• A robust model for estimating standing crop across vegetation types

      Vermeire, L. T.; Ganguli, A. C.; Gillen, R. L. (Society for Range Management, 2002-09-01)
      Some recent investigations have shown the visual obstruction (VO) measurement method to be an effective means of estimating herbage standing crop non-destructively in tallgrass prairie. Although the method is rapid and inexpensive, visual obstruction models have been limited to tallgrass prairie and sandhills range types. Our primary objective was to evaluate the relationships between visual obstruction and standing crop in shortgrass plains and mixed prairie. Secondly, we wanted to determine whether these models could be integrated with tallgrass prairie models without appreciable losses in prediction capabilities. We conducted 44 trials on non-burned sites with various levels of grazing pressure. Each trial was composed of 20 randomly selected plots and served as 1 experimental unit to represent pasture-level standing crop estimation. Visual obstruction readings were taken from a modified Robel pole placed centrally at the back of 0.1-m2 quadrats and vegetation was clipped to estimate standing crop. Trial standing crop was regressed on visual obstruction and models were compared among range types. Visual obstruction explained 91 and 89% of the variation in shortgrass plains and mixed prairie standing crop, respectively. A single visual obstruction model effectively estimated herbage standing crop across range types and produced a coefficient of determination of 0.93. Although greater precision may be obtained from models developed for specific sites, the ability of a single visual obstruction model to predict standing crop across years, management schemes, and range types indicates visual obstruction models may be successfully employed on a regional basis.
    • Fire and nitrogen addition increase forage quality of aristida purpurea

      Dufek, N. A.; Vermeire, L. T.; Waterman, R. C.; Ganguli, A. C. (Society for Range Management, 2014-05)
      Purple threeawn (Aristida purpurea Nutt.) is a native perennial bunchgrass with limited forage value that dominates sites with disturbed soils and persists with repeated severe grazing. Fire and nitrogen addition have been used to reduce threeawn and can increase grazing utilization of threeawn by livestock. We evaluated effects of fire, spring urea addition, and phenological stage on purple threeawn forage quality 1 yr postfire on two similar sites in southeastern Montana during the 2011 (site 1) and 2012 (site 2) growing seasons. Fire (no fire, summer fire, fall fire) and rate of nitrogen addition (0, 46, 80 kg N·ha-1) were arranged in a completely randomized, fully factorial design. Samples were collected at five phenological stages throughout each growing season. Forage quality was assessed using nutrient analyses of crude protein (CP), net energy (NEm), and total digestible nutrients; antiquality analyses of neutral detergent fiber (NDF), acid detergent fiber, and silica; in vitro fermentation for organic matter disappearance (IVOMD) and NDF disappearance; and gas production (asymptotic [maximum] gas production, fractional rate of gas production, lag time, and average fermentation rate). In vegetative stages, summer and fall fire increased CP from 6.2% to 12.1% and 13.0%, respectively, and NDF decreased from 72.1% to 69.4% and 68.2%, respectively. Summer and fall fire reduce silica content from 7.0% to 4.1% and 4.3%, respectively. Purple threeawn IVOMD increased by 14.0% and 13.0% following summer and fall fire, respectively, compared to nonburned plots. Nitrogen addition increased CP from 7.5% to 8.0% and 8.4%, respectively, with 46% and 80 kg N·ha-1, respectively. In vitro fermentation and gas production variables did not change due to nitrogen addition. Fire generally improved purple threeawn forage quality to a greater extent than did nitrogen addition. Results indicate fire can potentially improve the suitability of purple threeawn as a forage species.