Roseann T. Wallander

Effects of livestock grazing on rangeland grasshopper (Orthoptera: Acrididae) abundance

Livestock may impact habitat quality for grasshoppers by reducing food availability and by altering microclimate and potential oviposition sites. A 5-year study was conducted to create consistent grazing impacts on replicated plots and measure their effects on plant cover, microclimate, and grasshopper abundance. Cattle were used to produce two levels of grazing intensity that were compared to ungrazed controls. Differences in plant cover were greatest immediately after grazing each summer, grasshopper microhabitats tending to be shadier, cooler, less windy, and more humid in the ungrazed plots. The grasshopper assemblage included five of the worst pest grasshopper species in North America: Ageneotettix deorum, Aulocara elliotti, Melanoplus sanguinipes, M. packardii, and Camnula pellucida. Most species had greater abundance on ungrazed pastures, particularly during the 4–6 weeks after grazing each year. However, A. elliotti was often more abundant in heavily grazed areas early in the year when early instars were present and in late summer when adults were predominant. There was no strong evidence that the effect of grazing on grasshopper abundance increased over the 5-year study. At this time, all changes in grasshopper numbers cannot be directly attributed to particular habitat characteristics that changed after grazing, but the results suggest that grazing management could be used to reduce pest grasshopper densities.

Effects of invasive forb litter on seed germination, seedling growth and survival

Summary Two Eurasian forbs, Euphorbia esula L. and Centaurea maculosa Lam., continue to spread in the northwestern United States despite extensive and expensive control efforts. We investigated whether litter from these forbs and associated soils suppress germination and growth of two native perennial grasses (Pseudoroegneria spicata [Scribn. & Smith A. Love], Pascopyrum smithii [Rydb.] A. Love), which may partly explain the success of these invasive forbs. Seed germination was unaffected, but roots were shorter at higher leachate concentrations. The physical presence of litter (Euphorbia, Centaurea, none) did not affect seed germination or number of leaves of seedlings of the four species, but it affected seedling heights; the tallest seedlings were those covered with Centaurea litter; the shortest were those without litter. As a mechanical barrier, litter type did not affect survival or number of leaves of pregerminated seeds, but litter inhibited shoot growth. Seed germination of the four species on soils from infested or noninfested areas differed; Euphorbia germination, albeit low ( Die Ausbreitung zweier Unkrauter eurasiatischer Herkunft (Euphorbia esula L. und Centaurea maculosa Lam.) in den nordwestlichen USA konnte trotz umfangreicher Gegenmasnahmen bisher nicht aufgehalten werden. Wir untersuchten, ob diese Krauter durch Streuproduktion oder andere Bodenveranderungen eine negative Wirkung auf Keimfahigkeit und Wachstum zweier heimischer mehrjahriger Graser (Pseudoroegneria spicata [Scribn. & Smith A. Love], Pascopyrum smithii [Rydb.] A. Love) ausuben, was zum Teil den Erfolg dieser Unkrauter erklaren konnte. Saataufgang war nicht beeintrachtigt, aber Wurzeln waren kurzer und wiesen hohere Exsudatkonzentrationen auf. Die pure Anwesenheit von Euphorbia- oder Centaurea streu hatte keinen Effekt auf Auflauf oder Blattanzahl der Samlinge aller vier Arten, sowohl auf Boden aus verunkrauteten Arealen als auch auf vorher unkrautfreien Boden. Streu beeinflusste aber die Samlingshohe: die hochsten Samlinge traten in Verbindung mit Centaurea Streu auf, die niedrigsten unter streulosen Bedingungen. Die Streuart hatten keinen Einfluss auf Uberlebensfahigkeit oder Blattzahl vorgekeimter Samen, jedoch beeintrachtigte das Vorhandensein von Streu die Sprosslange. Die Keimfahigkeit der vier Arten hing davon ab, ob die Boden aus verunkrauteten oder unkrautfreien Gebieten stammten. Die Euphorbia-Keimrate, obwohl generell niedrig (

Spotted knapweed seed viability after passing through sheep and mule deer.

