Showing papers in "Agronomy Journal in 1996"

Utilization of Allelopathy for Weed Management in Agroecosystems

Abstract: Biorational alternatives are gaining increased attention for weed control because of concerns related to pesticide usage and dwindling numbers of labeled products, particularly for minor-use crops. Allelopathy offers potential for biorational weed control through the production and release of allelochemics from leaves, flowers, seeds, stems, and roots of living or decomposing plant materials. Under appropriate conditions, allelochemics may be released in quantities suppressive to developing weed seedlings. Allelochemics often exhibit selectivity, similar to synthetic herbicides. Two main approaches have been investigated for allelopathic weed suppression. One is use of living rotational crops or mulches that interfere with the growth of surrounding weeds [e.g., tall red fescue, Festuca arundinacea Schreb.; creeping red fescue, F. rubra L. subsp. commutata; asparagus, Asparagus officinalis L. var. altilis); sorghum, Sorghum bicolor (L.) Moench; alfalfa, Medicago saliva L.; black mustard, Brassica nigra (L.) Koch; and oat, Avena saliva L.]. Attempts to select germplasm with enhanced suppressive ability have been limited. The second is use of cover crop residues or living mulches to suppress weed growth for variable lengths of time (e.g., winter rye, Secale cereale L.; winter wheat, Triticum aestivum L.; and sorghum). Cover crop residues may selectively provide weed suppression through their physical presence on the soil surface and by release of allelochemics or microbially altered allelochemics. The ability to understand the physiological basis for allelopathy in a crop plant may allow the weed scientist or ecologist to work closely with molecular biologists or traditional plant breeders to selectively enhance the traits responsible for weed suppression.

Interactions Involving Allelopathy in Cropping Systems

Abstract: Allelopathic inhibition typically results from the combined action of a group of allelochemicals which, collectively, interfere with several physiological processes. The objectives of this paper are to summarize research that illustrates the joint action of allelochemicals, and to provide evidence that both the amount and detrimental action of these compounds depends on the extent of associated abiotic and biotic stresses. Allelopathy is strongly coupled with other stresses of the crop environment, including insects and disease, temperature extremes, nutrient and moisture variables, radiation, and herbicides. These stress conditions often enhance allelochemical production, thus increasing the potential for allelopathic interference. In the paradigm of interactions, the data indicate that crops are more sensitive to allelopathy when moisture, temperature, or nutrient conditions are less than optimal. For example, the inhibition threshold concentration for ferulic acid to affect seedling growth was reduced with even minor moisture stress (ψ = - 0.15 MPa) or a growth temperature at the higher end of the normal range for a species. Under greenhouse conditions, additive inhibition resulted from the joint action of ferulic acid with low levels of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], atrazine [6-chloro-N-ethyl-N'(1-methylethyl)-1,3,5-triazine-2,4-diamine], or trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoro-methyl)benzenamine]. Interactions of multiple stresses in crop environments will determine the relative impact of allelopathy. Allelopathystress interactions also have implications for herbicide and residue management strategies, crop rotations, biological control measures, and tillage practices that can contribute to a more sustainable agriculture.

Nitrogen Release from Grass and Legume Cover Crop Monocultures and Bicultures

Abstract: The use of grass-legume bicultures grown as winter annual cover crops may provide farmers with additional cover crop management options regarding the availability of cover crop residue N. A 2-yr field experiment was conducted to determine dry matter (DM) accumulation, chemical composition, and N release from grass and legume cover crops grown in monoculture (rye, crimson clover, and hairy vetch) and biculture (rye-crimson clover and rye-hairy vetch). Air-dried plant material was placed on the soil surface in 1-mm mesh nylon bags for 1, 2, 3, 4, 6, 8, and 16 wk. Following retrieval, mesh bag contents were analyzed for total N, C, cellulose, hemicellulose, and lignin concentrations. The 2-yr mean cover crop DM production was in the order of rye-hairy vetch > hairy vetch > rye-crimson clover > rye > crimson clover. The greatest cover crop N content (2-yr mean) occurred with hairy vetch monoculture (154 kg N ha -1 ), compared with a low of 41 kg N ha -1 for the rye monoculture. When grown in biculture with rye, hairy vetch accumulated more DM and biomass N compared with crimson clover, both as a proportion of the biculture and as DM yield. In general, the order of N release rates (rapid to slow) was hairy vetch > crimson clover = rye-hairy vetch > rye-crimson clover = rye. Estimates of N (kg ha -1 ) released from cover crop residue after 8 wk of field decomposition, averaged over 2 yr, were 24 for rye, 60 for crimson clover, 132 for hairy vetch, 48 for rye-crimson clover, and 108 for rye-hairy vetch. Results of this study demonstrate only slight reductions in N release from grass-legume bicultures compared with legume monocultures.

