Photo of the Week – June 30, 2011

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.

10 thoughts on “Photo of the Week – June 30, 2011

  1. Good post Chris. Your tally of grazed and ungrazed milkweed plants touches on one of the things we’re looking at down here in the southeast part of the state. We’re quantifying how frequently as suite of grasses and forbs are clipped in each patch of the patch burn pastures as well as the control pastures (where the entire pasture is burned once every three years). The working hypothesis is that the frequency of being grazed is the same for both types of pastures (patch burn or control) at the pasture scale, but that it differs at the patch scale.

    Starting with your milkweed numbers as an example, lets assume you’re using a two-patch system (I know you’re not but the numbers provided in your post make this assumption the most logical for the purposes of this exercise). At the pasture scale, you observed defoliation of 70% of the milkweed plants. But at the patch scale you observed defoliation of 83% of the plants in one patch type and 43% of the plants in another patch type. The working hypothesis that I mentioned previously could be tested by data from a comparison pasture that isn’t being patch-burned to see if:

    a) Defoliation across the entire pasture is 70% and defoliation within each patch-scale portion of the pasture is 70%. This would indicate that herbivory is functioning differently in the patch-burn compared to the comparison pasture.


    b) Defoliation across the entire pasture is higher or lower than 70%. This would also indicate that herbivory is functioning differently within the two types of pastures.


    c) Defoliation across the entire pasture is 70% and defoliation within one patch-scale portion of the pasture is higher than 70% while in another patch-scale portion it is lower than 70%. This would indicate that herbivory in the patch-burn pasture is functioning similarly to how it functions in the comparison pasture.

    • So do you have a general idea of what you’re seeing so far? I see a lot of between-year variation in the species and extent of grazing that forbs get. Very interesting – largely tied to effective stocking rate. Not actual stocking rate, but the stocking rate relative to weather conditions, etc., that influence how much forage is available at any time during the season. During periods when grass growth is slow and cattle get “ahead” of the grass, I see more forb grazing. Sometimes that lasts for a season, sometimes only for a week. In other years (like last year) the grass grows so fast we end up being “understocked” the whole year, and they don’t even graze all the big bluestem – let alone any forbs. Great intriguing stuff to watch!

      • With regard to your question of do we have a general idea of what we’re seeing, I know you’re not going to like my answer – I haven’t looked at the data yet.


        One impression I got during the first year of the study, when some of the pastures were initially overstocked a little, seemed to support some of what you relate – the higher the stocking rate the greater the breadth of the diet.

  2. Great post Chris. I have also spent a lot of time looking at what cattle graze and what they do not. No data, just observations. I have noticed a huge difference in grazing preferences between herds. Some herds will go around and nip off all of the Canada thistle seedheads for about a 2 week period while other herds will completely ignore it. I think the herds that graze the thistle seedheads have encountered it before and been “forced” to eat it based on the look of the pastures they came from. They have taught their calves to eat it during that 2 week period so it carries forward each year. The other herds may not have encountered it before or been forced to eat it to realize that for 2 weeks, those juicy Canada thistle seedheads are the best thing out there!

    I have seen the same thing on selectively grazing on other plant species. For example smartweeds and dock are listed in most forage quality analysis as unpalatable or even toxic to cattle. Some herds are “tuned in” to smartweeds and dock and it is one of the first plants they will go after in wetland units while other herds under similar conditions may ignore it. This is likely influenced by where they have grazed before and whether they have encountered smartweed before. As you have discussed in previous posts, there are many other factors involved as well, with stocking rates being a big one. Higher rates may force them to eat the smartweed where lighter rates would allow them to ignore it. Timing is also a big one. Earlier in the growing season, smartweeds and especially dock are heavily grazed by both deer and cattle. Once the growing season progresses, cattle will continue to graze the already grazed smartweeds and dock, but will begin to ignore the ugrazed plants. I believe it is due to accumulations of bitter substances (toxins?) in the plant tissue. I can even smell it when I walk through it once it starts to flower. A likely defensive strategy to allow flowering and seed production? This also happens to be the time when many forage samples are taken that result in smartweed being listed as unpalatable or toxic. Jeff Drahota, the Wildlife Biologist at Rainwater Basin Wetland Management District looked at the nutrient values in wetland plants through the summer and answered the question many of the cattlemen and women we work with have, is there anything for a cow to eat? His data is listed at the following site: Plants that are often thought of as worthless for grazing, such as cattails, is actually pretty good stuff early on before it starts to flower.

    There is also a growing number of ranchers and researchers looking at the question of selective grazing in cattle and if that can be intentionally influenced to advantage and actually “train” cows to eat noxious weeds and other plants that are generally considered as toxic, unpalatable, or worthless as grazing forage. Here are a couple that were interesting. I am shure googling would provide an evenings worth of entertainment on the subject of “how to teach cows to eat weeds”.

    Click to access COWS%20EAT%20WEEDS.pdf

    Applying this on the landscape to manage prairies depends on a good partnership and working relationship with the cattlemen and women that do all of the work. I’m not telling you something you don’t already know and practice.

    I would agree, very interesting stuff!

    • Tom – thanks for chiming in! Great info. I agree – training cattle to eat weeds can be an important part of suppressing some species. It won’t kill most of them (unless they’re short-lived plants anyway ) but can sure help slow spread down their spread.

  3. You and your readers might be interested to know that common milkweed was given the name Asclepias syriaca by Linnaeus, who found the plant growing in southern Europe and erroneously thought it originated in Syria. Linnaeus was fooled because unbeknownst to him, by his time the plant had already been introduced into Europe from North America. I first heard this story several years ago from famed botanist Robert Mohlenbrock of Illinois.

  4. Having spent Thursday afternoon walking the trails with Chris, I agree now is a great time to see the prairies and the results of all the good work. I learned a lot and appreciate the information Chris shared.


  5. This year appears to be a very good year for Prairie Fringed Orchids. I usually check the bloom of PF in a twenty acre virgin prairie. In past years I would find only a few plants , seven at the most this years I lost count at 60.


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