Low pod numbers and inefficient use of radiation are major constraints to high productivity in Crambe crops

Although there have been several attempts to develop commercial production of high erucic acid (HEA) oil from Crambe since the 1950s, this species has not yet become an established crop. Recent reviews suggest that higher yields are needed to meet the market competition from HEA rapeseed oil and the on-farm competition from other crops. Our remit was to identify the major constraints to high productivity in Crambe crops. We studied Crambe crops under favorable growing conditions and focused on the duration of the developmental stages, leaf area growth, radiation interception and on the relations between assimilate sources and sinks. The radiation use efficiency (RUE) was used as a reference value and a tool to identify constraints to seed formation. The basic crop data collected and the derived relations with temperature and intercepted radiation were integrated in a simple crop model to estimate the attainable yields for a range of years. The major constraint to high productivity in Crambe appeared to be the inefficient use of radiation during seed formation. For the period from emergence to mid-flowering the RUE was 2.2 g MJ⁻¹ against only 0.9 g MJ⁻¹ for the subsequent period to seed ripeness. That inefficient use of radiation was shown to result from the high proportion of radiation intercepted by photosynthetically less active stems and senescing leaves. Seed filling appeared to depend on actual photosynthesis by the pods; however, pod numbers and total pod area were too small to take over crop photosynthesis from the leaves. The simulations suggested that yields were also sensitive to exceptionally low temperatures during early growth because leaf area growth was retarded and a high proportion of radiation was wasted in the period with highest receipts. According to the growth model, an average annual production of 3250 kg ha⁻¹ was feasible for a range of years.     

Relations between summer crops and ground cover legumes in a subtropical environment 1. Effects of a Vigna trilobata ground cover on growth and yield of sorghum and sunflower

An annual form of Vigna trilobata (L.) Verdc., a semi-domesticated herbaceous legume, was used to provide additional ground cover under rainfed crops in a subtropical environment. This study reports effects of V. trilobata on growth and yield of sorghum (Sorghum bicolor (L.) Moench) and sunflower (Helianthus annuus L.). Crops were sown on a vertisol in south-east Queensland in October and January at densities of 10 and 5 plants m⁻² for sorghum and 5 and 2.5 plants m⁻² for sunflower, and with or without ground cover legume. Crop rows were 0.8 m apart. V. trilobata seed was broadcast at 150 seeds m⁻² when the crops were sown.While a full profile of soil water at sowing, and 330 to 424 mm rainfall during crop growth, led to vigorous growth of both crops and ground cover, crops from both sowings experienced water stress from about anthesis to the middle of grain fill. At the higher crop density, yields of sorghum grain were 453 and 355 g m⁻² and yields of sunflower seed were 161 and 227 g m⁻² after sowing in October and January, respectively. Yields were decreased to 87%, 70%, 76% and 90% respectively in the presence of up to 330 g m⁻² of dry matter of V. trilobata. Sorghum yields were marginally lower at the lower density and decreased relatively more in the presence of ground cover. Neither the yield of sunflower seed, nor the effect of ground cover on it, was affected by density.Competition from ground cover decreased radiation interception by crops through decreasing their leaf area index. It did not appear to affect the efficiency of conversion of intercepted radiation to dry matter. Tiller number in sorghum was decreased by ground cover, except in the low density stand sown in October. Effects of ground cover on the contribution of tiller heads to yield were more severe, leading to a halving of their contribution at the lower density. Lower grain yields in the presence of ground cover were due to lower numbers of seeds m⁻² in both crops. Seed size was also smaller in sunflower, but the percentage of oil was not affected.We conclude that under well-watered conditions in south-east Queensland the decreases in yields of summer crops in competition with V. trilobata are likely to be outweighed by the anticipated long-term benefits from using ground cover to decrease soil erosion. The need for information on the processes and consequences of competition under drier conditions is emphasized, so that we can evaluate the broader potential for ground cover legumes in the subhumid subtropics.     