Nitrogen Deficiency Detection Using Reflected Shortwave Radiation from Irrigated Corn Canopies

Abstract: Techniques that measure the N status of corn (Zea mays L.) can aid in management decisions that have economic and environmental implications. This study was conducted to identify reflected electromagnetic wavelengths most sensitive to detecting N deficiencies in a corn canopy with the possibility for use as a management tool. Reflected shortwave radiation was measured from an irrigated corn N response trial with four hybrids and five N rates at 0, 40, 80, 120, and 160 kg N ha -1 in 1992 and 0, 50, 100, 150, and 200 kg N ha -1 in 1993. A portable spectroradiometer was used to measure reflected radiation (400-1100 nm in 1992, 350-1050 nm in 1993) from corn canopies at approximately the R5 growth stage. Regression analyses revealed that reflected radiation near 550 and 710 nm was superior to reflected radiation near 450 or 650 nm for detecting N deficiencies. The ratio of light reflectance between 550 and 600 nm to light reflectance between 800 and 900 nm also provided sensitive detection of N stress. In 1993, an inexpensive photometric cell, which has peak sensitivity to light centered at 550 nm, was also used to measure reflected radiation from a corn canopy. Photometric cell readings correlated with relative grain yield (P < 0.001, r 2 = 0.74), but more research will be required to develop procedures to account for varying daylight conditions. These results provide information needed for the development of variable-rate fertilizer N application technology.

Row Spacing Effects on Light Extinction Coefficients of Corn, Sorghum, Soybean, and Sunflower

Abstract: In many crop models, light intercepted by a canopy (IPAR) is calculated from a Beer's Law equation : IPAR = PAR x [1 - exp(-k × LAI)], where k is the extinction coefficient, PAR the photosynthetically active radiation, and LAI the leaf area index. The first objective of this study was to investigate the effect of row spacing on k for corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], soybean [Glycine max (L.) Merr.], and sunflower (Helianthus annuus L.) to provide information for modeling. Data from literature and from an experiment conducted at Temple, TX, were evaluated. The second objective was to investigate effects of time of day and stage of crop development on k for different row spacings. Seeds of all four species were sown in rows 0.35, 0.66, or 1.00 m apart. Measurements of canopy light interception were taken near solar noon on two dates before anthesis. At anthesis, extinction coefficients were determined at 0845, 1015, and 1145 h (solar time). The extinction coefficient showed a linear decrease as row spacing increased. For each crop, the effect of row spacing on k was described by one linear regression for most data. Stage of crop development and stage of development × row spacing interaction did not significantly affect k during the period of measurements. The effect of time of day was significant for all four crops, and the time of day × row spacing interaction was significant for soybean and sunflower. Thus, modeling light interception for different row spacings should account for these effects.

Canopy Light Reflectance and Field Greenness to Assess Nitrogen Fertilization and Yield of Maize

Abstract: Assessment of crop N requirements is necessary to develop production systems with optimal N input. A field experiment with six maize (Zea mays L.) hybrids grown at three N fertilizer rates (0, 100, and 200 kg N ha -1 was conducted on a well-drained sandy loam of the Grenville series (coarse-loamy, mixed, mesic Typic Eutrochrepts) on the Central Experimental Farm at Ottawa, ON, in Canada (45°23' N, 75°43'W) for 3 yr (from 1991 to 1993) to evaluate whether canopy reflectance and greenness can measure changes in maize yield response to N fertility. Canopy reflectance, leaf area and greenness were measured on 11 dates from 4 wk before to 4 wk after anthesis. Grain yield at harvest was also measured. Direct radiometer readings at the 600- and 800-nm wavelengths or a derived normalized difference vegetation index [NDVI = (800 nm - 600 nm)/(800 nm + 600 nm)] best differentiated N and hybrid treatments at most sampling dates. Canopy light reflectance was strongly correlated with field greenness at almost all growth stages (field greenness being a product of plant leaf area and leaf greenness measured with a chlorophyll meter, in this case a SPAD-502). Both canopy light reflectance and field greenness measured preanthesis were correlated with yield at harvest. Light reflectance measured after anthesis differentiated hybrid differences in leaf senescence. Our data suggest that light reflectance measurements prior to anthesis may predict grain yield response and provide in-season indications of N deficiency.

Chemistry and Mechanisms of Allelopathic Interactions

Abstract: Allelopathy, originally defined as chemically elicited interactions between plants or fungi, is mediated by many types of compounds with different sites and modes of biochemical action. Although it is possible to measure a generalized effect, it is unlikely that deeper insight into the basic mechanisms of this phenomenon will he obtained until problems are more clearly and specifically delineated. Future work should include identificaflon of the compounds found in specific interactions and their evaluation in bioassay systems that use organisms actually involved in the response. A broader range of compounds and test organisms must be considered, and the biochemical sites and mechanisms of interaction must be evaluated, even for compounds that appear to be inactive. Once active compounds have been identifled, their release, movement, uptake, and effects on source and target species in the allelopathic system should be examined. The effects of plant density require additional investigation. Studies in which plants are grown in various combinations and ratios can ultimately provide information about the major sources of allelopathic interaction in communities, the role of allelopathy in succession, and the nature and extent of coadaptation of organisms. Understanding the role of allelopathy in succession will require information about the chemistry and biology of interaction, and a better understanding of the ecological factors involved. Agricultural systems can supply much needed information, as they are similar to natural systems but usually are simpler and better defined. In turn, a better understanding of allelopathy is important in many phases of modern agriculture, especially in the tropics. By attacking allelopathic interactions at several organizational levels, the effects collectively known as allelopathy can be understood, even in complex natural communities. Major advancement in understanding allelopathy will be accomplished only by the combined effort of investigators from many disciplines.

