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 Alfalfa is stressed by weed interference during establishment and in thinning stands. Alfalfa does not occupy all of the ground area at these two stages, so weeds are able to establish and compete with alfalfa for soil nutrients, water, light, and space. Yield reductions of alfalfa due to weed competition in new stands often exceed 1000 pounds per acre, and severe infestations can cause stand failure. Yield reduction in thinning stands results in loss of 1/2 to 1 pound of alfalfa for every pound of weeds produced. A number of weed species can compete with alfalfa. The weed species present in any field depends on soil type, environmental conditions, the previous cropping history, and the weed management practices. Weeds that have been documented in Oklahoma alfalfa fields are as follows: Bushy
wallflower, flixweed
(Photo
40), shepherdspurse
(Photo
41),
tansy
mustard, and pepperweeds (green,
veiny). Mustards are a problem in fall-planted
alfalfa and in thinning stands. Mustards emerge in late summer and early fall as soil
temperatures decrease and fall rains occur. Seedlings grow as rosettes in fall and early
winter, then they initiate upright growth in late winter, flowering in April and
May. Mustard flowers have four petals and four sepals. Pepperweeds have no petals, so
they appear green when flowering; shepherdspurse (Photo
41) has four white petals, and the other listed mustards have four
yellow petals. The mustards can be effectively controlled in
fall-planted alfalfa with herbicides applied to actively growing rosettes
in October and November. In thinning established stands, mustards
can be controlled with residual herbicides applied in January and February
when alfalfa is dormant. Buckhorn
plantain, common
chickweed, curly
dock (Photo
42), cutleaf
eveningprimrose (Photo
43), dandelion,
henbit,
musk
thistle, plains
coreopsis, prickly
lettuce (Photo
44), red horned
poppy, and spiny
sowthistle. Buckhorn plantain, curly dock, and dandelion are
perennials. It is important to control seedlings of these perennials
in fall planted alfalfa with herbicides applied in October and
November. The rest of the listed weeds are annuals, and most of
these weeds respond best to postemergence herbicides applied in the fall. Palmer amaranth (Photo 45), redroot pigweed, tall waterhemp, and tumble pigweed, in both new spring stands and thinning stands. The pigweeds start emerging in spring with warming soil temperatures and rainfall events in April and May, and new flushes of seedlings can occur throughout the summer whenever rainfall events (or irrigation) keeps the soil surface moist for several days and the soil surface is not shaded by standing forage. Pigweeds are a big problem in thinning stands, since there can be emergence and establishment of these weeds after each cutting of alfalfa, if there is good soil moisture at cutting. Other Summer Annual Broadleaf Weeds Buffalobur,
common
ragweed, dodder
(Photo
46),
horseweed,
kochia
(Photo
47),
lambsquarters (common,
slimleaf), and
Russian
thistle. These weeds
usually have only one big flush of germination. Kochia, lambsquarters, and Russian thistle
emerge in late winter, while buffalobur, common ragweed, and dodder emerge during the
spring. Cheat
(Photo
48), downy
brome (Photo
49), Japanese
brome, and rescuegrass. The
annual bromes emerge in late summer and early fall as soil temperatures decrease and
fall rains
occur. They tiller in fall and early winter, then joint (initiate upright growth) in
late winter, with flowering in May and June. Cheat and rescuegrass are fairly common in
fall-planted alfalfa, while downy brome is a major weedy grass in thinning
stands. In seedling alfalfa stands, best control of bromes is
achieved with early fall applications of herbicides. In thinning
established stands, bromes can be controlled with some of the residual
herbicides applied in January and February when alfalfa is dormant. Annual
bluegrass, little
barley, jointed
goatgrass, ryegrass, and
volunteer
wheat (Photo
50). Annual bluegrass and little barley are minor
problems in thinning stands of alfalfa. Jointed goatgrass, ryegrass, and
volunteer wheat are all
major competitors in fall-planted alfalfa; goatgrass and ryegrass are also
a problem
in thinning stands. In seedling stands, best control of all listed
grasses is with fall applications of herbicides. Ryegrass can also
be controlled with spring applications. Barnyardgrass, bermudagrass, crabgrasses (smooth or large; Photo 51), cupgrasses (prairie or southwestern), foxtails (green or yellow; Photo 52), goosegrass, johnsongrass, sandburs, sprangletops (bearded or red), and windmillgrass. Most of these summer grasses can be a problem in new spring stands and thinning stands. The only exception is windmillgrass, a perennial which only infests thinning stands. Germination of the summer grasses is similar to pigweeds in that they start emerging in spring and new flushes of seedlings can occur throughout the summer when conditions for germination are favorable (high soil moisture). Bermudagrass and johnsongrass are perennials that can be selectively controlled by repeated herbicide treatments. Windmillgrass occurs primarily in thinning stands and is not controlled with selective postemergence herbicides. The other weeds listed are annuals. These annuals must be controlled while small and actively growing. A complete listing of alfalfa weeds by common name, Latin name, and competitiveness is listed in Table 2-4. Additional information on weed life cycles, sampling procedures, and photographs can be obtained through the Oklahoma Alfalfa Calendar on the Internet at Oklahoma Alfalfa.
