Animal Manures
Animal Manures
for Increasing Organic Matter and Supplying
Nutrients
Once cheap fertilizers
became widely available after World War II, many farmers, extension agents, and
scientists looked down their noses at manure. People thought more about how to
get rid of manure than how to put it to good use. In fact, some scientists
tried to find out the absolute maximum amount of manure that could be applied
to an acre without reducing crop yields. Some farmers who didn’t want to spread
manure actually piled it next to a stream and hoped that next spring’s floodwaters would wash it away. We now know that manure, like money, is better
spread around than concentrated in a few places. The economic contribution of
farm manures can be considerable. On a national basis, the manure from 100
million cattle, 60 million hogs, and 9 billion chickens contains about 23
million tons of nitrogen. At a value of 50 cents per pound, that works out to
a value of about $25 billion for just the N contained in animal manures. The value
of the nutrients in manure from a 100-cow dairy farm may exceed $20,000 per
year; manure from a 100-sow
farrow-to-finish operation is worth about $16,000; and manure from a
20,000-bird broiler operation is worth about $6,000. The other benefits to soil
organic matter buildup, such as enhanced soil structure and better diversity
and activity of soil organisms, may double the value of the manure. If you’re
not getting the full fertility benefit from manures on your farm, you may be
wasting money.
Animal manures can have very
different properties, depending on the animal species, feed, bedding,
handling, and manure-storage practices. The amounts of nutrients in the manure
that become available to crops also depend on what time of year the manure is
applied and how quickly it is worked into the soil. In addition, the influence
of manure on soil organic matter and plant growth is influenced by soil type.
In other words, it’s impossible to give blanket manure application
recommendations. They need to be tailored to every situation.
We’ll
start the discussion with dairy cow manure but
will also offer information
about the handling, characteristics, and uses of some other animal manures.
MANURE HANDLING SYSTEMS Solid versus Liquid
The type of barn on the farmstead frequently
determines how manure is handled on a dairy farm. Dairy-cow manure containing a
fair amount of bedding, usually around 20% dry matter or higher, is spread as a
solid. This is most common on farms where cows are kept
in individual stanchions or
tie-stalls. Liquid manure-handling systems are common where animals are kept in
a “free stall” barn and minimal bedding is added to the manure. Liquid manure
is usually in the range of from 2% to 12% dry matter (88% or more water), with
the lower dry matter if water is flushed from alleys and passed through a
liquid-solid separator or large amounts of runoff enter the storage lagoon.
Manures with characteristics between solid and liquid, with dry matter between
12% and 20%, are usually referred to as semisolid.
Composting
manures is becoming an increasingly popular option for farmers. By composting
manure, you help stabilize nutrients (although considerable ammonium is usually
lost in the process), have a smaller amount of material to spread, and have a
more pleasant material to spread—a big plus if neighbors have complained about
manure odors. Although it’s easier to compost manure that has been handled as a
solid, it does take a lot of bedding to get fresh manure to a
20% solid level. Some
farmers are separating the solids from liquid manure and then irrigating with
the liquid and composting the solids. Some are separating solids following
digestion for methane production and burning the gas to produce electricity or
heat. Separating the liquid allows for direct composting of the solids without
any added materials. It also allows for easier transport of the solid portion of
the manure for sale or to apply to remote fields. For a more detailed
discussion of
composting, see chapter 13.
Some dairy farmers have
built what are called “compost barns.” No, the barns don’t compost, but they
are set up similar to a free-stall barn, where bed-ding and manure just build
up over the winter and the pack is cleaned out in the fall or spring. However,
with composting barns, the manure is stirred or turned twice daily with a
modified cultivator on a skid steer loader or small tractor to a depth of 8 to
10 inches; sometimes ceiling fans are used to help aerate and dry the pack
during each milking. Some farmers add a little new bedding each day, some do
it weekly, and others do it every two to five weeks. In the spring and fall, some nearly all of the bedding can be removed and spread directly or built into a
traditional compost pile for finishing. Although farm-ers using this system
tend to be satisfied with it, there is a concern about the continued
availability of wood shavings and sawdust for bedding. More recently,
vermicomposting has been introduced as a way to process dairy manure. In this
case, worms digest the manure, and the castings provide a high-quality soil
amendment.
