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Aerobic rice describes a management adaptation to reduced irrigation water supplies but, due to reduced intervals of flooding in this system, this requires revised weed management approaches to reduce costs and provide effective weed control. One approach is to make the crop more competitive and reduce the effects of weeds on the crop by using higher rice seeding rates. A study was conducted in the Philippines and India in 2008 and 2009 to assess the relations of seeding rates (15-125kgha-1) of hybrid and inbred varieties to crop and weed growth in aerobic rice. Plant densities, tillers, and biomass of rice increased linearly with increased in seeding rates under both weedy and weed free environments. Weed biomass decreased linearly with increasing seeding rates from 15 to 125kgha-1. Panicles and grain yields of rice in competition with weeds increased in a quadratic relation with increased seeding rates at both locations; however, the response was flat in the weed free plots. A quadratic model predicted that seeding rates of 48-80kgha-1 for the inbred varieties and 47-67kgha-1 for the hybrid varieties were needed to achieve maximum grain yield when grown in the absence of weeds, while rates of 95-125kgseedha-1 for the inbred varieties and 83-92kg seed ha-1 for the hybrid varieties were needed to achieve maximum yields in competition with weeds. On the basis of these results, seeding rates greater than 80kgha-1 are advisable where there are risks of severe weed competition. Such high seeding rates may be prohibitive when using expensive seed, and maximum yields are not the only consideration for developing recommendations for optimizing economic returns for farmers. Results of the present study do suggest however that increasing seeding rates of aerobic rice does suppress weed growth and reduce grain yield losses from weed competition. This information could be incorporated in integrated crop management packages to manage weeds more effectively.

Rice ( Oryza sativa L.) followed by chickpea ( Cicer arietinum L.) or a fallow is one of the predominant cropping systems in the rainfed lowlands of India. Crop rotation experiments over 3 years (1996–1998) to quantify N supply and demand under rainfed lowland rice–chickpea and rice–fallow cropping systems on a loam Alfisol and a clay Vertisol in Raipur, India were conducted under direct-seeded rice culture. The rice growth, yield, development and N accumulation were affected most by N rates (0, 40, 80, 120 kg ha −1 ) followed by cropping system (rice–chickpea, rice–fallow) and soil types (Alfisol, Vertisol). The incorporation of chickpea in the cropping system helped in accumulating a greater amount of soil N than fallow. The rice yield, dry matter and N accumulated were significantly higher in rice–chickpea than rice–fallow systems on both soils and in all years. The lowest rice yields were recorded in 1997 due to unfavorable rainfall distribution. The total rainfall was the highest in this season, but most of it occurred during a short period at an early growth stage. The post-heading rains were lowest in this season and resulted in the lower rice yield as compared with that of 1996 and 1998. This indicates the significance of rainfall distribution in controlling yield in a rainfed environment. The rice yields were lower on Vertisol than Alfisol during periods of drought. The performance of chickpea was also better in Alfisol as compared with that in the Vertisol due to its better soil physical attributes. The residual effect of N applied to the preceding rice crop was non-significant on all yield, growth and N accumulation parameters of chickpea. The N balance computed from the top 70 cm soil layers indicated less N loss in the rice–chickpea system as compared with that in rice–fallow. The recovery efficiency at the highest N rate (120 kg N ha −1 ) was higher for the rice–chickpea (57–61%) than that of rice–fallow (49–53%) system. The improved N balance for rice–chickpea system from third year onwards was due to switch to dry seeding and improved soil N status. The inclusion of legume and the effective capture of biologically fixed N and soil N through direct-seeded rice system in rainfed lowlands may help in improving the rice yield of resource poor farmers.

Crop rotation experiments were conducted over 2 years to quantify N supply-demand under rainfed lowland rice–chickpea and rice–fallow cropping systems on a loam Alfisol and a clay Vertisol in India. Significant differences among N rates (0, 40, 80, and 120 kg N ha−1) and soils were observed with rice for grain yield, total biomass and grain N uptake in both years. Low N response, low grain yield, low N uptake, and a short grain filling phase during the 1995 wet season was due to post-heading water stress. The stress was more pronounced on the Vertisol and at high N rates. This resulted in lower N content at maturity than at heading. The loss of biomass N from plant implied that apparent N recovery (AR) and physiological nitrogen use efficiency (PNUE) may differ significantly based on whether maximum N accumulation or total N uptake at maturity was used. Plant N recovery, both by the N difference method and 15 N technique, revealed much lower recovery of fertilizer N (21–27%) with rainfed lowland rice than with irrigated dry-season rice. The residual effect of N applied to the preceding rice crop was nonsignificant on all yield, growth, and N uptake parameters of chickpea. The performance of chickpea was better on the Alfisol than the Vertisol, principally due to soil physical attributes. The better performance and longer growth duration during 1994–1995 dry season as opposed to the 1995–1996 season was attributed to higher rainfall (92 mm versus 39 mm) and rainfall during the critical pod-filling to maturity phase. Mineralization and nitrification of N was negligible during the rice–fallow period due to the dry conditions and low organic matter content. This was corroborated by the similarities in N balance between rice–chickpea and rice–fallow system. The PNUE of rice was 33 to 57 kg grain per kg N absorbed compared with 25 to 27 kg grain per kg N absorbed of chickpea, owing mostly to higher N concentration of chickpea grain and the higher energy cost due to biological N fixation.