Long-Term Tillage, Crop Rotation, and Nitrogen Fertilizer Effects on Wheat Yield under Rainfed Mediterranean Conditions

Abstract: The combined long-term effects of tillage method and crop rotation on crop yield have not been studied in rainfed systems under Mediterranean climates. A field study was conducted from 1988 to 1994 to determine the effects of tillage (TILL), crop rotation (ROT) and N fertilizer on wheat (triticum aestivum L.) yield in a rainfed Mediterranean region. Tillage treatments include no tillage (NT) and conventional tillage (CT). Crop rotations were wheat-sunflower (Helianthus annuus L.) (WS), wheat-chickpea (Cicer arietinum L.) (WCP), wheat-fababean (Vicia faba L.) (WFB), wheat-fallow (WF), and continuous wheat (CW), with N fertilizer rates of 50, 100, and 150 kg N ha -1 . A split-split plot design with four replications was used. Differences in rainfall during the growing season had a marked effect on wheat yield. Amount of rainfall during the vegetative period for wheat (November-February) was highly correlated with yield because of the high water-retention capacity of Vertisols (Typic Haploxerert). In dry years, wheat yield was greater under NT than under CT ; the opposite was true in wet years. The TILL x ROT interaction was also significant in the drought years ; the wheat yield under NT was greater for CW and the WFB and WF rotations than under CT. Wheat yields ranked by crop rotation were : WFB > WF >> WCP > WS >> CW. Wheat did not respond to N fertilizer when rainfall was below 450 mm during the growing season. Using these results strategies can be developed for establishing the N fertilizer rate applied to wheat as a function of rainfall, the preceding crop, and residual N in soil in order to optimize wheat yield and reduce nitrate pollution to groundwater.

Allelopathy in Forage Crop Systems

Abstract: Secondary plant metabolites and their degradation products are important in all agroecosystems including those with forage crops. Allelopathy affects cropping systems and interseeding of one species into established sods of another. Autotoxicity and heterotoxicity are types of allelopathy. Alfalfa (Medicago saliva L.) has been investigated as both an autotoxic and heterotoxic species. Reestablishment of alfalfa immediately after alfalfa has usually resulted in poor stands due to autotoxicity, and several other forage species also exhibit autotoxicity. Many forage species have shown heterotoxicity, both between forage species and weed species. Several investigators have identified various allelochemicals and/or families of allelochemicals as being responsible for allelopathic reactions. Identifying such chemicals would aid in developing resistant forage cultivars and in maintaining a productive and profitable crop. Crop rotation, cover-crop management, interplanting, double cropping, no-till planting, and nonrotational cropping systems are involved with allelopathic effects. Some alfalfa cultivars possess some resistance to these allelochemicals. Therefore, a breeding program could provide resistant germplasms. Various forage grasses and some weed species have demonstrated allelopathic effects on alfalfa, and alfalfa has allelopathic effects on some weed species. This alone may provide an insight for herbicide studies in alfalfa production.

OILCROP-SUN: A Development, Growth, and Yield Model of the Sunflower Crop

Abstract: Crop simulation models are important tools for agronomic management strategy evaluation, particularly under rainfed conditions. This paper describes the structure and testing of OILCROP-SUN, a process-oriented model of the sunflower (Helianthus annuus L.) crop. The model simulates organ biomass and N content, leaf area index, and soil water and N balances with a daily time step. Daily weather data, soil physical and chemical characteristics, and initial contents of water and N are required inputs. Three cultivar-specific parameters synthesize crop responses to temperature and photoperiod during development, and two cultivar-specific parameters define potential grain number per capitulum and potential kernel growth rate. Tested against independent data sets, the model generated good predictions of the durations of the sowing-emergence (range 7 to 32 d; root mean square error [RMSE], observed vs. simulated: 3.7 d) and emergence-anthesis phases (range 52 to 146 d; RMSE between 2.9 and 4.3 d, depending on the cultivar). The model also simulated seasonal crop biomass and leaf area dynamics for irrigated and rainfed crops sown at two densities satisfactorily, as well as aerial biomass (range 5 to 15 t ha -1 ; RMSE 1.7 t ha -1 ) and grain yield at maturity (range 1.0 to 6.2 t ha -1 ; RMSE 0.8 t ha -1 ) for irrigated and rainfed crops sown at a range of population densities (0.5 to 10 plants m -2 ). We conclude that OILCROP-SUN can be used to simulate biomass and grain yields over a wide range of conditions.

