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Title: Simulating CO2 effects on range plant growth and water use with GP-FARM-range model

Author
item Andales, Allan
item Derner, Justin
item Ahuja, Lajpat
item Hart, Richard

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2015
Publication Date: 7/7/2015
Citation: Andales, A.A., Derner, J.D., Ahuja, L.R., Hart, R.H. 2015. Simulating CO2 effects on range plant growth and water use with GP-FARM-range model. Rangeland Ecology and Management. 68:423-431.

Interpretive Summary: Adjustment of stocking rate is the primary management tool available to ranchers that has the biggest impact on rangeland vegetation and livestock production. Profitability and sustainability can be achieved by appropriately stocking cattle herds on native rangelands. This study explored the possibility of using the improved USDA-ARS Great Plains Framework for Agricultural Resource Management (GPFARM) rangeland-cattle computer model to predict the effects of different stocking rates on forage production and steer weights on semiarid northern mixed-grass prairie in southeast Wyoming. The computer model was run for three stocking rates (light: 16 steers per 80 hectares; moderate: 4 steers per 12 hectares; heavy: 4 steers per 9 hectares). Model accuracy was 65% to 74% in predicting peak standing crop (PSC) (1991 – 2001) and 96% to 99% in predicting average grazing season (mid-June to mid-October) weights of Hereford yearling steers (1982 – 2001). Possible causes of errors in the model predictions are inadequate responses of the plant model to grazing and water stress. The model was also used to predict steer weights at extremely high stocking rates (4 steers per 6 hectares and 4 steers per 4 hectares) that have not been tested in the area. The model predicted that these rates would result in substantially decreased forage production and lower animal performance, with a pronounced decline in steer weights at the very high stocking rate of 4 steers per 4 hectares. It was demonstrated that the model can be used to explore the interactions among the environment, stocking rate, rangeland productive capacity, and steer performance.

Technical Abstract: Adjustment of stocking rate is the primary management tool available to ranchers that has the biggest impact on rangeland vegetation and livestock production. Profitability and sustainability can be achieved by appropriately stocking cattle herds on native rangelands. Mechanistic simulation models of the rangeland-livestock production system that incorporate interactions among soil, plants, animals, and the atmosphere can help ranchers determine the appropriate stocking rates. This study explored the possibility of using the improved USDA-ARS Great Plains Framework for Agricultural Resource Management (GPFARM) rangeland-cattle model to simulate interactions between stocking rate, forage production, and steer weights on semiarid northern mixed-grass prairie in southeast Wyoming, U.S.A. Comparisons of simulated peak standing crop (PSC) (1991 – 2001) and average grazing season (mid-June to mid-October) weights of Hereford yearling steers (1982 – 2001) to experimentally-obtained values from three stocking rates (light: 16 steers'80 ha-1; moderate: 4 steers'12 ha-1; heavy: 4 steers'9 ha-1) indicated that the model could explain 65% to 74% of the variability in PSC and 96% to 99% of the variability in steer weights. Likely causes of errors in simulated PSC are inadequate responses of the plant model to grazing and water stress. The model can be used to explore the interactions among the environment (especially precipitation), stocking rate, rangeland productive capacity, and steer performance. The model indicated that the stocking rates used in this experiment in southeast Wyoming were low enough to obtain acceptable steer weights. However, simulations of higher stocking rates (4 steers•6 ha-1 and 4 steers•4 ha-1) indicated that implementation of these rates would result in substantially decreased forage production and lower animal performance, with a pronounced threshold response at the very high stocking rate of 4 steers•4 ha-1. These simulation results provide further evidence that the recommended moderate stocking rate (4 steers'12 ha-1) is sustainable and does not adversely affect rangeland productive capacity or animal performance.