Growth of sugarcane under high input conditions in tropical Australia. I. Radiation use,biomass accumulation and partitioning

There is little detailed information on yield accumulation in sugarcane under high-input conditions, which can be used to quantify the key physiological parameters contributing to yield variation. Sugarcane is grown under plant and ratoon crop conditions. This study analysed canopy development, radiation interception and biomass accumulation of two contrasting cultivars of sugarcane under irrigation during the same season under plant and ratoon crop conditions. Over the 15 month season, 11 crop samplings were conducted. Biomass partitioning to stalk was also measured to determine to what extent differences in partitioning between cultivars under ratoon and plant crop conditions contribute to differential productivity. The key findings were: (1) The ratoon crop accumulated biomass more quickly than the plant crop during the first 100 days of growth due to higher stalk number, faster canopy development and greater radiation interception. For similar reasons, cultivar Q138 had higher early biomass production than cultivar Q117 in the plant crop. (2) Early differences in biomass accumulation due to crop class became negligible at about 220 days because maximum RUE of the plant crop (1.72 ± 0.01 g MJ⁻¹) was 8% higher than in the first ratoon crop (1.59 ± 0.08 g MJ⁻¹). The higher maximum RUE in the plant crop was consistent with a higher crop growth rate (35.1 ± 2.3 versus 31.0 ± 3.4 g m⁻² d⁻¹) during the linear phase of biomass accumulation. (3) Biomass accumulation, which ceased about 300 days after planting/ratooning and 140 days before final harvest, attained similar levels of 53–58 t ha⁻¹ in all four crops. (4) The plateau in biomass was associated with loss of live millable stalks, and not a cessation in the growth rate of individual stalks. The crops continued to intercept radiation while on the biomass plateau, so that average RUEs at final harvest were much lower than the maximum values. (5) There was no effect of crop class or cultivar on the fraction of biomass in the leaf and millable stalk components. This study emphasizes that maximising early radiation interception and biomass accumulation will not necessarily lead to higher yield in an environment where biomass production reaches a plateau well before final harvest. Loss of live millable stalks late in the crop cycle results in poor utilisation of intercepted radiation.     

Growth and yield of perennial cassava in the subhumid subtropics

Growth and tuber yield of perennial cassava crops of up to 4 years' duration were compared with those of annual crops in a subhumid subtropical environment where low winter temperature limits crop growth to 8–9 months per year.Resumption of growth by perennial cassava was rapid; sprouting occurred at many nodes in spring, and its canopy intercepted more solar radiation than did young annual crops. However, total biomass production during a season was generally slightly less than that of annual crops.After four seasons of cropping, four annual crops combined produced 49.4 t ha⁻¹ of tuber dry weight. Corresponding yields for two 2-year crops, one annual plus one 3-year crop, and one 4-year crop, were 44.7, 40.3 and 38.1 t ha⁻¹, respectively. The higher productivity of the annual crops was partly related to irrigation applied in the first three seasons to the annual crops. In the fourth season when no irrigation was applied to any crop, the annual increments of tuber weight in 2- and 4-year crops were similar to the tuer yield of the annual crop. It was concluded that productivity of a 2-year crop is 90–100%, and that of 3- and 4-year crops is 80–90%, that of annual cassava.     

Making sense of yield trade-offs in a crop sequence: A New Zealand case study

Crop growth is driven by the capture and utilisation of solar radiation. The most productive crop sequences are those that maximise the interception and use of solar radiation. However, there are yield trade-offs because of the timing of transitions between successive crops. A longer duration of one crop will mean that the following crop is sown later and will therefore produce a lower yield. Maximising the yield of a sequence involves a compromise between the yields of successive crops. We describe a case study of a forage cropping rotation in New Zealand, demonstrating how simulation models can be used to define the best compromise between the yields of successive crops, and thereby maximise the total yield of the full sequence.A case study using a series of long-term simulation experiments for four diverse environments in New Zealand was undertaken in a continuous, summer maize - winter cereal, cropping sequence. Maize sowing dates and hybrid durations, and cereal sowing and harvest times were varied systematically. The actual simulated crop and sequence yields varied from site to site, but there was a consistent trend identifying the most productive combinations of sowing date and hybrid duration. The sequence of comparatively late sowing date of maize (1 December) and a long-season hybrid maximised the total yield of the sequence. The highest sequence yields were achieved by balancing the need to capture a high level of annual solar radiation and the need to have a large proportion of solar radiation captured by maize, which has the greater RUE in summer. This analysis illustrates how crop simulation models can be used to design and understand the processes that give the most productive cropping sequences.     

