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When savannas in Latin America are brought into cultivation, rice (Oryza saliva L.) can be sown with the perennial grasses palisadegrass [Brachiaria brizantha (Hochst. ex A. Rich) Stapfl and signalgrass (B. decumbens Stapf) to harvest a grain crop while establishing a pasture to suppress weeds and provide grazing in subsequent years. However, these Brachiaria spp. can reduce upland rice yields. Rice cultivars need to be competitive with Brachiaria spp. to maintain yields but must allow Brachiaria spp. sufficient growth for pasture establishment. Field studies were conducted during 1994 and 1995 on a Typic Haplustox oxisol soil in the Eastern Plains of Colombia to evaluate the competitiveness of upland rice cultivars and to identify rice traits for competitiveness. Ten (1994) and 14 (1995) upland rice cultivars were grown with and without signalgrass in 1994 and palisadegrass in 1995. Rice cultivars differed substantially in their competitiveness. Rice yield losses ranged from 18 to 55%, and Brachiaria aboveground biomass ranged from 1.4 to 3.2 Mg ha -1 dry mass. Competition for light was critical; rice photon flux density interception, leaf area index [≥45 d after emergence (DAE)], and number of tillers (≥60 DAE) were correlated with competitiveness. No tradeoff between high yield potential and competitiveness was detected in these upland rice cultivars. Early maturity of rice is a desired characteristic for the rice-Brachiaria spp. association. The development of more-competitive cultivars appears to be a viable approach for reducing herbicide dependency and improving profitability of Latin American rice-pasture intercropping systems.

The repeated use of a limited number of herbicides has led to the development of resistant weed species in California rice (Oryza saliva L.). Competitive rice cultivars could help reduce herbicide dependency and decrease selective pressure for resistance. We conducted field experiments to determine if differences in competitive ability existed between two semidwarf cultivars of water-seeded rice. Cultivars M-202 and A-301 were grown on Stockton clay soil at two seed rates (84 and 168 kg ha 1 ) in 1996 and four seed rates (56, 112, 168, and 224 kg ha -1 ) in 1997. Molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) and propanil [N-(3,4-dichlorophenyl)propanamide] were used to control a mixed infestation of watergrass [Echinochloa oryzoides (Ard.) Fritsch, E. phyllopogon (Stapf) Koss] in 1996 and 1997, respectively. Molinate (0 and 4.5 kg a.i. ha -1 ) was applied in 1996, and propanil (0, 1, 2 and 4 kg a.i. ha -1 ) was applied in 1997. M-202 was taller, produced more leaf dry weight, had greater light interception, and reduced watergrass biomass more than A-301 in both years of the study. The more competitive M-202 also had higher yields than A-301 under weedy and weed-free conditions. Watergrass growth was not affected by rice seed rates in either year. This study suggests that herbicide rates could be reduced and weed control could be improved if more competitive cultivars were developed for water-seeded rice.

Kochia [Kochia scoparia (L.) Schrad.; syn. Bassia scoparia (L.) A.J. ScottJ is a weed that infests cereal crops in the Great Plains of the USA, often severely reducing yields. Herbicides have controlled kochia, but recently kochia has developed resistance to many herbicides. Nonherbicide alternatives are therefore needed for the integrated management of kochia. Greenhouse and growth chamber competition studies were conducted between kochia, a C 4 weed, and barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) to determine the environmental conditions that would render kochia most vulnerable to competition by a small-grain crop. Replacement-series experiments between kochia and wheat or barley were conducted under various temperature, soil moisture, and light conditions. Unlike wheat, kochia growth and photosynthesis were suppressed under cool temperatures. Barley suppressed kochia more than wheat did because of its larger canopy, despite its lower photosynthetic rates. Under high radiation conditions and warm temperatures, growth and photosynthesis were greater for kochia than wheat. Warm temperatures also increased dark respiration and reduced water use efficiency under low radiation conditions, however, thus limiting kochia's competitiveness under a closed canopy. Water stress did not affect competition, although net photosynthetic rates of kochia were greater at photosynthetically active radiation (PAR) values > 400 μmol m s 1 . Growth and CO 2 exchange rates varied among four different kochia accessions, but growth of all accessions was reduced by shade. Results suggest that a leafy, cold-tolerant crop or cultivar, grown early in the season to produce necessary ground cover, should provide opportunity to suppress kochia.

