Weed Control in Seedling Alfalfa, Using Post Emergent Herbicide
Treatments
Mark Brubacher, Seth Woodward,
California State University, Chico, Students
Jerry Schmierer,
University of California Cooperative Extension, Farm Advisor, Colusa County
Abstract
A series of four seedling alfalfa postemergent herbicide trials were established in the
fall of 1998. Two of the trials were located near the town of Grimes in Colusa County, CA.
The other two trials were conducted on the California State University Chico farm in Butte
County, CA. The trials utilized five herbicides and combinations of these five. The
herbicides used included Bromoxymil (Buctril), 2,4-DB (Butoxone), Sethoxydim (Post),
Clethodim (Prism), and Imazethapyr (Pursuit). With the exception of only one combination,
the trials found that combining herbicides did not significantly increase the overall
effectiveness of the treatments. One of the four experiments was designed as a timing
trial. The two to three true leaf stage of the alfalfa seedlings was found to be the
optimal time to apply herbicide treatments.
Introduction:
California produced over $1 billion worth of alfalfa in 1997. (Putnam &
Schmeirer,
1998) Alfalfa production is California's seventh most productive agricultural enterprise.
(Putnam, 1998) Controlling weeds in alfalfa at the seedling stage is one of the most
important factors in producing a high yielding, good quality stand of alfalfa. By
competing for light, nutrients, and water, weeds can reduce yields and the value of
alfalfa. If left unchecked, weed infestations can untimely render a stand of alfalfa
worthless. (Cudney et al., 1990) The Sacramento Valley produces approximately 10% of the
state’s alfalfa.
Each of the four trials conducted, utilized combinations of herbicides. Herbicide
combinations have several advantages over a single material application. Herbicides have
different modes of action and can be selective in the type of plants they affect.
Therefore, a combination of herbicides may control a broader range of weed species. Lower
doses of two combined herbicides can usually be applied more efficiently than a higher
dose of a single herbicide. Lower doses of herbicides reduce damage to the crop and lower
the active residues left behind in the crop and soil, ultimately reducing environmental
impacts. Lastly, combining herbicides greatly reduces the ability of weeds to develop
resistance to treatments. (Jordan et al. 1994) A study performed by Orloff and
Cudney,
found that combination treatments of 2,4-DB amine plus Pursuit, Sethoxydin, or
Pronamide,
provided the most complete control of weed species. (Cudney et al. 1990)
Young weeds are far easier to control than well-established weeds. In fact, late
application of post emergent herbicides is the most common reason for herbicide failure.
(Orloff et al. 1995) One of the trials conducted at the CSUC farm was a timing trial. The
optimal time for herbicide applications is well established. The objective of this timing
trial is to provide the local growers with information on weed susceptibility at different
growth stages, in the event that the grower failed to apply herbicide at the optimum time.
Four post emergent herbicide trials in seedling alfalfa were established in the fall of
1998. The primary objective of these alfalfa herbicide trials is to provide the farmers of
the Sacramento Valley reliable and current information relating to weed control in
seedling alfalfa.
The primary hypothesis to be tested in these experiments is that combination herbicide
treatments provide more complete weed control when compared to the application of a single
product. The second hypothesis is that the application timing of herbicides alters the
effectiveness of herbicide treatments. The later the application, the less effective the
herbicide will be.
Materials and Methods:
Two of the four experiments were conducted on the CSU Chico teaching farm in Butte
County, California, the other two trials were conducted on the Tom Ellis farm in Grimes,
in Colusa County, California. The seedling alfalfa fields used for the trials were
prepared and planted in the fall of 1998. The fields were set up for flood irrigation, and
the checks were spaced fifty feet apart. The four experiments were set up in similar
fashion. Each of the trials was set up as a randomized complete block design. All of the
treatments within the trials were replicated four times. Each of the individual plots
within each block was 10 feet wide by 25 feet long. For details on the herbicides and
herbicide combinations used, as well as the rates and dates of application see Tables 1
through 4. A pressurized CO2 backpack sprayer was used to apply the herbicides. The boom
width of the sprayer was 6.7 feet, leaving a 3.3 foot wide control between each treatment.
With the exception of Trial 2 conducted on the CSUC farm, all treatments were applied
at the 2 to 3 true leaf stage of the alfalfa seedlings. Trial 2 was designed as a timing
trial. This trial included six treatments, which were applied at three growth stages of
the alfalfa development. The first timing was applied December 17, 1998 when the alfalfa
was at the 2 -3 leaf stage. The second timing was applied on January 27, 1999 when the
alfalfa was at the 5 -7 true leaf stage. The final application was applied on March 4,
1999 when the alfalfa had reached a 10 - 11 true leaf stage.
The treatments within the trials were analyzed using a visual rating scale of 0 through
10. Zero representing no weed control, 10 representing 100% control, and 8 representing
economic control. The overall ratings were determined by visually assessing the
effectiveness of weed control and the negative effects of alfalfa suppression. The
effectiveness of each herbicide treatment on individual weeds within each plot was also
determined using a rating of 1 to 10. Negative effects on the alfalfa caused by the
treatments was also rated on a scale of 1 to 10, with 0 representing no suppression and 10
representing 100% kill of the alfalfa seedlings. The data was analyzed using a
multi-factor Anova statistical analysis program for a completely randomized block design.
Results And Discussion:
Overall Results: In general, the single treatments of Pursuit and combinations
containing Pursuit received the highest overall ratings. The highest overall rating
between the four trials was 8.5. This rating was given to Pursuit at the 2.16-oz/a
concentration in Trial number 1 at the CSUC farm. Buctril 2EC provided reasonable weed
control. In Trial 3, both concentration levels of Buctril 2EC provided greater than
economic control. The poorest overall weed control was provided by the WP formulations of
Buctril and Butoxone.
