Canopy height and its relationship with leaf area index and light interception in tropical grasses

Photosynthetically active tissues, mainly green leaves, are the major component of forage growth and development. The amount of these tissues in a forage plant is influenced directly by cutting management, i.e. cutting frequency and stubble height. The normal recommendation is to cut (or graze) forage whenever it reaches a given stubble height. Brougham (1956) stated that the critical leaf area index (LAI) is reached when the forage canopy is intercepting 95% of the photosynthetically active radiation, and the forage is near its maximum growth rate without shading itself. Alternatively, the optimum LAI occurs when the forage reaches the maximum point of mass accumulation, indicating the time to start grazing or cutting. Generally, the critical and optimum LAI values are quite similar, but not necessarily the same (Brown and Blaser 1968). This trial evaluated the relationships among canopy height, LAI and light interception in 10 tropical grasses.


Introduction
Photosynthetically active tissues, mainly green leaves, are the major component of forage growth and development.The amount of these tissues in a forage plant is influenced directly by cutting management, i.e. cutting frequency and stubble height.The normal recommendation is to cut (or graze) forage whenever it reaches a given stubble height.Brougham (1956) stated that the critical leaf area index (LAI) is reached when the forage canopy is intercepting 95% of the photosynthetically active radiation, and the forage is near its maximum growth rate without shading itself.Alternatively, the optimum LAI occurs when the forage reaches the maximum point of mass accumulation, indicating the time to start grazing or cutting.Generally, the critical and optimum LAI values are quite similar, but not necessarily the same (Brown and Blaser 1968).This trial evaluated the relationships among canopy height, LAI and light interception in 10 tropical grasses.

Methods
The experiment was carried out at the Animal Science Department of the Universidade Federal Rural de Pernambuco.The evaluated species were: Brachiaria decumbens, Brachiaria hybrid cv.Mulato II, Brachiaria brizantha cv.Xaraés, Brachiaria brizantha cv.Marandu, Panicum maximum cv.Tanzânia, Panicum hybrid cv.Massai, Sorghum vulgare, Sorghum bicolor, Pennisetum purpureum "Common" and P. purpureum cv.Roxo.Light interception, LAI and mean leaf angle (MLA) were measured using a Plant Canopy Imager CI-120 from CID Bio-science ® , simultaneously with canopy height, at 1week intervals over 2 months, in experimental plots ___________ Correspondence: José C.B. Dubeux Jr., University of Florida, North Florida Research and Education Center, 3925 HWY 71, Marianna, FL 32446, USA.Email: dubeux@ufl.edumeasuring 2.8 x 1.8 m.Measurements were replicated 4 times per plot in every weekly evaluation.Correlation analyses were performed using Sigmaplot 12.0, and the correlation magnitude was based on Franzblau (1958).
In contrast with our findings, Galzerano et al. ( 2010) found, for Panicum maximum cv.Áries and Cynodon nlemfuensis, very strong correlation between canopy height and LAI, with coefficients of determination of 0.76 and 0.88, respectively.Engel et al. (1987), studying a cool season grass, Bromus inermis, found that LAI and light interception were strongly correlated with season of the year and fertilization level.They also found that forage mass per unit area correlated well with light interception of 95%.In the same study, the authors reported that, during the reproductive phase of B. inermis, tillers became elongated and the canopy became more erect and open, requiring higher values of LAI to intercept the same radiation as absorbed by a smaller and denser canopy.
Table 1.Correlations between canopy height, light interception, leaf area index and mean leaf angle of 10 tropical forage grasses (T-test, with linear correlation).Falster and Westoby (2003) reported that small values for mean leaf angle are directly related to higher indices of light interception.In our study, LAI and mean leaf angle correlated positively (P<0.05) and affected Brachiaria hybrid cv.Mulato II, Panicum maximum cv.Tanzânia, Panicum hybrid cv.Massai, Brachiaria decumbens and Brachiaria brizantha cv.Xaraés.Mean leaf angle affected negatively (P<0.05)light interception for only 3 species: Brachiaria brizantha cv.Marandu, Brachiaria hybrid cv.Mulato II and Pennisetum purpureum "Common".

Conclusions
Canopy height seldom correlated well with light interception and leaf area index across a range of tropical grasses.Other measurements on the canopy, either directly as herbage mass or indirectly as disk settling height (Santillan et al. 1979), could be alternative measurements to correlate with light interception in tropical grasses.