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).