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Table of Contents
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.
2 Approximate CHUs required to reach various stages of corn development.
3 Estimated date to reach various stages of development based on long-term heat unit accumulations for an average 2,800-CHU-M1 region and anticipating a May 1 planting date.
2 Estimated date to reach various stages of development based on long-term heat unit accumulations for an average 2,800-CHU-M1 region and anticipating a May 1 planting date.
3 NA: not available, kernels not formed until after pollination.
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
There are several methods used to count corn leaves:
1 Expressed as a percent of the uniform spacing and emergence treatment.
Uniform seeding depth is a critical factor in achieving uniform emergence Plate 7.
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.
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.).
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.
Research was conducted at Elora and Woodstock, 2000-01.
When walking corn fields keep the following in mind:
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.
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.
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,
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.
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