Managing Water for Weed Control in Rice (1/3)

Managing Water For Weed Control in Rice (1/3)
(This article first appeared in California Agriculture, 1990, 44:5:7-10)
J. F. Williams, S. R. Roberts, J. E. Hill, S. C. Scardaci, G. Tibbits
A strategy of slightly deeper water, no draining, and lower grass herbicide rates can help rice growers maintain weed control and sustain high yields while lowering the economic and runoff costs of herbicides.
Over the past decade, a rice cultural system has developed to maximize the performance of modern short-stature cultivars. The system includes precision leveling, very shallow flooding, temporary draining, and high fertilization rates. Unfortunately, this cultural system also promotes weeds that may require high rates and multiple applications of herbicides for control, and that increases the economic, social, and environmental costs of growing rice. Growers are spending more for weed control, and in 1981 objectionable levels of rice herbicide residues were discovered in public waterways. Rice growers must now follow increasingly strict regulations to keep residues within allowable limits. They rely mainly on tailwater return systems and ponding on fallow land, but the industry needs additional tools to control rising production costs and to keep up with residue "performance goals" that get lower every year.
From 1985 through 1987, we examined the management of water in California rice production, looking for ways to reduce herbicide use while maintaining crop performance. Specific research objectives included determining rice and weed response to water depth and drainage; estimating the degree of control provided by water alone; and evaluating cultivar responses to different water depths. Our results have economic and environmental value for conventional and alternative rice producers.
With funding from the Statewide IPM Project, we established a large-scale, multiuse water management facility in a commercial rice field in Sutter County, California. The 34-acre site included 36 individually irrigated and drained basins arranged with 18 basins on each side of a central irrigation canal. On one side of the canal, the basins received no herbicide treatment; on the other side, basins received annual aerial applications of 5 lb/ac molinate (Ordram 10G) for watergrass control and 1 lb/ac bentazon (Basagran) plus oil for broadleaf and sedge control. Each set of 18 basins comprised three replicates of six water management regimes that represented a range of those typically used in California rice production; the same randomization was used every year:

shallow - continuous 2-inch water depth
moderate - continuous 5-inch water depth
deep - continuous 8-inch water depth
early drain - drain 2 days after planting (DAP) for 4 to 5 days, and reflood to a 5-inch continuous water depth
late drain - flood to 5 inches until about 20 DAP, drain until about 30 DAP, and reflood and hold at 5 inches continuous water depth
lowered - flood to 8 inches for 21 DAP, lower to 5 inches, and hold

Treatments 1 through 3 (shallow, moderate, and deep) represent the full range of water depths used in California, and were established before rice emergence so the plants would have to grow through the entire depth of water. Early drain (treatment 4) provides an aerated environment during stand establishment, favoring root growth over shoot growth, speeding root penetration into the soil. Late drain (treatment 5) simulates a practice followed when rice fields are stressed: draining helps relieve the stress and promotes recovery. Lowered (treatment 6) was a commonly used method before the advent of herbicides, and takes advantage of the differing ability of rice and weeds to emerge through deep water. By lowering the water at a strategic time, the grower gives the rice an advantage over the weeds. Each treatment was held for approximately 75 days after planting, and then all water levels were raised to about 8 inches for the remainder of the water season. Figure 1 shows all treatments in one replication at mid-season. The lower basins were treated with a conventional herbicide regime described previously. The upper basins had only water for weed management (no herbicides).

Figure 1. Aerial view of the trial showing all treatments (1-6) in one replication at mid-season. The lower basins were treated with a conventional herbicide regime as described in the text. The upper basins had only water for weed management (no herbicides).

Pre-soaked seed (cultivar M-201) was sown by air at 150 lb/ac. We rated weed control visually on a 1 to 10 scale, where 1 equals no control and 10 equals complete control. We also collected data on rice plant population, days to 50% heading, height, lodging, grain moisture, grain yield, water temperature, rice growth, and cultivar performance.
Weed control
The initial weed composition was a uniformly distributed stand of common rice weeds (figure 2): barnyardgrass, smallflower umbrellaplant, redstem, and ducksalad were most common and uniformly present. Watergrass, not present in the native weed population, was sown at 10 seeds/ft² in 1985 in 10 x 80-foot plots in each basin. Roughseed bulrush was not uniformly distributed at the start, but it spread throughout the trial site during the course of the experiment. Weed growth in response to water treatments is discussed in water management effects on rice weed growth and corresponds reasonably well with the control ratings discussed here.



Figure 2. The initial weed composition was a uniformly distributed stand of common rice weeds (clockwise from upper left): barnyardgrass, smallflower umbrellaplant, redstem, and ducksalad were common and uniformly present. Photos by Jack Kelly Clark

Grass control ratings in the 10 x 80 foot plots (table 1) improved with increasing water depths. The best weed control without herbicides was in the deepest water, while shallow water and early drainage gave poor control. Molinate gave adequate control of grasses in all water treatments except shallow and late drain. Cold weather lowered the herbicide's activity and resulted in poorer control in 1985, reducing the 3-year mean for that treatment. Barnyardgrass control in late drain was also lower in 1987. Under optimal conditions for herbicidal activity, molinate provided excellent control in all water treatments.

