Clipping Does Not Simulate Grazing

Posted by :Beth Burritt On : November 20, 2015
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Clipping studies are used by some groups to point out the potential ill effects of grazing on plant health. According to Trlica and Rittenhouse (1993), plant ecologists often focus on the defoliation aspect (removal of plant material) of grazing and how individual plants respond to defoliation. A large number of studies have used clipping treatments, rather than grazing animals, to defoliate plants. Ecologists often assume clipped plants and grazed plants responded equally, but many studies have shown clipping cannot be used to simulate grazing.

Finding an article that compared clipping to grazing meant going back to the 1960s. Compared to clipping, livestock 1) tend to graze plants to different heights, not to a single uniform height; 2) remove less plant material; 3) eat specific plant parts rather than the whole plant; 4) may change plant size and form by selecting certain individual plants within a species; 5) affect the build-up of plant litter differently; 6) add nutrients to the soil through urine and feces; 7) trample the area they graze; 8) change forage preferences during the grazing season; 9) affect the competition from neighboring plants through grazing; 10) do not graze all plants continuously throughout the grazing season (Jameson 1963).

In 1975, Rickard et al. reported that clipping studies do not simulate how cattle graze plants. Especially if animals are able to select among multiple plant species and plant parts over a large area. Working in south-central Washington, a 9” precipitation zone, they concluded that it would take many years of moderate grazing before shifts in plant species composition and abundance would be noticed.

Some researchers have tried to mimic grazing through clipping. They found that these clipping treatments were not as severe as the treatments where plants were clipped to a uniform height. For example, when half of a bluebunch wheatgrass plant was clipped the basal area increased by 18.6%. However, if the entire plant was clipped the basal area decreased by 7.8%. The basal area of unclipped plants increased by 5.2%. In this study, plants were clipped to a 3” stubble height just before seedheads emerged (Clark et al. 1998). When compared to unclipped plants, Stroud et al. (1985) reported that simulated grazing of western wheatgrass did not decrease above or below ground production (roots and rhizomes) provided season-long utilization (clipping) was below 80%. Tiller numbers, however, increased 28% on unclipped plants compared to plants under simulated grazing. Simulated grazing involved clipping individual plant tillers one to four times during the growing season. Clipping intensity on tillers was 33%, 67%, or 100%. For most simulated grazing treatments, season-long utilization ranged between 64 and 79%.

After an extensive review of the literature on herbivory, Maschinski and Whitham (1989) came to the following conclusions: A plant’s response to herbivory is flexible. Herbivory can be detrimental, neutral, or even beneficial for a plant depending on conditions and its ability to replace tissue eaten by herbivores. The effect of grazing on plants depends on the frequency, intensity, competition, nutrient availability, timing of grazing, and the weather.

References

Clark, PE, WC Krueger, LD Bryant, and DR Thomas. 1998. Spring defoliation effects on bluebunch wheatgrass: II. Basal area. Journal of Range Management 51:526-530.

Jameson, DA 1963. Responses of individual plants to harvesting. Botanical Review 29(4): 532-594

Maschinski, J and TG Whitham. 1989. The continuum of plant responses to herbivory: the influence of plant association, nutrient availability, and timing. American Naturalist 134:1-19.

Rickard, W.H., D.W. Uresk, and J.F. Cline. 1975. Impact of cattle grazing on three perennial grasses in south-central Washington. Journal of Range Management 28:108-112.

Stroud, DO, RH Hart, MJ Samuel, and JD Rodgers. 1985. Western wheatgrass responses to simulated grazing. Journal of Range Management 38:103-108.

Trlica, MJ and LR Rittenhouse. 1993. Grazing and plant performance. Ecological Applications 3:21-23.

Sauer (1978) More Than Just Dead Leaves

Posted by :Beth Burritt On : November 2, 2015
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Currently, I’m selecting material for my blog posts from a recently published article, Carter et al. (2014). Authors of the paper represent Grand Canyon Trust, Western Watersheds, Foundation for Deep Ecology, Kiesha’s Preserve and Wild Utah Project. Carter et al. (2014) is a peer-reviewed paper published in a new, rather obscure (in my opinion) journal, International Journal of Biodiversity. After reading the paper, I questioned and researched some of the authors’ statements and the articles they used to support those statements. I begin with Sauer (1978). (Note: Words in bold are the points I address in my post.)

According to Carter et al. (2014): “Grasses with attached dead leaves are more productive than grasses from which the dead leaves have been removed. Loss of these dead tissues to grazers increases thermal damage to the growing shoots and reduces the vigor of the entire plant [28].” According to the reference section in Carter et al. (2014) reference 28 is Sauer (1978).

Sauer graphic

Clipping to the crown is much shorter than 80 percent utilization. Normally, the recommended utilization level for livestock grazing is 50 percent.

Dead Leaves: Sauer (1978) didn’t just remove dead leaves from dormant bluebunch wheatgrass (BBWG), he removed all dead plant material: leaves, stems, nodes, sheaths and inflorescences.

Grazers: Carter et al. (2014) doesn’t define grazers. In Sauer (1978), BBWG plants were not grazed they were clipped with scissors to the crown leaving no stubble. Clipping was much more severe than almost any livestock grazing.

Thermal damage: Sauer (1978) doesn’t mention thermal damage or plant vigor in his paper.

