Mesotrione is a new callistemone herbicide that inhibits the HPPD enzyme (p-hydroxyphenylpyruvate dioxygenase) and introduces a new naturally selective tool into weed-management programmes for use in maize. Mesotrione provides control of the major broad-leaved weeds, and it can be used in integrated weed-management programmes depending on the grower's preferred weed-control strategy. At post-emergence rates of 150 g AI ha-1 or less, mesotrione provides naturally selective control of key species that may show triazine resistance (TR), e.g. Chenopodium album L, Amaranthus species, Solanum nigrum L, as well as species of weed that show resistance to acetolactase synthase (ALS) inhibitors e.g. Xanthium strumarium L, Amaranthus spp and Sonchus spp. The data presented show that resistant and susceptible biotypes of these species with resistance to triazine herbicides, such as atrazine, simazine, terbutylazine and metribuzin, or ALS-inhibitor herbicides, such as imazethepyr, remain susceptible to mesotrione. These results confirm that there is no cross-resistance in biotypes with target site resistance to triazine or ALS-inhibiting herbicides. It is important that herbicide choice and rotation becomes an integral part of planning weed management, so as to minimise the risks of crop losses from weed competition, build-up of weed seed in the soil and the further development of weed resistance across a range of herbicide modes of action.

Maize (Zea mays L.) hybrids were grown at a high plant density (98,800 plants/ha) to determine the influence of several morphological and physiological traits on barrenness, the major factor limiting grain yields at high stand densities. Correlation and multiple regression analyses indicated density‐tolerant maize genotypes would be characterized by rapid completion of silk extrusion, coincidence of pollen‐shed and silk extrusion, rapid growth of the fust ear and first‐ear silk, prolificacy, reduced tassel size, and efficient production of grain per unit leaf area. Breeding populations composed of these “plant types” should allow isolation and development of high‐yielding and density‐tolerant maize genotypes.

Planting densities change with technology: higher fertility levels, good weed control, and improved hybrids which stand and yield well at high, planting rates have permitted use of considerably higher densities of corn (Zea mays L.) in recent years. Reports of high yields from populations of 50,000 or more plants per hectare are becoming common (Lang et al., 1956; Everett and Crowder, 1965; Pendleton, 1965; Rutger and Crowder, 1967a; Nunez and Kamprath, 1969). Lang et al. (1956) also noted that hybrids respond differentially to plant populations. Maximum corn grain yields are commonly obtained at leaf area indices (LAIs) of 3.5 to 4.5 (Eik and Hanway, 1966; Nunez and Kamprath, 1969). However, researchers have reported maximum dry matter accumulation for corn at much higher LAIs (Williams et al., 1965b; Rutger and Crowder, 1967a; Williams et al., 1968). Williams et al. (1965b) found crop growth rate usually approached asymptotic values at high LAIs, even when such indices approached 18 (plant density of 283,000 plants per acre or approximately 700,000 plants per hectare). The decline in corn grain yield accompanying LAIs above approximately 4.0 or 4.5 is due to a reduction in ear number and/or size. Nunez and Kamprath (1969) found a linear reduction in leaf area per plant as the plant population increased. They also reported a linear increase in LAI as plant population increased over the range of 34,500 to 69,000 plants per hectare, whereas a quadratic equation defined more pre cisely this relationship as the range became greater (34,500 to 102,500 plants per hectare). The optimum plant population or LAI for grain yield In general, literature concerning the effect of planting date on bar renness is quite limited, but numerous recent reports support the yield superiority of early dates. Data presented by Rossman (1965) and Rossman and Cook (1966) demonstrates the superiority of early May plantings in Michigan. May 1-9 plantings average 9% greater yields than May 12-20, 16% greater than May 22-31, and 27% higher than June 1-11 plantings. These general results are supported by the data of Dungan (1944), Stringfield and

The lowered radiation and temperatures of the subtropical winter season induce a physiological dwarfism in sugarcane (Saccharum spp.). This reduction in growth can be overcome to increase yields by application of gibberellic acid (GA3).In Hawaii, cultivars are commercially developed for culture in specific environmental zones defined by their mean daily radiation. In this study, cultivars selected for culture under leeward conditions of high radiation and temperatures were more dwarfed by the adverse winter conditions and gave a greater response to GA3 than did cultivars selected for culture under the lower radiation and temperatures of higher elevations and windward zones.

Seven sugarcane (Saceharum sp. hybrids), cultivars grown on 1,600 ha of the four sugarcane growing islands in Hawaii received 140 g/ha gibberellic acid (GA3) in single or in two haft‐rate applications during the wnter of 1976. Stalks harvested 4 to 6 months after the last GA3 application were divided into supposedly responding and non‐responding segments. Analyses of lengths and weights for each segment and for the sum of the three segments showed significant increases due to treatment. The effect of GA3 was not limited to the “responding” segment, so that whole stalk data were necessary to calculate the response of yield to treatment. In addition to increasing weights, GA3 treatments improved the sucrose content of juice. These gains resulted in a calculated yield improvement of 0.93 to 1.47 metric tons sugar/ha.