• Evaluation of GPFARM for Simulation of Forage Production and Cow-Calf Weights

      Andales, Allan A.; Derner, Justin D.; Bartling, Patricia N. S.; Ahuja, Lajpat R.; Dunn, Gale H.; Hart, Richard H.; Hanson, Jon D. (Society for Range Management, 2005-05-01)
      A modeling approach that assesses impacts of alternative management decisions prior to field implementation would reduce decision-making risk for rangeland and livestock production system managers. However, the accuracy and functionality of models should be verified before they are used as decision-making tools. The goal of this study was to evaluate the functionality of the Great Plains Framework for Agricultural Resource Management (GPFARM) model in simulating forage and cow-calf production in the central Great Plains. The forage production module was tested in shortgrass prairie using April-October monthly biomass values from 2000 through 2002 for warm-season grasses (WSG), cool-season grasses (CSG), shrubs, and forbs. The forage module displayed excellent (99% explained variance) agreement in the 2001 calibration year in tracking growth and senescence trends of WSG and CSG, which constitute the vast majority of the aboveground biomass. Less agreement (35%-39% explained variance) was observed for shrubs and forbs. The model-explained variances of biomass in 2000 and 2002 (verification years) were 80% for WSG, 67% for CSG, 78% for shrubs, and 82% for forbs. Further development is needed to improve predicted plant response to environmental stresses. The cow-calf production module was tested in northern mixed-grass prairie using June-November monthly average cow and calf weights from 1996 through 2001 for March-calving, moderately stocked Hereford pairs. Overall, GPFARM performed well and tracked cow (81% explained variance) and calf (94% explained variance) pre- and postweaning weights. The GPFARM model has functional utility for simulating forage and cow-calf production with satisfactory accuracy at semiarid-temperate sites, such as southeastern Wyoming and northeastern Colorado. Continued development will focus on improving plant response to environmental stresses and testing the model’s functionality as a decision support tool for strategic and tactical ranch management.
    • Seedling Growth of Two Honey Mesquite Varieties Under CO2 Enrichment

      Derner, Justin D.; Tischler, Charles R.; Polley, H. Wayne; Johnson, Hyrum B. (Society for Range Management, 2005-05-01)
      Seedlings of 2 varieties of honey mesquite (Prosopis glandulosa var. glandulosa and P. glandulosa var. torreyana) were exposed to 2 concentrations of atmospheric carbon dioxide (CO2) (368 and 704 μmol mol-1) in environmentally controlled glasshouses under near-optimal temperature and soil water conditions to determine if CO2 enrichment alters above- and belowground growth responses. CO2 enrichment substantially enhanced both above- and belowground growth variables of both varieties for all harvest dates (8, 16, and 24 days postemergence). This growth enhancement was greater for aboveground variables (21%- 35%) at the first harvest, greater for belowground variables (36%-40%) at the second harvest, and similar for both above- (13%-68%) and belowground (10%-40%) variables at the last harvest. Differences in temporal growth enhancement associated with CO2 enrichment suggest changing carbon allocation priorities, with initial carbon investment allocated primarily aboveground to develop photosynthetic machinery, and later carbon allocations predominately directed toward increased investment in roots. The absence of significant CO2 X variety interactions at any harvest date provides evidence that CO2 enrichment did not exaggerate growth responses between the 2 varieties. These results suggest that varietal differences in rooting and other characteristics did not modify the size advantage of the glandulosa over the torreyana variety, as the absolute differences in sizes did not change as a function of CO2 treatment. Although CO2 enrichment did not exaggerate growth differences between varieties in this species, it is evident that honey mesquite seedlings possess the capacity to respond markedly to CO2 enrichment. The greater root depth of honey mesquite seedlings exposed to CO2 enrichment confers a competitive advantage to mesquite seedlings over grass seedlings, assuming that C3 and C4 grass seedlings will not respond as vigorously to CO2 enrichment. As such, this species should continue to aggressively encroach into grasslands in future CO2-enriched environments.