Invasive Species
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The widespread abundance of cheatgrass on the foothill and semidesert ranges of the Intermountain Great Basin region causes concern to range managers of this area. During the past 20 years, crested wheatgrass has been used to replace cheatgrass on some of the more favorable sites. However, many soils supporting extensive areas of cheatgrass are so low in productivity it is believed that they should be managed as an annual grass type and used for the most effective purpose which appears to be spring range for livestock.
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Yearling cattle gained weight satisfactorily on cheatgrass range under rotational (moderate) and continuous (moderate and heavy) grazing systems during a 3-year study. This study was designed to determine effects of these systems on the rangeland-not on individual plant species. Assignment of these systems to different pastures each year precluded evaluation of long-term vegetal response to the treatments. Weight gain was greatest in late spring. Grazing capacity of the range and cattle gain per acre increased through the summer, then declined. Yearly variation in production of forage and beef was apparently due to weather. Grazing capacity and beef production increased under continuous heavy grazing, but possible vegetation changes not evaluated in this study make heavy grazing undesirable.
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Comparisons were made of grazing capacities and sheep gains on both cheatgrass range and on native bunchgrass range during early spring. Capacities were similar on these two range types in wet years, but were almost 60% greater on bunchgrasss range during dry years. Sheep gained an average of 0.32 lb. per head per day over three spring periods on the two vegetation types. In 2 of the 4 study years, they lost weight during the first 2-week grazing period. Sheep gains per acre were similar on the two range types.
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Grazing management can be complicated with very sophisticated grazing system designs, but in this document we discuss a simple method for managing livestock to control annual grasses while allowing perennial grasses to reoccupy the sites and generating more animal production. It’s called “Green and Brown” grazing to manage annual grasses: graze when invasive annual grasses are green and desired
species are brown. This strategy is also known as time-controlled, short-duration, high-intensity grazing.
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Determining the ecologic and economic importance of cheatgrass is a challenge to range research. This poorly understood species produces a large volume of herbage over extensive acreages and is undoubtedly the most important forage plant on southern Idaho ranges. Its probable effect upon grazing, watershed, wildlife, recreation, and timber production further adds to its importance. Many additional facts are needed before we can fully determine the importance and the desirability of this newcomer. If cheatgrass is found to be desirable a program for its proper management should be developed. Should it be found undesirable, methods of replacement must be found. The first and most important immediate problem now facing research is finding out how cheatgrass ranges might be managed to hold the soil in place and to maintain soil productivity.
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In the more open Pinus ponderosa and Pseudotsuga menziesii forests, cheatgrass colonization may often occur in openings in the understory alone. Colonization in the more shady A. grandis and Thuja plicata forests is unlikely, however, unless the opening extends through both the understory and the overstory. As a result, cheatgrass is unlikely to increase in any of these forests unless the scale and incidence of disturbance increases substantially.
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Proper grazing management appears urgent in preventing or delaying further encroachment of cheatgrass (Bromus tectorum) into perennial vegetation on western grazing lands and subsequently gaining site dominance. In mixed stands with desirable perennials, livestock grazing might be directed to (1) reducing cheatgrass competition by concentrating grazing of cheatgrass during dough seed stage, providing perennials still have opportunity to complete their life cycles, or (2) basing grazing on the needs of perennials while mostly ignoring cheatgrass. When cheatgrass domination results in a closed community, alternatives appear limited to (1) managing as annual grassland, or (2) revegetation using intensive cultural practices. In conjunction with revegetation, livestock might conceivably be employed for “graze out” in site preparation and I or for selective plant control during germination and emergence of the seeded perennials. However, with these possible limited exceptions, grazing is concluded not to be an effective general tool for cheatgrass control.
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This paper provides a historical perspective of the influence of cheatgrass invasion on western rangelands (1930-1950). This was a period of awakening interest by range scientists. Range managers, the livestock industry, and scientists have always bad a love-bate relationship with cheatgrass. It provides the bulk of the forage on many ranges, yet it is the symbol of environmental degradation. Trying to cope with the endless ramifications of cheatgrass invasion, dominance, persistence, and potential community decline keep forcing scientists to critically evaluate the ecological principles upon which range management is based.
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The range livestock industry has adapted to the presence of invasive annual grasses, especially where infestations are extensive. Light to moderate livestock grazing provides enough standing grass at the end of the grazing season to limit soil erosion and conserve soil moisture and nutrients. Elsewhere, however, where infestations of invasive annual grasses are less extensive or less advanced, opportunities exist for using targeted, or prescribed, livestock grazing to suppress annual grass plants. This chapter focuses on using prescribed livestock grazing to suppress invasive annual grasses on sites where these grasses are considered weedy invaders.
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Although this study helped pinpoint optimal defoliation parameters for cheatgrass control, it also called into question the potential for livestock grazing to be an effective seed-bed preparation technique in native plant reseeding projects in cheatgrass-dominated areas.