Submitted by: agronomy, Wed May 19 09:18:54 1999


STAGING SMALL GRAINS

The recent heavy rains have resulted in heavy weed infestations
and it is obviously time to apply post-emergent herbicides.
Monitoring crop development and correct staging is crucial to insure
proper application timing of these herbicides and pesticides in
general. Staging small grains, and wild oat, is based on
both vegetative and reproductive development. Vegetative stages are
defined by the number of leaves produced on the main stem and the
number of tillers on a plant. All leaves must be counted even if
missing.
Leaf stage is are defined by the number of leaves on the main
stem only and can be described in multiple ways. When using the Haun
scale count all fully developed leaves and the next leaf as a
fraction of the last completely developed leaf; using this system the
plant in Figure 1 would be in the 4.5 leaf stage. When using the
Feekes scale a new leaf is counted when it reaches one-half the
length of the leaf below it; using this method the plant in Figure 1
would be in the 5 leaf stage.
Each tiller produced in addition to the main stem is numbered
when it becomes visible. There are two types of tillers: those
arising from a crown leaf axis and those arising from the coleoptiler
node. Only tillers arising from a crown leaf axis are counted when
staging. These tillers are also surrounded by a small membranous
structure, called a prophyll, that is useful to distinguish axillary
tillers from main stem tillers. When present there will only be one
coleoptiler tiller. Following this system the plant in Figure 1 is in
the two tiller 4.5 leaf stage.
The flag leaf stage is reached when the last leaf on a tiller has
emerged. This is followed by jointing which begins when stem
elongation occurs and the growing point emerges from the soil
surface. Boot stage occurs when the flag leaf sheath becomes
visible and continues until head emergence, or flowering.
A small grain plant enters its reproductive stage when flowering
starts. In wheat this is generally 3 days after heading; whereas in
barley flowering occurs just before heading while still in late boot
stage.
There are six developmental stages that occur after flowering in
small grains: watery stage, milk stage, soft dough stage, hard dough
stage, kernel hard stage, and harvest ripe. Hard dough is the stage
in small grains when physiological maturity (PM) is reached, and
occurs at 30 - 35 percent moisture. Once PM has been reached the crop
will not assimilate additional dry matter into the kernel.
Swathing to facilitate drying and hasten harvest, it
should be done at or after this stage.

GDD AND CROP DEVELOPMENT
Growing Degree Days (GDD) correlates plant development with heat
units (daily temperature extremes). Based on plant emergence and
historic temperature trends GDD can be used to predict the
time when a crop will reach a certain developmental stage, however,
the actual stage is best determined by visual evaluation.
Using heat units to predict plant development functions on the
premises that the actual number of GDD for a crop to reach maturity
remains relatively constant across environments even though calendar
days change. Growing degree days are based on daily high and low
temperatures and are calculated for one day as follows: (high temp. +
low temp.)/2 -minimum base temp = GDD in degrees Fahrenheit.
When calculating GDD there are minimum and maximum base
temperatures for each crop that are used when temperatures reach
extremes. When the temperature falls below the minimum base
temperature or exceeds the maximum base temperature for the crop in
question then the minimum or maximum base temperature is used in
the calculation. For example if you are calculating the GDD for corn
when the high temperature was 78o F and the low temperature was 45o F
then the GDD would be (50 + 78)/2 - 50 = 14. The low temperature
of 45o is not used because it is below the minimum base temperature
of 50o F for corn. Table 1. gives minimum optimum and maximum base
temperatures for several cereal crops.
In small grains early growth is the most sensitive to high
temperatures and 70o F should always be used as the maximum
temperature until the crop has reached the two leaf stage. Following
the two leaf stage the maximum temperature will be higher.

Table 1. Minimum base, optimum and maximum growth temperatures in
Fahrenheit of several crops for use when calculating growing degree
days.
Growth Temperatures (F)
Crop Base Optimum Maximum
Wheat 32 76 90
Barley 32 70 86
Oats 32 70 86
Corn 50 86 108

There is good agreement of what minimum base temperatures should
be used when calculating GDD for the cereal crops, however, there is
often disagreement of what maximum base temperatures are appropriate.
Use of GDD is based on a model that assumes developmental rate
increases linearly to a maximum temperature and then remains
constant. In reality, developmental rate in any crop does not
increase linearly and remain constant but will increase to an optimum
rate and then decrease. Disagreement occurs over the question, should
the optimum temperature for development be used as the maximum
temperature when calculating GDD or should the temperature at which
development rate falls to near zero. I prefer to use optimum growth
rates as the maximum when calculating GDD, as is commonly done with
corn at 86o F as the maximum temp.
When calculating GDD in North Dakota the maximum temperature
will make little difference in many years. If the daily temperature
rises above 90o F three or four days during the entire growing season
and does not significantly exceed the optimum temperature the rest of
the season the total accumulated GDD will differ little with method
of calculation.
When using GDD to determine crop growth stage accumulation
should start the day after planting. Small grains require about 180
GDD for germination and emergence. A wheat plant requires about 140
GDD for each leaf whether accumulated in four days or ten days, and
about 2400 GDD to reach maturity, which can range from 83-100
calendar days.
This is important since the number of GDD that have accumulated
for your crop will be different from the total accumulation for the
growing season. The North Dakota Agricultural Weather Network
(http://www.ext.nodak.edu/weather) provides daily temperatures that
can be used to determine the accumulated GDD. GDD listed for wheat
are for the entire season and should be adjusted for planting date.
The maximum temp used by NDAWN is 95o F for wheat and 86o F for corn.
Growing degree days calculated for crops should not be confused
with insect degree days (DD or IDD). Insect degree days start
accumulating when the minimum temperature for the insect in question
is reached.

Michael D. Peel, NDSU Extension
Agronomist, Small Grains
mpeel@ndsuext.nodak.edu