Abstract
The objective of this study was to evaluate the occurrence of the tiller size/density compensation mechanism in Tifton 85 bermudagrass swards grazed by sheep under continuous stocking. Treatments corresponded to four sward steady state conditions (5, 10, 15, and 20 cm of sward surface height), maintained by sheep grazing. The experimental design was a complete randomized block with four replicates. Pasture responses evaluated include: tiller population density, tiller mass, leaf mass and leaf area per tiller, and herbage mass. Tiller volume, leaf area index, tiller leaf/stem ratio, and tiller leaf area/volume ratio were calculated and simple regression analyses between tiller population density and tiller mass were performed. Measurements were made in December, 1998, and January, April, and July, 1999. The swards showed a tiller size/density compensation mechanism in which high tiller population densities were associated with small tillers and vice-versa, except in July, 1999. Regression analyses revealed that linear coefficients were steeper than the theoretical expectation of -3/2. Increments in herbage mass were attributable to increases in tiller mass in December and January. Leaf area/volume ratio values of Tifton 85 tillers were much lower than those commonly found for temperate grass species.
Highlights
Defoliated grass swards exhibit tiller size/density compensation (SDC) with change in defoliation height (Bircham & Hodgson, 1983; Davies, 1988; Matthew et al, 1995; Hernández Garay et al, 1999)
This pattern of self-regulation can be viewed as means by which pastures regulate leaf area index (LAI) in response to change in defoliation height, since tiller population density (TPD) is the component of the sward LAI in which changes in sward structure can be readily expressed (Matthew et al, 2000)
Under very severe grazing, some grasses may lack organic reserves for bud release to increase tiller population density and pastures may collapse (Matthew et al, 1995). This tiller size/density compensation issue has been discussed in terms of the -3/2 self-thinning rule originally described by Yoda et al (1963), according to whom tiller mass and tiller population density would be inversely correlated, following a linear regression line with a -3/2 slope when considered in a logarithm scale
Summary
Defoliated grass swards exhibit tiller size/density compensation (SDC) with change in defoliation height (Bircham & Hodgson, 1983; Davies, 1988; Matthew et al, 1995; Hernández Garay et al, 1999). Under very severe grazing, some grasses may lack organic reserves for bud release to increase tiller population density and pastures may collapse (Matthew et al, 1995) This tiller size/density compensation issue has been discussed in terms of the -3/2 self-thinning rule originally described by Yoda et al (1963), according to whom tiller mass and tiller population density would be inversely correlated, following a linear regression line with a -3/2 slope when considered in a logarithm scale. This relationship has been studied recently for some temperate species, but for tropical grasses there are few data available
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