Light interception, canopy architecture, and carbon assimilation in Xaraés palisadegrass [Brachiaria brizantha (A. Rich.) Stapf. cv. Xaraés] pastures as affected by rotational grazing strategies

Author(s)

Bruno Carneiro e Pedreira

• Document type: Master's Dissertation
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2006-07-03

Examining board members

Carlos Guilherme Silveira Pedreira; Gustavo José Braga; Sila Carneiro da Silva

• Advisor: Carlos Guilherme Silveira Pedreira

Abstract

Yield formation in pastures is a complex process at it involves factors of both physiological and morphological nature, as well as interactions among them. By understanding individual processes and how they interact it is possible to assess the assimilatory potential of the sward, using through modeling and simulation. The objective of this research was to evaluate the agronomic performance and to model the photosynthetic potential of vegetative swards of Xaraés palisadegrass as a function of canopy architecture and light environment, in pastures under intermittent grazing, where defoliation frequency was dictated either by levels of light interception or by chronological time, in order to try to rationalize management practices under a physiological standpoint and operational practicity. In addition, plant morphological and physiological responses were described in terms of their mutual interplay, as treatments ultimately affected forage yield. The study was conducted at Escola Superior de Agricultura "Luiz de Queiroz", USP campus in Piracicaba, SP, on a kandiudalfic euthrudox using a one-year-old pasture of Xaraés palisadegrass where agronomic, morphological and physiological traits, such as foraqe yield, photosynthesis, leaf area index (LAI), light interception (LI) and leaf angles were studied. The experimental design was completely randomized with three treatments and three replications, for a total of nine experimental units (paddocks) of 120 m² each, which were mob grazed whenever canopy light interception reached 95% or 100%, or every 28 days. Grazing strategies resulted in different seasonal forage yields. The 100% LI treatment made for longer rest periods and less frequent defoliations, resulting in highest (22,760 kg DM ha -1) forage productivity, compared with the 95% LI and the 28-d treatments (mean 17,700 kg DM ha-1). LI-based treatments resulted in less variable pregraze sward heights than the calendar-based treatment, and this suggests that sward height may be successfully used as a management guide. Across treatments sward height was highly correlated with LI (r=0.84) and with LAI (r=0.92) in all treatments, and so were LAI and LI (r= 0.95). Grazed at a higher frequency (95% LI) the canopy responded with maximum levels of assimilation from the midpoint to the end of the rest period, probably due to a lower mean LAI (2.08), which made for a favorable environment and less competition for light. Thus, optimal management seems to be related with higher canopy photosynthesis during the rest period. (AU)

• FAPESP's process: 04/04717-8 - Luminous relationships, architecture and assimilation of carbon by canopies of xaraes grass [Brachiaria brizantha (hochst ex a. Richi.) Stapt] in response to grazing strategies, under intermittent occupation
• Grantee: Bruno Carneiro e Pedreira
• Support Opportunities: Scholarships in Brazil - Master