The Microhistological Technique: Testing Two Central Assumptions in South-Central New Mexico

Abstract

Analysis of the botanical content of diets of livestock, big game, small mammals, and insects has become common in research. The better known dietary analyses utilize either stomach, fecal or esophageal fistula samples. The respective advantages and disadvantages of these procedures have been recognized, and the use of a particular procedure is dictated by the type of study, subject animal, available facilities, time, manpower, and information desired. Methods for quantifying the diet sample obtained by an analytical procedure have been generalized as visual appraisal, manual separation, microhistological techniques, and microscope point techniques (I’heurer et al. 1976). Of these general categories, visual appraisal and manual separation are limited by the ability of the researcher to quantify individual species in the sample diet. Hence, these methods may contain large inherent personal errors. The microscope point techniques require statistical regression procedures, determination of species weight/volume relationships, and overestimation correction tactors. The microhistological techniques have become the most favored method. Sparks and Malechek (1968) have outlined the basic procedure for this method. However, its accuracy in determining dry-weight composition of masticated forage samples was not examined. The objective of this study was to examine two central assumptions of the microhistological technique and determine if these assumptions were satisfied when determining dry-weight composition of masticated forage samples from blue grama (Bouteloua grucilis) foothill mngeland. These assumptions were that percent frequency can be used to estimate particle density and the percent of identifiable material is constant for each species. Methods Samples were collected from esophageal-fistulated steers grazing native shortgrass rangeland in south-central New Mexico. The study area was located on the Fort Stanton Experimental Ranch. The blue grama-dominated rangeland has been described by Allison et al. ( 1977). Dietary sampling was conducted during 1975 and 1976. Three periods representing three different plant growth stages+arly summer, late summer, and mid winter-were selected for study. Handling and slide preparation of sample material as well as calculations for frequency, particle density, relative density, and percent dry-weight followed the procedure outlined by Sparks and Malechek (1968). Five slides were prepared for each sample period, and 20 systematically located fields were sampled for particle frequency and density on each slide to give 100 fields for each period. Reference material for all species present was collected during late summer. Reference slides, drawings, and photographs were utilized in identifying species within sample diets. When a particle observed in a field could not be identified, the slide was moved so that identification was improved by observing the entire particle.