Association of tropical forage grasses in pastures: agronomic aspects that determine plant performance

• Autor(a) principal: Alex Marciano dos Santos Silva
• Data de Publicação: 2023
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://doi.org/10.11606/T.11.2023.tde-13042023-103333

Botanical richness and diversity regulate many ecosystem functions and offer the opportunity for sustainable intensification of perennial pastures through simultaneous or partial cultivation of grasses in the same field, which can result in increased productivity, reduced fertilizer and pesticide inputs, and less influence of edaphoclimatic variations. However, the existing information is mostly directed to associations of annual temperate grasses or association between grasses and legumes, with little information for associations of tropical perennial forage grasses. In addition, many studies do not explore the \"how and why\" of the interactions and agronomic responses among plant species that make up the association. In this context, the general hypothesis of this study was that three species of tropical perennial forage grasses with complementary strategies of exploration of niches, above and below ground, cultivated in association and managed under moderate defoliation regime do not compete for resources (light and nitrogen) and have greater herbage production than the monocultures individually. The general objective was to describe the dynamics of agronomic performance of the three tropical perennial forage grasses cultivated in monoculture and association aiming to identify the interactions that shape the use of resources (light and nitrogen), a determinant aspect of the botanical proportion and productive performance in these pastures. The study was carried out in Piracicaba, São Paulo, Brazil. The treatments were Andropogon gayanus cv. Planaltina (andropogon grass), Panicum maximum cv. Massai (massai grass) and Brachiaria brizantha cv. BRS Piatã (piata grass) cultivated as monocultures and in association (the three grass species in equal proportions based on the number of viable seeds) with four replications. Defoliation management was common to all treatments and corresponded to a pre-cutting height of 35 cm and a post-cutting height of 17.5 cm. We also described the calibration of a DNA-chloroplast based method to determine whether the botanical proportion of root samples from DNA artificial mixtures of these grasses can be accurately estimated and applied the method to data collected in the field to determine below ground botanical proportion and its relationship to the corresponding above ground botanical proportion. The results indicated that the association was shaped by competition for light (little competition for nitrogen), with massai grass showing greater botanical proportion and productive performance. Including the monocultures, piata grass had the lowest number of cycles and the greatest herbage accumulation per cycle. On the other hand, andropogon grass, massai grass, and the association showed greater number of cycles with smaller herbage accumulation per cycle, resulting in similar total herbage production for all treatments. The functional traits of the grass species in monoculture indicated that massai grass has greater foliage angle and smaller leaf area index in the upper 10 cm, which resulted in a greater proportion of light in the vertical profile of the canopy, favoring rapid leaf elongation. The fact resulted in greater tiller population density, resulting in shading on the others two grass species in the association and causing competition for light. The results of the calibration method indicated that the below ground botanical proportion was accurately estimated, and there is a high correlation between below ground and above ground botanical proportion. In conclusion, the association presented a productive performance similar to the monocultures. The grass species functional traits are important predictors to explain resource acquisition strategies and should be considered when choosing grass species that will be used to compose new associations.