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Two-hundred rice farming households from eight lowland rice villages in North-West Cambodia were surveyed in 2020 to determine changes in farmers’ knowledge, weed management practices and weed seed contamination in seed kept for sowing. The major yield constraints cited by farmers were lack of water, inability to manage water and competition by weeds. Water shortages result in reduced crop establishment, non-optimal herbicide application timing and poor weed control. Reduced tillage, drill planting and use of pre-emergence herbicides can improve weed management. The adoption of drill planting improves crop establishment and enables the use of pre-emergence herbicides. Possible reasons for changes in weed problems include the change to a two-crop rice system in the wet season and spread of seeds by harvesting machines. The main weed seed contaminants of rice seed kept for sowing were Echinochloa crus-galli, E. colona, Fimbristylis miliacea, Ischaemum rugosum and Melochia corchorifolia. F. miliacea was the only species effectively removed by village cleaning methods. Although prevalent in rice fields, Cyperus. iria with &lt, 0.2 seeds per 500 g and Leptochloa chinensis with nil contamination were insignificant contaminants of seed samples. The majority of farmers in the study area are relying on repeated use of a narrow range of post-emergence herbicides, thus leading to increased severity of weed problems in dry direct-seeded rice. Integrated weed management is required to reduce over-reliance on post-emergence herbicides. This will require engagement with the local input supply network to introduce advice on improved weed management and sustainable herbicide use.

The objective of this work was to determine the value of improved establishment methods and herbicide applications as alternatives to high seeding rates to improve weed suppression in rice. Field experiments were carried out in 2010 and 2011 to determine optimal seeding rates and seeding methods with and without weed competition in wet-seeded rice. Under wet seeding conditions, drum seeding at 80 kg ha&minus, 1 was the most profitable treatment for both weed-free and unweeded rice. Although pre-emergence herbicides are beginning to be adopted in wet-seeded rice, they are seldom used in dry direct-seeded rice in Cambodia. Experiments were carried out in 2018 and 2019 to test crop tolerance and the efficacy of butachlor, oxadiazon, pendimethalin and pretilachlor applied post-sowing and pre-emergence to dry direct-seeded rice. Oxadiazon and butachlor, with the option for a post-emergence herbicide, provided effective weed control and a high grain yield in dry direct-seeded rice. Pretilachlor did not effectively control weeds under dry seeding conditions. Although pendimethalin exhibited good weed control, crop damage was a risk in poorly prepared seedbeds which typify Cambodian rice systems. With an effective integrated weed management strategy, it might be possible to safely reduce seeding rates below 80 kg ha&minus, 1 using drum or drill seeding machines.

One-hundred farmers from lowland rice systems of Battambang province in Cambodia were surveyed in 2017 using a structured questionnaire with the objectives to (1) determine farmers' current knowledge and weed management practices and document the effect of adopted agronomic practices on management of weeds in rice, and (2) quantify the extent of weed seed contamination in farmers’ own saved paddy seed lots. To estimate the level of contamination by weed seeds, a one kg paddy seed sample was collected from each surveyed farmer. All farmers practiced broadcast direct-seeded rice (DSR), with an average seeding rate of 181 kg ha−1. For sowing the rice crop, 82% of farmers used their own saved seeds or bought seed from their neighbour. All the paddy seed samples were contaminated with seeds of 34 weed species with an average of 1,070 weed seeds kg−1 of paddy seed. The most common weed contaminants in the seed samples were Oryza sativa f. spontanea (weedy rice), Fimbristylis miliacea, Echinochloa colona, Echinochloa crus-galli, and Ischaemum rugosum. Weeds, in their rice field, were considered a major problem by 93% of farmers with 70% of farmers indicating a yield loss of >20% due to weed competition. All farmers followed a post-emergence based herbicide program for weed control with no use of pre-emergence herbicides. Farmers (75%) relied on the advice of input dealers on the selection and use of herbicides. Knowledge gaps were found among farmers on herbicide application techniques including selection of the right sprayer, nozzle tips, and sprayer calibration. Although 94% of farmers responded that they were aware of pesticide exposure risk, use of boots and gloves, as personal protective equipment, during spraying was low (10 and 54%, respectively). The fertilizer use was lower than recommended rates (50% of recommended N and around 40% of recommended P and K). An exploitable rice yield gap of 1.3 t ha−1 (40%) and 1.1 t ha−1 (30%) was found in the wet and dry season, respectively. These results suggest that integrated weed management (IWM) and optimum fertilizer use can play an important role in closing the rice yield gap in Battambang. IWM options using clean/certified seeds free from weed seeds, optimum fertilizer, selection and application of appropriate pre- and post-emergence herbicides at the right time, amount, and accurate application techniques can improve weed control and hence enhance the rice yield in Cambodia. Farmer training is needed to close their knowledge gaps and to educate them on IWM, especially, to manage difficult-to-control weeds such as weedy rice.