It's Time for Frost Seeding: Using Legumes to Meet Nitrogen Needs

By Criag Sheaffer and Jochum Wiersma et.al

“Frost seeding” is the seeding of legumes and grasses on the soil when it is frozen during late winter and early spring. With frost seeding the seed becomes buried in the soil through alternating freezing and thawing events. When favorable temperatures (45 F and above) occur, the seed germinates and emerges. It's considered a simple, low-cost approach to improve pastures and to add red clover to overwintering grains. Establishing nitrogen-fixing legumes provides additional advantages when fertilizer prices are high as they allow nitrogen to be fixed from the atmosphere and can replace at least a portion of costly nitrogen fertilizer.

Frost seeding for pasture improvement/renovation

Frost seeding is often recommended for pasture improvement or renovation when tillage or using a no-till drill is not practical. The probability of successful frost seedling is typically less than conventional spring seeding because you aren’t preparing a seedbed or using a drill that ensures better seed to soil contact. We have observed seedling survival rates from 0 to 60% with frost seeding. The number of freeze-thaw cycles is an important factor influencing success but other critical management considerations include:

• Improving seed to soil contact after seeding. Freeze-thaw cycles are critical for frost seeding to be successful. Ideally, the existing sod should be suppressed by aggressive grazing and hoof traffic in the fall, but even if that has not occurred, running livestock on frost-seeded pastures in the spring can improve the success of frost seeding by removing senesced vegetation and by improving seed to soil contact through hoof traffic.
• Improving initial establishment with rotational grazing. After germination, the new seedlings are at a competitive disadvantage for light and moisture compared to the established grasses in the sod. Short-interval rotational grazing to a 4-inch height in the spring and summer when the grass canopy reaches a 10-inch height helps the new seedlings get established as competition from the established sod is reduced.
• Prioritize legumes like clovers for frost seeding into pastures. Use a Rhizobia trifolii inoculant when seeding clovers to ensure effective nitrogen fixation. Over time, most pastures become primarily grasses and a sod-bound condition can occur. Pasture productivity and forage quality can best be improved by introducing perennial legumes. 
  •  Legumes have potential to fix nitrogen from the air to be used for their own growth. An average rate of biological nitrogen fixation for most legumes in pure stand is 50-100 lb N per acre. When mixed with grasses, legumes can transfer some nitrogen to grasses in the pasture which can substitute for application of nitrogen fertilizers. 
  • Legumes contain more protein than grasses, are often more palatable, and have a higher digestibility. 
  •  Red and white clover are relatively easy to establish, tolerate low pH and fertility, and have a high number of seeds per pound. This means a few pounds go a long way. Seeding 3 -4 lb of each legume in a mixture will provide over 80 seeds per ft2 of seed (Table 1). Seeding rates can be increased to allow for challenging seedbed conditions. 
  • Other legumes like alfalfa and birdsfoot trefoil can be used for frost seeding and can be substituted in the mixture in some soil conditions. While birdsfoot trefoil has tolerance to low soil pH, it has low seedling vigor and may not establish well. Alfalfa has a high level of productivity and is persistent but is most sensitive in terms of poor soil fertility and low pH.
• Introducing new grasses into pastures has benefits if there is considerable open space. However, all-grass pastures will still need nitrogen fertilizers to be productive so it may be advantageous to establish grass-legume mixtures. Annual and perennial ryegrass are easiest to establish because of their high seedling vigor. Orchard grass also has good seedling vigor and can be frost-seeded and will offer greater long-term persistence than the ryegrasses.
• Soil sample after seeding. Soil fertility may limit the forage yield potential of your pastures. Although best done before frost seeding, a spring soil sampling after seeding may identify pH, P, K, or S levels that can affect plant vigor. These can be adjusted through fertilization.

Finally, be patient with frost seeding. Because of all the factors above there is risk of establishment failure. Even if seedlings establish, it will require 60 days to observe appreciable new forage growth. Details of frost seeding pro’s and cons as well as strategies for improving pastures are well described at the following University of Minnesota's website, Frost seeding forages.