Water treatment	Barnyardgrass & watergrass	Roughseed bulrush	Smallflower umbrellaplant	Redstem	Ducksalad
*Table 1. Control ratings for major weeds^, 1985-1987 combined analysis**
Without herbicides
Shallow	1.6 g	5.8 bcd	4.0 d	4.2 c	6.2
Moderate	4.9 e	5.8 bcd	7.7 bc	4.4 c	5.1
Deep	7.7 c	6.4 bc	9.0 ab	6.9 b	6.4
Early drain	3.5 f	6.8 b	7.6 c	5.2 c	5.9
Late drain	5.3 d	5.4 cd	4.2 d	4.7 c	5.9
Lowered	6.1 d	4.7 d	7.8 bc	5.2 c	4.9
Mean	4.8 b	5.9 b	6.7 b	5.1 b	5.7 b
With herbicides
Shallow	8.3 bc	10.0 a	7.9 c	10.0 a	7.8
Moderate	9.0 ab	9.8 a	8.9 abc	9.8 a	8.4
Deep	9.7 a	10.0 a	9.8 a	9.4 a	8.5
Early drain	8.5 bc	10.0a	8.8 abc	10.0 a	7.8
Late drain	7.8 c	9.6 a	7.7bc	10.0 a	7.6
Lowered	9.0 ab	9.8 a	9.0 ab	9.8 a	8.3
Mean	8.7 a	9.9 a	8.9 a	9.8 a	8.1 a
LSD, 5%
Herbicide	***	***	***	***	***
Water	***	ns	***	ns	ns
Herbicide and water	1.04	1.2	1.4	1.2	ns
CV, %	16.2	16.2	18.9	16.4	19.3
*Echinochloa crus-galli (barnyardgrass), E. oryzoides (watergrass), Scirpus mucronatus (roughseed bulrush), Cyperus difformis (smallflower umbrellaplant), Ammannia spp. (redstem), Heteranthera limosa (ducksalad) Means for treatment are not significantly different at P = 5% if followed by the same letter.

Estimates of the two Echinochloa species' populations at the end of each season revealed that barnyardgrass responded more than watergrass to increasing water depths: more of the surviving grass was watergrass in deep water (45%) than in shallow water (14.9%). The absolute amount of both species was less in deeper water, but the relative increase of watergrass in deep water suggests selective pressure in favor of watergrass, which growers generally consider harder to control.
Roughseed bulrush had spread throughout the untreated section of the site by the beginning of the second year. This weed was relatively unresponsive to water depth (table 1) and was the dominant weed species in nonchemical deep treatments that suppressed grasses. Grass weeds dominated roughseed bulrush in shallow water, but where grass weeds are absent, roughseed bulrush can be a serious rice competitor in shallow water if not controlled. Bentazon controlled it very well each year in every water treatment.
Smallflower umbrellaplant (table 1) responded well to water depth, with the poorest control achieved in the shallow and late drain treatments without herbicides. Bentazon also provided poor control in shallow and drained plots, so these treatments may stimulate the development of smallflower umbrellaplant and reduce the efficacy of herbicides.
Redstem ratings were only slightly lower (table 1) in deeper water than in shallow and drained treatments. Bentazon gave nearly complete control in all water treatments. Water depth is not critical in managing this weed, although it may be some help.
Ducksalad is a common rice weed that may have dense growth in open water areas, but because of its short stature it competes poorly in normal-density rice stands. Though ducksalad responded slightly to increasing water depths the first year, domination by more competitive weeds in later years rendered multiyear ratings less meaningful (table 1). Overall, ducksalad did not show any strong response either to water depth or to drainage. Bentazon provided only partial control of ducksalad in all water treatments, and there was no interaction of water and herbicide.
An increased water depth suppresses some rice weeds more than others. As measured by visual ratings, barnyardgrass, watergrass, and smallflower umbrellaplant are strongly affected, redstem species are slightly to moderately affected, and ducksalad and roughseed bulrush are slightly or not at all affected by an 8-inch continuous water depth treatment. Figure 3 shows the effect of shallow (1), moderate (2) and deep (3) water on weeds on the non-herbicide half of the trial. Note that in the shallow treatment weeds have over-grown the rice, whereas in the deep treatment, the rice is clearly visible. Other common rice weeds noted in the study were sprangletop (strongly affected) and California arrowhead (slightly or not at all affected).


Figure 3. The effect of shallow (1), moderate (2), and deep (3) water on weeds on the non-herbicide half of the trial. Note in the shallow treatment weeds have overgrown the rice, whereas in the deep treatment, the rice is clearly visible.

Increasing water depths also increased the herbicide's efficacy on watergrass and smallflower umbrellaplant. Conversely, shallow water and drainage increased the severity of weed infestations and reduced herbicidal efficacy. Bentazon activity against roughseed bulrush, redstem, and ducksalad was not influenced by water treatments.

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Modified: 4 Sep 1997 Comments to jayoung@ucdavis.edu