We saw the first soybean flowers of the year yesterday at the Arlington Ag Research Station. As we enter the soybean reproductive growth phase
there are a few
things to keep in mind. The first is that soybean will produce flowers
for ~3 to five weeks, depending upon planting date and environment.
During that time soybean will abort anywhere from 20 to 80% of the
flowers that they
produce. Generally it is the first and last flush of flowers produced
that are most likely to be aborted.
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| R1 soybean growth stage |
Next, the timing
window for glyphosate applications in our early planted soybean is
quickly closing. Glyphosate labels indicate that applications can be
made through R2 or full flower. The R3
growth stage begins when one of the four top nodes with a fully
developed leaf has a 3/16 inch long pod. Applications made after the R3
stage begins are off-label
applications. On average it takes ~ 4 days to move from
R1 (beginning flower) to R2 (full flower) and ~10 days from R2 to the
start of R3 (beginning pod).
Last but not least,
wheel track damage made from ground applications may start to reduce yield. Sprayer wheel traffic
from first flower (R1) through harvest can damage soybean plants and
reduce yield (Hanna et al. 2008). Our research suggests that an adequate
soybean stand (more than 100,000 plants per acre) planted in late April
though mid-May can compensate for wheel tracks made when a field is
sprayed at R1. Yield loss can occur, however, when wheel tracks are made
at R1 or later in thin soybean stands (less than 100,000 plants per
acre) or late planted soybeans. Regardless of stand, plants could not
compensate for wheel tracks made at R3 (early pod development) or R5
(early seed development). The average yield loss per acre is based on
sprayer boom width (distance between wheel track passes). In our trials
yield losses averaged 2.5, 1.9, and 1.3% when sprayer boom widths
measured 60, 90, and 120 foot, respectively. Multiple trips along the
same wheel tracks did not increase yield loss over the first trip.
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| Spraying soybean at the R1 crop growth stage |
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| Wheel track damage to drilled soybean at R1 |
Hanna, S., Conley,
S. P., Shaner, G., and Santini, J.
2008. Fungicide application timing
and row spacing effect on soybean canopy penetration and grain yield. Agronomy Journal: 100:1488-1492.
