CONTROLLING DIFFICULT WEEDS

IN ESTABLISHED ALFALFA

Robert E. Wilson(1)

ABSTRACT

Control of difficult weeds in established alfalfa is primarily the utilization of all management options available to the producer. Good farming practices, including a weed free, firm seedbed are the primary first steps to controlling difficult weed species. Correct identification of the problem weed species and then a good understanding of the biology of that species will insure the best probability that your control efforts will be successful at the lowest possible resource cost. Understanding and selection of the most effective herbicide regime are a part of that control effort.

INTRODUCTION

Herbicides are an integral part of any weed management program in cropping systems. Herbicides need to be used in conjunction with other tools such as cultural controls and sanitation. The better we use all of the tools, the better chance we have of producing high quality, clean hay.

A grower needs to first know the weed species he has a problem with. Weed species will differ from area to area. They also differ from neighbors because of management practices such as crop rotations, past weed control efforts, or other reasons. Numerous weed identification guides are available.

A grower should keep good records of where weed species occur in a field. The newer precision farming techniques are making this easier for growers through such things as remote sensing, computerized mapping, etc. but that technology has not been widely adopted in Nevada at this time.

Weed control in alfalfa fields must begin long before the stand starts to thin out. The herbicides that are currently available can effectively control most weeds in established alfalfa without harming the crop, but removing weeds from a heavily infested, poor stand of alfalfa will probably be disappointing to the grower. The result is often a very light stand of alfalfa with a total forage yield which is less than before it was treated.

Weed control prior to planting and during the first season of establishment is the key to cost-effective weed management during the stand life of an alfalfa crop. Herbicide treatments then are tools for maintaining a highly productive and mostly weed-free stand, rather than a tool to catch up from heavy weed infestations that are more easily controlled without the alfalfa crop present. With this approach, the strong competitive influence of a vigorous, uniform stand of established alfalfa is utilized to its full potential in preventing or slowing the invasion of weeds.

Response of weeds to any of the available products may be altered by growing conditions, weed populations, type of irrigation, genetic variations of weeds, soil type, pH, organic matter, time of application, and application rate. Weed control generally decreases as the season progresses.

Growers can determine the products of choice by using Table 1. The left column lists the most common Nevada weed problems. The product families and the weed effectiveness are across the chart.

To avoid building a resistance to a particularly effective herbicide in your fields, a herbicide management strategy should be adopted. Start by rotating herbicides, rather than by cutting back or increasing application rates. Do not rotate to another herbicide with the same mode of action.

By choosing products from different groups in subsequent years the problems of weeds resistant to a particular product can be minimized or eliminated.

Table 1.(2) Chemical Names
Established Alfalfa Stand Weeds
Common Winter Annual Weeds (Germinate in the Fall and Weed are Found in 1st Cutting)
bulbous bluegrass (Poa bulbosa) - -(3) - - - - - - - - - - - - - - - - - - L* - - - - - - - -

downy brome

(Bromus tectorum)

G E E E - - F -- P E G-E E - - P P E
hare barley (foxtail) (Horeum leporium) G E E E - - E - - F-G E E E - - P P E
bilobed/snow speedwell (Veronica biloba) G - - F - - - - G - - - - P - - E P F - - G
blue mustard (Chorispora tenella) G P P P - - G - - - - F-G P E P P F-G G

fiddleneck

(Amsinckia spp)

- - - - - - - - - - L - - L* - - - - - - - - - - L - -
filaree (Erodium spp) G - - G - - - - - - - - L* F P G P G - - F
flixweed/tansey mustard (Descurainia spp) F-G P E P - - E - - E G P E P E G G
netseed lambsquarter (Chenopodium berlandieri) E G-E E G-E - - E - - F G G E P E G E
shepherd's purse (Capsella bursa-pastoris) E P E P - - E - - E P P E P P F-G E
tumble mustard (Sisymbrium altissimum) E P E P - - E - - E G F-G E P E G E
clasping pepperweed (Lepidium perfoliatum) G - - G - - - - - - - - L P - - G P G - - G
prickley lettuce (Lactuca serriola) E P E P - - E - - P P P E P E F E
purslane (Portulaca oreracea) G G G-E G - - E - - G G G - - P G F E
wild oats (Avena fatua) - - - - - - - - - - - - - - L* - - - - - - L - - - - - -
Common Summer Annual Weeds (Germinate in Spring and are Found in 2nd and 3rd Cuttings)
barnyardgrass (Echinochloa crus-galli) G E E E - - E - - F-G E E - - L P P E
green/yellow foxtail (Setaria spp) L L - - - - - - L - - L - - - - L L - - - - - -
lovegrass/stinkgrass (Eragrostis cilianensis) - - - - - - - - - - L - - - - - - - - - - - - - - - - - -

witchgrass

(Panicum capillare)

