Almost 30 herbicides are in California rice growers weed control arsenal. It takes that many to control weeds in the various growing regimes and with the constant challenge of weed resistance to herbicides.

Fortunately for growers and pest control advisers at the annual Biggs, Calif., Experiment Station rice field day recently, there was a scorecard to keep up with it all.

The trials conducted directed by University of California Davis weed scientist A.J. Fischer covered three water management regimes; continuous flooding; pinpoint flood management; leathers method of flooding. A separate trial off the station addressed the growing issue of herbicide-resistant watergrass.

Continuous flooding

In continuous flooded rice where water is maintained at four inches, except when drainage is necessary for 70 percent weed exposure to a foliar herbicide application. The combination of Cerano followed by either Londax, SuperWham (propanil) or Granite GR provided excellent broad-spectrum control at a station location where all weeds are susceptible and there are no apparent resistance issues.

Granite GR was available for the first time in California this season.

Other excellent broad-spectrum combinations were: Bolero followed by either Regiment, Granite GR or Shark; Shark tank mixed with Granite GR; Abolish followed by SuperWham, Regiment or Granite GR; Ordram followed by SuperWham; Granite GR alone or followed by either Stam (propanil) or Clincher.

Good control was also achieved with combinations of Cerano followed by Grandstand or Granite SC; Ordram tank mixed with Shark; Bolero followed by SuperWham; or three way combinations of Bolero followed by Shark followed by either SuperWham or Regiment.

Granite worked very well at this site and was less phytotoxic to rice than the granular.

According to the UC weed scientists, Cerano causes mild to severe bleaching of rice but the crop usually grows out of it. Regiment and Granite cause stunting and darkening of rice, while Granite also causes stunted root growth for a period of time after application. The crop seems to recover from these effects, according to the researchers.

Pinpoint flood

In the pinpoint flood management system, rice was water seeded. The water was drained for early foliar herbicide treatments. Fields were then re-flooded within 48 hours. The drainage period allows certain weed species to germinate in the aerobic environment. This is particularly true for sprangletop, which is generally found in continuously flooded rice.

Other problem weeds with this water management are barnyardgrass and smallflower umbrellasedge. Residual herbicides can be especially helpful when drain time is prolonged due to time needed to re-flood large fields, according to Fischer.

Regiment plus Abolish in a tank mixed applied at the 3- to 4-leaf stage continues to produce excellent watergrass control due to the synergistic nature of this mixture. It was also “very safe” on rice at this timing. Regiment alone at the same growth state provided excellent control of watergrass at the Biggs station research field.

Sequential applications of Regiment followed by Clincher also provided excellent control of watergrass and sprangletop; Regiment followed by SuperWham resulted in good broad-spectrum control except sprangletop. Depending on initial weed spectrum, the sequence can be reversed with propanil as the first herbicide with either Clincher or Regiment as a follow-up treatment.

Clincher followed by either Stam, Granite SC or Regiment provided excellent broad-spectrum control as also did Stam plus Granite SC. It worked very well at the Biggs research station site and was less toxic than Granite GR. Granite SC is a liquid formulation of the same chemistry in Granite GR. It was less phytotoxic to rice than the granular on the Biggs research plot.

A tank mix of Clincher, Stam and Granite SC also gave broad-spectrum control. Tank mixtures of Whip with SuperWham or Whip with Regiment eventually followed by SuperWham provided broad-spectrum control.

Another broad-spectrum (including sprangletop) control tank mix is SuperWham and Abolish.

In another experiment, Prowl H20 was tested for residual control of germinating grasses in a pinpoint flood system. Prowl is a dinitroanaline soil active herbicide that could be useful when re-flood is delayed due to poor availability of water. Otherwise, it has short residual activity under flooded conditions, according to Fischer. It can be tank mixed with foliar-active herbicides for control of emerged weeds.

Prowl H20 in tank mix with SuperWham or Clincher controlled watergrass and sprangletop and was safe on rice. Prowl can be useful for watergrass and sprangletop control in pinpoint systems.