Distinguishing Resource Competition and Chemical Interference: Overcoming the Methodological Impasse

Abstract: Understanding allelopathy may hold the key to new weed management strategies. However, the difficulty of distinguishing chemical interference from competition has hindered studies of allelopathy in natural and cultivated plant communities. Experimental rigor has increased, but has yet to provide unambiguous proof of allelopathy. The complexities of allelopathic interactions, as illustrated by ongoing investigations in the Florida scrub, make it unlikely that clear examples will be forthcoming. While conclusive proof of chemical interference may not be attainable, the challenge of obtaining strong supportive evidence remains. Progress is needed in bioassay methods that distinguish allelopathy from other interference mechanisms. Phytotoxic effects are density-dependent in a manner inconsistent with resource competition, suggesting that allelopathy can be distinguished by characteristic growth responses across planting densities. In monoculture, greater phytotoxicity at low plant densities causes deviations from expected yield-density relationships. In mixed culture, the target-neighbor method, in which differing densities of a neighbor species are planted around a target plant, has been used to study phytotoxic effects. In the presence of an applied phytotoxin, increased growth of sensitive target plants as the density of insensitive neighbors increases is inconsistent with a hypothesis of resource competition and provides compelling, though not conclusive, evidence for chemical interference. Once evidence of allelopathy is obtained from plant growth studies, supportive analytical data must be obtained from analyses of toxin concentrations and flux rates in the soil and rhizosphere. The use of adsorbent materials originally applied to the analysis of organic pollutants should allow the measurement of allelochemical flux rates in addition to static concentrations.

SUNDIAL: A PC-Based System for Simulating Nitrogen Dynamics in Arable Land

Abstract: SUNDIAL (SimUlation of Nitrogen Dynamics In Arable Land) is a user-friendly, PC-based version of the Rothamsted Nitrogen Turnover Model. It comprises a menu-driven system that allows agricultural advisers, even with minimal computing experience, to enter details of a particular field or farm and simulate N turnover. The processes involved are described by a set of parameterized zero and first-order equations. The addition of a novel facility for calculating crop parameters enables instantaneous estimation of parameters for rough but immediate simulations of a new crop, graphical visualization of the expressions used in SUNDIAL, comparison of the parameterized expressions with experimental mesurements, and further refinement of parameters by fitting individual expressions to measured data or by iteratively adjusting parameters to improve the fit of simulated results to soil and crop N and 15 N measurements. The facilities in SUNDIAL for displaying the various outputs in graphical form make it particularly useful for examining the impact of different management strategies on the N cycle in arable agriculture. Graphical facilities include balance sheets, graphical plots, pie charts, and flow diagrams.

Whole-plant physiological and yield responses of maize to plant density

Abstract: Modern compared with older maize (Zea mays L.) hybrids tolerate more plant density stress, but more information is required on how modern hybrids interact with plant density. Field experiments were established in 1991 (warm-dry) and 1992 (cool-wet) to evaluate whole-plant physiological, dry matter (DM), and grain yield responses of four commercial hybrids at low (4.5 plants m -2 ), medium (6.75), and high (9.0) plant densities. As plant density increased, leaf CO2 exchange rates (CER) declined 10 to 20% under mild and 20 to 30% under warm dry conditions. Compared with the high plant density, the low plant density had a 40% lower leaf area index from midvegetative to early grain filling, which offset higher photosynthetic efficiency, resulting in lower crop growth rates during vegetative development and 25% less DM accumulation at silking. When averaged across hybrids, the low plant density averaged 15% lower DM and grain yields than the high plant density. Hybrid x plant density interactions were observed for DM and grain yields. The more prolific hybrids showed mostly linear DM and grain yield responses to plant density, whereas a single-eared hybrid showed quadratic responses in both years. Another single-eared hybrid, which did not respond to density, had low leaf CER at all densities, a reduction in kernels per plant at the medium density, and increased barrenness at the high plant density in 1991. Apparently, modern hybrids interact with plant density, regardless of growing conditions, and some modern hybrids do not tolerate density stress in dry years.

