Fertilizer N recovery is sometimes greater for NO3–N than for NH4–N forms by no-till wheat (Triticum aestivum L.) in the semiarid northern Great Plains and is perhaps affected by preferential N immobilization. This study was conducted to determine whether fertilizer N immobilization (FNI) was affected by N form (NH4–N vs. NO3–N) and glucose-C availability. Laboratory incubation (26 d, 10°C) was conducted on a Tamaneen clay loam (fine, smectitic, frigid Typic Agriustolls) collected from a field site where greater recovery of NO3–N compared with NH4–N fertilizer forms (i.e., NaNO3 vs. urea) had been previously observed by wheat. Soil (20 g) was treated with 2 mg of N as 15N-labeled NaNO3 and (NH4)2SO4 in factorial combination with a gradient of 0, 10, or 100 mg of glucose-C. Inorganic N (NH4–N + NO3–N) concentrations were comparatively stable over the 26-d incubation without C but exhibited a biphasic time response with C, falling rapidly during the first phase to a lower step second phase. Overall, inorganic N concentrations were lower for (NH4)2SO4 than NaNO3. Fertilizer N immobilization vs. incubation-day relationships conformed to an exponential rise to maximum functions. At 26 d, FNI was equivalent to 0.87, 27.6, and 65.9% for NaNO3 and 8.8, 41.2, and 65.3% for (NH4)2SO4 at 0, 10, and 100 mg glucose-C, respectively. Fertilizer N immobilization ratios, FNI(NH4)2SO4/FNINaNO3, averaged (1–26 d) 10.7, 1.8, and 1.5 for 0, 10, and 100 mg glucose-C, respectively. This study found that NH4–N was preferentially immobilized over NO3–N where available C was limited and likely contributing to greater fertilizer N recovery of NO3–N forms by wheat.

Recently completed NH 3 volatilization field studies with urea suggest the urease inhibitor N ‐( n ‐butyl) thiophosphoric triamide (NBPT) has prolonged activity in alkaline relative to acidic soils. A laboratory incubation experiment was conducted at two temperatures (0.5 and 20°C) to determine whether NBPT degradation was affected by soil pH (alkaline vs. acidic) and to determine the impact on urea hydrolysis. Soil (10 g) from the surface horizon of a loam (pH 5.5) and a silt loam (pH 8.4) was placed in bottles and incubated up to 28 d in constant‐temperature chambers. A third soil was constructed by adding CaCO 3 (0.3 g) to the loam soil (pH 8.2). Urea (20 mg) with and without NBPT (20 mg) was applied to each soil. High‐performance liquid chromatography–mass spectrometry analysis of KCl extracts revealed that NBPT degraded rapidly in the acidic soil compared with the alkaline soils (silt loam and loam with added CaCO 3 ). Exponential decay constants for the acidic soil were 5.2 and 3.9 times larger than decay constants for the alkaline soils at 0.5 and 20°C, respectively. Urea hydrolysis rates were reduced by NBPT, and the response was greater for the alkaline soils ( P < 0.0001). Hydrolysis was inhibited by 17.0 and 86.2% at 20°C and by 53.3 and 92.1% at 0.5°C for the acidic and alkaline soils, respectively. This study confirmed field observations that NBPT persistence and activity is greater in alkaline soils. Additional studies, including chemical hydrolysis and sorption, are needed to clarify the processes responsible for the rapid disappearance of NBPT from extracts of acidic soils.

