- Author: Bruce Linquist
We all know it has been a warm year. In fact, in 2014 the average daily temperature for May and June was 72.2 degrees (CIMIS-Colusa). This is 2.4 degrees warmer than the average of the last 20 years and 3.3 degrees warmer than the average of the last 5 years. What does this mean for crop development? In general, rice develops at a faster rate with increasing temperatures. In the statewide variety trials (funded by the Rice Research Board) we have seen that the crop is progressing at a faster rate than previous years. This year the crop reached PI about 3 to 7 days earlier than in previous years. The difference between this years and others is especially evident in the early plantings (early May). Based on past experience, we expect the crop to reach heading and harvest earlier as well - assuming on big weather changes. Based on this, growers should plan practices accordingly. Also, be sure to monitor the crop rather than just relying on days after sowing.
A few weeks ago I was on the phone with a grower worried that he had put too much nitrogen on his rice for the cool year we were having. Then, the weather turned and got really hot. So, are we in a cool or warm year? What is the trend so far? It is hard to come with an answer just by comparing the daily average temperatures for this year to the historical average temperatures.
A way to discern what is going on is to look at the amount of heat units or degree days (DD) accumulate so far. Using the Colusa CIMIS station and a lower developmental threshold of 55oF, I calculated how many DD we have accumulated since May 1st.
Looks like 2013 is on the warmer side. The year 2011 was a characteristically mild year, while 2012 was closer to the historical average.
When looking at May and June individually, you realize that most of the extra DD were accumulated in June. May was closer to 2012 and to the historical average.
What are some of the implications of this? Early planted fields are already pass PI. Fields that have not reached PI yet should be evaluated for nitrogen topdress need. Warmer conditions promote growth and nitrogen use, and you don't want your crop to run out of fuel before the end of the race.
We've seen that during mild years blast can become a problem in some areas. Warmer temperatures seem to discourage explosions of the disease. If you are considering a treatment, scout your fields to identify infections and talk to your neighbors to see what is happening in the area.
In the past few days we've seen some unusual high temperatures in the Sacramento Valley, and looks like the hot weather is going to continue for a few more days. By mid morning temperatures are reaching 80o F, by 2 pm we are close to 90o F, and late in the afternoon temperatures can easily reach 100o F or more. Rice at the moment is heading (40% headed by August 12, according the USDA's Statistics Service report), putting some rice at risk of high-temperature sterility.
Anthesis (the opening of the flowers or spikelets) is the most susceptible stage of rice to high temperature damage. Very high temperatures cause indehiscence of anthers (anthers not opening), reduce pollen production and viability, and can dry the germinating pollen tube before fertilization occurs. In rice, spikelet sterility by heat occurs during and up to 3 hours after anthesis. Once fertilization is complete, spikelet sterility does not occur. Generally, anthesis is believed to occur between 9:00 am and 2:00 pm. In one study*, anthesis in M-202 was found to occur between 9:30 and 10:30 am. Given these times, rice would be susceptible to high temperature spikelet sterility from morning to mid afternoon.
In the above referenced study, when the average daytime temperature during anthesis was 95o F, spikelet sterility increased by more than 70%. Unfortunately, the study did not indicate the temperatures right at anthesis, just the daytime average. It is believed that temperatures should be above 104o F during anthesis to cause sterility. We had a couple of days last week when temperatures reached this level, putting rice at risk of spikelet sterility. We'll have to wait to evaluate if this actually happened.
*Prasad, P. V. V., K. J. Boote, L. H. Allen Jr., J. E. Sheehy, and J. M. G. Thomas. 2006. Species, ecotype and cultivar differences in spikelet fertility and harvest index of rice in response to high temperature stress. Field Crops Research 95: 398-411
The season is advancing and fields are starting to go into tillering. I have always been amazed at the capacity of rice plants to adapt to the conditions of the field, producing more tillers in thin stands and less tillers in dense stands. Tillering is one of the important stages that can be most influenced by management practices. Below is a brief review of how the tillering process occurs and its implications for rice management.
Tillering marks the end of the seedling stage. It starts when the fourth true leaf is fully emerged. At this stage, the nodes are all “compressed” close to the ground—the length between nodes (internode length) is less than 0.04 inches. In theory, each node has the capacity to produce a leaf, a tiller and a root; however, tillers and their roots emerge later than leafs. A tiller and its root start growing when the leaf from the third node above is emerging. This means that when the 4th leaf starts emerging (from the 4th node), the 1st node's tiller and root start growing. When the 5th leaf emerges, the 2nd node's tiller and root start growing, and so on.
Tillers emerging in this way from nodes in the main culm are called primary tillers. These emerge all through the vegetative growth phase, but stop when plants reach panicle initiation (the starting point of the reproductive phase). These tillers emerge and branch out from the base of the plant. After panicle initiation, tillers may continue to emerge from preexisting tillers, filling out the space among plants. These tillers are called secondary tillers.
The tillering capacity of rice plants varies with variety, plant spacing, fertility, weed competition and damage from pests. Some varieties are intrinsically better at tillering than others, and these differences can make a variety more “plastic”, adapting better to thin or dense stands. Late maturing varieties have longer periods of tillering than early maturing varieties.
Each tiller has the potential to produce a panicle; however, not all do. Some tillers die before flowering because of shading by other tillers or weeds; the surviving tillers will produce a panicle.
Plants in dense stands produce fewer tillers than plants in thin stands (Fig. 1). In dense stands, the duration of the tillering phase, and in turn, the maturity period for panicles, is reduced. This might be beneficial because it reduces the variability in maturity among panicles. However, if stands are too dense, one runs the risk of disease and lodging. Under low plant densities, more tillers are produced. This can result in a longer period of panicle maturity and a higher number of ineffective tillers (tillers that don't produce a panicle).
Fig. 1. Relationship between plant stand, number of tillers per plant and yield. Butte County, 1984-1985.
Yields under thin or dense stands may not vary much. Because of their tillering capacity, plants can compensate for a thin stand by producing more tillers, and still reach high yields, as shown in Fig. 1. Research in California has shown that to avoid the problems related with thin and dense stands, take full advantage of the tillering capacity of rice plants, and produce good yields, plant densities must be between 15 to 20 seedlings per square foot.