Fallow Crop Research Articles

Nitrogen dynamics and crop growth on an Alfisol and a Vertisol under a direct-seeded rainfed lowland rice-based system

Rice ( Oryza sativa L.) followed by chickpea ( Cicer arietinum L.) or a fallow is one of the predominant cropping systems in the rainfed lowlands of India. Crop rotation experiments over 3 years (1996–1998) to quantify N supply and demand under rainfed lowland rice–chickpea and rice–fallow cropping systems on a loam Alfisol and a clay Vertisol in Raipur, India were conducted under direct-seeded rice culture. The rice growth, yield, development and N accumulation were affected most by N rates (0, 40, 80, 120 kg ha −1) followed by cropping system (rice–chickpea, rice–fallow) and soil types (Alfisol, Vertisol). The incorporation of chickpea in the cropping system helped in accumulating a greater amount of soil N than fallow. The rice yield, dry matter and N accumulated were significantly higher in rice–chickpea than rice–fallow systems on both soils and in all years. The lowest rice yields were recorded in 1997 due to unfavorable rainfall distribution. The total rainfall was the highest in this season, but most of it occurred during a short period at an early growth stage. The post-heading rains were lowest in this season and resulted in the lower rice yield as compared with that of 1996 and 1998. This indicates the significance of rainfall distribution in controlling yield in a rainfed environment. The rice yields were lower on Vertisol than Alfisol during periods of drought. The performance of chickpea was also better in Alfisol as compared with that in the Vertisol due to its better soil physical attributes. The residual effect of N applied to the preceding rice crop was non-significant on all yield, growth and N accumulation parameters of chickpea. The N balance computed from the top 70 cm soil layers indicated less N loss in the rice–chickpea system as compared with that in rice–fallow. The recovery efficiency at the highest N rate (120 kg N ha −1) was higher for the rice–chickpea (57–61%) than that of rice–fallow (49–53%) system. The improved N balance for rice–chickpea system from third year onwards was due to switch to dry seeding and improved soil N status. The inclusion of legume and the effective capture of biologically fixed N and soil N through direct-seeded rice system in rainfed lowlands may help in improving the rice yield of resource poor farmers.

Nitrogen management in 'adequate' input maize-based agriculture in the derived savanna benchmark zone of Benin Republic

Although the West-African moist savanna zone has a high potential for crop production, yields on farmers' fields are, on average, far below this potential, mainly due to the low use of external sources of nutrients. Since the mid-1990s, it has become clear that in order to upgrade crop production to levels needed to sustain the growing population without further degrading the soil resource base, inorganic fertilizers are required. Due to the physico-chemical nature of these soils and the relatively high cost of inorganic fertilizers, a general consensus exists in the research and development community that these inorganic inputs need to be complemented with organic matter. Here, we explore options to produce organic matter in-situ and evaluate the impact of combining inorganic and organic sources of N on maize yields, focusing on the densely populated derived savanna (DS) benchmark of Benin Republic. Although most of the farmers (93%) in this benchmark use inorganic fertilizer, applications rates are low (on average, 27 kg N ha−1). A significant response to N was observed for 96% of the studied farmers' fields. Grain and herbaceous legumes were observed to produce between 383 and 8700 kg dry matter ha−1 in the benchmark area. Inoculation with Rhizobia and inorganic P additions were shown to significantly improve biomass production on sites with low contents of Rhizobia and P. Although maize grain yield was observed to increase significantly following a legume compared with following a maize crop or natural fallow, these increases were insufficient in the case of a cowpea crop or were obtained at the cost of leaving the field `idle' for a whole year in the case of a herbaceous Mucuna fallow. Topping up a cowpea haulms equivalent of 45 kg N ha−1 with 45 kg urea–N ha−1 was shown to give maize yields similar to the yields obtained after applying 90 kg urea–N ha−1 on the poorest fields. Moreover, on these fields, a positive interaction between cowpea–N and urea–N sources of 200 kg grain ha−1 was observed. On the richest fields, the effects of applied organic matter and fertilizer were additive. Agroforestry systems are alternative cropping systems that produce organic matter in-situ. As tree roots go down below the rooting depth of food crops, sub-soil fertility was observed to influence tree biomass production. Yield increases in tree-crop intercrop systems – such as alley cropping – in the absence of inorganic inputs are often reduced by the occurrence of tree-crop competition. In cut-and-carry systems, where tree prunings are harvested from a field adjacent to the crop land, increases in maize grain yield caused by addition of those prunings were observed to be on the low side. Mixing these residues with urea, however, was shown to lead to added benefits of about 500 kg grains ha−1, relative to the treatments with sole inputs of organic matter or urea. Although residue quality was shown to affect maize N uptake in a pot trial, its impact under field conditions was minimal for the range of considered residue qualities. In an alley cropping trial, maize yield was shown to be sustained on a non-degraded site and enhanced on a degraded site, when a minimal amount of mineral fertilizer was added with the prunings, whereas fertilizer application alone failed to do so in both cases.