Monoculture of irrigated rice (Oryza sativa L.) in Latin America has led to serious weed problems and intensive herbicide use. Yield loss prediction enables the economic analysis of weed control, providing a basis for strategic use of herbicides and diversified weed management, but site differences restrict predictions to the environments for which the models are calibrated. We developed an algorithm for predicting rice yield losses based on early assessments of multispecies weed infestations emerging in successive flushes within variable crop stands. Rice was drill-seeded near Palmira, Colombia, at 75, 150. 200, 250, and 300 kg ha 1 , and grown under nonflood intermittent irrigation for two growing seasons. Five common weeds were allowed to emerge at 15, 24, and 30 days after rice emergence (DAE), when farmers usually decide about early, intermediate, and late herbicide applications. Yield losses were predicted using hyperbolic models with independent variables deseribing the mixed-weed infestations in terms of density (no, of plants m -2 ), leaf area index, dry matter m -2 , relative density [weed/(weed + rice)], relative leaf area (RLA), and a visual estimate of relative ground cover (RC,). With early weed emergence (15 DAE), weed density accounted poorly (r 2 = 0.77) for yield loss. Regression fits improved when RLA (r 2 = 0.86) and RC, (r 2 = 0.90) were used as independent variables to describe weed infestations in terms of the light apportionment between rice and weed canopies. Measured in terms of RC, late-emerging (24 and 30 DAE) weeds had lower effects on yield loss variability (r 2 = 0.69 and 0.59, respectively). Although subjective, RC, is easier to estimate than RLA, and was the best single variable to describe the competitiveness of a mixed-weed infestation. An additional variable was needed only when yield losses were predicted from weed density. Predictions based on RLA were further improved (by 36%) when each species' RI was measured separately and the model was extended for additive effects of all species. Yield loss predictions using empirical equations cannot be extrapolated widely across different locations; however, these data suggest that meaningful independent variables can strengthen the usefulness of hyperbolic equations for predicting rice yield losses over a range of situations, such as mixtures of weed species at various densities and times of emergence, and different rice seeding rates.

Continuous rice (Oryza sativa L.) cropping in Latin America and the Caribbean has resulted in serious weed problems and herbicide overuse. Competitive rice cultivars could help reduce herbicide dependence. A study was conducted during 1994 and 1995 at Palmira, Colombia, to (i) assess the competitiveness of semidwarf irrigated rice plant types adapted to Latin America and the Caribbean's direct-seeding systems, (ii) identify plant traits responsible for such competitiveness, and (iii) detect adverse effects of competitiveness on rice yield potential. Pregerminated seed of 10 and 14 semidwarf rice cultivars was sown on drained puddled soil in 1994 and 1995, respectively. Cultivars were grown weed-free or with junglerice [Echinochloa colona (L.) Link] (40 viable seeds m -2 , broadcast immediately after seeding rice), and were intermittently irrigated to keep the soil near saturation. Rice and junglerice biomass, leaf area index, tiler number, and height were recorded at 20, 40, 60, 90, and 120 days after emergence (DAE). Rice cultivars differed in their competitiveness against junglerice. Average yield losses ranged from 27 to 62% under saturating junglerice infestations of up to 5.9 Mg DM ha -1 . Leaf area index, tiller number, and canopy light interception recorded in competition, and not much before 40 DAE, correlated positively with rice competitiveness. Competitive semidwarf cultivars can substantially reduce the number of herbicide applications in systems where suboptimal water control does not allow weed suppression by flooding. Breeding to enhance rice competitiveness appears as a valid objective, since competitive and also highly productive cultivars were identified in this study.