Hawkins' Meta Analysis Should be Retracted

---

​Holistic Planned Grazing (HPG) is a specific grazing methodology developed over several decades by Allan Savory and his collaborators. HPG has gone through several iterations throughout the years as flaws in the process were revealed and subsequently fixed. The current version of HPG is detailed in the “Holistic Management Handbook: Healthy Land, Healthy Profits” (referred to as the “HM Handbook” from now on) and the “holistic decision making” aspect of HPG is laid out in the HM Textbook (the most recent version is called “Holistic Management: A Commonsense Revolution to Restore Our Environment”).

The version of HPG presented in these two books is the version of HPG which most managers are using right now, and which is advocated by the two major HPG promotional organizations; the Savory Institute and Holistic Management International.  Our organization is dedicated to providing hope and support to those who need it the most. We believe that everyone deserves a chance to live a fulfilling life, and that through our efforts, we can make a real difference in the world. Join us in our mission to change lives and make a lasting impact on the world.

​Hawkins claims in her paper that “If adaptive management is used, there is little to distinguish HPG from other rotational, high-density, time-controlled grazing approaches such as short-duration high-intensity grazing, intensive rotational grazing, cell grazing, and strip grazing.”  (Hawkins 2017) This statement is false, as anyone who is familiar with the grazing approaches in question should immediately realize. There are many, many aspects to Holistic Planned Grazing which are found nowhere else. Sure, there are many other grazing approaches which have superficial similarities to HPG (animals are high density, and moved between many paddocks frequently, for example), but, as in the case of the chemo vs poison example I gave above, there are details unique to HPG which make all the difference!

​The HM Handbook is not at all ambiguous about the methodology of HPG. You should read the foundational texts yourself, as I cannot adequately describe the process in this short article. But, for reference, here are a few key aspects which define HPG:

• First, a Holistic Context is developed to guide all decision making. The Holistic Context is constantly consulted throughout the creation of the grazing plan and during the actual implementation of HPG on the ground. Holistic management is not a synonym for “adaptive”, which is clear if you read the HM Textbook. (Savory and Butterfield 1998)

• Recovery periods (a.k.a. rest periods) are critical variables in HPG, and are carefully determined based on the specifics of each Holistic Context and the ecological understanding presented in the HM Textbook. A minimum recovery period is used when growth is at its fastest, and a maximum recovery period is used when growth is at its slowest (when growth stops the entire grazing methodology changes, this range of recovery periods is only meant for use within the growing season). A guideline given in the HM Handbook is that plants are ready to be grazed again “when [they] look like ungrazed plants growing nearby”, but advice is also given about when you would want to lengthen that period or shorten it based on your Holistic Context. (Butterfield et al. 2006)

• Different advice is given for different climates. Climates are broadly classified into “brittle” and “non-brittle”. In brittle-environments it is made clear that “herd effect” (“ the result of a change in animal behavior… usually brought about by some actual management action – using an attractant, or crowding animals to ultra-high density.”) is of paramount importance for successful regeneration or sustainability of pastures. Herd effect is still a useful tool in non-brittle environments, but not as strictly required for success as it is in brittle environments. (Butterfield et al. 2006; Savory and Butterfield 1998)

• In all environments the use of fire is heavily discouraged, with an entire chapter devoted to it, detailing why it is almost never the right tool to use, in the HM Textbook. (Savory and Butterfield 1998)

• Two different grazing methodologies are given in the HM Handbook: one to be used during the dormant season (called a “closed plan”), and one to be used during the growing season (called an “open plan”). (Butterfield et al. 2006)

• HPG always involves planning the season’s moves on something called a “grazing plan & control chart”. This planning is done according to a very clear 17-step process, which can be found only in the HM Handbook. This planning process primarily centers around ensuring that your chosen recovery periods are maintained for all paddocks, but it also includes many other variables in the planning of herd movements (ex: birthing and breeding periods, labour considerations, livestock health, wildlife habitat considerations, crop field needs, etc.) and, depending on the Holistic Context, these other variables may take precedence over maintaining your rest period. (Butterfield et al. 2006)

