Weeds in slash‐and‐burn rice fields in northern Laos

Abstract

Summary Slash‐and‐burn farmers in northern Laos consider weeds, insufficient rainfall and rodent damage as the most important constraints to upland rice (Oryza sativa L.) production. Labour inputs of 140‐190 days ha‐1 for weed control result in low labour productivity. Average weed cover observed in rice fields was 5.6, 4.1, 2.1, 1.7, 0.7 and 0.7 cm m‐1 (transect) for Chromolaena odorata (L.) King and Robinson, Ageratum conyzoides L., Commelina spp., Lygodium flexuosum (L.) Sw., Panicum trichoides Sw. and Corchorus spp, respectively. Chromoiaena odorata, which was introduced in the 1930s, has become the main fallow species and is considered a desirable fallow plant by most farmers. Average fallow periods reported for the 1950s, 1970s and 1992 were 38, 20 and 5 years respectively. Reduced fallow periods in the last decades have re‐suited in a marked increase in weeding requirements. Above‐ground biomass for rice stem, herbaceous plants and trees after rice harvest was 168, 67 and 60gm‐2 in 1991 and 115, 43 and 24 gm‐2 in 1992. Weeding at 14‐day intervals did not increase rice grain yield.

Yield Performance of Upland Rice, Maize and Job’s Tears in a Rainfed Upland Ecosystem in Mountainous Laos

“…Furthermore, upland farmers also regard Job's tear as a crop that requires minimal care for weeding (Douangsavanh & Bouahom 2006). Since weed infestation is one of the significant constraints on S&B agriculture, and often occurs in fields with low yields after short fallows (Roder et al 1997), the introduction of Job's tears is highly profitable in infertile soil conditions that many farmers are currently experiencing.…”

“…These varieties are generally tall, with few tillers and a low harvest index, and suited to fertile soil (Roder et al 1996, 1997Saito et al2006a, Asai et al 2009b; moreover, these varieties achieve a yield performance of 3-4 t ha -1 under favorable conditions (Roder et al 1996, Saito et al 2006a, Asai et al 2009b), but perform very poorly when grown in fields with infertile soil, as many farmers are currently experiencing (Sengxua et al 2007, Saito et al 2006a, Asai et al 2009b. In screening trials in the Lao-IRRI project, several traditional indica varieties were identified as promising varieties with better yield than traditional tropical japonica checks under intensive continuous rice cropping systems (Linquist et al 2006).…”

Genotypic Variation in Ability to Recover from Weed Competition at Early Vegetative Stage in Upland Rice

“…It yields an average of 1.7 t ha -1 and accounts for about half of the total rice growing area in the region (National Statistical Center, 2004). Weeds are a major constraint to upland rice production (Fujisaka, 1991;Roder et al, 1997). Dominant weed species in this region are Chromolaena odorata (L.) King and Robinson, Ageratum conyxoides L., Comelina spp.…”

“…Lygodium flexuosum (L.) Sw., Panicum trichoides Sw., Corchorus spp. (Roder et al, 1995(Roder et al, , 1997. The farmer relies largely on hand weeding in combination with the use of traditional cultivars with a large stature, which are considered to be strong competitors against weeds (Roder et al, 1996).…”

“…Consequently, no improved upland rice cultivars have been released in this region (Saito et al, 2007). The labor requirement for weeding is typically 150 d ha -1 and about 50% of the total labor input (Roder et al, 1997;Saito et al, 2006a). Saito et al (2006a) reported that in the fi rst two months of the rice growing season, the total labor input was 107 d ha -1 , indicating that two persons have to weed continuously during the fi rst two months (average fi eld size is about 1 ha for rice).…”