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Unlike annual crops in which vigor and productivity must be maintained for only one growing season lasting several months, alfalfa is a perennial forage legume that, once established, is typically expected to remain highly productive for 5-7 years. This expectation requires consideration of both short-term and long-term consequences of management decisions, especially those related to pest control. Effective pest control is possible only with a comprehensive integrated pest management (IPM) program that targets insect pests, weeds, and plant pathogens. The ability of plants to survive stresses imposed by pest infestations depends greatly on the initial vigor of the stand at establishment. From the time that plants emerge, they are subjected to insect feeding, diseases, and competition with weeds. Alfalfa is quite attractive as a host for phytophagous insects. Species such as the alfalfa weevil and aphids may cause moderate to severe damage at any time during stand life (see "Insect Management" in this chapter). As alfalfa stands age, infections by several types of pathogens tend to become more prevalent. This is particularly true of soil-borne fungal pathogens such as Phytophthora sp. and Fusarium sp. (see "Alfalfa Disease" in this chapter). In addition, the incidence of soil-borne pathogens may be increased as a result of damage by insects. For example, damage by soil insects to the roots of alfalfa plants provides points of entry for fungi, leading to increased levels of infection. Finally, as alfalfa stand densities decline due to stresses imposed by insect pests and pathogens, weeds exploit bare soil areas, grow, and compete with alfalfa. Neither weedy grasses nor broadleaf weeds survive and grow in healthy, full stands of alfalfa. Weeds can grow only after open spaces form in the alfalfa canopy as a result of stand decline. Typically, there is little weed growth in established stands until 3-5 years after establishment, when stand densities decline below 25 stems per square foot (see "Weed Management" later in this chapter). Once weeds have space to grow, they become competitive and are an important sign of stand decline. It is important; therefore, that insect populations are monitored and controlled in a timely manner every year in both full and thinning stands since alfalfa cannot compete with weeds if stressed from insects. Because the occurrence of different pests is often interrelated, staying focused on good pest management usually provides both immediate yield savings and long- term benefits through improved stand longevity. Individual vs. Combined Pest Effects Field experiments have been conducted in central Oklahoma to determine individual and combined effects of foliar insect pests (alfalfa weevil, blue alfalfa aphid, spotted alfalfa aphid), and cool-season weeds (downy brome and shepherdspurse) on alfalfa. Beginning in the second stand-year, four levels of pest management were used (see Table 2-1 for treatments). The effect of controlling pests on seasonal yields i.e., tons of alfalfa forage per acre (minus weed content) and stand densities in stems per square foot were determined for five years (the second through sixth stand-years). In the second stand-year, stem densities for the various levels of pest management were similar (30-32 stems per square foot), and no alfalfa yield reduction due to weeds was evident. There was a loss of 0.5 tons of alfalfa per acre in the 2nd year when insects were not controlled (Table 2-1). By the 4th stand-year, stem densities decreased to 25-26 stems per square foot, with no evidence of stand reduction due to pests.
However, in the fourth year, there were measurable alfalfa yield losses resulting from both weed and insect pests (0.2 and 0.5 tons per acre, respectively). By the sixth stand-year, average stem densities in plots where both weeds and foliar insects were controlled had declined to 16 stems per square foot. This stand decline over time was attributed primarily to infection by soil-borne fungi. (Note: Currently there is no fungicide labeled for control of diseases in established stands. Using disease resistant varieties is the only effective option). By the sixth stand-year, stress from insect pests and weeds reduced both stem densities and forage yield of alfalfa. When foliar insect pests and weeds were not controlled, stem density averaged just nine stems per square foot with a 4.5 ton per acre reduction in yield of alfalfa (Table 2-1). These results emphasize the need for integrated pest management programs that consider all types of pest infestations.
Establishment of a healthy stand with complete alfalfa ground cover is critical to pest management and to profitable hay production. The capability of alfalfa to withstand stresses imposed by pest infestations depends greatly on initial stand vigor. Soil testing should be conducted as land is prepared for planting so that the needed amounts of fertilizer and lime can be incorporated (see Chapter 4, "Fertilizing Alfalfa" for details). Following careful preparation of a firm seedbed (see Chapter 3, "Stand Establishment" for details), high-quality seed of a multiple-pest resistant variety should be planted. Perhaps the most important pest management decision to be made over the life of an alfalfa stand is to plant an improved, multiple-pest resistant variety selected from the list updated annually by Oklahoma Cooperative Extension Service specialists. Farmers should accept no substitute for the most recently recommended improved varieties with the highest available levels of resistance to insects and pathogens. In field experiments evaluating stand life and productivity, alfalfa forage yields in the sixth year averaged over six tons per acre for improved varieties compared to 2.2 tons per acre for the season with Oklahoma common, which has no pest resistance. Timely grazing of alfalfa stands can provide cost effective means of reducing stress from insects. For a modest investment in labor and fencing materials, grazing in late winter through early spring (February through March) can greatly reduce populations of insect pests such as alfalfa weevil and aphids. Cleanly grazing stands in fall to early winter to remove dead foliar growth left from the previous season and new crown growth, consistently reduces populations of alfalfa weevil larvae and aphids. This type of grazing can reduce the number of insecticide applications required to keep the insects below economic threshold levels, or in some years, can eliminate the need to spray for insects. Returns from grazing include the value of forage consumed by livestock and savings resulting from reduced pesticide applications. Early first harvesting can be a good alternative to spraying for control of alfalfa weevil larvae in some years. In most years egg hatch and larval development is completed by mid-April when alfalfa typically begins to bud in southern and central Oklahoma. Development of weevil infestations may be delayed enough that haying at early bud stage may be considered as an alternative to spraying. Haying early is most effective when weather remains warm and dry, promoting rapid drying of forage for baling. Such conditions also expose weevil larvae to the heat of the sun with little foliar growth for food or protection. Pesticides are the only effective means of control available when insect pest populations exceed economic threshold levels and significant losses in productivity are likely to occur. The economic threshold level is reached when there is enough pest infestation that the potential loss from that infestation exceeds the cost of a chemical application. At this infestation level, spraying becomes profitable. Infestations of alfalfa weevil larvae or blue alfalfa aphids can reach economic thresholds in many fields in Oklahoma on an annual basis, particularly in those not cleanly grazed in winter or early spring. Thus, it is important that fields be scouted regularly during March and April and that timely applications of insecticide are made. It is critical that sprays be timed properly according to the "shake-bucket" method to attain maximum returns on investments for insecticides (see OSU Current Report No. 7177 for details). If sprays are applied too late (once extensive plant damage has occurred), significant losses of alfalfa yield will result. In addition, profitability of applying insecticides tends to decrease in mid-April since populations of insect pests are decreasing and activity of beneficial insects such as lady beetles and parasitic wasps is increasing. Unlike annual problems with insect pests; weeds are not normally competitive with a full stand of alfalfa in the first 3-4 years of stand life. Herbicide treatments applied during this period usually provide no return on investment. Once stands begin to thin and weed production at first harvest is greater than 5 percent, it is important that cool-season weeds (downy brome, cheat, shepherdspurse) be controlled each year (see "Weed Management in Alfalfa" in this chapter). To maintain productive stands for 6-7 years, it is critical to plant an improved variety, maintain proper soil fertility, keep fields scouted for pests, and make timely applications of both insecticides and herbicides when needed.
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Alfalfa Production Guide for
the Southern Great Plains, 2001
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