Document Type

Thesis - University Access Only

Award Date

1998

Degree Name

Master of Science (MS)

Department / School

Animal Science

Abstract

A study was conducted to evaluate the effects of grazing history (timing, frequency and severity) on survival and biomass production of individual western wheatgrass [Pascopyrum (Agropyron) smithii (Rydb.) A. Love] parent tillers during the grazing season, and on number, survival and biomass production of daughter tillers (individual root-shoot systems arising from buds at the parent tiller base) the subsequent growing season. The study was conducted at the Cottonwood Range and Livestock Research Station in western South Dakota. Data were collected on 1382 individual parent tillers during the grazing season of 1996, and on 1378 daughter tillers generated by those parent tillers during the 1997 growing season. Four treatment combinations [high and low stocking rates (1 .67 AUM/ha and 0.67 AUM/ha, respectively) with high and low stocking densities (short duration and season-long grazing, respectively)] were utilized to acquire a range in values of the grazing history variables. High percents (>40%) of biomass removed during the initial grazing of a tiller resulted in a greater percentage of parent tillers that did not survive. Grazing early in the growing season was also more detrimental than later. Delaying initial grazing until late in the season resulted in biomass production during the year of grazing similar to production on ungrazed tillers. Earlier grazing significantly lowered that year's biomass production. Grazing that was delayed until late in the season also resulted in the greatest number of daughter tillers and greatest total production in the following growing season. Grazed tillers were more likely to generate daughter tillers, and delayed initial grazing, multiple grazings and high percent biomass removed (>40%) result in a greater likelihood of parent tillers to generate multiple daughter tillers. Daughter tillers were more likely to survive throughout the 1997 growing season if the parent tiller was initially grazed in mid-season or the severity of grazing was low (<=0.02 g) or high (>0.055 g). Low or high severity of grazing also resulted in the greatest daughter tiller biomass. Theories on which specialized grazing systems are based focus on reduced frequency and severity of grazing on individual tillers so as not to decrease plant vigor. When measuring plant vigor based on persistence in the plant community, frequent and/or severe grazing may not be as great an issue as had been thought, especially when moisture is not limiting. Production from rhizomes was not evaluated in this study, but research indicates that rhizome production may be compromised with grazing (Stroud et al. 1985; Demer and Briske, unpublished data). This implies that overall number of new tillers generated may not differ between grazed and ungrazed tillers of western wheatgrass. Timing is a critical factor, with delayed initial grazing allowing more daughter tiller generation and greater biomass production the year of grazing and the subsequent growing season. The above-average precipitation conditions during this study make long-term interpretation of these results difficult. During dry years, the production of multiple daughter tillers may be a considerable disadvantage through increased competition between "siblings" for limited moisture.

Library of Congress Subject Headings

Grazing

Wheatgrasses

Wheatgrasses -- Reproduction

Format

application/pdf

Number of Pages

129

Publisher

South Dakota State University

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