Fertility and Nutrients
Apply manure to native rangeland?
By Top Crop Manager
Short-term possibly. Long-term maybe. Alberta Public Lands says no. Here is how to assess the risks and benefits.
There are approximately nine million acres of native grassland in the Brown
and Dark Brown soil zones in Alberta and Saskatchewan. With the expansion of
the intensive livestock industry, interest in applying hog or cattle manure
to native rangeland has also grown. The assumption is often made that rangelands
are low in nutrients and, therefore, grassland production could be increased
with application of manure.
|Injecting liquid hog manure into pasture. Photos Courtesy Of
Ed Bork, University of Alberta.
"If you review the research that has been done for the past 50 years,
some of the trials actually had a nice response to livestock manure, but I think
you have to look at the long-term effects, not the short-term," says Ross
McKenzie, agronomy research scientist with Alberta Agriculture, Food and Rural
Development (AAFRD) at Lethbridge. "Native rangeland is very fragile and
it doesn't take much to put nutrient cycling out of balance. In the long-term,
you would be changing the entire nutrient cycling system. Over time, applying
more nitrogen and phosphorus may produce a shift to invasive species resulting
in a very negative effect to the overall biodiversity of the native range. Native
range has been a sustainable system that has worked for 10,000 years, and I
think we should leave it alone."
Rangelands are a natural ecosystem in which nutrients have reached a steady
state or dynamic equilibrium. Essential nutrients like nitrogen (N), phosphorus
(P), potassium (K) and sulphur (S) are in balance within the soil, vegetation,
animals and climatic effects. Virtually all N in the soil is stored in organic
matter. Nitrogen required for plant growth comes from organic matter breakdown.
Additional sources of N come from fixation by native legume plants and other
soil micro-organisms, and atmospheric additions by lightning storms.
For soil P, about half the P in surface soil on rangeland is contained in the
organic matter, with the remaining P tied up in soil minerals. Much of the P
for plant growth comes from breakdown of organic P, with a small amount released
from soil minerals. Approximately 75 to 95 percent of the nutrients are cycled
back to the soil in urine and feces of grazing livestock.
Native plants are generally low yielding species that are adapted for survival
in semi-arid to sub-humid climates on soils low in available nitrogen. However,
Edward Bork, an associate professor of range science in the Agricultural, Food
and Nutritional Science Department at the University of Alberta in Edmonton,
says research has shown that native species do respond to added manure much
like cultivated grass species, and that they even have a tendency to preferentially
direct nutrient additions into expanded root growth. This enhanced root growth
may be a benefit to the plant in the future by increasing growth and drought
McKenzie agrees that adding nutrients in the form of livestock manure, or inorganic
fertilizer for that matter, can produce a yield response, but repeated applications
in the long-term may also result in a shift in the ecosystem balance, and the
soil nutrient dynamics will no longer be in equilibrium.
For example, McKenzie says that an application of 12t/ac of livestock manure
can result in the doubling of forage yield of a Stipa-Bouteloua grass prairie
and the yield can persist for a number of years. Higher rates of 20t/ac to 30t/ac
increased yields even more, and increased crude protein content in the year
|Pasture response to 20kg/ha N (left) and 160kg/ha.|
Bork also found yield increases when manure was applied to native grassland.
His research was conducted in the southeastern region of Alberta between Hanna
and Drumheller. It found that liquid hog manure applied to both native rangeland
and tame pasture resulted in similar relative yield increases, although absolute
increases in yield tended to be greater on tame pasture. The two native rangeland
sites were representative of moist Rough Fescue Grassland and dry Mixedgrass
Prairie communities. Two tame pastures, alfalfa-meadow bromegrass and a crested
wheatgrass-alfalfa, were also included in the study.
"Both semi-arid tame pastures and native rangelands responded positively
to liquid hog manure application, with improvements in forage quality, primarily
crude protein, as well as forage biomass, for up to two years after application,"
explains Bork. "Overall, we did not see any plant species shifts of concern
in the native communities, except for pasture sage, which demonstrated an increase
in canopy cover the first year after manure application. This increase was short-term
in nature and disappeared by the second growing season."
Bork says the study indicated that forage crops, both tame and native range,
exhibited the ability to effectively utilize added nutrients while having a
substantial capacity to resist change in plant species composition following
a one-time addition of liquid hog manure.
