How is grass productivity above and below ground affected by grazing at different heights or by leaving different residuals after grazing? A study at UW-Madison found no simple answer to this question. Productivity of pasture grasses varies across grazing management strategies and species.
Nadia Alber with the UW-Madison Nelson Institute for Environmental Studies experimented with grazing management of different cool season grass species at the U.S. Dairy Forage Research Center farm near Prairie du Sac, Wisconsin. She grazed 1,000 pound Holstein heifers on paddocks where half were seeded with ‘Bartura’ meadow fescue, and half with ‘Bronc’ orchardgrass. Grazing began when the grass was either 12 inches tall or 24 inches tall and continued from April to October in 2009 and 2010. The heifers grazed off either 50% or down to 1.5 inches of stubble (100%). Alber measured above- and below-ground production using a rising plate meter and root cores.
There’s a lot of information packed into the two graphs below showing what happened to above- and below-ground production for the two different grasses in 2009 and 2010. First, you can see that production was affected by rainfall. Production was much lower in 2009, with 26.5 inches of rainfall than in 2010 when there was 40 inches of rainfall. The exception was that in the drier year (2009) meadow fescue below-ground production increased if grazing started when the grass was 12 inches high and then grazed down to stubble (100%). Alber can’t explain why this might have happened. Another unexpected result was that in the drier year (2009), both grasses produced greater above-ground biomass with 100 percent grazing at both 12- and 24-inch grazing heights than they did with 50 percent grazing. Of course, this results in longer recovery times, and given what we know about long-term over-grazing, this probably isn’t sustainable.
The graphs also show that the below-ground production of orchardgrass remained relatively constant regardless of how it was grazed, but meadow fescue showed increased below-ground production when grazed at 24 inches in 2010. This was true at both the 50 and 100 percent defoliation levels, indicating that these two grasses have different potentials for storing carbon in the soil. Alber hypothesizes that the structure of orchardgrass allows it to maintain photosynthesis and a level of carbon supply sufficient to support growth without depleting below-ground carbon, unlike meadow fescue, but more research is needed to test her theory.
Take Home Message
The differences shown between just these two species demonstrate once again that there is no such thing as a simple, cookbook approach to grazing that will maximize production above- and below-ground for all cool-season grasses. While it’s complicated to try and manage when you have multiple species, don’t let that convince you to go mono-culture. Different species provide different benefits. Here meadow fescue provides more root mass, which is good for feeding our soil, while orchard grass provided more forage, which is good for feeding our livestock. Feeding both is critical to our success.
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