Weedy Rice in California: An Update

In the past year, the California rice industry has begun to deal with a new pest: “weedy rice”, also known as “red rice”. Weedy rice is a common weed in rice-growing regions of the world, and when infestations are high, it can significantly reduce yields. In the southern USA, losses have been as high as 60% when uncontrolled. In 2016, through the efforts and cooperation of rice growers and Pest Control Advisors, weedy rice has been identified on over 10,000 acres in Butte, Glenn, Colusa, Sutter, Yuba, Placer and Yolo counties. Although 10,000 acres may sound like a lot, it is still only a small percentage of the total rice acreage in California (about 2%).

What makes "weedy rice" unique is that it is the same species as domesticated rice (both are Oryza sativa L.). This means that growers can only control weedy rice through non-chemical means, since any herbicides applied to control the weedy rice will also kill the crop. In the California rice cropping system, where chemicals are the primary method of weed control, growers can use cultural practices such as as a stale seedbed before the rice season. However, this can delay planting by as much as a month. Alternatively, growers can fallow or rotate with another crop, and if the infestations is high, this may be the best option. During the growing season, if weedy rice is found in the field, the only option that growers currently have is to hand-pull it out. 

How does a grower know if they have weedy rice? Weedy rice can be identified before flowering, when all grass-control herbicides have been applied: 

• If grassy weeds remain in the field, check for an auricle and ligule (see photo below).
• If none are present, then the grassy weed is likely a watergrass species. 
• If an auricle and ligule are present, it may be weedy rice, and it is time to get help with identification. A PCA or UCCE Rice Advisor should be able to assist in identification. 

Once rice has headed (produced seed), weedy rice panicles and the panicle of the crop will look similar, so growers should look for any that are different than the planted variety. Again, a PCA or UCCE Rice Advisor can assist in identification. If the field is a certified seed field, then the California Crop Improvement Association (CCIA) should be called to identify the suspect plants. 

Many growers have asked why it is important to control weedy rice, since weedy rice is still rice, and therefore, edible. There are a number of ways that weedy rice can impact rice production: 

1) Reducing milling quality: Due to the extra milling required to remove the red-colored bran, the number of cracked and broken kernels will increase, therefore decreasing the value and the price paid to the rice grower. If the rice is to be milled and sold as brown rice, large amounts of red bran can reduce the milling yield significantly. 

2) Hybridization with domesticated varieties: Weedy rice can cross with domesticated varieties in the field. If there is a high number of weedy plants in a field, the odds that this will occur is even greater. The hybrids (between weedy rice and domesticated varieties) may have different characteristic than their parents (more vigorous growth, for example).

3) Yield decreases: Since weedy rice shatters (falls off of the panicle before harvest), once the population reaches a critical threshold in the field, yields can decrease significantly. 

4) Weed management cost: Weedy rice cannot be managed by chemical means. Therefore, any control efforts have to be through cultural practices. One of the most effective methods is to hand-pull it out of the field. Labor, as we all know, is very expensive.

Weedy rice is a manageable pest in California rice, but it will only be possible through the joint efforts of rice growers, PCA's and members of the rice industry. It will take accurate identification in the field, as well as timely and sustained control efforts in the field.