• HPG does not end with the creation of the grazing plan on the chart; after the plan is implemented there is continual monitoring, and adaption involved which are essential to success with HPG. The two most common things which are monitored and changed throughout the growing season are stocking rate and rest period (which determines grazing period). The HM Handbook gives specific indicators to determine if stocking rate or rest periods need to be adjusted. In the academic range science literature, this practice of adjusting stocking rates and/or rest periods throughout the season is usually referred to as “Adaptive Grazing”. HPG is always adaptive grazing, but adaptive grazing is not always HPG. (Butterfield et al. 2006)

Now that we better understand what HPG is (a highly specific set of recommendations and processes detailed in the HM Handbook and HM Textbook), lets turn our attention to Heidi-Jayne Hawkins’s meta-review:

She analysed twenty-three studies from around the world and found that HPG has no benefit over Continuous Grazing in terms of animal and plant production (Hawkins 2017). Since her study is very specifically about “Holistic Planned Grazing ™”, we would expect that the studies analysed in her meta-analysis would be actual studies of “Holistic Planned Grazing™”. This is, alarmingly, not the case at all!
​
*Please note, I was not able to find an online copy of “Gammon and Roberts 1978”. This study was published 10 years before the earliest version of Allan Savory’s Holistic Management, so, barring the use of a time machine, it is impossible for this study to have been studying the specific approach referred to as “Holistic Planned Grazing”.

*Also note, the study “Clatworthy 1984” is a reference to the “Charter Grazing Trials” in Zimbabwe which were personally overseen by Allan Savory. This occurred before he had developed the modern versions of Holistic Management or Holistic Planned Grazing, but because it was overseen by Savory himself, we are going to treat it as a completely legitimate study of “Holistic Planned Grazing” which he created.

(Photo, left column: Fence line comparison, Lebowa, S. Africa, conventional management, left - reducing cattle density - vs HPG, right, - increasing cattle density with planned grazing)

Let’s compare the grazing methodologies used in the twenty one remaining studies (see the notes above about the other two studies) with Holistic Planned Grazing:

1. Was a Holistic Context developed and used to guide the grazing management?
   Since the development of Holistic Management, Allan Savory has been consistent: grazing management cannot be divorced from the holistic context, or holistic decision making, without negative consequences. This point is heavily reinforced in all official HPG/HM courses and literature. (Butterfield et al. 2006; Savory and Butterfield 1998)
   
2. Out of the twenty one studies, none mention that a “Holistic Context” or “Holistic Management” or “Holistic Decision Making” or anything similar to these concepts were at all involved in the grazing management. This makes them, at best, studies of “Planned Grazing” not “Holistic Planned Grazing”.
   ​
   Was the grazing planned on a “Grazing Plan & Control Chart” according to the 17-step process from the HM Handbook?
   Not a single study in the meta-review mentions “Holistic Management Handbook”, nor the “grazing plan & control chart” nor anything about the 17 steps to creating a grazing plan with HPG. So not only are the grazing methodologies used not “holistic” they are also not even “planned grazing”!

​Grounds for Retraction
At this point we can already say without reservation that Hawkins’s supposed meta-review of “Holistic Planned Grazing” was nothing of the sort, and therefore her conclusion that “Holistic Planned Grazing does not improve production” is unfounded, and the paper should be retracted.

​Let’s go easy on Hawkins; lets relax our definition of HPG a bit, and see whether the studies she assessed use grazing methodologies that are at least close to HPG…
There are seven simple questions we can ask to determine how close the grazing methodologies used in each of the studies were to actual Holistic Planned Grazing, as it is defined in the official texts and courses:

• Anderson et al. 1988 : “The 10 paddocks within the rotation cell were monitored as each was being grazed. On the day preceding grazing, and each day while the heifers were grazing the paddock, visual estimates of standing crop height were made at a minimum of 10 locations in the paddock. When grazing had reduced the average standing crop height by 30-35% from that estimated initially, cattle were moved to the adjacent fresh paddock” This means the herd movements were not based on any grazing chart, neither were they based on maintaining a certain recovery period. The movements were based on only looking at the current paddock being grazed, and how much forage had been taken from that paddock. This is not, and has never been, how HPG works. You will never find in any of the HM or HPG literature the advice to move animals based on the criteria used in the Anderson et al. study. In HPG the percentage of standing crop taken in each grazing is only monitored as an indicator of whether the stocking rate should change, it has no bearing on the timing of animal movements. (Butterfield et al. 2006; Savory and Butterfield 1998)

