Corn: Development

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Pub 811: Agronomy Guide > Corn > Development

Excerpt from Agronomy Guide for Field Crops
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Table of Contents

• Introduction
• CHU Season-Ending Dates
  • Table 1-17. Vegetative Growth Stages in Corn
  • Table 1-18. Reproductive Growth Stages in Corn
  • Corn Leaf Stages
  • Table 1-19. Comparative Growth Stages
  • Table 1-20. Corn Yield Response to Plant Spacing and Emergence Variability
    
• Uniformity of Emergence
• Uniformity of Spacing
  • Table 1-21. Effect of Planting Speed on Spacing Standard Deviation, Population and Corn Yield
    
• Uniformity of Emergence
• Uniformity of Spacing
  • Table 1-22. Effect of Planter Type on Corn Spacing Variability (Moldboard)
  • Table 1-22. Effect of Planter Type on Corn Spacing Variability (No-Till)
• Row Widths
• Replant Decisions
  • Table 1-23. Effect of Planting Speed on Spacing Standard Deviation, Population and Corn Yield

Introduction

The vegetative and reproductive growth stages in corn are described in Table 1-17, Vegetative Growth Stages in Corn and Table 1-18, Reproductive Growth Stages in Corn.

CHU Season-Ending Dates

The end of the growing season is defined as the first occurrence of a killing frost (-2°C), or the date when the daily average temperature has historically (30-year normals) fallen below 12°C. In the 30- year data used for CHU calculations, the season is terminated approximately 10% of the time by an occurrence of -2°C. This approach is identical for the CHU (traditional) and CHU-M1 (May 1 start date) calculations see Revised Ontario Crop Heat Units.

Table 1-17. Vegetative Growth Stages in Corn

Illustration
Stage	VE	V1	V4
Leaf Collars1	0	1	4
Leaf Tips	1	3	7
Leaf Over	0	2	6
CHUs Required2	180	330	630
Target Date3	May 14	May 26	June 12
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.

Table 1-17. Vegetative Growth Stages in Corn

Illustration
Stage	V6	V8
Leaf Collars1	6	8
Leaf Tips	10	11
Leaf Over	8	10
CHUs Required2	780	930
Target Date3	June 20	June 28
Notes	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.

Table 1-17. Vegetative Growth Stages in Corn

Illustration
Stage	V12	VT
Leaf Collars1	12	(varies)
Leaf Tips	15	-
Leaf Over	14	-
CHUs Required2	1,170	1,310
Target Date3	July 2	July 12
Notes	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

Corn Leaf Stages

Counting the leaves on a corn plant sounds like an easy task, but there are a few complications that can cause miscounting. It is important to know which leaf-counting method is being referred to on pesticide labels or in other production information.

Table 1-19, Comparative Growth Stages, shows comparative growth stages using different methods of counting leaves.

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 Plate 7.

Plate 7. Uneven planting depth. Uniform seeding depth is critical to achieving uniform emergence.

Uneven emergence affects crop performance, because competition from larger, early-emerging plants reduces the yield potential of smaller, later-emerging plants. Research indicates that 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 1-20, Corn Yield Response to Plant Spacing and Emergence Variability, shows the results of a University of Guelph study that examined the relative impact of emergence and in-row spacing variability on corn yield. 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.

Uniformity of Spacing

It is widely believed that uniform in-row plant spacing is necessary to produce top corn yields. However, a recent study conducted by the University of Guelph challenges the notion that significant increases in row plant spacing variability results in large yield losses. The relative yields shown in Table 1-20 indicate that yield losses are about 1% if the stand contains two out of six plants (33%) that are clustered as doubles and 2% if three out of six plants (50%) are clustered as triples. Doubles in this study were defined as two plants spaced about 3 cm (11/3 in.) apart situated next to a gap of about 38 cm (15 in.) and triples were three plants spaced 3 cm from each other next to a gap of 58 cm (23 in.). This study clearly demonstrated that even if half the stand were clustered as triples, that yield losses were minimal when overall population is not affected and emergence is uniform. The lower individual yields of corn plants that were part of clusters were almost completely compensated for by higher yields of single plants that were situated next to the gaps.

Recent University of Guelph research suggests that a 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 to 2001. It should be noted that there are some studies that suggest significant yield losses associated with increasing plant stand variability. Dr. Bob Nielsen (Purdue University, Indiana) has 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).

Results of a 1998-2000 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 (31/3 in.) with 95% of fields having standard deviations that were less than 11.7 cm (42/3 in.). The 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 (43/4 in.). This supports the Ontario research findings shown in Table 1-20 that 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 (Table 1-21, 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. Similar results were also observed in a University of Guelph study where increasing planting speed from 6.5-12 km/h resulted in a 1-2.5 cm (1/2-1 in.) increase in plant spacing and no effect on corn yield in conventional tillage systems. However, the higher planting speed did decrease no-till corn yield by 0.2 t/ha (3.1 bu/acre). This suggests that slower planting speeds may be necessary for planting equipment to uniformly place seed in seedbeds with more variable soil and/or surface residue conditions that are typical of reduced or no-till planting systems.

¹ An absolutely perfect stand, where every plant is exactly 18 cm (71/4 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 (71?4 in.), then the standard deviation would be 5 cm (2 in.).

