Managing Water For Weed Control in Rice (2/3)
J. F. Williams, S. R. Roberts, J. E. Hill, S. C. Scardaci, G. Tibbits
Rice growth and performance
The combined potential for stand loss, slow emergence, and weak rice plants make rice growers dislike deep-water culture. In standing water, growth is always stressful during seeding establishment, and deep water can jeopardize the crop. It is important, then, to determine a safe limit for water depth that will maximize the suppression of weeds without unacceptable risks. Several of our measurements demonstrated the effects of water management on rice growth. Plant population at 28 DAP (table 2) was not significantly affected by water depth, although there was a small but significant reduction in herbicide-treated plots. Even though populations varied greatly from year to year, stands were adequate for maximum crop performance each year in every treatment.

Table 2. Effect of water management, with and without herbicides, on agronomic performance of M-201 rice
Water treatment	Plant population #/sq. ft.	Days to 50% heading	Plant height (cm)	Lodging (%)	Harvest moisture (%)	Grain yield @14% moisture (lb/ac)
Without herbicides
Shallow	29.2	103 a	70 c	14.7	28.1 a	2,079 e
Moderate	28.6	97 bcd	80 ab	5.6	26.1 b	4,502 cd
Deep	27.6	94 f	80 ab	1.0	20.9 cd	6,366 b
Early drain	32.7	98 bc	81 a	9.6	25.7 b	4,501 cd
Late drain	29.4	98 bc	80 a	1.9	24.2 b	3,958 d
Lowered	31.0	96 cd	80 a	6.0	24.3 b	5,216 bc
Mean	29.7 a	97.6 a	78.4	6.8 a	24.9 a	4,437 b
With herbicides
Shallow	32.2	96 cd	80 ab	1.0	21.3 cd	8,813 a
Moderate	27.7	93 fg	78 ab	1.0	19.6 d	9,007 a
Deep	25.1	92 g	77 ab	1.0	19.9 cd	8,835 a
Early drain	27.2	93 fg	77 ab	1.0	20.2 cdf	9,080 a
Late drain	25.3	94 ef	79 ab	1.0	21.6 c	8,622 a
Lowered	24.4	93 fg	76 b	1.0	19.6 d	8,543 a
Mean	27.1 b	93.4 b	77.8	1.0 b	20.4 b	8,815 a
LSD, 5%
Herbicide	*	***	ns	***	***	***
Water	ns	***	*	ns	***	***
H x W	ns	1.7	3.9	ns	2.0	1,166
CV, %	19.4	1.9	5.2	203	9.3	18.7
Means for treatments are not significantly different at P = 5% if followed by the same letter.

Rice growers see rapid development as a sign of successful stand establishment, and equate slow emergence and development with poor performance. Time to 50% emergence was estimated visually at 9, 14, and 19 to 21 DAP for shallow, moderate, and deep treatments, respectively. The overall appearance of the crop in shallow water during stand establishment was that of a rapidly covering stand of vigorous plants; in deep water, stands looked thin and plants spindly, with leaves lying on the water and much water surface exposed. These visual differences quickly disappeared as the crop developed. Leaves lying on the water surface have been associated with leaf miner damage, although we did not observe this problem in our study.
Measurements of leaf stage, number of tillers, and biomass accumulation over time (fig. 4) show that rice developed more quickly in shallow water than in deeper water. These differences were greatest early in the season, and diminished with time, eventually becoming similar at all water depths.
Despite its slower start, rice in deep water produced heads about 4 days before rice in shallow water (table 2). Temperature differences did not explain this phenomenon, since the deep water was generally cooler than the shallow water. Earlier heading may have been a stress reaction. Grain moisture content at harvest was greater with shallow water, a further indication that water depth affects maturity. Height and lodging were affected by weed competition, but not directly by water treatment.
Grain yield (table 2) was affected by herbicide treatments and water management. The average grain yield for all water treatments without chemicals was half that of the chemically treated plots. There were no yield differences among water treatments where herbicides were used, but in nonchemical plots, some water treatments did better than others. The highest-yielding nonchemical treatment was deep, which produced 72% of the yield of the deep treatment with chemicals; the lowest-yielding was shallow, which yielded 23.5% of the yield of the shallow treatment without chemicals.
Although the 3-year mean yields of chemically treated plots did not differ, yields did differ within individual years. In 1985, shallow and late drain treatments yielded less because low temperatures during and after herbicide application reduced the chemical's efficacy; in 1986, yields for the deep treatment were lowest because adverse soil conditions (possibly straw residue) affected crop development. From this, we conclude that there is a risk of poor weed control if water is too shallow or remains drained for too long a time, and a risk of poor crop performance if water is too deep. The objective of rice water management is to seek a balance between these two risks and thus maintain optimum weed control and maximum yields.
Cultivar performance
In small, replicated plots in 1986-1987, neither seedling vigor, plant height, lodging, nor yield was affected by water depth across six cultivars hand-sown at 150 lb/ac. Grain moisture at harvest was lower in deep than in shallow water. However, when the same cultivars were evaluated at different seeding rates at the three water depths, we got somewhat different results (table 3, main effects only). Increased seeding rates, averaged across six cultivars and three water depths, produced the highest yield at the highest rate. Yield decreased with increasing water depth at the low rate, but was less affected by water depth at seeding rates of 150 or 225 lb/ac.

