functioning, and arresting early anther development. Another corn line expresses the barstar gene which encodes a barnase ribonuclease inhibitor^{Footnote 158}. Crossing the barnase ribonuclease expressing plants with fertile barstar-expressing plants restores the fertility of F1 hybrids.

Insect tolerant corn expresses insecticidal protein(s) derived from the soil bacterium Bacillus thuringiensis (Bt) and/or ribonucleotides that interfere with and kill corn pests. Early Bt corn products expressing the Cry1Ab protein targeted the European corn borer (Ostrinia nubilalis)^Footnote 33 . Subsequently, other Bt proteins conveying resistance to European corn borer have been authorized, including Cry1A(c), Cry1A.105, and Cry1F. Some Bt proteins convey tolerance to other lepidopteran corn pests including:

• Western bean cutworm (Striacosta albicosta)
• black cutworm (Agrotis ipsilon)
• fall armyworm (Spodoptera frugiperda)
• corn earworm (Helicoverpa zea)
• beet armyworm (Spodoptera exigua)
• true armyworm (Mythimna unipuncta)^Footnote 28

Bt proteins active against the Western and Northern corn rootworms (Diabrotica virgifera virgifera and Diabrotica barberi) include Cry3Bb1^Footnote 34 , Cry34/35Ab1, mCry3a, and eCry3.1Ab. In 2015, corn with a RNA interference mode of action was authorized for unconfined release in Canada targeting corn rootworm^Footnote 35. A list of insect tolerant corn products available in Canada and the pests they target is maintained by the Canadian Corn Pest Coalition^Footnote 28.

Herbicide tolerant corn which allows growers to spray certain broad-spectrum herbicides after corn emergence was first authorized for unconfined release in Canada in 1996. Tolerance to herbicides is introduced by altering enzymes that already exist in corn to no longer be inhibited by the herbicide^{Footnote 4 Footnote 111 Footnote 148}, or to introduce enzymes that break down the herbicide before it can damage the crop. Corn has been modified to express phosphinothricin acetyltransferase (PAT) enzymes, modified acetolactate synthase (ALS) enzymes, or modified 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) enzymes, which convey tolerance to glufosinate-ammonium, imidazolinones, and glyphosate herbicides, respectively. Other herbicide tolerant traits include tolerance to dicamba by incorporating the gene which encodes a DMO enzyme, tolerance to 2,4-D and certain aryloxyphenoxypropionate (fop) herbicides through the introduction of the AAD-1 protein, and tolerance to sethoxydim by altering the ACCase enzyme.

Corn is not covered by the variety registration system in Canada, having been exempted in 1996^Footnote 3. The Canadian corn industry (through the Canadian Seed Trade Association) maintains a database of commercially available corn hybrids^Footnote 3.

4.3 Cultivation and use as a crop

Corn is cultivated worldwide and represents a staple food for a significant proportion of the world's population. Corn grown for grain is the number one cereal crop worldwide with 1.13 billion tons produced in 2017^{Footnote 57 Footnote 113}; Canada was the 13th-highest producer with 14.1 million tons^Footnote 57. According to the 2016 Census of Agriculture^{Footnote 144}, 32,172 farms in Canada planted corn; in total, nearly 1.85 million hectares were planted (including corn for grain, silage, and sweet corn).

The majority of grain corn production occurs in Ontario and Quebec. In Ontario, the estimated seeding areas for grain corn are 891,300 hectares, of which 786,100 hectares are genetically modified. In Quebec, the estimated seeding area for grain corn is 382,500 hectares, of which 350,900 hectares are genetically modified^{Footnote 146}. Seeding rates vary depending on the variety of corn, fertility, and rainfall or irrigation. On soils with slow internal drainage, tillage can significantly increase the rate of soil drying and warming, and subsequently increase the possibility for earlier planting and the fast, uniform emergence of plants. Cultivation practices in corn often result in bare soil, which is susceptible to erosion by wind or water. No-till or other reduced tillage practices are used for some portion of corn production^{Footnote 44 Footnote 117}.

