My first job out of college was with the Agronomy Department at the University of Wisconsin and it was mostly working on managed grazing research projects. Mostly, we compared the forage quality and the responses of fish, "bugs", insects, and birds in continuously grazed pastures to pastures in a few different grazing rotations. This was my first real taste of really doing science. I had an undergraduate biology degree and you do some science but you do not really do science. This was particularly true way back when I was in college, undergraduate research opportunities were very limited. I volunteered and did what I could to gain experience but in this job I did real science and it was what really got me ready for research in graduate school.
When I say "we" in terms of these grazing studies, I mean I did the repetitive part - the collecting of vegetation samples, planting and harvesting of plots, helping sample stream habitat and macroinvertebrates, grinding vegetation samples and running them through the spectrometer - all the stuff that is vital to science but is not the fun part of science. Science is about repetition and I was the relatively cheap labor that did the repetitive parts. Through this job, I learned a lot. I learned that science is repetitive and not always exciting. But I also learned about statistics and study design. And most of all, I learned I did not want to be on the "grunt" forever, rather I would prefer to be the one designing the experiments and analyzing the data. I think I always knew that - I knew I wanted to teach at the college level - but this experience strengthened that.
Grazing is often a way to take advantage of land that is poorly suited for other agriculture like row crops. When most of us think of grazing, the massive open ranges of the Western United States where rainfall or access to irrigation water limits agricultural options often comes to mind. "Out West" they put cattle or sheep out over huge expanses of land that sound ridiculously large here in the Midwest. Looking at the dozen largest ranches in the US, they range from King Ranch in Texas at 825,000 acres (nearly 1,300 square miles) to the "smallest" of the 12 is nearly 59,000 acres (over 90 square miles). The Anna Creek Station in Australia is more than seven times the size of the King Ranch (5,851,000 acres; 9,142 square miles). I remember one of the Wisconsin farms we sampled was about 3,000 acres (about 4.7 square miles) and that felt huge. Of course a "grass farm" in Wisconsin grows a lot more grass per acre than does one in Texas, the western United States, or most of Australia.
Here in Wisconsin, grazing is often limited to marginal farmland - the stream valleys that flood too often and may have some wetlands that get in the way of row crop agriculture. And many of the valleys are not wide enough to allow for a lot of corn and soybean fields - certainly not at the scales that today's planters and combines operate most efficiently. There is a reason that farmland on the ridges sells for about twice what the land in the valleys does in much of the Driftless. That said, because of the precipitation that Wisconsin receives, a lot of grass can be grown per acre so our stocking rates are much higher than more arid grazing lands.
Rotational Grazing Systems (and some Photosynthesis)
First, we should define a few important terms. Continuously grazed pastures are exactly that, they are grazed continuously. Here in Wisconsin, this typically results in pastures that are overgrazed where little vegetation is left ungrazed - only those things cows do not like to consume like chicory and thistle - are left ungrazed. In Western states, where grazing animals are grazed in massive pastures or open range lands, continuous grazing can - but certainly does not always - look a lot more like rotational grazing as animals move on as they deplete the vegetation in one place. The vegetation that was grazed then has time to regrow before the next grazing season. This is essentially what native grazers - like American Bison (Bison bison) here in North America and the ungulates of Africa - do naturally.
In rotational grazing systems, pastures are divided into paddocks and grazing animals - usually cows in Wisconsin - are rotated to different paddocks on a schedule. There are many different ways to rotationally graze based on the farmer's grazing and ecological goals, characteristics of the land and vegetation, and a host of other factors. Some grazing systems incorporate fallowing (not grazing) some paddocks, others are more intensive; again depending upon the ecological and economic goals. As with any agriculture, decisions are largely based on their economics.
Another important idea to understand is that of warm-season (C4) and cool-season (C3) grasses. The difference between warm- and cool-season grasses is how carbon fixation occurs in photosynthesis. As the names imply, cool-season grasses tend to do best in the cooler months where temperatures and sun intensity are moderate and soil moisture is abundant whereas warm-season grasses grow fastest in the warmest parts of the summer, which are often drier as well. Cool-season grasses - like those in your lawn - use the ancestral C3 pathway in which carbon dioxide (CO2) from the atmosphere is "grabbed" by the enzyme RuBisCO. RuBisCo is the most common enzyme on Earth - that is how important it is. Because the last two parts of the enzyme's name are carboxylase / oxygenase, RuBisCO has an affinity for both CO2 and oxygen (O2) which is problematic because oxygen molecules far outnumber carbon dioxide molecules in the atmosphere (21% vs. about 420 parts per million or 0.042%). "Grabbing" oxygen instead of carbon dioxide causes plants to lose energy through the photorespiration process.
Limitations of C3 fixation and that C3 plants are not terribly efficient with limited soil moisture allowed for the evolution of an alternative fixation pathway, C4. Warm-season grasses - our native prairie grasses (and corn) - have an enzyme, PEPCase, which essentially creates an environment where RuBisCO can't help but "grab" CO2. Remember even in our world with elevated CO2, there are still only about 420 parts per million of CO2. C4 photosynthesis is a "work around" to keep CO2 high near RuBisCO so it can't "screw up". The C4 pathway causes plants to lose less water to evapotranspiration and is more efficient in higher sunlight and temperatures, thus the name, warm season grasses.
