Showing papers on "Sowing published in 2000"

Are plant populations seed-limited? A review of seed sowing experiments.

Abstract: We define seed limitation to be an increase in population size following seed addition. Here, we briefly consider how theoretical models deal with seed limitation and how seed sowing experiments can be used to unravel the extent of seed limitation in natural systems. We review two types of seed addition experiments: seed augmentation studies where seeds are added to existing populations; and seed introductions where seeds are sown in unoccupied sites. Overall, approximately 50% of seed augmentation experiments show evidence of seed limitation. These studies show that seed limitation tends to occur more commonly in early successional habitats and in early successional species. Most of the studies have concentrated on simply categorising populations as seed- or microsite-limited, but we believe that seed sowing experiments could be used to reveal much more about community structure, and we discuss possible future directions. In 53% of introduction studies (where seeds were sown at sites from which the species was known to be absent) the introduced species was recorded in at least one of the experimental sites following sowing. However, of the subset of studies where both seedlings and adult plants were recorded, 64% of sites contained seedlings while only 23% contained adults. This implies that, for many species, conditions for establishment are more stringent than conditions for germination. The successful establishment of plants in unoccupied patches indicates the potential for immigration to enhance local diversity (the spatial mass effect). Few studies continued monitoring for long enough to determine whether or not self-sustaining populations were successfully established, and no study attempted to link introduction sites to a putative natural source of propagules, or considered the dynamics of the metapopulation as a whole.

A comparative study of seed number, seed size, seedling size and recruitment in grassland plants

Abstract: In this study we analyse relationships between seed number, seed size, seedling size and recruitment success in grassland plants. The often hypothesised trade-off between seed size and seed number was supported by a cross-species analysis and by an analysis of 35 phylogenetically independent contrasts, derived from a data-set of 72 species. Apart from among-species relatedness, we also controlled for possible confounding effect of plant size that may influence both seed size and seed number. A sowing experiment with 50 species was performed in the field. The seeds were sown in a grassland and subjected to two treatments, disturbance and undisturbed sward. Evidence for seed-limited recruitment was obtained for 45 of the species. Disturbance had a significant, or nearly significant, positive effect on recruitment for 16 of the 45 species. The relative recruitment in undisturbed sward increased with increased seed size, and both recruitment success and seedling size were positively related to seed size. We suggest that a trade-off between competitive ability and number of recruitment opportunities follows from the trade-off between seed size and seed number, through a causal chain from seed size via seedling size to recruitment success. The relationships between seed size, seed number and recruitment may be an important underlying mechanism for abundance and dynamics of plant species in grassland vegetation. This is an example of a direct link between evolutionary life-history theory, and theory of plant community structure.

Is small-scale species richness limited by seed availability or microsite availability?

Abstract: To evaluate the relative roles of seed availability and competitive interactions in creating within-community patterns of species richness in unproductive grassland, we conducted a sowing experiment in a dry calcareous (alvar) grassland, where both the number of arriving seeds and the number of arriving species were approximately doubled compared to the natural seed rain. Also, in half of the plots, 36% of the vegetation and bryophyte cover was removed to simulate disturbance. Sowing significantly increased species richness and the number of seedlings in plots. Disturbance increased the number of seedlings but had no significant effect on species richness. In the first year, the highest number of seedlings was found in disturbed and sown plots. The dynamics of seedling numbers differed among species. Of the 15 sown species, seedlings of nine species were found in some plots. The number of seedlings of two species were not dependent on treatments, those of three species depended only on sowing, and for four species there was a significant positive interaction between sowing and disturbance. The establishment of sown species was not dependent on initial species richness or number of adult ramets in experimental plots. It was concluded that, though the behavior of individual species may differ, the local deficiency of seeds may be an important force generating small-scale community patterns of calcareous grasslands.

Base temperature and thermal time requirements for germination and emergence of temperate pasture species

Abstract: The base temperature (Tb) and thermal time (Tt) requirements for germination and emergence of temperate herbage species were defined using a linear model of development rate against temperature. A Tb of ≤ 4°C was found for all species. The Tt requirement for germination was lower for legume than grass species and generally lowest for small seeded species. The poor relationship (R2 = 0.42) between Tt requirements for germination and 50% field emergence indicated that germination results could not be extrapolated to predict the rate of field emergence. The Tt for 50% field emergence was highest for the small‐seeded grass species cocksfoot (220°Cd) and timothy (200°Cd). The times (days and Tt) for emergence of white clover and perennial ryegrass were similar from five autumn sowing dates but 57 days after sowing on 21 March 1996 ryegrass seedlings were 12 times heavier. The implication of these results for establishment of pasture mixtures is discussed together with the need for accurate measurement...

