Summer
Annual Forage Grasses for Emergency Crops
Richard
Leep
Department
of Crop and Soil Sciences
Michigan
State University
Summer
annual grasses are used for summer pasture,
green chop, hay, and silage.
Annual grasses are normally used as emergency
forage. The
most common annual grasses used in Michigan
are sudangrass, hybrid sudangrass, sorghum-sudangrass
hybrids, and forage sorghum.
Desirable
characteristics such as rapid growth, excellent
drought resistance, and good response to fertilizer
and water, make summer annual grasses attractive
to use in an overall management scheme for forage
production.
Sorghum-sudangrass
hybrids produce about the same amount of feed
as sudangrass when used for pasture.
When used for green chopped forage, yields
of sorghum-sudangrass hybrids usually exceed
sudangrass or forage sorghum.
Forage sorghums are best suited for silage. Making sorghum-sudangrass into hay is difficult because of the slow
drying time.
Sudangrass
True
sudangrasses have fine stems, tiller extensively
when conditions permit, and can regrow rapidly.
Thus, they are more suited to pasturing
than other types of sorghum.
Piper is a variety which has been in
existence for years.
Piper has low prussic acid content and
is generally regarded as safe to graze.
Hybrid
Sudangrass
Hybrid
sudangrasses result from a cross among true
sudangrass strains that are available primarily
as commercial varieties. They are similar to true sudangrass varieties,
but yield slightly more in a three-cut green
chop or hay system.
Their prussic acid content is generally
between that of Piper sudangrass and sorghum-sudangrass
hybrids.
Sorghum-Sudangrass
Hybrids
Sorghum-sudangrass
hybrids are the most numerous of the various
types of summer annual grasses.
Most of these are available as commercial
hybrids. They are high producing forage grasses, but
more than 50 percent of their yield usually
comes from their stems. Their rate of regrowth after repeated clippings
or grazing is lower than that of sudangrass. Thus, sorghum-sudangrass hybrids sometimes
gain or milk less than those consuming other
summer annuals, apparently due to a lower energy
content. When
these hybrids are cut at immature stages, quality
is higher but yields are much lower.
Forage
Sorghum
Forage
sorghums are usually tall growing, and mature
late in the growing season.
Often called “sweet sorghum”, forage
sorghums often have sweet and juicy stems, and
many have relatively small grain heads.
Forage
sorghums usually yield more silage dry matter
per acre than corn without irrigation.
However, yields of TDN per acre are usually
lower from forage sorghums than from corn.
Grazing
forage sorghums is not recommended. They usually contain much higher levels of
prussic acid than other summer annual grasses
and can be dangerous to graze even when plants
are completely headed, especially when young
shoots are present.
Forage sorghums can be cut for hay, although
their stems dry very slowly after cutting.
Utilization of Summer Annuals
Summer
Pasture
sudangrass
and sorghum-sudangrass can provide supplemental
summer pasture when cool-season grasses go dormant
and the feed supply is short.
Sudangrass
and pearl millet produce better pasture than
sorghum-sudangrass because they are usually
leafier. They also provide a more uniform supply of feed for grazing and support
higher daily gains or milk production.
Sorghum-sudangrasses produce higher yields,
but are better used to support livestock on
maintenance or lower productivity levels.
Graze
the summer annual grasses in a short, rotational
grazing system.
Subdivide fields into three or more pastures
so that each pasture can be grazed down in 7-10
days. Stagger
the date of planting each pasture by about 10
days so that grazing will begin on each pasture
when growth is at the appropriate height.
This rotation system allows maximum production
of quality forage.
Graze
sudangrass when it reaches 15-20 inches in height
and sorghum-sudangrass hybrids when they are
18-24 inches tall. Danger from prussic acid poisoning will be
low when grazing is delayed until grass is this
tall. Graze down rapidly to 6 inches of stubble before moving livestock
to a fresh pasture, and do not graze regrowth
until 18 inches of growth accumulates.
If growth is more than 36 inches tall,
harvest as hay, green chop, or silage since
grazing cattle will trample and waste much of
the forage.
Regrowth will be more rapid following
cutting this taller growth than if it is trampled.
Summer
grazing lasts about two months. During this time each acre of these pasture
can provide feed for one to six mature dairy
or beef animals. Grazing management and soil fertility and moisture
will determine total production.
Sudangrass,
sorghum-sudangrass hybrids, and forage sorghum
pastures are not recommended for horses because
kidney ailments may develop.
Green
Chop
Sorghum-sudangrasses
are well suited to a green chop program.
Under a 3-4 cut system, the forages produce
higher yields than other summer annual grasses. Field losses are less from green chopping than
from grazing or haying.
However, the fast rate of growth of sorghum-sudangrass
results in variable amounts and quality of feed
throughout the growing season.