Leathers method

The Leathers' method is generally used where there is an incidence of strong winds during the establishment period of rice. The wind causes wave action, which can dislodge the young rice plant and cause uneven stands. Mimicking the Leathers' method, water in the research trial field was dropped after seeding to just a skim for rice establishment. Foliar herbicides were applied at 1- to 2-leaf stage of rice. Permanent flood was established 6 days after initial herbicide applications. This longer aerobic period encourages even greater weed pressure, especially sprangletop, barnyardgrass and smallflower. Prowl H20 and Abolish are residual herbicides that provide good grass control. Abolish will also help control smallflower. Prowl H20 will not control established weeds so tank mixes with foliar active herbicides are necessary. Prowl H20 is a different chemistry than other rice herbicides so it may be useful on resistant watergrass. Applications with Prowl were made onto drained ground. At the susceptible watergrass site Prowl H20 provided 80 percent watergrass and sprangletop control, while Abolish provided 76 percent control. Tank mixture of Prowl H20 with SuperWham gave good control except for bulrush.

These sequential combinations provided good to excellent broad-spectrum control: Prowl H20 tank mixed with Clincher followed by SuperWham; Prowl H20 Clincher followed by Regiment; Granite SC tank mixed with Clincher; Abolish followed by SuperWham; Abolish tank mixed with; Clincher followed by SuperWham; Clincher followed by Regiment. Granite SC provided better control of watergrass (96 percent) than Prowl alone, but failed to control sprangletop. Clincher provided near complete control of watergrass and total control of sprangletop.

Granite is a broad spectrum ALS-inhibiting herbicide performed excellently except on sprangletop, which it is not meant to control. It can be tank mixed with Clincher for control of that weed. Poor control of certain Londax-resistant weeds or resistant watergrass may occur and sequential treatments with propanil may be necessary. Granite should not be applied in mixture or sequence with another ALS inhibitor (Londax, Regiment, or Sempra), warned Fischer. The SC formulation may be available as soon as 2006.

Resistance research

Herbicide resistant research was conducted at a Glenn County grower's field heavily infested with herbicide resistant late watergrass (“mimic.”)

In continuously flooded rice, propanil alone only controlled watergrass by 50 percent at this site. Regiment tank mixed with Abolish provided 80 percent control of resistant watergrass. Granite GR followed by SuperWham controlled watergrass by 84 percent, bulrush by 71 percent and ducksalad by 81 percent. However, the watergrass control with this treatment was mostly due to the SuperWham application, according to Fischer. Watergrass at this site is presumably resistant to Granite.

In the pinpoint flood experiment on this same site, sequential combinations tended to work the best. SuperWham followed by Clincher controlled watergrass by 83 percent but failed to control bulrush. Regiment followed by Clincher controlled watergrass by 84 percent and also provided 78 percent control of bulrush. However, this is poor resistance management, since watergrass is resistant to both herbicides and this should result in an increment of the resistant population for the next season.

Regiment tank mixed Abolish followed by SuperWham provided the best watergrass, Bulrush and ducksalad control (84, 98 and 90 percent respectively). SuperWham by Regiment provided the best watergrass control (93 percent).

Drill-seeded rice

M206 was drill-seeded; flushed with water and drained twice for establishment, immediately after seeding and again 10 days later. A final permanent flood (3 to 4 inches) was applied when rice was at the 5-leaf stage (25 days after seeding). Aquatic weeds are not generally a problem early in drill seeded rice, while the main weeds in this system were watergrass and sprangletop. Prowl H20 alone applied as a delayed pre-emergent. This was 7 days after initial flush of water and rice weeds had not emerged. This single application of the dinitroanaline controlled 72 percent of the watergrass and 92 percent of the sprangletop. Control was improved when Prowl was tank mixed or in sequential combinations with foliar-active herbicides.

Prowl H20 plus Clincher applied early post emergent (two days after the second flush of water and drain, with rice at the 1- 2-leaf stage) controlled watergrass by 93 percent and sprangletop by 96 percent. Prowl H20 plus SuperWham controlled watergrass by 95 percent and sprangletop by 85 percent. Prowl H20 seven days after the initial flush of water followed by Clincher at the 5- to 6-leaf stage (prior to permanent flood) controlled watergrass by 99 percent and sprangletop by 97 percent.

The tank mix of Abolish plus SuperWham two days after the second flush of water and drain provided 92 percent watergrass and 98 percent sprangletop control. Abolish at seven days after initial flush followed by Regiment applied post permanent flood provided 95 percent control of watergrass and 97 percent control of sprangletop. Abolish seven days after initial flush followed by SuperWham five to six days prior to permanent flood was also an excellent treatment, although watergrass control was weaker. Clincher followed by SuperWham prior to permanent flooding controlled watergrass by 99 percent and sprangletop by 91 percent.

e-mail: hcline@primediabusiness.com