Anthony LeBude, NC State University, Bugwood.org
There are a few species that may require special consideration. In general, most programs begin with glyphosate, which tends to be more consistent than paraquat (Gramoxone). Liberty has a narrow fit, mostly for marestail control, but does not add much for cover crops. Herbicide effectiveness ratings for some common cover crops are provided in Table 1. Here are some considerations as you get into the field this year.
Guidelines for Glyphosate
All cover crops should be actively growing and capable of intercepting the herbicide spray (e.g. not covered with crop residue). Remember to use a sufficient rate, which generally ranges from 0.75 lb. ae to 1.5 lb. ae/acre. The 22 fl. oz. rate of Roundup or 32 fl. oz. rate of Credit, Rascal, Clearout, etc. = 0.75 lb. In general, application alone in good quality water along with appropriate adjuvants (surfactant + AMS) is best and reducing the carrier volume to 10 gal/acre can increase activity. Do not add 28 or 32% UAN or other fluid fertilizers to the spray tank. If the water source has a high pH (8 or greater), consider adding an acidifying agent to the spray solution. Avoid tank mixing with higher-rate (> 0.25 lb.) clay-based herbicides (WDG, WG, DF, DG, F) like atrazine, simazine, and metribuzin. Other herbicides such as 2,4-D, dicamba, clopyralid, Balance or Corvus, Resolve or Basis Blend, etc. are OK.
Some research suggests that wheat is less susceptible to control with glyphosate than cereal rye. In general, make sure you have a sufficiently high glyphosate rate and follow other guidelines to maximize herbicide activity. The Roundup Powermax label recommends 32 fl. oz. (1.125 lb.) up 18-inch tall wheat. Performance is better for applications made prior to the boot stage of growth. Gramoxone can also be effective, but rate, adjuvant, spray volume, nozzles, timing (prior to tillering or after boot), and the addition of atrazine or metribuzin are important for effective control.
Annual ryegrass continues to be somewhat challenging to control. Glyphosate is the preferred herbicide and paraquat (Gramoxone) does not provide consistent control. Application during sunny warm days is best and cloudy weather will slow activity. Under cool conditions, it may take 2 to 3 weeks to kill the ryegrass and a second application may be necessary. Previous research suggests that small ryegrass is easier to control, but mild air temperatures 1 to 2 days before, during, and 1 to 2 days after application are likely more important. Apply glyphosate at 1.25 to 1.5 lb. ae/acre following the guidelines provide previously
Hairy Vetch, Red Clover, and Crimson Clover
For control of clover or other legume cover crops, glyphosate alone will not kill most legumes, but it is useful in mixture with other herbicides. Gramoxone alone is also not very effective on legumes and should be mixed with atrazine or metribuzin for increased performance. Dicamba (Banvel/Clarity) is one of the best herbicides for control of legume cover crops. It is often a necessary tank-mix partner with glyphosate for control of red or white clover. A 2,4-D ester formulation will effectively control hairy vetch and field peas. I am less familiar with crimson clover control and unsure if 2,4-D is adequate or dicamba is necessary. Both 2,4-D ester and dicamba can be tank-mixed with glyphosate without loss in activity and can be used in corn. Use a minimum of 12 fl. oz./acre of Banvel or Clarity or 2,4-D ester tank-mixed with glyphosate. For corn, apply dicamba or 2,4-D ester 7 to 14 days before planting or 3 to 5 days after planting for greater crop safety and plant corn at least 1.5 inches deep. Clopyralid is also effective on legumes and is a component of several corn herbicides. Dicamba and clopyralid are not suitable for soybean and 2,4-D ester (1 pt.) must be applied at least 7 days ahead of soybean planting.
Canola is sometimes included in cover crop mixtures and generally over winters in our region. Canola can be somewhat challenging to control in spring because glyphosate is somewhat weak and big canola can be difficult to control. Application timing is important to achieve adequate control. Usually adding 1 pt. of 2,4-D ester to glyphosate will provide adequate control of smaller canola. Gramoxone plus atrazine or metribuzin should also provide good control of smaller canola. Dicamba is not effective on canola or other brassica species.
Table 1. Effectiveness of herbicides for control of common cover crops (based on Penn State research or our best guess). Control ratings: 10 = 95-100%; 9 = 85-95%; 8 = 75-85%; 7 = 65-75%; 6 = 55-65%; and N = less than 55%.
|Rate* (lb./acre)||Annual ryegrass||Winter rye||Winter wheat||Crimson clover||Red clover||White clover||Hairy vetch|
|*0.75 lb. Glyphosate = 32 fl. oz. of a 41% glyphosate; 0.5 lb. paraquat = 2 pt. Gramoxone SL; Clopyralid is a component of Stinger, Hornet, and Surestart/Tripleflex.|
|Glyphosate + 2,4-D or||0.75 + 0.5||8||9||9||10||8||8||10|
|Glyphosate + dicamba||0.75 + 0.5||8||9||9||10||9||9||10|
|Paraquat +||0.5 +||7||8+||8+||9||8+||7||9|
|Atrazine or||1 or|