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During
December and January, seed dealers were unusually busy with soybean
orders. Typically, decisions on soybean seed aren’t made before
Christmas and supply of seed far exceeds demand. This year the demand
for planting seed generated by high soybean commodity price, the need
for a second crop following wheat harvest and an ever decreasing supply
combined to make soybean variety selection chaotic. After the dust
settles from this process, growers throughout the state will be left
with soybean varieties varying greatly in maturity, plant type, vigor
and disease resistance. Many of these growers will plant these soybeans
in fields that haven’t seen soybeans for many years. To make the most
of the high price, growers need to consider the characteristics of the
varieties they will plant and match them as judiciously as possible to
the conditions in which they will be grown.
Maturity
Group/ Planting
Date
Soybeans
are photoperiod sensitive, which means reproductive growth in soybeans
is triggered by changing day lengths. Critical night length varies
between varieties, allowing them to be categorized into maturity groups.
Each maturity group has a zone of adaptation in which normal
reproductive growth occurs when planted according to traditional
planting dates. In the United States, these zones range from Maturity
Group 00 along the Canadian border to Maturity Group VIII along the Gulf
Coast. A variety planted south of its zone of adaptation will flower and
mature earlier. Alabama has three zones of adaptation with MG VI adapted
to the northern part of the state, MG VII to the central and southern
parts, and MG VIII adapted to extreme south Alabama.
Over
the past 15 years, growers in the midsouth and southeast have planted
early maturity groups outside their zones of adaptation to match the
critical reproductive stages of soybeans with historical rainfall
patterns. This early soybean production system has given growers in
Alabama the flexibility to plant group IV and V varieties throughout the
state. Success with this system depends on matching planting date with
maturity group. Generally, early planting of soybeans should begin in
the middle of April with mid to late-group IV varieties. Areas in the
extreme northern part of the state have tried earlier plantings and
earlier maturity groups, but the results have not been consistent. Mid
to late-group IV varieties planted from the middle of April to the first
of May normally mature in late August to early September.
As
planting date shifts into May, growers should change to a group V or VI
variety. Planting these maturity groups earlier than May 1st have not
shown an advantage and can result in short stature plants at harvest.
Group V varieties planted in early May will be ready for harvest in late
September to early October and group VI varieties in early to
mid-October. When double cropping behind wheat, mid to late-group V or
early-group VI varieties should be used in the northern part of the
state and late groups V, VI and VII varieties should be planted in the
central and southern parts of the state. If planting is delayed into
July in areas where an early frost is possible, group IV varieties can
be planted late. They are indeterminate and will continue vegetative
growth after flowering.
Row
Spacing / Seeding Rate
When
deciding on row spacing for soybeans, growers should consider the
interactions between maturity group, planting date and plant type. The
perception that soybeans planted in narrow-rows will yield more than
wide-row plantings exists in some areas in Alabama.
According
to Larry Heatherly with the USDA Agricultural Research Service,
"Choice of row-spacing should not be based solely on the
presumption that narrow-row soybean systems will yield more than
wide-row systems.… The majority of row-spacing research in the
midsouthern U.S. has shown only a slight yield advantage for narrow-rows
in conventional plantings."
This
being said, certain varieties and planting dates will require narrow-row
spacing to maximize yield and allow proper management.
The
first consideration in row-spacing should be compatibility with
row-spacing in a grower’s rotational crops. Beyond that, narrow-row
spacing (20 inches or less) will be required in situations where
achieving canopy closure is difficult. These situations include planting
varieties with upright growth characteristics, planting in fields with
droughty or heavy clay soils, planting group V varieties too early and
planting group IV or V varieties late in the season. Generally,
indeterminate group IV varieties have a narrow, upright plant type that
does not branch well and planting determinate group V varieties before
May 1st results in reduced plant height at maturity. Both of these can
result in an incomplete canopy in wide-rows. Canopy closure is critical
for light interception and weed management.
Unlike
corn and cottonseed, which are now packaged according to seeds per bag,
soybeans are still bagged according to pounds per bag. When deciding on
seeding rates, growers should make decisions based on seeds planted per
acre.
According
to Heatherly, "Data collected in the midsouthern U.S. indicate that
a density of 80,000 to 120,000 uniformly distributed plants per acre is
adequate for maximum yield."
This
holds true across row-spacing, but growers must account for differences
in the efficiency of drills versus planters and adjust seeding rate
accordingly. Generally, drills will have poorer seed placement and
coverage, and require higher seeding rates to achieve equal stands. This
year, many seed companies have experienced seed quality problems. Check
the germination rate of your seed and adjust seeding rates accordingly.
Brad
Meyer is an agronomist for Agri-AFC. Contact him at bradm@agri-afc.com.
REFERENCES
Soybean Production in the
Midsouth; Larry
G. Heatherly & Harry F. Hodges; CRC Press LLC; 1999. |
