Manure and Alfalfa -
  a complicated management mix

Dairy producers typically have a significant number of alfalfa acres and lots of manure. These two entities create a unique problem. Both are sources of nitrogen (N) for cereal crops. Alfalfa in the rotation has the overall effect of reducing the N recommendation for corn. As stated many times before, the need for more than about 40 lbs. N per acre is rare for first-year corn following alfalfa. This is the equivalent of about 10 tons of solid dairy manure or 4000 gallons of liquid manure. While corn following corn offers the most opportunity for efficient manure application, many producers do not have enough acres in this situation to make a big dent in manure inventories. Additionally, where no manure storage system is in place, there is a need to apply manure year around. For these reasons, many producers are now looking to the alfalfa crop as an outlet for applying manure in an economically and environmentally sound manner. Two options exist and are discussed here.

Apply manure before alfalfa establishment

Applying manure to alfalfa prior to seeding breaks tradition from the standpoint that manure-N is being applied to a legume. However, such an application can result in significant P and K contributions for the seeding and subsequent years. Also, a majority of the manure N will be utilized by the crop rather than leached to groundwater as would be the case where large applications are made following alfalfa and before corn.

A number of recent research studies have examined the potential for pre-seeding manure applications. In Minnesota, a four-year study resulted in increased alfalfa yields on both medium and high P and K testing soils where manure applications up to 12,000 gallons per acre were applied. The yield increases also tended higher than increases associated with comparable rates of commercial fertilizer applications. Similar responses have also been obtained in several Wisconsin experiments where up to 24,000 gallons per acre of manure were applied. Increased yields in all cases carried through the first full production year. It's thought that secondary or micronutrients contained within the manure may be responsible for the yield benefits above that of applied fertilizer. Other factors may include improved soil tilth, earlier N availability, and increased microbial activity.

The application of manure to alfalfa prior to seeding should only be used where the crop is direct seeded or the companion crop is removed as forage. Applying high rates of manure where the companion crop is harvested as grain will likely result in significant lodging of the small grain. Weed infestations from manure applications made pre-seeding have proven to be variable. Research experiences show little difference in weed pressure between control and manured plots, however, some producers have noted increased infestations as a result of manure applications prior to alfalfa seeding.

Topdress manure onto established alfalfa

Topdress applications of manure are often made because of the necessity to spread manure during the summer months. They can result in improved yields and stand persistence, however, they may also be detrimental to an alfalfa stand. The latter effect may originate from several sources. The first potential injury is from the manure's N. Plant growth will slow or stop where free ammonia from the manure is released. Manure also contains many salts that when placed in contact with plants, will burn or desiccate leaf tissue.

Less visible, but just as important, is the potential for crown damage caused by driving application equipment over plants. Large equipment will also contribute to soil compaction, whereby plant growth and nutrient uptake are affected. Both liquid and solid dairy manure can cause problems. While liquid manure generally "coats" the alfalfa foliage more thoroughly, there is more localized damage from solid manure where large clumps are left in the path of the spreader.

With the potential for alfalfa plant injury, Michael Schmitt, University of Minnesota Extension Soils Specialist, offers these criteria to select the best suited fields for manure applications:

Age of stand: There is more to risk by making applications to a younger stand that has a higher yield potential compared to an older stand.

Alfalfa density: Poorer stands of alfalfa tend to have more grasses contributing to overall yield. Grasses are more tolerant to topdressed manure applications than alfalfa and will directly benefit from manure N. With thin alfalfa stands, manure applications may increase forage yield at the expense of the legume in the stand.

Soil nutrient status: Where fertility is limiting production, the overall effect of a topdressed manure application may be better production because the response from applied nutrients was greater than other injury related factors. Therefore, choose those fields with the lowest fertility.

When making topdress manure applications to alfalfa, apply relatively low rates-----3,000 - 7,000 gallons of liquid manure or no more than 10 tons of solid manure per acre in a single application. Additionally, applications should be made immediately after removing a cutting so manure contacts the soil instead of the foliage. This will reduce the risk of salt burn and avoids palatability problems. Make applications only when the ground is firm and dry to minimize compaction problems. Summary

The alfalfa crop provides an additional option for producers to apply manure in a manner that is agronomically, economically, and environmentally beneficial. Applications made prior to seeding is an under utilized management practice that results in an efficient means to apply phosphorus, potassium, and other nutrients needed by alfalfa. Research results have shown yields may increase more than when only inorganic fertilizer sources are used. Topdressing manure on established alfalfa fields is more risky but can provide benefits if fields are properly selected and application guidelines are followed. Because of alfalfa's high value, a corresponding high level of management is needed with either manure application option to limit crop injury and subsequent yield potential.