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Simulating bimodal tall fescue growth with a degree‐day‐based process‐oriented plant model

J. R. Kiniry, S. Kim, A. S. Williams, T. R. Lock, R. L. Kallenbach
Grass and forage science 2018 v.73 no.2 pp. 432-439
Festuca arundinacea subsp. arundinacea, biomass, cold season, cool season grasses, cultivars, dormancy, growing season, growth curves, growth models, pastures, perennials, photoperiod, plant development, simulation models, soil, summer, temperature, Midwestern United States
Plant growth simulation models have a temperature response function driving development, with a base temperature and an optimum temperature defined. Such models function well when plant development rate shows a continuous change throughout the growing season. This approach becomes more complex as it is extended to cool‐season perennial grasses with a dormant period and bimodal growth curves. The objective of this study was to develop such a bimodal growth model for tall fescue (Schedonorus arundinaceus (Schreb.) Dumort) in the Midwest USA based on multiyear measurement trials. Functions for bimodal growth were incorporated into the ALMANAC model and applied to tall fescue using published tall fescue yields for a variety of sites and soils. Fields of cultivars “Kentucky 31” and “BarOptima Plus E34” were divided into paddocks and sampled weekly for dry‐matter accumulation. These biomass estimates were used to derive weekly growth values by differences between sequential weekly samplings. The measured values were compared to a single tall fescue simulation each year on one soil. Using these results, the ALMANAC model was modified and tested against mean reported tall fescue yields for 11 sites, with one to three soils per site. When we introduced midsummer dormancy into ALMANAC, we assumed dormancy began on the longest day of the year and lasted until the photoperiod was 0.68 hr shorter than the longest. ALMANAC simulated previously reported tall fescue yields well across the range of sites. Thus, ALMANAC shows great promise to simulate bimodal growth in this common cool‐season grass.