Manure on Alfalfa ---- a good or bad idea?

Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension

     Manure has traditionally been targeted for corn acres during the course of crop rotation. The reason has simply been that corn uses and needs all of the nutrients supplied by manure (especially nitrogen). A recent trend has been toward using more manure on fields in the alfalfa years of rotation. Some of the reasons for this have been:

• alfalfa CAN use all of the nutrients
• established alfalfa fields are available during the growing season.
• alfalfa fields may be more accessible than other fields.
• there may not be enough corn acres to utilize N supplied by manure

     There are three points in time when manure can be applied to alfalfa. Let's take a look at each of these situations separately.

Fall of terminal alfalfa year
     The fall of alfalfa plowdown has traditionally been a dumping grounds for manure and likely one situation where gross over application of N occurs. There are few instances where a terminal stand of alfalfa won't supply at least 120 lbs. of N per acre to a subsequent corn crop. This means another 40 lbs. per acre is needed to meet the N needs of the corn. This is the equivalent of about 10 tons of solid dairy manure or 4000 gallons of liquid dairy manure per acre. In many situations, four or five times this amount of manure gets applied. Over application of manure during this terminal year of alfalfa comes at a cost to both the environment and the profitability of the crop enterprise.

Prior to 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 alfalfa establishment and in subsequent production years. Research trials in Wisconsin and Minnesota suggest applications of 12,000 to 24,000 gallons per acre can be made in the fall (preferable) or spring prior to seeding. Such a management strategy will reduce or eliminate the need for topdress P and K during the alfalfa production years. Another interesting result in nearly all of these research trials has been that manure has significantly increased alfalfa yields above and beyond simply major nutrient contributions. In other words, when a the same amount of nutrients were applied using commercial fertilizer, yields were not as high as those plots receiving the manure.

     It's thought that secondary or micronutrients contained within the manure may be responsible for the yield benefits above that of applied commercial 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 often results 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 sometimes made on recently harvested alfalfa fields because of the necessity to spread manure during the summer months. Why does this happen? Often it is because not enough manure storage capacity was constructed initially (an easy place to save money) or cow numbers have outgrown the existing storage structure. Although applying manure to established alfalfa has been done successfully, there are numerous potential risks. These include:

1. Stand reduction - salt and free ammonia cause damage to or kill new alfalfa growth.

2. Very narrow application window - applications must be made immediately after harvest to avoid plant damage. Either wet or dry weather after application can cause problems.  In the picture above, wheel tracks were killed when a heavy rain occurred immediately after manure application was made.

3. Soil compaction - especially if soils are wet. Trucks or spreaders loaded with manure transfer a lot of weight at the tire and soil surface interface.

4. Plant crown damage - caused from wheel tracks

5. Odor - many of the odor related summer spreading complaints initiate from situations where manure is being spread on recently harvested alfalfa fields. Here's one situation where you can't plow it down.

6. Palatability - although sometimes cited as a disadvantage, there is no research data to confirm such a fact. In short --- probably a non-issue.

7. Spread of Johnes' disease - overall, the risk of spreading Johnes' from topdressing alfalfa with manure is classified as "low". However, the risk factors increase when a high number of clinical animals are in the herd, forage is fed to young stock, spreading is done on 1st or 2nd crop rather than the 3rd/last, soil is "harvested", or when manure remains on the forage tissue.

To avoid dealing with the above risks, consider viewing manure applications to established alfalfa as merely a "last resort".

For more information contact Mike Rankin