Continuous pastures are typically C3 grasses such as fescues, orchardgrass, timothy, reed canary grass, and even Kentucky bluegrass - the grass most lawns are dominated by. Warm season grasses include big bluestem, little bluestem, Indiangrass, switchgrass, and a host of other native grasses. Managed grazing systems can use either warm- or cool-season grasses and many used both, knowing that in different parts of the year, different pastures will provide better forage. However, most places we are familiar with are covered in cool season grasses.
What I learned about grazing while working at the University of Wisconsin is that continuously grazed pastures are darn-near biological deserts. Much of the research was examining the effects of different grazing systems on grassland birds and trout streams in Southwestern Wisconsin. Grazing had a very strong effect on bird communities and setting aside some non-grazed land had an even stronger effect (Temple et al. 1999). Continuous pastures were dominated by Killdeer (Charadrius vociferus) and other birds of wide open landscapes. The effects were less significant on trout streams - but watershed conditions had a strong effect. Another way to say this, managed grazing contributes to watershed conditions but individual pastures had less effect than expected.
Being in an Agronomy department, we were studying forage quality, because if managed grazing systems do not produce good quality forage, they will not be implemented by farmers. And I think that is an important idea to remember. Both warm and cool season grasses have their advantages as forages. Warm season grasses are generally more digestible and cool season grasses are higher in proteins. Maybe most importantly, they can provide forages at different times of the year - though that does require more land and many grazers have cows mostly on marginal lands. This is particularly true as commodity prices are as high as they have been.
The most basic question is, "If managed grazing is so much more beneficial, why is it so much less common?" The simple answer is that managed grazing requires more land and more effort. A bit more complex answer is that it is always difficult to overcome the status quo. Many decisions are made simply because "that is how we have always done things". But anyone that has maintained a prairie knows that there is some work and cost involved in establishing a native warm-season grass prairie.
Grazing and Trout Streams
For trout streams, grasslands, like pastures, do three important things, 1) they hold soil and prevent bank erosion, 2) they are a source of terrestrial insects, and 3) they provide shade and cover, particularly on smaller streams. I have written about the importance of terrestrial insects and how there is a succession of terrestrial insects over the course of a season. Warm season grasses do a better job of the three important things I highlighted above. The roots of cool season grasses do little to prevent erosion whereas warm season grasses are much deeper rooted.
There is no question that agriculture is the single largest impact on trout streams and thus trout fishing in the Driftless area. That is not a judgement, rather it is just a fact. This is not surprising as agriculture covers about half of the US land area (source) and that number is even greater in the Driftless Area. Climate change - rightfully - gets most of the attention, we have long known that agricultural impacts are at least as important. As I have written about before, the valleys of the Driftless Area have been covered in cultural sediment from early agricultural practices and that we are in a better place now than we have been in recent decades, in large part due to improved agricultural practices.
In general, grazing impacts - particularly managed / rotational grazing - are much lower on the list of impacts than are row crop agriculture. Particularly at a time where we are seeing an increase in row crop acreage and field size and a decline in contour strip acreage in the Driftless (link). I will say a move to more managed grazing will be an improvement to Driftless stream corridors - not just for the trout but for the birds, bees, and other insects.
Links for More Information
Agouridis, C. T., S. R. Workman, R. C. Warner, and G. D. Jennings.2005. Livestock grazing management impacts on stream water quality: a review. JAWRA Journal of the American Water Resources Association 41: 591-606.
Lyons, J, B. M. Weigel, L. K. Paine, and D. J. Undersander. 2007. Influence of intensive rotational grazing on bank erosion, fish habitat quality, and fish communities in southwestern Wisconsin trout streams. Journal of Soil and Water Conservation 55: 271-276.
Medina, A. L., J. N. Rinne, and P. Roni. 2005. Riparian restoration through grazing management: considerations for monitoring project effectiveness. In Monitoring Stream and Watershed Restoration. P. Roni, editor. American Fisheries Society, Bethesda, Maryland 2005: 97-126.
Nerbonne, B. A., and B. Vondracek. 2011. Effects of local land use on physical habitat, benthic macroinvertebrates, and fish in the Whitewater River, Minnesota, USA. Environmental Management 28: 87-99.
Nussle, Se., K. R. Matthews, and S. M. Carlson. 2015. Mediating water temperature increases due to livestock and global change in high elevation meadow streams of the Golden Trout Wilderness. PloS ONE 10: e0142426.
Saunders, W. C., and K. D. Fausch. 2007. Improved grazing management increases terrestrial invertebrate inputs that feed trout in Wyoming rangeland streams. Transactions of the American Fisheries Society 136: 1216-1230.
Sovell, L. A., B.Vondracek, J. A. Frost, and K. G. Mumford. 2000. Impacts of rotational grazing and riparian buffers on physicochemical and biological characteristics of southeastern Minnesota, USA, streams. Environmental Management 26: 629-641.