Growers are always looking for effective
and lower cost options to produce their crops. As cover crop
use increases, their management becomes an important component
of many farming systems. Timing and method of termination are
the two most important factors of cover crop management. This
research investigates the use of a roller as an alternative cover
crop kill method and the optimum growth stage for its use on
three cereal cover crops.
Cover crop use in the United States is on the
rise, especially in conservation tillage systems. Due to this increase,
growers are looking for effective ways to manage cover crops, while
reducing input costs. Mechanical roller-crimpers have been shown
to be effective in southern Brazil and Paraguay in conservation
tillage systems. However, in the United States the use of the roller
is a relatively new cover crop kill method. The killing of some
cover crops at certain growth stages has been evaluated using herbicides
to a certain extent, however, more research is needed, especially
related to the roller and the potential to reduce herbicide inputs.
During 1998-1999 at two locations in east-central
Alabama, five cover crop kill methods were evaluated on three different
cover crops at three growth stages. Cover crop biomass production,
kill method efficacy, and soil water conservation were evaluated.
loamy sand and Cahaba sandy loam
plot design with four replications
wheat, black oat
stages 8 (flag leaf), 10.51 (anthesis), 11.3 (soft dough)
two herbicides (paraquat and glyphosate), and two reduced chemical
rate (half label rate) combinations with the roller
When termination occurred as late as soft dough
stage (Feekes stage 11.3), the roller was as effective as herbicides.
However, this late stage may not provide growers with enough time
to plant a cash crop. The early milk stage (Feekes stage 10.54),
prior to soft dough, may prove more beneficial since it provides
more time for planting, conserves soil water, and provides effective
kill. The roller provides additional benefits as it lays residue
flat on the soil surface providing maximum soil coverage; to prevent
erosion, decrease soil water losses, provide weed control, and
facilitate planting. Economically, the roller and the roller+herbicide
(half rate) treatments provided a significant savings ($5.25 ac-1
average) in the cost of cover crop termination.
There were no significant differences between
the cover crops when the roller was used. Plant height and maturity,
(i.e., differences in growth stage) were the main factors determining
the roller’s effectiveness.
Identifying more cost effective and perceived environmentally friendly
techniques for cover crop management can increase their use. This
study was conducted to determine the effectiveness and economic viability
of using a mechanical roller-crimper as an alternative kill method
for cover crops. Three cover crops, rye (Secale cereale L.), wheat
(Triticum aestivum L.), and black oat (Avena strigosa Schreb.) were
evaluated in terms of ease of kill and optimum time of kill using
a roller-crimper, two herbicides (paraquat and glyposate), and two
reduced chemical rate (half label rate) combinations with the roller.
During 1998-1999, the study took place at two locations in east-central
Alabama, using a split-split plot experimental design with four replications.
Three Feekes’ scale growth stages were used to determine optimum
time of kill: 8.0 (flag leaf), 10.51 (anthesis), and 11.2 (soft dough).
Percent kill measurements were taken 14 d after treatment application.
Black oat reached maximum biomass at anthesis (7660 lb ac-1), while
rye and wheat continued to increase biomass significantly through
soft dough (8480 lb ac-1 and 9340 lb ac-1, respectively). There was
a significant interaction between growth stage and kill method; by
soft dough, kill methods were equally effective due to accelerating
plant senescence (95% mean kill across kill methods). The label rate
of glyphosate and 1/2 label rate+roller combination produced the
best kill mean, 91% and 89%, respectively, at all growth stage levels
across all cover crops. However, at anthesis, the label rate of paraquat
and 1/2 label rate+roller combination were as effective (mean 89%
kill) as glyphosate. This study shows that it is possible to reduce
the use of herbicides and implement effective alternative kill methods
for cover crops.
