Article: Vegetation, cattle, and economic responses to grazing strategies and pressures

Authors: Manley, W.A. Hart, R.H. Samuel, M.J. Smith, M.A. Waggoner, JR. and Manley, J.T.

Published: Journal of Range Management 50(6), November 1997

The problem of replication is especially acute in the area of grazing research.  Replication is a fundamental tenet of experimental laboratory sciences like chemistry and physics.  Replication is not just a statistical tool used in the context of a single series of experimental trials.  Replication describes the process whereby other scientists at other locations can read published methods and results and replicate those same results under identical conditions.  In lab science, this process can be challenging but is commonplace.

In grazing and range science we are in the unenviable position of having many years of published results that are, for all practical purposes, difficult if not impossible to replicate.  This is in part because of our inability to control for confounding variables like drought and the outbreaks of plant and animal diseases.

Unfortunately, the problem of replication is exacerbated by the poor standards of the research community.  The problem of replication becomes clear when we look at the description of the three different “grazing systems” used for the purposes of the study under consideration:

“The season-long treatments had no pasture subdivision.  The rotationally deferred pastures were subdivided into 4 paddocks.  A different paddock was deferred from use each year until the end of the growing season…while cattle grazed the other 3 paddocks simultaneously.  For the balance of the grazing season, all 4 paddocks were grazed simultaneously.

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.

Three stocking rates were applied: 1) light, 0.16 to 0.23 steers ha 2) moderate, 0.42 steers ha; and 3) heavy, 0.56 steers ha.  Steer numbers varied under the season-long light treatment because this pasture included replacement steers which were moved to other pasture treatments as needed when steers originally assigned to those pastures became ill or died.  Grazing pressure was determined each year by dividing the number of steers ha on each treatments by the mean PSC of forage in Mg (thousands of kg) ha.”

This description leaves a few critical questions unanswered: How big were the paddock sizes?  How many animals were included in each herd?  Without this critical information, it is impossible to determine the stock density used for the purposes of this experiment, and therefore impossible to evaluate or replicate the true nature of these grazing treatments.  Any attempt to simulate Holistic Planned Grazing, as this study claims to, must deal with and report variables like paddock sizes, herd size, and even animal behavior.

In general, the results seem to favor a season-long light grazing strategy, with particularly favorable results for litter cover using this management system.  However, variability between treatments and from year to year is very high. A prototypical example of these fluctuations is summarized in the following statement: “Under light grazing in 1992 and 1993, cover of cool-season graminoids decreased until it was no greater than under heavy grazing, but in 1994 it was again significantly higher.  We could not identify any climatic or management variables to account for this fluctuation.”

The results from this study are inconclusive at best and very difficult, if not impossible, to replicate.  Inconclusive and non-replicable research results seem characteristic of many studies that attempt to measure the efficacy of Holistic Planned Grazing, perhaps even those studies that show HPG in a favorable light.  This reality begs the question: Are there more effective ways to get more reliable and replicable results from ecological research?

A final and useful point raised by the research is embodied in this quote: “…because regrowth occurs for only a few weeks to a few months on semi-arid rangelands regrowth rarely has a significant effect on forage quality or quantity.  [Other studies have shown] that western wheatgrass and blue grama tillers are rarely grazed more than once or twice in an entire grazing season, regardless of grazing strategy.”  This statement raises some difficult questions about the management of recovery periods through Holistic Grazing Planning, and requires follow-up analysis and investigation.