I got the chance to spend a couple days in Iowa last week, talking about conservation grazing with staff of the Iowa Department of Natural Resources. They invited me to join a two day workshop discussing various ways to use grazing for conservation objectives. My main role was to kick off the meeting by providing various examples of objectives that can be addressed through grazing. Beyond that, I was asked to participate in the remainder of the workshop and contribute thoughts and ideas as appropriate. I am grateful to have had the opportunity to participate, and came away with a better appreciation for the challenges faced by Iowa prairie managers.
I thought I’d share some of what I covered in my presentation. Essentially, I focused on two broad categories of prairie management objectives that can be addressed through cattle grazing. Those are:
- Reducing grass dominance to increase plant diversity
- Increasing heterogeneity of habitat
Reducing Grass Dominance
Dominant grass species can sometimes suppress prairie plant diversity by monopolizing soil and light resources. Two categories of prairies seem particularly vulnerable to this: 1) prairies that have been degraded by chronic overgrazing or broadcast herbicide use, and 2) restored (reconstructed) prairies. In Nebraska and Iowa, dominant grasses can include non-native invasive species such as smooth brome (Bromus inermis), Kentucky bluegrass (Poa pratensis), tall fescue (Schedonorus arundinaceus), and reed canarygrass (Phalaris arundinacea), as well as native species such as big bluestem (Andropogon gerardii).
When attempting to reduce the dominance of these grasses, it’s important to be clear about what you’re trying to accomplish. If the ultimate goal is to increase plant diversity, it’s not enough to just suppress the vigor of grasses. In order to be successful, a variety of other plant species have to colonize territory abandoned by that weakened grass. A late-spring prescribed fire can temporarily suppress the growth and vigor of smooth brome or Kentucky bluegrass, but often results in robust growth of big bluestem later that season. Trading a dominant invasive grass for an aggressive native grass may not be success if wildflower diversity remains low.
Grazing can play an important role in increasing plant diversity by repeatedly defoliating major grass species that limit plant diversity. The timing, stocking rate, and frequency of grazing can all be adjusted based on the grass species and objectives at a particular site. As an example, we sometimes combine an early spring prescribed fire with intensive grazing (through about June 1) to suppress cool-season invasive grasses such as smooth brome and Kentucky bluegrass. If big bluestem is abundant in the same place, we’ll leave cattle in for much of the summer as well, but at a lower stocking rate. The strategy is to suppress both the invasive cool-season grasses and the native warm-season big bluestem while allowing other plants to thrive and expand their footprint.
At low stocking rates, cattle tend to keep big bluestem closely cropped, but don’t target most wildflower species. We usually see an abundance of new plants growing in and amongst the weakened brome, bluegrass, and bluestem during the year of grazing and the following year. Those new plants include both short-lived “opportunistic” plants and longer-lived perennial plants. The result is a bump in plant diversity. If we repeat the same kind of treatment every few years, we can often maintain a richer plant community than we can with other management options such as fire or mowing alone.
There are countless ways to employ cattle grazing to weaken dominant plants and stimulate higher plant diversity. I’ve written about other examples previously. You can find a couple of those here and here.
Increasing Habitat Heterogeneity
Cattle grazing can create habitat structure that other management options such as fire and mowing can’t. As they work to meet their nutritional needs, cattle graze some plant species (mostly their favorite grasses) preferentially. Stocking rate, or the intensity of grazing, correlates with grazing selectivity. At low stocking rates, cattle are free to eat only what they really want, resulting in closely cropped patches of grass interspersed with taller clumps of less palatable grasses and wildflowers. When stocking rates are higher, cattle are forced to eat a wider range of plant species, creating a more uniformly short vegetation structure. Both the “lower-stocking-rate-patchy-habitat” and “higher-stocking-rate-uniformly-short-habitat” can be valuable to wildlife and invertebrate species.
The ideal situation is to provide the widest possible range of habitat types within a prairie, or within a series of adjacent or connected prairies. That way, regardless of their habitat needs, most wildlife and invertebrate species will be able to find a place to live. Changing the location of each of those various habitat types from year to year helps keep any species (plant or animal) from becoming so abundant that it impacts other species to the point of reducing diversity.
Because of the unique vegetation structure created by grazing, a wider range of habitat types can be created with grazing than with either fire or mowing. However, it’s also very important to ensure that grazing doesn’t have a detrimental impact on plant diversity in the name of creating wildlife habitat. Significant periods of rest from grazing and careful monitoring of grazing impacts and populations of sensitive plant species are important. If conservation is the primary goal, grazing should be used only when there are specific objectives to meet, not as a default strategy.
I’ve written much more on the topic of creating heterogeneous habitat with grazing in previous posts as well, and you can find a couple examples here and here.
Setting Useful Objectives – And Then Using Them
Regardless of the management tool(s) being employed, the biggest challenge for a prairie manager is to set clear objectives and then follow up on them. Start by defining the outcome you want (different habitat structure, more plant diversity, etc.) and then describe precisely what success looks like. Monitoring doesn’t have to mean spending hours on your knees with a plot frame, it just means measuring the outcome you desired.
For example, if you want more habitat diversity, you could start by listing the types of habitat structure you want (tall/dense, short sparse, patchy forbs with short grass, etc.) and how much of the prairie you’d like to be in each category. Then, you could make a rough map of how the site looks before the treatment and estimate percentages of each habitat type. After your grazing, fire, or mowing treatment, make another map and see if you reached your objective.
If plant diversity is important, decide how you will measure that. This is where a plot frame and repeated sampling across a prairie can be helpful, but there are simpler ways as well. You could pick out 3-5 small areas (less than 10 square meters) that you can find each year and then annually list the plant species you find in each area to see if that number changes over time. You don’t have to identify all the species, just list how many there are. If you are using grazing, it’s also important to figure out which plant species are favorites of the cattle and use that information to ensure that your management allows those plants enough rest from grazing that they can bloom and make seed every few years.
Most importantly, your objectives should drive the adjustments you make to management from day to day and season to season. If you can define what you want, you can see if your management is moving you in the right direction. It’s fine to change objectives as you learn, or as conditions change. In fact, in our Platte River Prairies, while we have some broad objectives (plant and habitat diversity), we set new specific objectives and management strategies each year to respond to what we’re seeing on the ground.
Cattle grazing is just another tool that can be used for the conservation of prairies. It’s not appropriate for all prairies or situations, but can help meet some objectives in ways that other tools (fire, mowing, herbicides) can’t. Conservation grazing differs from ranching in that income doesn’t have to be a major part of the decision-making process each year. On land where conservation is the primary objective, managers can decide when and how to employ grazing (or not) based purely on the conservation challenges they face.
Thanks again to the Iowa Department of Natural Resources for inviting me to their conservation grazing discussion last week. I was impressed by the thoughtfulness and creativity of the staff I met, and I look forward to hearing more about their prairie management and restoration work down the road.