Spotted knapweed (Centaurea maculosa Lam.), an introduced perennial plant, has invaded large areas of rangeland in the northwestern United States. Grazing animals may disseminate the weed by transporting seeds in their digestive system and depositing them in their feces. In this study percent viability and emergence of spotted knapweed seeds that passed through mule deer (Odocoileus hemionus hemionus) and sheep (Ovis aries) were determined. Percent viability included seeds that germinated and seeds that tested positive with tetrazolium. In the first trial, we pulse dosed 3 mule deer and 4 ewes with 5,000 spotted knapweed seeds each. Seed recovered from manure collected daily for 10 (days after dosing was tested for percent viability. We recovered 11% of the knapweed seeds from the 3 mule deer, and 4% from the sheep. Based on high variability in (0 to 26%) percent viability of recovered seed, we thought that our drying the manure at a 50 degrees C may have killed some of the spotted knapweed embryos. To determine if drying at 50 degrees C affected viability, we pulse dosed 41 rams with 5,000 spotted knapweed seeds each in a second trial. One subsample of manure was washed the same day to recover seeds and then dried at 35 degrees C, a second subsample was dried at d 50 degrees C, washed, and then dried at 35 degrees C. We recovered 17% of the spotted knapweed seeds from the 4 rams. No viable seeds were recovered from manure heated at 50 degrees C, and no viable seeds were recovered more than 2 days after dosing. Percent viability of seeds recovered from manure dried at 35 degrees C ranged from 0 to 22%. In both trials, percent viability of recovered seeds was lower compared with seeds that did not pass through animals. Sheep and mule deer can ingest, transport, and disseminate viable seeds of spotted knapweed in their feces.

Effects of sheep grazing on a spotted knapweed-infested Idaho fescue community.

Spotted knapweed (Centaurea Maculosa Lam.), a Eurasian perennial forb, is replacing many native perennial grasses, such as Idaho fescue (Festuca idahoensis Elmer.), in foothills of the Northern Rocky Mountain region. Our objective was to determine if 3 summers of repeated sheep grazing would reduce spotted knapweed without impacting the dominant, associated native perennial grass. Each summer, small pastures were grazed for 1-7 days in mid-June, mid-July, and early September. Areas repeatedly grazed by sheep had lower densities of seedlings, rosettes, and mature spotted knapweed plants than ungrazed areas. In addition, the proportion of young plants in the population was less in grazed than ungrazed areas. Basal areas of spotted knapweed plants were greater in grazed (8.2 cm2) than ungrazed areas (4.0 cm2). There were fewer spotted knapweed seeds in soil samples from grazed areas (12 seeds m-2) than from ungrazed (49 seeds m-2). Idaho fescue plant density increased 40% in grazed areas from 1991 to 1994, but leaves and flower stems on these plants were 38% and 17% shorter, respectively, than in ungrazed areas. By 1994, frequency of Kentucky bluegrass (Poa pratensis L.) was 35% greater in grazed than ungrazed areas. Grazing did not alter the amount of litter; however the amount of bare soil increased from 2.2 to 5.6% in grazed areas, while it decreased from 4 to 1% in ungrazed areas. Three summers of repeated sheep grazing negatively impacted spotted knapweed, but minimally affected the native grass community. A long term commitment to repeated sheep grazing may slow the rate of increase of spotted knapweed in native plant communities.

Recovery of leafy spurge seed from sheep

Sheep are often used to graze North American rangelands infested with leafy spurge (Euphorbia esula L.), a long-lived perennial forb from Eurasia. Our objective was to determine if sheep grazing infested rangelands disperse leafy spurge seed by transport in their fleece or by depositing seeds in their feces. Twenty-four yearling Targhee ewes grazed a 2.4 ha native bunchgrass range site infested with leafy spurge from late-May through mid-August of 1993 and 1994. Six of the 24 ewes were shorn in October 1993. To recover leafy spurge seeds from those fleeces, we used a standard method to test wool for vegetable matter. On average, 38 seeds were recovered per fleece. During these summers, 6 small groups (n = 4 sheep per group) each grazed 3 separate paddocks. We estimated the density of leafy spurge seed before the groups were moved into 1 of 3 paddocks. After the sheep were moved into a paddock (day 0), we collected fresh feces from each group on or about day 4, 10, and 14. Feces were then washed over sieves to recover leafy spurge seeds. All seeds were tested for germinability and viability. The number of viable seeds excreted daily per ewe was estimated. In 1993, 1,796 +/- 405 (S.E.) leafy spurge seeds m-2 were produced in the field, whereas in 1994, 399 +/- 63 (S.E.) leafy spurge seeds m-2 were produced. The summer of 1994 was much drier than the summer of 1993. We estimated that 41 to 144 leafy spurge seeds were excreted daily per animal in mid-July 1993. Viability of seeds in the feces averaged 5%, whereas viability of seeds collected from seed stalks was 42%. We estimated that the ewes excreted from 2 to 41 leafy spurge seeds daily at the peak in mid-July 1994. Viability of seeds excreted during 1994 averaged 24%, whereas viability of seeds collected from seed stalks was 68%. Sheep can pick up leafy spurge seed in their fleece, and will consume and pass viable seed. However, viability of seed recovered from feces was highly variable and almost always lower than seed collected in the field. Despite reduced seed numbers and viability, sheep have the potential to spread leafy spurge and should be managed accordingly.

