Weeds can be significant pests in an established alfalfa crop: they reduce alfalfa’s yield, quality and retail value. Many weeds are less palatable or nutritious for livestock than alfalfa, or are toxic. Weeds establish in alfalfa when the alfalfa is both growing and dormant. This fact sheet is intended to facilitate management decisions that reduce economic losses in established alfalfa due to weed pests.
Integrated Weed Management (IWM) is the best long-term approach to managing weeds. IWM involves the use of all available weed management options, such as planting improved or appropriate varieties, maintaining appropriate fertilization practices, crop rotation, tillage practices, and using the appropriate mechanical, biological, chemical controls, and prevention of weed infestation. There is no single, stand-alone weed control practice that can successfully provide long-term weed control in established alfalfa. Simply put, there are too many weed species that can occupy an alfalfa stand, and their highly variable life cycles and survival strategies guarantee that some will establish, particularly as old plants die-off. Using multiple approaches to weed management will result in the best control of more weed species and prevent weeds from developing resistance to any specific control strategy. If you use only one or two strategies there are two potential outcomes: 1) the target species will experience genetic selection and resistant biological types will evolve and become abundant; and/or 2) weed species which tolerate the control method will increase, further decreasing the crop. Applying the principles of IWM can minimize the overall economic impact of weeds, reduce herbicide use and provide acceptable economic returns to the producers.
The development of an IWM program is based on five general principles:
- Implement management actions and practices that limit the introduction and spread of weeds
- Encourage a dense, healthy stand of alfalfa
- Utilize the least toxic weed management techniques first
- Rotate herbicides so weeds are not regularly exposed to the same mode of action
- Change and adjust your IWM plan to reflect changes in conditions, rather than using one plan indefinitely
Effective weed management requires knowledge of the field’s history – what weeds are or have been present and the longevity of their seed, accurate weed identification and an understanding of weed biology. Field history is important in identifying potential problems. Field assessment is necessary to determine what techniques will be most important to ensure a healthy alfalfa field. An active weed control program starts by properly identifying weeds in the field. Scouting for weeds is important in all seasons, so identification and control of the weed can occur before weed density increases beyond an economic threshold and damages the crop, or goes to seed and creates a long-term problem.
Annual weeds can be either summer or winter annuals. Winter annual weeds germinate in early fall through early spring (September to March). They grow rapidly in the spring and are usually a problem only in the first cutting. Summer annual weeds start to germinate as temperatures rise in late spring (April to May) and germination and growth may continue all summer. Other weeds such as biennial weeds require two growing seasons to to complete their life cycle. Vegetative growth occurs during the first growing season and seed production usually occurs the second growing season.
Perennial weeds live more than two growing seasons and often decades. All perennial weeds reproduce from seed, but many also re-grow from other plant parts such as roots, rhizomes, stolons and tubers. These structures have buds that can produce new plants, and often only a small part of the structure is needed to produce a new plant if it is disturbed by tillage. To control such weeds, growers must eliminate all plant parts that can reproduce. This includes the seed and the vegetative parts that can produce a new plant.The first and most important step in a weed-management program is planting alfalfa into fields that are free of perennial and other persistent weeds. If weeds, particularly perennial weeds, become well established in alfalfa they are difficult to manage.
In Nevada, common summer annual weeds include: lambsquarters (Chenopodium album), pigweed (Amaranthus spp.), lovegrass (Eragrostis spp.) and foxtails (Setaria spp.). Winter annuals include flixweed (Sescurainia Sophia), shepherdspurse (Capsella bursa-pastoris), tansy mustard (Descurainia pinnata), prickly lettuce (Lactuca serriola), filaree (Erodium cicutarium) and cheatgrass (Bromus tectorum). Common biennial weeds include several thistles such as musk thistle (Carduus nutans). Common perennial weeds include Russian knapweed (Acroptilon repens), dodder (Cuscuta spp.), Kentucky bluegrass (Poa pratensis), Hoary cress (Cardaria draba), Canada thistle (Cirsium arvense), dandelion (Taraxacum officinale) and perennial pepperweed (Lepidium latifolium). While this list is extensive, familiarity with these species can help identify common weed problems early, before they cause economic damage or become long-term persistent problems. Seedlings are much easier to control than established, mature plants that have dispersed seed.
