Controlling Weeds in Wheat Stubble in Extreme Western Kansas
Controlling Weeds in W heat Stubble in Extreme Western K ansas
Merlin A. Dillon
Roy E. Gwin
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Controlling Weeds in W heat Stubble in Extreme Western Kansas
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Controlling Weeds in Wheat Stubble
in Extreme Western Kansas
Merlin A. Dillon, crops research agronomist.
Roy E. Gwin, Jr., superintendent,
Tribune Branch Experiment Statfon.
Average rainfall between wheat harve's't and
frost at Tribune, Kansas, is about 6.5 inches-30
percent of the total expected during the entire
fallow period in a fallow-wheat system. Much
of this late summer and fall moisture is wasted
if weeds and volunteer wheat are allowed to
grow undisturbed. Since moisture is usually the
limiting factor in crop production, preserving
additional water during fallow should increase
crop yields. Or, the fallow period might be
shortened. For example, fallow-wheat-sorghum
(FWS) involves two crops in three years with
two fallow periods of about 11 months each.
Whereas, fallow-wheat or falldw-sorghum
systems produce two crops in four years.
The study reported here compares several
cropping systems, involving fallow, wheat, and
sorghum (Table 1). The tests will run for several
more years, so these results are only preliminary.
Thus far we have harvested sorghum four years,
1972, 1973, 1974, and 1975, and wheat three
years, 1973, 1974, and 1975. Sorghum yielded
well in 1972, 1973, and 1975, but an early fall
freeze in 1974 markedly reduced yields.
AGRICULTURAL EXPERIMENT STATION
Kansa$ State University, Manhattan
Floyd W. Smith, Director
age wheat yields were high all three years, 31
to 42 bu/a (Table 2).
When conventional tillage was used, wheat
after sorghum (FWS) yielded the same as wheat
after conventional fallow (fW), 33 bu/a. In
the same FWS system, sorghum yielded 46 bu/a
compared with 53 bu/ a in the fallow-sorghum
rotation (FS). But, two~crops (wheat and
sorghum) were produced in three years rather
than two crops in four years. The
conventional systems did not include complete weed
control in the wheat stubble following harvest.
In- the two experimental systems, FWS+ and
FW+, blading plus a residual herbicide (an
80% atrazine product) controlled weeds in the
wheat stubble. Equivalent late summer and fall
weed control could have been accomplished
with additional tillage in lieu of the herbicide.
But the atrazine aided in weed control the next
spring and, in the FWS+, in the sorghum crop,
Experimental fallow-whei!lt-sorghum. Atrazine
applied at 2.5 lb/ a (product) to new wheat
stubble, then bladed. Disked once before
planting sorghum. Residual atrazine was the
weed control. Sorghum cultivated if
necessary before wheat planting. No herbicide
used between sorghum and wheat.
Conventional fallow-sorghum. Sorghum
stubble bladed as needed until frost, then listed
to prevent winter wind erosion. Ridges disked
twice before planting sorghum. !gran applied
to control weeds. Sorghum cultivated.
although in some years spring tillage was
needed to control grass weeds.
More moisture was stored in the soil in the
FWS+ system than in the co~J_ventional FWS
system, probably largely because the usual
system did not control after-harvest weeds. The
increased moisture contributed to increased
yields. The FWS+ plots had 4.1 inches of soil
moisture at planting time, compared w ith 2.9
for the conventional system, an increase of 41
percent. Likewise, sorghum yields increased
from 46 bu/ a to 53 bu/ a, a 17 percent increase.
Whe·at yields in these systems averaged 38 bu/ a
in the FWS+ and 33 bu/a for FWS. However,
the difference was from only 1975 yields.
Wheat yields under the two systems did not
differ in 1973 and 1974 (Table 2).
In the widely used fallow-wheat system,
wheat stubble is often left untouched until the
spring after ·ha rvest. Our work shows weed
control in wheat stubble is as important in FW
as in FWS. We used the same treatment
described in the FWS+ system, except the atrazine
rate was reduced from 2.5 to 1.5 lb/a product.
Average soil moisture at wheat planting
in'reased 32 percent (from 4.4 to 5.8 inches), and
average wheat yields increased 21 percent (33
to 40 bu/ a) .
Fall weed control was not advantageous
every year; the benefit depended on amount
and distribution of rainfall. Figure 1 gives the
amount and distribution of precipitation for
1971-1975. Note the wide variation and that
the average was less than long time figures.
When moisture after harvest was low, weed
growth was limited even without control
measures. When fall rain was above average, 1972,
and 1973, fa.ll weed control led to increased
crop yields (FW wheat yie lds in 1974 and 1975,
and FWS sorghum yields in 1973 and 1974,
Table 2). Fall weed control in 1971 and 1974
(when rainfall was low) increased neither
moisture nor yields. However, fall weed control has
never depressed yields.
Atrazine persists relatively long in the soil.
Its rate of breakdown depends on many factors,
Table 2 .
Effects of cropping system on soil moisture and yields of wheat and grain sorghum at Tribune, KS.
'* Top statistical g rovp each year.
1. See Table 1 for complete description of sntems.
3. l.S.D. at S% level lor the averages.
2. Sorghum bu/a X 56 + wheat bu/a X 60 + number of years in the sequence.
Small difforences should not b e overemphasized. Lea•l •lgnificant differences (l.S.D.) are shown at the bottom of each table. Unless two values diff.,.
by at least the L.S.D. shown, little confidence can be placed in the superiority of one over the other.
including temperature, moisture, organic mat~
ter, and application rate. It is commonly usf
for weed control in corn, sorghum, and s6m-..
fallow systems. However, it is not registered
for use in wheat-fallow system in western
Kansas, and its use is not recommended.
Our results show the importance of controlling
fall weeds. Until atrazine or some other
herbicide is registered and found effective, wheat
stubble s·hould be bladed to control weeds and
Yields from the various cropping systems are
sometimes difficult to compare directly. We
converted grain yields to pounds per acre per
year <lb/a/ yr, Table 2). Highest yield was from
FWS+, 1768 lb/ a / yr. That was 249 lb/ a/ yr
or 16 percent more than conventiona I FWS at
1519 lb/ a/ yr. Both FWS and FWS+ yielded
substantially more than FW (990 lb/a /yr) and
FW+ (1200 lb/ a / yr). Again the advantage of
fall weed control is readily apparent when the
FWS + and FW+ are compared, respectively,
with FWS and FW.
Ultimately net return, not total yield, will
decide acceptance or rejection of a practice. When
two crops, wheat and sorghum, are involved
relative prices and price changes make accurate
comparisons difficult. Possible additional costs
for herbicides, tillage, seed, harvesting, and
orher items such as interest must also be
considered. Gross returns can be easily calculated
from yields given in Table 2. Use current or
expected commodity prices. In any event and
with virtually any reali stic price structure, it
seems that FWS+ (1768 lb grain/ a /yr) will net
substantially more per acre than will FW (990
lb/ a/ yr).
Th is is a progress report of continuing
research at the Tri b une Branch Experiment Station.
We will continue to work and continue to bring
results to farmers, producers, and other