Maize and cowpea were grown in mixed and pure stands, using the same overall density in all plots. Four fertilizer treatments N 0 P 0 , NjPo, N^ and N^i were imposed in experiments carried out during the early growing seasons of 1976 and 1977 at Ibadan in the rainforest zone of Nigeria. There was no competition between the crops for nitrogen or phosphorus and yields in mixtures were significantly higher than the means of the species monocultures, as shown by relative yield total (RYT) and C (complementation) values.Mixed cropping, which is widely practised in the tropics, has numerous advantages including higher yields, lower variability of yield from season to season, a better spread of production over the growth period, less susceptibility to disease or lodging, and improved quality of the crop product. The commonest combinations of crop adopted by farmers in Nigeria in mixed cropping practices are maize/ cowpea, maize/yams and maize/cassava. The special nutrient relation between cereals and legumes in mixtures is of interest in the tropics, where farmers invest little on fertilizers.Reports on maize/cowpea inter-cropping are few (e.g. Enyi, 1973) but maize is known to require N fertilizer more than any other macro-nutrient (Fayemi, 1966;Voss et al., 1970) whereas cowpea does not respond to N but responds to P (Ezedinma, 1964), whilst response to K by both crops is very low (Voss et al.). The aim of this study was therefore to find the effect of varying N and P levels on maize (Z ea ma J s L.) and cowpea (Vigna unguiculata Walp) grown in mixed and pure stands. MATERIALS AND METHODS Experimental methodsMaize cv FARZ 23 and cowpea cv Ife brown were chosen for the study in trials carried out at Moor Plantation, Ibadan, in the rainforest zone of Nigeria in the early seasons of 1976 and 1977. Weather data for both years are given in Table 1.The first experiment was sown on 26 May 1976 on the flat, at 30 cm between rows and 30 cm within rows in 4-2x6 m plots previously cropped to cassava. Two seeds were sown per stand and later thinned after 2 weeks to one seedling per stand. Maize and cowpea were grown in mixtures of pairs of components planted simultaneously in 1:1 proportion, and their monocultures were sown at the same total density. The soil had a pH of 6-7, 0-09% total N and 14-0 ppm available P.Four fertilizer treatments were imposed: N 0 P 0 , N^,,, N 0 P l9 and NjP l5 where N x = 80 kg N ha~x applied as ammonium sulphate and P x = 40 kg P 2 O 5 ha" 1 as single

Field experiments were carried out in the early and late growing seasons in the rainforest zone of Nigeria to find out the relationship between lodging, morphological characters and yield of maize.Morphological characters associated with lodging were plant height, ear height and length of basal internode. There was a negative correlation between lodging and yield. Much of the lodging recorded, especially in the early growing season, was due to weak stems. Earliness, diameter of stem and leaf area had no relationship with lodging.

Experiments were made in a glasshouse and in the field to estimate the effect of weed competition on cowpea performance.In the glasshouse experiment, plant height, number of nodes, green leaves, peduncles, weight of poda and seeds of cowpea were decreased by competition from Euphorbia heterophylla and the decrease was greater with increasing density of the weed associate. Generally cowpea did not respond to application of fertilizers. In the field experiment, competition from the natural weed flora affected the number of days to 50 % flowering and yield components of the four cowpea varieties studied. The yield of the climbing variety, Dinner, was least affected by competition whilst the semi-erect variety, Ife brown, was the most affected. INTRODUCTIONWeed control is expensive in the tropics because little is known about herbicides especially for grain legumes. Weed competition in crop plants is usually severe because of the wide range of adaptability of the weeds which are native to the environments.Weeds may also serve as reservoirs for insect pests and plant pathogens. In the rainforest zone of Nigeria competing weed species in grain legume include Euphorbia heterophylla, Boerhavia diffusa, Amaranthus spinosus, Acanthospermum hispidium, Talinum triangulare, Tridax procumbens, Cyperus rotundus and several other sedges.There is no record of the losses incurred by the farmers due to weed competition on cowpea varieties grown in Nigeria. The present work describes (1) a glasshouse experiment in which one of the most abundant weeds, E. heterophylla, was grown at varying density and with varying levels of fertilizer with a recommended cowpea cultivar (Vigna unguiculata (L.) Walp), Ife brown: (2) a field experiment where the performance of cowpea cultivars with contrasting growth habits were assessed with and without weed competition.

Two maize cvs, FARZ 23 and FARZ 25, were grown at three densities (37,000, 53,000 and 80,000 plants/ha) in 1979 and 1980. Leaf area index (LAI) increased with increase in plant population and was at a maximum at mid-silk. Grain yield was highest at 53,000 plants/ha. There was no relation between LAI and grain yield but there was a positive correlation between LAI and total dry matter yield.Most farmers in the humid tropics grow their crops at wide and random spacings because of the system of cropping, which involves growing two or more crops together. These crops are introduced onto a piece of land at various times, and allowance is usually made for such introductions. However, as management practices improve and more farmers grow their crops sole, specific plant populations are more likely to be used.The situation in the ordinary farmer's plots is made even more complicated by his pattern of sowing 3-5 grains of maize per stand without thinning. Bartlett's (1980) survey of farming practice in the forest zone of Nigeria reported that one-third of farmers used a spacing of about 90 by 90 cm with 2-3 plants per stand; a smaller proportion (24%) reported a modified spacing of 90 by 60 cm; 15% used 75 by 75 cm; and none used the recommended 90 by 30 cm with one plant per stand. Such management practices are bound to affect the crop.The importance of plant density as a factor determining growth and yield of maize has been well established elsewhere (e.g. Ferry and Janick, 1971;Mock and Heghin, 1976;Moll and Kamprath, 1977;Milbourn et al., 1978) but only few reports are available from Nigeria (Fayemi, 1963;Chinwuba, 1967;Egharevba, 1977), where no definite recommendations are at present made. In a few research establishments and Ministries of Agriculture maize is grown at a population of 37,000 plants ha" 1 (90x30 cm), at which spacing improved maize varieties would probably not exploit the factors necessary for growth to the maximum. Hence this study examined the effect of density on growth and yield of maize.

Dactvlis glomerata and Holcus lanatus were grown together in boxes with either full competition, root competition only, shoot competition only or no competition between them. The boxes were either fertilized (150 kg ha^'N, 100 kg ha"'K, 80 kg ha" ' P) or not fertilized. The experiment was continued for 13 months, and a total of six cuts were taken.H. lanatus was more competitive than D. glomerata throughout the experiment, though the efTect declined after flowering during the second growing season. The effects of root competition were generally much greater than those of shoot competition and tended to increase during the first year. However, the effects of root competition declined, relative to shoot competition, after flowering in the second growing season.Fertilizer applications slightly increased the root competitive ability of //. lanatus relative to D. glomerata but slightly reduced its shoot competitive ability.