Soils: October 2002

Proper management of alfalfa is essential not only for sustaining high-level forage production, but for stand longevity. Under good management, a stand of alfalfa can remain highly productive for five to seven years. To reach this goal, both short- and long-term management is required, especially in the areas of stand establishment, soil fertility and pest management.

Stand Establishment
The ability of plants to survive stresses imposed by environmental conditions and pest infestations greatly depends on the initial vigor of the stand at establishment. There are several factors to consider before seeding a field to alfalfa.

• Site selection and soil testing have a large impact on stand success and level of production, so choose a site that is well drained, deep, fertile, free from damaging herbicides (residual activity from previous crop) and has a loamy to sandy loam texture.
   
• Take soil samples that well represent the area. Remember, the greatest margin of error associated with soil testing is the method of gathering the sample from the field.
   
• Apply fertilizer and lime according to the soil test analysis.
   
• Apply enough phosphorus and potassium to satisfy the crop's needs before planting.
   
• Begin seedbed preparations by plowing, leveling and draining low areas. It is important to create a mellow, firm seedbed with small clods.
   
• Select a seed variety that is adapted to the area.
   
• Make certain that the seed is clean and has good germination.
   
• An important step, which is often forgotten, is inoculating seed with rhizobium bacteria specific to alfalfa.

The best time for planting is in the fall from Aug. 15 to Sept. 15, and second best is in the spring from March 15 to April 15. Seeding rate should range from 10 to 15 pounds per acre. When planting, cover the seed with no more than one-half inch of soil. Calibrate and adjust the drill to place the seed in a good environment to guarantee good seed-to-soil contact. Finally, be sure to control weeds, insects and diseases.

Soil fertility
Soil testing is the only method of determining the nutrient-supplying status and the pH of your soil. Optimum soil test indices for alfalfa are 65 pounds per acre phosphorus (P), 350 for potassium (K), and a soil pH of around 7. If stands with a low soil pH and deficiencies in P and/or K are not corrected, alfalfa yields will decrease significantly and stand life will be shortened. High-yielding alfalfa removes large amounts of nutrients from the soil (Table 1). Frequent monitoring of the soil's nutrient status by testing is the best way to determine which nutrients are naturally being supplied to the crop and how much fertilizer is needed to keep the stand productive.

Alfalfa does not need much nitrogen (N) fertilizer. No more than 20 pounds of N per acre should be applied for establishment. Once seedlings form nodules on their roots, they can fix their own N from the atmosphere. Nitrogen fixation is a result of a symbiotic relationship between the alfalfa plant and rhizobium bacteria, thus the importance of proper seed inoculation at planting.

As noted in Table 1, large amounts of P and K are removed from the soil annually. Phosphorus deficiencies are best corrected by applying and incorporating a three-year supply of fertilizer prior to planting. Additional P fertilizer should be applied in the fall prior to the third or fourth year of production. Potassium fertilizer should be applied annually, as directed by the soil test, to support production for one year. Make K fertilizer applications each fall in preparation for spring growth.

Pest Management
The term "pest management" is broad, encompassing insect pests, weeds and disease. Effective pest control is possible only with an integrated pest management (IPM) program that targets the three categories mentioned above. We already know what individual pests can do to limit production, but it is important to remember that in a plant with two pest problems at the same time, stress will be evident three-fold.

Currently, there is no fungicide available for disease control in alfalfa. The only effective option for avoiding disease is to plant an improved disease-resistant variety. Oklahoma State University research has shown that alfalfa forage yields in their sixth year of production averaged more than 6 tons per acre for improved varieties, and only 2.2 tons per acre with Oklahoma common, which has no disease resistance. Common alfalfa diseases include phytophthora root rot (most common), bacterial wilt, fusarium rot, anthracnose, Texas root rot, viruses and nematodes.

Insects can drastically reduce yield in a short period of time. Timely grazing in the dormant season, to clean up dead foliar growth left over from the previous season, can greatly reduce populations of insects such as alfalfa weevil and aphids. It also may reduce the number of insecticide applications needed to keep insects below the economic threshold. The economic threshold level is reached when there are enough pests present that the potential loss from the infestation exceeds the cost of a chemical application. Pesticides are the only option for control when insects exceed the economic threshold. It is important to frequently scout your fields from February through April and make timely pesticide applications when necessary. Common alfalfa insects include the alfalfa weevil, blue alfalfa aphid, spotted alfalfa aphid and foliage-feeding caterpillars.

Weed problems in alfalfa usually occur at establishment and in thinning stands. Rarely will weeds be competitive in a full stand of alfalfa in the third or fourth year of production. Herbicide applications during this productive period generally do not provide a return on the investment. Once a stand begins to decline and weeds in the first harvest are greater than five percent, an herbicide treatment may be warranted. Yield reduction from weed competition in new stands often exceeds 1,000 pounds per acre. Yield loss in a thinning stand is one-half to one pound of alfalfa for every pound of weeds present. Again, it is critical to scout fields often and treat when populations exceed the economic threshold.

Focusing management on stand establishment, soil fertility and pest control will result in a high-yielding stand of alfalfa that should persist for several years. Most of the comments here regarding alfalfa management were gathered from Alfalfa Production Guide for the Southern Great Plains. This book is available through the Oklahoma Cooperative Extension Service's Division of Agricultural Sciences and Natural Resources at OSU. Additional sites regarding alfalfa production are listed below.

Timely Management Information
alfalfa.okstate.edu/index.htm (alfalfa alerts, alfalfa production calendar, etc.)

Alfalfa Production and Pest Management in Oklahoma E-826
okstate.edu/OSU_Ag/agedcm4h/pearl/e826

Alfalfa Stand Establishment Questions and Answers E-949
alfalfa.okstate.edu/pub/stand-949/stand-est.htm