Effect of previous experience on sheep grazing leafy spurge

Herbivores develop preferences for certain plant species in several ways. One way is to learn from role models, another is through postingestive feedback. In this study we determined (1) whether yearling sheep exposed to leafy spurge (Euphorbia esula L.) as lambs graze it more readily than yearlings that were not exposed to it as lambs, and (2) whether this difference, if present, persists through the grazing season. On a leafy spurge-infested, Idaho fescue (Festuca idahoensis Elmer) range site, we compared the grazing behavior of naive groups of Targhee yearlings with grazing behavior of experienced groups for 3 years. Using focal animals, we assessed diet selectivity by counting bites and by determining time spent grazing different forage types. During the grazing season, nutritive value (crude protein, neutral detergent fiber, in vitro dry matter digestibility) of leafy spurge was always higher than nutritive value of Idaho fescue. Experienced yearlings spent more than four times as much time grazing leafy spurge in early summer compared with naive yearlings (averaged across 1992 and 1993, experienced 13.6% vs. naive 2.9%), whereas in mid- (39% vs. 31%) and late summer (37% vs. 38%) both groups spent similar amounts of time grazing leafy spurge. Neither group readily grazed the plant in early summer. Experienced yearlings had higher bite rates of leafy spurge than naive yearlings during the early summer sampling periods in 1992 and 1993 (averaged across 1992 and 1993, 15.9 vs. 5.0 bites min−1) but their bite rates were similar by mid-summer (22.7 vs. 20.6 bites min−1). In 1994, we observed their grazing behavior every 5 days for 35 days. Initially, naive yearlings spent less time grazing leafy spurge than experienced yearlings, but were spending similar amounts of time grazing leafy spurge by day 25. Previous experience with leafy spurge, compared with no experience, resulted in a slight but short-lived advantage in early summer use of the species. Within 3–4 weeks, sheep that had never previously been exposed to leafy spurge readily grazed the plant.

Sheep grazing spotted knapweed and Idaho fescue.

Spotted knapweed (Centaurea maculosa Lam.), an Eurasian perennial forb, is replacing many native perennial grasses, such as Idaho fescue (Festuca idahoensis Elmer.), throughout the Northern Rocky Mountain region. Our objective was to determine sheep use of spotted knapweed and Idaho fescue during 3 consecutive summers (1991-1993). Each summer, 3 small spotted knapweed infested pastures were grazed for 5-8 days in mid-June, 2-6 days in mid-July, and 1-6 days in early September. Nutritive value of spotted knapweed leaves and flowerheads were consistently higher than of Idaho fescue. Nutritive value for both species declined as the summer progressed. The sheep readily grazed spotted knapweed, but they also grazed other plants, including the native Idaho fescue. They did not consistently graze 1 species more than another, which may have reflected daily weather patterns, slight differences in forage nutritive value, or cyclic grazing patterns which are often associated with plants containing secondary compounds, such as spotted knapweed. At the end of many grazing periods, heights of grazed spotted knapweed plants were greater than those of Idaho fescue, which reflected how the sheep grazed leaves and avoided fibrous stems of mature spotted knapweed plants, whereas they were not selective when grazing Idaho fescue. Although the sheep did not graze spotted knapweed exclusively, probably because animals seek diverse diets, their use of this noxious weed may help restore a balance in competitive relations between this noxious weed and native grasses. DOI:10.2458/azu_jrm_v54i1_olson

