Role of TimeExtraordinary as it may seem, until this key insight was first articulated, there was no formal definition of overgrazing anywhere in the world. Indeed, it was (and still is) believed by most people to be a function of too many animals on a piece of land.
Overgrazing is about the frequency with which a plant is bitten then re-bitten by a grazing animal. A fully recovered grass plant is in balance—an amount of root mass below the soil surface supports a similar amount of leaf matter above the soil surface.
When the plant is severely defoliated, the plant becomes unbalanced. The little remaining leaf area cannot support the large root system that remains. The plant deals with this imbalance by translocating energy from the roots upwards, to form fresh leaves that are ready to begin photosynthesis, but in the process the energy depleted roots die, and the living mass of roots below ground comes back into balance with the depleted mass of leaves above ground. The more severe the above ground grazing was, the more root system will be destroyed.
It takes time to build a new root system (the old roots are not rebuilt - completely new ones are built). This building occurs on a sigmoid curve (which is a doubling of a doubling of a doubling—1 unit today becomes 2 tomorrow, and 4 the day afterwards). The curve below shows this process.
Holistic grazing planning respects the time needed for a new root and leaf system to be constructed. If the plant is bitten again before new roots are fully built, the entire root system is damaged. If this occurs too frequently the root system becomes weakened, and plant death may ensue.
Furthermore, the rate of recovery is variable, depending on things including but not limited to the severity of the previous graze, the plant species, the season, rainfall, humidity and temperature. For instance, the following chart shows that when growth is very rapid, not only must plants be grazed sooner, but their dry matter yield per hectare or acre is greatest. Conversely slow growth results in the lowest yield per hectare or acre.
The green or ‘Rapid’ curve shows that those plants would be fully recovered in one unit of time, whereas the ‘Moderate’ and ‘Slow’ curves have hardly begun to recover. Were those plants to be bitten after one unit of time, they would be at grave risk of overgrazing. Indeed, the ‘Slow’ growth plants require three units of time for full recovery.
Overgrazing then, can occur under two conditions: when animals remain in a paddock too long during periods of rapid growth, and more damagingly, when animals return to a paddock too soon in periods of slow growth.
The second insight then is that overgrazing occurs to plants when they are bitten a second time before fully recovering from a previous bite. Overgrazing happens only to plants, not to paddocks (a particular paddock cannot be overgrazed, only individual plants within it), and it occurs ‘plant, by plant, by plant’.
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