QUESTION #32

How important is weed control with herbicides for alfalfa stand establishment?

SHORT ANSWER: Competitive weeds should be controlled in new alfalfa plantings. Weeds can cause stand failure if they shade alfalfa plants. In addition the presence of weeds may reduce forage quality and yield of first-cut hay. 

EXPLANATION: Weeds compete with seedling alfalfa for nutrients, water, and light, so it is usually profitable to control competitive weeds in seedling stands. Since seedling alfalfa is not highly competitive, one or two competitive weeds/ft² can cause a significant reduction in alfalfa yield. 

Weed control should begin with site selection. A good site should be fairly weed-free (even in fence rows). Perennial weeds should be controlled 1 or 2 years before planting alfalfa. Also, annual weeds should not be allowed to produce seed in crops preceding alfalfa.

Long before alfalfa is planted, good alfalfa producers fight problem weeds. Nevertheless, the most important stage for weed control begins with site selection and seedbed preparation. 

Some weeds escape control and cause problems. Unfortunately, no exact threshold exists for weeds indicating the need for herbicides. Competitiveness of weeds depends on the weed species present, soil nitrogen level, date of emergence, and growing conditions (temperature and soil moisture). 

During warm wet weather several competitive weeds/ft² can cause stand failure if they outgrow and shade alfalfa. On the other hand, alfalfa can grow with a high density of certain weeds without detrimental effects on stand establishment. Table 6 contains examples of several weed densities and their potential effects on fall-planted alfalfa. 

After alfalfa seedlings are well established with a high probability of surviving until first harvest, the use of herbicides depends on how the hay will be used. Producers intending to use first cutting for high producing dairy cows should strive for weed-free stands. If, however, first cutting hay will be fed to beef cattle, a few weeds may not be an economic concern. 

Often producers do not scout and properly control insect problems in weedy fields. Combined stresses from weeds and insects results in seedling alfalfa plants that are often severely stunted and even killed. This results in significant reductions in alfalfa production at first harvest and is a major reason for poor stands. 

Weed control in spring-sown alfalfa is critical. Essentially all fields will have enough serious summer weeds (pigweed and grasses) in seedling alfalfa to cause stand failure, if not controlled.

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DI'JA'NO (Pronounced in some areas "did you know") 

QUESTION #33 

What chemical weed control options are available for alfalfa stand establishment?   

SHORT ANSWER: The only herbicide available for preplant application is EPTAM. 

Postemergence options include PURSUIT, BUTYRAC 200, and POAST PLUS. 

EXPLANATION: Herbicide options for weed control in seedling stands include: 

1) EPTAM, applied preplant with thorough soil incorporation. EPTAM is effective on most grasses and some broadleaf weeds such as henbit. It is a good option for both spring- and fall-planted alfalfa. Estimated herbicide cost (3.5 pt/A) is $12.50/A. EPTAM is weak on pigweeds, so spraying with PURSUIT or BUTYRAC 200 may be required for acceptable pigweed control. 

2) PURSUIT, applied after alfalfa has reached the 2nd trifoliolate leaf stage and before weeds are 3" tall. PURSUIT is effective on many broadleaf weeds (except waterhemp) and will give some control of warm season grasses. However, it is not highly effective on cool-season grasses. It is a good option for spring-planted alfalfa. Estimated herbicide cost (4 oz/A) is $20/A. 

3) BUTYRAC 200, applied after alfalfa has three leaves and weeds are still small (< 6"). BUTYRAC 200 is effective on most small annual broadleaf weeds except henbit, kochia, and Russian thistle. It is a good option when weed problems are only pigweeds and other susceptible broadleaf weeds. Estimated herbicide cost (2 pt/A) is $8/A. 

4) POAST PLUS, applied after alfalfa has two leaves and grassy weeds are small (< 4"). POAST PLUS is a good option when the weed problems are only weedy grasses. Estimated herbicide cost (1.5 pt/A) is $9.15/A. 