Spectral Properties of Leaves Deficient in Iron, Sulfur, Magnesium, and Manganese

Abstract: In crop plants, deficiency of an essential element may drastically reduce growth rate and yield. Research on the use of leaf spectral properties in the detection of crop mineral deficiency is needed. The objective of this study was to examine the effects of Fe, S, Mg, and Mn deficiency on reflectance (R), absorptance (A), and transmittance (T) spectra of barley (Hordeum valgare L.), wheat (Triticum aestivum L.), corn (Zea mays L.), and sunflower (Helianthus annuus L.) leaves. Plants were grown in the greenhouse in nutrient solution. Chlorophyll (Chl), Fe, S, Mg, and Mn concentrations and spectral properties were determined on the youngest fully expanded leaf when deficiency symptoms were clearly manifested. In all species, mineral deficiency affected leaf concentration of the deficient element and also of other elements. Nutrient defidency reduced Chl concentration and A, and increased R and T. Iron deficiency severely affected all species, and corn was the species most sensitive to all deficiencies. Reflectance, A, and T spectra of leaves were correlated with leaf Chl concentration. Our results suggested that all nutritional deficiencies reduce leaf Chl concentration, and subsequently this reduction increases leaf R and T, decreases leaf A, and shortens the red-edge position, defined as the inflection point that occurs in the rapid transition between red and near-infrared. Modifications in leaf spectral properties were not characteristic of nutrient defidency, but were always observed in the same wavelengths.

Simulating Effects of Genes for Physiological Traits in a Process-Oriented Crop Model

Abstract: Recent improvements in crop simulation techniques and in understanding of crop genetics suggest the possibility of integrating genetic information on physiological traits into crop simulation models. By using known genotypes, rather than empirically fitted cultivar-specific coefficients, a simulation model should permit more explicit testing of hypotheses concerning the genetic basis of adaptation of cultivars to different environments or production systems. This paper describes and evaluates GeneGro, a version of the dry bean (Phaseolus vulgaris L.) crop simulation model BEANGRO version 1.01 modified to incorporate effects of seven genes affecting phenology, growth habit, and seed size: Ppd, Hr, Fin, Fd, and Ssz-1, and two more genes for seed size inferred from indirect evidence. Thirty cultivars were calibrated for BEANGRO using data from 14 trials conducted in Colombia, Guatemala, Mexico, and Florida. The resulting cultivar-specific coefficients of BEANGRO were replaced with information on specific genotypes of cultivars to create the gene-based model. With cultivar differences specified only by the seven genes, GeneGro explained 31% of observed variation for seed yield, 58% for seed weight, 84% for days to flowering, 85% for days to maturity, 52% for maximum leaf area index, and 36% for canopy dry weight at maturity, but 0% for harvest index. In testing the effectiveness of GeneGro after overall trial and cultivar effects were accounted for through regression analysis, all simulated data except for seed weight showed significant relations with observed data (P ≤ 0.01). Our results indicate that for certain traits surprisingly few genes must be characterized to simulate cultivar differences as accurately as with the BEANGRO model. Furthermore, they suggest a potential for developing models similar to GeneGro for studying the effects of genes on adaptation in other crops.

Salinity Effects on Four Sunflower Hybrids

Abstract: Sunflower (Helianthus annuus L.) is becoming an increasingly important source of edible vegetable oil throughout the world because of its high polyunsaturated fatty acid content and no cholesterol. The increasing demand for this oil may promote increased hectarage of sunflower in the western USA, where some soils are saline or have the potential to become so. Since there is little information concerning the response of sunflower grown under saline conditions, a 2-yr field plot study was conducted. Six salinity treatments were imposed on a Holtville silty clay (clayey over loamy, montmorillonitic [calcareous], hyperthermic Typic Torrifluvent) by irrigating with Colorado River water artificially salinized with NaCl and CaCl2 (1:l by weight). Electrical conductivities of the irrigation waters both years were 1.4 (control), 2.0, 3.0, 4.0, 6.0, and 80 dS m-‘. Seed yield and oil content of the seed were measured. Relative seed yield of four hybrids was unaffected by soil salinity up to 4.8 dS m-’ (electrical conductivity of the saturation extract, EC,). Each unit increase in salinity above 48 dS m-t reduced yield by 5.0%. These results indicate that sunflower is appropriately classified as moderately tolerant to salinity. Yield reduction was attributed primarily to a reduction in seeds per head. Oil concentration in the seed was relatively unaffected by increased soil salinity up to 10.2 dS m-‘. Sunflower appears to be well adapted for growth under moderately saline soil conditions. S UNFLOWER (Helianthus annuus L.), a New World plant, has been developed into a valuable source of edible oil and meal. In 1992, world production of sunflower oil was about 7.8 million tonnes. As an edible vegetable oil, only soybean [Glycine max (L.) Merr.] and rapeseedcanola (Brassica napus L. and B. campestris L.) oil production exceeded that of sunflower (USDA, 1993). In the USA, about 839000 ha of sunflower were harvested in 1992, with oilseed hybrids constituting about 88% of the harvest and nonoilseed hybrids making up the remaining 12%. Most production in the USA is in Minnesota, North Dakota, South Dakota, and Kansas (USDA, 1993). However, with the increasing popularity of edible vegetable oils that, like sunflower, contain high percentages of polyunsaturated fatty acids and low cholesterol, the potential