Weed Management in Seedling Stands Competitive weeds should be controlled in new alfalfa plantings to help insure establishment of a full stand of alfalfa. It takes more than 11 seedling alfalfa plants per square foot to give maximum alfalfa production at first harvest after fall planting. In Oklahoma, a planting rate of 12 pounds of alfalfa seed per acre into a good seedbed should give 14-19 seedling plants per square foot by November and 35 to 45 alfalfa stems per square foot by the following July. A full stand of alfalfa in Oklahoma is defined as a stand with at least 30 stems of alfalfa per square foot over the entire field. Winter weeds such as henbit and chickweeds in fall-planted alfalfa have seedling vigor that is similar to alfalfa, so a few of these weeds per square foot can be tolerated. However, the seedlings of most weeds are more vigorous than alfalfa. For example, just one plant per square foot of taller-growing weeds like mustard can result in a 10 percent yield reduction of alfalfa and six per square foot can result in 60 percent yield loss of alfalfa. A summary of competitiveness of several weeds in fall-sown alfalfa is listed in Table 2-4. The exact competitiveness of weeds is difficult to predict, since it depends on date of emergence of weeds and alfalfa, weather conditions (rainfall, temperature), and soil fertility level. With extended wet and warm weather conditions, when weeds outgrow and shade the alfalfa, several competitive weeds per square foot can result in the death of many alfalfa plants, thus stand failure. The first step toward obtaining a weed-free alfalfa stand is to plant weed-free alfalfa seed into a weed-free seedbed. A good alfalfa site should be fairly weed-free (including fence-rows). Weeds should be controlled in rotational crops preceding alfalfa planting to minimize weed seeds and plants of perennial weeds. This is especially important in western Oklahoma where weather conditions can be too dry for proper seedbed preparation during summer before planting. In these areas it is critical that deep tillage operations and perennial weeds be controlled in rotational crops at least two years before planting alfalfa. Perennial weeds such as curly dock and bermudagrass are difficult to control in established alfalfa. Another reason to control weeds in seedling alfalfa is to get producers focused on total pest management in new stands. Often producers do not scout and properly control insect problems in new stands of alfalfa. Combined stresses from weeds and insects result in seedling alfalfa plants that are often severely stunted or killed. This results in significant reductions in alfalfa production at first harvest and is one of the causes of poor stands. There are a number of selective herbicides available that can be used to control weeds in seedling alfalfa. Information about good herbicide options is listed on the Internet at Weed Control Suggestions, in the "Extension Agents’ Handbook of Insect, Plant Disease and Weed Control" (Circular E-832, which is updated annually), and on the web at Oklahoma Alfalfa.
Traditionally, most of the alfalfa in Oklahoma was been planted in the fall, and weeds have not been a major problem. However, with the shift toward minimum tillage in wheat (grown in rotation with alfalfa) plus planting wheat early for grazing, winter weeds are now a common problem in fall plantings of alfalfa. Postemergence herbicides are a better option for controlling weds in fall-planted alfalfa that preplant herbicides. Since there are good postemergence herbicides available for control of various cool-season weeds, it is possible to determine weed problems in October and then treat only when problems exist. Thus, it is important to scout new plantings of alfalfa for emerged weeds once a week after planting and then apply the appropriate herbicide (s) in the fall to ensure the establishment of a full stand of alfalfa. In most cases, it is profitable to control all competitive weeds in fall-planted alfalfa. Alfalfa planted in early September and kept pest-free will produce 4-5 tons per acre of weed-free hay in the first season. In addition, establishing a full stand of healthy alfalfa can extend the productive life of the stand by 2-3 years. Putting a dollar amount on profitability for the duration of the stand is not easy. It depends on weed infestation at establishment (weed species present, competitiveness of weeds, number of weeds), weather conditions during stand life, management practices of producers, and how hay is marketed. However, short-term partial budgets can be used to estimate and provide some guidance on controlling weeds. Top prices are paid for weed-free, high-quality alfalfa sold to dairies and horse owners. First harvest forage yields from fall-planted alfalfa average 1.75 tons of hay per acre. Value of first harvest hay would be $140 (1.75 tons at $80 per ton for weed-free hay). Alfalfa hay with a few weeds (5-15 percent weeds) is normally discounted $19 per ton when top quality hay is in short supply. This represents a loss of $33.24 (1.75 tons at $19) at first harvest. Cost to control weedy grasses or broadleaf weeds is approximately $29 per acre. Thus, the premium paid for first-harvest hay ($33.24) would cover the cost of controlling various types of weed problems in fall-planted alfalfa. Added benefits of controlling weeds in fall-planted alfalfa include15 percent increase in alfalfa yield at second cutting, assurance of a productive stand, giving high quality hay for a number of years, and an extended stand life. If hay is not going to be marketed as top-quality alfalfa hay, but is likely to go to beef cattle, then some weeds in newly established alfalfa can be tolerated. Seedling alfalfa must be scouted about one month after emergence to determine the weed infestation. Some examples of infestations and recommendations are as follows:
