This June issue of Walking Your Fields newsletter contains articles about: planting date impact, nitrogen loss, glyphosate restrictions and early weed control. Articles are written by DuPont Pioneer agronomists in North Dakota and Minnesota and are distributed on behalf of DuPont Pioneer account managers and Pioneer sales reps.

1. A late spring and wet conditions have delayed soybean
planting in some areas. Soybeans are ‘day length sensi-
tive’ plants. This means flowering and maturity are trig-
gered by the length of the night, or dark period. When
adapted maturity varieties are planted before the middle
of June, flowering is triggered by shorter days (actually
longer nights) after June 21. For each three to five day
delay in planting, flowering and maturity are delayed
only about one day. That means that if you plant the
same variety on May 10, and 30 days later on June 10
the maturity of the later planting is delayed about six to
ten days. This happens regardless of the maturity of the
variety.
Soybean Development
Flowering usually begins six to eight weeks after seedling
emergence, and continues for three to four weeks. How-
ever, flowering will occur earlier if soybeans are planted
late. Warm temperatures accelerate development, espe-
cially flowering. A full season soybean variety will normal-
ly flower in the first 10 days of July. If there is a hot period
in mid June, it can flower up to two weeks earlier. When
this happens, and if there is good weather for the rest of
the season, higher yields result because of the extended
reproductive period. The soybean plant may flower 10
days earlier, but it won’t necessarily mature 10 days earli-
er because of day length controls. Generally, yield poten-
tial increases as the length of the flowering to maturity
stage increases.
Warm weather in August or early September does not
hasten maturity much, unless it causes water deficit
stress. Sometimes, temperature is blamed for stress, be-
cause hot temperatures often accompany drought. A cool
fall does not delay maturity much either, although yields
may be reduced if the cool weather is accompanied by
heavy rain, causing disease. The flowering period is influ-
enced more by temperature, but maturity is more strongly
influenced by photoperiod. The rapid shortening of days
starting in mid August drives the soybean to maturity;
temperature has only a small influence on maturity.
KEY SUMMARY POINTS
 Soybeans are day length sensitive plants that adjust
their final maturity to compensate for late planting. De-
layed planting will delay the onset of flowering by only
a matter of days, even when planting is delayed for
weeks. Harvest dates will only be delayed by about
one day for each three to five day delay in planting.
 Normal maturity soybean varieties should be planted
at least until about June 10. Earlier varieties would be
induced to flower before sufficient vegetative growth is
achieved to produce top yields.
 Even normal maturity varieties may be shorter when
planted in June. When the normal vegetative period of
six to eight weeks is shortened, the effect on the soy-
bean plant is usually reduction in height.
 Warm temperatures will induce earlier flowering and a
longer flowering period, cool temperatures will induce
later flowering and a shorter flowering period. A long-
er flowering period is associated with higher yields.
 Maturity is strongly influenced by photoperiod (day
length). Warmer or cooler than normal temperatures
do not greatly influence maturity. The rapid shortening
of days starting in mid-August drives the soybean to
maturity; temperature has a small influence.
Derek Crompton
Field Agronomist
Zach Fore
Field Agronomist
Mike O’Leary
Product Agronomist
June 10, 2013 - Issue 3
WALKING YOUR FIELDS® newsletter is brought to you by your local account manager for DuPont Pioneer. It is sent to customers throughout the growing season,
courtesy of your Pioneer sales professional. The DuPont Oval Logo is a registered trademark of DuPont. PIONEER® brand products are provided subject to the
terms and conditions of purchase which are part of the labeling and purchase documents. ®, TM, SM Trademarks and service marks of Pioneer. © 2013 PHII.