Genotypic Interactions with Potassium and Nitrogen in Cotton of Varied Maturity

Abstract: The development of late-season deficiency symptoms in cotton (Gossypium hirsutum L.) fields has become more frequent in the Mid-South and Far West U.S. production regions. In this study, the objectives were to determine how yield and quality of lint produced by cotton genotypes of varying maturities are affected by different rates of soil applied K and N fertilization. Eight cotton genotypes representing a range of maturities and regional adaptations were studied in Mississippi in 1991 and 1992. All plots received a preplant application of 112 kg ha -1 N and half the plots also received a 38 kg ha -1 sidedress application of N. Within each N treatment, half the plots received 112 kg ha -1 K surface applied and preplant incorporated, with the remaining plots receiving 0 kg ha -1 K. Averaged across years and N treatments, the K deficiency associated with the 0 K treatment reduced lint yield (9%), boll mass (7%), lint percentage (1%), and seed mass (4%). Varying the N fertilization did not benefit these parameters. The high N treatment reduced lint yield 3% (P = 0.07) and lint percentage 1% (P = 0.06) when coupled with the 0 K treatment. All genotypes suffered yield reductions caused by the K deficiency, except for 'HS 26', which was not adapted for production in the Mississippi Delta. Potassium deficiency produced reductions in fiber elongation (3%), 50% span length (1%), uniformity ratio (1%), micronaire (10%), fiber maturity (5%), and perimeter (1%) in all genotypes. Nitrogen application above the 112 kg ha -1 rate did not increase the lint yield under the growing conditions of our study. The data indicate that genotype was not of importance when dealing with K fertility. The deficiency itself, however, must be dealt with to avoid significant reductions in the yield and quality of fiber produced.

Analysis of Aerial Photography for Nitrogen Stress within Corn Fields

Abstract: Spatial variability of soil parameters within fields complicates N fertilizer recommendations for corn (Zea mays L.) production. Thus, the ability to identify differences in crop N status within corn fields could lead to efficiencies in N fertilizer application and decreased ground water pollution. In this study, we digitized aerial color photographic transparencies using an eight-bit scheme to generate digital counts for the red, green, and blue primary colors in the photographs at the R5 growth stage and related them to grain yield. Digital count responses were relative to the N treatment in which grain yield plateaued. Experiments were conducted in 1992 and 1993 for four irrigated corn hybrids with five N rates on a 6-ha field near Shelton, NE. Red and green digital counts relative to those for the high N treatment provided better prediction of yield response than relative blue counts in both years. In 1993, black-and-white photographs taken with a filter centered around 536 nm also predicted yield response to N well (r 2 = 0.93). These findings permit the use of low cost aerial photographs to characterize variability in crop N status throughout entire fields.

Lower Limit of Soil Water Availability

Abstract: Estimating the available soil water reservoir is needed for developing a strategy for optimum management of rainfed crops in marginal dry regions. Previous research on measurement of the lower limit water content of soil water availability (required for calculation of the total soil water reservoir) has shown unexplained discrepancies between field estimates of the water content at which plants are dead or dormant due to soil water deficit and laboratory estimates of the water content at a water potential of - 1.5 kJ kg -1 . Whether such discrepancies are due to sample size, handling, or the choice of -1.5 kJ kg -1 as the lower limit is not clear. This study was designed to determine what the limit is under controlled conditions, and to test the appropriateness of the - 1.5 kJ kg -1 value. We used in situ soil psychrometers to measure soil water potential (Ψ) as water was withheld from cotton (Gossypium hirsutum L.) plants in a rain-sheltered lysimeter monolith and from sorghum [Sorghum bicolor (L.) Moench] in glasshouse pots. The lower limit Ψ was - 2.2 kJ kg -1 in the lysimeter experiment and 2.1 m. No Ψ oscillations, corresponding to so-called hydraulic lift or nocturnal water transport between soil layers, were found at any depth in the lysimeter (0.5-2.1 m). Our results suggest that for water balance calculations requiring absolute accuracy for the water balance of the soil reservoir, the choice of the - 1.5 kJ kg -1 soil water potential is appropriate and corresponds closely to the field lower limit of soil water availability.

Corn yield is equal in conventional, reduced, and no tillage after 20 years

Abstract: Reduced tillage has increased dramatically over the past several years and is expected to continue to increase in the future. Continuous no-till may become a popular tillage system with growers to facilitate compliance with government programs to control soil erosion. The objective of this research was to evaluate the long-term effects of four tillage systems and five fertilizer regimes on corn (Zea mays L.) yield. A 20-yr continuous-corn tillage x fertility study was conducted from 1970 to 1990 on an Ebbert silt loam (fine-silty, mixed, mesic Argiaquic Argialbolls), an imperfectly drained soil at the Belleville Research Center, Belleville, IL. Starter fertilizer did not increase corn height within a tillage system. Height was greater in no-till compared with conventional till (moldboard plow), reduced till (chisel plow), or alternate till (2 yr no-till, 1 yr moldboard plow) with or without a starter fertilizer. There was no difference in population among tillage systems due to fertilizer treatment. Corn population was lower in no-till compared with conventional till regardless of fertilizer treatment. Starter fertilizer did not increase yield in any tillage system. Corn yield averaged 5 to 7% lower in no-till compared with conventional till or reduced till where a starter fertilizer was applied. There was no difference in yield among tillage systems when NPK was broadcast. Corn yield was equal in conventional till, alternate till, reduced till, and no-till with fertilizer applied broadcast on an imperfectly drained soil. Continuous no-till with an imperfectly drained soil does not reduce corn yield.