Effect of sowing date on the optimum plant density of winter wheat.

Abstract: Pressure on financial margins in UK wheat production is driving a review of all inputs, and seed represents one of the largest financial inputs in wheat production. The potential savings through exploiting the crop's ability to compensate for reduced population are, therefore, attractive. Field experiments were carried out at ADAS Rosemaund (Herefordshire, UK) in 1996/97, 1997/98 and 1998/99 to investigate the effect of sowing date on dry matter growth and yield responses of winter wheat to reduced plant population. There were three target sowing dates (late-September, mid-October and mid-November), six seed rates (20, 40, 80, 160, 320 and 640 seeds m -2 ) and four varieties (Cadenza, Haven, Soissons and Spark). Grain yield was significantly affected by plant population with a mean reduction from 9.2 to 5.5 t ha -1 as plant number was reduced from 336 to 13 m -2 . In addition, there was a significant interaction between plant density and sowing date. There was, however, no interaction between variety and plant population in terms of yield, except when lodging affected high plant populations of lodging susceptible varieties. The experiments demonstrated scope for reducing plant populations below the current target of 250-300 plants m -2 ; however, the degree of reduction was dependent on sowing date. Over the three years, the average economic optimum plant density was 62 plants m -2 for late-September, 93 plants m -2 for mid-October, and 139 plants m -2 for mid-November sowings. Compensation for reduced population was due to increased shoot number per plant, increased grain number per ear and to a lesser extent increased grain size. Higher economic optimum plant densities at later sowing dates were due to reduced tiller production and hence ear number per plant. The other compensatory mechanisms were unaffected by sowing date.

Potential of Early-Maturing Soybean Cultivars in Late Plantings

Abstract: Late planting reduces soybean [Glycine max (L.) Merr.] yields in soybean-winter wheat (Triticum aestivum L.) double-cropping systems. We evaluated the hypothesis that the use of early-maturing soybean cultivars to shift reproductive growth into a more favorable environment would avoid some or all of this yield penalty. Soybean cultivars Hardin and Kasota [maturity group (MG) I], Burlison and Elgin 87 (MG II), Pioneer 9392 and Probst (MG III), and Stressland and Pennyrile (MG IV) planted in 38-cm rows were used in a 3-yr irrigated experiment with two planting dates (early, mid-May; late, late June) at Lexington, KY (38° N lat). Delayed planting reduced yield (7-36%) of all cultivars as a result of fewer seeds m -2 . Cultivars from MG I and II did not produce higher yields in the late plantings. A combination of narrow rows (19 cm) and high seeding rates (105 seeds m -2 ) had no effect on yield of cultivars from MGs I and II in either planting date. However, early maturity did provide an earlier harvest date without significant yield loss. Seed number was significantly related to crop growth rate (CGR) during flowering and pod set (r 2 = 0.36) and to length of flowering and pod set (r 2 = 0.56). Radiation use efficiency (g dry matter MJ -1 intercepted photosynthetically active radiation) was generally reduced in the late plantings for MG III and IV cultivars but not for MG I and II. Early-maturing cultivars in an irrigated environment did not reduce the yield penalty associated with late plantings.

Intercropping and pest management: a review of major concepts

Abstract: Intercropping is the agronomic practice of growing two or more crops in the same field at the same time (Andrews and Kassam 1976). Crops may be planted without regard to rows (mixed intercropping), in alternating rows, or with different crops alternating within the same row. Relay intercropping refers to the planting of one intercrop species before another so that their life cycles partially overlap (Kass 1978). The broader term “polyculture” includes intercropping but also encompasses combining crops and weeds intentionally and combining crops with beneficial noncrop plants, such as cover crops or nursery crops (Andow 1991a). Perrin and Phillips (1978) included mixtures of crop cultivars in their definition of intercropping, because such combinations may possess some of the advantages associated with conventional intercropping.

