Effects of Emergence Time in Nursery and Density of Watergrass (Echinochloa oryzoides (Ard.) Fritsch) on Competitive Ability with Rice (Oryza sativa L.) (II. End Season Studies: Yield and Competitiveness)

Weed–crop competition as one of the main reasons for crop loss is often influenced by many factors including emergence time and density of competing species. Patterns of weed seedling emergence affect the outcome of weed–crop interference interactions. Information on weed seedling emergence in relation to crop seeding will assist in developing an optimum weed control program.Weeds emerge simultaneously with a crop, have the greatest potential impact on crop production. Results from Chauhan & Johnson (2010) showed the advantage for weeds over rice in situations where these species emerge earlier in the growing season because of significantly greater biomass production comparing to those emerging later in the season. Minimal shading and competition for nutrients and soil moisture are associated with early emergence.The importance of density in competition studies is because of the relationship among plant yield, number of individuals, and resources present. Increasing the density may enhance the plant’s share of the total resource pool and reduce resource availability for adjacent plant. Thus, to analyze competition between the crop and weeds, the variation in density should be considered. Season-long competition with watergrass at densities of 86 and 36 plants/m2 reduced rice yields by 59% and 46%, respectively.Understanding relative aggressiveness of component species is required for the integrated weed management and it would improve weed management strategies. For watergrass as a relatively new-introduced weed species in paddy rice fields of Guilan province, it is essential to investigate the effects of the emergence time in nursery, to determine weed seedling ages at the time of transplanting, as well the amount of weed seedling translocation characterizing weed density in the field.

Factorial arrangements of watergrass seedlings ages at the time of transplanting (10, 20 and 30 days), and plant proportions of weed:rice (0:4, 1:3, 2:2, 3:1, and 4:0) were designed as a randomized complete block with three replications to study yield characteristics and competitive ability of watergrass and rice over two years of a field experiment. Individual plants of each hill constituted as a single experimental unit. The area between individual hills was hand-weeded to avoid competition from other species. At rice maturity stage, plants were harvested, and rice and watergrass panicles were separated from stems by hand. Panicles were dried to a constant weight at 75 ℃, and weights were determined. Grain weights standardized to 12% moisture content. Biological yields of rice and watergrass were also measured after sampling their above-ground parts from an area of 1 m2 and drying at 75 ℃ until constant weight. Data were subjected to ANOVA, and means were separated using Fisher’s Protected LSD at P < 0.05. Competitiveness of the species was assessed based on the relative yield (the ratio between the production of the species in the mixture and in monoculture), relative yield total (total relative yield of the two associated species), aggressivity index and replacement series diagrams (models describing the possible outcomes of the interaction of two species when grown in a replacement series).

For each species, the greater ratio at the planting proportion, the higher biological yield, grain yield, relative yield, and aggressivity index, but the lower harvest index. When grown in monocultures, and 1 weed: 3 rice of the second year, the two species produced similar grain yield; however in the other mixtures watergrass produced more biological and grain yield compared to the rice. Harvest index of rice was greater than watergrass in all planting proportions. In the first year, trials of 30-day watergrass seedling ages represented the highest values of biological-, grain-, and relative yield, and the lowest values of grain and relative yield of rice and harvest indices of both species. Also, for all watergrass seedling ages in both years, weed production was more than rice.

Investigating grain and biological yield, and relative yield of both species, and aggressivity index of rice, beside replacement series diagrams exhibited higher competitiveness for watergrass comparing to rice. Therefore, it is important to maintain watergrass control in order to insure the sustainability of transplanted rice production.