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 <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⁻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⁻¹ using drum or drill seeding machines.

Two varieties of mungbean (Vigna radiata (L.) Wilczek) are most commonly grown in Battambang Province North-West Cambodia: KPS-2 (Thailand); and DX-208 (Vietnam). From the buyer’s point of view, the ideal variety would have large shiny seeds and from the farmer’s point of view, resistance to pod-shattering for single-pick or mechanical harvesting is a priority. KPS-2 has resistance to pod-shattering but small seeds and DX-208 has large seeds but has pods that shatter readily. The ideal variety would have both traits. This study evaluated 17 released mungbean varieties from Cambodia, Thailand, Vietnam and Australia for grain yield, seed weight and resistance to pod-shattering in a series of experiments from 2019 to 2021. In 2019, the four Cambodian varieties were evaluated alongside 11 Australian public varieties. Five of the Australian varieties were rejected because of low seed weight, dull seed coat and unacceptable color. Cambodian variety CMB-1 was rejected because of dull seed coat and indeterminate maturity. In 2021, the six remaining Australian varieties were re-evaluated together with locally grown varieties (DX-208, KPS-2) and Cambodian varieties (CARDI Chey, CMB-2 and CMB-3). The seed weight for Emerald was very similar to that for DX-208. Seed weights for CARDI Chey, CMB-3, King and Shantung were not significantly different to KPS-2. In laboratory testing for resistance to pod shattering, Delta, Emerald and KPS-2 were the most resistant and DX-208 was the most susceptible to pod shattering. The Australian varieties Delta and Emerald are recommended for further testing across other mungbean growing regions of Cambodia before submission for registration and commercial release.