Planting Weeds?

Weeds are one of the greatest deterrents to successful rice production, causing an estimated loss of 46 million tons of rice per year. With their irrepressible drive to foil even the strictest weed control system, weeds compete with crops for the limited minerals necessary for a successful harvest. Labor invested in weed control occupies vast amounts of time. However, new information tells us that farmers' practices specifically employed to reduce weed growth account for some weed problems.

The establishment of weeds through contaminated rice seed has long been overlooked by farmers. During harvest, weed seeds similar in maturity and size are harvested and processed with rice seeds, thereby contaminating them. As a result, several major rice weeds are inadvertently disseminated.

A 1985-1986 study conducted by the International Rice Research Institute in Guimba, Philippines, found that samples of farmers' processed rice seed were contaminated with various weed seeds. The present study was conducted in the same area to examine farmers' practices regarding seed selection and processing.

The Gathering

In the 1991 wet season, 70 1-kg rice seed samples were collected from 56 farmers, randomly selected and stratified by water source. Samples were taken from farmers' stock of processed and stored seed. The weed seeds were manually separated from the rice seed, identified by species and counted. Seeds too small to be counted were weighed. Farmers were selected for interviews using four criteria: seed selection and processing methods, rice cultivars used, water source and perceptions regarding the spread of major weeds.

Farmer Practice not Perfect

Farmers selected seed from areas in their own or neighboring fields that appeared to be high-yielding, healthy and relatively weed-free. Most seed was machine threshed. Farmers saved seed after a portion of the selected sheaves had been fed through the thresher--some manually threshed--to reduce contamination by seeds that may have lodged in the machine. To further eliminate weed seed, more than half of the samples were winnowed, and fewer samples were placed in water to separate the seed that floated.

Despite these efforts, 97% of the samples were contaminated with weed seeds of Echinochloa glabrescens (79% of the total samples), Ischaemem rugosum (71% of total samples), Echinochloa crus-galli (L.) P. Beauv. (30% of samples) and F. miliacea (26% of samples). Among samples from irrigated vs rainfed systems, there was no difference in weed seed content except for sedge seeds which were found in significantly higher amounts in the rainfed systems. Of four criteria used to select farmers, only seed source (owned vs. exchanged) was highly significant.

Sixty to seventy-five percent of the farmers thought that the Echinochloa species was disseminated via contaminated seed rather than through the soil. Most (91%) thought that I. rugosum was soil- rather than seed-borne. Farmers' lack of awareness that I. rugosum is a seed contaminant may have contributed to farmers being less careful in eliminating it from their rice seed.

Farmers' rice seed samples were cleaner than those obtained in 1985-86, but were similar in terms of level of contamination by E. glabrescens and I. rugosum. The 1991 study sampled grain that was selected, processed and stored by farmers for use as seed. Farmers' seed selection and processing practices appear to be relatively effective in producing seed which is as clean as that in marketing stations.

The mean seed count for Echinochloa spp. of 87 seed/kg translates into 7,000 plants/ha (assuming 100 kg rice seed/ha and 80% germination). This does not necessarily represent a substantial problem in transplanted fields which normally have 250,000 hills/ha, but transplanted Echinochloa spp. outgrows rice seedlings, and appears on rice hills where removal is difficult and competition is high. The transplanted weeds' chances of reproduction are enhanced, which can exacerbate problems in subsequent crops.

The Future

Ninety-five percent of farmers' rice seed was contaminated by weeds despite practices to reduce contamination. The recent spread of I. rugosum in the Guimba may be due to rice seed contamination and the failure to recognize such contamination.

Weed seed contamination clearly needs to be reduced throughout the rice-growing world. Needed steps include:

  • increasing awareness concerning weed seed contamination and appropriate weed control methods;
  • hand removal of weeds that escape normal weeding practices;
  • developing methods to clean rice seed;
  • adoption of crop rotation schemes that reduce seed-contaminating weed species;
  • setting norms to indicate maximum limits of weed seed contamination; and
  • strict enforcement of seed laws

    Similar studies need to be conducted in countries without well-established seed certification systems if technologies to reduce weed seed contamination are to be developed. Not only do farmers, seed growers and seed merchants need to address the problem of weed seed contamination in rice seed, scientists and national policy makers need to be aware of the problem to direct wider efforts toward solutions.

    S. Fujisaka, R.A. Guino, R.T. Lubigan and K. Moody. 1992. Farmers' rice seed management practices and resulting weed seed contamination in the Philippines. Seed Sci. & Technol. Vol 21, pp.149-157.


    Sam Fujisaka
    Social Sciences and Agronomy-Plant Physiology Divisions
    P.O. Box 933, Manila, PHILIPPINES
    Fax: 63-2-8178470