Cereal cover crops are useful to growers in many ways (Reeves, 1994), however,
growers must have an effective and cost efficient way to kill covers when
they are ready to plant their cash crop. Mechanical rollers have been used
effectively on millions of acres of conservation tilled land in southern
Brazil and Paraguay (Derpsch et al., 1991). In the United States, the roller
is a relatively new cover crop kill method but there is growing producer,
as well as commercial, interest in this implement. The objectives of this
study were three-fold: 1) determine the effectiveness and economic viability
of the roller compared to herbicides as a cover crop kill method; 2) determine
the optimum kill time for three cover crops in terms of growth stage; and
3) identify any differences in ease of kill for three cover crops using the
The study was conducted at two locations in east-central Alabama, on a Compass
loamy sand (coarse-loamy, siliceous, subactive, thermic Plinthic Paleudults)
and a Cahaba sandy loam (fine-loamy, siliceous, semiactive, thermic Typic
Hapludults), using a split-split plot experimental design with four replications.
Whole plots were three small grain cover crops: rye (Secale cereale L.),
wheat (Triticum aestivum L.), and black oat (Avena strigosa Schreb.). Three
easily identifiable Feekes growth stages (Large, 1954) were the subplots:
8 (flag leaf), 10.51 (anthesis), and 11.2 (soft dough). Sub-subplots were
five kill methods: roller only, glyphosate at 3 pt ac-1 (label rate), paraquat
at 1 qt ac-1 (label rate), roller+glyphosate at 1.5 pt ac-1 (half label rate),
and roller+paraquat at 0.5 qt ac-1 (half label rate). Herbicide treatments
were applied first, immediately followed by rolling on specified plots. The
roller used was a drum roller with horizontal welded blunt steel metal strips,
which made it possible to crush the cover crop, facilitating kill by leaving
plant stems intact, yet discouraging regrowth.
Cover crops were planted into a stale seed
bed at a rate of 90 lb ac-1 on 18 November 1998, using an eight
foot grain drill. Kill treatments
were applied when at least sixty-five percent (65 %) of the plot
was at the desired growth stage. At each growth stage, prior to kill
treatment, two-1/4 meter square biomass samples within each subplot
were taken for each cover crop. Percent kill measurements were taken
using a visual rating method at 7, 14, 21, and 28 days after treatment
(DAT). Visual measurements were made using a 0-10 scale, with 0 being
no kill and 10 being complete kill. In addition plant moisture content
was determined to backup the visual percent kill measurements. Gravimetric
soil water content measurements (Gardner, 1986) were taken 28 DAT
to determine the amount of soil water available to a cash crop planted
after the cover crop. Soil samples were taken in the top 3 inches
of soil (cash crop seed zone) in each sub-subplot using a hand-held
There were no significant location interactions observed, so data
were averaged over locations. All data were analyzed using an analysis
of variance (ANOVA) with SAS (SAS Inst., 1988); means were separated
using the least significant difference (LSD) test at P# 0.10.
A significant cover crop X growth stage interaction was observed (P # 0.05).
Black oat reached maximum biomass at anthesis (7660 lb ac-1), while rye and
wheat continued to increase biomass significantly through soft dough (8480
lb ac-1 and 9340 lb ac-1, respectively). The early maturity of black oat may
be beneficial to growers as it allows for a larger planting window for cash
A strong linear relationship between plant moisture content and visual percent
kill ratings was observed (R2=0.58). The visual ratings will be presented
here. Percent kill measurements were taken at 7,14, 21, and 28 DAT; however,
after 14 DAT there were no significant increases in percent kill (P # 0.05).
Consequently, only the 14 DAT measurements are presented.
There was a significant cover crop X growth stage X kill method
interaction (P # 0.01); by soft dough, kill methods were equally
effective due to accelerated plant senescence (95% mean kill across
cover crops and all kill methods). The label rate of glyphosate and
1/2 label rate+roller combination produced the best kill mean, 91%
and 89%, respectively, at all growth stage levels across all cover
crops (Fig. 1). At anthesis, the label rate of paraquat and 1/2 label
rate+roller combination were as effective (mean 89% kill) as glyphosate.