Analysis of growth and yield of maize,sunflower and soybean grown at Balcarce,Argentina

The objectives of this study are: (1) to analyze the capacity of maize, sunflower and soybean to produce dry matter and seed yield, including the responses to shading during flowering, and shading and thinning during seed filling; and (2) to evaluate effects of plant density and sowing date on growth and yield. This report integrates data obtained during seven years of research at Balcarce Experimental Station, Argentina. In these experiments, nutrients and water were not limiting to growth. Maize produced the most biomass because of sustained ground cover and high light conversion efficiency. It also had the largest harvest index on a dry weight basis. When dry matter was expressed in glucose equivalents, differences among harvest indices for the three crops were smaller. Flowering in maize, seed filling in soybean and flowering and seed filling in sunflower were critical periods in determination of grain yield. Sunflower had more capacity than maize to compensate for fewer grains through greater grain weight. Maize and sunflower had low stability in grain number at less than optimal plant densities. Finally, delays in sowing date significantly reduced grain yields of the three crops. These reductions were due to decreases in number of grains per m² and in grain weight.     

Alfalfa (Medicago sativa L.) seed yield and quality under different forage management systems and irrigation treatments in a Mediterranean environment

Alfalfa (Medicago sativa L.) is widely grown as a forage crop due to its good quality characteristics and high adaptability. However, seed yield is generally considered to be of secondary importance and is characterized by fluctuating yields with often poor seed quality. A field experiment using five alfalfa cultivars (Equipe, Iside, Lodi, Robot, Romagnola) was carried out in 1995, 1996 and 1997 at Foggia (southern Italy) to evaluate the effects of four defoliation practices (H1: crop mown during early plant growth; H2: plant desiccation by chemical agent at the end of February; H3: crop mown at early flowering; H4: never cut) and two irrigation treatments (I: irrigation applied from April to beginning of seed filling; NI: non-irrigated control) on seed yield, seed yield components and seed quality (as determined by seed germination with and without accelerated ageing (AA)). The relationships between yield components (stems per m², pods per stem, seeds per pod, 1000-seed weight) were determined by path-coefficient analysis. Irrigation significantly increased seed yield; on average doubling the control yield over the three seasons. However, the potential seed yield (calculated from seed yield components) was, on average, five times the actual seed yield. The two mowing treatments produced consistently higher seed yields than either desiccated or untreated swards. Cultivar differences were evident for seed yield, with Equipe having the highest value (40% higher than the mean of the other cultivars). Irrigation improved the yield primarily because it led on average a greater than four fold increase in the density of stems (the most influential yield component). By contrast, seeds per pod and 1000-seed weight increased in the absence of irrigation. Defoliation treatments had little effect on stems per m², seeds per pod and seed weight, whereas pods per stem were reduced by desiccation. Path analyses calculated across irrigation treatments and years revealed that stems per m² had the largest positive direct effect on alfalfa seed yield under each harvest management (path-coefficient values ≥0.89), and its indirect effects on seed yield via other traits were negligible. Seed quality, as measured by germination percentage both before and after AA, remained relatively consistent across both defoliation and irrigation treatments and cultivars, but was anomalously low in the 1997 irrigated crops. Overall, the highest seed yields were obtained under irrigated conditions when crops were mown during early growth or at early flowering. The potential seed yield of alfalfa varieties is sufficient to guarantee a profitable seed harvest. However, the harvest efficiency of the combine-harvester was low (20% of the potential seed yield); thus, more appropriate harvest techniques should be used.     

Climate Change and Potato Production in Contrasting South African Agro-ecosystems 1. Effects on Land and Water Use Efficiencies

Explorations of the impact of climate change on potential potato yields were obtained by downscaling the projections of six different coupled climate models to high spatial resolution over southern Africa. The simulations of daily maximum and minimum temperatures, precipitation, wind speed, and solar radiation were used as input to run the crop growth model LINTUL-Potato. Pixels representative for potato growing areas were selected for four globally occurring agro-ecosystems: rainy and dry winter and summer crops. The simulated inter-annual variability is much greater for rainfall than for temperature. Reference evapotranspiration and radiation are projected to hardly decline over the 90-year period, whilst temperatures are projected to rise significantly by about 1.9 °C. From literature, it was found that radiation use efficiency of potato increased with elevated CO₂ concentrations by almost 0.002 g?MJ^?1?ppm^?1. This ratio was used to calculate the CO₂ effect on yields between 1960 and 2050, when CO₂ concentration increases from 315 to 550 ppm. Within this range, evapotranspiration by the potato crop was reduced by about 13% according to literature. Simulated yield increase was strongest in the Mediterranean-type winter crop (+37%) and least under Mediterranean summer (+12%) and relatively warm winter conditions (+14%) closer to the equator. Water use efficiency also increased most in the cool rainy Mediterranean winter (+45%) and least so in the winter crop closer to the equator (+14%). It is concluded from the simulations that for all four agro-ecosystems possible negative effects of rising temperatures and reduced availability of water for potato are more than compensated for by the positive effect of increased CO₂ levels on water use efficiency and crop productivity.     

