Dry Matter Response Research Articles

Influence of Nitrogen on Productivity and Nutritive Value of Forage Chicory

Chicory (Cichorium intybus L.) is highly productive and responsive to N fertilization under midsummer conditions in the eastern USA. We conducted a field experiment for 3 yr on a Ramsey soil (loamy, siliceous, subactive, mesic Lithic Dystrudept) in southern West Virginia to determine if fertilizer N influenced forage chicory nutritive value and NO3–N concentration. Each N rate (0, 80, 160, 240, or 480 kg N ha−1) was replicated three times in a randomized block design. Swards were clipped at 6‐wk intervals during the growing season. Swards were virtually pure chicory in the first year (1994) regardless of N rate. By the third year (1996), chicory ranged from about 40% (0 N) to less than 5% (480 kg N ha−1) of swards. Botanical composition changes in the sward influenced dry matter (DM) response to N rate and herbage nutritive value. Dry matter production increased with N rate in 1994, but was not affected by N in 1996 when chicory was not a major sward component. More than 70% of total annual DM production in 1994 occurred after the first harvest, but by 1996 was less than 50%, reflecting productivity patterns typical of cool‐season swards. Nitrate concentrations in herbage were greatest (3.5 g kg−1) in 1995, a relatively dry year, and least (2.3 g kg−1) in 1996, when there was less chicory in the sward. Crude protein (CP) and in vitro organic matter disappearance (IVOMD) values indicated high forage quality throughout the course of the experiment.

Tropical Soda Apple (Solanum viarum) Herbicide Susceptibility and Competitiveness in Tall Fescue (Festuca arundinacea)1

Tropical soda apple (TSA) was evaluated for response to 28 herbicide treatments. Treatments containing picloram or triclopyr controlled eight-leaf, 16-leaf, and 1-yr-old TSA greater than 90% 8 wk after treatment (WAT). Control of 1-yr-old TSA did not increase 8 WAT when triclopyr was mixed in diesel fuel rather than water. In greenhouse additive interference experiments, populations of 0, 1, 2, 4, 8, 16, 32, and 64 TSA plants/700 cm2 of tall fescue had no effect on tall fescue height. TSA height was affected by TSA population, and intraspecific TSA competition was expressed as etiolation at densities greater than 4 plants/700 cm2. Averaged over five periods of competition, predicted yield losses of tall fescue were 14, 16, 29, and 31% and 1, 11, 19, and 23% for 8, 16, 32, and 64 TSA plants/700 cm2, respectively, for each experiment. Differences in tall fescue dry matter response between experiments were attributed to ambient temperature. Dry matter per individual TSA plant decreased from 1.7 to 0.3 g as TSA density increased from 1 to 64 plants/700 cm2. Percent canopy coverage of TSA relative to an area of 700-cm2 surface increased proportionally as tall fescue coverage decreased. After 10 wk of competition, TSA monopolized the canopy with coverage of 92 and 94%; tall fescue coverage was limited to only 7 and 5% in experiments I and II, respectively.Nomenclature: Picloram, 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid; triclopyr, [(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid; tropical soda apple, Solanum viarum Dunal #3 SOLVI; tall fescue, Festuca arundinacea Schreb. # FESAR ‘Kentucky 31.’Additional index words: Additive competitive design, chemical control, herbicides, weed density, Solanum khasianum C. B. Clarke var. chatterjeeanum Sengupta, SOLVI.Abbreviations: TSA, tropical soda apple; WAT, weeks after treatment; WATP, weeks after transplanting.

Response of Bambara Groundnut (Vigna subterranea (l.) Verdc.) to Phosphorus Fertilisation in Botswana

Soils in Botswana are known to be poor in phosphorus. Information is lacking on the P requirements of bambara groundnut (Vigna subterranea) in Botswana soils and soil moisture can also limit P uptake. Elsewhere the response of bambara groundnut to P fertilization is contradictory. The effects of phosphorus (P) fertilization on growth and yield of bambara groundnut were studied in a pot and a field experiments using a low P loamy sand (P-Bray 6.2 mg P kg-1) at Sebele Botswana. In the pot experiment, the response to two bambara groundnut landraces ('Diphiri Cream' and 'Zimbabwe Red') to P fertilization (0-480 mg P pot-1) was investigated. In the field experiment, the response of 'Diphiri Cream' to P fertilization (0-80 kg P ha-1) was investigated under two different soil moisture regimes (unsupplemented rainfall and supplementary irrigation) with samples taken at 28, 49,78, 99, days after sowing (DAS) and the final harvest at 126 DAS. In the pot experiment, terminated at 51 DAS, there was a positive response of shoot dry matter (DM) to P fertilization while the two landraces did not differ significantly. Root DM, nodule fresh weight and shoot P and nitrogen (N) concentrations were not affected by P fertilization. In the field, P fertilization had no effect, while supplementary irrigation increased all plant growth parameters except root DM. Total seed yield was 2.8 t and 4.2 t ha-1 for unsupplemented rainfall and supplementary irrigation treatments, respectively. Shoot P and N concentrations were not affected by soil moisture level. The positive effect of P in the pot and not in the field experiment may be an indication of low P supply of plants in pots due to a small rooted soil volume. The response in the pot experiment may indicate a 'starter P effect', whereas lack of response in the field may indicate unavailability of applied P to bambara groundnut. These aspects are discussed in this paper. UNISWA Research Journal of Agriculture, Science and Technology Vol. 4 (2) 2000: pp 202-207