Short-Season Soybean Yield Compensation in Response to Population and Water Regime

Abstract: Short-season soybean [Glycine max (L) Merr.] production systems, such as double cropping and late sowing, require high populations to optimize yield, but effects of high populations on seed number and seed mass are unknown. We evaluated plant population effects on yield compensation, stability of harvest index, assimilate partitioning for seed number, and seed-filling characteristics for 2 yr near Keiser, AR. The study had two cultivars, two levels of irrigation, and three row spacings that each had five levels of population ranging from 6 to 134 plants m -2 . Increasing population reduced yield per plant but increased yield per unit area. Harvest index was relatively constant across populations for a given year and irrigation regime, and yield was closely associated with biomass at maturity. At high populations, plants maintained individual seed mass by reducing the proportion of shell mass per pod. Final individual seed mass, seed growth rate (SGR), and the length of effective filling period did not change with increasing population for irrigated or nonirrigated treatments. Reductions in yield caused by low population density were due to low seed number. Seed number per square meter was directly proportional to the ratio of crop growth rate (CGR) to SGR. For short-season production, high populations ensured early canopy coverage and maximized light interception, CGR, and crop biomass, resulting in increased seed number and yield potential.

Wheat Nitrogen Use Efficiency in a Bed Planting System in Northwest Mexico

Abstract: Wheat (Triticum aestivum L.) in the Yaqui Valley, northwest Mexico, is planted as a winter crop using a raised-bed, furrow-irrigated system and high fertilizer N rates. Wheat residues are usually burned before planting maize (Zea mays L.) as a summer crop. The N use of wheat planted following conventional tillage using a raised-bed system (CTB) incorporating both wheat and maize residues was compared with wheat planted using permanent raised beds (PB) under four residue management treatments: all straw (wheat and maize) left as stubble, straw partly removed (maize residues removed; wheat residues retained), all straw removed, and all straw burned. Each wheat plot was split into seven N fertilizer (N t ) treatments: five applied at planting (0, 75, 150, 225, and 300 kg ha 1 ) and two at the 1st node stage (150 and 300 kg ha -1 ). Maize received a uniform N f application of 150 kg ha -1 . The N use efficiency of wheat with 150 kg N f ha -1 at the 1st node stage was superior to basal applications at the same rate. Permanent bed-all straw left as stubble and PB-all straw burned had the highest average wheat grain yields (5.57 and 5.52 Mg ha 1 , respectively), N use efficiency (28.2 and 29.1 kg grain kg -1 of N supply, respectively), and total N uptake (133 and 137 kg ha -1 , respectively). Total N uptake for 150 and 300 kg N f ha -1 at the 1st node stage was 14 and 8% greater, respectively than at planting. In most tillage-straw treatments, 21% of the difference in wheat grain yields was due to the N supply component at low N rates; at high N rates, 97% was due to N use efficiency.

Optimizing Soybean Plant Population for a Short‐Season Production System in the Southern USA

Abstract: Soybean [Glycine max (L.) Merr.] production systems that utilize short-season cultivars for double cropping and late sowing often have insufficient time to establish a complete canopy prior to reproductive development. Our objectives were to evaluate plant population as a tool to manage crop growth, maximum biomass (BM), the time required for canopy closure, and yield. Field tests were sown on 8 July 1997 and 26 June 1998 at Keiser, AR (35 degrees 67' N, 90 degrees 83' W) in 0.19-, 0.57-, and 0.95-m rows with maturity group IV soybean cultivars Asgrow 4922 (A4922) and Manokin. Yield from irrigated and nonirrigated treatments increased as population density increased from 7 to 134 plants m-2, except when lodging occurred. Populations recommended for early-season sowing (25-35 plants m-2) resulted in many plots not achieving 90% light interception (LI), especially in 1998 when weather was hotter and drier than in 1997. The time required after emergence to begin linear crop growth (t(b)) was dependent on LI, and as density increased, t(b) decreased. The values of t(b) varied from 16 to 27 d in 1997 and 22 to 37 d in 1998, with up to 12 d difference in achieving >90% LI. In this short-season production system, yield, crop growth rate between R1 and R5, BM, and t(b) were dependent upon the early establishment of a high LI. Losses attributable to excessive delays in canopy establishment and slow crop growth could be minimized by using high populations in narrow rows. Our research indicates that higher populations than are traditionally recommended provide a way to optimize grain yields in time-constrained systems.

Expression of Tolerance for Meloidogyne graminicola in Rice Cultivars as Affected by Soil Type and Flooding.

