Influence of tillage systems on weed seedling emergence pattern in rainfed rice
Introduction
Rice (Oryza sativa L.) is a principal source of food for more than half of the world population, especially in South and Southeast Asia and Latin America. It is grown on approximately 153 million hectares globally, of which 134 million hectares are in Asia (Rao et al., 2007). Change in rice establishment method from traditional manual transplanting of seedlings to direct-seeding has occurred in many Asian countries in the last two decades in response to rising production costs, especially for labor and water. In different production systems, dry rice seed can be sown (direct seeded) into a prepared seedbed, after tillage or under zero-till (ZT) conditions (Rao et al., 2007).
With dry direct-seeding, fuel costs are further saved by sowing rice under ZT or reduced tillage conditions. In addition to reducing fuel and labor costs, these conservation tillage systems may reduce soil erosion, improve soil physical and chemical properties and conserve soil moisture (e.g., Chauhan et al., 2006a). Changes in tillage practices, however, influence vertical distribution of weed seeds in the soil (Chauhan et al., 2006b), and this may affect the relative abundance of weed species in the field (Froud-Williams et al., 1981). A large proportion of the weed seed bank remains on or close to the soil surface after crop planting in ZT systems (Chauhan et al., 2006b), which may promote greater emergence of weed species that require light to germinate. With conventional tillage (CONT), however, seedling emergence depends partly on the effect that tillage has on seed burial as deeply buried seeds may not be able to emerge. In a pot study, for example, seedlings of Digitaria ciliaris (Retz.) Koel. (southern crabgrass) did not emerge from a seed burial depth of 6 cm (Chauhan and Johnson, 2008a). There is little research data available however in relation to the weed species occurring in rice under different tillage systems.
A trend toward reduced tillage is likely to continue and therefore research is needed to understand the effects of tillage on shifts in weed flora. The aim of the research was to determine the effects that different tillage systems for dry seeded rice have on seedling emergence pattern of some major weed species of rainfed rice.
Section snippets
Location and weed seeds
Field studies were conducted during the wet (rainfed) season of 2007 and 2008 on the farm of International Rice Research Institute, Los Baños, Philippines. The tillage systems compared were CONT, minimum tillage (MINT) and ZT. The characteristics of soil and rainfall during the duration of experiments are presented in Table 1.
The effect of tillage on seedling emergence was evaluated on four grass species and four broadleaved species; all are widespread weeds of rice growing under rainfed or
Effect of tillage on vertical seed distribution
There were no interactions between the years and other treatments, and therefore, data was pooled over the years and the treatment means across years are presented. The vertical distribution of seeds was influenced (P < 0.001) by the interaction between tillage systems and sampling depth. The very limited soil-disturbance in ZT resulted in 77% of the beads being retained in the top 2 cm soil layer, whereas the consequence of the soil-disturbance CONT resulted in 62% of the beads being buried to a
Conclusions
The influence of tillage systems on weed seedling emergence pattern is thought to be mainly due to effects on the vertical seed distribution in the soil. Soil tillage affects vertical seed placement zone, which in turn, affects weed seedling emergence pattern and weed population. Small-seeded species mainly emerge from the surface layer; they tend not to emerge from deep depths, probably because they have insufficient reserves to emerge from such depths. Small-seeded species also often require
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