Dr. Whitney Brim-Deforest CALIFORNIA

Best Practices For Early Season Weed Control In California Rice

Specialists Speaking

Dr. Whitney Brim-Deforest CALIFORNIA

Early season weed control is vital to keeping yield potential high in California rice. Most of our yield potential is lost to weed competition in the first 30 days. The vast majority of that yield loss is due to watergrass species (Echinochloa spp.) including barnyardgrass, late watergrass, early watergrass and coast cockspur (Walter’s barnyardgrass). Other weed species are also important, but the impact on yield loss is not as high.

Control of the grass species is becoming increasingly difficult in our system, due to widespread resistance. The new species, coast cockspur, seems to have high tolerance to many of our registered products as well (although a full profile of this weed has not yet been conducted). The key to grass control is to use combinations of products upfront, early in the season, to reduce growth and therefore, competition with young rice plants. The best combinations are sequential applications or tank-mixes and same-day applications (if label allows) of herbicides with different modes of action.

A good option for early weed control is Abolish (thiobencarb) applied as a pre-emergent pre- flood, followed by a granular product soon after planting. Some good granular options for grass control include Cerano (clomazone), Zembu (pyraclonil), Bolero (thiobencarb), and Granite GR (penoxsulam). Butte (benzobicyclon + halosulfuron) or Cliffhanger (benzobicyclon) are other products that provide suppression and are good in combination with other chemistries. Cerano (clomazone) has the least widespread resistance of the granular options, and while Zembu (pyraclonil) is too new to understand the complete resistance profile (if any), preliminary data suggests that it controls most grass biotypes when applied at the correct timing. It is a new mode of action for grass control in California (a PPO-inhibitor), so if resistance to other products is suspected in a field, it is a good candidate herbicide for rotation.

Again, combinations are the best idea for controlling grasses early. Some combinations that work well are Cerano (clomazone) applied day of seeding, followed by Butte (benzobicyclon +halosulfuron) or Cliffhanger (benzobicyclon) soon after. Some phytotoxicity has been observed; however, the rice recovers. Zembu (pyraclonil) followed by Cerano (clomazone) is another good combination, as long as the two products are appropriately spaced (see label for timings).

Applications of these two products close together can cause phytotoxicity. A good practice can also be to do a pinpoint application around the three- to four-leaf stage of rice after a granular application. Although it may be difficult to do this with certain granular products with long water-holding periods, it may be possible to do with some of the products with shorter water-holding periods. These include products like Abolish (thiobencarb) applied pre-flood, or Cerano (clomazone) applied day of seeding. Butte (benzobicyclon + halosulfuron) also has a shorter water-holding period, as does Cliffhanger (benzobicyclon), which has no water hold.

If conducting a pinpoint drain, make sure to expose at least 70% of the weed foliage, and reflood as soon as possible after application (24-48 hours if possible). Good herbicides to apply at this timing for grass control include propanil, Clincher (cyhalofop), Regiment (bisbyribac-sodium), and Loyant (fluorpyrauxifen-benzyl). Again, these should be applied in tank mixes for best control.

For good, early season weed control that has the greatest impacts on yields, focus on grass control with many modes of actions applied in a short window, maximizing control while decreasing selection for resistant biotypes.   ∆

Dr. Whitney Brim-Deforest CALIFORNIA