Abstract: The effects of different water regimes on the pathogenicity of Meloidogyne graminicola on six rice cultivars were determined in two soil types in three greenhouse experiments. Two water regimes, simulating continuous flooding and intermittent flooding, were used with five of the cultivars. All cultivars were susceptible to the nematode, but IR72 and IR74 were more tolerant than IR20 and IR29 under intermittent flooding. All were tolerant under continuous flooding. UPLRi-5 was grown under multiple water regimes: no flooding; continuous flooding; flooding starting at maximum tillering, panicle initiation, or booting stage; and flooding from sowing until maximum tillering or booting. In sandy loam soil, M. graminicola reduced stem and leaf dry weight, root dry weight, and grain weight under all water regimes. In clay loam soil, the nematode reduced root weight when the soil was not flooded or flooded only for a short time, from panicle initiation, or booting to maturity, and from sowing to maximum tillering. In clay loam soil, stem and leaf dry weight, as well as grain weight, were reduced by the nematode under all water regimes except continuous flooding or when the soil was flooded from sowing to booting stage. These results indicate that rice cultivar tolerance of M. graminicola varies with water regime and that yield losses due to M. graminicola may be prevented or minimized when the rice crop is flooded early and kept flooded until a late stage of development.

Windbreaks enhance seed dispersal into agricultural landscapes in monteverde, costa rica

Abstract: Planting windbreaks on degraded tropical lands may accelerate natural successional processes by attracting seed-dispersing animals and increasing the seed rain of forest plants. I examined the effects of planted windbreaks on seed deposition patterns in dairy farms in Monteverde, Costa Rica, by comparing annual seed rain patterns in windbreaks and adjacent pastures. I also examined the effects of windbreak connectivity and remnant trees on seed rain patterns. Traps placed in 19 windbreaks and in seven pastures collected more than 3 million seeds (of 199 species) during the year-long study. Windbreaks received significantly greater densities and species richness of seeds of tree and shrub species than pastures. Bird-dispersed seeds occurred in greater densities and species numbers in windbreaks than in pastures; in contrast, the seed rain of bat-dispersed seeds did not differ between windbreak and pasture habitats. Windbreak connectivity had no detectable effect on seed dispersal patterns, whereas the presence of remnant trees increased the number of tree species but not the density of tree seeds. This study suggests that windbreaks can dramatically increase the deposition of tree and shrub seeds within the agricultural landscape, and that the location of windbreaks within the landscape may be important in determining seed deposition patterns.

The Role of Soil Seed Banks in the Rehabilitation of Degraded Hillslopes in Southern Wello, Ethiopia1

Abstract: The species composition in the soil seed bank of degraded hillslopes in southern Wello, Ethiopia, was assessed using the seedling emergence method and compared with that of the standing vegetation. Surface soils were sampled at 0-to 5-cm depth from 49 plots of four physiognomic vegetation classes (hereafter vegetation classes): forests, shrublands, grasslands, and degraded sites. Soils were spread on sterile sand in a glasshouse and watered. Emerging seedlings were recorded for five months until no new seedlings emerged. A total of 3969 seedlings belonging to 71 species and 30 families germinated. The species composition of the seed bank was dominated by 53 herb species (75%) compared to 2 tree species which accounted for only 3 percent of the total number of species. Seedling density differed significantly among vegetation classes and ranged from 391 to 7807 seeds/m2. Mean species richness also differed significantly among the vegetation classes. Forty-two species were found to be common to the seed banks and the standing vegetation; however, correspondence between species numbers and composition of the seed banks and the standing vegetation was poor. Although most of the species that germinated in the seed banks were herbs and grasses, they can develop a vegetative cover and contribute to reduction of soil erosion. Regeneration of the tree species (some of which have seed viability up to four years) however, requires both time and the presence of mature individuals. Together with hillside closure and soil conservation measures (e.g., terracing), planting of native woody seedlings might help to expedite rehabilitation of degraded hillslopes devoid of trees and shrubs.

Evapotranspiration and Crop Coefficients of Wheat and Sorghum

Abstract: Wheat and sorghum are important cereal crops next only to rice in India and several other Asian countries. Improved water management requires accurate scheduling of irrigation, which in turn requires an accurate measurement of daily crop evapotranspiration ETc. Thus, the first objective of this study was to measure daily, weekly, and seasonal crop ETc of wheat and sorghum directly from sensitive weighing-type lysimeters. Experiments were conducted in a set of two electronic weighing-type lysimeters, 2 × 2 m in surface area and 2-m deep, to measure the hourly evapotranspiration of wheat and sorghum from 1991 to 1995 at Karnal, India. The average daily ETc of wheat varied from 4 mm/d at milking stage. The peak ETc of wheat was 4.6 mm/d and it occurred 16 weeks after sowing at the reproductive growth stage when leaf area index was 3.5. The measured seasonal ETc of wheat was 336 mm. In the case of sorghum, ETc was 3 mm/d at the initial stage, achieved a peak value of 6 ...