Urea is frequently surface applied to winter wheat (Triticum aestivum L.) during cold weather months (October–April) in the semiarid northern Great Plains. This study was conducted to quantify NH3 volatilization loss from surface-applied urea (100 kg N ha−1) and urea amended with N-(n-butyl) thiophosphoric triamide (NBPT) during this period. Ammonia emissions were quantified by a micrometeorological integrated horizontal flux method with samplers placed on a mast in the center of circular plots (20-m radius). Cumulative NH3 losses from urea varied but averaged 20.5% of applied N across 12 trials. The largest losses (30–44% of applied N) occurred after urea was applied to high-water-content soil surfaces, followed by a period of slow drying with little or no precipitation. Emissions occurred for a prolonged period often lasting >42 d. Periods (1–2 wk) of high NH3 flux (>30 g N ha−1 h−1) frequently occurred when mean daily soil temperatures (1-cm depth) were –2 to 5°C. In one trial, 24.3% of applied N was lost after urea was applied to a 140-mm snowpack. Ammonia losses were moderated by applying urea to dry soil surfaces. If precipitation events that followed were light (<8 mm), losses were reduced to 10 to 20% of applied N. If the events were heavy (>18 mm), then losses were <10%. Coating urea with NBPT (1 g kg−1) reduced cumulative NH3 losses by 66%. Volatilization protection lasted 2 to 3 wk on acidic soils, and >7 wk on an alkaline soil. This study demonstrated that significant NH3 losses from surface-applied urea could occur during cold weather months.

Urea placement in band or nests has been shown to enhance N use efficiency, but limited work has been done to assess its affect on N(2)O emissions. This study compared N(2)O emissions from urea prills applied to an Amsterdam silt loam (fine-silty, mixed, superactive, frigid Typic Haplustolls) using broadcast, band, and nest placements. Experiments were conducted in greenhouse pots (200 kg N ha(-1)) and in canola (Brassica rapa L.) seeded fields using rates of 100 kg N ha(-1) (recommended) and 200 kg N ha(-1). Urea placement affected N(2)O emission patterns and cumulative N(2)O losses in the greenhouse and field. Urea prills placed in nests, and sometimes bands delayed N(2)O production with peak flux activity occurring later, and elevated emission activity being more prolonged than for broadcast applications. Differences were more obvious at 200 kg N ha(-1). These effects were attributed to a delay in urea hydrolysis and inhibition of nitrification. The fraction of applied urea-N lost as N(2)O for broadcast, band, and nest placements applied at the recommended rate averaged 2.0, 2.7, and 5.8 g N kg(-1) N, respectively. The fraction of applied urea-N lost as N(2)O averaged 2.9, 10.4, and 9.2 g N kg(-1) N for broadcast, band, and nest placements when urea-N rate was increased from 100 to 200 kg N ha(-1), respectively. Greater N(2)O production with nest placement may in part be due to significant soil NO(2)-N accumulations. Potential benefits to crop fertilizer use efficiency that come with placement of urea in concentrated zones may lead to enhanced N(2)O production.

Field measurements of N2O emissions from soils are limited for cropping systems in the semiarid northern Great Plains (NGP). The objectives were to develop N2O emission-time profiles for cropping systems in the semiarid NGP, define important periods of loss, determine the impact of best management practices on N2O losses, and estimate direct N fertilizer-induced emissions (FIE). No-till (NT) wheat (Triticum Aestivum L.)-fallow, wheat-wheat, and wheat-pea (Pisum sativum), and conventional till (CT) wheat-fallow, all with three N regimes (200 and 100 kg N ha(-1) available N, unfertilized control); plus a perennial grass-alfalfa (Medicago sativa L.) system were sampled over 2 yr using vented chambers. Cumulative 2-yr N2O emissions were modest in contrast to reports from more humid regions. Greatest N2O flux activity occurred following urea-N fertilization (10-wk) and during freeze-thaw cycles. Together these periods comprised up to 84% of the 2-yr total. Nitrification was probably the dominant process responsible for N2O emissions during the post-N fertilization period, while denitrification was more important during freeze-thaw cycles. Cumulative 2-yr N2O-N losses from fertilized regimes were greater for wheat-wheat (1.31 kg N ha(-1)) than wheat-fallow (CT and NT) (0.48 kg N ha(-1)), and wheat-pea (0.71 kg N ha(-1)) due to an additional N fertilization event. Cumulative losses from unfertilized cropping systems were not different from perennial grass-alfalfa (0.28 kg N ha(-1)). Tillage did not affect N2O losses for the wheat-fallow systems. Mean FIE level was equivalent to 0.26% of applied N, and considerably below the Intergovernmental Panel on Climate Change mean default value (1.25%).

