Research and IPM

Assess the potential for reduced herbicide use when watergrass emergence and early growth are delayed.

We conducted two pot experiments under field conditions. These were carried out in large flooded basins outdoors at Agronomy Field Headquarters following the same basic design followed in 2001 (weeds as targets in rice neighborhoods, competition between species manipulated by fertilization timing and light deprivation or water depth in different pots). The first experiment was designed to determine the effect of delayed fertilization and light deprivation on the competitiveness of late watergrass (Echinochloa phyllopogon) against rice (Oryza sativa, Cv. M-202). Rice and watergrass seedlings were germinated simultaneously, but the watergrass were subject to full sun or 50% shade or to delayed vs. preplant fertilization (equivalent to a rate of 150 lbs/ac) for one week. The stands were planted with one rice and one watergrass as targets within a stand of 7 rice plants in 10" pots and grown to rice maturation. The second pot experiment was similar, but used simultaneous plantings of the same target design, using water depth and fertilization delay as the main variables. Both late and early watergrass (E. oryzoides) were used. The field experiment was designed to simulate high water depth combined with fertilization delayed to midtillering compared to a control of standard water depth and preplant fertilization. Six checks of each treatment were planted; rice was sown at 200 lbs/ac but watergrass was derived solely from the seedbank.Watergrass was exclusively late watergrass; a few other weed species were present but only at low densities.

The first pot experiment showed that week long delays in fertilizer and reductions in light available did not significantly affect the outcome of competition. Variability within treatments was too high to demonstrate significant effects of light or fertilization reduction. The second pot experiment showed that early watergrass is so competitively superior to rice that neither water depth nor fertilization delay significantly affected the outcome. The watergrass matured successfully and earlier than the rice, while rice grain production was substantially reduced. These results are consistent with reductions observed in the field. On the other hand, late watergrass was significantly affected by water depth and fertilization delay. Evidently, there is potential for these management treatments to affect late watergrass competition in rice. The field experiment with rice and late watergrass was even more clear evidence of the effect of delayed fertilization. Preplant fertilizer incorporation resulted in heavy stands of vigorous watergrass and a corresponding reduction in rice. Delayed fertilization reduced watergrass vigor so that the two species were much more even in relative density. However, delayed fertilization did carry a penalty in reduced yields, jointly with the effect of competition by watergrass.

Considered together, the 2002 experiments illustrate how different early and late watergrass are as competitors with rice. Where early watergrass is the species of concern, manipulation of water and fertility does not seem to offer much potential for culturally-based weed control. Viewed in reverse, the conventional practice of limited water depth and heavy preplant incorporation of fertilizer establishes conditions where herbicide control of early watergrass is absolutely necessary. With the number of feasible herbicides being curtailed by the rapid rise in resistance, conventional practices are increasingly unsustainable. Where late watergrass is the weed of concern, manipulation of fertility and water depth offer hope for improved control and limited use of herbicides. These results suggest that delaying fertilization by skipping preplant fertilization may play a substantial role in reducing the competitive advantage of late watergrass.

We conducted two pot experiments under field conditions. These were carried out in large flooded basins outdoors at Agronomy Field Headquarters following the same basic design followed in 2001 (weeds as targets in rice neighborhoods, competition between species manipulated by fertilization timing and light deprivation or water depth in different pots). The first experiment was designed to determine the effect of delayed fertilization and light deprivation on the competitiveness of late watergrass (Echinochloa phyllopogon) against rice (Oryza sativa, Cv. M-202). Rice and watergrass seedlings were germinated simultaneously, but the watergrass was subject to full sun or 50% shade or to delayed vs. preplant fertilization (equivalent to a rate of 150 lbs/ac) for one week. The stands were planted with one rice and one watergrass as targets within a stand of seven rice plants in 10" pots and grown to rice maturation. The second pot experiment was similar, but used simultaneous plantings of the same target design, using water depth and fertilization delay as the main variables. Both late and early watergrass (E. oryzoides) were used. The field experiment was designed to simulate high water depth combined with fertilization delayed to mid-tillering compared to a control of standard water depth and preplant fertilization. Six checks of each treatment were planted; rice was sown at 200 lbs/ac but watergrass was derived solely from the seedbank. Watergrass was exclusively late watergrass; a few other weed species were present but only at low densities.