• Badini et al. 2007: This was not an actual study of real-world results with different grazing methodologies, it was a computer simulation. HPG was created to bring benefits in the real world, not a simulation which was so limited that it could not even “simulate the effect of pasture degradation factors such as selective overgrazing and physical damage of soil and pasture due to animal activity”! Besides the obvious limitations of using a simulation, the criteria used to determine when to move the animals was, once again, based on a threshold of biomass in the current paddock being reached. As mentioned above, this is absolutely not how animals are moved in HPG. (Butterfield et al. 2006; Savory and Butterfield 1998)

• Teague et al. 2010: Besides the heavy use of fire in this study (which is almost never a part of HPG) the movement of animals was, once again, not based on a grazing plan or on maintaining a certain rest period, but instead the decision to move was based on “The Grazing Manager (TGM) optical utilization score (scale of 0 to 5) of Kothmann and Hinnant (1999)”. This is an entirely different methodology than HPG. (Butterfield et al. 2006; Savory and Butterfield 1998)

…. Now we are down to 18 studies. Four studies, out of the original twenty three, have been shown to be not even remotely studies of “Holistic Planned Grazing™”,  and one has been confirmed as a legitimate study of HPG.

Question 2: Did any of the studies implement anything remotely resembling a Holistic Context/Holistic Decision Making?

​None of the papers mentioned anything resembling a Holistic Context, Holistic Management or Holistic Decision Making except for Teague et al. 2011 which says “They plan their grazing management within an adaptive, goal-oriented management framework using basic knowledge of plant and animal physiology and ecology” in reference to those using planned rotational grazing. Presumably, if they were testing this same method in their study they would have also used an “adaptive, goal-oriented management framework”, although there is no confirmation of this or further mention of it in the paper.

Question 3: If they didn’t use a “Grazing Plan & Control Chart” or the 17-step grazing planning process, did they at least plan the grazing on some sort of chart and/or in a way that ensured full recovery for each pasture before the next grazing?

None of the papers make any reference to planning the grazing on any sort of chart. However, in email correspondence with the author, I discovered that “Badgery et al. 2017a,2017b” did utilize a grazing chart (although it was not the same type of chart used in HPG). Also in “Dowling et al. 2005” “The paddocks within the ‘cell’ were managed according to the rules established for time-control grazing (all cooperating managers had completed a common time-control grazing training course)” so I will assume they did use some sort of grazing chart, since that is a typical component of time controlled grazing courses.

As for basing the movement schedule primarily on ensuring full recovery for the plants before they are re-grazed (a key part of HPG’s planning process):

• Six of the studies gave no indication of how the grazing schedule was created (Derner and Hart, 2007; Heidschmidt, 1982; Jacobo et al, 2006; Thurow et al. 1988; Vermiere et al. 2008; White et al. 1991)

• Five of the studies did not base the grazing schedule primarily on ensuring adequate recovery for the plants, these were all studies of the same grazing experiment in Wyoming: “The time-controlled rotation treatments were subdivided into 8 paddocks, with length of grazing on each paddock determined by estimated forage availability and rate of forage growth. These parameters were not actually measured, nor was any attempt made to develop mathematical guidelines, but when more forage was available or when forage growth slowed in a given paddock, steers grazed that paddock longer.” (Derner et al. 2008; Hart et al. 1988; Hart et al. 1993b; Hart et al. 1993a; Manley et al. 1997)

• Four of the studies (Badgery et al. 2017a,2017b; Dowling et al. 2005; and Teague et al. 2011) were not totally clear, but at least give me good reason to believe that they were using recovery period as a primary criteria for the grazing schedule.

• Three of the studies (Cassels et al. 1995; Gillen et al. 1998; McCollum et al. 1999), all from the same grazing experiment in Oklahoma, were clear that they did use recovery period as the primary variable in their grazing schedule. I received this in email correspondence with one of the authors: “Our planning for the rotational movement started with our target rest period. If we wanted a 30 day rest period for the first cycle and we had 8 paddocks, the grazing period for each paddock was 30/(8-1)=4.3 or about 4 days per paddock. We did not base movement on forage remaining in the paddock or any animal indicator. We wanted any grass tillers grazed during a given grazing period to have adequate rest before cattle entered the paddock again. A given paddock was always “moderately” grazed during a cycle. We never tried to graze down to some target stubble height or uniformity.”