Scouting during fall and winter for weeds and insects is critical in newly planted fields. Competitive weeds should be controlled with a herbicide when population densities of these weeds approaches one weed per square foot. Infestations of blue and spotted aphids should be monitored closely and controlled with insecticides before they reach the economic threshold since insect damage results in seedlings that are not competitive with weeds. Grazing of fall-planted alfalfa before it flowers the following spring is not recommended since it retards root-growth and may decrease stand density. However, grazing can be used as a salvage treatment if aggressive weeds are not controlled in the fall. Once weeds start shading alfalfa, it will stop growing. Grazing maybe less damaging than shading by weeds. Grazing or cutting the first harvest early, before winter weeds mature, is another way to minimize the damage by weeds during establishment. This is particularly true with cheat, volunteer wheat, and mustards. Many weeds are palatable and have acceptable protein levels if grazed or harvested before they mature.
Spring-Planted Alfalfa Seedlings of most summer weeds (crabgrasses, foxtails and pigweeds) are more vigorous than alfalfa and must be controlled in all spring-planted alfalfa. Weeds are always a problem in spring-planted alfalfa, so using a preplant herbicide is good insurance. Postemergence herbicides are available, but they are more expensive and not as effective as preplant herbicides. Failure to control weeds in spring-planted alfalfa will often result in stand failure. Spring establishment of alfalfa should be avoided. Pest stresses from weeds, insects, and diseases are much greater in the spring, plus environmental conditions for alfalfa establishment are not as favorable in the spring. As a result, most stands established in the spring are not as productive as those established in the fall. In addition, stand life of spring-established fields is usually shorter.
Weed Management in Established Stands Full Stands Weeds are not competitive in properly managed full stands of established alfalfa. Properly managed stands have a soil pH of about seven, with proper soil fertility and effective pest management. Weeds germinate in full productive stands, but are not able to grow due to competition and shading by the alfalfa. Also, frequent mowing for hay coupled with the relatively short regrowth time of alfalfa makes established alfalfa competitive even with established weeds. For example, mowing at 1/10 bloom stage, under high moisture conditions, along with an adequate fertility program has been noted to suppress perennial weeds like johnsongrass. Thus, in established alfalfa fields, the presence of weeds is a sign of poor management or a thin, diseased stand. The full stand life of a multiple-pest resistant variety that is managed properly varies from 4 to 8 years, depending on location in the state. In general, stand life increases from east to west across the state. The shorter stand life in eastern Oklahoma is attributed to greater incidence of diseases associated with more rainfall. Thin Stands When alfalfa stem density declines below 30 stems per square foot, alfalfa does not occupy all of the area, and production is decreased. When this happens, weeds are able to grow in areas not occupied by alfalfa and compete with it; thus, an additional loss of alfalfa yield results. For each pound of weeds produced in these thin stands, there is a loss of ½-1 pound of alfalfa. The first weed problems to occur in thinning stands are often winter annual weeds. These weeds include henbit, mustards, and downy brome. A practical way to determine that a stand is no longer a full stand is when weedy grasses and broadleaf weeds make up more than 5 percent of the forage at harvest time. When this happens, the most economical decision is to start controlling winter weeds with a herbicide. First-harvest forage yields from stands with 20-30 alfalfa stems per square foot average about 1.5 tons of hay. Value of this hay, weed-free, is $120 per acre (1.5 tons at $80/ton). Weedy alfalfa hay with 15 percent weeds is, on average, discounted by 20 percent; thus the value of 1.5 tons with 15 percent weeds is only $96 (1.5 at $64/ton). The loss due to weedy hay is $24 per acre ($120-$96). A dormant application of a herbicide costs about $18/acre. Thus, the cost of controlling winter weeds is more than covered by the $24 saved by cutting weed-free hay. Added benefits of controlling winter weeds in thinning stands include a 15 percent increase in alfalfa yield at second harvest and several years of extended stand life, provided good management practices are continued during this period. It is very important to keep insect damage in alfalfa to a minimum, since it has been proven that weed growth increases in insect-stressed alfalfa. Also, it is important to keep soil nutrient levels adequate