Manure from hogs can also be
handled in different ways. Farmers raising hogs on a relatively small scale
sometimes use hoop houses, frequently placed in fields, with bedding on the
floor. The manure mixed with bedding can be spread as a solid manure or
composted first. The larger, more industrial-scale farmers mainly use little to
no bedding with slatted floors over the manure pit and keep the animals clean
by frequently washing the floors. The liquid manure is held in ponds for
spreading, mostly in the spring before crops are planted and in the fall after
crops have been harvested. Poultry manure is handled with bedding (especially
for broiler production) or little to no bedding (industrial-scale egg
production).
Storage of
Manure
Researchers have been
investigating how best to handle, store, and treat manure to reduce the
problems that come with year-round manure spreading. Storage
allows the farmer the opportunity to apply
manure when it’s best for the crop and during appropriate weather conditions.
This reduces nutrient loss from the manure, caused by water runoff from the
field. However, significant losses of nutrients from stored manure also may
occur. One study found that during the year dairy manure stored in uncovered
piles lost 3% of the solids, 10% of the nitrogen, 3% of the phosphorus, and 20%
of the potassium. Covered piles or well-contained bottom-loading liquid
systems, which tend to form a crust on the surface, do a better job of
conserving the nutrients and solids than unprotected piles. Poultry manure,
with its high amount of ammonium, may lose 50% of its nitrogen during storage
as ammonia gas volatilizes unless precautions are taken to conserve nitrogen.
Regardless of the storage method, it is important to understand how potential
losses occur in order to select a storage method and location that minimize environmental
impact.
CHEMICAL
CHARACTERISTICS OF MANURES
A high percentage of the nutrients in feeds
passes right through animals and ends up in their manure. Depending on the
ration and animal type, over 70% of the nitrogen, 60% of the phosphorus, and
80% of the potassium fed may pass through the animal as manure. These nutrients
are available for recycling on cropland. In addition to the nitrogen,
phosphorus, and potassium contributions given in table 12.1, manures contain
significant amounts of other nutrients, such as calcium, magnesium, and sulfur.
For example, in regions that tend to lack the micronutrient zinc, there is
rarely any crop deficiency found on soils receiving regular manure
applications.
The values given
in table 12.1 must be viewed with some caution, because the characteristics of
manures from even the same type of animal may vary considerably from one farm
to another. Differences in feeds, mineral supplements, bedding materials, and
storage
systems make manure analyses
quite variable. Yet as long as feeding, bedding, and storage practices remain
relatively stable on a given farm, manure nutrient characteristics will tend to
be similar from year to year. However, year-to-year differences in rainfall can
affect stored manure through more or less dilution.
The major difference among
all the manure is that poultry manure is significantly higher in nitrogen and
phosphorus than the other manure types. This is partly due to the difference in
feeds given poultry versus other farm animals. The relatively high percentage
of dry matter in poultry manure is also partly responsible for the higher
analyses of certain nutrients when expressed on a wet ton basis.
It
is possible to take the guesswork out of estimating manure characteristics;
most soil-testing laboratories will also analyze manure. Manure analysis
should become a routine part of the soil fertility management program on
animal-based farms. This is of critical
importance for
routine manure use. For example, while the average liquid dairy manure is
around 25 pounds of N per 1,000 gallons, there are manures that might be
10 pounds N or less OR 40
pounds N or more per 1,000 gallons. Recent research efforts have focused on
more efficient use of nutrients in dairy cows, and N and P intake can often be
reduced by up to 25% without losses in productivity. This helps reduce nutrient
surpluses on farms using only needed P.
