### 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.