Effect of sheep grazing on a leafy spurge-infested Idaho fescue community

Leafy spurge (Euphorbia esula L), an aggressive Eurasian forb, is invading native upland ranges dominated by bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love) and Idaho fescue (Festuca idahoensis Elmer) in the Northern Rocky Mountain Province. Our objective was to determine cumulative effects of 3 summers (1992-1994) of repeated sheep grazing on a leafy spurge infested-Idaho fescue community. Targhee ewes were systematically rotated through paddocks in small pastures. Leafy spurge stem and Idaho fescue and bluebunch wheatgrass plant densities, and frequencies of other species were determined before grazing in 1992, 1993 and 1994, and in 1995, 9 months after grazing ended. Number of viable leafy spurge seeds in seedbanks was determined in 1992 and 1995. Plant heights were measured each year. Density of leafy spurge seedlings was low in grazed and ungrazed areas in 1992 and 1994, whereas density was higher in ungrazed than grazed areas in 1993 and 1995. Grazing did not increase or decrease density of mature leafy spurge stems from 1992 to 1995. Grazing increased density of Idaho fescue but reduced density of bluebunch wheatgrass. Kentucky bluegrass, (Poa pratensis L.), Sandberg bluegrass (Poa sandbergii Vasey), annual bromegrasses (Bromus spp.), and sedge (Carex spp.) frequencies increased in grazed areas. Number of viable leafy spurge seeds in the seedbank was lower in 1995 than in 1992; this reduction was greater in grazed than ungrazed areas. Three years of repeated sheep grazing reduced numbers of leafy spurge seed in the seedbank and seedling densities, but had no effect on density of mature leafy spurge stems. There was minimal effect on the cool season native grasses, possibly because the site was grazed primarily in midsummer when these grasses are dormant. Thus, a long-term commitment to repeated sheep grazing may help to control leafy spurge, although grazed sites should be monitored regularly to ensure that other undesirable species do not increase at the expense of the native plant community.

Effects of livestock grazing on grasshopper abundance on a native rangeland in Montana.

ABSTRACT Livestock grazing can affect habitat quality for grasshoppers through effects on food and oviposition site availability, microclimate, and other factors. Because of this, some authors have suggested that grazing programs can be used to help manage pest grasshopper populations. In a 6-yr study, we controlled access of cattle to replicated experimental plots on an Agropyron spicatum/Poa sandbergii pasture to create consistent year-to-year differences in postgrazing plant cover, with resultant affects on microclimate. After sampling grasshoppers multiple times after grazing treatments each summer, we found evidence of between-treatment differences in grasshopper abundance for the entire assemblage during 4 of the 6 yr. Some species, including Melanoplus sanguinipes (perhaps the worse rangeland grasshopper pest in the western United States), tended to be more abundant on ungrazed plots, whereas Melanoplus gladstoni often had greater densities on heavily-grazed plots. The effect of grazing on grasshopper densities in this study was lower in magnitude and less consistent among years than in a study we conducted simultaneously at a nearby site where the vegetation was dominated by the exotic species crested wheatgrass (Agropyron cristatum). Our results generally support proposals that grazing could be used to reduce pest grasshopper densities, although the effectiveness of a particular grazing scheme may vary among sites, years, and grasshopper and vegetation assemblages.

Biomass and carbohydrates of spotted knapweed and Idaho fescue after repeated grazing.

Spotted knapweed (Centaurea maculosa Lam.), an aggressive Eurasian forb, is replacing many native perennial grasses such as Idaho fescue (Festuca idahoensis Elmer.) on foothills of the Northern Rocky Mountains. We assessed biomass allocation, carbohydrate reserves (total nonstructural carbohydrate concentrations - TNC), and carbohydrate pools (TNC X biomass) as indicators of cumulative effects of 3 summers (1991-1993) of repeated sheep grazing on spotted knapweed and Idaho fescue. In early May 1994, we excavated 30 spotted knapweed and Idaho fescue plants previously exposed to repeated grazing and 30 ungrazed plants of each species. On grazed Idaho fescue plants, shoot (P < 0.02) and root (P < 0.06) biomass were 38 and 27% less than on ungrazed plants. In contrast, shoot (P = 0.26) and root biomass (P = 0.85) of grazed and ungrazed spotted knapweed plants were similar. Although grazing resulted in some minor differences in total nonstructural carbohydrate concentrations and carbohydrate pools of shoots, total nonstructural carbohydrate concentrations and pools of crowns and roots were similar for grazed and ungrazed plants of each species. Thus, carbohydrate concentrations or pools were not sensitive indicators of the response of Idaho fescue or spotted knapweed to the cumulative effects of repeated grazing. In contrast, aboveground biomass could be used to indicate the response of Idaho fescue to repeated grazing. By reducing shoot and root biomass of Idaho fescue but not spotted knapweed, repeated grazing may reduce the ability of Idaho fescue to compete with spotted knapweed when both species are grazed.