Vigor: Sauer (1978) reported that clipping all standing dead from BBWG did not change the number of flower stalks or the height of the flower culms plants produced than when compared to unclipped plants. BBWG vigor can be determined by combining the number of flower stalks with the maximum length of flower culms (Mueggler 1975). Basal area has also been used to measure vigor (Clark et al. 1998). Sauer (1978) reported the basal areas of clipped plants were not different from unclipped plants. Thus, BBWG vigor was probably not affected by removing dead material from BBWG plants.

So what did Sauer (1978) find? He found that removing all standing dead material decreased the weight of new leaves and stems by 28%, decreased the loss of standing dead by 21% and decreased leaf length by 25%. Sauer concluded that standing dead is not a deterrent to growth and beneficial to bluebunch wheatgrass.

References:

Carter, J., A. Jones, M. O’Brien, J. Ratner, and G. Wuerthner. 2014. Holistic Management: Misinformation on the Science of Grazed Ecosystems. International Journal of Biodiversity. http://dx.doi.org/10.1155/2014/163431

Clark, PE, WC Krueger, LD Bryant, And DR Thomas. 1998. Spring defoliation effects on bluebunch wheatgrass: II. Basal area. J. Range Manage. 51:526-530.

Mueggler, W. F. 1975. Rate and pattern of vigor recovery in Idaho fescue and Bluebunch wheatgrass. J. Range Manage. 28(3): 198-204.

Sauer, R.H. 1978. Effect of removal of standing dead material on growth of Agropyron spicatum. Journal of Range Management 31:121–122.

Misusing Mueggler

Posted by :Beth Burritt On : October 28, 2015
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Mueggler (1975) has been cited to further the notion that all livestock grazing causes irreparable harm to plants in arid environments unless plants are rested for years after grazing. Below I give a recent example of authors misusing Mueggler (1975) from the following publication:

Carter, J., A. Jones, M. O’Brien, J. Ratner, and  G. Wuerthner. 2014. Holistic Management: Misinformation on the Science of Grazed Ecosystems. International Journal of Biodiversity. http://dx.doi.org/10.1155/2014/163431

These authors state: “… bunchgrasses in arid environments are more likely to die if they are heavily grazed by domestic animals [34” AND “Native, western USA bunchgrass species such as bluebunch wheatgrass (Pseudoroegneria spicata) and Idaho fescue (Festuca idahoensis) are sensitive to defoliation and can require long periods (years) of rest following each period of grazing in order to restore their vigor and productivity [34”

Reference 34 is identified in Carter (2014) as: Mueggler, WF. 1975. Rate and pattern of vigor recovery in Idaho fescue and Bluebunch wheatgrass. Journal of Range Management 28:198-204.

Why do I think these statements are misleading? Public land agencies do not condone heavy grazing. Mueggler doesn’t study the likelihood of plants dying from heavy grazing. I couldn’t find any information concerning it in the paper. Defoliation isn’t defined above, but I assume they mean grazing since it is used later in the sentence. However, Mueggler is not a grazing study. It’s a clipping study. Grasses are clipped in or near the boot stage of development, when grasses are most susceptible to both clipping and grazing.

To apply this clipping study to any livestock grazing in an arid environment, one must assume:

  • Clipping and grazing affect plants equally.
  • The species of concern are either bluebunch wheatgrass or Idaho fescue. If not, then all plants respond the same way to grazing.
  • All plants in the community are grazed to the same level of clipping used by Mueggler.*
  • The entire plant, and not just part of the plant, will be grazed.
  • Plants are grazed at or near boot-stage of development.
  • The same climatic conditions that occurred during Mueggler are present.
  • The same history of grazing, that occurred prior Mueggler, exists.
  • Plants grow on the same aspect, slope, and soil type as in Mueggler.
  • Soils have the same nutrient levels as those present in Mueggler’s study.
  • Plants are growing in the same plant community that occurred at Mueggler’s study site.
  • Grazing a plant not in full vigor, which is a very vague term, is detrimental to plants and the grassland as a whole at a landscape level.

*Bluebunch wheatgrass was clipped to remove 50% of the plant by weight. For Idaho fescue, the clipping was so severe that it cannot be equated with any grazing management plan.

The statements made by Carter et al. (2014) ignore grazing studies by Ganskopp and Bedell (1981) in eastern Oregon. They showed that: 1) Lightly (0%-25% utilization) grazed Idaho fescue and bluebunch wheatgrass (BBWG) produced as much or more height, weight, and seed stalks as ungrazed plants; 2) No differences were detected between moderately (25%-70% utilization) grazed and ungrazed BBWG and fescue plants; 3) Heavy grazing (above 70% utilization) reduced production and height of fescue, but did not significantly impact BBWG; and 4) No plants died from grazing treatments.

Also, Rickard et al. (1975) working in eastern Washington concluded that clipping studies do not mimic cattle grazing. They also reported that many years of moderate grazing could likely be sustained before pronounced shifts in plant species composition and abundance, including BBWG, are noticed.

Finally, a clipping study by Clark et al. (1998) discovered plants that when only ½ of their basal area was clipped to a 3” stubble height during mid-boot, it increased their basal area by 18.6%. In comparison, the basal area of unclipped plants only increased by 5.2% by the end of the growing season.

References

Clark, PE, WC Krueger, LD Bryant, and DR Thomas. 1998. Spring defoliation effects on bluebunch wheatgrass: II. Basal area. Journal of Range Management 51:526-530.

Ganskopp, DC and TE Bedell. 1981. An assessment of vigor and production of range grasses following drought. Journal of Range Management 34:137-141.

Rickard, WH, DW Uresk, and JF Cline. 1975. Impact of cattle grazing on three perennial grasses in south-central Washington. J. Range Manage. 28:108-112