Alfalfa is a key crop in many rotations within Wisconsin, but if not successfully removed it can be troublesome in subsequent crops. This is especially true in no-till systems. This experiment was conducted on Roundup Ready alfalfa, as concern exists in the ability to remove this crop without the use of glyphosate as this is the most common herbicide used to remove alfalfa.
Fall Alfalfa Removal Using Herbicides
Mark Renz, Extension Weed Scientist, University of Wisconsin-Madison
Alfalfa is a key crop in many rotations within Wisconsin, but if not successfully removed it can be troublesome in subsequent crops. This is especially true in no-till systems. This experiment was conducted on Roundup Ready alfalfa, as concern exists in the ability to remove this crop without the use of glyphosate as this is the most common herbicide used to remove alfalfa.
We established a trial at Arlington Field Research Station to evaluate the ability of several growth regulator herbicides in removing alfalfa for no-till production systems. Applications were applied at two timings in October that represent typical timings and environmental conditions for alfalfa removal in Wisconsin. The October 5th timing had good environmental conditions conducive for herbicide absorption/translocation and mortality. In contrast the October 19th timing was applied when conditions were sub-optimal with maximum air temperatures below 50 F the day of and the day after application. A range of growth regulator herbicides and rates were evaluated (See Table 2 for details). All treatments were applied to plots that were 10 ft wide by 30 ft long using a hand held CO2 powered backpack sprayer that delivered 15 gallons/A of spray solution. Other site and environmental conditions are summarized in Table 1.
Table 1.Environmental conditions for fall herbicide applications at Arlington, WI.
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Timing of treatment
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10/5/06
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10/19/2006
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Height of alfalfa
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4-6 inches tall
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5 – 7 inches tall
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Air/Soil Temp at time of application
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Air = 59 F; Soil =57 F
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Air = 39 F; Soil =40 F
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Max/Min air temp day before application
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Max= 62F; Min= 43F
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Max = 53F; Min= 35F
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Max/Min air temp day of application
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Max= 57F; Min= 34F
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Max = 39F; Min= 31F
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Max/Min air temp day after application
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Max= 62F; Min= 29F
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Max = 47F; Min= 25F
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In May, the effectiveness of treatments was evaluated. Percent cover of alfalfa was visually estimated, and number of crowns that had green foliage present were counted in each plot. Table 2 summarizes the average (each treatment was replicated four times) values along with significant differences (p<0.05). Only Weedmaster applied at 2 pt/a didn’t have any resprouting plants, however all herbicides were effective at limiting resprouting of alfalfa at the appropriate rate and timing. The early October timing had significantly better results than the later timing. 2,4-D and Banvel performance was reduced at the later timing at the lowest rate. Weedmaster applied in late October did not have any significant reduction in the number of crowns, but control was much more variable between plots. Only Weedmaster applied at 1.5 pt/A in late October had 5 crowns or less resprout the following spring.
Table 2. Alfalfa cover and # crowns May following fall herbicide applications at Arlington, WI.
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Treatment
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Rate
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Applic. date
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% cover
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# crowns / 300 ft2
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1.0
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pt/a
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10/5
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5.0
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cd
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45.8
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c
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2,4-D Amine + NIS
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2.0
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pt/a
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10/5
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1.3
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fg
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8.5
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de
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2,4-D Amine + NIS
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3.0
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pt/a
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10/5
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0.8
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g
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2.0
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e
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Banvel + NIS
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1.0
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pt/a
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10/5
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1.0
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g
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5.0
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de
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Banvel + NIS
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1.5
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pt/a
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10/5
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1.0
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g
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2.3
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de
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Banvel + NIS
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2.0
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pt/a
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10/5
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0.5
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g
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0.5
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e
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Weedmaster + NIS
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1.0
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pt/a
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10/5
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1.0
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g
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5.8
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de
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Weedmaster + NIS
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1.5
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pt/a
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10/5
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0.3
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g
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0.3
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e
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Weedmaster + NIS
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2.0
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pt/a
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10/5
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0.0
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g
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0.0
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e
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2,4-D Amine + NIS
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1.0
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pt/a
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10/19
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26.3
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b
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103.5
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b
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2,4-D Amine + NIS
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2.0
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pt/a
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10/19
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4.3
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cd
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42.5
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cd
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2,4-D Amine + NIS
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3.0
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pt/a
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10/19
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1.0
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fg
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8.3
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de
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|
Banvel + NIS
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1.0
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pt/a
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10/19
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5.0
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c
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58.8
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c
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Banvel + NIS
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1.5
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pt/a
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10/19
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2.0
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de
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30.5
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cde
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Banvel + NIS
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2.0
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pt/a
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10/19
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0.8
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fg
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8.8
|
e
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Weedmaster + NIS
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1.0
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pt/a
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10/19
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2.5
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fg
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23.0
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cde
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Weedmaster + NIS
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1.5
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pt/a
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10/19
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1.0
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ef
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4.3
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de
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Weedmaster + NIS
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2.0
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pt/a
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10/19
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-
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|
-
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|
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UTC
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-
|
-
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-
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1000.0
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a
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95.0
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a
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Abbreviations: NIS = nonionic surfactant (applied at 0.25 % v/v)
Significance
This experiment clearly shows that environmental conditions can alter the level of control with growth regulator herbicides in alfalfa. Applications when conditions promote herbicide absorption and translocation (temperatures at least in the 50s) are the most desirable. Unfortunately time constraints often make for applications during non-ideal conditions. Realize that this IS NOT RECOMMENDED, but if this does occur a reduction in control will likely result. The level of reduction will vary from field to field due to a range of environmental conditions. It would have been interesting to compare the difference in effectiveness between these growth regulator herbicides and glyphosate, but the use of a Roundup Ready alfalfa stand prevented us from including this treatment.
This study found that rates needed to be increased when applied later in the fall under less than ideal environmental conditions. I think another point to consider is what level of control is acceptable? How many alfalfa plants surviving are acceptable in your production system? While a few volunteer alfalfa plants are unsightly, they likely would not cause a loss of yield.