Semi-arid Tropical Environments Research Articles

Improving efficiency of water use for irrigated rice in a semi-arid tropical environment

Irrigation water accounts for almost 40% of total variable production costs for rice ( Oryza sativa L.) cropping in the Burdekin River Irrigation Area, northern Australia. Increasing the efficiency of water use would improve the economic viability of growers and long-term environmental benefits would also be likely due to lower water tables and decreased salinisation in irrigation areas. The aim of these studies was to maximize grain yield by optimizing its functional components: water use, efficiency of water use for dry matter production (WUE dm) and harvest index (HI). The responses of dry matter and yield in rice (cv. Lemont) to five methods of irrigation were studied in a wet and dry season in the region. Applying a permanent flood at sowing, the 3-leaf stage (traditional) and prior to panicle initiation were compared with two unflooded methods: saturated soil culture (SSC) and intermittent irrigation at weekly intervals. Saturated soil culture consisted of growing rice on raised beds of height 0.2 m and width 1.2 m, with water maintained in the furrows (0.3 m wide) some 0.1 m below the bed surface. The results of these studies show that it is not necessary to flood rice to obtain high grain yield and quality. The trend was for yield to increase with water supply, but there was no significant difference in yield and quality between SSC and traditional flooded production, although SSC used about 32% less water in both seasons. Therefore the efficiency of water use for grain production (WUE g, g m −2 mm −1) was higher in SSC than in traditional flooded production in the wet season and a similar trend existed in the dry season. There were no differences between SSC and the traditional method of irrigation in any of the grain quality components measured, indicating that this water saving method did not lower grain quality. Weed growth was generally higher in unflooded treatments, although weed populations in SSC and traditionally flooded rice were equivalent in the dry season, suggesting that weeds can be controlled in SSC. Saturated soil culture provides a viable alternative to flooded rice production for growers in semi-arid tropical environments. Substantial reductions in variable costs of production are attainable by reducing water use without reducing yield and quality.

The Chokla Sheep in India

SummaryThere are approximately 41 million sheep in India, 0.51 million of which are made up by the Chokla population. This breed is generally found around Churu, Jhunjhunu, Sikar and bordering areas of the Bikaner, Jaipur and Nagaur districts of Rajasthan. The animals have a light to medium-sized body, a small head and are reddish brown or dark tan in colour as far as the middle of the neck, with a white body. The skin is pink. The ears are small to medium in length and tubular in shape. The Chokla sheep are hardy and well adapted to arid and semi-arid tropical environments and are suited for the region where migration is a common practice. The Chokla is a comparatively fine-wool producing type among the Indian sheep. The wool produced by Chokla animals is heterogeneous in quality and generally mixed with coarser fleece of other sheep for their utilisation as carpet wool.

Effect of water availability pattern on yield of pearl millet in semi-arid tropical environments

Throughout much of the semi-arid tropics, fluctuations in grain yield can largely be attributed to differences in timing and intensity of drought stress. Since seasonal rainfall in these environments is often poorly related to grain yield, the aim of this paper was to establish a relationship between water availability and grain yield for pearl millet (Pennisetum glaucum (L.) R. Br.), grown across 24 semi-arid tropical environments in India. We used a simple soil water budget to calculate a water satisfaction index (WSI) throughout the season. The cumulative WSI at maturity explained 76% of the variance in grain yield. This was three times as much as explained by actual rainfall, because WSI accounted for differences in water losses and pan evaporation. A classification of environments into four groups of water availability patterns explained 75% of the environmental sum of squares for grain yield. For a subset of 13 environments, environmental differences in grain number could also be explained by water availability patterns, whereas differences in grain mass were related to both water availability and temperature. Our results indicate that cumulative WSI, which is an integrated measure of plant-available water, can provide an adequate estimation of the environmental potential for yield in environments where grain yield is mainly limited by variable availability of water.

Effect of water availability and temperature on the genotype-by-environment interaction of pearl millet in semi-arid tropical environments

Grain yields in drought-affected environments are often characterized by large genotype-by-environment (GE) interactions, caused by a highly variable timing and intensity of stress occurrence. The aim of our study was to assess the effectiveness of simple meteorological environmental descriptions in providing an understanding of the environmental contribution to the GE interaction of pearl millet (Pennisetum glaucum (L.) R. Br.) in the semi-arid tropics. The experiment comprised fourteen genotypes, grown across 24 environments in India. Data on water availability throughout the season and maximum temperatures during grain filling were used to describe the environments. Escape from drought and heat stress were the major factors determining the GE interaction. Both the timing and intensity of stress occurrence were important. Knowledge of the environmental causes of the GE interaction may facilitate the identification of adaptive plant traits, and may also enable a more rational choice of test locations for breeding programs.

