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yr Ϫ1 ; Wolf and Fiske, 1995; Brejda, 2000). The compromise between yield and quality does not apply to bio-Management practices for biomass production of bioenergy grasses mass production for bioenergy feedstock because the may differ from management for forage. Our objective was to determine the yield and stand responses of 'Alamo' switchgrass (Panicum goal generally is to maximize production of lignocelluvirgatum L.) to N and P fertilization as affected by row spacing. A lose (Sanderson et al., 1999a). Thus, management praccombination of five rates each of N and P were applied to plots during tices that maximize biomass production may differ from 1992 to 1998 at Stephenville, TX and 1993 to 1995 at Beeville, TX.that for herbage production. Annual yield of several Three row-spacing treatments were applied as subplots. Biomass proswitchgrass cultivars in Texas fertilized with 134 kg N duction was determined each year with a single harvest in late summer. ha Ϫ1 ranged from 8 to 20 Mg ha Ϫ1 , depending mainly Tiller density and tiller mass were measured during 1993 to 1996 at on seasonal rainfall variations (Sanderson et al., 1999b). Stephenville. Biomass production was not influenced by the additionAlamo switchgrass was the most adapted cultivar for of P. Biomass production response to N at Beeville was greater in the south-central USA. narrow rows than wide rows during the establishment year only. Bio-Phosphorus fertilizer recommendations for switchmass production responses to N were quadratic in 5 of 7 yr at Stephenville and linear at Beeville. A maximum yield of 22.5 Mg ha Ϫ1 occurred grass depend on soil pH, P supplying power of the soil, during 1995 at Stephenville at 168 kg N ha Ϫ1 . Lodging occurred atand soil test P (Brejda, 2000). In the central Great Plains, both locations but only at the 224 kg N ha Ϫ1 rate. Tiller density and P recommendations for switchgrass ranged from 0 to mass increased as row width increased. Tiller mass also increased 35 kg ha Ϫ1 , depending on soil test P (Brejda, 2000). In Oak Ridge Natl. Lab. managed by Martin Marietta Energy Systems.

Switchgrass (Panicum virgatum L.), a warm‐season perennial grass native to North America, has potential as a biomass energy crop. Our objective was to develop harvest management recommendations for biomass feedstock and forage production. ‘Alamo’ switchgrass was established in 1992 at Stephenville and Dallas, TX. Four harvest frequencies (one to four cuts per year) and three final autumn harvests (Sept., Oct., or Nov.) were imposed from 1993 to 1996. Tiller densities were counted each spring. Neutral detergent fiber (NDF) and crude protein (CP) concentrations were measured in 1993 and 1994. Concentrations of NDF were lowest (avg. = 640 g kg−1) and of CP (avg. = 110 g kg−1) were highest in May‐harvested biomass. Forage quality of regrowth decreased with age, reaching NDF concentrations of 790 g kg−1 and CP of <20 g kg−1. Total seasonal yields decreased as harvest frequency increased; however, a severe drought reversed this trend at Dallas in 1996. The highest yields (15–20 Mg ha−1) occurred with a single harvest in mid‐September. Delaying the final harvest until November reduced yields. Harvest date and frequency did not affect tiller density, although tiller density decreased from 900 to 650 and 630 to 310 m−2 at Dallas and Stephenville, respectively, during 1994 to 1997. Thus, a single mid‐September harvest should maximize biomass yields in the south‐central USA. A two‐cut (spring‐autumn) system may allow a farmer to use initial growth as forage and the regrowth for biomass, but total yields would be reduced. More frequent harvests would reduce yields further.

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A model for forage yield with adequate details for leaf area, biomass, nutrients, and hydrology would be valuable for making management decisions. The objectives of this study were to develop Alamo switchgrass (Panicum virgatum L.) parameters for the Agricultural Land Management Alternatives with Numerical Assessment Criteria (ALMANAC) model and demonstrate its accuracy across a wide range of environments. Derived plant parameters included potential leaf area index (LAI), potential biomass growth per unit intercepted light, optimum nutrient concentrations, and growth responses to temperature. The model's simulated yields accounted for 79% of the variability in measured yields for one‐cut and two‐cut harvest systems from six diverse sites in Texas in 1993 and 1994. Simulated yields for three locations differed in sensitivity to potential LAI, heat units to maturity, radiation use efficiency (RUE), and soil depth. The ALMANAC model shows promise as a management tool for this important forage and bioenergy crop.