Planting Date Impact on Soybean
Development and Maturity
Photo: Pat Branick, DuPont Pioneer

2. Heavy spring rainfall can cause concerns about nitrogen
(N) loss in corn fields and raises questions about the
need for supplemental N applications. There are two
types of N loss: 1) Leaching—movement with water be-
low the root zone, and 2) Denitrification—loss to the
atmosphere. In general, leaching losses are more likely
on sandy soils where water can move through the profile
quickly. Denitrification is more likely on medium and fine
textured soils that are not well drained. The exact extent
of N losses through leaching and/or denitrification follow-
ing the heavy rains is difficult to determine. Both of these
loss processes occur through the nitrate form of N, so the
potential for significant loss is determined by the amount
of the N supply that was in the nitrate form when the ex-
cess rainfall occurred. Losses depend on many factors
such as when the N was applied, the forms of N applied,
soil characteristics, and how wet the soil is.
Where fertilizer N was applied before planting, the timing
of the application and the form of N used are important in
determining the risk of loss. Keeping in mind that losses
occur through the nitrate form of N, the timing of nitrate
formation is an important consideration in evaluating po-
tential losses. Fall-applied fertilizer N has a high risk for
loss following excess rainfall because most or all of the N
would be in the nitrate form by mid-May. For spring pre-
plant applications, ammonium forms of N such as anhy-
drous ammonia or urea are converted to nitrate-N in
about four to six weeks. Urea usually is converted to ni-
trate more rapidly than anhydrous ammonia. Nitrogen
solutions (28% UAN) contain half of the N as urea and
the remainder as ammonium nitrate. Essentially, when
this fertilizer is applied, it contains 75 percent of the N as
ammonium and 25 percent as nitrate.
Denitrification losses can occur within a few days if the
soil remains saturated or flooded and nitrate-N is present.
Warm temperatures and extended periods of saturated
conditions favor high losses. Work in Illinois suggests that
four to five percent of the nitrate-N present can be lost
each day the soil remains saturated. Table 1 from the
University of Nebraska provides some estimates of deni-
trification losses at various temperatures and times of
saturated soil conditions.
Options for applying supplemental N when it is needed
include traditional side-dressing with anhydrous ammonia
or N solutions. Urea-ammonium nitrate solutions (28%)
can also be applied as a surface band or as a broadcast
spray over the growing crop. Dry N fertilizers such as
urea or ammonium nitrate can also be broadcast applied
to the crop. Leaf burning from solution or dry broadcast
applications should be expected. Applying the dry materi-
als when foliage is dry will help to minimize burning. Basi-
cally, broadcast N rates should be limited to 90 lbs. N/
acre for corn with 4 to 5 leaves and to 60 lbs. N/ acre for
corn at the 8-leaf stage. Under N deficient conditions,
corn will respond to supplemental N applications through
the tassel stage of development if the N can be applied.
Feel free to contact your local Pioneer Sales Repre-
sentative to assist you in assessing your N needs.
(Source: Portions of this article are from Dr. Larry G. Bundy, U
of WI Extension Service, ‘Evaluating Nitrogen Losses Following
Excessive Rainfall.’)
To help you assess potential fertilizer N losses, download
the simple and reliable Supplemental Fertilizer N Work-
sheet from the University of Minnesota. http://
bit.ly/17iNb9A. (Source: Schmitt and Randall, U of MN.)
Glyphosate on Corn
Corn can be damaged by
spraying glyphosate too
late in the growing sea-
son. Ears appear to have
aborted kernels or scat-
tered pollination.
 Corn may be sprayed
broadcast or over the
top with glyphosate
until it reaches V8
stage or 30 inches in
height (whichever
comes first).
 Corn can be sprayed
between 30 and 48
inches with a ground
sprayer employing drop
nozzles that direct the
spray to the base of the
corn plants and avoids
direct application of
spray solution into the whorl of the corn plant.
Table 1. Estimated denitrification N losses influ-
enced by soil temperature and days saturated.
Soil Temp (°F)
Days
Saturated
N Loss
(% of applied)
5 10
55-60
10 25
75-80
3 60
5 75
7 85
9 95
Source: Shapiro, University of Nebraska.
Restrictions on Spraying Glyphosate
Ear on left shows irregular kernel
set resulting from a late glyphosate
application. Ear on right is from
unsprayed plant in same field. Pho-
to: Clyde Tiffany, DuPont Pioneer.
