Weedy rice is morphologically similar to cultivated rice (Oryza sativa L.). It has biological characteristics that identify it as a weed. Its important weedy characteristics, that is, early and heavy seed shattering, makes it very difficult to control. Weedy rice has not been reported to be an important weed problem in transplanted, flooded rice. However, the shift to direct‐seeded rice (DSR) due to water issues and high costs of labor has increased reports of weedy rice becoming an expanding important problem in Vietnam, Malaysia, Thailand, and the Philippines. Experts believe that the growing adoption of DSR in Asian countries will result in the rise of weedy rice as one of the top troublesome weeds in rice production. Early and recent surveys in the Philippines have indicated the urgent need to increase awareness of weedy rice among farmers to allow the implementation of a number of effective location‐specific weed management strategies. These surveys and other studies conducted since weedy rice was first reported in 1991 confirmed that weedy rice and grass weed species caused major problems in DSR areas. About 35% of the 4.56 M ha harvested area in the country is planted with DSR. As cultivated and weedy rice are close relatives, it would be very difficult to implement management options very early in crop growth. However, a deeper understanding of the underlying traits of weedy rice can help develop a holistic approach toward effective and economic weed management.

Cyperus rotundus L. was reported as dominant weed in upland rice areas and a minor problem in the rainfed lowland. However, it has evolved as second mostdominant weed in rainfed and irrigated lowland rice fields (rice-vegetable system) in three villages of San Jose City, Nueva Ecija.  Its occurrence in the monoculture rice production system has never been explored especially in irrigated lowland rice fields of Nueva Ecija.  Field surveys and interviews were conducted in 2005- 2006 to determine the prevalence of lowland ecotype C. rotundus and how farmers manage it in irrigated lowland rice fields of Aliaga, Nueva Ecija. In 2004, lowland rice fields of Nueva Ecija where the rice-rice cropping system was commonly established were surveyed. Aliaga was surveyed to calculate the incidence and map the distribution of the weed. To check whether C. rotundus was a problem, 40 randomly selected rice farmers were interviewed using a guided questionnaire. Results of the survey revealed that 9 of 26 villages in Aliaga had 1 - 10% infestation of C. rotundus; 3 had 11 - 20%; and 5 had more than 20%. Infestation was high in areas with low elevations. Even taller than cultivated rice, C. rotundus was the most commonly encountered and dominant weed species and had already existed for 5-20 years in the field. Herbicide application (MCPA at a rate of 1-1.5 L ha) was the most commonly used weed management practice against the weed. Keywords - Cyperus rotundus, weed control, herbicide, prevalence, farmers’ practices, ecotype, Nueva Ecija, Philippines

A survey of different weedy rice biotypes was conducted in rice field areasof Nueva Ecija from CY 2004-2006 to determine their prevalence, agronomiccharacteristics and biology. Surveys and quadrat samplings were conducted inrandom alongside of the roads to gather data on prevalence, height, numberof panicles/plant and density/m of weedy rice as well as the cultivated onesthey were associated. Studies on seed germination test, biology and agronomical characteristics of different weedy rice biotypes were also conducted at thelaboratory and screenhouse of Crop Protection Division, Philippine Rice ResearchInstitute.  Five weedy rice biotypes designated as WR-NE 1, WR-NE 2, WR-NE3, WR-NE 4 and WR-NE 5 were found distributed almost everywhere in ricefields of Nueva Ecija. Among the biotypes, WR-NE 1 was the most common inmany rice fields. Prevalence of all biotypes around the province ranged from 1to 17%. In the field, all biotypes were found taller but had fewer panicles plantthan cultivated rice varieties. In the laboratory, all weedy rice biotypes emergedearlier than PSB Rc82 and IR64. Under screenhouse condition, all weedy ricebiotypes were taller, produced more leaves and culms plant than the cultivatedrice. WR-NE 5 was the tallest biotype while WR-NE 1 and WR-NE 2 had themost number of leaves and culms plant-1-1. All biotypes matured earlier but hadno seed shattering characteristics. The biotypes had comparable values in termsof flag leaf areas and LAI/plant with PSB Rc82. Shoot dry weight and number ofpanicles/plant of all biotypes was significantly higher than the control varieties.WR-NE 4 and WR-NE 5 had the longest panicles, longer and wider grains.Grains/panicle was highest in WR-NE 5 (102 grains/panicle). Grain lengthwidthratio (GLWR)of all biotypes, however,weresignificantly lowerthan thetwocontrolvarieties.WR-NE1 and WR-NE4 had the longest length of awnwhileWR-NE4 and WR-NE5 had the highest grain weight(both at 24 g/1000grains).The growthand developmentof the fiveweedyrice biotypes weremuchadvantageouscomparedto selected cultivarssuggesting their competitivenessandthreatin achieving the potential yield and quality of harvestedcultivatedrice. Keywords - Plant ecology, prevalence, agronomic characteristics, weed biology, biotype,Oryza sativa L., weedy rice, experimental design, Philippines