EFFECTS OF MANURING ON SOILS Effects on Organic Matter
When considering the influence of any residue
or organic material on soil organic matter, the key question is how much solids
are returned to the soil. Equal amounts of different types of manures will have
different effects on soil organic matter levels. Dairy and beef manures
contain undigested parts of forages and may have significant quantities of
bedding. They, therefore, have a high amount of complex substances, such as
lignin, that do not decompose readily in soils. Using this type of manure
results in a much greater long-term influence on soil organic matter than does
a poultry or swine manure without bedding. More solids are commonly applied to
soil with solid-manure-handling systems than with liquid systems because
greater amounts of bedding are usually included. A number of trends in dairy
farming mean that manures may have less organic material than in the past. One
is the use of sand as bedding material in free-stall barns, much of which is
recovered and reused. The other is the separation of solids and liquids with
the sale of solids or the use of digested solids as bedding. Under both
situations, much less organic solids are returned to fields. On the other hand,
the bedded pack (or compost barn) does produce manure that is high in organic
solid content.
When
conventional tillage is used to grow a crop such as corn silage, whose entire
aboveground portion is harvested, research indicates that an annual
application of 20 to 30 tons of the solid type of dairy manure
per acre is needed to
maintain soil organic matter. As discussed above, a
nitrogen-demanding crop, such as corn, may be able to use all of the nitrogen
in 20 to 30 tons of manure. If more residues are returned to the soil by just
harvesting grain, lower rates of manure application will be sufficient to
maintain or build up soil organic matter.
An example of how a manure
addition might balance annual loss is given in figure 12.1. One Holstein “cow
year” worth of manure is about 20 tons. Although 20 tons of anything is a lot,
when considering dairy manure, it translates into a much smaller amount of
solids. If the approximately 5,200 pounds of solid material in the 20 tons is
applied over the surface of one acre and mixed with the 2 million pounds of
soil present to a 6-inch depth, it would raise the soil organic matter by about
0.3%. However, much of the manure will decompose during the year, so the net
effect on soil organic matter will be even less. Let’s assume that 75% of the
solid matter decomposes during the first year, and the carbon ends up as
atmospheric CO2. At the beginning of the
following year, only 25% of the original 5,200 pounds, or 1,300 pounds of
organic matter, is added to the soil. The net effect is an increase in soil
organic matter of 0.065% (the calculation is [1,300/2,000,000] x 100). Although
this does not seem like much added organic matter, if soil had 2.17% organic
matter and 3% of that was decomposed annually during cropping, the loss would
be 0.065% per year, and the manure addition would just balance that loss.
Manures with lower amounts of bedding, although helping maintain organic matter
and adding to the active (“dead”) portion, will not have as great an effect as
manures contain-ing a lot of bedding material.
USING MANURES
Manures, like other organic residues that
decompose easily and rapidly release nutrients, are usually applied to soils in
quantities judged to supply sufficient nitrogen
for the crop being grown in
the current year. It might be better for building and maintaining soil organic
matter to apply manure at higher rates, but doing so may cause undesirable
nitrate accumulation in leafy crops and excess nitrate leaching to groundwater.
High nitrate levels in leafy vegetable crops are undesirable in terms of human
health and the leaves of many plants with high N seem more attractive to
insects. In addition, salt damage to crop plants can occur from high manure
crops, manures should be incorporated into the soil in the spring immedi-
ately after spreading on the surface. About half of the total nitrogen in dairy manure comes from the urea in urine that quickly converts to ammonium (NH4+). This ammonium represents almost all of the readily avail-able nitrogen present in dairy manure. As materials containing urea or ammonium dry on the soil surface, the ammonium is converted to ammonia gas (NH3) and lost to the atmosphere. If dairy manure stays on the soil surface, about 25% of the nitrogen is lost after one day, and 45% is lost after four days—but that 45% of the total represents around 70% of the readily available nitrogen. This problem is significantly lessened if about half an inch of rainfall occurs shortly after manure application, leaching ammonium from the manure into the soil. Leaving manure on the soil surface is also a problem, because runoff waters may carry significant amounts of nutrients from the field. When this happens, crops don’t benefit as much from the manure application, and surface waters become polluted. Some liquid manures— those with low solids content—penetrate the soil more deeply. When applied at normal rates, these manures will not be as prone to lose ammonia by surface drying. However, in humid regions, much of the ammonia-N from manure may be lost if it is incorporated in the fall when no crops are growing.
