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Project description

Cultural manipulation of crop/weed competitive relations in a rice cropping system. (02CC011)
Program UC IPM competitive research grants program
A.J. Fischer, Vegetable Crops, UC Davis
Host/habitat Rice
Pest Ricefield Bulrush Scirpus mucronatus
Discipline Weed Science
Cultural Controls
Start year (duration)  2002 (Three Years)
Objectives Confirm last year's results on the morpho-physiological responses of rice (cv. M-202) to the moisture limitation resulting from extended periods of field drainage during the early season.

Assess if such responses would confer upon rice a competitive advantage over bulrush when both species are grown together under imposed moisture stress.

Provide an objective and accurate criterion that farmers can use to time the duration of the drainage period to optimize non-chemical control of ricefield bulrush while maintaining satisfactory grain yield.

Explore if the different root dynamics of rice and bulrush relate to differential competitiveness for nitrogen, and if those differences can be accentuated to favor rice competitiveness through manipulation of fertilizer application timing during stand establishment.

Final report Ricefield bulrush (RFB) is a major semi-aquatic weed in organic rice crops in California. It has been suggested that organic farmers may successfully suppress RFB competition with rice using temporary moisture stress. However, the ecophysiological basis for defining the agronomic management of such an approach are unknown, and farmers, so far, rely on risky empirical tactics with mixed results. A two-year study in large flooded pots placed outdoors was conducted with rice and RFB, alone and in competition, exposed to a series of drought regimes. At the end of each drought regime, plants were reflooded and allowed to recover until final harvest. A brief drought (8-10 days) imposed by 33 to 35 days after seeding had little effect on rice growth and a moderate effect on its yield, but had a devastating effect on the ability of RFB growing in competition with rice to recover when reflooded, thus releasing rice from its competition. The deeper-rooted rice plants were able to rapidly dry the soil surface, while still capturing moisture from the deeper profile. RFB with only superficial roots was thus driven to severe moisture stress, which caused rapid death of aboveground green plant parts. Thus, physiological and morphological differences between rice and RFB would enable a carefully managed strategy of imposing temporary moisture stress to suppress this weed and allow rice to resume growth virtually weed-free. Subsequently, two field studies were conducted to identify optimal drought durations and intensities that suppress the growth of RFB while causing minimal rice growth reductions due to drought stress. Temporary drought reduced the competitive ability of RFB by causing greater leaf necrosis and slower re-growth than that of rice. However, if drought stress is not properly managed, rice growth reductions due to drought stress can exceed that which would be caused by RFB competition. Delayed drought durations in slow-drying areas may extend periods of low rice physiological activity and delay maturity to a time when temperatures are cooler, which may affect reproductive processes. Imposed drought stress may increase rice yield if used in situations where RFBwill be competitive, which may include RFB densities greater than 40 plants sqm or when expected rice yield loss due to RFB exceeds the anticipated drought stress duration.

A field study was established to identify optimal drought durations and intensities that minimize rice yield loss associated with Scirpus mucronatus (ricefield bulrush) growth and drought stress. Among the drought treatments, S. mucronatus necrosis was consistently greater than that of rice and S. mucronatus regrowth after each drainage period was less than that of rice. Using growth observations from this study, a process-based growth model was developed to evaluate the effects of drought duration, drought intensity, and species densities on rice yield. According to model predictions, rice yield loss associated with S. mucronatus competition was most greatly reduced by drought intensity, which was indicated by levels of necrosis at the end of the dry-down periods. However, rice yield loss associated with drought stress was most greatly increased by drought duration as predicted rice yield loss increased by approximately 1% per day of drought stress. Therefore, the expected proportional rice yield loss due to S. mucronatus growth in a nonstressed environment may be used to indicate the maximum duration of drought stress that should be imposed for weed control (e.g., no more than 30 d of drought stress if 30% yield loss is expected from S. mucronatus). Overextending drainage periods may result in greater yield loss from drought stress than would have occurred from S. mucronatus growth in a non-stressed environment. Imposed drought stress may be most effective when fields can be dried quickly with a minimum drought duration time. Rice yield loss may be further reduced by increasing rice population density.

Field experiments were established at the Rice Research Experiment Station in Biggs, California, and in growers' fields to explore the potential of induced drought stress as an alternative means for controlling ricefield bulrush. Mixed communities of bulrush and rice were drained until bulrush necrosis reached 30, 60, or 90%. The relationship between leaf necrosis and light use efficiency was defined to demonstrate the effects of drought stress on daily plant growth. These results indicated that most of the reduction in plant growth due to drought stress occurs between 0-10% leaf necrosis. However, at this level of drought stress, bulrush necrosis was approximately six-fold greater than that of neighboring rice plants. Therefore, 30% bulrush necrosis was generally associated with 5% rice necrosis. Since there was a 1:1 relationship between rice yield loss and percent rice necrosis, it was important to minimize drought stress to rice. Therefore, it was concluded that 30% bulrush necrosis was adequate for control. In summary, imposed drought stress gave rice a competitive advantage over bulrush and may be used as a management tool to control bulrush when herbicide options have failed. However, utilization of this approach for weed control may require intensive monitoring to prevent unnecessary rice yield loss.

Ricefield bulrush is a major weed in organic rice crops (cv. M-202). One approach that organic rice growers have had some initial success in managing this weed is to subject the weed (and corresponding rice crop) to drought, which seems to have a greater impact on bulrush than rice. Previous work supported that drought may be a useful tool in managing bulrush. However, the effectiveness of this method and the duration of drought needed to suppress the weed while minimizing adverse effects on rice yields are not known. This experiment is designed to test the impact of drought in effectively managing bulrush in organic rice crops and to assess whether an appropriate tool can be found for growers to use to manage the weed. This concept may also find applicability with conventional rice with the extra benefit of potentially reducing water use as a result of the extended non-flood period.

We conducted a pot experiment under field conditions in the Agronomy Field Headquarters, UC Davis. Monoculture and competitive mixes of rice and bulrush were initially grown for 32 days in saturated conditions and then subjected to 0, 8, 14, 21 and 27 days of drought. At each interval, plant physiology, above and below ground plant morphology, soil and environmental conditions were attained. Moreover, at each interval additional plants were removed from the drought regime and placed back under saturating conditions and permitted to continue development to assess the impact of competition and drought on rice yield.

Bulrush in the competitive mixes showed 50% death by day five and by day eight over 80% was killed, while in monoculture bulrush plants remained relatively unaffected. Belowground analysis showed that during the drought rice had developed more roots in the upper soil profile than bulrush and deeper roots as the drought progressed. The consequence of this was that rice quickly drew water from the soil, depriving bulrush of water, thus affecting bulrush.

Rice yield results gave some indication that bulrush may have been sufficiently suppressed by the drought. Yields were much higher in plants that were resaturated in water after exposure to eight and 14 days of drought than control plants, which had remained saturated for the whole duration of the experiment.

Results showed that environmental and soil components were not convincing indicators in determining the duration of drought required to suppress bulrush. Physiological parameters revealed the impact of the drought on plants, but perhaps the best indicator may be the amount of live-leaf length in rice left after drought exposure. Early analysis revealed a positive relationship between this indicator and rice yields where a greater live-leaf length corresponded to higher yields. This does require further investigation.

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