COMPONENTS OF EARLY COMPETITION BETWEEN UPLAND RICE (ORYZA-SATIVA L) AND BRACHIARIA-BRIZANTHA (HOCHST EX ARICH) STAPF

In Latin America early weed competition is a constraint upon upland rice production, leading to intensive herbicide use. Breeding for rice competitiveness may help reduce the need for herbicides. Two consecutive screenhouse pot experiments were conducted with 13 upland cultivars grown alone or in mixtures with Bracharia brizantha to identify plant characteristics for early rice competitiveness which could serve as selection criteria. Rice growth reduction from a. brizantha competition was evident in all cultivars by 30 d.a.e., and the traits most related to cultivar competitiveness were: total above-ground plant biomass > leaf weight > stem weight > leaf area. Plant height and tillering were least affected by competition. Rice and B. brizantha partially avoided competition since the competing canopies did not fully intercept the incoming light. Traits measured in monoculture did not correlate well with the competition-induced growth reductions. In breeding for competitiveness, selection according to the above traits should be conducted on rice growing in competition, rather than in monoculture. A measure of plant biomass or leaf area at 30 d.a.e. appears to provide the most convenient estimates of early interference.