Cultural weed control measures are intended to disturb the targeted weeds and benefit the crop. Cultural techniques are designed to establish and maintain a vigorous crop and to use the crop to exclude weeds. These management actions include selective grazing, cultivating, mowing, burning, fertilization, crop rotation, variety selection, seeding rates, weed free seed, seed with high germinability, proper seed bed, and planting at the proper seed depth. During the winter growers may turn livestock into specific fields or areas of the field to graze and rotate them among areas to prevent overgrazing. Fields can be lightly cultivated in spring with a harrow to disrupt seedling weeds. This should be done when the alfalfa is dormant to avoid injury to crowns and new stems, and subsequent yield reductions and/or the introduction of diseases. Mowing can either prevent or delay weed seed production for many species. The result will depend upon the species specific response to removal of the terminal growing points and regrowth potential at the time the weed is cut. Burning or flaming of fields in the spring, when the alfalfa is still dormant, can help control winter annual weeds without harming the alfalfa.
Growers sometimes seed grasses or grains into older established alfalfa stands. Growers have the ability to manage in favor of the preferred (most valuable) species in the mixture. A short-statured winter wheat or winter barley can be seeded in the fall, while spring wheat or other grains can be planted very early in the spring. These plantings can help suppress weed growth by out competing weeds with a beneficial crop when alfalfa fields are slow-growing or dormant.
Nutrient management can aid in weed control. Growers who apply recommended amounts of fertilizer prior to planting and/or growth each year provide their forage crops the opportunity to optimize growth and competitively exclude many weeds. Over or under-applying fertilizer can harm the forage plants and provide the weeds an advantage. Fertilization should always be based on soil and plant nutrient levels and how much nutrient is expected to be removed by the crop or from leaching during harvest.
In the summer, the pre-harvest irrigation should occur early enough to allow the soil to dry out to prevent compaction by harvest equipment, but late enough to provide adequate moisture (particularly deep soil moisture) for re-growth to occur following harvest. A dry surface soil at harvest also minimizes the germination of weed seeds when the canopy is short and the quantity and quality of sunlight are high. Delaying irrigation following a harvest may help suppress summer annual grasses by giving the alfalfa time to grow back and shade the ground before the grasses germinate. Irrigation management for weed control, however, is site-specific and dependent upon the soil texture, crop needs and management issues. What works on a loamy soil may be less useful on a sandy or clay soil. Always evaluate your specific circumstances.
Harvesting alfalfa can either mean cutting the vegetative growth for hay with machinery or harvesting the forage by grazing animals. Both methods can impact weed management programs.
There are several factors that will affect a grower’s decision to cut a forage stand for hay, including: desired hay quality, anticipated length of the growing season and the hay market. Cutting schedules are a compromise between producing high quality hay and maintaining a high density of large vigorous alfalfa plants. Short cutting intervals reduce alfalfa vigor and increase bare ground, which facilitates the establishment of weeds. Longer cutting intervals promote large, more vigorous alfalfa plants and fewer safe-sites for weeds to establish. A large percentage of the alfalfa grown in Nevada is produced for the dairy market for high quality alfalfa, so it is cut at short intervals to maximize nutrient content and minimize cellulose and other less digestible structural components, which increase at the bloom stage. When harvesting fields with weed patches, growers should consider the growth stage of the weeds before cutting a weed-infested area. If the weeds are flowering, growers should avoid the patches to eliminate contaminating the equipment harvested forage with weeds. If the weeds are vegetative, cutting may be the best way to reduce weed vigor and prevent seeding. Equipment sanitation is important to avoid spreading weed seeds.
Appropriate timing of the first cut can promote a healthy stand throughout the season and prevent the establishment of weeds. The initial cut should be made before weeds flower and produce seeds. Timing of the final cutting is also very important. The final harvest should be early enough in the fall to allow at least four inches of regrowth before the stand goes dormant. If a cutting is made too late in the season, alfalfa regrowth is limited, which stresses the alfalfa plant and reduces its competitive ability.
