Previously in our on-going series on the science of plant growth and grazing we talked about timing grazing and haying based on the secrets of photosynthesis, and how the height of the sun changes how tall our pasture canopy should be in order to capture the most sunlight possible. Now we’re going to take those two parts and turn it into something you can use to help you plan your rotational grazing system.
Here’s what we know so far: The green leaves of the forb or grass are the solar collectors gathering energy for the plant. When leaves are completely removed from a plant, new growth is dependent entirely on stored energy reserves. When some leaf area is left, plant regrowth is often faster since it is powered by photosynthesis and reserve carbohydrates. What this means to us as graziers is that the shorter we graze our pastures, the longer it will take them to recover.
Here’s an example from a year in a pasture in West Virginia on a mixed grass-clover pasture. The pasture was allowed to grow to 3000 lbs/acre of dry matter (DM) forage mass. Then it was grazed to 1500 pounds. From the graph you can see that it took about 30 days for the grass to rebound from 1500 pounds to 3000 pounds DM. Of course sunshine isn’t the only thing a plant needs to grow. You can see that when dry weather hit in mid-September, regrowth slowed and plateaued at 2000 lbs/acre DM.
So how do we optimize pasture yield?
Optimal pasture yield is achieved by grazing plants when they reach a height corresponding to a high point in energy reserves then grazing the pasture to a height which removes much of the forage but leaves adequate residual leaf area for photosynthesis and stubble with energy reserves. The optimum pre- and post-harvest forage height depends on the forage species or mixture you have.
For a mixed orchardgrass-white clover stand we would allow growth to a 10-inch height then graze the stand to a 2-inch residual height. The low residual height allows high utilization of the standing forage and will set back the orchardgrass a little, allowing the white clover to be more competitive. You can see this in the graph to the right as the red line of the grass dips below the clover’s green line. If the white clover becomes too competitive and we want to favor the orchardgrass, we can leave a 4-inch stubble. This enables the orchardgrass to have a more vigorous regrowth, shading the clover, and reducing the clover in the stand. (One more complicating factor is our soil. If the clover is to respond to this management, there must be adequate soil pH, phosphorus, and potassium.)
Grazing to Set Back Pasture Growth for Seeding Clover
We can reduce pasture growth rate by grazing the pasture when it has low carbohydrate reserves. For example, graze a paddock down to a low residual forage mass (2-inch stubble). Then allow the pasture to grow back to about half the normally recommended height; a 5-inch height when we would normally allow it to come back to 10-inches. At this point the energy reserves will be at about their lowest point. Graze the pasture again to a 2-inch stubble. This removes the leaf area when there is little reserve energy in the plants. This results in a growth rate about half that which would occur in the stand if it had high energy reserves. If clover was no-till seeded prior to the second grazing it would have less competition and a better chance to establish.
Coming Next Week
Till now we’ve focused on what the plants need from us to maintain growth and store energy. Next week we’re going to cover how we’ll cover cell growth in plants and how that growth affects what our livestock get from the plants.
Are you grazing in arid areas?
The drier your environment, the less your plants grow, meaning you have different grazing challenges. So we’ll be sharing some information more pertinent to our rangeland readers in upcoming issues.