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Grassland productivity across brittle environments

by Jim Howell

These photos demonstrate differences in standing biomass, ground cover, and stock density in highly brittle environments with different levels of biological productivity.

The properties shown are managed under planned grazing. Stock densities are very high relative to conventional management.

  Stock density Growing season Dormant season Soil surface in dormant season
Cold temperate steppe
low production
14" (360 mm) precipitation, western Colorado, U.S.A.
grazing cattle
Cold temperate steppe: stock density. 400 yearlings on 35 acres (14 ha) for 2 days. Once long recovery periods create adequate forage, use high stock densities and short grazing periods to achieve even forage utilization and well-distributed animal impact.
knee-high grass & flowers
Cold temperate steppe: growing season. Steppes tend to get their precipitation spread out over the whole year, so rains rarely build on each other. Biological productivity is much lower than tropical grasslands that get the same precipitation in a four-month summer growing season. On the cold steppe, cool temperatures and a short growing season also limit productivity.
dry knee-high grass
Cold temperate steppe: dormant season. Due to slow growth, recovery periods between grazings must be long -- up to several years in some cases -- to avoid grazing plants before they're fully recovered.
short plants, scant litter
Cold temperate steppe: soil surface in dormant season. Low productivity makes providing adequate litter to protect the soil surface a challenge. Overrest is less of a problem in this type of environment, since accumulating enough standing forage to shade and weaken plants can take years.
Mild temperate steppe
low production
12" (300 mm) precipitation, southern New Mexico, U.S.A.
grazing cattle
Mild temperate steppe: stock density. 400 dry cows on 250 acres (100 ha) for 3 days. As with cold steppe, use long recovery periods followed by short grazing periods at high stock densities to produce even forage utilization and well-distributed animal impact.
short grass, bare spots
Mild temperate steppe: growing season. Erratic and spread-out precipitation limits productivity. Mild temperatures create faster evaporation than on the cold steppe, further limiting usable moisture.
short dry grass, much bare ground
Mild temperate steppe: dormant season. Though biological productivity is low and recovery periods must be long, grazers must eventually return or standing dead growth will shade the plants' growth points.
short plants, scant litter
Mild temperate steppe: soil surface in dormant season. As with the cold steppe, providing enough litter to protect the soil surface can be a challenge.
High rainfall tropical savanna
high production
35" (890 mm) precipitation high veld, Zimbabwe
dense cattle herd
High rainfall tropical savanna: stock density. 300 dry cows on 1/4 acre (1/10 ha) for 30 minutes provide very high animal impact -- essential in these high-productivity areas to get standing growth grazed down or trampled before the next growing season. In my experience it's nearly impossible to impact this sort of country too much.
dense tall green grass
High rainfall tropical savanna: growing season. Abundant rainfall combines with warm temperatures to create terrific growing conditions and very high biological productivity. Plant growth is so fast during the rainy season that it's far more challenging to maintain pasture quality than to prevent overgrazing of plants.
tall dry grass, grazed short at left
High rainfall tropical savanna: dormant season. With such favorable conditions during the growing season, abundant plant material accumulates rapidly, and heavy rains create an algae-covered cap quickly on exposed soil. If those plants and soils aren't disturbed during the following dormant season, they are a major liability when the next growing season arrives. Here the area to left has been grazed at very high density; area to right has not. A one-strand electric fence separates them.
dense litter
High rainfall tropical savanna: soil surface in dormant season. The huge bulk of organic material produced every year guarantees an abundant supply of litter-making material. It takes a lot of animal impact to knock down this much dead growth. If left standing, it would shade the resprouting grasses.

All images copyright Jim Howell

Although all three environments are brittle, differences in biological productivity require different management of the tools of grazing and animal impact.

In low-production environments, plants may require long recovery periods between grazings due to typically slow (in the cold environments) or very erratic (in warmer environments) growing conditions. It is relatively easy to produce widespread overgrazing of plants when animals return too soon. Plants must recover to the point that sufficient material is present to both feed the animals and replenish the litter that protects the soil surface.

With slow or erratic growth, plants and soil surfaces take a long time to begin suffering from excessive rest, or overrest, compared to high-production environments. Because forage quality is usually high, and because mature soil caps are not usually present, very high stock densities are generally less critical to achieving a well-disturbed soil surface and even level of forage utilization. Nonetheless, the bigger and more concentrated the herd, the better for overall ecosystem process functioning

In contrast, plants typically grow so fast in high-production environments that overgrazing is less of a problem. Vast amounts of low quality forage usually ensure an adequate supply of litter, but plants must be eaten or trampled down before the next growing season or they become a major liability. So do undisturbed soil surfaces, which can develop a thick algae cap in just one season. Dealing with these two major challenges requires very high stock density and heavy animal impact every year. See my landscape brittleness and productivity article for more information.

-- Jim Howell
2003

Jim Howell and his wife Daniela ranch outside Montrose, Colorado, U.S.A. They run tours of holistically managed farms and ranches around the world. Jim also writes a regular column for Holistic Management In Practice magazine. Contact him at howelljd@scranchtours.com.


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Posted 12 April 2003
URL: managingwholes.com/photos/howell1/photos/big.htm