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 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 4 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 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.

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.

Grazers ingest seeds of invasive forbs and may contribute to their spread by depositing viable seeds in uninfested areas. Some mature seed pass through the gastrointestinal (GI) tract of ruminants, but grazers consume flowerheads of invasive species from anthesis to dehiscence. We collected seed from the Eurasian leafy spurge (Euphorbia esula L.) at 3 stages of maturity (soft dough, hard dough, mature). With seed collected from these different stages, our objectives were to determine effects of 1) rate of passage through the GI tract of sheep on leafy spurge seed recovery, germinability and viability, 2) residence time in sheep rumen on seed germinability and viability, and 3) acid pepsin digestion, simulating the lower GI tract, on seed germinability and viability after different residence times in the rumen. More seed from the later stages of maturity were recovered in the manure. The greatest number of seed recovered only represented 3.9% of the number of ingested seed. Few seeds were recovered after day 4. Soft dough seed in manure would not germinate and was not viable, whereas hard dough and mature seed collected from manure during the first 4 day were viable. Pepsin had a slight effect on the number of mature seed recovered, but eliminated viability of recovered seed. Viability of non-pepsin treated seed from the hard dough and mature stages declined with greater residence time in the rumen. Thus, managers should be aware that livestock ingesting hard dough as well as mature seed may be dispersing viable weed seed.

Large ungulates grazing native rangelands during winter must balance forage intake, a source of energy gain, with minimizing thermal stress, a source of energy loss. Another source of energy loss, energy expended, is related to time spent in different activities. Natural or man-made shelter may alter time spent in different activities. Our objectives were to determine if access to windbreaks altered diurnal activity patterns of beef cattle grazing a windy, foothill range site during winter, and if diurnal activity patterns could be related to weather. We observed diurnal activity patterns of cattle with and with out access to windbreaks (windbreak, nonwindbreak) for two winters on an exposed pasture in southwest Montana. Actual use of windbreaks was only recorded in Winter 2. Windbreak cattle did not use the windbreak until day 16; subsequently, time spent behind windbreaks ranged from 0 to 30%. Activity patterns of windbreak and nonwindbreak cattle differed in subtle ways, indicating they used similar behaviors to minimize energy expended and to maximize energy gain. Time spent grazing was inconsistent between the two winter trials, possibly reflecting large differences in body condition when they entered each winter. For individual groups, time spent grazing (Winter 1, 33–96%; Winter 2, 18–94%) and standing (Winter 1, 0–63%; Winter 2, 0–78%) varied widely on a day-to-day basis, reflecting either an immediate response to that day's weather, or possibly a compensatory response to the previous day’(s’) weather, especially following cold, windy days. Wind velocity had minimal effect on grazing time, presumably because high wind velocities were associated with relatively warm days, or the animals were in sufficient condition to tolerate high wind velocities. Instead of minimizing energy expended by lying down during extreme cold, cattle spent more time standing, which maximizes heat gain from solar radiation. During winter, cattle behavior is a tradeoff between maximizing energy gain (thermal and food) and minimizing energy loss (thermal and metabolic). Key words: Activity, behavior, cattle, shelter, thermal, weather

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 (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.

Cattle often graze foothill rangelands in northern latitudes as a cost-effective alternative to being fed hay. In a 2-yr study, we determined whether exposure to winter wind elicits a stress response in pregnant cows (5–7 yr old) grazing a foothill rangeland. We measured several indicators of stress-response of cows which had (windbreak) or did not have (non-windbreak) access to windbreaks. In Winter 1 (1966–1997), backfat thickness increased on windbreak and non-wind-break cattle. Body condition scores of both groups were lower at the end of the trial. In Winter 2 (1997–1998), backfat thickness decreased for windbreak and non-windbreak cattle. Body condition scores did not change. Weight changes were minor both winters. Changes in backfat thickness indicated that cattle in Winter 1 were catabolizing more lean muscle tissue, whereas cattle in Winter 2 were catabolizing more fat. Cattle entered Winter 1 in lower body condition and mass than in Winter 2. Cattle with access to windbreaks tended to have a stronger cell-mediated immune response than non-windbreak cattle. Birth and weaning weights of calves, and calving interval were similar for windbreak and non-windbreak cattle. Neither winter elicited a strong stress response from cattle grazing this foothill rangeland, with or without access to windbreaks. Key words: Beef cattle, backfat, body condition, weight loss, immune response