- - - - - - - - - - - - - - - - - - - - - - L - - - - - -
kochia (Kochia scoparia) G G-E G-E G-E - - E - - G F P - - P G-E E G

knotweed

(Polygonum erectum)

G G E G - - F - - L G F - - P P F G

common mallow

(Malva neglecta)

F - - SO - - - - - - - - L P F F P G - - SO
millet, wild proso (Panicum miliaceum) - - - - - - - - - - - - - - - - - - - - - - L - - - - - -

nightshade

(Solanum spp)

P P E P - - - - - - E G G - - P G G E

field pennycress

(Thlaspi arvense)

E P E P - - E - - E G P E P E - - E

pigweed

(Amaranthus spp)

E G-E E G-E - - E - - E P G E P E F-G E

Russian thistle

(Salsola iberica)

F-G G-E G-E G - - G-E - - L G P - - P E G E
sunflower (Helianthus annuus) F-G P G P - - F - - G P P - - P G F-G E
dodder (Custuta spp) P P P P - - P - - P F-G P P P P - - P
Common Perennial Weeds(4)
foxtail barley (Hordeum jubatum) F - - G - - - - - - - - - - G P F P P - - SO
johnsongrass (Sorghum halepsense) - -

L

SO

- - - - - -

L

SO

- -

L SO

L*

- - - - - - L - - - - - -
Kentucky bluegrass (Poa pratensis) G L* - - - - - - - - - - - - G F G P P - - SO
quackgrass (Agropyron repens) P-F P F P - - P - - P G F-G F - - P P F
field bindweed (Convolvus arvensis) SO - - SO - - - - - - - - - - - - P SO P - - SO - -
dandelion (Taraxacum spp) P-F P F-G P - - - - - - P P P F-G P F P F
perennial ryegrass (Lolium perenne) - - - - - - - - - - - - - - - - - - - - - - L - - - - - -
plantain (Plantago spp) SO - - SO - - - - - - - - - - - - - - SO P SO - - SO
poison hemlock P - - - - - - - - P - - - - P - - G P F - - P
Russian knapweed (Centaurea repens) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

 

 

Herbicides Families and products Registered on Alfalfa in Nevada

ACCase Inhibitors (Group 1)

Cyclohexanediones: sethoxydim (Poast®, Poast Plus®)(5) Selective postemergence. Absorbed rapidly by the leaves. Addition of an oil concentrate or other adjuvants enhances absorption. Readily translocated throughout the plant. Interferes with growth. Degraded rapidly in broadleaved plants.

ALS Inhibitors (Group 2)

Imazethapyr (Pursuit®) Inhibit the formation of

Microtubule Assembly Inhibitors (Group 3)

Dinitroanilines: benefin (Balan®, Balan DF®), pendimethalin (Prowl®), oryzaline (Surflan®) or trifluralin (Treflan®, Treflan TR-10®,Tri-4, Trifluralin) Most are used in preplant soil incorporation treatments. All are susceptible to volatilization, photodecomposition, or microbial and chemical breakdown unless protected by immediate incorporation. The chief symptom of injury is usually severe root inhibition of seedlings, leading to plant stunting.

Phenoxy-acetic acids (Group 4)

Phenoxys: 2,4-DB (Butyrac®), Selective postemergence. These are one of the types of growth hormone herbicides. They are usually foliar applied because they are translocated in the food stream. They move from the applied leaves to the terminal meristems of leaf, shoot, and root. The first symtom of injury is usually stem twisting followed by deformities in terminal tissues.

Photosystem II Inhibitors (Site A) (Group 5)

Triazinones: metribuzin (Lexone®, Sencor®) hexazinone (Velpar®) Used primarily as preemergence soil-applied treatments although several products are also widely used postemergence. These herbicides inhibit photosynthesis by blocking the Hill (PS-II or dark) reaction which is the splitting of water. Triazine injury on corn causes yellow leaf margins and tips which then turn brown. Soybean injury appears as yellowing of outer leaf margins. If the entire leaf turns yellow, veins may remain somewhat green.

Uracils: terbacil (Sinbar®) Selective control of annual weeds. Most readily abosrbed by the roots and less so by the leaves. Primarily translocated through the xylem. Accumulated in the leaves. Inhibits photosynthesis.

Photosystem II Inhibitors (Site A - Nitrils (Group 6)

bromoxynil (Buctril®) Selective postemergence. Foliar contact herbicide. Causes blistered or necrotic spots within 24 hours and extensive leaf tissue destruction later. Mottled chlorosis may appear around the necrotic areas.

