Herbicides for Rice: Active Ingredients, Groups and Timing by Rice Age

First come clean seed, clean bunds and canals, and a well leveled field (see pagtatanim ng palay). Next comes water, since a flooded field keeps many weed seeds from sprouting. Then you weed by hand or with a rotary weeder while the weeds are small. Our guide to weed management in rice covers each step.

A herbicide earns its place when those steps cannot keep up, for example after a season thick with weeds. Even then, the right product changes with the region and the weed, so follow your local agricultural officer.

The three timings: pre plant, pre emergence and post emergence

Each herbicide label names its timing. Spraying at the wrong time wastes money, and it helps breed resistant weeds.

• Pre plant. You spray weeds that are already growing, before you transplant or sow. Never use it once rice is in the field.
• Pre emergence. A pre emergence herbicide goes on the soil after planting, before the weeds come up. That is the 0 to 5 day window below.
• Post emergence. A post emergence herbicide goes on weeds that are up and have green, active leaves. Early post emergence, at 6 to 10 days, hits them at two to three leaves.

Shop by active ingredient, not by brand

The active ingredient is the chemical in the pack that kills the weed. Many brands sell the same ingredient under different names and prices, so the brand alone tells you little.

Every registered label lists the active ingredient and its amount. So bring the ingredient name to the agrivet store and compare the packs that carry it. A name like pretilachlor plus butachlor means two ingredients in one registered product.

Herbicide groups and why you rotate them

Herbicides also fall into groups by the way they kill a weed. The Herbicide Resistance Action Committee (HRAC) numbers each group. These are the groups on this page:

• Group 1, ACCase inhibitors, the grass killers: cyhalofop butyl, fenoxaprop P ethyl, metamifop and profoxydim.
• Group 2, ALS inhibitors: bispyribac sodium, chlorimuron ethyl, ethoxysulfuron, flucetosulfuron, imazosulfuron, metsulfuron methyl, penoxsulam, propyrisulfuron, pyrazosulfuron ethyl, pyribenzoxim and triafamone.
• Group 3, root and shoot growth inhibitors: pendimethalin.
• Group 4, synthetic auxins, the broadleaf killers: 2,4 D, MCPA and florpyrauxifen benzyl.
• Group 5, photosynthesis inhibitors: propanil.
• Group 6, photosynthesis inhibitors: bentazone.
• Group 13, pigment inhibitors: clomazone.
• Group 14, PPO inhibitors: carfentrazone ethyl and oxadiazon.
• Group 15, seedling growth inhibitors: butachlor, pretilachlor and thiobencarb.
• Group 27, HPPD inhibitors: tefuryltrione.
• Group 28, DHODH inhibitors, the newest group in rice: tetflupyrolimet.

Spray one group season after season, and the few weeds it cannot kill survive and take over. Resistance comes from the same kind used again and again, wrong doses and wrong timing. So rotate groups between seasons. A new brand with the same ingredient is not a rotation. Labels do not always print the group, so check the ingredient against this list.

The barnyard grass lesson

A 2017 review by weed scientist Aurora Baltazar tells the story of barnyard grass. In Philippine rice monocrop areas, farmers sprayed butachlor (group 15) and propanil (group 5) without a break for 7 to 9 years. In the 1990s, barnyard grass there became resistant to both. Farmers then moved to bispyribac (group 2) and cyhalofop (group 1). Resistant types are still found in direct seeded areas.

So if a herbicide fails on barnyard grass, do not raise the dose. Switch groups, and lean on water and hand weeding.

Weedy rice: no herbicide can single it out

Weedy rice is the same species as your palay, so no selective herbicide can kill it inside a rice crop. Prevention is everything: clean certified seed, a clean harvester and thresher, and pulling off types before their grains drop.

Herbicides for rice by age: the active ingredients that work

Every active ingredient in both tables is in a herbicide registered in the Philippines for rice, and the ones Filipino companies sell most come first in each cell. Count the rice age in days after transplanting or seeding, and find that row. Then read the column for your weed group. Our guide to types of weeds shows how to tell the groups apart.

Transplanted rice (lipat tanim)

Direct seeded rice (sabog tanim)

Mixed weeds and late weeds

After 30 days, the direct seeded table lists no grass herbicide and ends. So pull late weeds by hand and keep the water up.

Safe use: label, clothing, water and empty containers

Misused herbicides put people, animals and the environment at risk. So follow these steps:

Ask Anee before you buy a rice herbicide

A spray bought for the wrong weed group is money lost. So snap a photo in anee.io and ask Anee, the smart farm technician, which weed it is and which group it belongs to. Anee answers in Tagalog or English and reads your season first, so she knows your rice age.

The cropping calendar dates each spray window from your lot's own day zero. Also log each herbicide in inventory, so next season you see which group you used and can rotate.

Questions about herbicides for rice