Cultivation of corn requires a well-prepared seedbed. Corn rows are typically planted 38 to 96 centimeter (cm) apart, at seed depths of 4 to 7.5 cm^{Footnote 117}. Planting seeds less than 3 cm deep may lead to less favorable positioning of the growing point and first nodal roots, possibly resulting in rootless corn syndrome and predisposing the seed to herbicide injury. Furthermore, if planted too shallow, corn will be more susceptible to poor root establishment in coarse-textured soils that dry rapidly. Conversely, deeper planting, especially early in the season when soils are cold, can significantly delay emergence compared to planting at depths of 3 to 5 cm^{Footnote 117}.

Corn Heat Units (CHUs) can describe the accumulation of thermal units above a base temperature in a region for a growing season and the heat requirements of a particular corn variety^{Footnote 103}. In Ontario, the average CHUs in a growing season ranges from <2,500 in northern growing regions to ~3500 in southwestern Ontario^{Footnote 117}. Highest yields are obtained from corn planted late-April to mid-May in Ontario^{Footnote 117}.

Corn is commonly grown in rotation with other crops including cereals (typically wheat), soybeans, and dry edible beans^{Footnote 102 Footnote 117}. Crop rotation can help to decrease weeds, break up disease and pest cycles, and increase soil health^{Footnote 106 Footnote 117}. Studies have shown that corn yield increases when grown in rotations^{Footnote 5 Footnote 45 Footnote 126}. Corn-soybean rotations have demonstrated a yield advantage of approximately 10% over corn grown in the absence of crop rotation^{Footnote 117 Footnote 145}. Corn grown for grain or silage, and cultivated in the absence of crop rotation accounted for only 5% and 2% of Canadian production, respectively^{Footnote 145}.

Fertilizer is generally applied to the soil before corn is planted. Nitrogen can also be applied as side-dressing via surface or sub-surface placement near or between the plant rows. Corn responds well to fertilizer; rates depend on seasonal availability of soil moisture, cultivar selection, and environmental conditions^{Footnote 117}. The majority of nitrogen is usually applied in the spring, pre-plant, pre-emergence, or side-dressed before corn is over 30 cm tall. Fall application is not recommended because of the potential for high fertilizer losses^{Footnote 117}.

A Field Crop Protection Guide is published yearly by OMAFRA, recommending federally registered insecticides and fungicides for use in corn^{Footnote 118}. A wide range of herbicides can be used for weed control in corn. A Guide to Weed Control is published yearly by OMAFRA recommending federally registered herbicides for use in corn^{Footnote 116}.

Bt corn is commonly planted prophylactically for insect control^Footnote 72. In Canada, Bt corn hybrids were planted on 3.1 million acres in 2015, representing 85.8% of total corn acres^Footnote 51 . Insect pests developing resistance to Bt proteins expressed in corn has been a concern following the commercial release of Bt corn. Insect resistance management plans have been required by the CFIA to delay the evolution of resistance to Bt proteins by European corn borer and corn rootworms. Resistance by Western corn rootworm to Bt proteins is confirmed to multiple Bt proteins including Cry3Bb1^Footnote 59, mCry3a^Footnote 60 , and Cry34/35Ab1^Footnote 61. One resistant management strategy is to grow a specific percentage of refuge corn that does not express the Bt trait, either interspaced in the Bt corn field or planted in separate fields, or in structured areas within a Bt corn field^Footnote 28.

