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Serving: WI
harvesting alfalfa field
MORE ALFALFA: Decreasing stressors to new seedings caused by insects, diseases and weeds will increase the likelihood of higher future yields.

Scouting, fertility key to alfalfa yields

Field Fodder: Limiting stress boosts alfalfa yields.

By Dan Marzu

Last winter brought challenges with winterkill in many Wisconsin alfalfa fields. This caused several alfalfa stands to be terminated early and new seedings to be planted throughout the state.

Now that summer is upon us, it is important to manage new seedings and established hay acres to ensure adequate yields to supply feed for your farming operation. This can be accomplished through fertilizing the forage to provide the needed nutrients and scouting for timely control of insects, diseases and weeds that could decrease yields.

According to Nutrient Application Guidelines for Field, Vegetable, and Fruit Crops in Wisconsin, alfalfa removes 13 pounds of phosphate (P2O5) and 60 pounds of potash (K2O) per ton of yield. A soil test is the best method to determine soil nutrient levels as well as recommend the amount of fertilizer needed to support your yield goals. Alfalfa responds to incorporated nutrients in the top 6 to 8 inches. If nutrients were not applied prior to seeding, the Alfalfa Management Guide recommends applying them immediately after harvest and before regrowth.

Avoid applying nutrients on soft soil to prevent damage to alfalfa crowns. Split the application throughout the year if recommendations require 500 or more pounds of fertilizer to prevent salt damage to the plants. If manure is to be used as a fertilizer source, apply on your oldest stands limiting rate to 3,000 gallons, or 10 tons, per acre. Just as with fertilizer, apply manure immediately after harvest and prior to regrowth on dry soils. Make sure you are following the requirements of your nutrient management plan when making an application of any fertilizer or manure.

Decreasing stressors to new seedings caused by insects, diseases and weeds will increase the likelihood of higher future yields. In the publication Effect of Seeding Year Stress on Future Alfalfa Yields, Dan Undersander, retired University of Wisconsin-Madison Extension forage agronomist, reported that potato leaf hopper damage in the alfalfa seeding year decreased future yields by 0.25 to 0.5 ton per acre. Yield losses were higher when a second stressor was present. Bryan Jensen, UW-Madison Integrated Pest Management Program, and Damon Smith, UW-Madison plant pathologist, have developed a Generalized Calendar of Events for Insects and Diseases in Wisconsin for alfalfa. This is a great resource to aid in what to be scouting for as well as providing economic threshold levels to aid in deciding if a control measure is needed. Other resources include the UW-Madison Crop Manager and the Wisconsin Department of Agriculture, Trade, and Consumer Protection Wisconsin Pest Bulletin. Both of these provide up-to-date information on pests that may be in the fields during the growing season.

Use sweep net

When scouting for insects, use a 15-inch-diameter sweep net taking 20 sweeps at five locations throughout the field. Count each type of insect you find in the field after each 20 sweeps. Take the average by dividing by 100 and compare with the threshold level.

For example, if you calculated an average of 109 pea aphids per sweep and the threshold is 100 aphids per sweep, then you may consider cutting if the alfalfa is near the harvest date or apply an insecticide to control the insect. Many Extension offices have sweep nets available to use to help you determine insect pest numbers in your fields.

Leaf discoloration, stunted growth, reduced root mass and dead stems may be signs that a disease is present. It is important to examine the whole plant, including digging up the plant to look at the condition of the crown and roots. It may be necessary to submit a sample to the UW Plant and Disease Diagnostic Clinic to have a correct diagnosis, as some of these symptoms have similar characteristics of nutrient deficiencies, insect damage or other diseases. Your local Extension office will be able to help you with the process of submitting the plants to the lab.

Remember to always read and follow labeled directions when applying any pesticides.

With a little bit of time to scout for pests and plan out fertilizer applications, you may be able to increase the chance of having a healthy, productive forage source.

Marzu is the Extension agriculture educator for Lincoln and Langlade counties in Wisconsin.

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