In a mixture of crop and weeds, competition exists between plants for incident solar radiation, soil nutrients, and soil moisture. Integrated weed management manipulates cropping‐system factors such that the competitive relationship favors growth of the crop at the expense of the weeds; however, relatively few studies have been reported on the influence of cropping‐system factors on crop‐weed competition. The objectives of this study were to evaluate the impact of weed interference on the performance of four maize (Zea mays L.) hybrids and to examine the influence of soil N level on the competitiveness of the hybrids. Experiments were conducted during 1990, 1991, and 1992 at the Elora Research Station, Elora, ON, on a London loam (Aquic Hapludalf) soil that had been tile drained. Maize hybrids included an old hybrid (Pride 5, released in 1959) and three more recently released hybrids (Pioneer 3902, DEA, and Pioneer 3953). The maize hybrids were grown at 7 plants m−2 under three weed pressures and at two soil N levels. Weed pressures were established by varying the weed‐free period after planting: all season (weed free), planting to 5‐ to 7‐leaf stage of maize (medium weed pressure), and planting to 3‐ to 4‐leaf stage of maize (high weed pressure). Maize dry matter accumulation and grain yield were measured. Mean grain yield of four hybrids across 3 yr was 65% higher in the high‐N, weed‐free treatment than in the low‐N, high‐weed‐pressure treatment. The effect of weed interference on maize grain yield was higher at low than at high N. Maize hybrids responded differently to N level and weed interference; yield of the old hybrid was reduced more by low N and weed interference than yields of the new hybrids. Consequently, selection of maize hybrid and soil N level should be an integral part in the design of a weed management system.

Fly ash is a major waste of coal-power generation and its management is a major environmental and economic challenge, and it will become even more critical with a projected increase in the reliance on coal for power generation. The authors discuss how the unique physicochemical properties of ash can be strategically employed to ameliorate acidity and sodicity, and physical and fertility constraints, in agricultural soils. They show that with careful selection of ash type and methods and rates of application, mutually compatible with the soil and crop type, the often reported phytotoxicity due to high concentrations of certain trace metals can be avoided while maintaining the quality of produce and minimizing risk to the environment. Specific examples are presented to demonstrate where it is economical to use fly ash as a low-cost alternative to certain fertilizers and liming materials on farms. The authors also propose criteria for the selection of ash and for regulatory parameters that would ensure the safe and routine utilization of ash in plant production systems.

Recently, there has been growing interest in agroforestry systems due to their great potential to mitigate threats to household food and nutrition security from soaring food prices but also as carbon sinks. In Sri Lanka, smallholder farms such as homegardens constitute a majority of Sri Lanka's total annual crop and timber production. Despite Sri Lankan homegardens being considered desirable and sustainable land-use systems, their role in food and nutrition security is not yet entirely understood. By synthesising scientific articles and grey literature we sought the link between food security and homegardens by quantifying their products or services and ascertaining whether food security characteristics are assessed as direct or indirect impacts. The results show that 27 % of 92 identified articles directly quantified aspects that are relevant to food security. Another 51 % of the articles quantified indirect aspects that have relevance for food security, including climate, soil, ecosystem services, structural and floristic diversity and economic aspects. Twenty-two percent of the articles were categorised as being qualitative or conceptual and contained no direct assessments or quantification of food security. The presence of significant merits from homegardens includes providing food security throughout the year at low-cost while sustaining numerous ecosystem services. This benefits particularly the poor farmers. However, many studies are descriptive and only provide location-specific information on single research focuses such as plant species, yield and management. There are few comparisons with crop land, forests or other production systems, and there is even less empirical evidence and quantification of the food security and other benefits. Seven areas where more scientific focus would be beneficial are identified. Homegardens are strong in national policies and to reach a greater level of efficiency within these activities our findings suggest more emphasis on a higher degree of inclusiveness of relevant stakeholders and long-term engagements with context specific guidance.

Grain yield improvement of maize (Zea tnays L.) hybrids in Ontario has been associated with increased stress tolerance. Effects of shortterm water-deficit stress on canopy gas exchange of an old and a new maize hybrid were studied. Plants of Pride 5 (old) and Pioneer 3902 (new) were grown under controlled-environment conditions until tassel emergence and, subsequently, plants were transferred to enclosures, each containing the aboveground parts of four plants. Canopy photosynthesis, respiration, transpiration, and stem water potential were measured continuously for 7 d. Plants were either well watered or exposed to a water-deficit stress by withholding water supply until net canopy photosynthesis declined to zero and, subsequently, plants were rehydrated. Under well-watered conditions, the two hybrids did not differ significantly in terms of canopy photosynthesis, transpiration, and stem water potential. In contrast, hybrids responded differently to a water-deficit stress. Cumulative canopy photosynthesis and transpiration during the drying cycle were reduced by 21 and 31%, respectively, in Pioneer 3902 and by 34 and 31%, respectively, in Pride 5. The threshold stem water potential related stomatal closure, and thus, photosynthesis and transpiration, was lower for Pioneer 3902 than for Pride 5. During the recovery day, canopy photosynthesis was 53% higher and canopy transpiration was 31% higher in Pioneer 3902 than in Pride 5. Respiration per unit CO2 fixed was lower in Pioneer 3902 than in Pride 5 in all conditions.

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