The Rural Voice, 1988-08, Page 32T
WEATHER
STRESS
he other day I heard about a
fellow who is so concerned
about the water shortage that
he's drinking his whiskey straight.
As for farmers, we are already
seeing the nasty side effects of
"usually safe" low rates of residual
atrazine adversely affecting this year's
grain and beans. Our soil water is not
being replenished with clean, fresh
water — therefore no dilution and
decreased chemical degradation.
As well, with the high amounts of
soil evaporation plus water movement
to root areas and the soil surface via
capilliary action from the soil depths,
we're keeping all our residual atrazine
right in the root zone.
The final chapter on residual her-
bicide problems hasn't been written
yet. My major concern now is resid-
ual Treflan (all trifluralins) and Dual
levels for our fall wheat planting.
Depending on moisture between now
and spring as well as chosen tillage
systems, we may have problems with
various chemical products well into
next year. Perish the thought! Resi-
due testing isn't cheap, and we also
know it isn't fast.
The big question on everyone's
mind these days (as I write this July
14) is "will my corn recover and how
much, if we receive meaningful
rainfall?"
The amount of soil moisture
available to crops is determined by the
interaction of four factors: the amount
of moisture in the soil, the soil profile,
the moisture capacity of the crop, and
the demand for water by the atmos-
phere.
ON CORN
SOME FACTS
SOME SUGGESTIONS
Mervyn Erb
is an agronomist with CROPCO, a crop
consulting firm based in Centralia
For crop moisture to be adequate,
there must be more than enough soil
moisture available to meet the atmos-
pheric evaporative demand. On hot,
windy, sunny days with low humidity,
for instance, evaporation demand on a
crop is high, so a high amount of
available soil moisture must be present
if the crop is to avoid stress.
Water use is low early in the
season and reaches a peak in July.
Average use at that time is almost 0.2
inch per day and under high demand
may exceed 0.3 inch per day.
Shortly after emergence, a corn
plant shifts from depending on food
stored in the seed to depending on
food available in the soil. At this
stage, stress due to moisture shortage
seldom occurs because spring rainfall
is usually adequate and the plant's
moisture requirement is very low —
basically only enough to get perma-
nent roots growing.
If the top few inches of soil
become very dry when the plant is
small, early growth problems will
likely occur. However, some believe
that moderate moisture stress during
this period is actually advantageous,
since such stress may encourage early -
season root growth, which would
prove beneficial later if moisture
supplies become short. Further, a dry
surface soil utilizes more of the solar
energy to heat the soil.
Excess moisture in the early
vegetative stages may retard early -
season root development as well as
create aeration -nutrition problems.
Also, cool but above -freezing temper-
atures apparently have some detrimen-
tal effects at this time, but such effects
are not well documented. Late -spring
frost and freezing temperatures, how-
ever, can destroy a core crop if the
plant growing point has emerged
above the soil surface, which usually
occurs at the 6 to 8 -leaf stage of
growth. If the growing point is below
the soil surface, there is seldom per-
manent injury because the growing
point rarely freezes. The problem
we've seen, however, is that the frozen
tops and leaves "flop" over the whorl
and restrict any new leaf initiation.
Therefore the plant dies a slow death.
The effect of early -season stress
on the final core yield is difficult to
assess. Although moisture stress
reduces plant size, it has little effect
on final yield if good crop production
conditions prevail during the polli-
nation and grain -filling periods.
Relationships between weather and
yield are more significant in the late
30 THE RURAL VOICE