Agronomy of fibre hemp (Cannabis sativa L.) in Europe

Fibre hemp may yield up to 25 t above ground dry matter per hectare (20 t stem dry matter ha⁻¹) which may contain as much as 12 t ha⁻¹ cellulose, depending on environmental conditions and agronomy. Its performance is affected by the onset of flowering and seed development. Effects of cultivar and management on yield and quality were tested at three contrasting sites in Italy, the Netherlands and the UK in three years, making use of standardised protocols for experimental design and research methodology. Highest yields (up to 22.5 t dry matter ha⁻¹) were obtained in Italy when later cultivars were used. Attainable yields proved slightly lower in the Netherlands and much lower in the UK. The quality of the cellulose was relatively stable over the growing season, but lignification may proceed rapidly some time after flowering. Crop development was very rapid and crops maintained green leaf area for a long time, thus radiation interception was considerable. The radiation use efficiency changed during development. It was lower after flowering (about 1.0 g MJ⁻¹ PAR) than before (about 2.2 g MJ⁻¹ PAR). Growing earlier cultivars to obtain some seed set advanced the reduction in radiation use efficiency. Nitrogen proved to affect yield only slightly. A relatively small amount of fertiliser will be adequate to cover the crop’s needs. Plant density declined during growth in a site-specific manner when it was high initially. Very low plant densities may not show this self-thinning but reduced yield and (especially) quality. Final plant densities were proven to depend more on initial plant stands than expected from literature. This was true at all three contrasting sites and in the different years. Nitrogen and plant density hardly interacted within one site. Results suggest that hemp can yield large quantities of useful cellulose when ecologically adapted cultivars are sown in proper plant densities. The cultivation is environmentally friendly with little harmful accumulation or emission of chemical inputs. More research on ideotyping is required and breeding efforts should be broadened.     

Relations between summer crops and ground cover legumes in a subtropical environment 2. Effects of sorghum and sunflower on the growth of Vigna trilobata and Medicago scutellata

Vigna trilobata (L.) Verdc. is a low growing legume with potential as a ground cover for controlling soil erosion in cropping lands of the subhumid/semi-arid subtropics of northern Australia. When it was grown beneath sorghum (Sorghum bicolor (L.) Moench) and sunflower (Helianthus annuus L.) crops under well-watered conditions on a vertisol in southern Queensland it decreased their yields by about 20% in the first season. This paper reports the effects of competition from crops sown at a normal dryland density or at half the normal density on its growth.V. trilobata sown in pure stand in October produced 531 g m⁻² of above-ground dry matter, and sown in January produced 323 g m⁻², by the time the grain crops sown at the same time were mature. Yields under mature sorghum sown at the normal density (10 plants m⁻²) in October and January were 38% and 35% respectively of those from pure stands of ground cover sown at the same time. Under sunflower at the normal density (5 plants m⁻²), corresponding values were 42% and 22%. The decrease in dry matter yield attributable to competition for water, nutrients and/or radiation from the main crops was proportional to the decrease in intercepted radiation.V. trilobata responded to shade in several ways, but a substantial increase in specific leaf area, together with some additional partitioning of dry matter to leaf, was sufficient to ensure that its leaf area index under crops was similar to that in pure stands.Seed yields of 116 and 84 g m⁻² were obtained from pure stands of V. trilobata sown in October and January respectively. However, seed yields under crops sown at the normal density were only 4–18% of those measured in pure stands. Seed size was not affected by treatment.We conclude that, under well-watered conditions in southern Queensland, V. trilobata grown as a ground cover under crops of sorghum and sunflower should be able to produce sufficient biomass to decrease soil erosion, and also produce sufficient seed for regeneration in successive seasons. Its growth and suitability are also briefly compared with those of an alternative ground cover legume, Medicago scutellata (L.) Mill. (snail medic).     