5) BUTYRAC 200 mixed with POAST PLUS: The mixture of these two herbicides is the only good option for controlling a mixture of emerged broadleaf and grassy weeds in fall-planted alfalfa. However, to avoid damage to alfalfa and get a good alfalfa response, it is critical that the mixture be applied in fall and not in spring. Estimated herbicide cost of the mixture of 2 pt/A BUTYRAC 200 & 1.5 pt/A POAST) is $17.15/A. 

Fall-planted alfalfa: Postemergent weed control is a good option for alfalfa producers when weeds are normally not a problem. Cool-season weeds are often not as competitive as summer weeds and sometimes weed populations are too low to justify the cost of treatment. If problems with cheat or volunteer wheat are likely, then EPTAM is a good option since it is effective on cool-season grasses and some cool-season broadleaf weeds (henbit and chickweed). Soil moisture is usually lost during incorporation, and that could result in a marginal seedbed, especially during dry falls. 

Spring-planted alfalfa: There is usually a problem with both grasses and broadleaf weeds (pigweeds) in spring-sown stands. Since EPTAM is not effective on pigweeds, it may not be the best option. BUTYRAC 200 would still be needed for control of broad leaves after they emerge. Herbicides such as PURSUIT or a mixture of BUTYRAC 200 and POAST PLUS are good early postemergent options. 

Be sure to scout new fields for both weeds and insects and spray to control them when infestations merit control.

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QUESTION #34 

Is it profitable to control weeds with herbicides in fall-planted alfalfa?

SHORT ANSWER: Profitability of herbicides on seedling alfalfa depends on the number of weeds and the probable use of the hay. When selling alfalfa hay to dairies or feeding it to high-producing dairy cows, it is usually profitable to produce weed-free alfalfa. 

EXPLANATION: Top prices are paid for weed-free, high-quality alfalfa. Fall-planted alfalfa should yield 1&frac12; to 2 tons/A at first harvest; thus, at $80/ton for weed-free hay, the value of 1.75 ton would be $140. 

Dairy Hay -- Discount on weedy alfalfa (5-15% weeds) is normally from $15 - $23/ton. With this amount of weeds in first-cutting hay, producers could pay for their herbicide treatment cost with first- cutting profits. 

Other benefits of using herbicides for dairy quality hay include increased alfalfa yield at second harvest and being able to sell top quality hay when there is an adequate supply of hay. 

Beef Hay -- If hay will likely go to beef cattle, then newly-planted alfalfa needs to be scouted about one month after emergence to determine if it would be pay to control weeds. A few non-competitive weeds in hay for beef cattle can be tolerated. Table 6 (page 21) on "Competitiveness of Weeds" may assist in this determination. 

Several factors to consider when deciding on weed control in fall-planted alfalfa to be used for beef hay are: 

A. If more than one competitive broadleaf weed/ft² is found, then consider spraying in October or November if weeds are outgrowing the alfalfa. Competitive broadleaf weeds include mustards, cutleaf eveningprimrose, and prickly lettuce. 

B. If broadleaf weeds consist of henbit or chickweed, it would usually not pay to spray. These weeds are not highly competitive, and no herbicide is both economical and effective on them. 

C. If less than two weedy grasses (wheat or cheat) plants/ft² are present, it would not usually pay to spray. However, if weeds make up more than 50% of the fall growth, it would pay to spray. If more than six grass plants/ft² are detected and they dominate the alfalfa, then spray. Once alfalfa is shaded, it stops growing and stand loss can result. In May, if cutting is delayed due to warm wet weather, weed growth can take over an alfalfa stand. 

D. Be sure to harvest first cutting of weedy hay early to maximize quality and decrease the potential of damage to alfalfa by shading weeds. Ideally, such stands should be cut in April when weedy grasses are in the boot stage. 

Consider spraying only the borders of fields. In many cases weeds are only a problem around borders of fields. Scout fields during fall. If weeds are only around edges, then only spray borders. The first step in assuring of good stand life is to begin with a good stand across the entire field. 

It is critical to scout newly planted fields all fall and winter for insects as well as for weeds. Alfalfa infested with blue and spotted aphids is not competitive with weeds.

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