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Grazing Feedback
Mention "continuous grazing" and most environmentalists imagine bare land, fat cattle and polluted runoff. But the precise relationships between grass, cows and rain are much more complex. Two new studies being conducted by U.C. Berkeley scientists Barbara Allen-Diaz and Jim Bartholome promise to better pinpoint these relationships.
In the studies, pasturelands at two sites - one on East Bay Municipal Utility District (EBMUD) lands near San Pablo Dam and the other in East Bay Regional Park District lands surrounding Wildcat Creek - are being subjected to four different grazing and fire treatments. The goal is to measure impacts on native grasses, botanical composition, herbage productivity, and water infiltration and chemistry, and to assess whether limiting grazing to one season or eliminating it all together improves water quality and native grass growth.
"There's been very little quantification of the interaction of grazing animals, grass residue, nonpoint source pollution and watershed protection," says Allen-Diaz. "We want to look at the feedback loops between these factors."
The EBMUD experiment involves 13 hundred-foot-square paddocks, a few cows and a rainfall simulator; the Wildcat experiment involves four 70-80 acre pastures, a herd of cattle, a rancher and the whims of the weather and passersby.
"From an experimental point of view, the EBMUD paddocks are small and easily controlled," says Allen-Diaz. "The Wildcat pastures are much larger, more diverse and more realistic in terms of real life ranching conditions. The next experimental step will be a whole watershed."
Allen-Diaz and Bartholome have already applied for funding for that next step. But in the meantime, Allen-Diaz is busy writing up the first year's results from the EBMUD project. In this project, her research team allowed cows to graze different paddocks in different seasons. Some paddocks were grazed during the winter and spring, some in the summer and fall, some all year and some not at all. Half of each paddock was then burned. The timing of grazing and burning affects the growth of different plant species. Summer and fall grazing may be the best for promoting native species, for example, because by then the perennial natives have gone dormant and the annual exotic grasses have started to grow. Likewise, fall burning is best after the first rains, because it kills newly germinated annuals.
In the experiment, Allen-Diaz found a statistically significant relationship (95 out of 100 times) between grazing, burning and the standing biomass. There was little change in biomass on the continuously grazed plot (see table below).
Allen-Diaz says weather played a major role in the results. But her efforts to simulate weather - with the help of a collection of hoses and nozzles called a rainfall simulator - failed. "Even at the highest setting, which was 8-9 times the amount of rain we get in a winter storm, there was no runoff," she says. To address this in next year's experiments, Allen-Diaz is "redesigning our approach to raining."
Allen-Diaz also found that the native species purple needlegrass increased from 4.5% to 6.2% of cover on burned plots between 1993 and 1994, but decreased on unburned plots. However, this change only occurred in plant size, not in density. "I'd hoped to see more response to the different treatments in the native perennials, but it's only been one year," she says.
Though the two three-year projects were both begun in 1993, results have been slower to come in from Wildcat - a S.F. Estuary Project watershed management demonstration project - because passersby kept leaving the pasture gates open. With self-closing gates now in place, results should be forthcoming.
Contact: Barbara Allen-Diaz (510)642-7125
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