By Gene Schriefer
Cool season pastures go through a bi-modal growth curve -- a large amount of forage early in the season, a period of very low growth from mid-June through mid-August and then a smaller bump in forage growth in late summer and early fall. This is more exaggerated in the southwestern part of the state where it is warmer and drier than the rest of Wisconsin.
In a normal year, we expect and plan for some length of summer slump -- this period when growth rates are lower than livestock consumption rates. During and after the drought of 2012, producers were asking how they should have managed their pastures better or differently because of the weather. The right pasture management is the same regardless of drought or no drought, there is just less margin for error.
Grazing a paddock begins at a target height, let's say between 8 inches and 10 inches. We size the paddock such that it feeds a particular number of livestock for a period of 3 to 4 days, we move the livestock to the next paddock when there is about 4 inches of grass left -- this is the residual or the ungrazed portion. We do not return livestock to this paddock until it has recovered. Recovery in this example is between 8 inches and 10 inches. In spring, recovery may be as little as 16 days. In a normal summer maybe it's 24 to 30 days while under drought conditions it may take 45 to 60 days.
This residual is the plant factory, a savings account of stored energy the plant has invested into its own future survival. How much of the savings account of energy is depleted from over grazing (below 4 inches) impacts how delayed or reduced the building process (growing new leaf area) is.
The more leaf area the greater the amount of sunlight the plant can capture and the more energy it can store. When we have 8 to 10 inches of leaf or more, sunlight is excluded from the soil surface. The shade on the soil created by this leaf area creates a damp, micro climate beneath the plant. In 2012, I was reading 100°F soil temperatures at 2 inches soil depth in over-grazed paddocks, while taller paddocks were still at 80 degrees. At 77°F, root growth stops on cool season grasses, at 90°F, the shoot growth stops. If our management causes a plant to stop growing, how long will it take to recover?
In a more normal summer, this 20°F temperature spread seems to hold true, and creates conditions that allow plants to thrive, not simply survive. Warmer soil loses more moisture to evaporation than cool soil. Wind on exposed soil dries soil even further -- another way that taller forage conserves moisture.
What's the big deal?
Grazing research at the USDA/ARS - Dairy Forage Research Center demonstrates overgrazing from mid-June to August has the single greatest reduction in fall pasture growth. Every drought ends with a rain. If we have protected our residual through summer, the plant will rapidly respond with a period of vigorous regrowth into fall.
Overgrazed pasture may turn slightly green once rains return but yield is compromised.
When the next paddock is not ready for grazing (not 8 to 10 inches tall or whatever your target height is) your choices are: reduce feed demand or increase supply. When the weather warrants it, selling livestock or feeding stored feeds in summer is a strategic management decision, if it is to avoid grazing paddocks before paddocks have fully recovered. Managing to keep forage residual intact helps maintains plant vigor and allows for rapid regrowth once some rains return.
In most summers, our climate is a bit more forgiving in terms of moisture and temperature. Close to right management is often quite good. In a drought like 2012, there was no margin for error in how to manage pastures. In drought, consider leaving five to six inches of residual.
Comfortable livestock produce more, comfortable plants do too.
Schriefer is the Iowa County Extension agriculture agent.