The vegetative and reproductive growth stages in corn are described in Table 1. Vegetative growth stages in corn and Table 2. Reproductive growth stages in corn.
Table 1. Vegetative Growth Stages in Corn
| Stage | VE | V1 | V4 | V6 | V8 | V12 | VT |
| [insert file | Corn VE.eps | Corn V01.eps | Corn V04.eps | Corn V06.eps | Corn V08.eps | Corn V12.eps | Corn VT.eps |
| Leaf Collars | 0 | 1 | 4 | 6 | 8 | 12 | (varies) |
| Leaf Tips | 1 | 3 | 7 | 10 | 11 | 15 | — |
| Leaf Over | 0 | 2 | 6 | 8 | 10 | 14 | — |
| CHUs Required1 | 180 | 330 | 630 | 780 | 930 | 1,170 | 1,310 |
| Target Date2 | May 16 | May 25 | June 11 | June 18 | June 26 | July 8 | July 20 |
| Notes | • Emergence. • Days to emerge most often ranges from 6–21 days. • Uniform emergence essential to high yields. • Look for poor germination caused by chafer, wireworms, seedcorn maggot, seedcorn beetle, slugs, black cutworm. | • Start of critical weed-free period. • Growing point below ground. • Ensure herbicide selection is safe for crop stage. | • Ear initiation. • Growing point below ground. • Expansion of nodal root system will soon completely replace seminal root system. • Risk from cutworm and flea beetle damage has passed. | • End of critical weed-free period. • Lower leaves (1–4) dry up, may not be visible • Growing point at or above ground; more susceptible to frost injury. • Initiated ears and tassel now visible upon plant dissection. | • Side-dressing nitrogen and inter-row cultivation beyond this point pose threat of root pruning. • Beginning rapid stem elongation. • Risk from slug damage has passed. | • Crop becomes increasingly sensitive to yield reduction by heat or drought. • Size of ear and number of potential kernels being established. | • Tassel emerges. • Pollen shed begins 2–3 days prior to silk emergence. • Pollen viability reduced by drought and high temperatures. • Scout for corn leaf aphids, corn rootworm adults and goosenecking caused by rootworm larva. |
| 1Approximate CHUs required to reach various stages of corn development. 2Estimated date to reach various stages of development based on long-term heat unit accumulations for an average 2,800-CHU region and anticipating a May 5 planting date. | |||||||
Refer to the section on Corn Leaf Stages, for a description of the methodologies of corn leaf counting.
Table 2. Reproductive Growth Stages in Corn
| R Stage | R1 – Silking | R2 – Blister | R3 – Milk | R4 – Dough | R5 – Dent | R6 – Maturity |
| Description | Silks emerge from husks at tip of ear. | Kernels are white, filled with clear fluid and distinct from surrounding cob material. | Kernels begin to have yellow colour. Inner fluid is milky white. | Milky inner fluid becomes thicker and pasty. Outer edges of kernels become firmer. Some dents appear. | Majority of kernels are dented. Hard white layer of starch evident at top of kernel (milk line). | Hard starch layer evident from top to bottom of kernel. Black layer forms at base of kernel. |
| CHU Required1 | 1,480 | 1,825 | 2,000 | 2,165 | 2,475 | 2,800 |
| Target Date2 | July 20 | Aug. 3 | Aug. 11 | Aug. 18 | Sept. 1 | Sept. 18 |
| Kernel Moisture | NA3 | 85% | 80% | 70% | 55% | 30%–35% |
| Notes | • Pollination requires 3–7 days. • Silks continue to elongate until fertilized. • Environmental stresses very detrimental to yield. • Begin scouting for ear insect pests (corn earworm, fall armyworm). | • Kernels beginning dry matter accumulation. • Relocation of nutrients from the leaves and stem to the ear begins. • Firing of lower leaves may become evident. | • Rapid grain filling period. • Good plant health, clear skies and active photosynthesis add to kernel size and test weight. | • Top of kernel begins to firm up. • Killing frost may cause yield losses of 25%–40%. • Begin to assess ear rot incidence. | • Milk line advances toward tip as crop matures. • Whole plant moistures suitable for silage harvest. • 90% of grain yield reached by one-half milk line. • Examine fields for lodging, ear drop and stalk rots – if high, consider harvesting early. | • Physiological maturity. • Kernels have achieved maximum dry weight. • Moisture loss from kernels still required for suitable threshing. |
| 1 Approximate CHU required to reach various stages of corn development. 2 Estimated date to reach various stages of development based on long-term heat unit accumulations for an average 2,800-CHU region, and anticipating a May 5 planting date. 3 NA – not available, kernels not formed until after pollination. | ||||||
Corn Leaf Stages
Counting the leaves on a corn plant sounds like an easy task, but there are a few complications that can cause mistakes. It is important to know which leaf-counting method is being referred to on pesticide labels or in other production information.