Effect of phosphorus application in legume cover crop rotation on subsequent maize in the savanna zone of West Africa

The benefit of planted fallow with legume cover crops may be limited on P deficient soil. A trial was conducted at two P deficient sites in northern Nigeria to test the hypothesis that application of P to legume cover crop fallow can substitute for N application to subsequent maize. Mainplots consisted of leguminous fallows followed by unfertilized maize, or native (mostly grass) fallows followed by maize with 0 or 40 kg N ha−1 (Kaduna) and 0, 30 or 60 kg N ha−1 (Bauchi). Three rates of P (0, 9, and 18 kg ha−1) were applied to fallow sub-plots as single superphosphate. In the first year, dry matter accumulation of lablab (Lablab purpureus) responded to P application, while mucuna (Mucuna cochinchinensis) dry matter did not. Lablab mulch dry matter during the dry season was significantly increased by previous season P application while mucuna was not. Previous fallow vegetation was a significant factor for maize growth in the second year but the interaction with P applied to the fallow was not significant at P < 0.05. Substantial and similar yield increases were achieved with application of N fertilizer to maize and from application of 9 kg P ha−1 to previous lablab. Depending on local economic circumstances, a good use of expensive inorganic fertilizer might be to apply P sources to cover crop legumes to profit from additional N benefits as well as residual effects of applied P.

Tillage, Cover Cropping, and Poultry Litter Effects on Cotton: I. Germination and Seedling Growth

Inadequate and less vigorous crop stand is a constraint to adoption of conservation tillage in cotton (Gossypium hirsutum L.) production. We evaluated the effects of tillage (conventional till, mulch‐till, no‐till), cropping system (cotton–winter fallow, cotton–winter rye, Secale cereale L.), and N source and rate (ammonium nitrate and poultry litter; 0, 100, and 200 kg N ha−1) on cotton seedling emergence on a Decatur silt loam soil (Typic Paleudults) in northern Alabama, from 1996 to 1998. Cotton seedling counts under no‐till were 40 to 150% greater than those under conventional till at 1 and 2 d during seedling emergence. Cotton–winter rye cropping system had 14 to 50% greater seedling counts than cotton–winter fallow cropping during the first 4 d of emergence in 1998. Poultry litter source of N gave 17 to 50% greater cotton seedling counts than ammonium nitrate during the first 4 d of emergence in 1998. In all these cases, the differences progressively narrowed down by the 4th day of seedling emergence. Cotton seedling counts were significantly correlated to cotton growth parameters and lint yield, especially in the drier year (1998). These results were attributed to soil moisture conservation during seedling emergence. Our results show that conservation tillage improved cotton germination, emergence, dry matter, and lint yield. Therefore, no‐till with winter rye cover cropping and poultry litter can be used for achieving early cotton seedling emergence and growth in the U.S. cotton belt where dryland cotton production systems are on the increase and safe disposal of poultry litter is becoming an environmental problem.