N Loss Following Excessive Rainfall

3. Glyphosate on Soybeans
Spraying soybeans with glyphosate can occur through R2
stage. Reproductive phases are as follows:
 R1--Plants have at least one flower on any node.
 R2--Plants have at least one flower open on one of the
two uppermost nodes (Four days after R1).
 R3--A 3/16 inch pod at one of the four uppermost
nodes (Ten days after R2).
After you see the first flower, you have approximately two
weeks to spray glyphosate to stay on label. This recom-
mendation is a clarification of previous definitions of
spraying glyphosate on soybeans throughout flowering.
Application timing is critical when using glyphosate or
other post-emergence herbicides to control weeds in
corn. The weeds need to be emerged, but not too large
that they compete with corn and reduce yields. Usually
this control is accomplished with multiple applications of
glyphosate, and/or by using glyphosate in combination
with herbicides that provide residual weed control.
Critical Weed-Free Period
The critical period of weed control is the interval when
weed control is required to provide maximum yield. Weed
competition before this period will not affect yield, if
weeds are controlled by the start of the critical period. In
addition, weed competition after the critical period will not
affect yield.
Figure 1 shows when weeds were removed during the
first 4 weeks after planting, corn yield was 100 percent
(solid line). When the crop was kept weed-free for at least
7 weeks, corn yield was also 100 percent (dashed line).
It is difficult to predict the critical period, because it de-
pends upon the weeds (how competitive the different
weed species are, their density, when they emerge), and
the environment (soil moisture, nitrogen, etc.). Research
conducted on weed interference in corn indicates that
weeds should be controlled by at least the third week
after planting, and weed emergence should subsequent-
ly be prevented until about 8 weeks after planting (or
more accurately, 14-leaf or V12 corn). If a non-residual
herbicide like glyphosate is applied three weeks after
planting, a second herbicide application or cultivation will
probably be required to keep later germinating weeds
controlled, or a residual herbicide will need to be added to
prevent further weed germination during this window.
Herbicide Systems
According to research at University of MN by Dr. Jeffrey
Gunsolus, the longer the duration of weed competition,
the greater the impact on yield. His research concluded:
 The best economic return came from a pre-emergence
application of a residual soil herbicide (acetochlor) fol-
lowed by a post-emergence application of glyphosate
at the 5-inch weed stage.
 A one-pass application of a residual a soil herbicide
(acetochlor) plus glyphosate at the 1-inch weed stage
can maximize yields.
 One post-emergence application of glyphosate did not
maximize yields or returns in his study
(Gunsolus, 2005).
Always read and follow label directions and safety pre-
cautions.
Early Weed Control is Critical in Corn
Figure 1. Source: Reprinted with permission by Chris
Boerboom, U of WI. ‘Timing Post-emergence Herbicides in
Corn and Soybeans.’
For nearly a half century, the National Corn Grower’s
Association’s corn yield contest has provided corn grow-
ers an opportunity to compete with colleagues to grow
the most corn per acre ~ helping feed and fuel the
world. This NCGA contest has given participants the
recognition they deserve as well as an opportunity to
learn from their peers.
 June 15: Early entry deadline. Cost per entry $80.
 July 12: Final deadline to submit all entries. Cost
per entry $110.
Entry fees for Pioneer corn hybrids paid by Pioneer.
Enter a Pioneer®
brand hybrid in the yield contest and
all entry fees and membership dues are paid by Pioneer
directly to NCGA.
Talk to your Pioneer Sales Representative about
your entry.
On-line entries are available by following this link:
https://membership.ncga.com/CornyieldContest/
NCGA National Corn Yield Contest

4. WALKINGYOURFIELDS®
KB Crompton, Fore, O’Leary
DuPont Pioneer
Sales & Marketing
PO Box 466
Johnston, IA 50131
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Follow your DuPont Pioneer Agronomists @PioneerNDakota or @PioneerMN
In this issue:
 Planting Date Impact
 N Loss Following Rain
 Glyphosate Restrictions
 Early Weed Control
 NCGA Yield Contest
This Walking Your Fields®
newsletter is brought to you
courtesy of your Pioneer Sales
Representative and DuPont Pioneer
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