Why Frost Seed?
Frost seeding legumes and grasses is increasingly being used by graziers as
a means to improve pasture yields or change forage species composition within
the pasture. Frost seeding offers several potential advantages. These include
the ability to establish forage in an undisturbed sod, a reduced need for labor
and energy compared to conventional seeding methods, the ability to establish
forages with minimum equipment investment, a shortened "non-grazing"
period, and it is a method to maintain stands at productive levels with both
grasses and legumes.
Keys to Successful Frost Seedings
1. Seed - soil contact
As with any method of forage establishment, seed to soil contact is critical
for successful frost seedings. There are several management practices that can
be done to help insure good seed-soil contact. The first involves fall grazing
management. Pastures should be closely grazed in the fall or winter to open
stands and expose soil. Sod-type grasses like bluegrass are the most difficult
to make successful frost seedings, especially where a thick layer of thatch
covers the soil surface. It is in these situations where short duration animal
hoof action is sometimes needed to help "plant" the seed.
2. Reduce plant competition with new seedlings
New seedlings must be given a chance to establish without excessive competition
from plants already present in the stand. Reducing competition can be done in
several ways. First, grazing pastures down to 2 inches in the fall will help
to slow regrowth in the spring. Frost-seeded pastures need to be grazed regularly
in the spring and summer to allow for light penetration into the plant canopy.
However, it is also beneficial to move animals off pastures before young seedlings
are consumed prior to adequate root development.
The importance of reducing plant competition during the year of establishment
is seen in research done at Michigan State University (Leep, 1989). In this
study, red clover and birdsfoot trefoil were frost-seeded into a sod that was
either suppressed with a herbicide or cut from zero to four times following
seeding. The more frequent cutting regime resulted in stand densities similar
to or better than those of the herbicide-treated field plots (Table 1).
3. Species selection and seeding rates
In many cases, frost seedings are made to introduce or increase forage legume
species into a grass stand. Research and farmer experience has shown good results
with red clover and birdsfoot trefoil. Alfalfa, alsike clover, and white or
ladino clover have also been frost-seeded with varying degrees of success. Do
not frost seed alfalfa in situations where alfalfa plants already exist in the
stand. Autotoxicity will prevent new seedlings from becoming established. Several
studies have indicated that frost seeding both red clover and birdsfoot trefoil
together offer the advantage of long-term legume presence in a pasture. In an
Iowa State University research trial (George, 1984), red clover established
quickly and remained productive for the first two years while birdsfoot trefoil
became productive in the second and subsequent years following seeding (Table
2). Many producers routinely frost seed red clover every two to three years
to sustain legume production.
Table 1. Stand Density of Two Forage Legumes Following Frost Seeding (Michigan
State University)
Red Clover Trefoil
----------------------------------months after seeding-----------------------------------
Treatment 3 15 3 15
Established plants as % of herbicide treatment stand
0 cuts 69 51 74 52
2 cuts 79 82 88 96
4 cuts 103 107 117 114
Herbicide 100 100 100 100
Data averaged over 3 grass sods (smooth bromegrass, reed canarygrass, and orchardgrass
Table 2. Plant Density and Percentage Establishment for Legumes Frost-Seeded
into Grass in March, 1977 (Iowa State University)
Legume Plant Density
Legume Type Seeding Rate Sept. 1977 Sept. 1978
Lbs./acre Seeds/ft.2 Plts./ft.2 %* Plts./ft.2 %*
Red Clover 8 46 5.1 11 4.9 11
Red Clover 16 92 8.2 9 8.7 9
B. Trefoill 6 48 3.1 6 9.4 20
B. Trefoil 12 96 4.9 5 12.0 13
* Legume plants as a % of seeding rate
Frost seeding grasses.......
Wisconsin graziers sometimes find themselves in a position to try and introduce
grasses into old alfalfa stands. This offers some unique challenges because
most cool season grasses do not establish from frost seeding with the success
of legumes. A recent University of Wisconsin study compared frost seeding establishment
of several cool-season grass species into older, established alfalfa stands
(West and Undersander, 1997). In this two year trial, perennial ryegrass and
orchardgrass exhibited the best establishment success, smooth bromegrass was
intermediate in establishment, while reed canarygrass and timothy had the fewest
productive seedlings develop. Although smooth bromegrass was only intermediate
in establishment, its sod-forming growth habit would likely result in additional
plant formation from rhizomes.
Both perennial and annual ryegrass are good choices for frost seeding where
fast establishing, high quality pasture is desired. However, both of these species
will not typically overwinter in Wisconsin and should be seeded with the intent
of filling single season forage needs. Based on Wisconsin studies, there does
seem to be variety differences for annual ryegrass establishment success with
frost seeding. Current recommendations are to select forage-type, late maturing
annual ryegrass varieties. Reference Perennial Forage Variety Update for Wisconsin
(A1525) for additional information and ryegrass variety performance data.
