In the past few weeks I received several calls asking a version of the following question: how are stand, tillering and panicle size related to yield? A way to understand how these things interact is by discussing rice yield components and the factors that affect them. Yield components refer to the structures of the rice plant that directly translate into yield. These are: the number of panicles per given area, the number of spikelets (potential) grains per panicle, the percent of filled grains per panicle, and the weight of each grain.

The number of panicles per unit area (we usually talk about panicles/ft²) is determined by the number of established seedlings and tillers produced per seedling. In general, 60 to 70 panicles/ft² are needed to achieve good yields. How you get to this optimum number can vary. In a good stand (15 to 20 plants/ft²), plants will produce one to three tillers. In contrast, when the stand is poor (5 to 7 plants/ft²) plants may produce up to 12 tillers. The tillering capacity of rice plants help compensate for poor stands, but there is a price to be paid. When a lot of tillers per plant are produced, panicle maturity will be uneven, compromising grain quality at harvest. When stands are very dense, tillers may not even develop or may die before they can produce a panicle due to shading. Consequently only the main culm will produce a panicle. Other factors that can reduce tillering are nitrogen deficiency, weed competition and others pests and diseases.

The number of grains per panicle is determined by variety and stand density. Most California varieties commonly produce 70 - 100 grains per panicle; the higher the plant density the lower the number of grains per panicle. The number of grains per panicle is set during panicle differentiation, about a week after the green ring stage. Thin stands will promote the production of more grains per panicle (and more tillers), but since there is a genetic limit to the number of grains per panicle, plants in fields with very thin stands might not be able to produce enough grains per panicle to compensate for low panicle densities.

The percentage of filled grains per panicle can be affected by several factors. Empty grains, or blanks, can be the result of cold temperature during pollen formation (see the article “Water Management to Mitigate Blanking” in this newsletter). Later, temperatures above 104^o F during flowering can dry the germinating pollen tube and cause blanking. Other factors that can reduce the percentage of filled grains are excess N, panicle blast and armyworms feeding on developing grains. Grain weight is relatively constant. It cannot be increased to compensate for poor tillering or small panicles. However, grain weight can be negatively affected by draining the field too soon before harvest.

So how do these components relate to yield? Remember in large part, crop management affects only the first three variables.

YIELD = (Panicles/area) X (no. of spikelets/panicle) X (% filled grains/panicle) X (kernel weight)

With the approach of late summer and the possibility for lower nighttime temperatures, this is the time when blanking can occur. Keep in mind how water management helps to mitigate this problem.

Spikelet sterility, sometimes referred to as "blanking", occurs when the developing pollen grains are exposed to nighttime temperatures at or below 55 degrees F for several hours. Pollen is sensitive to low temperatures about 7 to 10 days after panicle initiation. The pollen is at the temperature sensitive stage when the collar of the flag leaf and collar of the previous leaf are aligned (Figure 1). While there are varietal differences in blanking susceptibility, in normal years, blanking is around 12 percent. Blanking can be detected in the field about 10 days after flowering. The occurrence of translucent hulls when the panicle is held up to the sun identifies unfilled grain. 

Figure 1. The low temperature sensitive stage of pollen development occurs when the collar of the flag leaf and the collar of the previous leaf align (center plant in photo).

Proper water management helps to mitigate the occurrence of blanking. About three weeks before heading, the base of the panicle is about 4.5 inches above the ground, and the tip of the panicle is about 10 inches above the ground. Raising the water level above the base of the developing panicle can help to reduce the incidence of blanking because the water acts as a heat sink. The minimum nighttime water temperature will be about 3-5 degrees F warmer than the minimum nighttime air temperature, depending on water depth. The warmer water temperature will also warm the air temperature immediately above the water level.  In a 1980 paper, UC Davis researchers found that shallow water (3-4 inches) resulted in 22.2 percent blanking among eight varieties, whereas deeper water (6-8 inches) resulted in 17.8 percent blanking among the same eight varieties. Currently, we are recommending a water depth of 6 inches at 7-21 days before heading to help reduce blanking. Given drought concerns this year, we suggest that growers allow the flood water to subside naturally rather than drain the fields. In practice, this means that water flow into the field can be stopped well in advance of the drain date. How far in advance will depend on the amount of water already in the field, as well as soil and field properties such as percolation. Therefore, raising the water before heading does not necessarily use more water provided the water is turned off earlier at the end of the season.

Variety and fertility management can also result in varying amounts of blanking. Varieties that tend to have lower levels of blanking have true genetic tolerance to cooler temperatures, and they generally are shorter in stature and mature early. High nitrogen rates may increase blanking by increasing vegetative growth and delaying heading. The increased vegetative growth draws away sugars that the plant would otherwise use to fill the grain. Keep in mind that different varieties and fertility practices could result in neighboring fields reaching the susceptible development stage for blanking at different times. Different varieties and fertility practices could result in neighboring fields that were planted at about the same time being more or less susceptible to low temperature events, therefore, resulting in different levels of blanking.

Unfortunately, blanking is not like thinning fruit trees – it does not result in larger grains where grain forms. A 1972 UC Davis study showed that panicles do not compensate for high blanking by producing larger grains. In fact, the study showed that grains from high-blanking panicles had weights that were 3 percent lower than grain from panicles where blanking was low.

Rice Straw - A New Method to Get Through a Drought

Veterans Memorial Hall

525 West Sycamore Street

Willows, CA

Tuesday, July 29, 2014

9am - noon

UC Cooperative Extension research has found that baling rice straw right behind the harvester greatly increases cattle's ability to utilize it. Making rice strawlage involves baling the straw the same day as the rice is harvested at 50 to 60% moisture and then placing it under a tarp cover.

With the present drought conditions, a meeting will be held to discuss how to make rice strawlage. It will cover the nutritional advantages of strawlage over straw, the challenges of baling at 50 to 60 percent moisture, additives that can be provided, how to stack and tarp it, nutrients removed from field with the process, and the cost associated with the process. Most important will be the discussion with two ranchers that fed rice strawlage last year.

Because of the high moisture associated with this product and transportation costs, it is assumed that the most prudent use will be by ranches adjacent to rice production areas. With limited water for rice straw decomposition this fall, this could also provide rice operations an alternative method of the straw management. The goal of the meeting is to give producers information that will allow them to implement rice strawlage during this fall's harvest. Both cattle and rice producers are encouraged to attend the meeting.

For more information contact Glenn Nader at 822-7515 or ganader@ucanr.edu

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.