Transformations and plant uptake of urine N and S in long and short-term pastures

A field trial was carried out to compare the transformations and plant uptake of urine N and S in a short-term pasture from within an arable/pasture ley rotation and a long-term pasture. Animal urine labelled with 15N and 35S was applied to microplots at both sites. These microplots were destructively sampled at various time intervals over 12 months and analysed for 15N and 35S. It is known that soil organic matter accumulates under short-term pastures compared with a long-term pasture in which accumulation and degradation are in balance. Consequently, it was hypothesised that immobilization of urine N and S is more intense in the short-term. However, in this study there was considerably less immobilization of 15N and 35S into soil organic forms under short-term pasture than long-term pasture. This was attributable to a greater pasture dry matter response to urine application under the short-term pasture (due to its inherently low N fertility) resulting in a greater plant uptake of 15N and 35S with less 15N and 35S consequently being available for immobilization. At both sites, all of the applied 35S was accounted for through plant uptake and recovery in the soil, but 21–48% of the 15N was unaccounted for and presumed to have been lost through gaseous emissions. It was concluded that accumulation of soil organic N and S under short-term pastures is likely to be attributable to turnover of plant residues (particularly root material) and does not appear to be related to immobilization in urine patches.

Response of lactating dairy cows to steam-flaked sorghum, steam-flaked corn, or steam-rolled corn and protein sources of differing degradability.

Protein sources with different degradabilities were fed to 48 lactating Holstein cows receiving 37 or 39% of dietary dry matter as steam-flaked sorghum (360 g/L), steam-flaked corn (360 g/L), or steam-rolled corn (490 g/L) in a 3 x 2 factorial arrangement of treatments. Cows were fed an alfalfa-based diet with 7% soybean meal or 5% of an animal-marine protein blend and 37 or 39% grain. Although not significant, cows fed flaked grain yielded a mean of 1.5 kg/d more milk than did those fed rolled grain. Gross feed efficiency was not affected by grain processing or protein source, but diets with the animal-marine protein blend had 9% higher estimated net energy for lactation than did diets with soybean meal. The greater gains in body weight and increased digestibility of the diets with the animal-marine protein blend verify the higher energy concentration of those diets. Yield of milk protein was increased by flaked grain or the animal-marine protein blend, and flaked grain increased percentages of lactose and solids nonfat. Increasing the ratio of rumen-degradable starch to rumen-degradable protein increased milk protein content and yield linearly and increased contents of lactose and solids nonfat. A linear response of dry matter, organic matter, crude protein, and starch digestibilities was observed as the ratio of rumen-degradable starch to rumen-degradable protein increased. These data show improved performance of dairy cows fed a high rumen-undegradable protein source with diets high in rumen-degradable starch from steam-processed grains.

Manure and fertilizer contributions to soil mineral nitrogen and the yield of forage maize

Fifteen field trials were conducted to evaluate soil mineral N measurement as a means for quantifying the total N supply to forage maize and so to form the basis for fertilizer recommendations on a crop-specific basis. In every trial, 4 rates of cattle manure N (nominally 0, 80, 160, 240 kg N per ha) and 4 rates of ammonium nitrate (0, 50, 100, 150 kg N per ha) were factorially combined. Soil mineral N measurements were made before manure application, at the time of maize drilling, 7-10 weeks after drilling and after harvest. Measurements on control treatments which received no manure or ammonium nitrate showed extensive net mineralisation of soil N (mean 140 kg N per ha) in the 7-10 weeks after drilling followed by a decrease due to crop uptake, and probably net immobilisation, of approximately the same amount by harvest. This net mineralisation was probably the reason why only one trial showed a significant dry-matter yield response to ammonium nitrate. Results indicated that , to be useful for N recommendations, soil mineral N measurements should be taken 7-10 weeks after drilling. Only if the amount of mineral N at this time is less than expected crop N offtake should fertilizer N be applied. A mean of around 64% of the N applied in ammonium nitrate could be accounted for in soil mineral N after harvest of the maize, although this was reduced to 24% in the single trial where a dry-matter response to ammonium nitrate was recorded.