Some grass species establish more rapidly than others. The ryegrasses and orchardgrass
contribute to forage yield during the seeding year. Smooth bromegrass and reed
canarygrass often need a full growing season before plants become productive.
Broadcasting grass seed can pose some unique problems. When mixed with legume
seed, grass seed will not "throw" as far and result in alternating
strips of grass and legume plants. For this reason, it is recommended to seed
grasses separate from legumes (i.e. make two passes) when using a broadcast
seeder. Grass species like smooth bromegrass are often more easily established
into an existing sod using a no-till drill early in the spring instead of a
broadcast seeder. Where grasses are drilled into existing forage stands, mix
light, large-seeded species like smooth bromegrass with oats to prevent bridging
and insure uniform flow through the drill.
Determining seeding rates.......
At equal seeding rates, the number of seeds that establish into productive plants
will be a lower percentage with frost seeding compared to conventional methods.
However, frost seeding pastures almost always involves a resulting forage composition
that will be a mixture since it is being done into some type of established
sod. For this reason, even seeding only one species usually does not require
rates as high as those seeded with conventional methods in a tilled seedbed
where a full stand is desired. Frost seeding rates then become a function of
existing sod condition, species being seeded, and the desired number of seedlings
in the final stand. Optimum seeding rates for specific pasture situations sometimes
need to be determined by trial and error over several years. Based on research
experience, recommended guidelines for seeding into existing forage stands are
presented in Table 3.
Table 3. Recommended Seeding Rates for Frost Seeding into an Existing Grass
or Legume Sod
Rate (lb./acre) Expected Established Plants*
Species SeededAlone As Part ofSeed Mixture Plants per square foot
Red Clover 4 - 8 3 - 4 2 - 5
Birdsfoot Trefoil 4 - 6 2 - 3 6 - 9 (in 2nd year)
Alfalfa 5 - 8 3 - 4 4 - 6
Ladino Clover 2 - 3 1 - 2 1 - 2
Alsike Clover 2 - 4 1 - 2 2 - 3
Perennial/Annual Ryegrass 8- 15** 2 - 3 10 - 12
Orchardgrass 3 - 4 1 - 2 4
Smooth Bromegrass 12 8 - 10 1 - 2
Reed Canarygrass Not recommended for frost seeding
Timothy Not recommended for frost seeding
* Expected plants based on "alone" seeding rates ** Use higher rate
in "bare ground" situations and lower rate in existing sods
4. Seeding Time and Method
The fundamental principle behind frost seeding is that alternating freezing
and thawing, along with spring rains, will help to incorporate the broadcast
seed into the soil surface. Seeding on top of snow is acceptable if the depth
is not too great. The risk of seeding on top of snow is that a rapid meltdown
may result in runoff of both water and seed. In Wisconsin, most frost seeding
is accomplished during March.
Be certain to inoculate legume seeds prior to seeding. Bacterial inoculant is
specific for each legume species. For example, alfalfa inoculant is not effective
on red clover.
There are many excellent tools for making broadcast frost seedings. These include
seeders that mount onto ATV's and tractor 3-point hitch mounted seeders. Conventional
roller and grain drill seeders can also be used but will require more trips
across pastures. When using spinner-type seeders, be sure to determine the effective
seeding width for each seed type or mixture. This will vary between species.
Summary
Frost seeding can be an effective, low-cost method to introduce new forage species
into an existing sod or maintain the current forage composition of pastures.
To be successful, good seed-soil contact needs to be accomplished by grazing
or clipping pastures close to the soil surface in the fall prior to seeding.
Best results are obtained in forage stands of non-sod forming species without
a thatch barrier. After frost seeding, keep competition to a minimum by frequent
grazings or machine harvests. Frost seeding success is usually obtained with
legume species like red clover or birdsfoot trefoil but some grasses such as
perennial or annual ryegrass, orchardgrass, and smooth bromegrass can be successfully
frost seeded with proper management. Base seeding rates on the current condition
of pastures and the desired density of forage species being established.
References
1. George, J. R., 1984. Grass sward improvement by frost-seeding with legumes.
Proceedings of the American Forage and Grassland Conference. p. 265-269.
2. Leep, R. H., 1989. Improving pastures in Michigan by frost seeding. Extension
Bulletin E-2185. 4p.
3. Undersander, D. J. et al., 1997. Perennial forage variety update for Wisconsin.
Extension Bulletin A1525. 29p.
4. West, D. and D. J. Undersander. 1997. Spring frost seeding. Proceedings of
the Wisconsin Forage Production and Use Symposium. Wisconsin Dells. p. 93-95.
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