Funding Bmitations
often restrict pasture replication in grazing research on rangeland.
Consequently, subsample error has been used to estimate treatment effects or
characterize populations. Assumptions associated with experimental designs
which utilize subsample error to make inferences are discussed and an example
evaluated. The appropriate experImentaI unit for inferential grazing research
is the pasture. Animals or vegetation aampIing within pastures must be
considered as subsamples in inferential grazing research. Pasture replication
must be used in intensive grazing trials to establish treatment dffferences or
provide adequate characterization. Following intensive trials, extensive,
unrepIicated trials implemented by private producers can be effective in
estabIishIng broad-based applicability. Unreplicated pasture trIaIs may also be
used for screening several treatments. Shrinking research budgets, increased
competition for research dollars, and increased complexity of grazing research
have contributed to efforts to enhance the efficiency of the research process.
Additionally, range and pasture research competes with other research for land
in organizations with finite land resources. Differentiating between efficiency
of experimental design and the ability to make the proper characterization or
inference is particularly important in range science. Minimizing cost in the
experimental design process often is done at the expense of inference. Most
books on experimental design or statistics provide few examples representing
experimentation at the population, community, or ecosystem levels of
organization (Hurlbert 1984). The principles of design for grazing studies are
most often violated. It is the purpose of this discussion to explore some
aspects of experimental design critical to evaluating efficiency and cost
versus interpretive credibility of grazing research. Hurlbert (1984) defined
experimental research as mensurative (sampling studies) or manipulative
(external treatment). Mensurative experiments involve only the making of
measurements at one or more points in space or time; space or time is the only
experimental variable or “treatment”. Cochran (1977) provided an excellent
review of sampling techniques for sampling studies. Manipulative experiments
always involve two or more treatments and have as goals making one or more
comparisons. Several references provide valuable information on the design of
manipulative experiments (Federer 1955, Cochran and Cox 1957, Cox 1958).
Measuring attributes of interest is important in both mensurative and
manipulative experiments. There are several references available on measurement
techniques (Brown 1954, U.S. Forest Service 1963, Greig-Smith 1964,
Mueller-Dombois and Ellenberg 1974, ‘t Mennetge 1978, Stubbendieck and Schacht
1984, Cook and Stubbendieck 1986). Experiments can be inferential, providing
information about a population; or they can be descriptive, providing
information about specific individuals within a population. Inferential Grazing
Trials Population of Inference The focal point of inferential research is the
characterization of or inference about some population. Steel and Torrie (1980)
defined a population as all possible values of a variable. The Authors are
research animal scientist, USDA, ARS, Rt. 2, Box 144A, Booneville, Ark. 72927;
professor, Department of Agronomy, University of Nebraska-Lincoln, Lincoln
685834915. Manuscript accepted 8 August 1985. JOURNAL OF RANGE MANAGEMENT
39(3), May 1996 selected population of interest becomes the population of
inference from which individuals are selected for experimentation. The
population could be all native pastures in eastern Nebraska for a grazing study
or all silty range sites in Vegetative Zone IV of Nebraska for a vegetative
survey. Explicit definition of the population of inference must be made prior
to the selection of the experimental material. If the population is well defined;
means, variances, covariances, probabilities and other statistics generated
from a research project can be properly interpreted. Once the population is
defined, sample units can be selected within the appropriate experimental
material. Sample units could be termed observational units in descriptive
research and experimental units in experimental research. Experimental Unit A
proper experimental design can be destroyed by failing to recognize what
constitutes the experimental unit (Nelson and Rawlings 1983). According to Cox
(1958) the experimental unit corresponds to the smallest division of
experimental material such that any two units may (independently) receive
different treatments in the actual experiment. Because responses are not
constant in biological research, sample units should be chosen at all levels
which can affect characterizations or responses. For example, treatment,
pasture, animal, year of measurement, year of treatment, the failure of main
effects to respond consistently within the grazing season, and appropriate
interactions are all potential sources of variability in a manipulative grazing
trial. Animals can be considered experimental units in mensurative and
manipulative research trials in which the forage resource has no differential
effect on the measured response. Breed comparisons, reproductive physiology,
insect or parasite studies are examples of such studies. Prior knowledge is the
basis for the assumption that pasture effects are negligible or of no interest.
In identifying the animal as the experimental unit and not replicating
pastures, the researcher foregoes the opportunity to evaluate potential pasture
X treatment interactions. Animals in manipulative grazing research on rangeland
are generally used as a treatment (e.g., stocking rate study) or as measures of
treatment effect (e.g., gain/ head, gain/area). Freegrazing animals in range
situations can seldom be considered as experimental units. Each animal must
receive a treatment to be considered an experimental unit in manipulative
research. In addition, each animal must be independent of other animals for the
response measured. If forage availability or selection are important factors in
the response variable, animals within a pasture cannot be considered
independent. As an example, forage consumed by one animal cannot be consumed by
another, implying a dependency. Thus, animals are actually repeated measures on
the same experimental unit. The experimental unit of a production system in the
measurement of animal production from grasslands must consist of an area of
land and the animals grazing on it as well as auxiliary facilities for such
management activities as supplemental feeding (Morley 1978). Pasture size and
number is a critical consideration for research facilities with limited land
resources. Pastures should be large enough to support adequate animal numbers
to provide the precision in response measurements (Peterson and Lucas 1960).
However, small pastures are more desirable than large, production-size pastures
for intensive grazing trials since land is restricted.