Agronomic traits and adaptation of the CROPGRO-Perennial Forage model to predict the growth of five Brachiaria genotypes

Abstract

Brachiaria spp. play a key role in animal production for grassland-based systems in tropical and subtropical regions. The evaluation of forage productivity and canopy characteristics allows us to understand pasture responses over time and identify high-yield genotypes with different defoliation management needs. Our objectives were to compare and explain differences in forage accumulation and canopy characteristics of five genotypes of Brachiaria spp. (Basilisk, Marandu, Mulato II, Piatã and Xaraés) in southeastern Brazil under three levels of nitrogen fertilization (220 kg of N ha-1 yr-1 in year 1 and 0 and 550 kg of N ha-1 yr-1 in year Year 3), and adapt the CROPGRO-Perennial Forage model (CROPGRO-PFM) to simulate the seasonal growth and production of these same genotypes, as well as to describe the adaptations and improvements made to the model. Both objectives were developed from an experiment of mechanically harvested plots with 15 cm stubble every 28 days and 42 days during the wet and dry season, respectively, from October 2013 to November 2014 (Year 1) and from September 2015 to September 2016 (Year 3). Response measurements included annual herbage accumulation, plant-part composition, canopy height, herbage bulk density, leaf area index, light interception, and tiller population density. The first objective showed us that there were differences between the genotypes, indicating great variability. Mulato II, Piatã and Xaraés were the most productive genotypes, producing an annual average of 12.1 Mg DM ha-1. The plots fertilized with N produced 2.3 times more forage than the non-fertilized plots. The main contrast occurred between the Basilisk and Xaraés genotypes. Basilisk showed higher tiller population density (1512 vs. 854 tillers per m2) and low leaf:stem ratio (2.4 vs. 12.8) in relation to Xaraés. While the second objective showed us that the model adaptation required the modification of parameters related to the partition between leaf and stem fractions, and the allocation intensity to shoot versus storage tissues based on photoperiod sensitivity. And after these modifications, the CROPGRO-PFM model performed well, providing realistic seasonal growth responses to seasonal climate variations and nitrogen fertilization. Morphological and structural characteristics of the canopy are valuable ways of understanding the management potential of different Brachiaria genotypes. And after adapting the model, we can apply it as a tool to simulate the growth and partitioning of the five Brachiaria genotypes in different scenarios.