1. Results Effect of Simulated Grazing Intensity on Dual-Purpose Winter Wheat Growth and Grain Yield Dillon Butchee and Jeff Edwards Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK Effect of Simulated Grazing Intensity on Dual-Purpose Winter Wheat Growth and Grain Yield Dillon Butchee and Jeff Edwards Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK Introduction Approximately 50% of the 2.4 million hectare Oklahoma wheat crop will be grazed by stocker cattle. Many farmers will choose to remove cattle from wheat pasture in late winter and harvest a grain crop (dual-purpose). In a dual-purpose wheat production system, grazing intensity can range from high to low. Grazing intensity affects wheat canopy closure, and canopy closure influences yield potential of wheat after grazing. This study evaluates the effect of mid-season canopy closure on final grain yield. Objectives 1.Determine the relationship between wheat canopy closure and grain yield 2.Evaluate the impact of simulated grazing intensity on winter wheat grain yield 3.Determine if plant growth habit (planophile or erectophile) affects crop response to simulated grazing intensities Materials and Methods Overley (erectophile growth habit) and Fuller (planophile growth habit) wheat were sown in a RCBD at 120 kg/ha on 21 September 2009 at Stillwater, OK. Grazing intensity was simulated using a rotary mower set to clip plots at 3, 7.5, or 12 cm. The mower included a bagging unit to catch clippings. Simulated grazing began 5 November 2009 and ended 5 March 2010. Canopy closure was determined before and after each mowing treatment using digital photography analysis (SigmaScan Pro V 5.0) and at Feekes Growth Stage 10.5 with a line quantum sensor. Plots were harvested 22 June 2010 with a small plot combine. Image 1. Light interception measurements made by digital photography analysis. Conclusions Fractional canopy closure values were lowest throughout the growing season for the most intense simulated grazing treatment (3cm) and increased as simulated grazing intensity decreased. Grain yield had an asymptotic response to fractional canopy closure measured at grazing termination. There was a large decrease in grain yield when fractional canopy closure at flowering fell below 0.9. Overley had lower canopy closure values for all simulated grazing regimens but produced grain yield equal to or higher than that of Fuller. Intensifying simulated grazing from a 7.5-cm mowing height to a 3-cm mowing height reduced yield of Overley by 42% and Fuller by 22%. For more information Dillon Butchee Graduate Student / Department of Plant and Soil Sciences Oklahoma State University dillon.butchee@okstate.edu Figure 4. Grain yield of cultivars at each mowing treatment. Columns with the same letter are not significantly different at the 0.05 level. Figure 3. Average fractional canopy closure at Feekes Growth Stage 10.5 of each cultivar at each mowing treatment. Canopy closure measured by a line quantum sensor. Figure 1. Average canopy closure for each mowing height. Canopy closure of all plots decreased during winter dormancy, but the decrease was much greater for more intensively grazed plots. Image 2. Stillwater location after simulated grazing treatments on 5 March 2010. Figure 2. Relationship between 5 March 2010 canopy closure of both cultivars and final grain yield.