Grazing
Grazing:
 A form of exploitation where the prey (primary producer) is
not killed.
 Typically involves low vegetation (grasses, herbs, algal
mats), not taller, woody plants (“browsing”).
Global distribution of grazed land:
Grazed grasslands are the largest single component (25%) of the
Earth's 117 million km2 of vegetated lands.
In the last 300 years, managed grazing systems have increased 6fold
by land cover, and they are rising still.
Grazing by biome
1.53 billion (2001) cattle occupy nearly 24
percent of earth’s landmass.
Add sheep & goat: 3.3 billion (2001)
They weight more than the entire human
population.
• Forest converted to pasture or production of cattle feed.
• Desertification of marginal rangelands in semi-arid and arid regions.
• Production of greenhouse gases (CO2, methane, nitrous oxide) 18% of emissions.
• Decreased water quality through runoff from fertilized fields and feed lots.
• Introduction of invasive species.
In the 17 Western States:
1870: 4.1 million beef cattle
4.8 million sheep
1900: 19.6 million beef cattle
25.1 million sheep
1930: great drought
Overgrazing – a global issue
Since 1945, 3.8% of Earth’s vegetated land have been seriously degraded
by overgrazing. The rate of abandonment of dryland due to degradation is
1 million km2 per decade and this rate is probably accelerating.
Overgrazing fundamentally changed the face of western states
18 years without cattle
30 years without cattle
San Pedro River, AZ in 1984
San Pedro River in 1998
Cattle-free for 12 years
Grazing in the Texas Hill Country
1932
2000
Woody Encroachment linked to Grazing:
 Grazing reduces grass biomass, so that seedlings of
woody plants achieve higher growth rates.
 Fire is suppressed on rangelands, so that seedlings and
saplings suffer lower mortality.
 Grazers compact soils and increase erosion by wind and
water, all of which lower grass productivity.
 Grazers often facilitate seed dispersal.
Overgrazing
Shrubs replace
grasses
Increased spatial
heterogeneity
Soil compaction,
loss of soil stability
Wind & Water
erosion
Nutrient loss
Irreversible land degradation.
Mechanics of grazing:

Cow has a specific bite width
given by the size of the mouth

Cow takes off a fixed percentage
of the height of the pasture

Cow takes a certain amount of
time tearing off a bitefull of grass

Cow takes time chewing
proportional to bite size
Co-evolution of grass and grazer
Many large grazers evolved after a global climate change which
replaced forests with grasslands at mid-latitudes
Grazers evolved several specialized organs to deal with low
quality, abrasive food:
high-crowned teeth to increase the ability
to tear apart abrasive grass leaves
Ruminant digestive system
Co-evolution of grass and grazer
Grasses evolved strategies to escape from grazers:
Below-ground storage of carbohydrate
to rapidly regrow after grazing
Altered morphology under heavy
grazing: highly branched and low
canopy
Red Queen Hypothesis
(the evolutionary arms race between consumer and consumed)
"It takes all the running you can do,
to keep in the same place."
The Red Queen said in Lewis Carroll's Through the
Looking-Glass.
Intake per bite
Intake as a function of pasture height:
Cattle take about 50% of the
standing biomass within the
bite area in one bite.
Grass height (“prey density”)
But there is an
ungrazeable
horizon.
Handling time
Handling time as a function of bite size:
Taking a bite takes
the same amount of
time, independent of
bite size.
Chewing time is proportional
to biomass per bite.
Bite size
Functional response curve for grazing:
Daily rate of consumption
per animal
Reflects limits of grazing time (ca. 13 hours per day) and
digestive capacity (ca. 8 kg dry matter)
Grazing limited by digestive
capacity
Grazing limited by time
Grass height
Non-grazeable horizon
this is the effect of
the ungrazeable horizon
Predator isocline:
Predator density
Isoclines for free-breeding grazers
Victim density
Predator isocline:
Predator density
Free-breeding grazers
Victim density
Predator isocline:
Predator density
Free-breeding grazers: the grass “refuge” stabilizes
Victim density
Taurine cattle (without hump)
were domesticated twice:
In the near-east, giving rise to all
European breeds (humpless
cattle)
In India (zebu, humped cattle)
Zebu cattle probably entered
Africa from India.
Old-world grasses co-evolved with domesticated cattle for about 70008000 years.
New-world grasses did not. This may explain the vulnerability of
American prairies to domestic cattle.
Alien plant origins:
Bromus spec
Botriochloa
Salsola spec
Timothy
Wild oat
Lehman lovegrass
Buffelgrass
Bermudagrass
Europe (near-east)
Europe (near-east)
Eurasia
Europe
Europe
South Africa
(India and Africa)
(Africa)
Grass height
Increasing stocking rates
Total daily rate of
grass consumption
How about captive animals with fixed stock density?
Grass growth or consumption rate
grass growth function
one cow
per acre
Grass height
Stable grass height
for grazing at 1
animal per acre.
Grass growth or consumption rate
two cows
per acre
Grass height
Grass growth or consumption rate
four cows
per acre
Grass height
Grass growth or consumption rate
stable
equilibrium
state
unstable
equilibrium state
five cows
per acre
Grass height
One and the same stocking rate can lead to two stable states:
the desired state, where the sward is fast-growing and the
cows eat all they can every day,