When grass is young and growing rapidly
it may contain 20 percent crude protein and
produce a highly succulent feed.
As the crop grows taller and nears maturity
the protein content may drop to 7 percent or
less, and a course, fibrous, low quality green
chop is produced.
Nitrates
can become a problem in a green chop program
under certain growing conditions.
Do not feed green chop that has heated
in the wagon, feed bunk, or stack, or that has
been held overnight.
Nitrates are converted to nitrites as
plants respire; nitrites are about 10 times
more toxic than nitrates.
Hay
For
good quality hay, harvest sudans and sorghums
before heads emerge or when they are 30-40 inches
tall. These hays will contain slightly less protein than alfalfa hay and
as much energy as good quality alfalfa hay. Use of a conditioner will aid in field drying. Field drying will usually take several days
to dry to satisfactory levels.
Silage
Forage
sorghums for silage usually have about 75 percent
of the energy value of corn silage per unit
of dry matter, while other summer annual grasses
have 60-75 percent of the value of corn silage. Most summer annuals need to be wilted or
mixed with dry feeds to make satisfactory silage. Silage is often cut after frost to reduce moisture, especially with
forage sorghums.
Seeding
Seedbed
Preparation
A
firm, well-prepared seed bed is needed for good
seed-soil contact and rapid germination.
Conventional , Minimum , or direct drilling
can be used for establishment.
Date
of Seeding
Sudangrass
and sorghum are warm-season grasses. Seed should be planted into soils when average
soil temperature is above 60 degrees F. Plan the seeding date to produce desirable feed when needed. Stagger planting dates to aid rotational grazing.
It takes at least six weeks after planting
before usable forage is available. Later plantings will result in lower yields due to summer droughts
and fall frosts.
Planting
rates
Recommended
planting rates depend on row spacing. Broadcast and narrow-row spacing are preferred
for sudangrass and sorghum-sudangrass hybrids
because they result in shorter plants with finer
stems. Total
forage yield will be similar for different row
spacing because sorghums and sudangrasses tiller.
Removing the primary growing point at
the first cutting enhances tillering. First-cut yields are usually higher for broadcast
or narrow-row seedings than for 20 - 40 inch
rows.
Planting
Depth
Seed
to a depth of 1-2 inches, depending on soil
moisture conditions.
Seeds planted too deep do not emerge
well and poor stands may result.
Fertilization
Annual
grasses have fertilizer requirements similar
to those of corn.
With rapid growth, apply sufficient nitrogen
at planting to ensure establishment and high
first-cutting or grazing yields.
Apply 40-80 pounds of nitrogen per acre
at planting and an additional 50 pounds after
the first cutting or grazing.
Phosphorus and Potassium should be applied
based upon soil test recommendations.
Prussic
acid poisoning
Cellular
damage to sorghums and sudangrasses from frost,
wilting, bruising, drought, excessive soil nitrogen,
or deficiencies in soil phosphorus or potassium
can result in prussic acid poisoning in cattle. Prussic acid poisoning consists of the following
sequence of events:
plant cells rupture and cyanic acid (HCN)
forms from cyanogenic glucosides; cattle consume forage with elevated HCN levels;
HCN is absorbed from the rumen; HCN binds to hemoglobin; asphyxiation and death occur. Poisoning is most likely after a frost when
animals consume the leafy regrowth.
Regardless of season, plants less than
18-24 inches tall should not be grazed.
Suspect forage should be harvested as
dry hay or silage.
Both harvest methods tend to reduce hydrocyanic
acid levels.
Nitrate
poisoning
High
dietary nitrate levels can overload the animal’s
ability to detoxify this chemical and can result
in death due to asphyxiation.
In the rumen, nitrate is reduced to ammonia,
which is absorbed into the bloodstream or converted
into microbial protein.
High dietary nitrate levels that overload
this microbial reduction system cause an accumulation
of nitrite in the rumen.
This nitrite is then absorbed into the
bloodstream where it binds to hemoglobin in
place of oxygen.
This deprives the tissues of oxygen and
causes abortions and asphyxiation.
Sorghums
and sudangrasses can accumulate high levels
of nitrate during environmental conditions that
decrease plant growth rate, including water
stress, lack of sunshine and high nitrogen fertilization. Plants usually absorb nitrogen as nitrates
and synthesize protein.
However, during stress, the synthesis
rates decrease and nitrates accumulate. Cattle should not be fed forages with nitrate
levels greater than 2 percent.
Nitrate analysis can be obtained from
numerous commercial laboratories.
Seed
Availability
Several
commercial suppliers of seed carry varieties
of sorghums, sudangrasses, hybrid-sudangrasses,
and sorghum-sudangrass hybrids.
Check with your local supplier for availability
and variety characteristics.
Michigan State University does not routinely
test varieties of annual grasses and therefore
does not provide variety recommendations.
