study the competition of weeds in canola (brassica napus l.) field using reciprocal of individual plant weight model and weed relative leaf area

in order to evaluate competition among some weed species and canola, a field experiment in astan ghods razavi's field in mashhad was done on 2007-2008. thirty 50×50 cm quadrates were used in two times (stem and flower production) to measure the canola dry weight, leaf area index and canola and weeds density with destruction and thirty similar quadrates were used for counting weed species and identification without destruction. in order to estimate dry weight and leaf area of non-destructive quadrates; a nonlinear hyperbolic function has been fitted between leaf area or dry weight of weed and canola (species exist in destructive quadrates) as a dependent variant and their number as an independent variant. for comparison of competition status and estimation of inter and intra specific competition coefficient for each species, multi linear regression function between the early season’ relative leaf area as an independent variant and the latter season’ the reciprocal of individual plant weight of weed and canola as a dependent variant has been fitted. regression results showed that reversed weed dry weight and relative leaf area have a proper fitting (r2= 0.87) for estimation of yield reduction and competition coefficients. in this research in spite of present of fumitory (fumaria officinalis l.), knot weed (polygonum aviculare l.) and malcolm stock (malcolmia africana (l.)) only wild oat (avena ludoviciana l.) inhibited canola growth. the positive effect of other weed species on canola growth was probably due to their inhibitive effect on wild oat as a main competitor for canola.