Seed growth of three perennial ryegrass cultivars sown on two dates and treated with trinexapac ethyl straw shortener

First‐year crops of diploid perennial ryegrass (cvs. Meridian, Bronsyn and Grasslands Impact) were sown on 1 April and 14 May 2008. Applications of trinexapac ethyl (TE) plant growth regulator at 0, 200 and 400 g a.i. ha^?1 were used to shorten stems to examine the impact of seed growth. Seed filling followed a consistent sigmoidal growth pattern with a lag phase of 127°C days, and linear duration of 390°C days. Time to 95% of final seed weight was 517°C days. Seed yield increases from TE were from higher numbers of first‐grade seeds m^?2, achieved by a higher rate of seed filling during the linear phase of 0·115 mg per °C day per spike. For all cultivars, the maximum stem dry weight occurred at 310–400°C days post‐anthesis, which suggest the stem was a strong sink. As seeds developed, their demand for assimilate increased and they drew more from the stem. At harvest, stem weights from TE treatments were 25% heavier than at anthesis, while untreated ‘Bronsyn’ and ‘Grasslands Impact’ stems were similar to those at anthesis. Thus, stems treated with TE contributed assimilates to increase seed yield but were still a net sink with assimilates in the stem at harvest. Trinexapac ethyl rate induced an inverse relationship between seed yield and stem height. This showed that competition for assimilate between stems and growing seeds limited the seed yield. Management or genetic factors that reduce stem height are likely to increase seed yields of perennial ryegrass.     

A crop model component simulating N partitioning during seed filling in pea

Seed N concentration is one of the main quality parameters in grain legume crops. Since few studies have aimed at modelling both seed and vegetative parts N concentrations, our objective was to model N partitioning between vegetative parts and filling seeds for pea (Pisum sativum L.) in field situations where both N nutrition and the plant genotype varied. A crop model component predicting the time courses of vegetative and seed N concentrations was built using knowledge concerning N partitioning during the seed filling period, which include a previously demonstrated relationship between the rate of individual seed N accumulation and the N availability within plants. A greenhouse experiment where assimilate availability was non-limiting was conducted with two genotypes. This experiment demonstrated the genotypic variability of one of the crop model component parameters, the maximum rate of individual seed N accumulation (SNR_max), allowing introduction of this parameter in the crop model component for the studied genotypes. Field experiments spanning 3 years and comprising various crop N nutrition and four genotypes were conducted to evaluate the crop model component. Observed seed and vegetative parts N concentrations ranged at harvest from 19.3 to 39.1 mg g⁻¹ and from 3.6 to 18.4 mg g⁻¹, respectively. N partitioning was well-simulated by the crop model component except when crops had deficient N nutrition. These results suggest that the parameter “NC_n-remob” (proportion of N in vegetative parts which is not available for remobilization to filling seeds), which is taken as constant in the crop model component, could depend upon the crop nutrition level. A sensitivity analysis highlights the need for a precise calibration of the parameters “NC_n-remob” and “SNR_max”. When the crop N nutrition level and further genotypic variability of these parameters are incorporated in the proposed crop model component, it will become a useful part of a pea crop model predicting yield and seed N concentration.     

Water and nitrogen limitations in soybean grain production II. Field and model analyses

A short-season and a long-season soybean (Glycine max (L.) Merr) cultivar were grown on two different soil types under both irrigated and water-limited conditions in a semi-arid tropical environment. In addition to differences in water holding capacity, the clay soil had less available soil nitrogen than the sandy loam soil. The experimental water regimes coupled with the differing soil types gave a wide range in yield response. A model analysis was done to simulate the crop growth to identify those factors that limited yield. Under irrigated conditions, the yields of crops grown on the clay soil were found in the model to be especially limited by low amounts of available soil nitrogen. With weekly irrigation of the sandy loam soil, the long-season cultivar in the simulation experienced brief episodes of water shortage which reduced the nitrogen fixation rate. An optimal irrigation schedule was simulated based on soil water depletion, which improved yield and saved water compared to the simulated weekly schedule. Terminal water deficits reduced yields more for the long-season cultivar both experimentally and in the simulations, as the short-season cultivar initiated seed growth earlier and produced greater seed mass before water shortage terminated crop growth.A model analysis of the water limitations showed that water deficits at the beginning of seed fill had the greatest effect on yields. Greater soil water storage as simulated by greater depth of water extraction resulted in increased yields. Increased nitrogen supply to the crop simulated by either greater soil nitrogen availability or increased nitrogen fixation rates resulted in substantial yield increases.     

