Two weed species in Nueva Ecija and Iloilo have developed resistance to three groups of herbicides that farmers regularly used to control them in the past.

PhilRice researchers Leylani M. Juliano and Dr. Madonna C. Casimero said the herbicides butachlor, propanil and their combination can no longer control barnyard grass species Echinochloa crusgalli and Echinochloa glabrescens if the recommended application rates are followed.

The two weed species are known in the dialects as marapagay (Ilocano), telebisyon (Nueva Ecija), paray paray (Cebuano and Ilonggo), and bayakibok (Tagalog).

The two weed species are almost identical in their physical appearance, except that the spikelets of Echinochloa crusgalli are red to purple, while those of Echinochloa glabrescens are green to brown. These are the most dominant weeds in Nueva Ecija and the second in Iloilo.

Juliano and Casimero interviewed farmers in Nueva Ecija (Aliaga, Rizal, Quezon, Bongabon, Sta. Rosa, Talavera and Science City of Muñoz) and Iloilo (Barotac Nuevo, Pototan and Dingle) where large areas are planted to direct seeded rice at least twice a year.

They found that many of the Iloilo and Nueva Ecija farmers have been using herbicides for as long as 15 and 20 years, respectively.

Iloilo and Nueva Ecija farmers have been using butachlor for 8.3 and 8.7 years, respectively. Over the years they had observed that the butachlor formulation was no longer effective in controlling the weeds and so they shifted to other herbicides.

Most of them (80 percent in Iloilo and 86.7 percent in Nueva Ecija) shifted to butachlor + propanil and have been using it for an average of 9 and 7.1 years, respectively.

Amazingly, 10 percent of the Nueva Ecija farmers have been using butachlor + propanil for 15 to 20 years. Some farmers have even been applying another brand of butachlor + propanil or a mixture of two brands 15 days after seeding.

In the screenhouse, Juliano and Casimero found that overall, 89.5 percent of the total weed populations collected and screened are resistant to butachlor and propanil and one population is developing resistance.

What's more, seven populations survived the application of four times the recommended rate of butachlor + propanil. Ten populations survived butachlor application up to four times the recommended rate. The populations resistant to butachlor + propanil were also resistant to butachlor. Six out of seven populations also survived the application of four times the recommended rate of propanil.

The farmers used higher doses of butachlor + propanil and increased the frequency of applications because they perceived that the herbicide formulation was no longer as effective as when it was still relatively new.

Juliano and Casimero said the continuous use of a single herbicide for several years provided selection pressure, resulting in resistant populations. Thus, farmers in these areas have been using higher rates and have increased the frequency of application.

"What was perceived as a result of reduced herbicide efficacy is actually resistance of the weeds to butachlor and then to butachlor + propanil," they added.