Species shift can occur in mixed communities
While those yield increases may be of benefit in the short-term on relatively
pure stands of native grass, McKenzie cautions that in mixtures of grasses,
forbs and shrubs, 30 percent or more of the fertilizer applied can be used by
unpalatable forbs and shrubs. Essentially, the manure is feeding the weedy,
undesirable species in the field.
On ungrazed rangeland, McKenzie says that manure application reduced the abundance
of ground cover species in favour of a few dominant grasses, especially western
wheatgrass. While western wheatgrass is a productive species under good moisture
conditions, in drought it is less productive and palatable than the other native
species that it replaced.
"Look at the biodiversity of native range. It can respond to the environment.
It can tolerate low nutrient levels or low rainfall. Most of the plant's energy
goes to storing reserves in the roots. That's why native grasses can come back
after droughts, while some of the invasive species will die out," explains
On grazed rangeland, research shows that nutrient additions resulted in an
increase of weedy species. Moderate to heavily grazed rangeland resulted in
the increase of early successional weedy species that responded to the increased
nutrient supply. Though little long-term research has been done on the effects
of manure on native grassland integrity, McKenzie says that considerable anecdotal
evidence suggests that manure applications will cause undesirable species shifts.
"You have to look at both sides. In the short-term, there may be an advantage
in increased forage yield, but in the long-term, you may be changing the ecosystem,"
says McKenzie. "That's why manure application is not permitted on Crown
rangelands in Alberta. Because it is viewed as an undesirable disturbance to
native plant communities."
Bork says that his research, funded by the Canada-Alberta Hog Industry Development
Fund, only had the mandate to examine short-term effects following a one-time
application of hog manure on native and tame grass species. He feels that it
would also be helpful to carry on this type of research over a longer period.
"My approach would be to find out if native grasslands have the potential
to sustain low application rates of liquid manure, around 20kg/ha to 40kg/ha
at intermittent periods such as every four to five years, or maybe even up to
10 years apart. The key question is whether repeated nutrient addition, even
at these conservative levels, eventually leads to the inability of native grasslands
to resist weed entry and spread, thereby threatening their integrity. Our preliminary
work with liquid hog manure indicates it does not," he says.
McKenzie and Bork agree that manure should not be applied to solonetzic or
saline soils. Manure tends to have a moderate level of salt and any addition
to these soils will only make the problem worse. Hog manure is higher than livestock
manure in sodium salts, and application to solonetzic soils may have a large
impact on soil quality. "I would not contemplate applying hog manure on
lands in southern Alberta with solonetzic properties, as these areas have severe
soil restrictions limiting plant growth and, as a result, little benefit to
forage yield is likely to occur with nutrient addition," emphasizes Bork.
Another consideration when looking at the land base for manure disposal is
which crops provide the best forage response to added nutrients. McKenzie says
that tame grass species are very responsive to added nutrients and that most
tame forages have poor fertility. "If looking for a land base to apply
manure, why not put it where you get the most forage response; on tame pastures,"
McKenzie also cautions that broadcast manure, if not incorporated, is a very
inefficient use of N, with more than 50 percent of the N lost to the atmosphere
through volatilization. In his research, Bork found that while disc injecting
liquid hog manure at least partly overcomes that problem, broadcasting of liquid
hog manure would suffer the same fate. As a result, McKenzie normally suggests
that if manure must be broadcast, as in the case of solid livestock manure,
that it be broadcast and immediately incorporated on annual land to minimize
nitrogen losses to the atmosphere, and to get the most value from all the nutrients
in the manure.
Additionally, McKenzie cautions that broadcasting manure on the surface without
incorporation leaves P on the soil surface which greatly increases the risk
of P movement into surface water during a rapid snow melt or intensive rain,
leading to the pollution of nearby surface waters.
AAFRD has produced an Agri-Facts sheet called Manure Application and Nutrient
Balance on Rangeland, which further discusses the issues. It can be found on
AAFRD's Ropin the Web site by conducting a search for the title, or Agdex 538-1.
The Agri-Facts sheet also includes a list of research reports with further information
on the practice.
In the end, the decision to apply manure to native rangeland is up to rangeland
managers, at least on private land. While there are pros and cons, they must
carefully consider the potential long-term consequences of disrupting a rangeland
ecosystem that has existed for 10,000 years.