Intercropping has been recognized as an important farming practice, particularly under rainfed conditions, for its greater stability in production, equal distribution of labour throughout the growing season and greater diversity of food and income sources. Evidence for higher productivity from intercropping through more efficient use of resources has been demonstrated (Andrews, 1972; De & Singh, 1979; Willey & Natarajan, 1978). Groundnutpigeonpea intercropping, which is prevalent in semiarid conditions of India on alfisols, does not produce good yields because of the traditional practice of having low pigeonpea density. Also, unsuitable genotype and poor arrangement of the crops often prove to be disadvantageous.

Sugarcane is usually grown after paddy in tropical and subtropical India; this rotation is adopted when sugarcane is planted in October (autumn planting). Soil puddling is required for paddy, but puddling disturbs the soil structure and impedes development of roots beyond the puddled layer. Adverse effects on growth and yield of sugarcane are therefore expected, but contrary to such expectations, sustained or increased yields of cane after paddy have been observed (Alvarez & Snyder, 1984; Anon. 1984; Snyder el al. 1986). It appears that certain other soil factors overcome the harmful effects of adverse soil physical conditions. Therefore, the present study was undertaken to investigate such factors.

The control of eyespot (Pseudocercosporella herpotrichoides (Fron) Deighton) is most effective after application of carbendazim (Hampel & Lücher, 1973; Taylor & Waterhouse, 1975) or prochloraz (Marshall & Ayres, 1986) at growth stages (GS) 30–31 (Zadoks, Chang & Konzak, 1974), although some control occurred after applying prochloraz with carbendazim at GS 37–49 (Marshall & Ayres, 1986). Growers in the U.K. are advised to apply eyespot fungicides only when severe disease is expected. This forecast is usually made at GS 30, and spraying is recommended when two or more leaf sheaths have been penetrated on at least 20% of shoots (Anon. 1985). However, subsequent weather conditions may result in severe eyespot developing when no spray was recommended, or may not favour eyespot and result in fungicide being wasted.

Cassava (Manihot esculenta Crantz) is a carbohydrate staple and cash crop for about 800 million people in the tropics. As food, its use is influenced by its content of potentially toxic cyanogenic glycosides that release hydrocyanic acid when enzymatically hydrolysed (Conn, 1969). Cyanide plays important role in the protection of cassava plants from attacks by animals and insect pests. The glycosides are synthesized mainly in leaves and translocated to all parts of the cassava plant (Bediako, Tapper & Pritchard, 1981). De Bruijn (1973) noted more than a 100% increase in the HCN content of stem bark above the incision after ringing, especially during the first 2 days, and a continued increase for at least 2 months. However, when leaves were removed, no such increase was observed. De Bruijn (1973) observed that younger plants showed a greater increase (165%) than older plants (65%). Leaf HCN did not increase after ringing, whereas root HCN decreased by about 20% in 2 weeks. The literature on cyanogenic glycoside translocation is relatively recent and limited. Consequently, the exploitation of the supposed direct relationship between leaf HCN and root HCN in the selection of low HCN clones in cassava breeding programmes needs to be carefully assessed. The assessment is complicated because the methods used by different workers for the determination of HCN vary in their efficiency. Thus, Cooke, Howland & Hahn (1978) found no correlation (r2 = 0·13) between leaf HCN and root HCN among 108 clones, yet over 88000 genotypes have been screened by analysis of leaf HCN, and leaf analysis has been used routinely in cassava improvement for 9 years (Hahn, 1983). Hahn also reported the range of HCN concentration within each leaf-picrate class (1 = 80, 2 = 80−200, 3 = 200 mg/100g fresh weight) but the ranges within each of these classes were too wide to enable effective selection for low HCN clones.

Brown planthopper, Nilaparvata lugens (Stal), is one of the most serious insect pests of rice (Oryza sativa L.) throughout Asia. Introduction of resistant varieties could be an effective means of minimizing losses from the pest. Several hundred rice cultivars resistant to the pest have been identified and the genetics of resistance has been analysed. Studies have shown that the genes for resistance to brown planthopper, bph-4, and to green leafhopper, Glh-3, are linked (Sidhu & Khush, 1979). Ideka & Kaneda (1983) reported that bph-2 for brown planthopper resistance was linked with the gene d2 for dwarfness. The present study sought to ascertain relationships between resistance to brown planthopper and tungro virus and grain characteristics.

Urea can be an inefficient nitrogen source, compared with ammonium nitrate, on calcareous soils (Ernst & Massey, 1960) and under dry conditions (Murphy, 1978) as a result of ammonia volatilization. However, the interaction between factors which can affect the performance of urea is not clearly understood. The risk of ammonia volatilization would appear to be higher on light soils with a low cation exchange capacity than on heavier soils (Fenn & Hossner, 1985). Field trials in the Netherlands indicated that a minimum of 5 mm of rain must fall within 2 days of application to give good dry-matter yields with urea (Van Burg, Dilz & Prins, 1982). In addition ammonia volatilization has been affected by initial soil moisture content in some studies (Fenn & Escarzaga, 1977) but not in others (Gasser, 1964).

The problem of drought presents a challenge to plant breeders as breeding is a major method by which yield improvement can be obtained in drought-prone areas. The plant responses to drought and the nature of drought resistance have received the attention of several workers (Kozlowski, 1968; Kramer, 1969; Levitt, 1972). There are reports in the literature which show that yield and drought resistance are controlled by separate genetic entities. Breeding thus requires identification of definite drought resistance attributes and their transfer to high-yielding and agronomically acceptable cultivars.

J. agric. Sci., Camb. (1986), 107, 285–297

The authors regret that Fig. 9 relating sucrose weight to total dry weight contained a few misplotted measurements. The sixth group of bivariate observations (five data points) were plotted a factor of 10 g/plant too light in the y-direction (sucrose weight). This error was not present in curve fitting, thus the fitted relationship (Fig. 9; Table 2) is correct fitted relationships on Figs 5 and 9 and thus also remain unchanged. Figure 9 should be presented as