Question 4:  Did they monitor and modify stocking rate throughout the season to match livestock needs with the forage available (ie. was the stocking rate “adaptive”)?

• Fourteen of the studies did not change stocking rate through the season based on observed forage shortages or excesses (Cassels et al. 1995; Derner and Hart 2007; Derner et al. 2008; Gillen et al. 1998; Hart et al. 1988; 1993a; 1993b; Heidschmidt et al. 1982; Jacobo et al. 2006; Manley et al 1997; McCollum et al. 1999; Thurow et al. 1988; Vermiere et al. 2008; White et al. 1991). A few of these did destock 100% occasionally during severe drought, but this is not the same as the HPG practice of closely matching livestock numbers to the needs of the land throughout the season. To be clear, HPG does not always require frequent stocking rate changes, but it DOES require the manager to constantly monitor the forage base to determine if stocking rate changes are needed. Sometimes changes are not needed for several years, but more often stocking rates will need minor adjustments more than once per year.

• Only four of the studies did use adaptive stocking rates (Badgery et al. 2017a,2017b; Dowling et al. 2005; Teague et al. 2011).

Question 5: Did they monitor and modify their rest period throughout the season based on the observed rates of growth in the first grazed paddock? (ie. Was the rest period “adaptive”?)

• Three of the studies do not mention whether their rest periods changed at all, or were static (Thurow et al. 1988; Vermiere et al. 2008; White et al. 1991).

• One of the studies used a fixed rest period (Derner and Hart 2007).

• The remaining thirteen studies did change their rest periods based on growth rates (Badgery et al. 2017a,2017b; Cassels et al. 1995; Derner et al. 2008; Dowling et al. 2005; Gillen et al. 1998; Hart et al. 1998,1993a,1993b; Heidschmidt et al. 1982; Jacobo et al. 2006; Manley et al. 1997; McCollum et al. 1999), but none of them say what exactly triggered the change to a different rest period, with several of them indicating these changes in the rest period were predetermined at the beginning of the season, and not based on actual observed growth in the first-grazed paddock during the current season (which is the practice advocated in the HPG methodology).

Question 6: Did they use fire, which the HPG literature makes very clear is not a good tool to use? (Savory and Butterfield 1998)

• Three of the studies mention that fire was used (“all units were burned 1 April 1990 and 20 March 1993”) as a part of the management (Cassels et al. 1995; Gillen et al. 1998; and McCollum et al. 1999).

• We will assume fire was not used in the remaining studies since it was not mentioned.

Question 7: If the study was done in a brittle environment, did they focus on creating “herd effect” as a primary management goal?
​
Herd effect is described in the HM Handbook as “The impact on soils and vegetation produced by a large herd of animals in high concentration or in an exited state… the result of a change in animal behavior and usually has to be brought about by some actual management action – using an attractant, or crowding animals to ultra-high density”. (Butterfield et al. 2006)

Out of the ten studies conducted in brittle environments (Derner and Hart, 2007; Derner et al. 2008; Hart et al. 1988,1993a,1993b; Heidschmidt et al. 1982; Manley et al. 1997; Thurow et al. 1988; Vermiere et al. 2008; White et al. 1991), none have any mention of “herd effect”. Considering that it takes out-of-the-ordinary management ​actions to create herd effect, we can safely assume none of the studies used herd effect if they did not mention it.

*Studies in non-brittle environments were given a 2.5 rating for the herd effect category since herd effect is unimportant in non-brittle environments, so whether these studies used herd effect or not should have no bearing on their rating relative to the other studies.

(Left: Research Comparison Table: rating papers cited in Hawkins 2017 by how closely the practices studies match to Holistic Planned Grazing)

(Left:Research Comparison Table: rating papers cited in Hawkins 2017 by how closely the practices studies match to Holistic Planned Grazing)

​Note: I will only be showing the overlay for her “plant biomass (kg/hectare)” and “animal gain (kg/hectare)” forest plots. The other two forest plots, showing “plant basal cover (%)” and “average daily gain (kg/head/day)” are not factors which are generally important to practitioners of Holistic Management or HPG since most are concerned with whole-ranch profits, which “forage production/hectare” and “animal production/hectare” are much better measurements of than the weight gain of individual animals or the number of stems per hectare. Of course, you can go and look at those two graphs yourself and overlay them with my rating of similarity to HPG, and you will find that there is little to no correlation between similarity of the grazing system to HPG and plant basal cover or individual animal gain. This is because HPG has been developed to benefit ranchers in the real world, where total forage production and total animal weight gain per hectare are far more important than plant basal cover or individual animal gain. Plant basal cover could be a useful performance measure in HPG since it is related to the function of the water cycle, but it would have to be coupled with measurements of surface litter cover and water infiltration rates to be useful.

