A weed is a plant whose virtues are not yet discovered. But people may soon discover that the lowly weed, which has long been considered a nuisance by farmers all over the world, has qualities that can be useful even to its adversary and frequent victim, the food crops.

For rice production alone, around 44 to 96 percent of a single season yield are lost due to weed infestation, depending on the rice culture. This is according to the International Rice Research Institute’s (IRRI) Handbook for Weed Control in Rice, published in 1991. According to the handbook, weeds accounts for more yield loses in rice fields than any other pest. Weed control cost is also the highest compared to other pests. Which is why for weed scientists, understanding how and why weeds thrive in cultivated lands is key to improving weed management and increasing crop productivity.

Noted weed scientist, Dr. Aurora Baltazar, shared this vision during her Professorial Chair Lecture at the Agriculture and Development Seminar Series (ADSS). Her lecture, held on August 3, 2010 at SEARCA, aimed at identifying weed characteristics and adaptation mechanisms, and using them to improve crop productivity and weed management methods. Dr. Baltazar stressed that important lessons could be learned from the weed’s superior ability to adapt, resist control, and multiply.

Super Weeds
Although all weeds, by definition, are unwanted plants, some weeds are more prolific and may cause more harm than others. Dr. Baltazar used the term “Superweeds” throughout her presentation, which she defined as “weeds with widespread global distribution,which are difficult to control with conventional means”. She pointed out two of such Superweeds that are common to the Philippines: Barnyardgrass (Echinochloa crusgalli) and Purple Nutsedge (Cyperus rotundus).

Both weeds have been wreaking havoc to farmers and their rice fields all over the world, since it quickly spread in the 1970’s. Dr. Baltazar explained that the proliferation of Barnyardgrass could have coincided with the shift from transplanting to direct-seeding methods of rice farming, while Purple Nutsedge spread by evolving a lowland ecotype, which adapted to flooded soil. Since then, both weeds are considered to be among the world’s worst weeds. The presenter reported that Barnyardgrass and Purple Nutsedge infest 36 crops in 61 countries and 52 crops in 92 countries, respectively. They also multiply rapidly. One plant of Barnyardgrass can produce up to 40.000 seeds, while a single Purple Nutsedge plant can grow 3 to 7 million tubers per hectare.

Learning from Weeds
Dr. Baltazar’s presentation focused on the adaptation mechanisms of the said weeds, in comparison with rice, highlighting the weeds’ tolerance to flooding, resistance to herbicides and competitiveness. She relayed that the Barnyard Grass and Purple Nutsedge’s ability to adapt to adverse environment is related to its high genetic diversity, while years of natural and human selection meant that food crops have more uniform genes. Genetic uniformity generally results to higher yields. Studies that compare crop and weed genomes also show traits that make weeds more competitive and efficient. According to Dr. Baltazar, “weeds are rich sources of genes which could be used to improve crop adaptation to adverse environment”.

She then suggested the manipulation of crop genes to increase its competitiveness and adaptive mechanisms. She proposed the silencing of specific traits in crops that make them less efficient in some environment, like flooding, or incorporating them with useful genes from weeds. Dr. Baltazar explained that the “identification of flood tolerant enzymes in weeds can pave the way for identification of genes that confer flood tolerance” and that plant breeders can “incorporate flood-tolerant traits into flood-susceptible crops such as certain rice cultivars and other flood-susceptible crops”.

Genetic manipulation can also be used to increase the weed competitiveness of crops. Dr. Baltazar supported the search for and development of allelopathic crops. She described allelopathy as the plants ability to secrete chemicals that will injure, or inhibit the growth of surrounding plants, like weeds. On the other hand, crop scientists can also modify the weed itself to reduce its competitiveness and make it “harmless”. Genes that inhibit growth, mimic herbicide action, or modulate hormone levels could also be incorporated to weeds.

Dr. Baltazar stipulated that the above suggestions can help farmers manage weeds with less herbicides and direct removal inputs. Understanding the physiology and morphology of weeds could help improve existing chemical and manual weed management strategies, avoid the development of herbicide-resistant weeds, and lessen the back-breaking work load and labor costs for farmers.

However, she noted that only a few researches are devoted to the study of weed genomes and that crop scientist or plant breeders would rather keep the week traits in food crops if it means more yield. This is because genes that confer competitiveness are in contrast to those that confer high yields.

Finally, Dr. Baltazar posed a question to the audience regarding the possibility of having weed competitiveness and high-yielding traits in a single cultivar. She said that through closer and wider collaboration among weed scientist, geneticists, plant breeders, agronomists, and plant physiologist, farmers of the future might be able to produce high yield, without being pestered by the troublesome weed.