Dave Jones
Director - Agronomic Services

Agronomic Update

June 23, 2010

Early-season Development Based on Degree Days (Heat Units)

Now that it’s officially the summer, we can reflect on the spring we had (or lack of it) and what an adventurous planting season it was. Late spring rains led to delayed planting in almost all rice growing areas of the state. Delayed plantings seemed to be aggravated by cool temperatures throughout April and May, so even if you did happen to get planted, it appeared the rice “just sat there” and didn’t grow very much. So how delayed was rice development by these cool temperatures? The best way to estimate just how the rice plant has progressed to this point is to look at degree-day accumulation, since it is a primary driver of plant development. In review, the Degree Day units listed in body of the table are calculated for three planting dates (May 1, May 15, and June 1) to June 21, using a minimum of 50o F (virtually no rice growth below this temperature) and a maximum of 98o F and are compared to the 25-yr average using the same parameters. The temperature measurements were taken at the following locations for the Counties listed in the table:

Glenn (Orland), Colusa (Colusa), Butte (Durham), and Sutter (Nicolaus).

According to the data in the table above, it appears that the May plantings lag significantly behind the 25-yr average, while the June 1 planting appears to be close to “normal”. To illustrate the amount of difference between the current years DD’s with the 25-yr average, I thought I’d redo the above table and include the % of “normal” for this years growing season.

As you can see, the two May planting dates are only averaging about 80% of normal, while the June 1 date is 95+% of average. Within the counties, it appears Butte lags farthest behind while Sutter is faring a little bit better than the others. Sounds pretty bad doesn’t it? But let’s try to put this in the perspective of the overall growing season, let’s take a single location and variety and determine where we stand in overall plant development. If we take the Colusa location and look at the M-202 variety and assume it matures in approximately 140 days when planted on May 1, it accumulates roughly 3175 degree day units. Based on the 25-yr. average (“normal” year) the rice would have accumulated around 28% (949) of its seasonal total heat units. This year, having only accumulated 767 heat units, rice has accumulated 23% of its seasonal total heat units. Well, looking at it that way, it’s not so bad after all, 2010 is only 5% (28-23%) delayed in growth due to weather. The biggest delay in plant development is due to delayed planting, so if your rice isn’t tillering, it’s because in normal years you planted two weeks earlier and it would be tillering by now. Give it a couple of weeks of warm weather and hopefully it will start catching up. Update coming in early July.