Realistic Expectations for Switchgrass

YIELD POTENTIAL

Switchgrass has been correctly reported to be a high-yielding perennial native grass that will persist almost indefinitely. However, some biomass yields have been taken out of context and extrapolated to amounts that are not well founded.

Prediction of switchgrass biomass yield in Oklahoma is not an exact science since yields vary widely as a function of precipitation. The best data to assist with predictions comes from a seven-year study conducted at two Oklahoma locations near Chickasha and Haskell. Average biomass yields for Alamo and Kanlow switchgrass varieties grown from 1994 through 2000 at the Eastern Research Station located near Haskell, OK (average annual precipitation 44 inches) were 7.6 and 7.9 tons/acre/year, respectively. Conversely, yield for the same varieties during the same period at the South Central Research Station located near Chickasha, OK (average annual precipitation 33 inches) was approximately 2 tons/acre/year less.

Biomass yield (tons/acre) for
common switchgrass varieties
adapted to the southern Great Plains

*44- and 33-in. annual rainfall, respectively.

There are important differences in biomass yield expectations based on switchgrass type. For instance, both Alamo and Kanlow switchgrasses are characterized as lowland types, whereas Blackwell and Caddo are upland types. As a general rule, the lowland types have greater yields than the upland types under similar conditions. However, in dry environments, the upland types, such as Blackwell switchgrass tend to be quite productive.

The following figures illustrate the year-to-year yield variation due to rainfall and the general differences between ecotypes. Yield generally follows the rainfall pattern. When yield during a particular year does not reflect the total rainfall, it is frequently due to distribution. Rain that comes after switchgrass heads will not contribute much to that year’s yield, but may contribute to the next year’s production.

Mean biomass yield (tons/acre)
for upland and lowland switchgrass types
grown at Chickasha, OK.

Mean biomass yield (tons/acre)
for upland and lowland switchgrass types
grown at Haskell, OK.

Switchgrass biomass yields depend on environmental conditions that occur during the growing season. In Oklahoma, precipitation decreases from the southeast to the northwest and is almost always the limiting factor in determining switchgrass biomass yield. The yield expectations in the table are based primarily on annual precipitation. The range of yields shows the effect of year and site conditions.

Expected long-term yield (tons/acre/year)
of upland and lowland switchgrass grown
in different regions of Oklahoma.

While rainfall is the primary determining factor affecting switchgrass yield, soil depth and fertility also are important. Soil depth determines, to a large extent, the amount of water that can be stored. One to two inches of water can be stored per foot of depth in most soils. If there is less than a foot of top soil to store water, regardless of the total rainfall, biomass yield will be limited with infrequent rain.

There is interest in using biomass harvested from erodible lands enrolled in a Conservation Reserve Program (CRP). Typically, these are restored grasslands planted with a mixture of warm-season native grass species, usually including switchgrass. Since these lands are classified as erodible, they also are highly variable in soil fertility, texture, and depth. In Oklahoma, biomass yields from plots established in a CRP have ranged from 0.35 ton/acre/year with zero lbs N/acre up to 0.80 ton/acre with 100 lbs N/acre with an average annual precipitation of approximately 25 inches. Conversely, studies conducted in South Dakota having monocultures of switchgrass in a CRP had biomass yields of 1.80 tons/acre with 20 to 25 inches of annual rainfall.

Figure 3. Extensive switchgrass tissues below
ground aid in reducing soil erosion.

STAND PERSISTENCE

There is limited information available on long-term persistence of switchgrass stands. However, there are a few common sense production practices that would seem to be crucial to maximize persistence. Good agronomic practices that follow a reasonable fertility program and proper harvest management, will generally result in switchgrass stands that should be economically productive for at least 10 years.