Application of zinc to six high-yielding rice varieties in calcareous soils produced significantly higher grain yield and interactions between varieties and Zn levels were significant. Yields of varieties IR 20, Jaya, Sita and Jayanti at 2.5 and 5.0 ppm Zn were similarly affected, but Archana produced significantly higher grain yield at 5 ppm Zn than at 2.5 ppm. Effects of Zn on straw yield were similar. Zn application invariably resulted in a higher Zn concentration in plants at all stages of growth and in grain in all varieties. Maximum Zn contents were observed at the tillering stage and minimum at harvest.

An evaluation of methods for replacing established uncapped, multiple stem, arabica coffee cv. French Mission with cv. Ruiru 11 arabica hybrid in the field was carried out at Ruiru, Kenya. When Ruiru 11 was raised on a single stem using the hedgerow system, a mixed stand of Ruiru 11 and French Mission gave the largest mean yields of clean coffee. The next largest yields were obtained by the complete replacement of French Mission with Ruiru 11 grown at a spacing of 2 × 1.5 m and trained on a single stem system. However, in economic terms, replacing the established coffee partially or sequentially with Ruiru 11 trained on a single stem proved to be the best method.

Métodos de reemplazo de café

Despite numerous altempts, the development of generalixed models capable of accurate predictions of the times from sowing to flowering (f) of crop plants in field environments has remained elusive. Models which seek to correlate ;ʃ with various states of environmental factors such as photoperiod, P, and temperature, T, using formal statistical procedures arc often complex. Here, we describe a family of photothermal responses (involving unambiguous parameters and limits) which quantify the linear, non-interacting effects of P and T not on ʃ but on 1/ʃ, i.e. on the rate of progress towards flowering. Based on these relations we suggest that the modelling of crop phenology will be simplified, more reliable and more biologically plausible.

The competition for fertilizer nitrogen between sugarcane and a companion crop (maize or potato) grown between the sugarcane rows was studied using nitrogen-15 labelled nitrogen in three field experiments in Mauritius. The effect of the timing of nitrogen application on nitrogen recovery by sugarcane was also investigated. Not more than 15 kg ha−1 of the 120 kg ha−1 nitrogen applied to the sugarcane was taken up by the companion crop but this was compensated for by the uptake of 8 kg ha−1 nitrogen applied to maize or potato. The present recommendations for nitrogen fertilization of pure stand sugarcane were found to be applicable to sugarcane intercropped with non-leguminous food crops. Though fertilizer nitrogen uptake by sugarcane was increased by delaying nitrogen application until after the harvest of the maize and potato, this was not accompanied by an increase in sugarcane yields.

Because of shortages of oxen for mouldboard ploughing, delayed planting of maize is common in Mangwende, Zimbabwe and reduces grain yield by 32%. On-farm experiments over four years tested the possibility of using a ripper tine, with atrazine herbicide, to allow smallholders to plant maize earlier. Compared to mouldboard ploughing, tine cultivation increased grain yield at 13 out of 18 sites. All these sites had less than 240 mm of rainfall in January (which coincided with crop anthesis) and long term rainfall records suggest tine cultivation should raise yields in two out of three years. Handweeding and atrazine treatments gave similar grain yields when used with tine cultivation, provided weeds were controlled within 14 days of crop emergence. Tine cultivation was economic at sites where the yield was greater than that with the mouldboard plough. The farmers could manage the combination of tine and atrazine use on a field scale.

Dry matter production and partitioning in old clonal tea bushes pruned and maintained at different heights were investigated. New tissues (leaves, new branches and small roots) consti- tuted a small fraction of the total dry matter in all bushes pruned and initially maintained at 70, 40, 25 and 10 cm high, 36 months after pruning. Leaf/frame dry matter ratio was larger and the shoot/root and frame/root ratios were smaller in the bushes initially maintained at 10 cm than in the bushes maintained at other heights. Yields per unit area of the plucking surface were greatest in the smallest bushes initially maintained at 10 cm in 42 months of recording but on the basis of the available space they were greatest in the large bushes initially maintained at 70 cm.

The adaptability of rice varieties to crop seasons in Taiwan was evaluated for 106 varieties. Average grain yield was higher in the first than second crop seasons. Indica varieties yielded better than Japonicas, particularly in the second crop season, when temperatures are higher. In both the Indica and Japonica groups, varieties developed in Taiwan outyielded those from overseas. Japonica varieties from Japan were lowest yielders in both seasons. Some IRRI varieties were outstanding in the first, while several Ponlais and Taiwan Indicas appeared promising in the second crop seasons.

Examples are presented of tropical tree commodity plantation crops, tea, coffee, rubber and oil palm, as models for farm system development. They are environmentally benign in comparison with annual food crops, have been organized on a large scale with smallholder participation, and make significant contributions to local and national economies. It is suggested that lessons learned in their development might be used in the large-scale development of other potentially major tree crops, including timber, fruit and nut producing species.

The information collected from an experiment on tea, in which two plucking intervals and three plucking standards were imposed, allowed analysis of the effects on the total value of output (Cloughley et al., 1983; Cloughley, 1983). The analysis can be extended and its practical relevance improved by incorporating harvesting and manufacturing costs. Using information collected in another experiment and readily available economic information the effects of incorporating these costs are presented. A method of simultaneously determining optimal plucking interval and standard is described.

Under controlled environment conditions a semi-dwarf wheat, Mexico 120, was subjected to temperature regimes representative of the extremes that occur during the dry (winter) season of the Ord River valley in northern Australia. High temperatures (35°C by day: 18°C by night) such as occur at the beginning and end of the dry season were compared with low temperatures (30°C by day: 13°C by night) such as occur in mid-season. Day length was constant at 12 hours. High temperatures from floral initiation onwards adversely affected floret development, and after ear emergence reduced grain filling to nil. Net assimilation rates were reduced by high temperatures. Major effects on yields were caused by high temperatures well before anthesis. The critical temperature after anthesis is apparently between 32° and 35°C.

An experiment is described in which the productivity of maize–bean and maize–potato mixtures in alternate rows was compared with pure stands of the component species in four cropping seasons. In seasons with low rainfall, yield from the mixtures fell short of that from pure stands, but in one exceptionally wet season a yield advantage was found for maize–bean mixtures. The poor performance of mixtures in low rainfall seasons appeared to result from large reductions in maize yield attributable to competition from the potato or bean. In the one high-rainfall season, no such reduction in maize yield occurred. An implication of these findings for farming systems with mixed cropping is discussed.