Water use patterns and agronomic performance for some cropping systems with and without fallow crops in a semi-arid environment of northwest China

Winter wheat ( Triticum aestivum L.) monoculture, characterized by a 2–3 month summer fallow (from the wheat harvest in early July to sowing in late September), not only lowers the overall precipitation-use efficiency because of the large amount of water evaporated from the bare soil during the fallow period but also has a high risk of the erosive action of rainstorms. To develop effective cropping systems as alternatives for farmers, thirty-two 3-year crop rotation systems with and without fallow crops were established and their water use pattern, agronomic performance and soil conservation characteristics were evaluated in a semi-arid region of northwest China. Compared with wheat monoculture, the 16 rotations without fallow crops produced more grain yield and greater water-use efficiency. Among the 16 rotations with fallow crops, seven patterns were significantly greater in grain yield and five patterns were significantly higher in water-use efficiency than wheat monoculture. Cropping systems showed a marked increase in evapotranspiration when fallow crops were added to the rotation largely because of better utilization of seasonal precipitation. On average, the 16 rotations with fallow crops utilized 17 and 27% more precipitation than the 16 rotations without fallow crops and wheat monoculture. The use of the fallow crops did not greatly influence the quantity of water stored in the soil for use by the subsequent wheat crop because their growth depends only on growing-season precipitation. From a perspective of soil conservation and productivity improvement, it appears highly viable to cultivate winter wheat followed by a 2–3 month fallow crop in 1 year and a summer crop cultivation in the next. This system allows the soil to be covered during both easily-eroded rainy periods but lies bare 6 months every 2 years. As most of this 6-month period is winter, not only is soil evaporation reduced but also the danger of erosion is low.

Spring Wheat Response to Tillage System and Nitrogen Fertilization within a Crop–Fallow System

Spring wheat (Triticum aestivum L.) production in the northern Great Plains generally utilizes conventional tillage systems. A 12‐yr study evaluated the effects of tillage system [conventional‐till (CT), minimum‐till (MT), and no‐till (NT)], N fertilizer rate (0, 22, and 45 kg N ha−1), and cultivar (Butte86 and Stoa) on spring wheat grain yields in a dryland spring wheat–fallow rotation (SW–F). Butte86 yields with CT exceeded NT yields in five out of 12 years with 0 and 22 kg N ha−1 applied, and four years with 45 kg N ha−1 applied. Stoa yields with CT exceeded NT yields in three out of 12 years with no N applied, four years with 22 kg N ha−1 applied, and only one year with 45 kg N ha−1 applied. Yields with NT exceeded those with CT in one year. Most years, yields with MT equaled those with CT. Responses to N tended to be greatest in years when spring soil NO3–N was lowest. Positive yield responses to N fertilization with CT occurred in three years with Butte86 and two years with Stoa; with MT, four years with Butte86 and two years with Stoa; and with NT, five years with Butte86 and three years with Stoa. Cultivars were not consistent in their response to tillage and N fertilization. These results indicate that farmers in the northern Great Plains can successfully produce spring wheat in a SW–F system using MT and NT systems, but yields may be slightly reduced when compared with CT systems some years.

Farmers indigenous practices for conserving Garcinia kola and Gnetum africanum in southern Cameroon

Traditional approaches to conservation of Garcinia kola and Gnetum africanum were assessed by a survey of 15 villages (three per province) in the humid forest zone of southern Cameroon. Four major land use systems [evergreen forest, degraded forest, bush fallow (10 years and over) and food crop fields] were identified as major niches for the species. The distribution of the plant stands varied from province to province. Gnetum africanum is intensively harvested (up to four times per week throughout the year) and reported to generate substantial income (averaging US$2,630 per household per annum). Indigenous practices used by farmers to protect the species include selective clearing during land preparation for cropping, sustainable bark harvesting of stands in wild population, transplantation of wildings, artificial propagation and recognition of individual property ownership on certain wild stands of G. kola.