Table 3. Comparative growth stages shows comparative growth stages using different methods of counting leaves.
Table 3. Comparative growth stages
| Leaf Tip | Leaf Over | Leaf Collar | Standing Height | Leaf Extended |
| 3 | 2 | 1 | 5-6 cm | 5-11 cm |
| 5-6 | 4 | 3 | 9-17 cm | 16-25 cm |
| 7-8 | 6 | 4-5 | 18-33 cm | 29-46 cm |
| 9-10 | 8 | 5-6 | 36-54 cm | 54-77 cm |
| 14-15 | 12 | 10 | 99-114 cm | 121-149 cm |
| Source: OMAFA Publication 75, Guide to Weed Control. | ||||
There are several methods used to count corn leaves:
- The leaf-tip method counts all leaves, including any leaf tip that has emerged from the whorl at the top of the plant.
- The leaf-over method only counts those leaves that are fully emerged and are arched over with the next leaf visible in the whorl but standing straight up.
- The leaf-collar method, used extensively in the U.S., refers to the leaf collar being visible. The leaf collar is the light green-to-whitish band that separates the leaf blade from the leaf sheath, which wraps around the stem. The stages for corn are referred to as V1, V2, V3, etc., where the V3 stage is a plant with three collars visible.
Uniformity of Emergence
Uniform seeding depth is a critical factor in achieving uniform emergence, see Figure 1[cmb1] . Uneven emergence affects crop performance, because competition from larger, early-emerging plants reduces the yield potential of smaller, later-emerging plants. Yields can be reduced by 5% when half the stand suffers from a 7-day delay in emergence and by 12% when half the population experiences a 2-week delay. Table 4. Corn yield response to plant spacing and emergence variability, shows the relative impact of emergence and in-row spacing variability on corn yield. In summary:
- If one of six plants (17%) had an emergence delay equal to two leaf stages (about 12 days), then overall yield reduction was 4%–5%.
- If one of six plants had emergence delays equal to four leaf stages (about 21 days), then overall yield was reduced by 8%.
- The sizes of yield reductions associated with delayed emergence were not significantly affected by the spacing variability of the stand (doubles and misses) within the corn row.
This study emphasized the fact that plants that are neighbouring a plant that is delayed in emergence do not compensate for the lower yield of the plant that is developmentally behind.
Table 4. Corn Yield Response to Plant Spacing and Emergence Variability1
| 1 Research was conducted at Elora and Woodstock, 2000–01. 2 Expressed as a percent of the uniform spacing and emergence treatment. | |||
| Plant Spacing | Emergence Delays | ||
| Uniform | 2-leaves (1 in 6) | 4-leaves (1 in 6) | |
| Uniform | 100% | 95% | 91% |
| Double (33% of plants) | 99% | 95% | 90% |
| Triple (50% of plants) | 98% | 94% | 90% |
| Source: Liu, Tollenaar, Stewart, Deen, University of Guelph. | |||
Uniformity of Spacing
It is widely believed that uniform in-row plant spacing is necessary to achieve high corn yields. However, a considerable number of studies challenge the notion that increased variability of in-row plant spacing results in large yield losses.
The relative yields shown in Table 4 indicate that when plants are less than perfectly spaced, those plants that have more space compensate for those that are given less space. Doubles are defined as two plants spaced about 3 cm (1.33 in.) apart situated next to a gap of about 38 cm (15 in.). Triples are defined as three plants spaced 3 cm from each other next to a gap of 58 cm (23 in.). A collection of research has further shown:
- Yield losses are about 1% if the stand contains two out of six plants (33%) that are clustered as doubles.
- 2% if three out of six plants (50%) are clustered as triples.
- 2.5 cm (1 in.) increase in plant stand standard deviation decreased yield by less than 0.08 t/ha (1.3 bu/acre), assuming equal plant populations. These results were consistent with earlier research conducted in Ontario during the late 1970s and in Wisconsin from 1999–2001.
- Dr. Bob Nielsen (Purdue University, Indiana) reported that every additional 2.5 cm (1 in.) of standard deviation over 5 cm (2 in.) decreases yields by 160 kg/ha (2.5 bu/acre). This suggests that significant yield losses are associated with plant stand variability.
