It’s a great time to hike the trails at The Nature Conservancy’s Platte River Prairies! Regal fritillaries are out in force, along with a number of other butterfly and insect species, and big wildflower season has begun, with many of the more showy species just starting to bloom. Find out more about the public trails and download directions and trail guides here. We’re only two hours west of Omaha, and just south of the Wood River I-80 exit (#300). If you’re passing through our area on the interstate and need a place to stop and stretch your legs, stop by!
Common milkweed in restored (reconstructed) prairie. The Nature Conservancy's Platte River Prairies, Nebraska. In the week since this photo was taken, many of the more showy wildflowers in our prairies have begun to bloom.
This season has been an intriguing one so far, ecologically speaking. Each time I walk the prairies I’m learning something new and surprising. The abundance of rain and the high groundwater level has meant lush growth and wet wetlands. The cool weather has meant delayed blooming for many plant species, leading to an interesting mix of flowers right now (a combination of species normally done by now and others that are blooming on time).
We weren’t able to get all the prescribed burning done that we wanted to this spring, so have been using alternative strategies to get the kind of disturbance impact we want on those sites. On the prairies where we did burn successfully, the patch-burn grazing impacts look really really nice. Much of my time in the field so far has been trying to interpret what I’m seeing in terms of the response of plants and insects to those management strategies.
My most recent attempt at ecological interpretation deals with our patch-burn grazing and milkweeds. Over the last 10 years, I’ve spent a lot of time collecting formal and informal data on the impacts of grazing on prairie plants. Up till now I’ve spent a little time thinking about milkweeds, but since I’m trying to see my prairies through butterfly eyes this year (a good idea, by the way – looking at your site through the eyes of various species) milkweeds have become a higher priority. I’ve known that milkweed flowers can be a target for cattle grazing, but now I’m looking more directly at how many flowers are grazed or ungrazed within our patch-burn grazing systems.
This week, I looked at one of our prairies under patch-burn grazing, and counted milkweeds (grazed and ungrazed) within both burned and unburned portions. It was still an informal data collection attempt, but instructive. I looked at about 150 common and showy milkweed plants (Asclepias syriaca and A. speciosa), and found that 83% of the flowers had been nipped off in the burned portions of the prairie, and about 57% in the unburned. Those are pretty high percentages in a system that is set up to encourage grazing in burned areas but not unburned areas, and contrasts with the selective preferences of grasses over wildflowers that we typically see. It’ll be interesting to watch what happens during the rest of the season.
There are various layers of interpretation here. First, the fact that the cows are eating blooms in the first place is intriguing because while that’s common in many grazing systems, it’s not common in our lightly-stocked patch-burn grazing system. The attractiveness of those flowers ito cattle is apparently very high. Second, the grazing of flowers in the unburned portions of the prairie is REALLY interesting because there is almost no other grazing taking place there. In fact, I wondered if the flowers were being grazed by cattle or deer, and had to check the exclosure we have on the site to confirm that there are no flowers grazed off there (there aren’t). It still could be deer, but I doubt it.
On the positive side of things, there are still milkweed flowers available throughout the site, even in the burned/grazed portions. There could be more, but from a pollinators standpoint, there are still milkweeds there. (And the grazed milkweeds are still alive and growing – they just don’t have flowers) Also, the grazing is not having a severe immediate negative impact on the plants – in fact several that were grazed earlier in the season have re-bloomed now. If those plants are prevented from flowering successfully for many years in a row, it could hurt the population, but periodic grazing shouldn’t be a big deal to these perennial plants.
The same prairie as above - with more context. You can see that most of the grass is grazed, but few of the wildflowers are. In the background, you can see the unburned portion of the prairie as a sliver of yellow. Grasses in that unburned area are about 2 1/2 feet tall - a stark contrast with the burned area where grazing is keeping them short. In another week or so, this prairie will be awash with color from all of the big wildflowers that are on the edge of blooming.
As I try to find management strategies that optimize biological diversity in prairies, one of the biggest objectives is to prevent any species, plant or animal, from being negatively impacted by our management year after year. I’ll continue to watch milkweed grazing as the season progresses, but it might be that these species are more vulnerable to grazing than most, and that they could be a good indicator that can help me tweak our management over time. The current plan under which this particular prairie is being managed calls for patch-burn grazing for two years, followed by one year of complete rest. Under that system, I’m not concerned about the long-term vigor of the milkweed plant populations because they’ll have at LEAST one year out of three to bloom and reproduce successfully (through both seed and rhizome). I’m also not worried about insects that use those milkweeds because in addition to those in the prairie, there are numerous milkweeds in exclosures, outside fencelines, and other locations in the very nearby neighborhood. Plenty of milkweed to go around.
So – this upshot is that it’s been valuable to look at the prairie from a perspective that forces me to consider species I hadn’t paid as much attention to in the past. I’m not seeing anything that makes me think we’re heading in the wrong direction, but milkweed flower grazing seems to be a good thing to add to the aspects of this and other prairies that are part of my annual evaluation efforts.
As an aside, the patch-burn grazing system we’re trying on the prairie mentioned above includes a fairly high stocking rate early in the season, followed by a lighter stocking rate in the summer/fall. In May, I saw fairly regular grazing of forbs such as compass plant, Canada milkvetch, Illinois bundleflower, and rosinweed. Now that the stocking rate is reduced, I see very little grazing on those plants, and rosinweed and compassplant are just getting ready to bloom. The milkvetch and bundleflower plants are growing strong, and should also bloom. Also, most of the milkweed plants that are being grazed are only getting the tops nipped off, so the vigor of the plant is not really being reduced much. It’s all very interesting to watch.