Effects of Emergence Time in Nursery and Density of Watergrass (Echinochloa oryzoides) on Competitive Ability with Rice (I. During Season Studies: Growth and Development)

Investigating the competitive effects of watergrass (Echinochloa oryzoides) as a new-introduced weed species in rice production of Guilan province is of major importance. Consequences of watergrass–rice competition are influenced by each species proportion and density; although could be greatly affected by relative weed seedling emergence time. Furthermore to analyze crop-weed competition, the variation in plant traits that confer competitive ability should be considered. Total dry matter and leaf area as the basic processes in vegetative growth, and plant height, dry weight, and leaf area index as the indications of the relative size, photosynthetic capacity, and productivity, affect competitive ability. Rice traits were associated with competitiveness against weeds included Initial biomass, crop growth rate, leaf area index, and biomass at tillering. Documenting watergrass and rice differences in growth or structure and plants changes over the course of a growing season contribute to explain the severity of watergrass as a weed in transplanted rice. The objectives of this study were to compare growth characteristics of weed and crop species and investigate growth pattern changes during the growing season.

During two years of a field study, the effect of watergrass seedling ages at the time of transplanting (10, 20, and 30 day), and planting proportions of watergrass: rice in each hill ( 0:4, 1:3, 2:2, 3:1, and 4:0, (weed: rice)) were examined as a factorial based on randomized complete block design with three replications. Growth characteristics of rice and watergrass under full-season competition evaluated by six and five destructive samples respectively in first and second years of experiment. At each sampling interval, plants were returned to the laboratory, where the height (by recording the highest extended leaf), the number of tillers (by hand counting total number of tillers) the leaf area index (by measuring the leaf area with an automatic area meter (Li-cor ,LI-2500)), and dry weight of leaves, stems and total plant (by drying at 75 C until a constant weight was reached) were determined separately for each weed and rice species. Sampled plots were not sampled at subsequent harvests. Below-ground biomass was not included in the measurements.

According to the results, tiller number, leaf area index, leaf dry weight, and total dry weight of rice increased with increasing seedling number in plant proportions; while final height did not affect. For watergrass, tiller number and total dry weight were also influenced by plant proportion, and increased as weed ratio at the planting proportion was raised. planting proportion caused no significant effect on final height, maximum leaf area index and leaf dry weight of watergrass, and differences among treatments disappeared over the growing season.At the planting proportion of 1:3 (weed: rice) tiller number of two species was similar in most samples, but was higher for weed than rice at the other mixtures. Maximum leaf area index, leaf maximum dry weight, and total dry weight of watergrass in all planting proportions were higher than rice. Weed seedling ages had significant effects on tiller number, leaf area index, and leaf dry weight of two species, and also total dry weight of watergrass at the first year, but such a effect did not observe in the second year.

Totally based on results, it could be stated that watergrass is more competitive than rice; where the relative aggressiveness of component species was significantly higher for rice than weed. this is probably because of more growth compensation ability of weed through rapid growth and high tillering when it germinate later in the growing season, and because of its stronger competitiveness relative to the rice.