The Buctril WP formulations resulted in significantly lower overall ratings as compared
to the Buctril 2EC formulations. The Butoxone 200 EC formulations out performed the
Butoxone 7500 WP formulations. In Trial 1, the EC formulation of Butoxone was
significantly greater than the WP formulation. In Trial 3, the EC formulation of Butoxone
received a higher rating, but it was not significantly greater then the WP formulation.
Alfalfa suppression: The application of 2,4-DB (Butoxone 200 EC.), resulted in the
highest levels of alfalfa suppression, in single applications and in combination with
other herbicides. Single applications of Butoxone 200 EC, received suppression ratings
between 30% and 75%, with an average suppression rating of 45%. Single applications of the
grass herbicide Clehodim (Prism), resulted in no alfalfa suppression. Suppression
resulting from the application of Imazethapyr (Pursuit) ranged form 0% to 15%, and
suppression resulting from the application of Bromoxynil (Buctril) ranged form 0% to 35%.
In general, there was no significant suppression difference between the EC formulations of
Buctril and the WP formulations of Buctril. This was not the case with Butoxone. The EC
formulations of Butoxone received significantly higher ratings as compared to the WP
formulations.
Effect of Application Timing: There is a definite relationship between timing and
the effectiveness of the herbicide treatments. Graph 1. Shows the correlation between
application time and the effectiveness of the herbicide treatments. With
the exception of Pursuit, the data shows an almost linear decline in the effectiveness of
the herbicide treatments over time. It is clear that the 2 - 3 true leaf stage is
the optimal time to apply herbicides.
Graph 1. The relationship between the timing of herbicide application
and overall weed control
The Effect of Combining Herbicides: One of the hypotheses being tested was that
herbicides would provide greater weed control when in combination, as compared to single
herbicide applications. Treatment 19 of Trial 1 at the CSUC farm, combined Prism and
Buctril. This is the only case of two combined herbicides providing significantly greater
weed control as compared to the expected weed control of either of the individual
herbicides. Some of the other herbicide combinations did provide greater weed control when
combined, but these results were not significantly different at the 0.5 level of
significance.
Another reason for combining herbicides is to control a broader spectrum of weeds.
Although many of the treatments and combinations containing Butoxone, Buctril, and Pursuit
worked well on broad leaf weed species, however, they missed the grasses present in the
plots. Prism was extremely efficient at controlling grass, but provided almost no broad
leaf control. Only when Prism was combined with one of the broad leaf herbicides was there
satisfactory control of both broad leaf and grasses.
Individual Weed Control: Wild radish (Raphanus raphanistrm L) was a fairly
easy weed to control, most of the herbicides provided sufficient control particularly when
in combination. Wild mustard (Brassica kaber L.C. wheeler) was controlled very well
by the EC formulations of Buctril and Butoxone, but Pursuit provided only close to 50%
control. Shepherd’s purse (Capsella Bursa-pastoris) was controlled well by
Pursuit and combinations containing Pursuit. Chickweed (Stellaria media (L.) Vill)was
a particularly resilient weed. Not one of the treatments provided economic control of
chickweed. Pursuit and combinations containing Pursuit provided reliable control of
Wortcress, with the EC formulation of Butoxone providing the most complete control of
Wortcress (Cornopus didymus). Only Prism and combinations containing prism
controlled the wheat (Triticum aestivum L.), Annual Bluegrass (Poa Balbosa L.) and
Canarygrass (Phalaris minor) present in the plots.
Conclusion
Only the combination of Prism and Buctril provided significantly greater weed control
as compared to individual applications of either of the two herbicides. As expected, the
2-3 true leaf stage of the alfalfa proved to be the optimal time to apply herbicide
treatments. In general Pursuit and combinations containing Pursuit provided the most
complete broad leaf weed control. When grasses were present in the fields, combinations of
Buctril 2EC and Prism provided the most complete grass and broadleaf weed control
Acknowledgements: Special thanks to the Chico State Farm and Tom Ellis for
providing their alfalfa fields for these herbicide trials.
References
Canevari, M., R. Chavarria., D. Colbert., and P. Verdegaal. Efficiency and Yield
Comparison of Herbicides in Seedling Alfalfa in the Central Valley of California.
University of California Cooperative Extension Service. 1988.
Carlson, H.L., and S.B. Orloff. Intermountain Alfalfa Management. UC. Davis Agriculture
and Natural Resources. 1995.
Cudney, D.W., and S.B. Orloff. Postemergence Herbicide Combinations for Grass and
Broadleaf Weed Control in Seedling Alfalfa. University of California Cooperative
Extension, Lancster CA. 1990.
Dalrymple, A.W., and S.D. Miller. Grass and Broadleaf Herbicide Combinations in
Seedling Alfalfa. Wyoming Agricultural Extention in Laramie. 1990.
Jordan, T.N., and G.F. Warren. Herbicide Combinations and Interactions. Purdue
University, 1994. Pages 232-234.
Putnam, Dan. History , Importance, and Production Dynamics of Alfalfa in California.
Department of Agronomy and Range Science, UC Davis, CA. 1998.
Putnam, D.H., and J. Schmeirer. Alfalfa Ice Cream in the Making. California Alfalfa
Workgroup. 1998.
Experiment Data:
Table 1, Table 2, Table 3, Table 4, Weather Data.