Common weeds in corn production in Canada include annual broadleaves such as

• giant ragweed (Ambrosia trifida)
• lamb's-quarters (Chenopodium album)
• cocklebur (Xanthium strumarium)
• ragweed (Ambrosia artemisiifolia)
• wild mustard (Sinapis arvensis)
• velvetleaf (Abutilon theophrasti)
• lady's thumb (Persicaria maculosa)
• wild buckwheat (Fallopia convolvulus)
• eastern black nightshade (Solanum ptychanthum)

In addition, annual weedy grasses in corn include

• giant foxtail (Setaria faberi)
• proso millet (Panicum miliaceum)
• fall panicum (Panicum dichotomiflorum)
• barnyard grass (Echinochloa crusgalli)
• green foxtail (Setaria viridis)
• yellow foxtail (Setaria glauca)
• old witch grass (Panicum capillare)
• crabgrass (Digitaria sanguinalis)^{Footnote 117}

Anthesis typically occurs approximately 60 days after emergence, and maturation of the crop is completed in 80 to 120 days, although this varies based on the latitude and environment^{Footnote 112 Footnote 141}. Corn reaches physiological maturity when an abscission zone forms between the kernel and the cob tissue (black layering), usually when grain moisture content is 31 to 33%^{Footnote 117}. Grain moisture content is important in determining the harvest date. Harvesting grain with over 28% moisture may result in poor quality grain; high-quality food-grade grain may need to be harvested when grain moisture is 20 to 22%^{Footnote 2 Footnote 117}. Corn can be dried using 3 types of grain dryers:

• in-bin
• batch
• continuous flow

Natural air drying may be used in warmer regions such as southern Ontario^{Footnote 117}. The drying temperature is dependent on the end-use of the corn, ranging from 45 to 120°C. Nutritional value is diminished once the temperature of corn grains exceeds 90 to 100°C^{Footnote 117 Footnote 141}.

Corn grown in North America is predominantly of the yellow dent type, a commodity crop largely used as animal feed, either as grain or silage, and for ethanol fuel production^{Footnote 2 Footnote 145}. Corn may be processed by wet or dry milling depending on the desired end product. Dry milled products include:

• cereal flakes
• corn flour
• corn grits
• corn-meal
• grits for beer production

Wet milled products include:

• high-fructose corn syrup
• glucose and dextrose
• corn oil
• starch
• beverage alcohol
• industrial alcohol
• fuel ethanol^{Footnote 152}

Corn starch also serves as raw material for industrial manufacturing processes, including thickening, gelling, and binding agents for products like puddings, wallpaper paste, aspirin, chalk, and biodegradable plastics.

4.4 Gene flow during commercial seed and biomass production

Corn is a cross-pollinating species^Footnote 75. Corn pollen is primarily wind-dispersed and is viable for up to two hours, remaining viable longer under high humidity levels^{Footnote 8 Footnote 100}. A single plant may produce up to 2 million pollen grains per day depending on the variety; however, most modern hybrids have substantially smaller tassels resulting in decreased pollen production^Footnote 90.

The outcrossing rate is influenced by wind, the distance between the pollen source and receptor, and whether flowering and pollen shed occurs simultaneously^{Footnote 76 Footnote 97}. Halsey et al.^Footnote 76 observed an outcrossing rate of less than 0.01% when 500 m separated the pollen source and receptor when both crops were flowering at the same time. Bannert and Stamp^Footnote 9 determined that at distances of 50 to 4500 m between pollen donors and receptor fields, the cross-pollination rate in a field was always lower than 0.02%. Temporal separation of 2 weeks in the timing of flowering between corn crops reduced outcrossing; when flowering was separated by 2 weeks, the outcrossing rate was less than 0.01% at 62 m^Footnote 76. A combination of physical and temporal separation (750 m distance and 2 weeks separation between flowering) can achieve 100% genetic purity^Footnote 76. Field trials in Germany indicated that planting tall sunflower crops as a buffer between corn fields does not effectively reduce gene flow between corn crops^Footnote 97.

In Canada, the production of seeds grown for pedigreed status must meet standards specified by the Canadian Seed Growers' Association's Canadian Regulations and Procedures for Pedigreed Seed Crop Production (Circular 6). Border rows are used for crop isolation purposes and are required when the female parent is less than 200 m from the contaminating corn. When using border rows, the spacing between rows should not be less than 40 cm in width and must stay consistent throughout the field. For hybrid seed production, border rows are not required when the seed (female) parent is more than 200 m from the contaminating corn. Hybrid seed production is accomplished by interplanting rows of the male and female inbred parents (that is, 1 row of male to 4 female rows). With border rows of the male parent, the allowable distance from contaminating corn can be reduced^Footnote 31. The minimum isolation distances required for hybrid corn is dependent on the number of pollen (male parent) border rows in a field. For corn inbred lines or single crosses, an isolation distance of 200 m is required between the inspected seed corn crop and a source of potential contamination that is shedding pollen at the same time.