crops, manures should be incorporated into the soil in the spring immedi-
ately after spreading on the surface. About half of the total nitrogen in dairy manure comes from the urea in urine that quickly converts to ammonium (NH4+). This ammonium represents almost all of the readily avail-able nitrogen present in dairy manure. As materials containing urea or ammonium dry on the soil surface, the ammonium is converted to ammonia gas (NH3) and lost to the atmosphere. If dairy manure stays on the soil surface, about 25% of the nitrogen is lost after one day, and 45% is lost after four days—but that 45% of the total represents around 70% of the readily available nitrogen. This problem is significantly lessened if about half an inch of rainfall occurs shortly after manure application, leaching ammonium from the manure into the soil. Leaving manure on the soil surface is also a problem, because runoff waters may carry significant amounts of nutrients from the field. When this happens, crops don’t benefit as much from the manure application, and surface waters become polluted. Some liquid manures— those with low solids content—penetrate the soil more deeply. When applied at normal rates, these manures will not be as prone to lose ammonia by surface drying. However, in humid regions, much of the ammonia-N from manure may be lost if it is incorporated in the fall when no crops are growing.
fertility. The availability of phosphorus and
potassium in manures should be similar to that in commercial fertilizers.
(However, some recommendation systems assume that only around 50% of the
phosphorus and 90% of the potassium is available.) The phosphorus and potassium
contributions contained in 20 tons of dairy manure are approximately equivalent
to about 30 to 50 pounds of phosphate and 180 to 200 pounds of potash from
fertilizers. The sulfur content as well as trace elements in manure, such as
the zinc previously mentioned, also add to the fertility value of this
resource.
Because one-half
of the nitrogen and almost all of the phosphorus is in the solids, a higher
proportion of these nutrients remain in sediments at the bottom when a liquid
system is emptied without properly agitating the manure. Uniform agitation is
recommended if the goal is to apply similar levels of solids and nutrients
across target fields. A manure system that allows significant amounts of
surface water penetration and then drain-age, such as a manure stack of
well-bedded dairy or beef cow manure, may lose a lot of potassium because it is
so soluble. The 20% leaching loss of potassium from stacked dairy manure
mentioned above occurred because potassium was mostly found in the liquid
portion of the manure.
Timing of
Applications
Manures are best applied to
annual crops, such as corn, small grains, and vegetables, in one dose just
before soil tillage (unless a high amount of bedding is used, which might tie
up nitrogen for a while—see the discussion of C: N ratios in chapter 9). This
allows for rapid incorporation by plow, chisel, harrow, disk, or aerator. Even
with reduced tillage systems, application close to planting time is best,
because the possibility of loss by runoff and erosion is reduced. It also is
possible to inject liquid manures either just before the growing season starts
or as a side-dressing to row crops. Fall manure applications on annual row
crops, such as corn, may result in
considerable nitrogen loss,
even if manure is incorporated. Losses of nitrogen from fall-applied manure in
humid climates may be as much as 25% to 50%—result-ing from conversion of
ammonium to nitrate and then leaching and denitrification before nitrogen is
available to next year’s crop. It was determined in modeling studies that fall
applications of liquid manure posed the greatest risk for nitrate leaching in a
dairy system in New York.
Without any
added nitrogen, perennial grass hay crops are constantly nitrogen deficient.
Application of a moderate rate of manure—about 50–75 pounds worth of available
nitrogen—in early spring and following each harvest is the best way to apply
manure. Spring applications may be at higher rates, but wet soils in early
spring may not allow manure applications without causing significant compaction.
