We are trying something this year that we have not ever tried. We are drill seeding rice into fields that have not been tilled this season. Why are we doing this? It is one way to save considerable amounts of irrigation water. We estimate that up to 0.5 ac ft/ac of water could be saved. Water savings are due to using existing soil moisture (from the winter/spring rains) and limiting the evaporation of water during the first month of the season by not having the field flooded. Other potential benefits include reduced tillage costs, the ability to plant early, and avoiding tadpole shrimp and midge injury.

We are looking at four different seedbeds to NT drill seed into.

1. Fallow stale-seedbed (FSS): field was fallowed in 2022. It was disked and leveled then. It was not flooded during the winter. No tillage was done during 2023.
2. No-till. We have three strict NT treatments. Rice was grown in 2022. After harvesting (harvested to limit ruts), the straw in the field was subjected to one of three treatments:
   1. Chopped (NT-Chop)
   2. Half removed to simulate baling (NT-Remove)
   3. Burned (NT-Burn)

          The no-till fields were all winter flooded.

At the time of planting: 

On May 2 the fields were all planted with a Great Plains no-till drill seeder (see above) at a rate of 150 lb seed/ac. 

There were a lot of winter weeds in the NT-Burn and FSS treatments (with the wet spring we got more winter weeds than normal). While we tried to get rid of them by spraying glyphosate before drilling, many were tolerant. In the NT-Chop and NT-Remove treatments there were little to no weeds.

Soil moisture varied tremendously between the different treatments. There was a lot of moisture under the NT-Chop straw (too much for optimal planting). In the FSS and NT-Burn the soil was the driest with soil moisture at about 2 inches below the soil surface. We probably could have planted to moisture in the NT-Chop but due to the layout of the experiment, we gave all treatments an initial irrigation flush after planting on May 4 and the flush was drained on May 8. 

We did not apply any herbicide at planting (either before or after the flush).

Just before a permanent flood:

We did not apply any irrigation water after the initial flush at planting until June 2 when we applied the permanent flood. The rice was at the 4-leaf stage by this time. We got a good stand in all treatments. The stand was lowest in the NT-Chop treatment but still good. The soil moisture in the NT-Burn and FSS was starting to dry out but we did not see any moisture stress. In the NT-Chop treatment, there was still a lot of moisture beneath the straw mat. The winter weeds were still present but we saw very few other weeds coming up. Just before permanent flood, we applied urea and herbicides (Prowl, Clincher and Propanil).

After the permanent flood, the plants almost doubled in height in the first few days and were well above the water line. As one can see in the pictures, the winter weeds persisted into the permanent flood. However, they did not appear to affect the stand of rice; but we will see as time progresses. 

We are encouraged by what we are seeing early on. Obviously, we still have a long way to go. This year we are looking at potential water savings, weed problems and their management, and fertility management in these systems.  If you are interested in this research and would like to visit the site, please let us know and we can try to arrange something. We will highlight this research at the Annual Rice Field Day at the end of August.

The 2023 Rice Production Manual is available for download here. The Manual was produced in association with the 2023 Rice Production Workshop; it describes the principles of rice production based on research conducted in the rice production areas of California.

I recently visited a field that had quite a bit of rice seed midge injury. In this case, the seedlings that were not injured were almost at the one leaf stage, and one could observe quite a few seeds still in the soil surface. Close inspection of these seeds showed that they had been hollowed out by rice seed midge. Some midge casings were observed as well. Later, I got some reports of more fields injured by rice seed midge in the Colusa area.

Seed injured by rice seed midge

Rice seed midge casings and injured seed

Rice seed midge has been considered a pest of rice for many years. However, it has been an infrequent pest. The thought was that pyrethroid insecticides controlled it. Nevertheless, in the past three years, I have seen considerable injury in fields that had been treated with a pyrethroid.

Ian Grettenberger, UC Davis Entomology Specialist, collected some midges from the field I mentioned above and exposed them to a full rate of a pyrethroid or an insect growth regulator for 48 hours. The larvae survived. We are not sure if the survival is due to low susceptibility or resistance build-up to the pesticides. Ian started doing some pesticide work last year but the results were not clear cut. More work is going to be necessary to improve on the management of this pest. Let us know if you have seen issues with seed midge recently so we can have a better idea of the current importance of this insect.

Coming out of 2022, where roughly half of all California rice fields were left fallowed last year, and others may have been fallowed for two seasons, many of us have questions about what weed management will look like in 2023. While we do not have data on what a 1- or 2-year fallow does to all of our major weed species, we have some preliminary and anecdotal data that might lend some insight.

For small-seeded weeds such as smallflower umbrella sedge, redstem, and ricefield bulrush, the fallow period will likely have no effect. A good anecdotal example is from a field in Davis that was planted for a long time in rice, followed by nothing being planted for over 10 years. Once the field went back into rice, ricefield bulrush was widely distributed throughout the field, and readily emerged. Ricefield bulrush seeds have a thick seed coat, making it easy for them to survive in the soil for a long period of time, and they have a high level of dormancy, increasing their persistence. In an unpublished overwintering experiment with smallflower umbrella sedge to determine if decomposition occurred over the winter in flooded conditions, the smallflower seeds did not decompose. Seeds of smallflower umbrella sedge are also found in the 1,000s per square foot of soil, so once established, they are difficult to get rid of. 

For larger seeded weeds including watergrass, a fallow may have an impact, but that impact will likely only be on those weed seeds near the surface of the soil, where they may decompose, and are at increased chance of predation. In a preliminary study conducted last year, we found that in fields where tillage occurred in the spring post-fallow, watergrass emerged, as seeds were brought up from deeper in the soil profile, where they would have remained dormant if tillage did not occur. In fields that were no-till in the spring, in at least one instance, the watergrass did not emerge at the same rate as in the tilled field. Similar results were seen with arrowhead, a perennial aquatic species. The effect of the tillage was to bring up the seeds from deeper in the soil profile, increasing emergence in the tilled area of the field. 

At this time, no anecdotal or preliminary data exists for sprangletop. However, due to the small seed size and high dormancy period, it is likely that sprangletop would not be greatly affected by a fallow period and would emerge as normal. 

Preliminary data from grower fields that were monitored over three years shows a reduction in weedy rice seed viability in the top 12 inches of soil. However, these fields have not been planted back into rice, so it is unknown whether the reduction of viable seed in the soil translated to reduced emergence. 

To summarize, it is unlikely the 1- to 2-year fallows will cause a great reduction in weed emergence, especially for small-seeded species. For larger-seeded species, there may be an impact, which can be increased by use of zero- or reduced tillage in the spring following a fallow period. 

"Originally published in Rice Farming, February 2023".