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).

Based on early projections, 478,000 acres of rice were planted this year. This is down 37,000 acres from last year. Lower acreage is in part due to a large rainfall event on May 4 and 5 which prevented some planting. This rainfall also delayed planting by about three days (50% planted on May 15 compared to the average of May 12 - according to the USDA). That said, the 50% harvest date was October 8 which is about average. Thus, the season length was a bit shorter (146 d from planting to harvest) which may be due to a warm summer and favorable harvest conditions with little to no rain.

Speaking of weather, July was an exceptionally hot month. For most rice, July coincided with panicle initiation through the booting stage. I regularly analyze weather data from CIMIS stations in the rice growing area of the Sacramento Valley. CIMIS has weather data going back to 1984. On average, July maximum temperature is 92.3 oF. This year, it was 97.6 oF and was the hottest on record since these records first started being tabulated. This also coincided with the warmest average nighttime temperatures (62.2 oF), which was 2 oF above average. The September average maximum temperature (91.2 oF) was also the hottest it has been since 1991 when it was 92.5 oF. However, September average nighttime temperatures were average.

Based on data from Dustin Harrel at the Rice Experiment Station, roughly 94% of the acreage was planted to medium grain varieties this year. The remainder was planted to short (4%) and long (2%) grain varieties. Of the medium grain varieties, M-206 was the most widely planted (27%). The other main medium grains (M-105, M-209, M-210, and M-211) represented between 14 and 19% of the medium grain acreage.

Talking with others in extension, this year we also saw a lot of fields where watergrass got out of control, in addition to a lot of redstem. Two relatively new herbicides were used this year: Zembu (pyraclonil) and Cliffhanger (benzobicyclon). While results were generally, in some cases there were issues that need to be worked out to ensure effective control. Pests and diseases were not a huge problem and were generally within typical ranges.

Based on personal communications with growers, farm advisors and mills, yields are a bit lower than average. Lower yields are likely due to a later planting date and warmer than normal growing season temperatures. High nighttime temperatures can reduce yields due to increased nighttime respiration. We have also heard reports of lower than normal head rice yields. Low head rice is being reported for all varieties but especially M-211. One reason for this is, as mentioned above, September temperatures (during grain fill) were warmer than normal.

Rice acres approved for seed production in 2024 totaled 24,000 acres, of which, 872 acres were in the Quality Assurance (QA) program. The seed production acreage in 2024 was a 3818 acre decrease from 2023, but it is worth noting that rice seed producers generally overproduce seed and there should be ample seed in 2025. 

In 2024, there was production of 38 rice varieties (9 long grain, 17 medium grain, 12 short grain). Seed production exceeded 1000 acres only for the 5 Calrose varieties. Of the Calrose-type varieties, the current ranking in acres approved is M-206 (5583 acres), M-211 (3980 acres), M-105 (3640 acres), M-209 (3431 acres), and M-210 (3003 acres). These 5 varieties made up 81% of the seed production. Some trends to note:

• M-206 saw a ~2K acre decrease in acres approved compared to 2023, and there are no new fields being transitioned into M-206 (for additional 2025 seed production).
• M-211 had a 606 acre decline in seed production, but also had 494 acres in transition to seed production for 2025.
• M-209 acres had a 206 acre decline in seed production, with 78 acres in transition to seed production for 2025.
• M-210 and M-105 had increases in seed production by 152 and 330 acres, respectively. M-210 and
• M-105 also had increases of 774 and 1011 acres, respectively, in transition to seed production for 2025.

To summarize, M-206 is trending downward, and M-210 and M-105 are trending upward.

More acreage details and analysis can be found on the CCRRF website: https://crrf.org/

Seed Field Inspections:

~100 acres of seed were rejected due to the presence of weedy red rice. There were additional partial rejections due to excessive weeds and other varieties. Several Certified class fields required roguing due to excessive off-types. Incidence of rice blast was rare this year. 

The Certified seed and QA programs ensure that every rice seed field is inspected by field inspectors from the California Crop Improvement Association, and every seed lot is tested, to ensure that planting seed meets industry expectations for quality seed.