​After re-examining Hawkins’s data with our new knowledge of what exactly HPG is and what it is not, we can draw a few clear conclusions:

1. Not a single study in this meta review looked at “Holistic Planned Grazing™” as it is defined in the HM literature, or by organizations like HMI or The Savory Institute (who hold the Trademark on “Holistic Planned Grazing™”), or even as it is described by Allan Savory himself.
2. Even if we ignore the official HPG literature and severely relax our definition of HPG most of the grazing practices categorized as HPG in this meta-review are still nowhere close to HPG (average rating 13 out of 40 in our rating of similarity to HPG, see above).
3. Given the above two points, Hawkins has no justification for her conclusion that “Holistic Planned Grazing does not improve production and therefore does not warrant the additional inputs (infrastructure and labour) that the approach requires” (Hawkins 2017). Her paper should be retracted.
4. If we look at Hawkins’s charts showing effect sizes for forage productivity and animal productivity per hectare (the two measures that HPG practitioners are actually concerned with) we see that the similarity of the grazing system to HPG is strongly correlated with a significant positive effect size. In other words, Hawkins’s meta-review actually supports the efficacy of HPG to increase forage and animal production!
5. There are only five studies, out of twenty three, which stand out as being fairly close in grazing methodology to HPG. Clearly there is simply not enough data available on the specific practice of HPG to warrant a meta-review. Given that all five of these studies show positive effects on production, more experiments studying actual HPG are warranted.

Anderson DM. 1988. Seasonal stocking of Tobosa managed under continuous and rotation grazing. Journal of Range Management 41: 78–83.

Badgery WB, Millar GD, Broadfoot K, Michalk DL, Cranney P, Mitchell D, van de Ven R. 2017a. Increased production and cover in a variable native pasture following intensive grazing management. Animal Production Science 57: 1812–1823.

Badgery WB, Millar GD, Michalk DL, Cranney P, Broadfoot K. 2017b. The intensity of grazing management influences lamb production from native grassland. Animal Production Science 57: 1837–1848.

Badini O, Stöckle CO, Jones JW, Nelson R, Kodio A, Keita M. 2007. A simulation-based analysis of productivity and soil carbon in response to time-controlled rotational grazing in the West African Sahel region. Agricultural Systems 94: 87–96.

Briske DD Ash AJ, Derner JD, Huntsinger L. 2014. A critical assessment of the policy endorsement for holistic management. Agricultural Systems 125: 50–53.

Briske DD, Bestelmeyer BT, Brown JR, Fuhlendorf SD, Polley HW. 2013. The Savory Method can not green deserts or reverse climate change. A response to the Allan Savory TED video. Rangelands 35: 72–74.

Briske DD, Derner JD, Brown JR, Fuhlendorf SD, Teague WR, Havstad KM, Gillen RL, Ash AJ, Willms WD. 2008. Rotational grazing on rangelands: reconciliation of perception and experimental evidence. Rangeland Ecology and Management 61: 3–17.

Briske DD, Derner JD, Milchunas DJ, Tate KW. 2011. An evidencebased assessment of prescribed grazing practices. In: Briske DD (ed.), Conservation benefits of rangeland practices: assessment, recommendations, and knowledge gaps. [Davis, CA]: United States Department of Agriculture, Natural Resources Conservation Service. pp 22–74.

Butterfield J, Bingham S, Savory A. 2006. Holistic Management handbook: healthy land, healthy profits (4th edn). Washington, DC: Island Press.

Carter J, Jones A, O’Brien M, Ratner, J Wuerthner G. 2014. Holistic Management: misinformation on the science of grazed ecosystems. International Journal of Biodiversity 2014: Art. ID 163431, 10 pages.

Cassels DM, Gillen RL, McCollum FT, Tate KW, Hodges ME. 1995. Effects of grazing management on standing crop dynamics in tallgrass prairie. Journal of Range Management 48: 81–84.