At flag leaf, the label rate of paraquat and the 1/2 label rate+roller
had a significantly lower kill mean (41% and 42%, respectively),
especially on black oat (24% and 27%, respectively). Cover crop plant
height was relatively low and plant stems were still elongating at
flag leaf, contributing to the low termination rate by the roller
alone at this growth stage. The roller was not able to effectively
crimp the plants at flag leaf, leading to the low kill mean (12%)
by the roller alone for all covers. Roller efficacy increased at
anthesis to 47%, but this was not enough to be a suitable kill method
at this growth stage.
The soil moisture content measured at 28 DAT is indicative of the amount of
soil water available at cash crop planting. The soils at the two locations
were different types, a sandy loam and loamy sand. However, since there were
no significant location interactions, results were averaged across locations.
For reference, the average field capacity of the two soil types is about
14.7% and the average permanent wilting point (PWP) is about 5% (Miller and
A significant cover crop X growth stage X kill method interaction
was observed (P # 0.01). Soil water content measurements at the flag
leaf growth stage were directly related to efficacy of kill method.
Ineffective kill methods resulted in depletion of soil water by still-growing
cover crops. Glyphosate treatments, which resulted in the best kill,
had the highest soil water content for all cover crops 28 DAT at
flag leaf (11%). However, in wheat, soil water following paraquat
treatments (9.5 %) was not significantly different than wheat treated
with glyphosate treatments (11.5%).
Paraquat treatments were especially ineffective at terminating black
oat, resulting in soil water depletion significant enough to likely
affect emergence of a cash crop if planted. At flag leaf, the roller
only treatment was the least effective kill method and therefore
resulted in the lowest soil water content in all cover crops (5%).
Considering an average PWP of 5%, soil at this water content would
not be adequately moist to plant a cash crop.
There were no significant differences in soil water 28 DAT of any
cover crop as a result of kill method at anthesis or soft dough.
However, soil water content was affected by cover crop, as a result
of straw biomass at both growth stages. A significant but poor linear
relationship was observed between cover crop growth (biomass production)
and soil water content (P # 0.01, R2=0.10). At anthesis, rye resulted
in greater soil water content (12%) than either black oat or wheat
(10% and 9%, respectively). At soft dough, soil water content within
wheat (10%) was less than under rye or black oat (12% and 11%, respectively).
These soil water contents would all be moist enough to plant a cash
This study shows it is possible to effectively terminate cover crops using
reduced herbicide inputs, especially when the cover crop is at an optimum
growth stage. Farmers may be able to decrease the use of herbicides when
implementing alternative kill methods for cover crops. At anthesis, it would
be possible to use the combination methods and still get an effective kill
(88% with roller+paraquat and 91% with roller+glyphosate), while reducing
the amount of chemical used, thereby decreasing costs. The average reduction
in chemical costs when using half rates and the roller, rather than full
label rates would be $5.25 ac-1 (reflecting current commercial prices). The
cost of using the roller alone can be estimated as $1.50 ac-1, which is the
cost of running a cultipacker (Prevatt et al., 1998). Use of the roller provides
benefits when killing cover crops as it lays residue flat on the soil surface,
providing maximum soil coverage, thereby preventing erosion, decreasing soil
water evaporation, and providing weed control. The use of a roller also facilitates
planting, by reducing hairpinning of residue when the planter runs parallel
to the roller.
When termination occurs as late as soft dough, which in most cases
is not practical due to cash crop planting windows, the use of herbicides
may even be eliminated. At this late growth stage, all kill methods
were equally effective (94% across all cover crops). The optimum
kill time, when using the roller alone, is some point after anthesis
prior to soft dough, possibly the early milk stage (Feekes growth
stage 10.54). There were no significant differences between the cover
crops in terms of percent kill when the roller was used, the main
determining factors were plant height and maturity, which are directly
related to growth stage.
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