General review of the year:

Walter's barnyardgrass AKA coast cockspur (Echinochloa walteri) continues to spread across the valley. Kassim AlKhatib and I continue to work on understanding more about the biology and management of this new weed, and will have more data coming in the future about its competitive abilities and tolerance for different irrigation methods.

A greenhouse screening shows it is resistant or tolerant to most of our registered rice herbicides. The herbicides still showing the greatest efficacy (alone) are: Cerano® (clomazone), Clincher® (cyhalofop), and SuperWham®/Stam® (propanil).

While field studies are being conducted to look at late-applied tank mix combinations or sequential applications of many registered rice herbicides, we currently have no species-specific data (between late watergrass, Walter's barnyardgrass, barnyardgrass, etc.) so recommendations would be the same regardless of the specific species found in a particular field.

For specific recommendations for your field, please submit samples to the UC Weed Science Research Herbicide Resistance Screening Program with Kassim Al-Khatib.

General control recommendations for all watergrass species are:

Best grass control:

• Abolish® + SuperWham®/Stam 80DF®

• Regiment® followed by SuperWham/Stam® (may cause injury on certain specialty varieties) Good grass control):

• SuperWham®/Stam 80DF® + Loyant® • SuperWham® /Stam 80DF® + Shark H2O® (some phyto)

• Regiment® + Clincher®

• SuperWham® /Stam 80DF® + Clincher ®

New weed species:

At this point in time, there is no concern with finding this plant in a field, as it is not considered noxious. However, no herbicide recommendations are available at this time. It has been found in a field that is using no-till planting methods, and may be showing up there due to the selection pressure caused by no-till planting, which causes a shift from annual weeds to perennial weeds over time.

Figure 1. Eastern annual saltmarsh aster (Photo Credit: PictureThis). 

Delta rice acreage has been steadily increasing, and yields are comparable with the statewide average. I estimate that acreage approached, if not exceeded, 15,000 acres in 2024. This article is my seasonal update on UCCE Delta rice research and observations.

Variety Trial: UCCE collaborates with the California Rice Experiment Station to evaluate commercial varieties and advanced breeding lines. The San Joaquin County Delta was one of eight locations in the 2024 statewide trial. The Delta is the only drill-seeded site and is a test site for very-early maturing varieties because it has cooler growing conditions than other rice growing regions of the state. Variety trial results will be available in early 2025.

Armyworm Monitoring: In 2024, we monitored for true armyworms on three Delta farms, and moth catches peaked around July 1st (Fig. 1). I observed that feeding damage was highly variable across the three farms but also across fields on the same farm. This has important implications for in-season management and highlights the importance of scouting for crop damage and the presence of worms. UC IPM guidelines (https://ipm.ucanr.edu/agriculture/rice/armyworms/#gsc.tab=0) provide monitoring guidelines and treatment thresholds. While a second peak has sometimes been observed in the Sacramento Valley, we have not observed a second peak after heading in the Delta.

Weedy Rice: We need to stay vigilant in our efforts to prevent the spread and manage weedy rice. Early in the season, weedy rice is often mistaken for watergrass because it grows taller than the cultivated rice. However, I have noticed that watergrass (and barnyardgrass) will head sooner than weedy rice, and weedy rice has a lime green color in full light. There is a video on the CA Weedy Rice website (https://caweedyrice.com/) that can help with identification, or call your local farm advisor if you would like help. In-season management includes rogueing or spot spraying before viable seed is produced. The organic herbicide Suppress is registered for spot spraying. Post-harvest management should include straw chopping, but not incorporation, and winter flooding. This will keep seed on the soil surface where it can potentially deteriorate over the winter. With Whitney Brim-DeForest and Luis Espino, I will host a meeting for the Delta rice industry in early 2025 to provide weedy rice research updates and management information. Stay tuned for the meeting announcement.

Cover Cropping: With funding from the CDFA Healthy Soils Program and CA Rice Research Board, we are evaluating whether cover cropping improves soil carbon and nitrogen dynamics in the rice system. We are also assessing cover crop species performance, like survivability and biomass production. Since rice may be grown over multiple seasons without rotation, cover crops may provide an opportunity to introduce plant diversity, including nitrogen-fixing legumes. There are three trial locations: in the Delta, Colusa County, and Butte County. While the 2022-23 winter season was excessively wet, which hindered cover crop establishment, the 2023-24 season started off dry, so sowing and establishment were successful. We observed that the brassicas emerged quickly and started covering the soil after just one month, but when rainfall became more frequent after the new year, the brassicas died off. In contrast, the two vetches and balansa clover started off slowly but had vigorous stands by early spring, despite the wet conditions. For more information on Delta trial results, please visit my blog article (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=59659). The 2024-25 winter season will be our third and final year of trialing.