The first pot experiment showed that week long delays in fertilizer and reductions in light available did not significantly affect the outcome of competition. Variability within treatments was too high to demonstrate significant effects of light or fertilization reduction. The second pot experiment showed that early watergrass is so competitively superior to rice that neither water depth nor fertilization delay significantly affected the outcome. The watergrass matured successfully and earlier than the rice, while rice grain production was substantially reduced. These results are consistent with reductions observed in the field. On the other hand, late watergrass was significantly affected by water depth and fertilization delay. Evidently, there is potential for these management treatments to affect late watergrass competition in rice. The field experiment with rice and late watergrass was even more clear evidence of the effect of delayed fertilization. Preplant fertilizer incorporation resulted in heavy stands of vigorous watergrass and a corresponding reduction in rice. Delayed fertilization reduced watergrass vigor so that the two species were much more even in relative density. However, delayed fertilization did carry a penalty in reduced yields, jointly with the effect of competition by watergrass.

Considered together, the 2002 experiments illustrate how different early and late watergrass are as competitors with rice. Where early watergrass is the species of concern, manipulation of water and fertility does not seem to offer much potential for culturally-based weed control. Viewed in reverse, the conventional practice of limited water depth and heavy preplant incorporation of fertilizer establishes conditions where herbicide control of early watergrass is absolutely necessary. With the number of feasible herbicides being curtailed by the rapid rise in resistance, conventional practices are increasingly unsustainable. Where late watergrass is the weed of concern, manipulation of fertility and water depth offer hope for improved control and limited use of herbicides. These results suggest that delaying fertilization by skipping preplant fertilization may play a substantial role in reducing the competitive advantage of late watergrass.

We conducted a pot experiment under field conditions. This was carried out in large flooded basins outdoors at Agronomy field Headquarters following the same basic design followed in 2000 (weed as targets in rice neighborhoods, competition between species limited to roots, shoot, or both in different pots). This experiment had the primary objective of measuring the relative importance of root vs. shoot competition between rice and two weeds (early watergrass, Echinochloa oryzoides, and redstem, Ammannia coccinea), with a water depth variable substituted for the target weed planting delay used in the first year. The two-year results of these experiments show that watergrass responds with greater reduction in biomass production, greater than predicted additively from root and shoot components alone, with delayed fertilization alone or delayed fertilization plus delayed germination/greater water depth compared to germination delay/water depth alone. This was a surprising result; we knew that germination delay was important, but the magnitude of the interaction effect with fertility, or the effect on watergrass growth of fertility alone proved larger than we expected. These results suggest that delaying fertilization by skipping preplant fertilization may play a substantial role in reducing early watergrass competitive advantage.

The results with redstem in 2001 confirmed the results seen in 2000. Unlike watergrass, redstem shows no strong impact of delayed fertilization or delayed germination, presumable because of its capacity to elongate later in the growing season. Although the analysis is still incomplete, our results suggest that what might work reasonably well for Echinochloa is unlikely to work well for Ammannia control.

We conducted three greenhouse/growth chamber experiments. The first was done in the growth chamber to determine if watergrass germination was affected by light and/or anoxia. This experiment showed no significant effect of light on watergrass germination, but seed germinated better under anoxic conditions.

The second was a greenhouse experiment to determine the interaction of water depth and light on watergrass growth in rice stands. Both rice and watergrass are significantly affected by combining greater water depth and reduced light, but watergrass is affected more.

The third experiment was conducted in the greenhouse to study the interactions of fertility, delayed germination and restriction of either or both of root and shoot competition on the growth, and canopy penetration of watergrass and redstem in rice.

This was a complex experiment which has yielded complex results. Watergrass responds to withheld fertility while redstem does not. Although watergrass is the more widespread and important weed, control programs combining fertility and water depth to suppress its early growth are promising. With its delayed stem elongation, redstem is not sensitive to fertility control or early canopy closure and may prove to be the harder weed to control.