Changes in Organic Matter of a Ferrallitic Tropical Soil Degraded by Cropping Systems: The Case of Southern Senegal

Understanding of the mechanisms involved in the restoration of degraded soils is necessary in order to assess the sustainability of a cropping system. For this purpose, an 8-year experiment was set up to test the efficiency of four cropping systems involving applications of fertilizer and/or manure and incorporation of crop residues and/or fallows on the regeneration of a Haplic Acrisol degraded by about 90 years of traditional cropping (groundnut-millet rotation without any input). The supply of nitrogen by the soil and particle size and chemical distribution were measured and used to characterize the soil organic matter (SOM) after 2 and 8 years of treatments, and then related to nutrients uptake. Two of the four cropping systems tested that were expected to regenerate soil fertility (Q1 and Q2: 4-year rotations including a plowed-in fallow, incorporation of crop residues, and mineral fertilization) both had a depressive effect on SOM content, although the losses of total C and N observed during the experiment (e.g .,-23% N and -20% N for Q1 and Q2, respectively) were less than that for the control (traditional cropping B, -33% N). Only the multiyear fallow did not result in further degradation of SOM, without, however, enriching the soil. On the other hand, an annual application of 10 Mg dry matter of manure ha -1 , combined with fertilizer, markedly increased the SOM (total N increased by factor 2 after only 2 years), an effect that seemed, however, stabilized at the end of the 8 years. Only a strong (10 Mg dry matter ha -1 ) and annual supply of manure, associated with mineral fertilization, had a positive effect on SOM, but its durability is not established.

Soil Quality Assessment after Weed‐Control Tillage in a No‐Till Wheat–Fallow Cropping System

Adoption of reduced‐tillage fallow systems in the western USA is limited by winter annual grass weeds such as downy brome (Bromus tectorum L.). Moldboard plowing is an effective means of controlling downy brome in winter wheat (Tritcum aestivum L.)–fallow systems. The purpose of this study was to assess the influence of plowing and secondary tillage operations, for the purpose of weed control, on soil quality attributes of a silt loam soil that had been cropped in a sub‐till or no‐till (NT) winter wheat–fallow system for more than 20 yr. Compared with undisturbed NT, downy brome populations in plowed NT decreased 97 and 41% in the first and third crops following tillage, respectively. Wheat yields in plowed NT treatments were 30 and 9% greater in the first and third crops following tillage, respectively, compared with undisturbed NT. Soil quality indicators assessed were organic C (OC), total N, inorganic N, pH, electrical conductivity, bulk density, water infiltration rate, and pore‐size distribution. Five years after tillage, soil OC decline in the 0‐ to 7.5‐cm depth was 20% in plowed compared with undisturbed NT; however, OC increased 15% in the 7.5‐ to 15‐cm depth and was not different in the 0‐ to 30‐cm depth. Total soil N followed similar trends. Soil inorganic N in plowed NT decreased 37%, and soil pH increased 9%, compared with undisturbed NT, at the 0‐ to 7.5‐cm depth. Occasional tillage with the moldboard plow in a reduced‐ or no‐tillage management system will help control winter annual grass weeds, while retaining many of the soil quality benefits of conservation‐tillage management.

AN IMPROVED COVER CROP-FALLOW SYSTEM FOR SUSTAINABLE MANAGEMENT OF LOW ACTIVITY CLAY SOILS OF THE TROPICS

The potential of planted leguminous cover crop fallow as an alternative to the natural regrowth fallow for sustaining the productivity of low activity clay (LAC) soils in the tropics as the fallow period shortens was tested at Ibadan in the forest-savanna zone of southwestern Nigeria. The study, initiated in 1990, compared an improved system (cover crop-fallow) and a traditional system (natural fallow), with the aim of developing an improved cover crop-fallow system for sustainable management of LAC soils of the tropics. Pueraria phaseoloides, an herbaceous, N-fixing, perennial legume with a slow initial growth habit, was selected as a test cover crop. Pueraria was sown with a maize-cassava intercrop at the same season. In a 2-year cycle, 1 year of cropping was followed by 1 year fallow with pueraria for the cover crop-fallow system or with natural regrowth (mainly Chromolaena odorata) for the natural fallow system. Maize-cassava intercropping without a fallow period (continuous cropping) was included as a control. No fertilizer was applied throughout the experimental period. Pueraria produced 253 kg N ha -1 after the fallow period, compared with 109 kg N ha -1 for chromolaena; pueraria was also better in recycling P (11 kg P ha -1 ) than was chromolaena (9 kg P ha -1 ). Natural fallow for 1 year increased the maize yield from 75 to 350% from 1992 to 1996, and the cassava yield from 9 to 130% compared with continuous cropping. Cover crop-fallow resulted in a 22 to 72% higher maize yield than the natural fallow from 1992 to 1996. Although in 1992 and 1994 cassava tuber yield was lower with the cover crop-fallow than with the natural fallow, with better pueraria husbandry in 1996, cover crop-fallow increased the cassava tuber yield by 41%. With residue burning, the N balance was 27 kg N ha -1 yr -1 for the pueraria cover crop-fallow system and -15 kg N ha -1 yr -1 for the chromolaena natural fallow system. Cover crop-fallow maintained soil organic carbon status better than the natural fallow. Integration of a pueraria legume cover crop did not affect soil pH. Results indicate cover crop-fallow with pueraria could be a better alternative to traditional natural fallow under shortened fallow periods for raising or maintaining productivity of LAC soils of the humid tropics.

