No abstract

Vesicular‐arbuscular mycorrhizal (VAM) fungi play a role in crop productivity and soil stability in agricultural ecosystems. This role is accentuated under adverse cultural or climatic conditions, such as grazing or drought. The purpose of this study was therefore to assess the effects of stress (grazing) on VAM‐fungal colonization of a major forage species. Changes in root colonization of crested wheatgrass [Agropyron desertorum (Frisch.) Schult.] plants by VAM fungi were measured under three levels of grazing pressure, ranging from no grazing to heavy, continuous grazing. Percent colonization of root length was determined by microscopy, and VAM‐fungal biomass spectroscopically. These findings were related to the changes in root/shoot ratios caused by grazing. The study site is characterized as a Wyoming big sage brush (Artemisia tridentata spp. Wyomingensis) ‐ grassland community on an Abgese loam (fine‐loamy, mixed, mesic Xerollic Haplargid) soil in central Nevada. Plants studied were introduced previously as part of a range improvement program. Colonization of root length declined significantly (P < 0.05) and seasonally (40.2%, June and 50.6%, October) in plants under heavy grazing as compared to ungrazed plants. Spores (60.8% June and 89.5% October) and biomass (35.0% June and 61.5, October) of the VAM fungi also declined with grazing. Colonization of wheatgrass was favored when the amount of photosynthetic tissue relative to root mass was high. Phosphorus concentration in plant tissues was not significantly affected (P < 0.05) by grazing. It is concluded that severe grazing adversely affects the colonization of crested wheatgrass by VAM fungi. This reduction in the fungal symbiont may have an effect on plant nutrition and soil structure and stability which needs further investigation.

No abstract

No abstract

Highlight: Forage sampling using fistulated grazing animals is a generally accepted technique to measure dietary forage quality and botanical composition, but is it a satisfactory technique to evaluate dietary mineral intake? Using a variety of diets which were fed to rumen-fistulated steers, the fistula samples had relatively larger concentrations of ash, Si, Na, P, Zn, and Co (PC 0.05) than did diet samples. Small decreases in the Mg and Ca concentrations of the fistula sample, as well as the small increases in N, K, Mn, Fe, and Mo values, were not generally different from diet concentrations. Regression equations predicting diet-mineral concentrations of all diets, given the concentration in the fistula sample, were accompanied by errors of S to 37% of the true value. Smaller errors can be expected when similar diets like alfalfa hay are used throughout a given study.The use of esophageal or rumen-fistulated animals for collecting forage samples is common in range nutrition. This procedure accommodates the animals' preference for individual plants and certain plant parts. When compared with clipped or hand-plucked forage samples, fistula collected samples often will contain more crude protein and total ash, but less soluble carbohydrates. Fistula samples are also useful in determining intake and botanical composition of the grazing diet. Researchers generally conclude that fistula samples are more representative of forage consumed by the grazing animal than are hand-plucked or clipped samples (Lesperance et al. 1974).Fistula sampling, however, introduces certain biases because of mastication, salivary additions, and rumen epithelial secretions. Additional biases associated with esophageal fistula collections may include leaching of forage nutrients through the screen-bottomed collection bag, incomplete sample collections (Hoehn et al. 1967;and Kiesling et al. 1969), and occasional rumen bolus regurgitation.Forage samples divided for direct chemical analysis and for ingestion by the fistulated animal and retrieved for analysis showed concentration changes in some forage quality parameAuthors are soil scientist, Snake River Conservation Research Center, Kimberly, Idaho 83341; and animal scientist, University of Nevada, Reno 89507.The report is a contribution from the Western Region, Agricultural Research Service, U.S. Department of Agriculture, in cooperation with the University of Nevada Agricultural Experiment Station, Reno.Manuscript received November 17, 1976.ters associated with ingestion (Lesperance et al. 1974). Esophageal-and rumen-fistula samples were higher in fiber and lignin, but lower in nitrogen-free extract (NFE), soluble carbohydrates, and Ca. Fiber and lignin increases and NFE decreases were at least partially attributable to sample preparation. The esophageal-and rumen-fistula samples should also have higher concentrations of ash, N, P, K, Na, and CI than do diet samples because of saliva mineral concentration (McDougall 1948). Mineral concentrations will change because of (1) a concentrating ...