RENOVATION AND INTERSEEDING OF GRASSES AND LEGUMES
By John Fouts
Sometimes, removing the stand and rotating to another crop is not desirable, because rotation crops may not be as profitable. Overseeding another forage species into a depleted alfalfa stand can extend stand life and improve the yield and marketability of the hay.
Generally,
the difficulty with mixed stands is the
difference in dominance, in performance or in establishment
rate of one species over another. Another
problem arises from differences in fertility
requirements. Grasses need nitrogen. Legumes fix nitrogen from the air in
root nodules. If a grass/legume
mix is fertilized for the grass production,
the result is essentially that the legume
becomes “lazy” and
rather than fixing nitrogen, it uses
nitrogen applied for the grass. This
can result in diminished vigor of both
the grass and the legume.
Overseeding
grasses into alfalfa usually creates
a mixture of hay that has a lower nutritional value
than alfalfa hay alone. This hay is generally not
suitable for lactating dairy cows, but is acceptable
for dry cows and appropriate for horses or other
livestock. Also, grass-alfalfa mixtures, especially
mixtures with cereals, produce higher yields than
legume-alfalfa mixtures.
Allelopathy
is the release of chemicals by certain
plants that inhibit the growth of competing plants. Autotoxicity is a form of allelopathy
that inhibits germination of the same
plant species. Alfalfa has an autotoxic effect on seedling alfalfa. This is why is has generally been recommended
to rotate out of alfalfa to a grain crop
for a couple of years before reseeding alfalfa. Research
has shown, however, that there is little
or no harmful effect of autotoxicity if seedings
are made at least several weeks after plowing or
spraying old alfalfa with glyphosate (Roundup)
to kill it. The
lack of any significant autotoxicity
with delayed seeding indicated that seeding
failures can be reseeded if there is
an interval of at least two weeks after plowing
or tilling, or three weeks after glyphosate application.
Allelopathy doesn’t
seem to be a problem between most grasses
and legumes.
Seedbed preparation is very important for successful establishment of any overseeded crop in alfalfa. Usually, a minimum amount of tillage is required. The biggest problem with interseeding grass or legumes into established stands of the other is the competition for space, nutrients and moisture. Obviously the established crop has the advantage because of its existing root system. To give the newly-seeded forage component a fair chance of establishment, the existing stand must be handicapped in some way. This can be accomplished by a fairly aggressive tilling that does not completely eliminate the established stand. A springtooth harrow or a light disk can do a good job. Timing of seeding is especially important to ensure that there is moisture available for the new seedlings. Seed coverage of ¼ - ½” is important for seed to soil contact and moisture availability to the seedling. This can be accomplished with a drill with depth control or by broadcast seeding followed by a light dragging with a section of chainlink fence. Spike-tooth harrowing will usually bury the seeds too deeply.
There
are circumstances during alfalfa establishment
that may create less-than-optimal stands
just after seeding. Seedling stands with a population
that falls below 10 plants per square foot may
be a good choice for overseeding. When overseeding
is timed so that existing alfalfa plants are still
small, less than six inches (15 cm) tall, and adequate
soil moisture is available for germination, the
success rate is high. Successful planting practices
have included the use of a disc-type grain drill
(or no-till drill) without tillage to place seed ½ to ¾ inch
(12.5 to 19 mm) deep, with minimal disturbance
of existing plants. If broadcast seeding
methods are used, the seed should be pressed into
the soil with a smooth or ring-type roller. Use
of a spring or spike-tooth harrow is not recommended,
since it can remove or damage many young plants
that have less than a six-inch (15cm) root.
Sod
seeding employs application of glyphosate
to burn back the established forage and reduce
competition. This effect can also be achieved by severe
grazing. Planting in this
method is done with a drill. Broadcast
seeding will not provide the desired
seed to soil contact. Overseeding can
also be done using a conventional grain
drill without seedbed preparation, provided
the soil is sufficiently soft to allow
penetration by the drill and covering
of the seed. It is important to overseed
into the stand as soon as possible after
the problem is identified. Overseeding
should occur when the existing plants
are small. Large alfalfa plants create
greater competition and allelopathy with the emerging seedlings.
The
key to preventing the need for renovation
is good pasture management. Controlled
grazing with adequate rest periods for
the pasture, stocking rate, should be one of the
main tools in pasture management. Overgrazing will deplete the vigor of
a plant. More information
on good pasture management can be found
in EB1297,
“Pasture Management Guide for Northeast Washington.”
For
more information, contact WSU Extension, (509) 477-2048.
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