Photosystem II Inhibitors (Site B - Phenylurea (Group 7)

diuron (Karmex®, Direx®) These photosynthetic inhibitor herbicies are relatively nonselective at high rates. Most are applied to the soil although a few also have foliar activity. They inhibit photosynthesis in much the same manner as triazines.

Thiocarbamates (Group 8)

Thiocarbamates: EPTC (Eptam®) All thiocarbamates are selective and incorporated to reduce surface loss. These are meristematic inhibitors. They are relatively volatile. They inhibit both cell division and cell elongation and may also alter plant hormone (gibberellic acid) distribution within the plants. Uptake occurs through seeds, shoots, or roots with shoots more affected than roots.

Photosystem I Electron Diverters (Group 10)

Bipyridiiums: paraquat (Gramoxone®, Gramoxone Extra®) Non-selective. Quickly disrupt cell membranes, causing plant wilting (often within a few hours) and then tissue death. Herbicide molecules carry strong positive charges so the molecules absorb very tightly by soil colloids resulting in virtually no soil activity. They increase their herbicidal activity when

EPSP Synthetase Inhibitor (Group 14)

Glyphosate (Roundup®, Roundup RT®) A relatively non-selective foliar applied herbicide which is very effective on annual and deep-rooted perennial grassy and broadleaved species. Readily absorbed by leaves and translocated throughout the plant. Inhibits photosynthesis and growth. Degradation in plants is very slow.

 

 

Ungrouped

Chloroacetamides: pronamide (Kerb®) Most are used selectively, either preemergence or preplant. Their action is to inhibit growth in the terminal leaves, shoots, or root meristems (meristematic inhibitors). They cause stunted, malformed seedlings. They interfere with normal cell division and elongation and with perotein synthesis. Uptake of these herbicides can take place through seeds, roots, and shoots. This group of herbicides does not translocate.

 

References

Herbicide families. 1983. Ag Consultant and Fieldman. February 1983.

Herbicide Handbook of the Weed Science Society of America. 4th Edition, 1979. 309 West Clark Street, Champaign, Ill.

Shaffeek, A. 1993. Crop Protection with Chemicals. Alberta Agriculture AGDEX 606-1.

Tickes, B. Weed Control During the Growing Season. Proceedings of the 1998 Nevada Regional Alfalfa Symposium, February 2-4, 1998, Atlantis Casino Resort, Reno, Nevada.

Williams, R.D., D. Ball, and T.L. Miller. 1997. Pacific Northwest Weed Control Handbook. Extension Services of Oregon State University, Washington State University, and the University of Idaho.

Wilson, R.E. Early Spring Dormant Alfalfa Herbicides and Herbicide Resistance. Proceedings of the 1998 Nevada Regional Alfalfa Symposium, February 2-4, 1998, Atlantis Casino Resort, Reno, Nevada.

Wheeler, G. Chemical Weed Control for Established Alfalfa. University of Nevada Cooperative Extension Fact Sheet 93-71.

Whitson, T.D., L.C. Burrill, S.A. Dewey, D.W. Cudney, B.E. Nelson, R.D. Lee, R. Parker. 1991. Weeds of the West. Western Society of Weed Science.

Whitson, T.D., S.A. Dewey, and P.K. Fay. 1993. Weed Control Handbook for Montana, Utah and Wyoming. 1993-94. Extension Services of Montana State University, Utah State University, and the University of Wyoming.

Yarish, W. and M.P. Sharma. Recognizing Herbicide Action and Injury. Alberta Agriculture, AGDEX 641-7.

1. 1 Extension Educator, University of Nevada, Reno, Cooperative Extension, White Pine County, 995 Campton, Ely, Nevada 89301. (702) 289-4459.

2. 4 This table contains herbicides that are registered for weed control on alfalfa in Nevada. The evaluations are based on field tests that have been conducted in Nevada, California, Arizona, Utah, Idaho, Washington, and Oregon. Product labels should always be consulted for more complete information and restrictions.

3. 2 Control Legend: P = poor or no control, -- = not enough information or not listed on label, SO = seedling control only, F = fair control, G = good control, E = excellent control, L* = on product label for limited control only, L = on product label, but no local control effectiveness tested by the university.

4. 3 Perennial weeds in alfalfa are easiest to control before an alfalfa stand is established.

5. 4 Family of herbicides, chemical name of herbicides, brand name of products. This is in no way intended to be an endorsement of any product, but simply for clarification. Endorsement of named products is not intended nor is criticism implied of similar products that are not mentioned.