USING CATTLE FOR WEED CONTROL IN PUBLIC LANDS

T.R. Seastedt

2/21/99

Boulder Open Space has shown substantial initiative to use cattle -- and particularly winter grazing by

cattle--to keep non-indigenous invasive plant species populations in check. In essence, the cows are

functioning as "biocontrols", equivalent to using a predator or parasite of a noxious weed to reduce the

weed's abundance. The success of this effort is particularly noticeable at the exclosure site east of

Cherryvale and just north of HW 36.

While this activity is clearly effective over the short-term, the possible problems associated with cattle

grazing need to be acknowledged. First, cattle are non-indigenous species that mimic some but not all of

the grazing characteristics of the extirpated ungulates. They don't graze in an identical manner and

fencing constraints will impose artificial grazing patterns. Second, intensive winter grazing (grazing that

removes or tramples senescent mid- or tallgrass warm-season grasses) is not a "natural" process, i.e., this

activity was not something bison would have done. While the shortgrass species may contain sufficient

protein in winter to sustain bison, the tallgrass species do not. This is, of course, the reason why the cattle will eat

anything green in sight. Heavy grazing by cattle increases soil nutrient availability. Moreover,

supplemental feeding appears to be common on at least some of the winter grazed areas. This supplemental

material can be viewed as fertilizer, a potential nutrient supplement provided to the site in cattle feces and

urine. It's absolutely critical to acknowledge that high nutrient availability in soils will not favor native

dominant grasses. Unfortunately, the consequences of fire supression and increased atmospheric deposition

of nutrients are also contributing to this very real change in soil chemistry.

My concern is that, should the cattle be removed from a site too early (before the native warm season

grasses are initiating growth), the fertilization and trampling effects will favor cool season species in

general (which were not the dominants of these grasslands) or nonindigenous species in particular. Even if

cattle are left on site until the initiation of warm-season growth, vagaries of climate (e.g., drought) or other

unforeseen variables can interfere with the growth of the desirable native species. Given the weedy seed

banks of these areas, the systems would then rapidly transform to systems dominated by non-native species.

I fear that chronic winter grazing will ultimately fail to meet its management objectives.

The solution, if there is one (and that's a very legitimate concern!), is to use more spring burns, and less

grazing. Fire will reduce nutrient (especially nitrogen) availability. Late spring burns will harm the cool

season species, and favor the warm season dominants. Mowing and haying (i.e., remove the standing dead

and cool season early growth) may also function to maintain the warm season dominants without causing

this inevitable eutrophocation effect.

The impacts of summer grazing by cattle seems less of a scientific issue. The fertilization effect is still there,

and now the cows will be consuming their preferred food items --often composed of native species-- and

leaving the less palatable weeds. Once an area is impacted by invasive non-palatable species,

cattle as a useful 'biocontrol' largely goes out the window.

Grazing by non-indigeous ungulates under the current conditions of environmental change must be

regarded as "an experiment". To understand experiments we need to monitor vegetation response and set up

reference plots and controls to understand how the site would respond without the grazers.

There's a huge literature, now well summarized, that support these statements. Ultimately, if we really want

a "native grassland", then I believe we must burn more and graze less. I can live with the alternatives, but

I believe the facts have got to be put out there where all can look at them.

grazing by cattle will both reduce seed production of knapweed and reduce

dispersal of the weed. The results look very promising. The grazing window is, however,

fairly restrictive.

FURTHER READING:
The keystone role of bison in North American Tallgrass Prairie. A.K. Knapp
et al. BioScience 49 (1): 29-38 (January 1999)