Kenaf seed storage duration on germination,emergence, and yield

Kenaf (Hibiscus cannabinus L.) is a potential alternative crop being developed for fiber production. Because planting area varies dramatically from year to year, seed supplies may greatly exceed use so that the excess seed must be stored for one to several years. The objectives of this study were to determine the effect of seed storage duration at 10 °C on germination, vigor, emergence, and yield. Replicated trials were established at Starkville, MS in 1999 and 2000 to evaluate field emergence and biomass yield of kenaf seed from five ‘Everglades 41’ (‘E41’) harvest year seed lots stored at 10 °C in ambient relative humidity for up to 4 years. Germination of these same seed lots under standard (20–30 °C) and cool (20 °C) temperatures, and seed vigor was evaluated over time. Field emergence was the same for the different seed storage durations up to 4 years, but was directly affected by drought conditions for each planted year. Biomass yields ranged from 12.39 to 14.57 Mg ha⁻¹ in 1999 and 16.82 to 18.47 Mg ha⁻¹ in 2000, but were not different between storage durations. Seed germination remained greater than 80% regardless of storage duration. Electrolyte leakage, based on conductivity, was 38–50% less with freshly harvested seed than seed stored for 4 years at 10 °C. However, neither the conductivity nor accelerated aging test were reliable predictors of field emergence. Kenaf seed stored up to 4 years at 10 °C retained germination rates acceptable for commercial use. Neither field emergence nor biomass yield was affected by seed storage duration.     

Growth,PAR use efficiency,and yield components of field-grown Vicia faba L. under different temperature and photoperiod regimes

N-fixing legume crops may be a good component of a general plan to improve cropping system efficiency. For this purpose, crop suitability to specific environments must be established. To estimate the yield potential we examined the growth and yield response of faba bean (Vicia faba L.) crops to different thermal and photoperiod regimes. Irrigated field experiments were conducted in northwest Spain for 3 years (2004–2007) with cv. ‘Alameda’ sown on five different dates in each year from mid-autumn to mid-spring. Environmental conditions experienced by plants across sowing dates were largely different. Sowing date had a great influence on biomass, grain yield and its components. This effect was associated with changes in PAR captured, PAR use efficiency (PUE) and biomass allocation to the different organs. Critical leaf area index (LAI_cr) tended to increase and the extinction coefficient, k, to decrease as the sowing date was delayed. Earlier sowing dates intercepted more radiation over the whole season than the spring sowing dates. Greatest crop growth treatments (2nd and 3rd sowing dates) had the highest values of PAR use efficiency probably due to more adequate temperatures for photosynthesis and a large number of reproductive sinks. The highest grain yield (7733 kg ha⁻¹) was obtained with the mid-February sowing date, which produced the most pods and seeds per m², the largest harvest index (62.0%), and large maximum leaf area index (5.41). Low yields of mid-autumn (1st) and mid-spring (5th) sowing dates were associated with reduced pods and seeds per m². Temperature and photoperiod had a large impact on faba bean growth, development, and yield. Best yields were obtained when abundant assimilate supply and moderate temperatures were available during pod set.     

Potato (Solanum spp.) in an isohyperthermic environment II. Response to planting date