Although corn will hybridize successfully with the subspecies of Zea mays when close to each other^{Footnote 163}, this is not a concern in Canada as no Zea mays subspecies occur in Canada^{Footnote 20 Footnote 65}.

4.5 Cultivated Zea mays as a volunteer weed

Corn is not capable of sustained reproduction outside of domestic cultivation^{Footnote 113}. Human intervention is required to disseminate the seeds beyond the field where corn was previously planted because the seeds (kernels) on the ear are enclosed within a husk. Corn is not listed as a noxious weed in the Weed Seeds Order 2016^Footnote 71 and is not listed as a noxious weed in the United States^{Footnote 153}. Individual kernels distributed along pathways during travel from fields to storage facilities may germinate but are not regarded as serious weeds^{Footnote 113}.

Stalk lodging or breakage, dropped ears, and kernel loss during harvest (for example, via improperly adjusted combines and header shatter losses of dry grain) can result in volunteer corn the following year. Volunteer corn can shade the planted crop, interfere with harvest, and harbour pests^{Footnote 37 Footnote 106}. Individual kernels may overwinter and germinate the following spring, but generally will not persist as a weed^{Footnote 113}. Frost often kills volunteer corn in areas with cold winters^Footnote 2.

Despite the introduction of herbicide tolerant corn hybrids in Canada, herbicide tolerant corn volunteers can be adequately controlled using herbicides with other modes of action and cultural/mechanical controls^{Footnote 117 Footnote 119}.

4.5.1 Cultural/mechanical control

Tillage is commonly used to control volunteer corn^{Footnote 117 Footnote 119}. There are different tillage systems used in Ontario, including conventional tillage, fall mulch tillage, spring mulch tillage, vertical tillage, and fall strip-tillage^{Footnote 117}. Cover crop mulches (such as crimper-rolled rye) can also be used by growers to control weeds or volunteers, although these are not as effective as tillage^{Footnote 119 Footnote 161}. Reducing equipment inefficiencies during harvesting such as properly adjusting combines can reduce the number of volunteers^{Footnote 113 Footnote 141}.

4.5.2 Chemical control

Several herbicides are registered in Canada for volunteer corn control in canola, chickpeas, field peas, flax, beans, lentils, safflower, soybeans, sunflower, industrial hemp, sugarbeet, forage legumes (alfalfa, red clover, and bird's-foot trefoil), rutabaga, potatoes, barley, wheat (spring, durum, and winter), and many more^{Footnote 79 Footnote 120}. The active ingredients contained in the registered products are from

• Group 1 (Fluazifop-P-butyl, Fenoxaprop- p-ethyl, Sethoxydim, Clethodim and Quizalofop-p-ethyl)
• Group 2 (Thifensulfuron methyl, Tribenuron methyl, Imazamox and Imazethapyr)
• Group 4 (MCPA)
• Group 5 (Metribuzin)
• Group 6 (Bromoxynil)
• Group 9 (Glyphosate)
• Group 10 (Glufosinate-ammonium) ^{Footnote 79 Footnote 119}

Group 1 herbicides, including the aryloxyphenoxypropionate (fop) and cyclohexanedione (dim) families, are most often used to control volunteer corn^{Footnote 119}.

Corn containing novel herbicide tolerance traits authorized in Canada as of 2020, include

• Group 1 – corn with tolerance to sethoxydim and "fop" herbicides
• Group 2 – imidazolinone and ALS-inhibiting herbicides
• Group 4 – 2,4-D and dicamba
• Group 9 – glyphosate
• Group 10 – glufosinate-ammonium^Footnote 36

Corn volunteers that are tolerant to herbicides can be controlled using herbicides with other modes of action or by mechanical means.

