Researchers concluded a six-year study exploring the effects different rice straw disposal methods had on continuous rice grown with and without a purple vetch cover crop. Their findings shed new light on how these cultural practices affect rice yield, rate of straw decomposition and soil microbiological properties.

The site of the study was a 15-acre portion of the Sills Farm in southern Sutter County. Rice residue was either fall burned, fall incorporated or spring incorporated. Each treatment was replicated on one-acre plots.

The straw and cover crop treatments were repeated each year on the same plots. Except for the first year, the variety grown was M-202. Planting date varied between mid- and late May. Data from the first year of the study were excluded because of the varietal difference and also because the straw management strategies had not been established.

This work was partially supported by a grant from the California Energy Commission and by the UC Sustainable Agriculture Research and Education Program. Some growers may have learned of the researcher’s principal findings at one of two farmer field days held during 1994. The following summarizes the results of the study.

Nitrogen Nutrition

The study demonstrated convincingly how vetch reduces the need for nitrogen fertilizer on each of the different treatments. In 1994 vetch reduced the amount of nitrogen fertilizer necessary to obtain maximum yields by 90 pounds/acre on the burned plots, 60 pounds/acre on the fall incorporated plot and zero pounds/acre on the spring-incorporated plot. Researchers calculated that the above-ground nitrogen content of the vetch in spring 1994 averaged about 35 pounds/ acre. Grain yields were high, ranging between 9,500 and 10,100 pounds per acre.

On the plots without a cover crop, maximum grain yields required 120-150 pounds nitrogen per acre. Yields at all nitrogen rates ranged between 500 and 1,000 pounds per acre higher on the fall incorporated plots than on either the burned or spring-incorporated plots but researchers are not sure why. The yield differences could not be linked to disease, lodging or any detectable nitrogen deficiency.

The higher yield on fall-incorporated plots was observed not only in 1994 but over the entire five-year period. Yields in burned and spring-incorporated plots were about equal. Researchers concluded that cover cropping consistently reduced nitrogen fertilizer requirements by an average of 60 to 90 pounds/acre.

Over the five-year period on the burned treatment, the maximum y ield oil the cover cropped plots (achieved with 60 pounds nitrogen per acre) was about 400 pounds per acre higher than on the noncovercropped plots.

Disease Incidence

In 1994 stem rot severity and sheath spot incidence were lower than in 1993. Unlike previous years, however, it appeared that stem rot was slightly more severe on straw-incorporated plots than on burned plots. Last year’s observations also showed that stem rot severity increased with nitrogen rate, while aggregate sheath spot decreased with increasing nitrogen. These findings were consistent with previous years’ observations. The cover crop had no effect on disease incidence or severity apart from its contribution to nitrogen fertility.

Straw Decomposition

Researchers’ findings over five years suggest that straw incorporation increases soil microbial activity and thereby accelerates the decomposition of rice straw. In one experiment in December 1993 the decomposition rate was slightly faster on straw-incorporated plots. Soil samples taken in March 1994 showed the highest rate of decomposition on the spring-incorporated vetch plots. They were lowest on the burned no-vetch plots.