Agronomic and microclimatic responses inside exclosure cages and their impacts on forage accumulation estimate on pastures under continuous stocking

Author(s)

Junior Issamu Yasuoka

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2020-05-26

Examining board members

Carlos Guilherme Silveira Pedreira; Ciniro Costa; Paulo Cesar Sentelhas; Sila Carneiro da Silva

• Advisor: Carlos Guilherme Silveira Pedreira

Abstract

Exclosure cages are the most commonly used technique to estimate forage accumulation (FA) on grazing experiments using continuous stocking method. However, differences in morphological, physiological, morphogenic, and microclimate responses between inside and outside of cages, the canopy condition at the time of grazing exclusion, and the exclusion period can affect the accuracy of the method in estimating FA. The aim of this study was to describe and explain how the exclusion of grazing for 14, 21, and 28 days using cages affects the structural and morphogenic characteristics, carbon assimilation, and microclimate on pastures of Mulato II brachiariagrass (Brachiaria brizantha × B. decumbens × B. ruziziensis) kept at 20 and 30 cm canopy height under continuous stocking and its implications in the FA estimate. Grazing exclusion affected leaf elongation rate (LER), which was the determinant of the greater leaf growth rate (LGR), expanded leaf length, and leaf area index (LAI) inside the cages compated to the grazed pasture. With greater LAI, light interception and estimated canopy photosynthesis were also greater inside the cages. As a consequence of the greater LGR and canopy photosynthesis, the forage accumulation rate inside the cages was greater than that of the pasture, resulting in overestimated FA. In addition, exclusion from grazing also affected leaf proportion, leaf and stem mass, average tiller weight, stem elongation rate, and mean stem length, that were greater in the longer exclosure periods. The opposite was recorded for dead material proportion. Differences in canopy architecture inside the cage did not affect air temperature and relative humidity. The cage structure reduced wind speed (WS) by 4.4% and global solar radiation (SR) by 5%; however, there was variation in SR between inside and outside the cage over a clear day. Smaller values of SR and WS resulted in less evapotranspiration inside the cage and also reduced plant growth. Total forage accumulation, stem proportion, dead material DM, tillers population density, leaf photosynthesis, leaf senescence rate, leaf lifespan, and the number of live leaves per tiller were not affected by the exclosure period. Overestimates of FA by the exclosure cage method derive from morphological and physiological changes in the ungrazed canopy, including LER, LAI, and canopy photosynthesis. Forage accumulation would be even more overestimated if SR and WS were not reduced by cage structure. (AU)

• FAPESP's process: 17/11288-6 - Do exclosure cages affect canopy microenvironment, morphogenetic responses and morphophysiological characteristics on pastures under continuous stocking?
• Grantee: Júnior Issamu Yasuoka
• Support Opportunities: Scholarships in Brazil - Doctorate