Controlling Herbicide-Resistant Weeds in California Rice Fields
By Emily McKay Johnson, Associate Editor
Whitney Brim-DeForest, UC ANR Cooperative Extension rice farm advisor for Sutter, Yuba, Placer and Sacramento Counties in California, currently works in all rice production areas across the state to identify problematic weeds in rice fields.
Given her background in weed science, Brim-DeForest explained California rice growers flood their fields for weed suppression, as well as use herbicides for weed control and management. “I’d say that we do have quite a few herbicides right now. As we continue to get new herbicide resistant weeds every year,” said Brim-DeForest, “we are starting to run out of options, especially for some growers who encounter herbicide resistance.”
Brim-DeForest believes herbicide resistance was first discovered in the early 1990’s, but “has become significantly problematic for growers within the last 20 years. Because of the herbicides we use and the limited number that we have, we have ended up with an increasing number of weeds that are herbicide resistant every year. Since about 2000, we’ve had a new species or herbicide that encounters resistance every year,” she stated.
Brim-DeForest treats a multitude of weed species in her line of work. “I would say the watergrass species is our biggest problem,” she noted. “We also have a weedy red rice that was discovered in the early 2000s. It is not widespread, but we do have a few fields with it,” she explained.
Farmers face many threats to their crops on a daily basis. Luis Espino, rice farm advisor UC ANR Cooperative Extension, Colusa County, said rice farmers are on the lookout for two caterpillar infestations during the year when armyworms invade rice fields. “The first one occurs sometime in June,” Espino said. “At that time all they do is just eat the foliage, and you can usually see it when you walk into a field. Nevertheless, the rice has a very good capacity to recover from that type of injury.” Espino’s UC Rice Blog explains it is difficult to accurately estimate yield losses due to early armyworm damage because it can reduce tillering, delay the crop, and cause uneven maturity.
“Heading” occurs when the rice plant prepares to enter its reproductive phase. The first sign, called the ‘booting’ stage, is when the leaf stem that conceals the developing panicle bulges. Then the tip of the developing panicle emerges from the stem and continues to grow. Rice is said to be at the ‘heading’ stage when the panicle is fully visible. Flowering begins a day after heading has completed. As the flowers open they shed their pollen on each other so that pollination can occur. Flowering can continue for about 7 days. (Source: Rice Development, Ricepedia.)
“The second infestation usually occurs in mid- to late-August when the rice is heading out,” Espino said. “At that time, armyworms can feed on the panicles, [causing the kernels to dry before filling], resulting in blanks [without kernels to harvest] on the panicles and broken panicle branches. That’s when armyworms are more important.”
Espino said that the first infestation this year was relatively large, making it harder for the treatments to control the armyworms as they devastated the rice fields. “There were some areas in fields where the rice was down to the waters,” Espino said. “so all the foliage was consumed, and sometimes only a little stem was left standing.”
During the second armyworm infestation, however, Espino said the rice fields were not as badly affected as they had been in the first attack. “We did see some fields with armyworm injury,” Espino said,“ and some farmers had to treat their fields. The numbers were just so big that the treatments were not controlling them.”