Table 1. Suggested seeding rates legumes and grasses frost seeded into pastures. Using a mixture of legumes may provide some long-term advantages. If a single species is used then adjust the seeding rate to achieve a minimum of 80-100 seeds per ft².

Frost seeding red clover into winter wheat and winter rye

Frost seeding red clover into winter small grains has become a standard practice in many regions where small grains can reliably overwinter. Frost seeding red clover into a winter cereal crop provides a cost-efficient way to add a nitrogen fixing legume to the cropping system. A typical sequence of events is as follows:

• Broadcast inoculated red clover seed in March-April to take advantage of the alternating freeze-thaw cycles (but not on top of snow).
• Red clover seeds germinate beginning in April as air temperatures increase above 40F. Seedlings develop but remain small due to competition for light and moisture from the small grain.
• The grain and straw are harvested from the cereal crop in July, reducing competition with the red clover seedlings.
• The red clover grows following harvest, providing at least one harvest (if desired) in summer or fall.
• The red clover can be killed and incorporated in fall or in early spring before planting the following year.
• The incorporated red clover provides a nitrogen credit of 75 lb N per acre for the following year, reducing N fertilizer needs. See: https://extension.umn.edu/crop-specific-needs/fertilizing-corn-minnesota

Research on frost seeding into winter cereals in Iowa has shown that red clover frost seeded in March can produce from 3.4 – 4.5 tons DM per acre to total forage from 2 cuttings (Table 2). In addition, an average of 64 lb per acre of biologically fixed N was present in the forage for incorporation! If left until the spring, the forage contained about 40 lb per acre of fixed N. Neither nitrogen contained in the roots nor soil (rotation benefits) were accounted for, so values are likely an underestimation of the 75 lb per acre fertilizer credit recommended by University of Minnesota. These values were determined even with a seedling success rate of only 20%. Seeding rates of 84-112 seeds ft-2 were recommended from this research.

Table 2. Red clover populations, summer, fall and spring yields and fall and spring yield of biologically fixed nitrogen when red clover was frost seeded into winter wheat and triticale in Iowa. Data is averaged for two years.

Yield of fall biological nitrogen fixation calculated as forage yield x 3.2% N in forage x 50% legume N from biologically fixed nitrogen.

Data from Blaser et al., 2006 & 2007; Agronomy Journal Volume 98 & 99. Schipanski and Drinkwater, 2011, Nutrient Cycling in Agroecosystems Volume 90

Frost seeding into winter cereals will work in Minnesota without affecting grain yields; however:

• The success of seedling establishment may be less because Minnesota has fewer cycles of soil freezing and thawing than regions further south. In addition, the frequency of temperatures less than 26 F following legume germination can damage developing seedlings.
• The forage yield potential of red clover will likely be less because of a shorter growing season and more variable precipitation. Cereal harvests from July to August limit the amount of time for regrowth; and low soil moisture levels may limit red clover regrowth. In addition, weed seed banks can have a significant effect on red clover yields and weeds emerging following grain harvest may require mowing to reduce competition and to prevent seed formation. In research conducted at three locations in Minnesota, fall forage yields ranged from 275 to 1200 lb per acre.
• Herbicide options are greatly limited because of the presence of both a broadleaf and a grass. When considering frost seeding or underseeding clovers in the spring, summer annual weeds are usually the primary concern because perennial and winter annual weeds were already controlled by tillage before seeding the previous fall. Winter wheat and winter rye are both very competitive with summer annual weeds and weed control is often not needed for summer annual weeds. When required, the safest option is bromoxynil applied prior to the second trifoliate of the underseeded legume.

Conclusions

Legumes added to pasture and winter grain systems can add nitrogen to cropping systems. This benefit is accentuated when nitrogen fertilizer prices are at an all-time high. Integrating red clover into small grain systems may be worth considering this year. The productivity of perennial pastures can also be enhanced with frost seeding legumes and sometimes grasses. The time to make these decisions is now.