Effect of Variety and Environment on Head Rice Yield

Objectives

The objectives of this project are to quantify the genetic, cultural, and environmental factors that affect head rice yields on California farms. The information generated from the large number of samples analyzed will be used to identify selection or management practices that lead to substantially improved head rice yields. Climatic data are used to develop regional maps for rice production and variety testing programs based on temperature contours during the rice growing season.

Data used in the studies of varietal and environmental influences on head rice yields were obtained from the Rice Growers Association (RGA) and the Butte County Rice Growers Association (BUCRA). Records from individual grain lots identified by growers at the time of grain delivery were sorted for variety, grain type, maturity class, harvest date, location, moisture content at harvest, and milling appraisal (head rice yield and total yield). Temperature records for the Sacramento Valley were obtained from the National Oceanic and Atmospheric Administration-National Weather Service (NOAA) stations and used to measure various temperature effects.

Degree days during the grain-filling period were defined as a measure of the percent of time temperatures ranged between 60°F and 95°F multiplied by actual temperatures.

Heat unit measurements at harvest were the degree days above 95°F for 14 days prior to harvest. Cold units were the degree days below 60° for the 10 days prior to flowering. Grain yield data for 1982 were obtained from BUCRA, as they have been for the previous three years.

Analyses were made to establish relationships, if any, between all of these various measurements. By this procedure, it was possible to identify factors regulating head rice yields such as varieties, temperature, location, and moisture content of the grain at harvest.

Head Rice Yield for Varieties

The 1982 overall average head rice (52.7 percent) and total rice (67.9 percent) were the lowest in four years. The average head rice for very early, early, intermediate, and late varieties .were 49.9, 53.6,.58.5 and 58.6 percent, respectively. These same measurements averaged over the four years 1979-82 were 49.4, 54.6, 57.1 and 59.0 percent. A significant relationship exists between maturity class and head yield.

Differences among maturity classes can be caused by the genetic makeup of the varieties or by the environmental conditions existing during their growth, grain filling or harvesting. It was possible to statistically separate the genetic influences from the environment because each variety was grown under many different 'environments. The separation for head rice yields among the variety maturity classes previously noted was evident for all harvest dates from September 8 through November. The same head rice yield differences among maturity classes held true for various temperatures at harvest time. While the analyses do not positively prove that head rice differences among maturity classes are due to genetic factors, there are very strong indications that this may be the case.

Moisture Content at Harvest

Another important factor affecting head rice was the moisture content of the grain at harvest. Although there was great variability in head rice for any level of moisture content at harvest, there was a clear trend for head rice to increase with an increase in moisture content of the grain at harvest. This relationship was true for all locations from Orland to Modesto. A statistically significant and positive relationship between moisture content and head rice was evident for nearly every variety. An interesting exception is California Belle. At moisture levels above 20 percent, head rice of this variety decreased. Because of a small number of samples, this response cannot be concluded with much confidence, and another year's records need to be examined to confirm the results.

High Temperatures at Harvest

In general, the very early and early maturing varieties were harvested over a wider range of harvest dates than were the intermediate and late varieties. However, for all four maturity classes, there was a trend for decreasing head rice with increasing temperatures during the 10 days before harvest.

Low Temperatures Prior to Flowering

Physiological studies reported in earlier years have shown that temperatures that fall below 60°F at night prior to flowering can cause blanking or floret sterility. In these studies, grain yields from the BUCRA data clearly showed that field yields declined as night temperatures dropped below 60°F during 10 days before flowering.

Contour Maps for Degree Days

Degree days (temperatures between, 60-95°F x duration) were calculated from 30-year averages for nine weather stations in northern California. Using these data, contour maps were drawn for critical periods during the rice growing season. These maps show three distinct regions. A cooler southwestern region from Sacramento to Modesto, a warmer northern region from Colusa to Orland, and a still warmer small central region surrounding Marysville. Generally speaking, the increments of degree day units are about 200 from April to May, 300 from May to June, 400 from June to July, 500 from July to August, 400 from August to September, 300 from September to October, and 100 from October to November. Degree differences between Marysville and Davis at the end of the season can be as great as 300 units, or the equivalent to a full month growing time in June. These data are useful for identifying areas best adapted for different varieties based on their maturity class and temperature sensitivity.