Nitrogen dynamics and crop growth on an alfisol and a vertisol under rainfed lowland rice-based cropping system

Crop rotation experiments were conducted over 2 years to quantify N supply-demand under rainfed lowland rice–chickpea and rice–fallow cropping systems on a loam Alfisol and a clay Vertisol in India. Significant differences among N rates (0, 40, 80, and 120 kg N ha −1) and soils were observed with rice for grain yield, total biomass and grain N uptake in both years. Low N response, low grain yield, low N uptake, and a short grain filling phase during the 1995 wet season was due to post-heading water stress. The stress was more pronounced on the Vertisol and at high N rates. This resulted in lower N content at maturity than at heading. The loss of biomass N from plant implied that apparent N recovery (AR) and physiological nitrogen use efficiency (PNUE) may differ significantly based on whether maximum N accumulation or total N uptake at maturity was used. Plant N recovery, both by the N difference method and 15 N technique, revealed much lower recovery of fertilizer N (21–27%) with rainfed lowland rice than with irrigated dry-season rice. The residual effect of N applied to the preceding rice crop was nonsignificant on all yield, growth, and N uptake parameters of chickpea. The performance of chickpea was better on the Alfisol than the Vertisol, principally due to soil physical attributes. The better performance and longer growth duration during 1994–1995 dry season as opposed to the 1995–1996 season was attributed to higher rainfall (92 mm versus 39 mm) and rainfall during the critical pod-filling to maturity phase. Mineralization and nitrification of N was negligible during the rice–fallow period due to the dry conditions and low organic matter content. This was corroborated by the similarities in N balance between rice–chickpea and rice–fallow system. The PNUE of rice was 33 to 57 kg grain per kg N absorbed compared with 25 to 27 kg grain per kg N absorbed of chickpea, owing mostly to higher N concentration of chickpea grain and the higher energy cost due to biological N fixation.

Prior crop and residue incorporation time affect the response of paddy rice to fertiliser nitrogen

Crop residues are an important source of nitrogen (N) for rice (Oryza sativa L.). The objective of this research was to determine how the supply of mineral N from different prior crops or fallow might affect the growth and yield of rice. The study also tested whether N use by rice might be improved by timing the application of inorganic fertiliser N to supplement the N mineralised after prior crops. Experiments consisted of fallow, or cereal or legume crops in the dry-season followed by wet-season rice; and fallow, or cereal or legume crops in the wet- season followed by dry-season rice. Urea at one-third of the rate required for optimum rice yield was applied at 3 times during the rice crop: sowing, permanent flood, and/or panicle initiation. The prior fallow and crop treatments significantly influenced the growth and yield of rice crops. After a fallow, the pattern of soil N mineralisation promoted vegetative growth but was limiting during grain-filling. In contrast, after a cereal crop, rice vegetative growth was limited but grain-filling was promoted. Legume prior crops promoted both vegetative and grain growth. The benefits derived from growing the cereal or legume crops before rice, in terms of replacing fertiliser N, were dependent on the time at which fertiliser N was applied to the rice crop. In particular, legume crops frequently nullified the rice growth responses to fertiliser N. The results demonstrated that fallow and prior crops can alter the amount and timing of mineral N supply to a rice crop. Farmers should consider including a legume crop in rotation with rice because legumes supply N, which increases rice yield and reduces the requirement for fertiliser N. Cereal crops also contribute N, although farmers who use a cereal rotation should monitor the soil and crop N status during early rice growth, and supply extra fertiliser N to alleviate N deficiency.

