By William Curran
As warmer weather approaches, start thinking about controlling winter annual weeds (especially common chickweed) in alfalfa. Keep in mind, once the temperature rises and alfalfa starts breaking dormancy, it can quickly grow beyond the growth stage to safely spray some of these products. In most cases, 2-inches of alfalfa regrowth is the maximum height listed. Spraying beyond this stage may cause stand and/or yield reductions. Keep in mind all of these herbicides are labeled for use in pure-stand alfalfa. However, only Aim, metribuzin and Pursuit can be applied to established alfalfa-grass mixes. Here are a few guidelines about available products:
Your best bet when controlling…
- Chickweed — Gramoxone SL, Pursuit, Raptor, Metribuzin, Aim (if chickweed is ALS-resistant, Pursuit and Raptor will not control it.)
- Annual bluegrass — Gramoxone SL
- Henbit/deadnettle — Gramoxone SL, Metribuzin
- Yellow rocket — Pursuit, Raptor, Metribuzin, Aim
Gramoxone SL 2L (paraquat) — May be applied at 1 to 2 pt/A to established “dormant” stands before 2 inches of spring regrowth. The weeds must be actively growing at the time of application. Gramoxone is also labeled at 1 to 2 pt/A for dormant application on new fall seeded stands. 1 pt/A can be applied between cuttings but no more than 5 days after harvest and before 2” of regrowth. Gramoxone will desiccate any green tissue including actively growing alfalfa and possibly cause stand and/or yield reductions. However, if chickweed infestations are thick, this may be a necessary option to help suppress it. Be especially cautious with new fall seedings. Do not tank-mix with metribuzin on newly seeded (less than 1-year old) alfalfa. Gramoxone is effective on low to moderate infestations of winter annuals including chickweed, henbit, deadnettle, and mustard species. Do not use on mixed stands.
Pursuit 70DG (imazethapyr) — Pursuit may be used for weed control in seedling (2 trifoliates or larger) or established alfalfa. Apply Pursuit at 1.08 to 2.16 oz/A plus adjuvants to actively growing weeds 1 to 3 inches in height. Pursuit performance improves with warmer temperatures. If spraying during extended cold periods, expect weeds to respond slower or the herbicide can have reduced activity. Pursuit is effective on many winter annual broadleaves including small chickweed. Pursuit may be used on established alfalfa-grass mixtures.
Raptor 1AS (imazamox) — Raptor may be used for weed control in seedling or established alfalfa. Apply Raptor at 4 to 6 fl oz/A plus adjuvants to small, actively growing weeds and to established alfalfa in the fall or in the spring. Any application should be made before significant alfalfa growth or regrowth (3 inches) to allow Raptor to reach the target weeds. Like Pursuit, Raptor performance is influenced by temperature. Therefore, try to apply the herbicide on warmer days and when weeds are actively growing. Raptor has a similar spectrum of winter annual weed control as Pursuit. Do not use on mixed stands.
Metribuzin 75DF (Dimetric, Glory, TriCor) — May be used on established alfalfa. Apply 0.5 to 1 lb/A before spring regrowth. Impregnation on dry fertilizer can improve crop safety and allow for slightly later applications (up to 3 inches spring regrowth). Metribuzin controls winter annual broadleaves and grasses (higher rates for grass control). May be used on mixed alfalfa-grass stands. Higher rates can potentially cause a reduction in grass stand.
Prowl H2O 3.8CS - has a label for use in seedling and established alfalfa. For seedling alfalfa (2-trifoliate to six inches tall), apply Prowl H2O at 1.1 to 2.1 pints/A prior to weed emergence. For established stands (i.e., alfalfa that was planted in fall or spring and has gone through a first cutting/mowing), apply 1.1 to 4.2 quarts/A prior to weed emergence. Applications can be made in the fall after the last cutting, during winter dormancy, or in the spring before alfalfa reaches 6 inches of regrowth and weeds have germinated. Prowl H2O will control many annual grasses and broadleaves including crabgrass, panicums, foxtails, lambsquarters, pigweed at the 1 to 4 pt rate. Other weeds such as annual bluegrass, common chickweed, dodder, henbit, prostrate knotweed, smartweed, and velvetleaf can be controlled with the 2 to 4 qt rate. Some stunting and yellowing to the alfalfa may occur after application. Do not harvest alfalfa less than 28 days after applying ≤2.1 quarts/A, or less than 50 days if >2.1 quarts/A. Only Prowl H2O has this label for use in alfalfa, NOT Prowl 3.3EC.
Aim 2EC (carfentrazone) — Apply 1 to 2 oz/A in alfalfa or alfalfa/grass mixes in spring or summer immediately after cutting up to 6 inches of new growth and when weeds are actively growing and no more than 4 inches in height, or rosettes are less than 2 inches across. Include necessary adjuvants. Aim may be applied when crop is in dormancy such as fall or spring application before breaking dormancy. Aim may be tank-mixed with fertilizers and other labeled herbicides, and insecticides. Follow most restrictive labeling of each tankmix partner.
Select 2EC (clethodim) - Apply Select at 6 to 8 fl oz to actively growing grasses that are less than 6 inches tall. Rates up to 16 fl oz may be used to control larger grasses, annual bluegrass or perennials such as quackgrass. Include a crop oil concentrate in the spray mixture. May be tank-mixed with Pursuit, Buctril, or 2,4-DB for broadleaf weed control. Do not use on mixed stands.
Sinbar 80W (terbacil) - May be applied to established alfalfa at up to 1.5 lb/A or to a new fall seeding at 0.33 to 0.5 lb/A. The reduced rate seedling year application is specifically aimed at chickweed. Sinbar is effective on a number of annual broadleaves and some grasses at the higher rates. Apply to ‘dormant’ alfalfa before 2 inches of spring regrowth. Do not use on mixed stands.
Velpar 2L (hexazinone) - Apply Velpar to established alfalfa at 2 to 6 pt/A during the dormant period before 2 inches of spring regrowth. Velpar is effective on annual broadleaves and grasses. Velpar is the most effective dormant season treatment for suppression of dandelion and dock species (higher rates for grasses and perennial suppression). Do not use on mixed stands.
Source:psu.edu