The following are a few thoughts about planting corn or sorghum
late this season with our inability to get cotton planted due to the extended
period of wet weather. It is a subject
that has gotten quite a bit of discussion lately, especially between us
entomologist. This was originally
published on the now blog based FOCUS on May 24, 2015.
Posted: 24 May 2015 06:44 PM PDT
Some cotton growers are considering planting corn or sorghum if
they can’t get all of the cotton
acres planted by the insurance cutoff date. This article is meant to provide a
few insect considerations when thinking about corn and sorghum. Dr. Dana
Porter, Ag Irrigation Engineer at Lubbock, is preparing an article on
irrigation considerations for release here in a few days.
Corn
The three biggest threats to late-planted corn on the southern
High Plains are fall armyworm, spider mites and mycotoxins (which are affected
by fall armyworm and corn earworm).
Any corn planted in June is considered to be “late corn” from an insect perspective. There is still time to
reach maturity with some of the less-than-full-season hybrids, so by late I
mean only as applied to insects. The first and perhaps most important
suggestion is to plant a good Bt
hybrid; fall armyworm numbers increase as the season progresses and this
insect can cause significant yield loss to non-Bt corn and corn that does not
have at least two Bt toxins in it. There are very good hybrids available with
two or more toxins and a list can be found here: http://www.texasinsects.org/uploads/4/9/3/0/49304017/traittable2015april6south.pdf).
Your seed dealer will be able to make specific hybrid recommendations. I would
avoid one toxin corn (Herculex and Yieldgard) because it won’t stand up well to high
fall armyworm numbers.
Fall armyworm is a major
threat. How
bad can it be? Our research at Lubbock has shown an average of 0.2 pounds of
yield loss per ear when fall armyworm punctures the side of an ear and feeds on
kernels. The losses come both from direct ear feeding and from the fungi that
come along with the insects and through the wounds they cause. In or trials,
52% of the yield loss was from fungi and 48% was from direct kernel damage by
the insect. These same research trials showed drastically higher levels of
mycotoxins in ears with side puncture FAW damage than ears with only tip
damage. Fall armyworm larvae can also damage the ear shank and cause ears to
drop to the ground.
The second suggestion is to pay particular attention to the Bt
corn refuge planting. All Bt corn with two or more toxins planted in the “cotton zone” (basically south of Amarillo) is supposed to
have a 20% non-Bt block planted in the field or in a closely adjacent field.
Strip refuges of four or more rows are also allowed. Single toxin corn is
supposed to have a 50% refuge. Even though our recent research makes us
confident that strip refuges have less fall armyworm damage than block refuges
(from the toxic Bt pollen falling in the strips and creating toxic kernels),
this protection is adequate with only low to moderate populations of fall
armyworm. Late planted corn should
expect heavier populations of fall armyworm, so a refuge, if it is to be
planted, should be a block refuge that can be sprayed if things get bad.
Also, even though seed blends are not supposed to be planted in the “cotton zone”, some fields are
planted to seed blends. None of the non-Bt seed in a seed blend counts toward
refuge in the cotton zone, so a correct refuge, if it is planted, should be
calculated as if there was no refuge seed in the seed blend.
We don’t
worry about corn earworm (cotton bollworm) much since it is mostly a tip
feeder, whereas fall armyworm is a tip feeder, punctures the sides of the ears
and does shank damage. Fall armyworm is the biggest caterpillar threat so
choose your Bt corn to protect against fall armyworm. Here are my rankings for
the efficacy of Bt hybrid types against both pests:
|
Caterpillar Pest
|
Bt
toxin combination in commercial hybrids
|
||||||
|
Cry1F
|
Cry1Ab
|
Cry1Ab+
Cry1F
|
Cry1Ab+
Vip3A
|
Cry1Ab+
Cry1F+
Vip3A
|
Cry1A.105+
Cry2Ab2
|
Cry1A.105+
Cry2Ab2+
Cry1F
|
|
|
Corn
earworm
|
F-P
|
F-P
|
F-G
|
VG
|
E
|
G
|
G-VG
|
|
Fall
armyworm
|
F-P
|
F-P
|
F-G
|
VG-E
|
E
|
G
|
VG
|
P=Poor, F=Fair, G=Good, VG=Very good, E=Excellent
The next threat, and one
that is always present whether corn is Bt or not, is spider mites. Bt corn has no effect
on spider mites. Mites can be a very serious problem and occasionally require
more than one miticide application. The good news is that late-planted corn is
somewhat less prone to reaching the economic threshold for spider mites than is
corn planted earlier in the season. Late planted corn should still be scouted,
but the spider mite threat is somewhat lower.
Mycotoxins are the next
worry in corn. Research has shown that corn growing under too much drought
stress is prone to developing high aflatoxin levels, and corn grown with too
much water is prone to developing high fumonisin levels. The relationship
between water stress and mycotoxins levels is not well understood. There is an
insect component, too, because the insects wound the ears and allow points of
entry for the fungi that make mycotoxins. If you are going to plant corn then
be certain you have enough irrigation capacity to keep it around at least 75%
ET during peak water demand; silking through grain fill. Mycotoxins at levels
beyond the Federal standards can either cause a dock at the elevator or make
the crop unmarketable. Dr. Dana Porter will write about irrigating corn later
in the week.
Sorghum
Sorghum is an excellent crop for late planting and the biggest
worry being voiced this year is sugarcane aphid. It is true that we have been
surprised how far north the sugarcane aphid has moved in 2015, and it is
possible it will be an early to mid-season arrival this year. (Last year we
dealt with it as a late season problem.) However, we have two excellent
insecticides to control the aphid and we have solid economic thresholds. If the
aphid arrives early to mid-season then it is likely that 2-3 applications will
be needed. If it arrives later, say in August, then one application might be
sufficient. There are sorghum hybrids resistant to sugarcane aphid, but the
resistance is not all that strong and these fields should be monitored as if
they were planted to susceptible
hybrids. The resistant hybrids are Sorghum Partners (Chromatin): SP6929, KS310,
NK5418, and K73-J6; Monsanto: DKS37-07 and Pulsar; and Pioneer*: 83P56. The
sugarcane aphid is not a reason to avoid planting sorghum on the southern High
Plains; it is manageable. Statewide sugarcane aphid news can be found here: http://txscan.blogspot.com .
Sorghum planted in June has a higher chance of needing sorghum
midge control than earlier planted sorghum. While pyrethroids are often used
for midge control, it should be noted that they can drastically enhance the
risk from sugarcane aphid by removing the biological control agents from the
field. Non-pyethroid insecticides should be used, and this is also the case if
headworms need to be controlled. These complexities are explained in three
videos posted here: http://www.texasinsects.org/sorghum-videos.html.
Thanks Pat!
Blayne Reed
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