The uptake and use of nitrogen by paddy rice in fallow, cereal, and legume cropping systems

In a previous paper, we reported that prior crops either increased or decreased the yield of paddy rice (Oryza sativa L.) and altered its response to fertiliser N. We considered that rice yield responses to prior crop might have reflected the uptake of crop residue N and the efficiency of its use to produce grain. Experiments consisted of dry-season grain or legume crops, or fallow, followed by wet-season rice (cv. Lemont); and wet-season grain or legume crops, or fallow, followed by dry-season rice. Urea at one-third of the rate required for optimum rice yield was applied at 3 stages of rice crop growth: sowing, permanent flood, and/or panicle initiation. Soil N supplied 4.1 to 6.5 g N/m2 to the rice crop, depending on the season. Rice also recovered 0 to 0.25 of the N in the residue of a prior maize crop and 0.23 to 0.57 of the N in grain legume residues or a legume green manure crop; the fraction was greater if fertiliser N was not applied. Increased N uptake was the major contributor to heavier yield. The relationship between grain yield and crop N content was mostly linear, and thus physiological efficiency of N use for rice grain production was essentially constant across the range of environments provided by fertiliser N and cropping system treatments in this study. In experiments where fertiliser N was applied, there were small effects of prior cereal and legume cropping treatments on physiological efficiency. In contrast, without fertiliser N application, physiological efficiency was increased by prior cereal and legume crops, which likely resulted from a greater congruence between the N demand of the rice crop, and the N supply from the soil and incorporated residue, when compared with a fallow treatment.

Rule-based management for simulation in agricultural decision support systems

Rule-based management systems can offer the farmer or consultant the opportunity to better approximate current or proposed management options, especially as they relate to dynamic conditions in farm fields and across the whole farm. Most existing attempts at rule-based management within simulation-oriented agricultural decision support systems (DSS) involve limited extension of fixed management dates in response to environmental conditions, or involve rules for implementing limited management events such as fertilizer applications or irrigations. A comprehensive rule-based management system for agricultural DSS was developed that allows simulated management events to occur in response to flexible producer-defined rules and to weather and management induced changes in the soil–crop system over time and space. The system provides a simple, English-based rules language, a rules development editor, and software to parse and interpret these rules and provide linkages to application DSS software packages such as Great Plains Framework for Agricultural Resource Management (GPFARM). Rules can be quickly and easily developed that cover management activities for individual management units (MUs) or groups of MUs. Feedback to the simulation package at each time step provides for generation of site- and time-specific management events across the application. Tests of the rule-based system on: wheat (Triticum aestivum L.), corn (Zea mays L.), fallow and wheat–fallow crop rotations used in eastern Colorado showed that management events were simulated within the correct time windows and in the proper sequence. Dates for simulated events varied as expected across each rotational cycle as a function of temporal conditions. Additional work is anticipated to allow dynamic calculation of event attributes, capture and implementation of producer time priorities, and a simplified menu system for the rule editor.

No-Tillage Increases Profit in a Limited Irrigation-Dryland System

Furrow irrigation is used on about 50% of the 22 million irrigated acres in the Great Plains. Irrigation water is pumped from the Ogallala aquifer and is in limited supply in relation to available land. Research has greatly improved irrigation water use efficiency since the mid 50s. The most recent and efficient system of furrow irrigation is Limited Irrigation-Dryland (LID) in which tailwater and rainfall runoff are almost eliminated using a combination of furrow dikes and irrigating only the top two-thirds of a field. Our objective was to compare effectiveness of conventional tillage, furrow diking, and no-tillage in fallow periods and crops in a winter wheat (Triticum aestivum L)-sorghum [Sorghum bicolor (L.) Moench.]-fallow crop rotation using a LID system of irrigation. No-tillage increased soil water storage during fallow periods compared with conventional tillage and furrow diking. This in turn increased sorghum yield 400 lb/acre as well as irrigation water use efficiency for sorghum. Wheat yield was not improved with no-tillage because of poor stands. Short-term variable costs with no-tillage were less than for furrow diking or conventional tillage. Annual profits for conventional tillage, furrow diking, and no-tillage were $66, 63, and 75/acre. This research showed that using no-tillage is better than narrow diking in a winter wheat-sorghum-fallow rotation using a LID irrigation system.

