(N) is the most difficult nutrient to manage in cotton
production. It has more impact on yields, earliness, and lint quality
than any other primary plant nutrient. It is also the most costly plant
nutrient applied per acre. Environmentally, potential nitrate-N leaching
into ground waters is a driving force behind water quality issues and
nutrient management planning policies. Because N is a primary constituent
in all protein, easily transforms into several chemical forms in the soil,
is biologically active, and can be mobile in the environment, farmers
have problems managing it in cotton production.
Growers have used a variety of techniques
to optimize N use efficiency in cotton, such as split or multiple N applications,
starter fertilizers, N fertilizer placement, foliar urea (46-0-0) applications
during bloom, petiole monitoring, plant leaf analyses, water management
(irrigation timing), plant growth regulators (e.g., Pix Plus®), cover
crops, and computer programs and plant growth models (e.g., Gossym-Comax®,
NLEAP®, EPIC®, COTMAN®)
Since it was founded in 1883,
the AAES has been studying N on cotton in an attempt to help Alabama growers
better manage this nutrient. The oldest, continuous cotton experiment
in the world, Alabamas Old Rotation (circa 1896) at Auburn University,
focuses on N management by rotating cotton with other crops and planting
winter legumes. However the Two-year Rotation and Rates of N-P-K experiments
also provide valuable information about N rates for cotton crops in Alabama.
Alabamas current standard N recommendations were developed from
these and other experiments in the 1950s and 1960s, modified in the 1970s
and refined in the 1980s. Therefore, if cotton responds to N rates differently
today, this might reflect effects of improved varieties, higher yields,
and different management.
To take a new look at the effects
of N rates on new cotton varieties with higher yield potentials, seven
years of cotton yield data from these experiments were summarized. Since
1996, Bollgard® and Roundup Ready® varieties have been used at
most locations. At all locations, the N source is ammonium nitrate (34-0-0)
and N rates are split with half applied at planting and half as a sidedress
at or near early squaring..
strongly indicate that the standard N recommendations are still appropriate
for maximum relative yields in these tests when cotton follows cotton
(figure 1). When cotton follows
soybean or peanut as in the Two-tear Rotation Experiment, predicting an
optimum N rate is more difficult. A good soybean or peanut crop may contribute
20 to 30 pounds N per acre to the following cotton crop. Because as many
as six months could elapse between soybean/peanut harvest in the fall
and cotton planting the following April or May, much of the residual N
may be lost from the soil. Data from N-rate treatments on the Two-year
Rotation at five locations verify the variable nature of residual N from
legumes. Since 1992, cotton on this experiment has always followed soybeans
or peanuts. Clearly, cotton response to N rates was highly variable following
a legume, but comments on the soil test report have always taken this
Due to the boll weevil eradication
program in Alabama, boll weevils have not been an economic factor for
Alabama cotton farmers since 1996. With the weevil out of the way, cotton
has the potential to set more late-season cotton bolls, thus increasing
yields and the demand for more N. However, data from these long-term experiments
have not indicated that boll weevil eradication nor the new, genetically
modified varieties have had any effect on cotton yield response to N rates
since 1996. In fact, highest yields were produced in 1992 or 1993 at all
sites except the Brewton Experiment Field.
Because of the rapid adoption of
new, genetically modified cotton varieties, there have been few opportunities
to evaluate their response to soil fertility variables. In 1996, the first
year Bollgard® varieties were available to Alabama producers, the
Rates of N-P-K experiment on a Lucedale soil at the Prattville Experiment
Field was modified to determine if any differences existed in response
to soil fertility variables between two varieties of similar genetic backgrounds.
All plots were split and a Bollgard® variety, Deltapine NuCotn 35B
(DP35B), was planted on half of each and a conventional variety of similar
genetics, Deltapine 5690 (DP5690), was planted on the other half of each
plot. This was repeated in 1997 and 1998.
The DP35B yielded an average of 85 pounds
more lint per acre per year than the conventional variety over all N rates,
but the differences due to variety would not affect the standard N recommendation.
This yield difference is most likely due to sub-threshold control of bollworm
and budworms by the Bollgard® variety.
Long-term, N-rate research at several
Alabama locations since 1992 supports the current standard N recommendations
used on soil test reports. For most sandy and loamy Alabama soils, the
standard recommendation is a total of 90 (plus or minus 30) pounds N per
acre during the growing season; 60 (plus or minus 30) pounds N per acre
is standard for the deep, red, silt, and clay loams of the limestone valleys
of North Alabama. Cotton following a good soybean or peanut crop will
benefit from some residual N, but predicting this response has been difficult.
While N recommendations based on a yield goal may apply for some crops, this clearly is not the case with non-irrigated cotton in Alabama. Producers should follow the standard N recommendation on new fields and make adjustments as experience and cropping systems dictate. Nitrogen rates do not need to be adjusted for the newer, genetically modified cotton varieties.