The potato has only recently been introduced into isohyperthermic environments, and is grown during the dry season. There is a need to determine the optimum planting time and the range of possible planting dates. Two contrasting cultivars, ‘Cosima’ and DTO-2, were studied to determine the influence of planting date on their growth and development. They were planted on five dates from early to late dry season (October-January) during two years, at 14° N lat. in The Philippines. Another objective, studied during the second year, was to compare seed produced under hot lowland conditions and stored for 8 months, with newly sprouted seed grown under cool highland temperatures.Potato plants grew faster and tuberized earlier for later planting dates. Physiologically young seed of Cosima and DTO-2 always produced plants with superior canopies. Tuber bulking was linear up to about 70 days after planting. Total dry-matter production was highest with young Cosima seed, exceeding 100 g plant⁻¹ for all planting dates except the last. Old Cosima and DTO-2 seed generally produced between 80 and 100 g dry matter plant⁻¹. Final tuber yields were highest with the mid dry-season planting (December) during both years with highland seed Cosima yielding 30 t ha⁻¹. Lowland seed of Cosima yielded over 21 t ha⁻¹ for the first 4 planting dates, significantly less than yields from highland seed. Yield differences increased with later planting. Although highland seed is recommended, lowland seed is an alternative for poor farmers. The final tuber yield of Cosima decreased by 12% with a minimum air temperature rise from 18.5 to 21° C, while DTO-2 was more adversely affected. Based on these results, minimum ait temperatures above 21 °C are unsuitable for potato production. Total intercepted solar radiation was positively correlated with final tuber yields and total dry-matter production. Efficiency of conversion into dry matter was about 70% of that in cool temperate climates.     

Evapotranspiration,yield, and water-use efficiency responses of Lesquerella fendleri at different growth stages

Lesquerella fendleri (Gray) Wats. is a potential new oilseed crop for the arid southwestern United States. Lesquerella seed oil with similar properties as castor oil is being considered as a domestic replacement for the imported castor oil. Development of new crops with low irrigation needs is of high priority. Because the most critical stage of sensitivity to moisture deficits has not been determined in Lesquerella species, the objectives of this study were: (i) to identify the most critical stage or stages for moisture deficit and, (ii) to determine the effect of moisture deficit on yield, yield components, oil and fatty acid composition. Two-year field studies were conducted at the New Mexico State University, Leyendecker Plant Science Research Center. The experimental design was a randomized complete block. The treatments consisted of (a) T1: Continuous favorable soil moisture [irrigated at 50% soil water depletion (SWD)]. (b) T2: Moisture stress (75% SWD) from establishment to initial flowering with no stress from flowering to final harvest (50% SWD). (c) T3: No stress imposed from establishment to initial flowering (50% SWD) followed by stress to final harvest (75% SWD). (d) T4: Moisture stress (75% SWD) from establishment to final harvest. The amount of water applied ranged from 810 to 729 mm for the first year, and 810 to 625 mm for the second year. Seed weight per plant and number of pods per plant were generally higher when water availability was maintained at or above 50% SWD throughout the growing season. Neither seed number per pod nor seed size was influenced by irrigation treatments. Lesquerella was more sensitive to water availability during flowering and seed development as a greater loss in seed yield occurred when irrigation was delayed to 75% SWD during that stage of development. Seed yield and dry matter production from the 2 year field studies were closely related to the seasonal cumulative evapotranspiration. For each millimeter of evapotranspiration, seed yield increased from 1.8 kg ha⁻¹ mm in 1994–1995 to 1.3 kg ha⁻¹ mm for 1995–1996. The dry matter production increased 13.4 kg ha⁻¹ for each mm increase in seasonal evapotranspiration during 1994–1995. This relationship was a second order polynomial with an R² of 0.86 during 1995–1996. The WUE_gr and WUE_dm were highest under the most favorable water availability conditions for growth and seed development. Delaying irrigation to 75% SWD throughout the crop growth period resulted in the lowest oil content. Lesquerolic acid content was not affected by irrigation during both the growing seasons.     

Potato Crop Response to Genotype and Environment in a Subtropical Highland Agro-ecology

Potato response to environment, planting date and genotype was studied for different agro-ecological zones in Lesotho. Field experiments were conducted at four different sites with altitudes ranging from 1,655 to 2,250 m above sea level during the 2010/2011 and 2011/2012 summer growing seasons. Treatments consisted of three cultivars that varied in maturity type, two planting dates and four sites differing in altitude and weather patterns. Various plant parts were measured periodically. To understand and quantify the influence of abiotic factors that determine and limit yields, the LINTUL crop growth model was employed which simulated potential yields for the different agro-ecological zones using weather data collected per site during the study period. Observed actual crop yields were compared with model simulations to determine the yield gap. Model simulations helped to improve our understanding of yield limitations to further expand potato production in subtropical highlands, with emphasis on increasing production through increased yields rather than increased area. Substantial variation in yield between planting date, cultivar and site were observed. Average tuber dry matter (DM) yields for the highest yielding season were above 7.5 t DM ha^?1 or over 37.5 t ha^?1 fresh tuber yield. The lowest yield obtained was 2.39 t DM ha^?1 or 12 t ha^?1 fresh tuber yield for cultivar Vanderplank in the 2011/2012 growing season at the site with the lowest altitude. Modelled potential tuber yields were 9–14 t DM ha^?1 or 45–70 t ha^?1 fresh yield. Drought stress frequently resulted in lower radiation use efficiencies and to a lesser degree harvest indices, which reduced tuber yield. The site with the lowest altitude and highest temperatures had the lowest yields, while the site with the highest altitude had the highest yields. Later maturing cultivars yielded more than earlier maturing ones at all sites. It is concluded that the risk of low yields in rain-fed subtropical highlands can be minimised by planting late cultivars at the highest areas possible as early as the risks of late frosts permit.     