Recruitment and survival of native annual Trifolium species in the highlands of Ethiopia

Germination and survival of indigenous annual Trifolium species were studied. Seedlings naturally emerging in the field and from sown seeds in pots were regularly counted and uprooted, and survival was studied by monitoring colour‐coded seedlings. Differences in recruitment of Trifolium species were strongly related to the rainfall pattern. In the fallow (crop) lands, no seedling survived the dry season between the short and main rainy periods during the year, while in the natural pasture, 8% of the seedlings survived into the main growing (rainy) season. Occasional rains occurring in the dry period (between the two rainy periods) also induced successions of germination and seedlings death, and therefore depleted the soft seed reserve in the soil by the beginning of the main growing season. As temperature fluctuations were minimal during the main rainy season, the rate of seed softening was low, affecting new germinations. This had a significant impact on the quality of the natural pastures on which livestock in the highlands are dependent.

Evaluation of EPIC's snowmelt and water erosion submodels using data from the Peace River region of Alberta

Water erosion due to snowmelt is a major form of erosion in boreal regions of the Canadian Prairie. Evaluation of erosion models is an essential step before recommending their use in local or regional assessments of erosion rates and control methods. Using inputs from a runoff study conducted at La Glace, Alberta (55°25'N, 119°10'W) from 1984 to 1986, we evaluated the Erosion-Productivity Impact Calculator (EPIC) for its ability to simulate runoff and sediment yield from snowmelt events. The model was initialized with soil profile data acquired at the study site and complemented with data from standard soil databases (Albright series; loam, Dark Gray Chernozem). Daily weather data were acquired from the nearest climatological station (annual precipitation = 475 mm). Management data were as reported and included combinations of conventional and reduced tillage, annual and perennial, and fallow cropping. Mean runoff volume measured in 1985 was 57 mm while in 1986 it was 76 mm. EPIC over-predicted runoff volume by 25% in 1985 but under-predicted it by 7% in 1986. The period in which snowmelt occurred (mid-March – beginning of April) was predicted correctly. Under the conditions of this study, with many cropping inputs obtained from different sources, the model was unable to reproduce the reported management effects on runoff and sediment yield. EPIC simulated springmelt soil temperature trends at 9-cm depth, although the predicted temperatures in 1985 were generally underestimated. Our results suggest that the EPIC model calculates adequate values of runoff volumes and sediment yields during snowmelt. Key words: Runoff, sediment yield, soil erosion, crop rotations

Pigeon pea for fallow improvement in slash-and-burn systems in the hills of Laos ?

Slash-and-burn farmers in the hills of Laos urgently need techniques that can sustain rice yields and reduce weed pressure under short fallow periods. For this, the potential of pigeon pea (Cajanus cajan (L.) Huth) as a fallow crop was evaluated in variety, establishment, and cropping systems studies conducted at Houay Khot station (19° N) from 1992 to 1995. Introduced perennial varieties were not superior to local material for the parameters evaluated. Some semi-perennial varieties produced grain yields up to 2.2 t ha-1 but were inferior in weed suppression and survival 12 months after planting. All varieties lost much of their weed competitiveness after the first picking of pods. In studies without weeding, < 10% initially established plants were present 15 months after planting. Compared with pigeon pea, Gliricidia sepium and Leucaena leucocephala performed better in biomass production, weed suppression, persistence, and self-regeneration after cultivation of rice. In a rotation study, rice yield was 1.3, 1.5, 2.3, 1.6, 1.8, 1.8, and 2.2 t ha-1 (LSD 5% = 0.5 t) for continuous rice, rice-pigeon pea intercropping, rice after pigeon pea, rice after cowpea, rice after Stylosanthes hamata, rice after maize, and rice after fallow, respectively. Compared with continuous rice, intercropping or rotation with pigeon pea reduced nematode (Meloidogyne graminicola) infestation of rice and weed biomass in the rice crop. In spite of the beneficial effects, rotation or intercropping systems with pigeon pea will only be viable if economic benefits from harvested grain or from using plant parts as livestock feed can be obtained. Because of high mortality and weed invasion pigeon pea is not suitable for fallow improvement if fallow duration is more than one year.