Disease Observations: In past years, I have been called out to fields to help diagnose diseases, which were later confirmed as stem rot, aggregate sheath spot, or rice blast. The 2024 season, however, was a relatively light disease year, and Luis Espino also observed that in the Sacramento Valley. Just to recap, it is important to scout for these diseases at late-tillering and early-heading because fungicide treatments are most effective when applied between late-boot and early-heading. Rice blast may be exacerbated by too much nitrogen, and stem rot and aggregate sheath spot by low potassium (K), so proper plant nutrition is a good strategy to mitigate disease. K can be limiting in some Delta soils, so one of my future goals is to do K fertilizer rate trialing to determine if it can reduce disease incidence and/or boost yields.

Herbicide Resistance Testing: UCCE, under the direction of Extension Specialist Kassim Al-Khatib, provides herbicide resistance testing for rice growers. If you suspect that weeds have developed resistance to certain herbicides, please collect mature weed seeds at the end of the season and submit them to your local farm advisor. 

Alternate Wetting and Drying: Earlier this year, I applied for funding from the Delta Science Program to evaluate the practice of Alternate Wetting and Drying (AWD) in the Delta. AWD is a management practice where a flooded field is temporarily drained during the growing season and then re-flooded. Research from other states and countries has shown that the practice can reduce methane emissions from rice fields, but there has not been research done in the Delta, with its unique soil and climate conditions. If the grant is awarded, this project would begin during the 2025 season.

I want to take this opportunity to thank all the growers who collaborated with us on these projects. I wish everyone a happy, healthy end to 2024.

Table 1. Rice acreage and yield.

*Rice acreage and yield according to the San Joaquin County (SJC) Agricultural Commissioner's Crop Reports. Rice acreage in SJC is primarily in the Delta region. Delta acreage in other counties is not included in these statistics. At the time of publishing, 2023 CDFA statewide data were not yet available (N/A).

 Figure 1. Delta true armyworm trap counts, 2016-2024.

This year we tested no-till (NT) drill-seeded planting of rice. This was our second year of these trials. No-till drill seeded planting offers some real opportunities to conserve water, plant early, save on tillage costs, and change the irrigation system which will shift weed species from a continuously flooded system (the system will be grass dominant).

This study was conducted at the Rice Experiment Station looking at N management, pests, diseases and weeds. We tested NT drill seeding into three different seedbeds and compared this to a conventional water-seeded system. The treatments of our study were:

1. Fallow stale-seedbed (FSS): field was fallowed in 2023. It was disked and leveled during the summer of 2023 and not flooded during the winter. No tillage was done in 2024 before drilling the rice in early spring.

2. No-till. We had two strict NT treatments. Rice was grown in 2023. After harvesting with care not to rut the field, the straw in the field was subjected to one of two treatments:

a.Chopped (NT-Chop)

b.Half removed to simulate baling (NT-Remove)

3. Water-seeded. Rice was grown in 2023. Straw was chopped and disked, and the field flooded during the winter to promote straw decomposition.

For the FSS and NT treatments, we planted May 1 using a NT drill seeder, flushed once after planting and then applied a permanent flood on May 30. Just before the permanent flood, we applied N fertilizer (urea) and herbicides (Pendimethalin, Super Wham and Loyant). On July 11 we applied Clincher. The water-seeded treatment was seeded on May 27 and managed conventionally. We harvested the FSS and NT plots on September 17 and the water-seeded plots on October 14.

The water-seeded treatment had the highest yield at 93 cwt/ac; this was followed by 84 cwt/ac in the FSS, 76 cwt/ac in the NT-Chop, and 74 cwt/ac in the NT-Remove. All of these treatments achieved maximum yields at N rate between 175 and 200 lb N/ac. These results are promising, but a bit different from last year. In 2023, we saw a yield reduction in the NT treatments, but the FSS and water-seeded yields were the same.

This year, several farmers tested these practices on their farms. While we do not have an exact comparison of yields at this time, the early indication is that the NT and FSS fields performed similar to water-seeded fields.

There are several reasons for using no-till practices. These include being able to plant earlier, saving water, using different herbicide formulations, and reducing tillage and herbicide costs. We were able to plant early; in fact, the FSS and NT treatments were the first planted fields at the Rice Experiment Station this year. Regarding water savings, the NT and FSS treatments saved about 6 inches of water. This water savings came in the first month after planting where we only flushed the field once (right after drill-seeding) and then let it dry up until permanent flood about four weeks later. These practices allow for the use of soil water as well as reduce evaporation. In water-seeded systems, a major pathway of water loss in the first month is evaporation.

Arthropods and Diseases

Arthropods were not an issue in any of the basins where we conducted the study. Tadpole shrimp or rice seed midge would not be expected to be a problem in the FSS or NT systems, but they can be a problem in the water-seeded system. In fact, in our study, we used the insecticide lambda cyhalothrin in the water-seeded treatments but not in the FSS or NT treatments. This is a savings that can be realized in drill-seeded systems.

