LINEAR ESTIMATION OF GIRTH AS A COVARIATE ON YIELD PARAMETERS OF RUBBER (Hevea brasiliensis Muell.Arg.): CORRELATION OF GIRTH WITH LATEX VOLUME AND WEIGHT

Abstract

Rubber, (Hevea brasiliensis Mull.Arg). plays an important role in the Sri Lankan economy as one of the main export agricultural crops in the country. Latex is the economically important part of rubber tree for which the girth increment is a very important factor to ensure higher latex yield. It is known fact that girth related to the latex yield of rubber. Therefore the current study was conducted to estimate the girth as covariate for rubber yield and to predict the correct out come of future experiments related to rubber. The relationships of latex yield and girth of rubber were estimated using a linear regression model. Three clones RRIC 100, RRIC 121 and PB 86 were used for the experiment. Latex yield was determined by latex volume of individual trees and Grams per Tree per Tapping (GTT). Each clone was categorized into 3 girth classes as poor girth (class 1,50-64 cm), medium girth (class 2, 65-79 cm) and high girth (class 3, >80 cm). The regression coefficient for RRIC 100 was +1.520 and +0.466 for GTT and latex volume respectively. The recorded regression coefficients for clone RRIC 121 were +0.690 for GTT and +1.192 for latex volume. Similarly PB 86 produced regression coefficients +0.602 for GTT and +1.582 for latex volume. The significance by positive correlation between latex yield and girth in tested clones suggested that tree girth can be taken as a covariate in rubber experiments.