Spring Seeding Decisions
Farmers
still wavering over the decision whether and when
to seed spring crops should take soil samples (which you’re doing anyway for future Conservation Security Program sign-ups,
aren’t you…), install a reliable rain gauge on your farm,
and add a soil thermometer to your nerd-farmer
pocket protector kit.
The first decision in this drought year is whether to seed
a crop. This depends on how much soil moisture is available to the crop,
and how much extra rain you expect to get during the growing season. Current
soil moisture should be reported on your soil test from this spring. As
spring crops generally do not root as deeply as winter crops, use only
the amounts of water and nitrogen reported from the top 3 ft when calculating
available moisture and residual nutrients.
The most definitive work for eastern Washington on calculating
expected crop yield from soil moisture was done by G.E. Leggett in 1959.
He concluded that a wheat crop needs 4 inches of moisture (from available
soil moisture and precipitation) in order to begin grain production and
mature. Thereafter every additional inch of water, from the soil or rain,
should add 6 bu/A for spring wheat and 7 bu/A for winter wheat (Cook and
Veseth, 1991, in Wheat Health Management).
Maximum spring wheat yield in bu/A = (Available soil
moisture + Rainfall
– 4) x 6
Maximum winter wheat yield in bu/A = (Available soil
moisture + Rainfall
– 4) x 7
Leggett conducted multiple trials over several years in different
rainfall areas, and concluded that his formula held pretty constant in
both annual cropping and summerfallow regions. Yields will vary among farms
and fields due to soil type and other environmental factors. One may be
a little more optimistic with the expected yield when growing current wheat
varieties that are more efficient water users than those used 45 years
ago. However, Cook and Veseth argued that newer varieties are more efficient
water users only at the upper end of production levels when water is plentiful.
Dry peas have moisture requirements similar to spring cereals,
but need more water for germination so should be seeded into a firm, moist
seedbed. Their roots go down to the 5 ft level, so they can extract a lot
of water from the soil (Muehlbauer, 1993. Food and Grain Legumes). Lentils
are more drought tolerant than are peas, but its roots extract water from
only the top two feet of soil (Lentil in Saskatchewan). For germination
each lentil seed absorbs its own weight in water within 36 hours. The plants
are drought sensitive at this stage and desiccation may reduce germination
rates. Garbanzos have higher moisture requirements than peas or lentils
for germination, but moisture levels less than field capacity will suffice.
However, they develop a deep taproot so are more drought tolerant during
the growing season and can respond to late-season rain ( Siddique. Univ
Western Australia. Abiotic stresses of cool season pulses in Australia).
Canola and mustard both use more water than do spring cereals.
Mustard tends to be more drought and tolerant than canola, but is more
cold sensitive earlier in the season (Aaron Esser, personal communication).
Other factors to consider in your planting decision include
your risk tolerance, how much fallow will work in your rotation, and how
many acres you want to seed to winter wheat this fall. Growers who have
decided to seed a spring crop must then decide whether to seed early to
take advantage of current soil moisture, or whether to wait until soil
temperatures are higher and there is less danger of frost damage to the
young plants.
The following table indicates the minimum soil temperatures
(in Fahrenheit) at which various crops will grow. Sources: Cook and Veseth
(Wheat Health Management), Kevin McPhee (USDA-ARS), Don Wysocki (OSU).
CROP |
Minimum
Growth Temp (F) |
Wheat |
32 |
Peas |
40 |
Lentils |
43 to 45 |
Mustard & Canola |
45 |
Corn |
50 |
Garbanzos |
50 to 53 |
Note that these are the minimum temperatures
for growth. Ideally the soil should be consistently in the desired range
before you seed. Take the soil temperature at seeding depth for that
crop, and include measurements from several aspects (north and south
facing slopes and draws) within a field. If the soil is at the desired
temperature in the morning then it is probably a realistic reading. However,
if the temperature peaks within range in the early afternoon, the soil
may well be too cool for most of the day. Most crops, especially canola
and mustard, get the best start if they can germinate and grow consistently
after seeding. Sitting in cold, wet soils makes the germinating plants
vulnerable to plant pathogens.
If you have already seeded, or are wondering how the forecast
weather might affect your crop emergence, you can calculate growing degree
days (GDD), also known as growing degree units (GDU) or heat units. The
GDD for a certain day is the average air temperature for the day minus
the base temperature for that crop (see table above).
GDD = (Daily max + daily min)/2 – crop base temp
To calculate how many GDD the crop has received at any time;
1. Obtain daily maximum and minimum Fahrenheit air temperatures from the planting date to the present at a location as close to the field as possible.
2.
Calculate the GDD for each day using the formula above and total them to
get the cumulative GDD for the crop. For example if the maximum temperature
for a day is 58 F and the minimum is 32 F, the GDD for wheat that day is
(58 + 32)/2 –
32 = 13 units.
Typically wheat in the Pacific Northwest requires about 144
GDD to germinate plus 90 GDD for every inch seeding depth for emergence.
Note: All the above calculations and GDD requirements are on the Fahrenheit
scale. Most sources use the Celsius scale, so ensure that you know which
system is used in any recommendations. (Cook and Veseth, Wysocki).
If you have further questions, contact Diana Roberts at 509-477-2167
or e-mail at robertsd@wsu.edu
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