- Results of a survey of 127 Wisconsin commercial corn fields with an average plant population of 73,500 plants/ha (29,750 plants/acre) suggested that plant spacing standard deviation averaged 8.4 cm (3.33 in.) with 95% of fields having standard deviations that were less than 11.7 cm (4.66 in.).
- Results of 24 research trials conducted along with the Wisconsin plant variability survey concluded that significant yield reductions begin to occur only when corn plant standard deviations exceed 12 cm (4.75 in.).
These results from other jurisdictions support Ontario research findings shown in Table 4. They suggest minimal yield impact of uneven plant spacing. Generally, within the range of plant spacing variability typically found in most Ontario corn fields that are at the target population, the reduction in yield potential due to plant stand variability is likely small.
Poor planter maintenance or high planting speeds are often identified as contributing to poor within-row spacing uniformity. Research conducted in Illinois and shown in Table 5. Effect of Planting Speed on Spacing Standard Deviation, Population and Corn Yield illustrated that with properly maintained planters, high planting speeds and slight variations in spacing uniformity had no impact on yield.
Table 5. Effect of Planting Speed on Spacing Standard Deviation, Population and Corn Yield
| (Average of 11 Illinois trials, 1994-96) | |||
| Planting Speed | Standard Deviation1 | Population | Yield |
| 5 km/h | 7.3 cm (2.9 in.) | 67,290 plants/ha (27,231 plants/acre) | 9.57 t/ha (152.5 bu/acre) |
| 8 km/h | 7.6 cm (30 in.) | 67,640 plants/ha (27,373 plants/acre) | 9.55 t/ha (152 bu/acre) |
| 11.3 km/h | 8.2 cm (3.2 in.) | 66,700 plants/ha (26,996 plants/acre) | 9.61 t/ha (153.1 bu/acre) |
| Source: E. Nafziger, University of Illinois and H. Brown. | |||
| 1 An absolutely perfect stand, where every plant is exactly 18 cm (7.25 in.) from its neighbour, would have a standard deviation of zero. If plants on average varied plus or minus 5 cm (2 in.) from the desired 18 cm (7.25 in.), then the standard deviation would be 5 cm (2 in.). | |||
Uniformity and timing of emergence, along with achieving target populations, generally have a greater impact on corn yield than uniformity of corn plant spacing. Planter maintenance and choice of attachments (i.e., coulters and residue row cleaners) should focus on achieving consistent seed placement and the creation of in-row seedbed conditions that ensure rapid uniform emergence. It is important to ensure that the planter is operating level and that all discs, depth-gauging wheels, and seed-firming devices are up to specifications, aligned and also operating at the correct depth or pressure.
Pre-planting management may also play a critical role in emergence uniformity. If the field is left too uneven, if residue is bunched, or if surface compaction has not been uniformly alleviated, even the most carefully prepared corn planter may not be able to consistently place seed and create in-row seedbed conditions that ensure rapid uniform emergence.
- Plants that emerge late, so that they are one or two leaves behind neighbouring plants, are likely to achieve a lower yield relative to uniformly emerged stands and may even yield less than later-planted but uniformly emerged corn.
- Relatively small investments in time and/or money for planter adjustments, such as installing new opener discs, levelling the planter, properly adjusting seed-firming wheels and proper seed depth placement, can significantly increase yield and returns.
Row Widths
Narrow Rows
Past research indicated that more northerly latitudes benefited the most from narrowing corn rows from the traditional 76–96 cm (30–38 in.) widths to 38–60 cm (15–24 in.) compared to mid-to-southern portions of the cornbelt. Most Ontario producers who converted to narrow-row production systems targeted 50 cm (20 in.) row spacing anticipating that the expected yield boost of 3%–8%, would cover the costs of converting planter and corn header. However, more recent studies conducted in Ontario by the University of Guelph and Pioneer Hi-Bred Ltd. have shown minimal yield advantage with 38 cm (15 in.) or 50 cm (20 in.) rows compared to 76 cm (30 in.) rows. The fundamental reason for moving to narrower rows is to enhance light interception. It appears that the total light interception once the canopy has fully developed is no greater in narrow rows than in wide rows. Any yield advantage experienced with narrow rows must come from earlier canopy closure and greater light interception in the late-June to early-July period.
Research has yet to find hybrids particularly suited for narrow rows. Increasing plant populations often resulted in comparable yield increases to traditional row widths. Yield improvements may be sporadic, and the justification of equipment costs may depend on other factors such as use of the narrow row planter for other crops (e.g., dry edible beans), numbers of acres to be planted and costs of equipment conversions. There is also the increased risk for stalk rots in narrow row systems.