in thinning stands to keep alfalfa productive. Pigweeds are the major broadleaf weed problem in thinning alfalfa stands. Pigweeds can usually be controlled, but scouting and timing of herbicide treatments are critical. Crabgrasses and foxtails are the most common summer grasses in thinning stands. On sandy sites, sandburs also can be a major problem. Windmillgrass often becomes the dominant summer grass in stands with less than 20 alfalfa stems per square foot. Control of weedy grasses with herbicides is more difficult than control of pigweeds, but it is possible. A better solution often is to consider using the hay as mixed grass-alfalfa hay for beef cattle. These grasses can actually supplement total forage production since they grow in areas not occupied by alfalfa. Control of summer weeds is difficult to achieve with dormant season applied herbicides, since most herbicides do not have enough soil residual activity to control weeds all season. However, some herbicides can be applied in January-February and still have enough residual to control some summer weeds, provided enough herbicide gets incorporated into the soil. Pigweeds are best controlled with Pursuit applied to seedling weeds after the first or second harvest. It will control the emerged pigweeds and provide summer-long control of later-emerging pigweeds. Butyrac 200 can effectively control actively growing small pigweeds, but must be reapplied after each harvest as new pigweeds emerge. Some effective options for summer grass control include using Zorial before grasses germinate in the spring and using either Poast Plus or Select as postemergence sprays. Zorial can control crabgrass all summer, but control of some of the other annual grasses is marginal. Poast Plus and Select have no soil residual activity, so new grasses emerging after each harvest must be sprayed before the alfalfa canopy closes. Economics of controlling summer weeds in thinning stands is not very predictable or favorable with nonirrigated alfalfa. Summer weeds usually are not a problem in first, second, and third harvests, but occur in later harvests during wet summers. Because production of forage at fourth, fifth, and sixth harvests is usually less than a ton per acre at each harvest, control may not be profitable. Summer weed problems are quite variable in thinning stands, being influenced by species, number, and dormancy of weed-seed reserves in the soil, rainfall events throughout the summer, and timing of hay harvest. Generally there are enough weed seeds germinating at each cutting in the spring to cause a significant weed problem during a wet summer. This means that weed control might be needed after each harvest, or that a long-life residual herbicide could be needed to control weeds for 2-3 months. Finally, herbicide treatment costs for control of summer weeds can be expensive, since multiple treatments and herbicide mixtures are often required to adequately control summer weeds. Herbicides applied for control of summer weeds usually are not profitable since summer hay yields are often too small to cover the herbicide cost.
Harvest of alfalfa by grazing should be integrated into every producer's management of established alfalfa. Flash-grazing of alfalfa in December or early January each year when the soil is dry or frozen is an effective way to utilize fall alfalfa production, decrease pest problems, and increase the profitability of an alfalfa stand. As stands thin below 30 stems per square foot and summer weeds start making up more than 10 percent of forage in July and August, grazing is probably the most profitable harvest alternative. Excellent stocker gains are possible even in weed-infested fields, and bloat problems are manageable in the summer when mature alfalfa is mixed with summer grasses. Harvest of spring growth by grazing in March through April can be an effective way to reduce spring insect problems. However, this practice is not usually recommended since bloat problems on essentially pure stands of alfalfa are very difficult to manage during this period and May weed problems are increased by early spring grazing. As stands thin to fewer than 20 alfalfa stems per square foot, fall drilling of wheat or ryegrass into the alfalfa and then harvesting the forage by grazing may be the most profitable option. These improved forage grasses are competitive with weeds and essentially replace them in the areas not occupied by alfalfa. As a result, the spring forage on these areas is significantly increased, and the mixture of grass and alfalfa is excellent forage for stockers. Grazing of these alfalfa-grass sites in early spring also effectively decreases alfalfa weevil and aphid populations below economic threshold levels, thus reducing the cost of pest control. Wheat would be a better choice than ryegrass for obtaining a June alfalfa hay harvest. Wheat does not decrease May alfalfa production and has limited regrowth after May harvest. As a result, a June cutting of essentially all alfalfa would be possible with wheat overdrilled. Ryegrass is more competitive with alfalfa, so significant ryegrass would be present in June. Having ryegrass overdrilled probably would be the best option if the area was only going to be grazed.
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Alfalfa Production Guide for the Southern Great
Plains, 2001
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