Weed floras of rice fields

Rice (Oryza sativa) is one of the predominant cereal crops of Nepal. It is cultivated mainly during June-July.

A major challenge faced by farmers is the emergence of a wide variety of weed floras in the rice fields, which can significantly lower the yield of crops.

Weeds are mainly classified into three groups: grasses, sedges and broad-leaved weeds. Echinochloa crus-galli, Echinochloa colona  and Cynodon dactylon are grasses; Cyperus rotundus, Cyperus iria and Cyperus difformis fall under sedges;  and Monochoria vaginalis, Ludwigia adscendens, Commelina bengalalensis, Ludwigia octovalvis and Lpomoea aquatic are broad leaved weed species that are commonly found in Nepal.

The abundance of weed floras varies with location, the method of rice establishment, cultural practices and associated environment. Hand weeding is a major weed control method. But it is a time-consuming process in which a lot of energy is required. Hence farmers often use herbicides in different parts of the country to get rid of weeds. Due to the lack of information about the appropriate use of herbicides, sometimes they affect crops, human health and even environment. Therefore farmers should be provided with training related to the use of right product at right time and water/ chemical ratio as well as equipment handling.

Herbicides are specific to certain types of weeds so identification of the dominant species is important. For example, Bispyribac (Nominee gold) is post-emergence herbicide which is sprayed at three-four leaf stage to control grass, sedges and broad-leaved weeds whereas 2, 4-Dichlorophenoxyacetic acid is commonly used to control the number of sedges and broad-leaved weeds.

Timing is very important while using herbicide, as too early or too late application can cause damage to the crops.

Glyphosate is used before planting rice whereas oxadiargyl and butachlor are more effective during pre-emergence of weeds. Farmers should read information label in the bottle before spraying.

Herbicide resistance among weeds in rice has not been reported in Nepal. Climate is changing and variable climates over the years have brought new dimensions to the agro-biodiversity. Weed survey is needed to adequately document the effects of weed floras on crops so that diversity and occurrences of these menacing agents in different regions of the country can be studied.