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MATERIALS AND METHODS Grazing animals prefer some plants to others. These choices are Cultivars and Experimental Design likely related to physical and chemical factors such as energy-dense The eight endophyte-free tall fescue cultivars included in carbohydrates contained in plants. This study quantified the nonstructhis study were 'Barcel', 'Kenhy', 'Kentucky-31' (KY-31), tural carbohydrate fractions in each of eight vegetatively growing, 'Missouri-96' (MO-96), 'Mozark', 'Stargrazer', and the experiendophyte-free, tall fescue cultivars (Festuca arundinacea Schreb.) mental selections C-1 and HiMag (Mayland and Sleper, 1993). and relates their sugar concentrations to cattle grazing preferences. On 20 Sept. 1991, these grasses were seeded in rows into The experimental area consisted of eight cultivar plots replicated three an irrigated, Portneuf silt loam loess soil (Durinodic Xeric times in each of three pastures. Within each pasture, forage was Haplocalcid) near Kimberly, in south-central Idaho (42Њ30Ј sampled between 0830 and 1000 h mountain daylight time (MDT) N, 114Њ08Ј W, elevation 1200 m). The experimental design during each of four seasons and 2 yr. Freeze-dried forage samples was a randomized complete block with three pastures (blocks), were extracted with hot water and an amylase (Clarase) solution. three replications nested within blocks, eight entries (main Sugars were quantified colorimetrically using potassium-ferricyanide plots), and six rows per main plot. Rows were 0.56 m apart and glucose-oxidase methods. Cattle grazing preferences among these and main plots were 3.35 by 6.7 m. tall fescue cultivars were related to the concentrations of total nonstructural carbohydrates (TNC) (r 2 ϭ 0.49, P Ͻ 0.05). Other sugar Harvesting and Sample Preparation fractions were not significantly related to grazing preference in this study. The nonstructural carbohydrate concentrations averaged over Vegetatively growing forage was clipped at a stubble height the entire study were glucose, 14; fructose, 5; sucrose, 40; fructan, 23; of 8 cm from randomly located 0.6-m sections of Rows 3 and insoluble starch, 24; and TNC, 129 g kg Ϫ1 . A forage selection criterion 4 in each plot. The forage in Block 1 was clipped on d 131, should include measures of the TNC because of their close relationship 165, 221, and 256 of 1993 and d 129, 164, 220, and 262 of 1994. to animal grazing preference.

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HiMag' tall fescue (Festuca arundinacea Schreb.) was selected for high Mg concentration to reduce grass tetany risk to ruminants, but neither animal preference nor consumption of HiMag were known. The objectives were to evaluate methods of quantifying preference and to determine intake and preference by cattle (Bos taurus L.) of HiMag relative to seven other tall fescues. All entries were free of a fungal endophyte INeotyphodium coenophialum (Morgan-Jones & Gams) Glen, Bacon & Hanna] that reduces cattle performance. The experimental design was a randomized complete block with three replications of eight cultivars nested within each of three test pastures. Six heifers grazed the vegetative to boot-stage pastures for 48 h in May, June, August, and September of 1993 and 1994. The pastures, located at 1200 m elevation, were furrow irrigated. Pre-and postgrazed forage were clipped and weighed to determine yield and utilization (48-h utilization < 50%). Preference scoring of 0 to 10 (0 to 100% of forage eaten) was done by four trained observers at 24, 30, and 48 h. The heifers quickly learned to distinguish between cultivars, and their order of preference was Kenhy > KY 31 > HiMag = Barcel = Cl = Stargrazer > M096 = Mozark. The cultivar x trial(year) interaction for preference indicated that cultivars responded differently to weather conditions, which in turn affected animal preference. Preference scoring had high repeatability and ranked cultivars similarly to the clip-and-weigh method of measuring utilization. Preference scoring was accomplished with 27% of the experimental error and only 6% of the time required for clip-and-weigh. Only 44% of the variation in preference score (PS) was explained by the model: PS = 8.8 -1

Increasing the amount of C in soils may be one method to reduce the concentration of C02 in the atmosphere. We measured organic C stored in southern Idaho soils having long term cropping histories that supported native sagebrush vegetation (N SB), irrigated moldboard plowed crops (IMP), irrigated conservation -chisel-tilled crops (ICT) and irrigated pasture systems (1?). The C02 emitted as a result of fertilizer production, farm operations and CO; lost via dissolved carbonate in irrigation water, over a 30 year period, was included. Net organic C in ecosystems decreased in the order IP>ICT>NSB>IMP. In this study, if NSB were converted to IMP, 0.15 g C m"2 would be emitted to the atmosphere, but if converted to IP , 3.56 g C m'2 could be sequestered. If IMP land were converted to ICT, 0.95 g C m"2 could be i sequestered in soil and if converted to I? 3.71 g C 111'2 could be sequestered. There are 2.6)(108 ha of land worldwide presently irrigated. If irrigated agriculture were expanded 10% and the same amount of raind land were converted back to native grassland, an increase of 3.4x109 Mg C (5.9% of the total C emitted in the next 30 yr) could potentially be sequestered. The total projected release of CO2 is 5.7x10'°Mg C worldwide during the next 30 years.Converting rainfed agriculture back to native vegetation while modestly increasing areas in f irrigated agriculture could have a significant impact on C0; atmospheric concentrations while maintaining or increasing food production.