Planting Equipment and Maintenance

Choice of planting equipment can have an impact on corn yield potential. Interest has increased in recent years in Ontario for use of lower-cost planting systems, such as air seeders, that could potentially plant all crops. Table 1-22, Effect of Planter Type on Corn Spacing Variability, contains the results of a study that evaluated the performance of three planters in conventional and no-till systems. Use of the air seeder resulted in greater plant spacing variability, delays in emergence and lower corn yields compared to use of the row crop planters. The yield reduction associated with the air seeder was larger in the no-till system, because the simpler planter design of the air seeder was not as capable as the row crop planters at maintaining uniform seeding depth and consistent seed furrow closure in the no-till system. There appeared to be a slight yield advantage with the newer row crop planter over the older model, suggesting that advances in planter design that result in more consistent seed placement increase corn yield potential.

Planter Descriptions:
¹ The vacuum meter was a John Deere 1750 MaxEmerge Plus row-crop planter manufactured in 1998 and equipped with a double-disk opener system, 2.5 cm-wide, angled closing wheels, fingered residue removers attached in front of the furrow opener, three coulters set at a 10-15-cm depth, and seed firmers.
² The finger-pickup planter was a John Deere 7000 row-crop planter manufactured in 1986 with similar components as the vacuum meter planter, except for the absence of seed firmers.
³ The air seeder was a Gandy Orbit-Air 6224 air seeder manufactured in 1990 and equipped with single-disc openers and a single, angled closing wheel. The air seeder did not have residue removers, coulters, or seed firmers.

When walking corn fields keep the following in mind:

• 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

By the mid-1990s, research conducted at various locations across the northern Corn Belt and Southern Ontario indicated significant yield advantages could be expected from narrowing corn rows from the traditional 76-96 cm (30-38 in.) down to row widths of 38-60 cm (15-24 in.). Results showed that narrow row advantages would be greater in more northerly latitudes, compared to results coming from the mid-to-southern portions of the Corn Belt. Most Ontario producers who converted to narrow-row production systems targeted 50-cm (20-in.) row spacing and anticipated paying for planter and corn header conversions with an expected yield boost of 3%-8%. More recently, studies conducted in Ontario by the University of Guelph and Pioneer Hi-Bred Ltd. have shown little to no 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. The perceived yield advantage of narrow rows must come from earlier canopy closure and greater light interception in the late-June to early-July period.

Adapted from University of Illinois data, E.D. Nafziger. 1994. Journal of Production Agriculture. Original data from Illinois was shifted 10 days later to reflect optimal planting dates in Ontario.
Example: A field is planted on May 4 with an expected plant population of 62,000 plants/ha (25,000 plants/acre). By the end of the month, the stand is reduced to 31,000 plants/ha (12,500 plants/acre) but uniform in size and plant distribution. The table would suggest that the expected yield for a final plant stand of 31,000 planted on May 4 would be 75% (a 50% reduction from the targeted population). By planting on May 29 at the same population (62,000), a yield of 89% can be expected. In this case, the cost of replanting may be recovered and therefore justified. Interpolate the data for planting dates or plant stands that differ from the listed values.

In addition, research did not indicate that there were specific hybrids particularly adapted to narrow rows. High plant populations within narrow rows often boosted yields, but yields were often increased on traditional row widths as well. 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.

Replant Decisions

There is no simple formula to aid in replant decisions, so each case must be dealt with individually. When contemplating a replant decision, consider the original planting date, target plant population, actual population, uniformity of plant size, distribution following damage, possible replanting date and cost of replanting (seed, fungicides/insecticides, fuel, etc).

The actual plant population can be estimated by counting the number of plants in a length of row that is equal to 1/1000 of an acre Appendix J, Plant Populations at Various Row Widths. This should be replicated at least five times per 10 ha (25 acre) in separate parts of the field. Calculate the average of these samples and then multiply the average by 1,000.

It is important when taking stand counts to observe the uniformity, plant size and distribution of the plants in the rows. How do the stand, plant size and distribution vary? Yields can be reduced by 2% if the stand has several 30-90-cm (12-36-in.) gaps. If the gaps are larger - 1.25-2 m (4-6 ft) - expect a 5%-6% reduction in yield when compared to a uniform stand. Yield reductions will be greater with more numerous and longer gaps between plants within the row.

Table 1-23, Expected Grain Yield Due to Various Planting Dates and Populations, shows the effect of planting date and plant population on final grain yield. Yields are based on stands that are normal in terms of uniformity of plant size and distribution. Grain yields for varying dates and populations are expressed as a percentage of the yield obtained at the optimum planting date and population: 64,200-76,200 plants/ha (26,000-30,000 plants/acre). Results will vary depending on location, environmental conditions, hybrid and other factors.

The availability of early-maturing hybrids with good yield potential and the cost of replanting are important factors in the replant decision. Consider whether the herbicide program allows for a switch to soybeans. If not, is a reapplication of corn herbicides required? What is the condition and health of the remaining crop? Before replanting, determine whether the conditions that caused the problem in the first place still exist (soil conditions, disease, insects, herbicide injury). If an insect or disease problem was the culprit, factor in the cost of an insecticide and/or fungicide treatment.