– Dr. Mark Sulc, Forage Specialist, Ohio State University
Applying manure to alfalfa is becoming increasingly common for a variety of reasons. There are several environmental, agronomic, and management advantages and potential concerns with applying manure to alfalfa. This article provides a brief synopsis of those issues, taken from the North Central Regional Research Report 346, “Applying Manure to Alfalfa”, authored by K.A. Kelling and M.A. Schmitt at the Universities of Wisconsin and Minnesota. At the end of this article, I provide information on how to obtain a copy of the complete report.
The advantages of applying manure to alfalfa include the following: 1) it provides substantial cropland for spreading manure throughout the summer months, 2) alfalfa can utilize the macro and micronutrients provided in manure, including excellent utilization of the nitrogen applied in manure, and 3) alfalfa’s deep root system extracts mobile nutrients such as nitrogen, sulfur, and boron at greater depths than corn. One nutrient concern, however, may be over application of potassium through the manure. This can lead to unacceptably high potassium levels in the forage. Soil test results are an essential tool to utilize when planning manure application to alfalfa or any other forage.
There are three basic timing strategies for applying manure to alfalfa and each has its advantages and risks. Careful management can improve the success of each of these strategies, as discussed below:
Applying Manure Before Alfalfa Establishment. This is a relatively new approach, and may provide the best combination of agronomic performance and reduced environmental risk. Research studies have shown that pre-plant manure applications generally have a positive effect on seedling-year alfalfa yield if weeds are adequately controlled. The yield response may be carried over into the first full production year. The increased weed pressure usually does not persist past the first cutting in the seeding year. In pure alfalfa stands, there are several good herbicide options to deal with this potential problem. When spreading manure ahead of seeding, avoid compacting soil and be sure that the manure is incorporated thoroughly in the soil so seed is not planted directly into areas with high amounts of manure. Do not apply more than 75 tons/acre of solid dairy manure or 20,000 gallons/acre of liquid dairy manure ahead of seeding to ensure healthy alfalfa growth and avoid environmental problems. If applying more than 40 tons/acre of dairy manure equivalent, apply at least 6 weeks before seeding. Other types of manure with higher salt and ammonia levels should be applied at lower rates than these.
Topdressing Manure on Established Alfalfa. This practice is risky because of potential for compaction injury, salt burn to alfalfa leaves, stand suffocation, and increased weed pressure. Some research results showed reduced alfalfa yields with topdressed manure on established alfalfa. Applications on frozen soils run the risk of large nutrient runoff losses, so exercise extreme care and judgment in those situations and avoid it if possible. Consider topdressing manure only on older stands with the most grass, which tolerates topdressed manure better than alfalfa (this may cause further loss of the alfalfa in the stand but tends to increase forage yield via stimulating grass growth). Topdress manure only where the nutrients are needed. Apply no more than 3000 gallons of liquid or about 10 tons of solid dairy manure per acre in a single application, and spread manure as soon as possible after harvest to avoid burn potential and palatability or forage quality problems with the forage regrowth. Topdressing manure after substantial regrowth is present may also negatively impact the ensiling process of haylage. Make sure equipment is adjusted for uniform application, and spread only when soils are firm to minimize damage to alfalfa crowns.
Applying Manure Immediately Before Terminating Alfalfa Stand. The most common strategy for applying manure to alfalfa ground is to spread it immediately before rotating the field to a grain crop, most notably corn. Advantages are that alfalfa injury is no longer a concern, labor is usually available in late summer to accomplish the task, and the field is still smooth and firm (before any tillage is performed). But, there is significant risk of loading the field with more nitrogen than the following crop can use, even if it is corn. Research has demonstrated that there is little, if any, response to additional nitrogen on corn following alfalfa. The environmental risk of nitrate leaching may be high with this practice. Limit the manure rate applied at the end of the alfalfa rotation to that amount which will supply the nitrogen required by the following crop AFTER accounting for the alfalfa nitrogen credit. Apply only to the very poorest hay fields where all alfalfa top growth has been harvested prior to manure application. The legume nitrogen credit increases as amount of topgrowth and alfalfa stand density increase. Apply manure immediately before primary or secondary tillage to reduce risk of direct manure runoff losses. Use a pre-sidedress nitrogen test before applying any additional nitrogen fertilizer to the following corn crop.
Reference: Kelling, K.A., and M.A. Schmitt. 2003. Applying Manure to Alfalfa. North Central Regional Research Report 346. For copies contact Keith Kelling, Department of Soil Science, University of Wisconsin, 1525 Observatory Dr., Madison, WI 53706-1299. Phone (608) 263-2795, email: firstname.lastname@example.org.