Emerging Weed Issues in Rice, 2021

 

Whitney Brim-DeForest, UCCE Farm Advisor, Sutter, Yuba, Placer and Sacramento counties

Unknown watergrass biotypes showed up in rice fields in 2017. Identification has proven difficult, so the emphasis has been on control measures.
Photo of panicles from the three species. From left: unknown species/biotype, barnyardgrass, and late watergrass.

A weed survey started in 2018 with the gathering of seed samples, including the unknown watergrass biotypes. This was in response to the growing list of new weeds identified in the past 5 to 10 years. 2019 saw the samples multiplied so that in 2020 they could be tested against an array of herbicides. The screening took place at the 1.5 leaf stage for the watergrass.

For growers, this preliminary screening implies that control of this new biotype/species will need to be prioritized early in the season. Possible treatments (keep in mind that these have not been field-tested and could cause phytotoxicity) could be: a stale seedbed using a non-selective herbicide; pre-plant Abolish® (thiobencarb) followed by Cerano® or Butte® or Granite GR®; Cerano® followed by Butte® or Bolero® or Granite GR®; or Butte® followed by Granite GR® or Bolero®. There is still a strong likelihood that a follow-up application may still be required later in the season, even with these early-season applications.

During 2020, 64 field soil samples were collected from across the rice growing region. When analyzed, these samples included the four major types of watergrass: the new biotype, junglerice, barnyardgrass, and late watergrass. Preliminary results (one replication) show that the new biotype has a similar number of tillers to barnyardgrass, is the tallest of all the species, heads at the same timing as barnyardgrass, has panicles that weigh the same as late watergrass, and significantly longer awns than barnyardgrass.