Nondestructive radiative transfer and canopy volume methods were compared with the destructive hand-clipping method to determine forage structure and phytomass. On a native range site, fifteen 1-m2 circular plots were located at each of five microsites. On a crested wheatgrass site, thirty 1-m2 plots were located in grazed and in ungrazed areas. At peak standing crop, all plots were measured with a LI-COR Plant Canopy Analyzer to determine leaf area index (LAI), diffuse non-intercepted radiation (DNIR), and mean tilt angle (MTA) of leaves. Then, plants within plots were measured with a ruler to determine volume. Finally, all phytomass within plots was harvested. At the native range site, plant volume was related with LAI and DNIR on four of five microsites. Phytomass was related with LAI and DNIR on two microsites. At the crested wheatgrass site, volume and phytomass were related with LAI, DNIR, and MTA on grazed plots. Only phytomass was related with LAI and DNIR on ungrazed plots. The Plant Canopy Analyzer measures canopy structure and phytomass; it is fast, and its data are transferred directly to a computer. Measuring plant volume is inexpensive and requires minimal training. Determining phytomass by clipping is accurate and requires minimal training, but it is time-consuming and destructive. Key words: Leaf area, canopy, volume, phytomass, radiative transfer

Weeds increase their dominance in a grazed plant community by avoiding herbivory and (or) by tolerating herbivory more than neighbouring plants. After defoliation, allocating carbon to shoots at the expense of roots may confer tolerance. We determined carbon allocation patterns of undefoliated and recently defoliated (75% clipping level) plants of the invasive leafy spurge (Euphorbia esula L.) growing with alfalfa (Medicago sativa L.), Kentucky bluegrass (Poa pratensis L.), or Idaho fescue (Festuca idahoensis Elmer). Plants were labeled with 13 CO 2 24 h after clipping to determine allocation patterns; all plants had equal access to the 13 CO 2 . Based on relative distribution of 13 C, defoliation did not affect the amount of carbon allocated to roots of E. esula. The amount of carbon allocated to shoots of E. esula was higher when growing with P. pratensis than when growing with the other species. Based on relative enrichment of 13 C, defoliation increased sink strength of remaining shoots on defoliated E. esula plants. Conversely, roots of unclipped E. esula plants were stronger sinks for carbon than roots of clipped plants. Even though defoliation increased "sink strength" of remaining shoots of E. esula, the amount of carbon allocated to the root system was unaffected by defoliation, suggesting that uninterrupted allocation of carbon to its extensive root system, not increased allocation to its shoot system, confers grazing tolerance.

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.

Oxeye daisy has invaded seeded pastures, roadsides, and mountain rangelands in western Montana. In 1990, we began a study to: (1) determine use of oxeye daisy and introduced perennial grasses by cattle; (2) determine effects of intensive cattle grazing on the number of oxeye daisy seeds in the soil; and (3) assess effects of intensive grazing on year-to-year changes in oxeye daisy and associated perennial grasses. Cattle grazed oxeye daisy but much of their impact was from trampling or removing stems. The number of oxeye daisy seeds in the soil seedbank was lower in 1992 than in 1990 in grazed areas, whereas the number was higher in ungrazed areas. Two years of intensive grazing reduced densities of oxeye daisy seedlings and rosettes, but did not change densities of mature stems. Intensive grazing had minimal impact on the introduced grasses.

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.

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-duly, 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 cm(2)) than ungrazed areas (4.0 cm(2)), 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 (Pea 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.

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 non-structural 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.