Clatworthy JN. 1984. The Charter Estate grazing trial: results of the botanical analysis. Zimbabwe Agricultural Journal 81: 57–67.

Derner JD, Hart RH. 2007. Livestock and vegetation responses to rotational grazing in short-grass steppe. Western North American Naturalist 67: 359–367.

Derner JD, Hart RH, Smith MA, Waggoner JW Jr. 2008. Long-term cattle gain responses to stocking rate and grazing systems in northern mixed-grass prairie. Livestock Science 117: 60–69.

Dowling PM, Kemp DR, Ball PD, Langford CM, Michalk DL, Millar GD, Simpson PC, Thompson RP. 2005. Effect of continuous and time-control grazing on grassland components in south-eastern Downloaded by [Ryerson University Library] at 09:52 14 October 2017 74 Hawkins Australia. Australian Journal of Experimental Agriculture 45: 369–382.

​Gammon DM Roberts BR. 1978. Characteristics of the herbage on offer during continuous and rotational grazing of the Matopos Sandveld of Rhodesia. Experimental Agriculture 4: 3–22.

Gillen RL, McCollum FT III, Tate KW, Hodges ME. 1998. Tallgrass prairie response to grazing system and stocking rate. Journal of Range Management 51: 139–146.

Hart RH, Bissio J, Samuel MJ, Waggoner JW Jr. 1993b. Grazing systems, pasture size, and cattle grazing behavior, distribution and gains. Journal of Range Management 46: 81–87.

Hart RH, Clapp S, Test PS. 1993a. Grazing strategies, stocking rates, and frequency and intensity of grazing on western wheatgrass and blue grama. Journal of Range Management 2: 122–126.

Hart RH, Samuel MJ, Test PS, Smith MA. 1988. Cattle, vegetation, and economic responses to grazing systems and grazing pressure. Journal of Range Management 41: 282–286.

Hawkins H-J. 2017. A global assessment of Holistic Planned Grazing™ compared with season-long, continuous grazing: meta-analysis findings. African Journal of Range & Forage Science 34: 65-75.

Heitschmidt RK, Frasure JR, Price DL, Rittenhouse LR. 1982. Short duration grazing at the Texas Experimental Ranch: weight gains of growing heifers. Journal of Range Management 35: 375–379.

Holechek, J., Hilton Gomes, Francisco Molinar, Dee Galt, & Valdez, R. (2000). Short-Duration Grazing: The Facts in 1999. Rangelands, 22(1), 18-22

Jacobo EJ, Rodríguez AM, Bartoloni N, Deregibus VA. 2006. Rotational grazing effects on rangeland vegetation at a farm scale. Rangeland Ecology and Management 59: 249–257.

Manley WA, Hart RH, Samuel MJ, Smith MA, Waggoner JW Jr, Manley JT. 1997. Vegetation, cattle, and economic responses to grazing strategies and pressures. Journal of Range Management 50: 638–646.

McCollum FT III, Gillen RL, Karges BR, Hodges ME. 1999. Stocker cattle response to grazing management in tallgrass prairie. Journal of Range Management 52: 120–126.
Savory A, Butterfield J (eds). 1998. Holistic Management: a new framework for decision making (2nd edn). USA: Island Press.

Teague WR, Dowhower SL, Ansley RJ, Pinchak WE, Waggoner JA. 2010. Integrated grazing and prescribed fire restoration strategies in a Mesquite Savanna: I. Vegetation responses. Rangeland Ecology and Management 63: 275–285.

Teague WR, Dowhower SL, Baker SA, Haile N, DeLaune PB, Conover DM. 2011. Grazing management impacts on vegetation, soil biota and soil chemical, physical and hydrological properties in tallgrass prairie. Agriculture, Ecosystems and Environment 141: 310–322.

Thurow TL, Blackburn WH, Taylor Jr CA. 1988. Some vegetation responses to selected livestock grazing strategies, Edwards Plateau, Texas. Journal of Range Management 41: 108–114.

Vermeire LT, Heitschmidt RK, Haferkamp MR. 2008. Vegetation response to seven grazing treatments in the Northern Great Plains. Agriculture, Ecosystems and Environment 125: 111–119.

​White MR, Pieper RD, Donart GB, White Trifaro L. 1991. Vegetational response to short-duration and continuous grazing in southcentral New Mexico. Journal of Range Management 44: 399–403.