the “overgrazed” state, where the sward is growing slowly and
the cows remain hungry.
Grass growth or consumption rate

1
five cows
per acre
2
Grass height
Grass growth or consumption rate
Less productive years can easily set the system into the
overgrazed state:
Theoretically
maximal yield:
not stable!
drier year
Grass height
Grass growth or consumption rate
Recovery from overgrazing is achieved only by drastically
reducing stock density:
Grass height
Grass growth or consumption rate
A safe stocking density is well below the maximal
sustainable yield for average conditions:
Grass height
Summary so far:
 In natural grazing systems, grasses and grazers have co-evolved to
sustain one another. Example: the ungrazeable horizon prevents
grasses from being eaten to extinction.
 Grazers are controlled by the state of the grassland and vice versa.
For example, grazer numbers decline when there is not enough
forage, either by death or migration. Grazers disappear before they
irreversibly damage the grassland.
 In managed grazing systems, grazer numbers (stocking rates) are
controlled by the rancher, making it possible for grass and soil
system to be damaged beyond repair.
 Things that are harmful:


Supplementary feeding during drought. This maintains an
unrealistically large herd.
Restocking too soon after drought. This subtracts time for grasses to
repair and recover.
Summary so far:
 Theory suggests that harvesting a resource at the maximal rate
maximizes the risk of over-exploitation.
 The pre-cautionary principle therefore suggest to set stocking rate
far below the value that would maximize production in an average
year.
 However, economic forces tend to work in the opposite direction.
The Tragedy of the Commons
Multiple individuals, acting independently and rationally in
their own self-interest, will ultimately deplete a shared
limited resource, even when it is not in anyone's long-term
interest for this to happen. – Hardin 1968.
Rancher 1
Rancher 2
Rancher 3
Each rancher, 10
cows
Sell 10 calves of
100 kg for $1000
Sell 10 calves of
100 kg for $1000
Sell 10 calves of
100 kg for $1000
Rancher 1 decides
to add one cow
Sell 11 calves of 99
kg for
$ 1089
Sell 10 calves of 99
kg for
$ 990
Sell 10 calves of
99 kg for
$ 990
Ranchers 2 and 3
follow the lead.
Sell 11 calves of 97
kg for
$ 1067
Sell 11 calves of 97
kg for
$ 1067
Sell 11 calves of 97
kg for
$ 1067
Some time later:
All ranchers have
20 cows.
Sell 20 calves of
40 kg for
$ 800
Sell 20 calves of
40 kg for
$ 800
Sell 20 calves of
40 kg for
$ 800
The tragedy is the result of privatizing profits,
while sharing the costs.
Summary:
 Co-evolution of grass and grazer produces stable grazing systems.
 Lack of co-evolution often causes species extinctions and
reorganization of biodiversity.
 Overstocking causes irreversible loss of species diversity, soil quality
and quantity. Unfortunately, there are economic incentives for
overstocking.
 This is what happened when cattle, sheep, goat were introduced to
the Americas, Australia and the Pacific Islands.
 Cattle production is still on the rise globally, at the cost of forest
conversion to species-poor grasslands, the spread of exotic,
invasive species (which often did co-evolve with cattle), methane
production (a greenhouse gas), irreversible land degradation.