C. rotundus is known to reduce grain yields of transplanted (TPR) and wet direct-seeded rice (WDSR) under flooded conditions. Information regarding its critical period of control is not yet known. Two identical experiments were performed to determine the critical periods of controlling C. rotundus in TPR and WDSR. Following the procedure of two crop-weed competition periods, the weed was grown with TPR and WDSR in a randomized complete block design arrangement with three replications. Under the critical time of weed removal, C. rotundus had the lowest shoot biomass when grown with TPR and WDSR from 0–10 DAT/DAS and the highest from 0–100 DAT/ 0–120 DAS. When grown from 0–20 up to 60 DAT/DAS, shoot biomass increased significantly thereafter. TPR grain yield was not affected by the weed from 0–10 up to 20 DAT but reduced thereafter by 10.7–40% from 0–30 up to 100 DAT. WDSR grain yield was also unaffected from 0–10 up to 20 DAS; however, it decreased thereafter by 15–34.1% from 0–30 up to 120 DAS. Under the critical weed-free period, the shoot biomass of the weed was highest from 10–100 DAT/DAS for both TPR and WDSR; it decreased significantly from 20–100/120 and up to 60–100/120 DAT/DAS. TPR grain yield decreased to 6.1% when the weed was present from 10–100 and up to 60–100 DAT; WDSR decreased by 16.9 and 4.1% from 10–120 and 20–120 DAS, respectively. There was no yield reduction in WDSR from 0–10 and up to 60–120 DAS. The classical method and statistical model suggest that the critical period for controlling C. rotundus is from 14–29 DAT in TPR and 18–26 DAS in WDSR. Appropriate and effective weed control techniques should be employed during these periods to avoid having more than 5% grain yield reduction.

The presence of different weedy rice (WR) have already been reported in other areas of the Philippines. However, reports documenting its presence and phenotypic characteristics in Central Mindanao are not yet available. This study investigated the incidence, biology, and phenotypic characteristics of WR in Central Mindanao. Survey and collection were conducted in rice field areas of South Cotabato, North Cotabato, Sultan Kudarat, and Maguindanao from 2016–2017. Eight WR variants with distinct grain characteristics were identified and designated in 2016 DS (dry season); 24 variants in 2016 WS (wet season); 31 variants in 2017 DS; and 24 WR variants in 2017 WS. The most prevalent variants were WR-SuK3 (2016 WS), WR-SuK2 (2016 DS), WR-Min15 (2017DS), and WR-SuK3 (2017 WS). Germination test and characterization were conducted at the Philippine Rice Research Institute, Maligaya, Science City of Muñoz, Nueva Ecija. The collected WR variants had variable grain characteristics that affect grain quality and germinated earlier than cultivated rice, which makes WR more competitive. Under screenhouse conditions, all variants matured earlier and had better agronomic characteristics than cultivated rice (NSIC Rc 222). Identified agronomic characters of WR collected from Central Mindanao will be useful for advising control strategies to farmers.