Wheat and Maize Yields in Response to Straw Management and Nitrogen under a Bed Planting System

Abstract: In the Yaqui Valley, northwest Mexico, the crop sequence that is becoming more common consists of planting wheat (Triticum aestivum L.) as a winter crop on a raised bed followed by maize (Zea mays L.) as a summer crop. In this area, straw of both winter and summer crops is commonly burned. The consequences of burning crop residues on crop yields in the Yaqui Valley have not previously been documented, and alternative practices have not been proposed. A 5-yr study was conducted at the CIANO (Centro de Investigaciones Agricolas del Noroeste) experiment station in Sonora, Mexico, to compare the effects of burning with other straw management strategies on wheat and maize yields. We tested two tillage systems (conventional-tilled bed, CTB, and permanent bed, PB), five straw management treatments (incorporated with CTB and straw as stubble, partly removed, removed, or burned with PB), and seven N treatments, five applied preplant (0, 75, 150, 225, and 300 kg N ha -1 ) and two at the 1st node stage (150 and 300 kg N ha -1 ) of wheat. Maize following wheat received a uniform application of 150 kg N ha -1 . The combination of PB and straw as stubble produced superior maize and wheat grain yields in high-yielding environments; in low-yielding environments, PB-straw burned produced greater wheat grain yields. Nitrogen fertilizer application of 150 and 300 kg N ha -1 at the 1st node stage of wheat increased grain yields compared with preplant N fertilizer applications. Permanent beds combined with retaining all crop residues in the soil as stubble have the potential to increase both wheat and maize yields in the Yaqui Valley.

Effect of lanthanum on aged seed germination of rice

Abstract: Attempts were made to promote germination of natural aged rice seeds by treating them with lanthanum nitrate. In tests to measure the germination rate, germination index, and vigor index of natural aged rice seeds were found to be significantly increased by lanthanum. It is treating aged rice seed with lanthanum nitrate that enhanced the respiratory rate and activities of superoxide dismutase, catalase, and peroxidase, and decreased superoxide O2*- and malondidehyde contents, and therefore reduced plasma membrane permeability. It suggests that lanthanum may be used to pretreat seed before sowing.

Relationship between Laboratory Seed Quality Tests and Field Emergence of Common Bean Seed

Abstract: In cool climates, early planting of bean (Phaseolis vulgaris L) is recommended to assure a longer vegetative growth period Early planting exposes seed to unfavorable conditions and the commonly used standard germination test cannot predict field emergence The objective of this work was to examine the relationships between various seed quality tests and field emergence of common bean seeds Thirty-nine strains and cultivars of bean were tested in the field and laboratory over 3 yr Each year seed samples were tested in the laboratory and then planted in the field at very early, early, and at optimal planting dates The viability of seeds was test by tetrazolium staining and was generally high; standard germination was above 80%, but germination in the cool test (at 10°C) varied from 0 to 99% The percentage of seeds with broken coats varied from 05 to 503 and conductivity ranged from 7 to 45 μS cm -1 g -1 Field emergence varied from 0 to 100% Soil temperature at sowing appeared to be not only the most important environmental factor influencing field emergence but also a factor able to be used to differentiate the field emergence potential of a seed lot Only the conductivity test could be used to predict seedling emergence in the field irrespective of soil temperature at sowing At temperatures ranging from 9 to 15°C, the first and the last count of the standard germination test and the broken seed coat test should be used in conjunction with the conductivity test to estimate field emergence potential

Velvetbean (Mucuna spp.) suppresses speargrass (Imperata cylindrica (L.) Raeuschel) and increases maize yield

Abstract: Speargrass, Imperata cylindrica (L.) Raeuschel, is a serious weed, threatening crop productivity in smallholder farms in West Africa. Since the use of more effective practices such as deep tillage and chemical control is beyond the means of resource-poor farmers who carry out most agricultural activities in this region, low-input alternative technology needs to be developed. Field studies were conducted during the 1993/1994 and 1995/1996 growing seasons to investigate the influence of three velvetbean accessions and two levels of fertilizer on the control of speargrass during the year of cover crop planting and one year later. The velvetbean accessions in 1993 were: Mucuna cochinchinensis and M. pruriens var. utilis. In 1995, M. pruriens var. IRZ was included in the study. M. cochinchinensis in 1993 and M. pruriens var. IRZ in 1995 had the highest ground cover rating early in the growing seasons whereas M. pruriens var. utilis had the lowest ground cover rating in both years. Inorganic fertilizer at30 kg ...