Although
the best use of manure is to apply it near the time when the crop needs the
nutrients, sometimes time and labor management or insufficient storage
capacity causes farmers to apply it at other
times. In the fall, manure can be applied to grasslands that don’t flood or to
tilled fields that will either be fall-plowed or planted to a winter cover
crop. Although legal in most states, it is not a good practice to apply manures
when the ground is frozen or covered with snow. The nutrient losses that can
occur with runoff from winter-applied manure are both an economic loss to the
farm and environmental concern. Ideally, winter surface applications of
manure would be done only on an emergency basis. However, research on frost
tillage has shown that there are windows of opportunity for incorporating and
injecting winter-applied manure during periods when the soil has a shallow
frozen layer, 2 to 4 inches thick (see chapter 16). Farmers in cold climates
may use those time periods to inject manure during the winter (figure 12.2),
although the windows of opportunity may be limited.
POTENTIAL
PROBLEMS
As we all know, too much of
a good thing is not necessarily good. Excessive manure applications may cause
plant-growth problems. It is especially important not to apply excess poultry
manure because the high soluble-salt content can harm plants.
Seedling growth is sometimes
retarded when high rates of fresh manure are applied to the soil immediately before
planting. This problem usually doesn’t occur if the fresh manure decomposes for
a few weeks in the soil and can be avoided by using a solid manure that has
been stored for a year or more. Injection of liquid manure sometimes causes
problems when used on poorly drained soils in wet years. The extra water
applied and the extra use of oxygen by microorganisms may mean less aeration
for plant roots, and loss of readily plant-available nitrate by denitrification
may also be occurring.
When manures are
applied regularly to a field to provide enough nitrogen for a crop like corn,
phosphorus and potassium may build up to levels way in excess of crop needs
(see table 12.2). When ammonium is properly conserved, the manure rate
necessary to meet crop nitrogen requirement is substantially reduced.
Correspondingly, phosphorus and potassium applications are moderated, reducing
or eliminating the accumulation of these nutrients in the soil.
When manure is
applied based upon needed or allowed P additions, as required by some nutrient
management plans, N-conserving management means that less fertilizer N will be
needed. Erosion of phosphorus-
rich topsoils contribute
sediments and phosphorus to streams and lakes, polluting surface waters. When
very high phosphorus buildup occurs from the continual application of manure at
rates to satisfy crop nitrogen needs, it may be wise to switch the application
to other fields or to use strict soil conservation practices to trap sediments
before they enter a stream. Including rotating crops, such as alfalfa—that do
not need manure for N—allows a “draw-down” of phosphorus that accumulates from
manure application to grains. (However, this may mean finding another location
to apply manure. For a more detailed discussion of nitrogen and phosphorus
management, see chapter 19.)
Farmers that
purchase much of their animal feed may have too much manure to safely use all
the nutrients on their own land. Although they don’t usually realize it, they
are importing large quantities of nutrients in the feed that remain on the
farm as manures. If they apply all these nutrients on a small area of land,
nitrogen and phosphorus pollution of groundwater and surface water is much more
likely. It is a good idea to make arrangements with neighbors for use of the
excess manure. Another option, if local outlets are available, is to compost
the manure (see chapter 13) and sell the product to vegetable farmers, garden
centers, landscapers, and directly to home gardeners.
Poultry and hogs
are routinely fed metals such as copper and arsenic that appear to stimulate
animal growth. However, most of the metals end up in the manure. In addition,
dairy farmers using liquid manure systems commonly dump the used copper sulfate
solutions that animals walkthrough to protect foot health into the manure
pit. The copper content of average liquid dairy manures in Vermont increased
about fivefold between 1992 and the early 2000s—from about 60 to over 300 ppm
on a dry matter basis—as more farmers used copper sulfate footbaths for their
animals and disposed of the waste in the liquid manure. Although there are few
reports of metal toxicity to either plants or
animals from the use of animal manures, if
large quantities of high-metal-content manure are applied over the years, soil
testing should be used to track the buildup.
Another potential
issue is the finding that plants can take up antibiotics from manure applied to
soil. About 70% of the antibiotics used in animal agriculture ends up in the
manure. Although the amounts of antibiotics taken up by plants are small, this
is an issue that may be of concern when using manures from concentrated ani-mal
production facilities that use considerable amounts of these substances
