Page %—cbrh ftSjj&fngl l&VurclaV? tfclober \l t jbs6
(Continued from Pago 18)
begin with, then this analysis
probably holds true. However,
if the Bt gene is put into a
hybrid that has lower yield
potential, then there may not be
a benefit from using the Bt
technology.
Put simply, it is quite possi-
ble that a superior non-Bt
hybrid will still outperform an
inferior Bt hybrid. In the end,
yield of the com hybrid will be
some combination of having a
good yielding hybrid to begin
with, and having the addition of
the Bt gene. Thus, hybrid trials
are still as important as ever.
If this new technology means
that in 50-80 percent of the com
fields of the U.S., using Bt com
will give economic returns in
most fields. But that still leaves
30-50 percent of the fields in
which that S7/acrc premium
may not pay off.
How does a grower maxim
ize returns on this $7/acre
premium?
Here is a rule of thumb to
help com growers maximize
their investment in Bt com:
com borer usually attacks the
earliest planted fields (the tal
lest) during its June or first gen
eration. and the latest planted
fields (the younger ones, esp
cially in in silk stages) in its
August or second generation.
Fields planted in between the
earliest and latest arc less likely
to be attacked. Also, because
tillage destroys much of the
com borer population, fields
near no-till may experience
higher com borer populations.
To give you a better feel for
which of your fields com borer
is attacking, tiy a few things,
and some of these you can do
now. Think about which fields
had the most June damage
the “shothole” damage. Were
these the earliest planted ones
in your area or farm? If so, these
would be good ones in which to
use Bt com next year.
For the August and Septem
ber damage (stalk tunnels, ear
damage), you can visit your
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e CARGILL
CARfS
Bt Corn:
fields now and get a feel for
where com borer attacked.
Were the ones attacked those
which were showing green silk
in August, when others were
drying down? Then these might
be good fields for Bt com.
Again, remember the rule of
thumb; com borer usually
attacks the earliest planted
fields in June and the latest
planted fields in August.
Some of our research at Penn
State is addressing this ques
tion: which fields (planting
dates) do cbm borer attack?
Bt com is a dramatic new
technology that all growers
should be interested in seeing
around for a long time, which
brings this article to its last sub
ject: Will insects become resis
tant to Bt com if enough of it is
planted in our fields? There is a
lot of theory, but my opinion is
that it is difficult to predict at
this time whether resistance
will occur.
All com growers have an
important role to play in this
“resistance management,” or
helping us to preserve this valu
able new technology: Watch
your fields and look for com
borer. If you have a field of Bt
com and see com borer dam
age, several possibilties exist.
First, perhaps the field is not Bt
com as you think, or that Bt
hybrid doesn’t have a high level
of Bt toxin. There are test kits
which allow you to know
whether that field has the Bt
toxin in the plants. Second, you
may have mixed Bt and non-Bt
seed, or alternated rows of Bt
and non-Bt, so that the larger
worms moved from non-Bt to
Bt plants (even with Bt plants,
the larger worms may survive).
Third, all Bt com hybrids are
not necessarily equal. Some
have the toxin in all plant parts,
whereas others may only have
the toxin in the living green tis-
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USEFULNESS OF
PSNT FOLLOWING
ALFALFA IN
WISCONSIN
Dr. Gregory W. Roth
Associate Professor,
Agronomy
Penn State
A 4-year study at the Univer
sity of Wisconsin recently pub
lished in the Journal of Produc
tion Agriculture revealed that
the presidedress soil nitrogen
test (PSNT) often overesti
mates the N requirements for
com grown following alfalfa.
ses, so that com borer may sur
vive on non-green plant parts.
These examples so far are
not cases of resistance. Howev
er, if the whole field is Bt com,
and you arc seeing many com
borer worms, you may have a
case of resistance. In any case,
it is important to contact the
producer of the seed and your
local extension agent and let
them know that you may have a
problem. In the end, it helps all.
com growers for all of us to be
aware of the potential of insects
becoming resistant to Bt com.
Any potato grower dealing with
potato beetle knows that insect
resistance can be a significant
economic concern. Let’s pre
serve this valuable new
technology.
In this study, researchers
L.G. Bundy and T, W. Andra
ski of the University of Wis
consin measured the response
of com to N at 24 sites between
1988 and 1991. At each site,
com followed alfalfa and a
sample to 1 foot was taken
when the com was 6 to 12
inches tall.
Where no fertilizer was
applied to the com, the PSNT
levels at sidedressing ranged
from 10 to 43 ppm nitrate-N
and at 11 of the 24 sites the
PSNT level was less than 21
ppm, indicating that the field
would be responsive to N
fertilizer.
In Pennsylvania, we use 25
ppm as a critical level to decide
whether fields would be
responsive. Com yields did not
respond to N at any of the sites
in the study, however, so the
PSNT overestimated the N
requirement at nearly one half
of the sites. The authors believe
that the failure of the test was
caused by slow mineralization
of residual N from the alfalfa at
some of the sites, probably due
to the lower soil temperatures
that arc often encountered in
the spring in Wisconsin.
Most studies indicate that
com following alfalfa rarely
responds to more than 30 to 50
pounds of additional N. The
authors suggest that a small N
application of 50 pounds per
>!
acre or less could be applied on
those fields following alfalfa
where the PSNT is below the
critical level. They feel the
PSNT is a good tool to convin
ce growers on the need to take a
credit for the alfalfa in their N
fertility program, since often it
will indicate that no additional
N is necessaiy.
Pennsylvania research has
revealed very similar results.
We have documented a number
of cases where PSNT level
were relatively low following a
legume sod, resulting in a high
er than expected N
recommendation.
Pr. Dick Fox and his col
leagues at Penn State have
found that to date the PSNT
overestimated N requirements
on 12 of 41 sites, or 29 percent
of the time when following a
legume sod that has not
received manure. Fox also attri
butes this to inadequate miner
alization of nitrate by the time
the sample is taken. In all but
one of the other sites the PSNT
correctly predicted that there
would be no response to addi
tional N.
Thus a high PSNT level in a
field following a forage legume
is a reliable indicator of adequ
ate N for the com crop. Howev
er, a low level following a for
age legume can be misleading
and often indicates the need for
more N than is really required.
Our recommendation has
been not to use the PSNT on
fields where com is following a
legume that has not been man
ured. On these fields, we sug
gest using the standard recom
mendations for com following
a legume of 20 to 50 pounds N/
acre depending on the expected
com yield and the per cent
legume in the old stand. PSNT
sampling should be concen
trated on fields that have
received manure where the test
has been found to do a reliable
job of indicating whether addi
tional sidedress N is required.