Sustaining productivity of a Vertisol at Warra, Queensland, with fertilisers, no-tillage or legumes 4. Nitrogen fixation, water use and yield of chickpea

Summary. Continuous cereal cropping in southern Queensland and northern New South Wales has depleted native soil nitrogen fertility to a level where corrective strategies are required to sustain grain yields and high protein content. The objective of this study was to examine the performance of chickpea in chickpea–wheat rotations in terms of yields, water use and N2 fixation. The effects of sowing time and tillage practice have been studied. Chickpea grain yields varied from 356 kg/ha in 1995 to 2361 kg/ha in 1988; these were significantly correlated with the total rainfall received during the preceding fallow period and crop growth. Almost 48% of total plant production and 30% of total plant nitrogen were below-ground as root biomass. Mean values of water-use efficiency for grain, above-ground dry matter, and total dry matter were 5.9, 14.2 and 29.2 kg/ha.mm, respectively. The water-use efficiency for grain was positively correlated with the total rainfall for the preceding fallow and crop growth period although cultural practices modified water-use efficiency. The potential N2 fixation was estimated to be 0.6 kg nitrogen/ha.mm from 1992 total dry matter nitrogen yields assuming all of the nitrogen contained in chickpea was derived from the atmosphere. Sowing time had a much larger effect on grain yield and N2 fixation by chickpea than tillage practice (conventional tillage and zero tillage) although zero tillage generally increased grain yields. The late May–early June sowing time was found to be the best for chickpea grain yield and N2 fixation since it optimised solar energy use and water use, and minimised frost damage. Nitrogen fixation by chickpea was low, less than 40% nitrogen was derived from atmosphere, representing less than 20 kg nitrogen/ha.year. The potential for N2 fixation was not attained during this period due to below-average rainfall and high soil NO3-N accumulation because of poor utilisation by the preceding wheat crop. Increased soil NO3-N due to residual from fertiliser N applied to the preceding wheat crop further reduced N2 fixation. A simple soil nitrogen balance indicated that at least 60% of crop nitrogen must be obtained from N2 fixation to avoid continued soil nitrogen loss. This did not occur in most years. The generally negative soil nitrogen balance needs to be reversed if chickpea is to be useful in sustainable cropping systems although it is an attractive cash crop. Sowing time and zero tillage practice, possibly combined with more appropriate cultivars, to enhance chickpea biomass, along with low initial soil NO3-N levels, would provide maximum N2 fixation.

Use of Incorporated and Non-incorporated Granular Trifluralin for Annual Grass Control in Barley (Hordeum vulgare), Wheat (Triticum aestivum), and Canola (Brassica napus)

The comparison of incorporation vs no incorporation on a mid-October application of the granular formulation of trifluralin to control infestations of wild oat and green foxtail was evaluated in spring barley, wheat, and canola in west central Saskatchewan over a 3 yr period. Incorporation treatments consisted of: no incorporation, one incorporation in fall, one incorporation in fall and a second in spring, one incorporation in fall and two additional incorporations in spring. All treatments eliminated green foxtail. In all three crops wild oat panicle counts were equivalent from incorporated and non-incorporated trifluralin. Wild oat fresh weights in crops grown on stubble were similar for incorporated and non-incorporated trifluralin. In fallow crops, wild oat fresh weight reductions were greater in three of nine site years with incorporation. There was little difference in crop yields from incorporated and non-incorporated trifluralin.