Plant analysis with standardized scores combines DRIS and sufficiency range approaches for corn

Abstract: The Sufficiency Range (SR) system and the Diagnosis and Recommendation Integrated System (DRIS) represent two competing approaches to interpreting plant analyses. Our objectives were (i) to show that the two approaches are complementary and (ii) to introduce a new plant analysis system that combines them, called Plant Analysis with Standardized Scores (PASS). The SR provides discrete, easily computed independent nutrient indices (INI). The DRIS determines continuous, difficult to calculate, but easily interpreted dependent nutrient indices (DNI), plus an overall balance index and a ranking of the relative deficiencies. Those differences are complementary; that is, the strengths of the one are the weaknesses of the other, and vice versa. The PASS system combines the strengths of the SR and DRIS by including an INI section and a DNI section, in which both types of indices are on the convenient DRIS scale. When we compared the ability to diagnose deficiencies and sufficiencies in 24 corn (Zea mays L.) dry matter or grain yield comparisons, PASS had a higher efficiency rating (82%) than the SR (41%) and DRIS (33%). The PASS system had a significantly higher proportion of correct diagnoses of deficient nutrients than the DRIS and a significantly greater proportion of correct diagnoses of sufficient nutrients than the SR. Furthermore, the PASS overall index, but not the DRIS, was significantly correlated with relative corn grain yield in 10 comparisons showing a yield response.

Winter legume cover crop benefits to corn : Rotation vs. fixed-nitrogen effects

Abstract: The use of winter legume cover crops for erosion control and to provide additional N to the soil is well established. Other potential benefits to legume cover crops besides N additions have been recognized, but have not been quantified. The objective of this study was to separate the fixed-N effects from the rotation effects in a winter legume cover cropping system. A field study was initiated in 1989 on a Norfolk loamy sand (fine, loamy, siliceous, thermic Typic Kandiudult) in east-central Alabama. Corn (Zea mays L.) was grown following (i) 'Tibbee' crimson clover (THfolium incarnatum L.), (ii) a partially ineffective-nodulating crimson clover, CH-1, (iii) rye (Secale cereale L.), and (iv) winter fallow. The plots were split into four rates of fertilizer N (0, 56, 112, and 168 kg N ha -1 ) in a split-plot experimental plan. An evaluation of different methods of distinguishing fixed-N vs. rotation effects of the winter annual legume cover crop to a subsequent corn crop was made. Regression analysis of the effect of N application rates on N 2 fixation by crimson clover (fertilized with 45 kg N ha -1 ) indicated that CH-1 clover biomass contained approximately 40 and 101 kg N ha -1 and Tibbee clover contained approximately 51 and 119 kg N ha -1 in 1990 and 1991, respectively. In both years of the study, crimson clover substantially increased corn yield compared with winter fallow, with a yield increase at the highest fertilizer N application level of 7 and 22% for 1990 and 1991, respectively. Estimates of yield increases due to rotation ranged from negative to 40%. The data indicated that winter cover crops improve corn yield and that besides soil N availability, there was very little difference between the beneficial effects of clover and the rye cover crops to corn.

Simulating Alamo Switchgrass with the ALMANAC Model

Abstract: A model for forage yield with adequate details for leaf area, biomass, nutrients, and hydrology would be valuable for making management decisions. The objectives of this study were to develop Alamo switchgrass (Panicum virgatum L.) parameters for the Agricultural Land Management Alternatives with Numerical Assessment Criteria (ALMANAC) model and demonstrate its accuracy across a wide range of environments. Derived plant parameters included potential leaf area index (LAI), potential biomass growth per unit intercepted light, optimum nutrient concentrations, and growth responses to temperature. The model's simulated yields accounted for 79% of the variability in measured yields for one-cut and two-cut harvest systems from six diverse sites in Texas in 1993 and 1994. Simulated yields for three locations differed in sensitivity to potential LAI, heat units to maturity, radiation use efficiency (RUE), and soil depth. The ALMANAC model shows promise as a management tool for this important forage and bioenergy crop.

