In the southern parts of Nueces
county, and western parts of the Coastal Bend , grain sorghum is showing severe
moisture stress and questions are being asked about possible options or fate of
this crop. One of the reasons we grow grain sorghum in the Coastal Bend is that
sorghum is drought tolerant and unfortunately we seem to have our share of
droughts. Currently we have grain
sorghum at all stages of development, however most is in the stage – panicle
initiation, just prior to flowering, at which time moisture stress can cause
significant yield reduction.
So let’s review some factors that
make grain sorghum so drought tolerant.
First the sorghum plant has an aggressive root system with a higher root
density that allows deep water extraction, in fact some research indicates that
90% of the total water used by the sorghum plant can come from a soil depth up
to 5.4 feet, however most comes from the top three feet of soil. Our problem
this year, we have very limited deep soil moisture as we learned from soil core
samples we took back in February. So we
knew going into this crop year that if we did not get timely rainfall events
after planting, our crops could become moisture stressed.
In addition to a good root system,
other drought tolerant traits of grain sorghum include stomata that remain open
at a wide range of leaf turgor, and stomata that close at higher levels of
stress, thus avoiding further water loss.
Then there is the leaf roll, that we all see occurring now, that also
avoids further water loss by decreasing surface area of leaf contact with
radiation. This leaf roll usually begins
after 10 to 14 days of water stress. Yet
with all of these traits, when there is no water, the crop suffers badly.
At
some point the moisture stress becomes so severe that the plant has reached a
point of no recovery, even if rain events begin soon. So when has the crop reached a point of
calling the insurance adjuster to the field?
According to USDA Grain Sorghum Loss Adjustment Standards Handbook, http://www.rma.usda.gov/handbooks/25000/2010/10_25210.pdf
, grain sorghum has reached the permanent wilting point when plants are damaged
to the point that the leaves remain tightly rolled throughout the night and the
four lower leaves of the plant are brown and brittle and during the day will
crumble when rolled between the hands. So
the bottom line is, when the crop reaches the permanent wilting point, the crop
is done.So if you have grain sorghum leaves that are crispy dry in the early morning after
having overnight relief and then remain that way throughout the day, you can
pursue the permanent wilt cause of loss with your crop insurance.
Could you harvest the stressed crop
for hay? Yes, that would be one
possibility, but there are factors to consider.
Before you choose this option, be sure to check with your crop
insurance provider.
Drought-stressed crops like grain
sorghum and corn may result in the accumulation of nitrate at levels that are
toxic to livestock. Nitrates naturally
occur in plants. At normal plant nitrate
levels, rumen microflora convert ingested nitrates into microbial proteins that
are then used by the animal. When the
uptake of nitrates exceeds the rate of use to sustain rapid growth, excessive
accumulation may occur, as is the typically the case when plants are stressed
due to drought—even though sorghum or other plants may not be growing, they are
still accumulating nitrate. Moreover, accumulation may be amplified when high
rates of nitrogen fertilizer were applied prior to the stress period.
Nitrate toxicity is often associated
with sorghum hybrids, sudangrass, johnsongrass, sorghumsudan hybrids, corn, and
carelessweed (pigweed). If you produce
or plan to use any of these crops for hay, testing for nitrates is strongly
recommended when the crop is produced under conditions of moisture stress.
Research at the Texas Veterinary
Medical Diagnostic Laboratory at Texas A&M University has found that it is
safe to feed forage containing up to 1 percent nitrate (on a dry-matter basis)
to healthy ruminants. Forage with a
higher percentage of nitrate could be fed if it is ground and mixed with low nitrate
forage to reduce the overall nitrate levels to less than 1 percent (dry-matter
basis). Forage containing 0.5 to 1
percent nitrate should not be fed to weakened cattle unless your veterinarian
has approved it. The 1 percent nitrate
level assumes that cattle are healthy, well conditioned, and being fed a
high-energy diet. An online reference
regarding Nitrates and Prussic Acid in Forages can be found at the web site; https://agrilifebookstore.org/
by typing E-543.
If forages contain too much nitrate
the animals cannot complete the conversion process to protein and nitrite levels
build up. The result is the formation of
methhemoglobin which can cause an animal to die from asphyxiation, or lack of
oxygen. The animal’s blood turns brown
instead of the normal bright red.
When sampling for nitrates, it is
important to remember that nitrate accumulates in the lower stem, and the least
is found in the leaves. If sampling
corn, grain sorghum, and sorghum-sudangrass standing forage, create a composite
sample from plant parts taken from at least 10 to 15 areas with the same
fertility and moisture conditions. Do
not mix plants from “good” and “bad” parts of the field. Create different
composite samples for these areas. Ship
samples to the laboratory in clean paper sacks. Do not use plastic bags because
the high moisture content will cause the samples to mold, which interferes with
the nitrate analysis.
To sample corn, grain sorghum and
sorghum-sudangrass in bales, the bale must be split open and the lower stems of
individual plants must be collected. If the bale is going to be ground before
it is going to be fed, then representative core samples can be taken with a
bale probe. Samples can be
submitted to the Texas AgriLife Extension Service Soil, Water, and Forage Testing
Laboratory. Submittal forms are available online at http://soiltesting.tamu.edu