4.5.3 Integrated weed management

Integrated weed management (IWM) utilizes a combination of biological, cultural, mechanical, and chemical weed control tactics to manage weed populations and increase economic returns. IWM strategies for volunteer corn can include

• the rotation of crops
• rotation of herbicide groups
• planting into weed-free seedbeds
• pre-plant cultivation
• inter-row cultivation
• pre-plant/pre-emergence/post-emergence herbicides^{Footnote 1 Footnote 117}

4.5.4 Biological control

No biological control methods for corn volunteers exist.

4.6 Means of movement and dispersal

Human intervention is required to disperse seeds due to the seeds being enclosed within a protective husk. Seed may be distributed along pathways of travel from field operation to storage facilities during harvesting^{Footnote 44 Footnote 113}. Deer, birds, and raccoons feed on corn crops; however, there is no evidence of significant dispersal by these animals^Footnote 2.

5. Related species of Zea mays

Wild relatives of corn are known as teosintes and include wild taxa from the 5 species in the genus Zea^{Footnote 136}. Corn is sexually compatible with other teosintes and can produce fertile hybrids^{Footnote 113 Footnote 163}. Of the Zea species, only corn is common to Canada and is present only as an agricultural crop^{Footnote 1 Footnote 2 Footnote 20}. Teosinte species are geographically restricted and occur only in México, Guatemala, Honduras, and Nicaragua^{Footnote 136}.

The closest relative of the genus Zea is Tripsacum, a genus of 11 species widely distributed between latitudes 42 degrees north and 24 degrees south^Footnote 46. The species Tripsacum latifolium and Tripsacum fasciculaum are present in Puerto Rico^{Footnote 153}. 3 species occur in the continental United States, namely

• Tripsacum floridanum (Florida gamagrass)
• Tripsacum lanceolatum (Mexican gamagrass)
• Tripsacum dactyloides (Eastern gamagrass)

T. floridanum is confined to Florida and T. lanceolatum is confined to the southern parts of Arizona and New Mexico^{Footnote 153}. Tripsacum dactyloides possesses a wide distribution across the eastern United States that includes Michigan and New York^{Footnote 153}, but it has not been reported in Canada^{Footnote 20 Footnote 22}.

5.1 Inter-species/genus hybridization

Crosses between corn and closely related genera are possible but extremely difficult^{Footnote 105}. Previous research has shown that corn can be crossed with Tripsacum species (T. dactyloides, T. floridanum, T. lanceolatum and T. pilosum); however, only through human intervention and with resulting hybrids possessing a high degree of sterility and genetic instability^{Footnote 56 Footnote 89 Footnote 113}. There is no evidence that hybridization occurs naturally. The first successful hybridization of corn and Tripsacum species was reported in the early 1900s with corn as the female parent^{Footnote 104}. The crossing of 2 relatives of corn Tripsacum dactyloides and Zea diploperennis is possible using a controlled standard pollination technique, with Z. diploperennis as the male parent and T. dactyloides as the female parent. Artificial culture techniques were used to germinate hybrid seeds^{Footnote 55 Footnote 58}. These hybrids were bred for resistance to disease and other pests, drought tolerance, and improved uniformity^{Footnote 55 Footnote 56}.

Hybridization between corn and other species of the Poaeceae family can only be achieved with human intervention^Footnote 77. There are no reports of corn hybridizing naturally with other species of the Poaeceae family. About 6% of crosses between corn and Coix lachryma-jobi produced hybrid seed, only when C. lachryma-jobi was the female parent^Footnote 96. Corn has also been crossed with hexaploid wheat (Triticum aestivum); however, endosperm did not form on the embryo^{Footnote 98 Footnote 167}. Corn has been crossed with hexaploid oat (Avena sativa), with hybrid plants only surviving following embryo rescue^Footnote 96. Crosses have also occurred with barley (Hordeum vulgare) and rye (Secale cereal) with corn as pollen donor; however, no embryos developed in the case of the H. vulgare × corn, and all S. cereal × corn embryos degenerated after 6 to 10 days due to lack of endosperm development^{Footnote 166}.

