Simulating Guinea Grass Production: Empirical and Mechanistic Approaches

Abstract

Tropical grasses are economically important for cattle production in Brazil, and accurate simulation models for tropical pastures can benefit forage researchers and farm managers by improving tropical forage production systems. This research calibrated and validated four modeling approaches of contrasting complexity to simulate mass production of Mombaça Guinea grass (Panicum maximum Jacq.). The models included three empirical agro‐climatic models (i.e., using cumulative degree days, photothermal units, and a climatic growth index) and a biophysical simulation model, Agricultural Production Systems Simulator (APSIM)‐Growth. Data sets for calibration and independent validation included frequent records of aboveground dry matter production during the 2005–2006 and 2010–2011 growing seasons from three trials. All models performed well during calibration (R2 = 0.78–0.86; coefficient of variation = 26–32.1%). During model validation, the R2 varied between 0.69 and 0.78, the agreement index was between 0.88 and 0.93, the coefficient of variation between 37.6 and 50.2%, and the mean bias error was between 6 and 470 kg ha−1. Even though all models were in agreement between simulated and observed results, APSIM‐Growth was able to simulate Guinea grass production across broader climatic, soil, and management (e.g., N fertilization) conditions.

Key factors that influence for seasonal production of Guinea grass

“…Since lower soil moisture values were recorded in the winter season when we observed the lowest average air temperature of 17.6 °C (4 th cycle), which is close to the minimum base temperature for Guinea grass of 15.6 °C (Araujo et al, 2013). Other studies have also re- (Araujo et al, 2013), three distinct phases were distinguished as shown in Figure 4.…”

“…Since lower soil moisture values were recorded in the winter season when we observed the lowest average air temperature of 17.6 °C (4 th cycle), which is close to the minimum base temperature for Guinea grass of 15.6 °C (Araujo et al, 2013). Other studies have also re- (Araujo et al, 2013), three distinct phases were distinguished as shown in Figure 4. In phase 1 the Guinea grass production (from 0 to 250 °C) was more correlated to the residual DM, followed by phase 2 (at 500 °C) characterized by similar correlations among the variables, ending with a phase 3, where DM production was better correlated with degree-days (from 750 to 1000 °C) (Figure 4).…”

“…In both trials, the highest and lowest DM productions were recorded for regrowth cycles in summer-fall and late fall to early winter, respectively. For more information, refer to Araujo et al (2013).…”

“…Air temperatures ranging between 12 and 17.5 °C generally provide slow or no growth for tropical forages cultivated in Brazil (Cruz et al, 2011;Araujo et al, 2013). Thus, for areas with mild temperatures in winter, irrigation alone cannot provide significant production since air temperature acts as the main limiting factor.…”

Climate change and future scenarios for palisade grass production in the state of São Paulo, Brazil

“…The vulnerability of tropical grassland-based animal production systems to climate changes would be better accessed by the use of mechanistic models. Although some mechanistic models have been parameterized for tropical forages (Pedreira et al, 2011;Lara et al, 2012;Araujo et al, 2013), parameters related to CO 2 effects on plant processes have not been adjusted yet. The refinement of the simulations, including more factors, especially the [CO 2 ], requires further experimentation.…”