Table 3. Effect of seedling rate and water depth on yield for rice cultivars
	Seedling rate (lb/ac)
Water depth	75	150	225	Mean
Shallow	9,839 a	9,557 abc	9,757 ab	9,716
Moderate	9,325 c	9,559 abc	9,839 a	9,537
Deep	8,693 d	9,378 bc	9,526 abc	9,496
Mean	9,286 b	9,496 ab	9,702 a	9,496
LSD, 5%
Water (W)	ns
Cultivar (C)	***
Rate (R)	***
W + C	***
W + R	***
C + R	***
W + C + R	ns

Long-grain cultivar L-202 was most sensitive to changes in seeding rate and water depth. Its yield increased with higher seeding rates and decreased with deeper water: yields in deep water at a high seeding rate were equal to or higher than in some treatments with lower rates and shallower water. These data suggest that for some cultivars additional seed may partially compensate for the effects of deeper water. Other cultivars tested (S-201, M-201, M-202, M-401, and A-301) were generally less responsive to seeding rate and water depth interactions.
Vigor, biomass production, and height may relate to a cultivar's response to increased water depths and seeding rates. L-202 has the least vigor, height, and vegetative growth of the cultivars tested, and was very responsive. S-201, in contrast, is taller, more vigorous, and more vegetative, but was less responsive to changes in seeding rate and water depth.
Summary and discussion
Clearly, deep water culture provides substantial control of some (not all) rice weeds, but cannot sustain maximum rice yields in the non-rotated cultural system currently used in California. However, water management is an important complement to a judicious herbicide program, and where conditions limit the efficacy of herbicides, deeper water and a lack of late drainage will improve weed control. Because weedy grasses are more sensitive than rice to water depth, growers will be able to reduce herbicide applications or dispense with them altogether in some situations. Over time, deep water treatments may select for E. oryzoides, which is somewhat more tolerant of deep water and more difficult to control with herbicides than E. crus-galli. To control weeds like roughseed bulrush that can tolerate deep water, growers will continue to rely on chemical control, crop rotation, and alternative systems. In a nonchemical rice system relying on deep water, roughseed bulrush will eventually become the dominant competitive weed.
While we achieved maximum yields in these trials with 8 inches of water, this depth could cause too much stress in fields with a history of difficulties in stand establishment resulting from such soil-related problems as salinity, alkalinity, and soil texture. We suggest 7 inches as a safer maximum depth that will allow rice to emerge and provide a good yield, but that will adequately suppress sensitive weeds. Additional seed may help offset the effects of growing rice in deep water, particularly for cultivar L-202. Growers using deeper water must also be vigilant for rice leaf miner, which is less of a problem in shallow water.
Mainstream adoption of these practices may be hindered by the perceived risks and difficulty of managing deep water, and because herbicides are so effective and so easy to use. Organic rice farmers and others who do not use pesticides are more likely to adopt these techniques. However, regulatory pressure and loss of agricultural chemicals should encourage mainstream rice growers to use all available tools to meet the challenge. We hope that rice growers will experiment with these practices at first, and eventually come to view them as useful tools to help them reduce costs, improve weed control, and reduce the chemical residues in their drainwater.
J.F. Williams is Cooperative Extension Farm Advisor, Sutter-Yuba counties; S.R. Roberts is Staff Research Associate, Agronomy and Range Science Extension, UC Davis; J.E. Hill is Agronomist, Agronomy and Range Science Extension, UC Davis; S.C. Scardaci is Cooperative Extension Farm Advisor, Colusa County; and G. Tibbits is a graduate student in Agronomy and Range Science at UC Davis.
The authors would like to thank the Scheidel Farming Company, and particularly Jack and Brett Scheidel, for their support and cooperation in this project.

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