Improving Rice-Based Cropping Pattern Through Soil Moisture and Integrated Nutrient Management in Mid-Tropical Plain Zone of Tripura,India

An experiment was conducted in three fallow paddy fields situated on the mid-tropical plain zone of a northeastern Indian state(Tripura) to provide rice fallow management options using leftover soil moisture and nutrients. The three experimental fields were managed by growing rice under the system of rice intensification as the rainy season crop and then groundnut, lentil, rapeseed and potato as the post-rainy season crops. Fertilization under the integrated nutrient management system and lifesaving irrigation at critical stages of each post-rainy season crop were provided. Results showed that the field water use efficiency values were 5.93, 2.39, 2.37 and 59.76 kg/(hm2·mm) and that the yield of these crops increased by approximately 20%, 34%, 40% and 20% after applying two lifesaving irrigations in groundnut, lentil, rapeseed and potato, respectively. Therefore, fallow paddy field can provide possible profitable crops during the post-rainy season by utilizing the residual moisture and minimum supplemental irrigation under improved nutrient management practices.     

Yield and land-use efficiency of a cassava/cowpea intercropping system grown at different phosphorus rates

Cassava (Manihot esculenta Crantz) is frequently intercropped with cowpea [Vigna unguiculata (L.) Walp subsp. unguiculata] in the tropics. Little is known about the influence of P fertilization practices on the efficiency of land use and yields in cassava/cowpea intercropping systems. Two experiments were conducted on a Typic Dystropept soil with the objective of determining the influence of P application rate on yield and P status of cassava and cowpea grown in sole and intercropping systems, and the influence on land use efficiency. Cassava yields averaged across P rates were reduced 29% from the 28 Mg ha⁻¹ sole crop yield when intercropped with cowpea in 1979–1980. Cowpea yields were reduced by 19–38% from a sole crop yield of 1522 kg ha⁻¹ when intercropped in 1979, and 29–38% from a sole crop yield of 1277 kg ha⁻¹ in 1980. The rate of P application had little influence on cassava yield, except in 1981 when intercropped cassava yields were greater than 40 Mg ha⁻¹. In 1981, increasing the rate of P application from 0 to 44 kg ha⁻¹ resulted in a cassava yield increase from 41 to 47 Mg ha⁻¹. In 1979 and 1980, increasing the rate of P application from 0 to 22 kg ha⁻¹ increased cowpea yield 44 and 92%, respectively, while increasing P rate from 66 to 132 kg ha⁻¹ increased cowpea yield 28 and 18%, respectively. In 1981 and 1982, increasing the rate of P application from 0 to 44 kg ha⁻¹ increased cowpea yield by 1052 kg ha⁻¹. Phosphorus concentration of cassava and cowpea leaf blades increased with increases in rate of P application from 66 to 132 kg ha⁻¹ in 1979 and 1980, and from 0 to 44 kg ha⁻¹ in 1981 and 1982. Intercropping cassava with cowpea resulted in a 30% increase in land-use efficiency when no P was applied, while land-use efficiencies resulting from intercropping were increased by 41–50% with P application rates of 22–132 kg ha⁻¹. Cassava proved to be well-adapted to low-P soils and very competitive even without P application, whereas cowpea required the addition of P for adequate growth and yield. High productivity and a good competitive balance between the two crops were reached with only 22 kg ha⁻¹ of P, showing the great potential of cassava/cowpea intercropping on acid, infertile soils in the tropics.