The only disease that occurred in the study area was stem rot. Stem rot severity was 20% lower in the FSS and water-seeded treatments than in the NT treatments. While the effect of the fungicide azoxystrobin on stem rot severity was not significant, its use reduced the severity of the disease 30% in the FSS and water-seeded treatments. Interestingly, we noticed that the timing of heading was not similar in all treatments. The NT treatments headed earlier than the FSS treatment. The fungicide application to all the drill-seeded treatments was made on the same date; at this time, the FSS treatment was at the very early heading stage while the NT treatments were past 50% heading. This may be the reason why we did not see an effect of the fungicide on the NT treatments. The differences in heading time may be due to differences in N availability between treatments.

Weeds

Weed management in this system is similar to managing weeds in a drill-seeded system. The main differences are:

•Necessary to manage winter weeds prior to planting (registered herbicides are glufosinate, glyphosate, saflufenacil (Sharpen), and 2,4-D). Always make sure to check the specific product label for use restrictions and registration.

oNOTE: oxyfluorfen is not an option due to plant-back restrictions (minimum of 10-month plant back period for rice).

•Effects of straw on weed emergence in the Chopped treatment (NT-Chop) (reduced weed emergence compared to the straw removed treatment)

•If repeatedly using No-till year after year, then perennial weeds are more likely to establish. Some we have noted initially include ricefield bulrush (roughseed) and tulles (cattails). 

This system is dominated by grasses, similar to our other drill-seeded systems. The main species we saw in 2024 were sprangletop and the watergrasses (specifically barnyardgrass). One of the predicted positives of the fallow treatment (FSS) is a reduction in watergrass emergence, however we do not have conclusive data on this currently.

There are no current recommendations for specific herbicide programs for these systems, but we will be doing a study next summer which will hopefully provide some specific combinations and sequences. Due to the inability to use granular formulations in this system, pendimethalin was applied upfront as a pre-emergent (please check the label for specific use instructions), followed by a foliar tank mix applied pre-flood (SuperWham and Loyant). We also followed up with a cleanup spray at tillering (Clincher).

The other option for a pre-emergent is Abolish (thiobencarb). For foliar tank mixes, there are many options for grass and sedge control, but keep in mind that sprangletop control is necessary, and the only two foliar options are: Clincher (cyhalofop) and Loyant (florpyrauxifen-benzyl). All programs should include one or both of these options, and rotation of chemicals (within and between seasons) is necessary to prevent the selection for herbicide resistance.

It's hard to quantify the effect this summer's heat has had on rice yields so far. The one thing I can say for certain is that there has been an effect; some growers are saying their yields are off by as much as 10 sacks, which counts for a lot in a year like this one, plagued by too much carry-over supply and rock-bottom commodity prices. Across the Sacramento Valley, we have been hearing yield gaps of about 5-10%. This season, there were many temperature-related factors that could have contributed to the yield effects we are starting to see in the Sacramento Valley, some of which occurred far before the heat spell we saw in July.

The wet weather this spring delayed plantings, which hurt yield potential before the rice was even planted. Yield potential in California is typically high due to the high solar radiation and long days, especially prior to the summer solstice. Delayed plantings means losing some of the available solar radiation early in the rice development stage. Research from Bruce Linquist's lab found that every day delay in planting can reduce yield by 0.23 to 0.26%, which can equate to over 21.1 lb/ac lost per day. In 2024, the 50% plant date was around May 17, a week later than the historical average date of May 10.

If you planted M-105 around May 17, the high July temperatures could have hit the rice during the reproductive stage through to flowering. However, high day temperatures (>100°F) can be damaging to rice at all stages in rice development.

Temperature Chart from Colusa Weather Station, June 10 – August 15, 2024.

In vegetative stages, high temperatures can result in reduced tillering and phytohormone imbalances – an effect of which can be stem elongation, which was definitely seen this year. This may have also resulted from rice putting more energy into vegetative growth versus using it for reproductive purposes. Tillering and yield are highly correlated; Soda et al 2018 reported panicle number and yield per plant decreased by 35% and 28%, respectively, in rice subjected to high temperature stress. Once the rice reaches panicle initiation and formation, high temperatures can result in reduced spikelet number and degeneration of the spikelets already formed. At flowering, high temperatures are the most destructive, causing high spikelet sterility. Satake and Yoshida reported that rice exposed to temperatures of 95°F for five days during the reproductive period failed to produce seeds (Satake & Yoshida, 1978).

High temperatures at maturity lowers starch accumulation and reduces grain fill, which can drop yields by 50% (Sreenivasulu et al 2015). This occurs for several reasons: the grain fill period can be shortened, conversion of sucrose to starch can be impeded, and photosynthesis can be inhibited, which leads to less carbon supply from vegetative organs to reproductive. Additionally, high daytime temperatures increase nighttime respiration rates, which can reduce yields as well. A side effect of the heat is that it can damage DNA in the rice seeds, which can delay germination of the rice when planted next year (Suriyasak et al 2020).