Many weed species are utilized by grazing animals. However, proper grazing management is very important to the longevity of the stand. Overuse will lead to an increase in many pest issues due to damage to the alfalfa plants in the field. Vigorous, healthy stands are more resistant to pest infestations. Grazing when the alfalfa plants are growing is most effective when high densities of animals are grazed for short times in small sections of the field and then moved into another section. This results in the highest forage quality and minimizes livestock damage to the plants. Grazing dormant alfalfa growth can sharply reduce weed populations and aid chemical weed control by exposing the soil and weed seedlings. Forage removal by livestock should be similar to cutting height to maintain alfalfa vigor through winter. Alfalfa should not be grazed repeatedly or continuously, which results in damage to the plant’s crown buds or prevents the plants from storing enough carbohydrates to survive the winter. This damage depletes root reserves, which reduces crop vigor and leads to a thin and weedy alfalfa stand. Grazing should also be avoided when the field is excessively wet.
Growers need to constantly evaluate alfalfa stand health and productivity to determine if a stand needs to be replaced. If yields decline significantly, market prices do not provide worthwhile returns, or crop quality is reduced due to high weed populations, a grower may decide to remove a stand. Regardless of the reason, stand removal provides an opportunity for weed control. Less than four plants per square foot or 40 stems per square foot indicates poor stand quality and may warrant stand removal. Stand removal provides an opportunity for a more comprehensive measure of weed control to be applied. For example, all plants in the field will typically be killed during the stand removal process, thereby reducing existing weed populations. Rotating to a grain crop for one or more years after stand removal will allow additional opportunities for the control of perennial weeds. Aggressive tillage can be applied before seeding and following harvest of the grain crop, disrupting weed production twice each year. Grain fields can also be severely grazed following harvest, further damaging existing weeds. A grower must recognize that animals can be a source of weeds if proper sanitation methods are not in place prior to the animal grazing the alfalfa fields. Animals can carry viable seeds for several days, so feeding weed-free forage or grazing in a weed-free area for seven days prior to introduction to the field will prevent spreading weed seeds. Grain crops are typically irrigated less than alfalfa, which weakens some weed populations. Finally, there are more broadleaf herbicide options for grain crops as compared to those available for alfalfa.
Herbicides should be integrated with proper cultural, mechanical, and/or biological weed control techniques to obtain the most effective, economical weed management results. The need for chemical control depends on many factors, including: weed species, their density and competitive effect, the potential market for the alfalfa, potential for shipment refusal at the destination, and time of year. Selecting the proper herbicide, applying it at the correct time and following all label directions are critical for successful weed control while minimizing the risk of crop damage.
Because the number and complexity of possible weed population in established alfalfa fields is so large, specific chemical recommendations are impractical in a publication of this size. The reader is advised to determine the most pressing weed problems in their alfalfa field and if herbicide use is warranted. If a herbicide application is the appropriate treatment action, select the herbicide(s) which best address the management situation.
Table 1 is a partial list of herbicides currently available for use on Nevada alfalfa fields. The products are separated into those that control grass, grass and broad-leaved or just broad-leaved weeds. The chemical class as defined by the Weed Science Society of America of each material is also provided. This information is useful when planning a herbicide-based, weed-management program. In order to avoid the development of herbicide resistance in weeds, select herbicides with different chemical classes in tank mixes or rotate between the different chemical classes in the annual control efforts. Always follow label instructions and have the most current information available before applying any herbicide.
Commercial alfalfa varieties with resistance to the herbicide glyphosate (Roundup) are currently available and being planted in Nevada. Known as “Roundup Ready” alfalfa, this technology provides the advantages of broad spectrum weed control with low risk of crop injury. However, there are drawbacks to this technology. The repeated use of the same herbicides or even of other herbicides with the same mode of action increases the risk of herbicide resistance in the targeted weed population. Resistance management strategies include the use of crop rotations, herbicides that have different modes of action, control of escaped weeds by tillage, and other cultural controls in order to forestall development of resistant weeds.
No matter which type of production system is used (standard or Roundup-ready varieties) a well-balanced, long-term weed management approach will incorporate several weed management techniques applied at the appropriate time. Normally more than one technique should be utilized including frequent scouting to identify newly arrived weeds that can be easily controlled before they become established. Preventing the establishment of new weed populations is the most cost-effective management technique available to alfalfa growers.
Table 1. List of active ingredients registered for use in alfalfa in Nevada, the number of products registered in Nevada and the mode of action group number to facilitate chemical rotation.
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