5.2 Potential for introgression of genetic information from Zea mays into relatives

Relatives of corn do not occur in Canada, although Tripsacum dactyloides is widespread across the eastern United States and found in counties close to Canada, including Niagara county in the state of New York and Macomb and Washtenaw counties in the state of Michigan^{Footnote 153}. T. dactyloides is extremely unlikely to form fertile hybrids with cultivated corn. All teosintes, including Zea mays ssp. mexicana and parviglumis, occur only in México, Guatemala, Honduras, and Nicaragua^{Footnote 136}.

In the absence of sexually compatible species related to corn in Canada, there is no potential for interspecific gene flow.

5.3 Summary of the ecology of relatives of Zea mays

Relatives of corn do not occur in Canada. T. dactyloides is widespread across the eastern United States; however, it is extremely unlikely to form fertile hybrids with cultivated corn^{Footnote 153}. T. dactyloides is not regarded as a noxious weed and is considered an endangered species in Massachusetts and Pennsylvania, a threatened species in New York, and a species of special concern in Rhode Island^{Footnote 153}.

6. Potential interaction of Zea mays with other life forms

The interactions of corn with other life forms in Canada are well characterized, and information is readily available on the management of many diseases and pests of corn.

Corn can host numerous pathogenic species that cause disease and decrease crop yield and quality^{Footnote 125}. The most common diseases of corn in Canada include^Footnote 1:

• anthracnose leaf blight and stalk rot (Colletotrichum graminicola)
• common rust (Puccinia sorghi)
• common smut (Ustilago maydis)
• Gibberella ear rot (Gibberella zeae)
• Northern corn leaf blight (Exserohilum turcicum)
• seed rots and seedling blights (Pythium, Fusarium spp., Diplodia spp., Penicillium spp., Aspergillus spp., and Rhizoconia spp.)
• stalk rots (Gibberella zeae, Fusarium and Diplodia maydis)

The European corn borer (Ostrinia nubilalis) was considered one of the most destructive insect pests of corn in Canada. However, widespread planting of Bt corn has been attributed to the area-wide suppression of European corn borer, causing its status as a primary pest to be questioned^{Footnote 143}. European corn borer is currently distributed in the southern parts of Canada from the Prairie provinces to the Maritimes. Corn rootworm is another important pest of corn in Canada, namely the western corn rootworm (Diabrotica virgifera virgifera) and the northern corn rootworm (Diabrotica barberi). These species feed on corn in the adult and larval stages of development and can be found in Ontario, Quebec, parts of the Atlantic provinces, British Columbia, and Manitoba^{Footnote 11 Footnote 62 Footnote 85}. Another insect pest of corn, western bean cutworm (Striacosta albicosta), was first detected in Ontario in 2008^Footnote 12 following a range expansion over the past century from its native locality in the southwest United States. The damage caused by larval feeding on tassel tissue and the corn ear reduces grain quality and leaves the ear susceptible to ear mold fungi which may produce harmful mycotoxins^{Footnote 123}. Other major insect pests including seed corn maggot (Delia platura), corn earworm (Heliothis zea), and several species of wireworms (Elateridae) are discussed in Agriculture and Agri-Food Canada's (AAFC) Crop Profile for Field Corn in Canada.

Please refer to the tables below for examples of interactions of Zea mays with other life forms during its life cycle:

• Table 1: Fungi
• Table 2: Bacteria
• Table 3: Viruses
• Table 4: Insects
• Table 5: Nematodes
• Table 6: Plants
• Table 7: Animals

Abbreviation list for tables

• BC – British Columbia
• AB – Alberta
• SK – Saskatchewan
• MB – Manitoba
• ON – Ontario
• QC – Quebec
• NB – New Brunswick
• NS – Nova Scotia
• PE- Prince Edward Island
• NL – Newfoundland & Labrador