By Kevin Jarek and Mark Renz

The alfalfa seed industry has introduced coated seeds over the last decade to improve establishment of alfalfa seedlings. These coatings can be light or heavy but are still sold in 50-pound bags. As a result, farmers are getting less pure live seed (PLS) in a bag.

While these coatings may enhance each individual seed’s ability to establish successfully, they reduce the total number of seeds in a bag when purchased by weight. Rhizobium bacteria, fungicide, colorant and polymers that bind the material are the most common constituents of coated seeds. Due to these changes, farmers have been asking University of Wisconsin-Extension educators: “What seeding rate should we be using to maximize our plant establishment?”

Seeding rate
The most recent alfalfa seeding recommendations for the Upper Midwest suggest that farmers should not be planting less than 10 pounds of PLS per acre to maximize plant establishment and overall yield. In contrast, industry recommendations often exceed those Midwest recommendations, resulting in more than 12 pounds of PLS per acre. As Wisconsin harvests about 1.25 million acres of alfalfa each year, the need to provide efficient and effective alfalfa seeding rate guidelines is critical.

The UW-Extension Team Forage’s Wisconsin Alfalfa Yield and Persistence project has revealed that most Wisconsin farmers are planting alfalfa at rates of 15 to 17 pounds of seed per acre at the time of establishment, regardless of the amount of coating or inert material (WAYP Program Summary Report, 2016). Seeding rates during the lifetime of the WAYP project (2007-16) show a range of 12 to 28 pounds per acre. With a range that wide, the question should not be “How many pounds of alfalfa seed should I be planting?” but instead, “How many pounds of PLS alfalfa should I be seeding at establishment?”

A typical 15-pound-per-acre seeding rate would result in about 75 seeds per square foot. Previous work examining alfalfa seeding rates revealed that once emergence is complete in three to four weeks, only 50% to 70% of seeds will have established as seedlings, leaving about 45 plants per square foot. Another 40% to 50% of those plants will no longer be present by the following spring, resulting in about 25 plants per square foot heading into the first full production year.

Survey developed
To help farmers answer the question about alfalfa seeding rate recommendations, a UW-Madison/UW-Extension Badger Plot Survey was developed. Members of the UW-Extension Team Forage Agronomy Workgroup recruited farmers in their respective counties across Wisconsin to participate in a statewide effort to collect on-farm data and track alfalfa plant and stem counts over the life of a newly established stand of alfalfa, from beginning to time of rotation. The effort enlisted the help of county agents tracking alfalfa fields in Calumet, Chippewa, Clark, Outagamie, Shawano and St. Croix counties. Agents and educators measured how many plants emerged and survived. Farmers were using different seeding rates with both coated and noncoated products, which resulted in a diverse set of data measuring how the number of alfalfa plants and stems diminishes over time.

The experimental design that was developed asked each participating farmer to seed alfalfa fields according to his or her own existing production practices. The partnering UW-Extension agent or educator was responsible for collecting a range of information from each field, including the seeding rate in PLS, plant stand counts measured in the spring and fall each year, and plant stem counts measured in the spring and fall each year.

Participating farms reported alfalfa seeding rates of 12.5 to 22.5 pounds per acre. When converted to PLS, the range was 8.8 to 17.8 pounds per acre for alfalfa fields across the state. The objective of this effort was to determine if alfalfa seeding rates higher than the Upper Midwest recommendation of 10 pounds of PLS per acre result in any significant difference in plant and/or stem counts during the life of the stand.

The top chart, “Plant counts in fall, year 2,” illustrates the number of alfalfa plants measured per square foot in the fall of the second full production year across three different PLS seeding rates. After analysis, no significant differences (P=0.23) were noted between plant counts at the PLS seeding rates identified.

Stem counts are often used to determine if an alfalfa field will produce an economic yield for the season. The number and size of stems often determine a field’s productivity over a growing season. The bottom chart, “Stem counts in fall, year 2,” illustrates the number of alfalfa plant stems measured per square foot in the fall of the second full production year across three different PLS seeding rates. After analysis, no statistically significant differences (P=0.15) were noted between plant stem counts and the PLS seeding rates identified.

Jarek is the Extension crops and soils agent in Outagamie County, Wis. Renz is an associate professor and weed scientist at University of Wisconsin-Madison.