Phosphorus transformations in a toposequence of lithosols and cambisols from semi-arid northeastern brazil

Concepts of phosphorus transformations in soils and landscapes have largely been developed in temperate regions on moderate slopes. Little is known about the P dynamics and availability in semi-arid tropical soils where rainfall extremes cause limited but periodically intensive leaching and erosion. We therefore studied the different forms of inorganic P (Pi) and organic P (Po) as related to mineralogy and landscape position in semi-arid northeastern Brazil. Samples were collected from a catena of Lithosols at upper, and Cambisols at mid and lower slopes. All soils were derived from syenite which had a high total P content near 5500 mg kg −1. Phosphaterich particles were identified by X-ray microprobe as fluor-apatite. The apatite contents of the A horizons were reduced in the weathering sequence from Lithosols to the older Cambisols, while the total P contents diminished to between 1200 and 2300 mg kg −. The lower-slope soils had also lost two thirds of the Ca, Mg and Fe contents from sand and silt fractions. Chemicals fractionation of P showed the lowest Ca-P contents in the C horizons of the Cambisols, indicating a maximum transformation of primary Ca-P in these lowermost horizons. Deposition of partly weathered materials from upper slopes explained some of the differences in the Ca-P contents between R and C horizons and the overlying solum at the mid and lower slope. Similarly, some Lithosols showed an exceptionally high Ca-P contents suggesting that these soils have been replenished with unweathered material from rock outcrops above. Microprobe analysis revealed P-rich silt particles containing Fe, Al and Ca, which may be explained by the impregnation of primary Ca-P with Fe-oxyhydroxides. This absorption of Fe by mineral particles would represent a short-cut in the transformations of primary P to secondary, resistant P forms. The observation of Fe-oxyhydroxide coatings in feldspars from saprolite indicates that such transformations did occur and may have general importance where primary and highly weathered minerals are intimately mixed through erosion, limited leaching and high weathering intensities typical of semi-arid tropical environments. Only 5% of the total P was found in organic forms, mostly in stable forms of low availability with little contribution to P fertility.

Environmental control of phenology and leaf growth in a tropically adapted maize

Phenology and leaf growth of the tropical maize hybrid Dekalb XL82 were examined as functions of photoperiod and temperatures. Data were collected from fully irrigated crops sown on three dates. The study was undertaken to obtain mathematical relationships describing phenology, and development and maintenance of leaf-area for use in crop-simulation models describing phenology, and development and maintenance of leaf-area for use in crop-simulation models describing the growth of maize in semi-arid tropical environments. The direct effect of photoperiod was on the thermal time from germination to tassel initiation, and subsequently on final leaf-number. Leaf appearance increased exponentially with thermal time, and the relationship was coincident for all sowings. The fully expanded area of each leaf was described as a function of leaf number, and the same relationship described each sowing, provided that account was taken of differences in final leaf-number across sowings. When account was taken of the leaves still expanding, the relationship between total per-plant leaf-area and leaf-number was similar across sowings. The relationship between leaf-area senescene and thermal time was significantly different among sowings, with the main difference accurring near maturity. The effects of water deficit on these relationships were also examined. Water deficit during early vegetative growth affected the rate of leaf appearance and leaf size, but not the final leaf-number nor leaf senescence. The main effect of water deficit during later growth was on leaf senescence.

An analysis of the effects of water deficits on grain legumes grown in a semi-arid tropical environment in terms of radiation interception and its efficiency of use

Differences in growth of soybean ( Glycine max cvs. Buchanan and Durack), green gram ( Vigna radiata cvs. Berken and CES-ID-21), black gram ( V. mungo cv. Regur), cowpea ( V. unguiculata cv. Red Caloona), lablab bean ( Lablab purpureus cv. Highworth) and pigeon pea ( Cajanus cajan cvs. Royes and insensitive ICP 7179) in response to water deficits were analysed in terms of the amount of photosynthetically active radiation intercepted ( I) and its efficiency of use in the production of above-ground dry matter ( E c). When water deficits developed slowly from seedling establishment and were unrelieved to maturity, reductions in I were more important than those in E c until at least 42 days after sowing. Thereafter reductions in E c were of greater significance than reductions in I. Integrated over the whole life cycle, the relative reductions in I tended to be greater than the reductions in E c although some grain legumes showed similar reductions in I and E c. In contrast, when water deficits developed rapidly when irrigation was terminated 6 weeks after sowing, the relative reductions in I were generally smaller than the reductions in E c. Over the whole crop cycle, differences in E c between grain legumes varied according to the phenology of the crop and the severity of water shortage. Whilst green gram, black gram, cowpea and lablab been attained the highest E c under well-watered conditions, only the early-maturing green gram and cowpea had the highest E c when water deficits developed immediately after seedling establishment. Differences in the overall efficiency of conversion of incident photosynthetically active wave radiation into combustible energy ( E i), which integrates the effects of water shortage on both canopy development and E c, were not necessarily related to phenology. Lablab bean matured late yet had an E i similar to the early-maturing green gram, and greater than that in soybean, black gram and pigeon pea. Values of E c and E i are compared with published data for different species and water regimes, and the implications for crop productivity in semi-arid tropical environments are discussed.