Tillage System and Crop Residue Effects on Surface Compaction of a Paleustoll

Abstract: Concern of excessive compaction of soils under conservation tillage (CT) has persisted as CT practices gain acceptance in major agricultural systems. The objective of the study was to determine the cumulative effects of tillage systems [moldboard plowing (MT), stubble-mulch tillage (ST), and no-till (NT)] and rates of crop residues on bulk density of the near-surface zone under annual winter wheat cropping. In the fall of 1983, tillage plots were established on Bethany (fine, mixed, thermic Pachic Paleustoll) and Renfrow (fine, mixed, thermic Udertic Paleustoll) silt loams on a 2% slope near El Reno, OK. Bulk density and water content were measured to a depth of 0.3 m in 0.025-m increments using gamma and neutron attenuation techniques during 1991 and 1992. After planting, MT and ST soils had the lowest mean surface density ; these soils consolidated as bulk density during winter wheat dormancy averaged 1.34 (±0.06), 1.30 (±0.06), 1.21 (±0.06) in 1991, and 1.31 (±0.06), 1.30 (±0.05), and 1.24 (±0.05) Mg m -3 in 1992 in MT, ST, and NT soils, respectively. Near the end of the season, surface density of ST and MT soils had increased by 1.6 and 4.8%, respectively. No-till soil showed a decrease of 5.4% during the same overwinter period and had the lowest bulk density in the end. The inherent reduction in field operation traffic in CT systems, and a decrease in tillage intensity proportionately reduced soil surface compaction under annual wheat cropping. Bulk density increased with soil depth in all treatments ; however, NT resulted in decreased soil density in and below an old restrictive layer more effectively than ST or MT management. Removal of crop residues resulted in higher bulk density, primarily in the 0.05- to 0.25-m depth of MT soils, and in the 0 to 0.05-m surface layer of NT soils. Maintaining high amounts of wheat residues at the soil surface helps improve the macroporosity of near-surface zones of soils under conservation tillage. Severe compaction of CT soils did not appear prevalent under winter wheat production to cause physical constraints to crop growth in subhumid regions.

Soil Temperature and Planting Date Effects on Corn Yield, Leaf Area, and Plant Development

Abstract: Early-season soil temperature has been reported to affect leaf appearance and expansion rates, and consequently corn (Zea mays L.) ontogeny. A 2-yr field experiment was conducted on a Drummer silty clay loam (fine silty, mixed, mesic Typic Haplaquoll) at the Agronomy and Plant Pathology South Farm of the University of Illinois at Urbana-Champaign. Treatments consisted of planting date (early May and early June) and soil temperature surrounding the corn growing point (5°C below ambient, ambient, and 5°C above ambient). Soil temperature was controlled using warm and cool water circulating through copper pipes buried next to the corn rows. Air temperatures were not controlled. Irrigation was used. The experimental design was a split plot in a randomized complete block with four replications. Ten plants were randomly selected within each replication and nondestructive measures of plant stage, individual leaf area, and leaf senescence were taken three times per week from emergence to V5, and once per week afterwards until complete leaf senescence. Growing degree days (GDD) were calculated using the modified growing degree day formula (MGDD). From planting to V5, MGDD were computed using the soil maximum and minimum daily temperatures from each treatment. After the soil temperature treatments were terminated, the MGDD calculations were based on maximum and minimum daily air temperatures. Lower early-season soil temperature delayed corn development and modified individual leaf area. Plants under these cooler conditions went through vegetative developmental stages faster per unit of accumulated MGDD, showing the effect of temperature on corn ontogeny. The results also show that the current method of calculating MGDD (with base temperature of 10°C) does not adequately predict corn development under exceptionally warm or exceptionally cool soils. Warmer early-season soil temperature linearly increased corn yield (β 1 = 0.14 Mg ha -1 °C -1 ). Leaves in the lower half of the canopy were larger under cooler soil temperature treatments (β 1 = -48.9 cm 2 °C -1 ). There was a linear increase in the size of the leaves in the upper half of the canopy of the plants under warmer temperatures (β 1 = 28.8 cm 2 °C -1 ).

Tillage systems for cotton on silty upland soils

Abstract: No-tillage systems for cotton (Gossypium hirsutum L.) production on sloping, upland sites have not been widely adopted in the Mid-South of the USA, even though conventional cotton production creates a serious erosion hazard. A field study was established following sod on a site with loess soils to evaluate tillage system effect on cotton yield and earliness. The sod was tilled prior to establishment of treatments, which included conventional (chisel, disk, bed, cultivate), ridge till (remove ridge tops at planting, cultivate postemergence to rebuild ridges), no-tillage [wheat (Triticum aestivum L.) cover seeded following cotton harvest, killed prior to planting], and minimum tillage (one pass with a mulch finisher prior to planting, cultivate postemergence). During the first year of the study, no-tillage cotton yields were lower and maturity delayed compared with conventional tillage. During Years 3 to 5, no-tillage crop yields were is to 42% greater and crop maturity was 6 to 10 d earlier than conventional tillage. Minimum tillage yields were similar to conventional tillage while ridge tillage was lower in two of the last 3 yr of the study. Results of this study indicate viable no-tillage production systems for cotton can be developed for highly erosive loess soils in the Mid-South.