Effects of N application on agronomic and environmental parameters in silage maize production on sandy soils

The current nitrogen (N) use in silage maize production can lead to considerable N losses to the environment. Maize growers fear that a reduction of N inputs needed to minimize N losses might depress yields. The objective of this study was therefore to quantify: (1) the response of silage maize dry matter (DM) yields to N, (2) the economically optimal N reserve, and (3) the trade-off between silage maize DM yield and N losses. The indicators of N losses used in this study were the difference between N input and N uptake and the post-harvest residual soil mineral N. Regression models were used to fit DM yields and N uptakes of silage maize measured in 25 experiments on sandy soils in the Netherlands to the sum (SUMN) of the soil mineral N reserve (SMN early) in March–April, plus mineral N in fertilizer, plus ammonium N in spring-applied slurry. The values obtained for the economically optimal SUMN in the upper 30 and 60 cm of soil were, respectively, 173 and 195 kg N ha −1, when we assumed that the value of 1 kg fertilizer N equals the value of 5 kg silage DM. The economically optimal SUMN was not significantly related to the attainable DM yield. The apparent N recovery (ANR) of maize averaged 53% at the economically optimal SUMN. The ANR rose considerably, however, when N was applied at lower rates, indicating that N losses may be considerably smaller in less intensive maize cropping. When maize was fertilized at 100 kg N ha −1 below the economic optimum, the ANR was 73%, the difference between the mineral N input and the N crop uptake decreased by 57 kg N ha −1 and the soil mineral N residue at the end of the growing season (0–60 cm) decreased by 24 kg N ha −1. The associated reduction in DM yield averaged 16%. Fertilizer prices would have to be as much as four times higher to make maize growers spontaneously reduce the application rates by a 100 kg N ha −1, however. It is concluded that adjusting the N input to a level below the economically optimal rate can reduce the risks for N losses to the environment associated with conventional maize production, with a limited effect on silage yields.

Bahiagrass response to applied nitrogen and harvest interval1

A growth model has been developed for warm‐season perennial grasses which relates seasonal distribution of dry matter to calendar time through the probability equation, and which relates total seasonal dry matter to harvest interval through a linear‐exponential equation. The model is used to describe response of ‘Tifton‐9’ bahiagrass (Paspalum notatum Flögge) to time and harvest interval at Quincy, FL. It is also shown that yield response to applied nitrogen (N) is described by the logistic equation, in agreement with earlier results for bahiagrass and other forage grasses. This model removes the restriction of harvest interval to 6 or 7 wk of the previous linear growth model. Maximum dry matter yield exceeded 20 Mg ha‐1. Maximum incremental dry matter response to applied N occurred at approximately 140 kg N ha‐1. A harvest interval of 8 wk provided peak dry matter production. The mean and standard deviation of dry matter distribution were 28.5 wk and 6.0 wk, respectively. Correlation coefficients of the models with data exceeded 0.99. Effects of harvest interval and applied N are easily accounted for with this model, which uses analytical functions and avoids the need for finite difference techniques.

Micronutrient status of a group of soils in Canterbury, New Zealand, as measured by extraction with EDTA, DTPA and HCl, and its relationship with plant response to applied Cu and Zn

The Cu, Zn and Mn status of 44 fields from the Canterbury region of New Zealand under winter wheat was investigated in Spring 1993. Micronutrient status was assessed using EDTA, DTPA and HCl as extractants. The Mn status of soils was generally high and unaffected by soil development. However, when samples were separated according to soil series, it was found that extractable Cu and Zn levels in both the 0–15 and 15–30 cm soil layers generally decreased with increasing soil development (increasing soil age and annual rainfall). Twenty-three percent of fields had EDTA-extractable Cu levels <1 μg g−1 whilst 34% had EDTA-extractable Zn levels <1 μg g−1.Twenty-two of the soils were used in a glasshouse experiment in which wheat was grown in the soils with or without the addition of added Cu and/or Zn. Plant dry matter responses to added Cu were recorded in soils with extractable Cu levels <1·1 μg g−1 EDTA, 0·4 μg g−1 DTPA and 0·9 μg g−1 HCl. Responses to added Zn occurred in soils with extractable Zn levels <0·8 μg g−1 EDTA, 0·25 μg g−1 DTPA and 1·4 μg g−1 HCl. Significant linear correlations were found between EDTA-, DTPA- and HCl-extractable Cu and Zn and Cu and Zn uptake respectively by wheat. Correlation coefficients were closer for Cu than for Zn uptake. It was concluded that many sites on the more strongly developed soils of the Canterbury Plains are potentially deficient in Cu and/or Zn and that these can be identified using conventional micronutrient soil tests.

Coupling of dry matter and nitrogen accumulation in millet and ryegrass1

The logistic model has proven very useful in relating dry matter production of warm‐season and cool‐season forage grasses to applied nitrogen (N). A recent extension of the model coupled dry matter and plant N accumulation through a common response coefficient c. The objective of this analysis was to apply the extended model to both warm season pearl millet [Pennisetum typhoides (Burm.) Staph and C. E. Hubb.] and cool season ryegrass [Lolium multiflorum Lam.], and to establish a common response coefficient, c, between accumulation of dry matter and plant N for the two grasses in rotation. Analysis of variance established the validity of this hypothesis. The model accurately described response of dry matter, plant N removal, and plant N concentration to applied N, with an overall correlation coefficient of 0.9954. Furthermore, the model closely described the relationship between yield and plant N removal. The logistic equation is well behaved and simple to use on a pocket calculator. It can be used to estimate yield and plant N removal in evaluation of agricultural practices and the influences on environmental quality.

Defoliation frequency and the response by white clover to increasing phosphorus supply 1. Leaf dry matter yield and plant morphology responses

A glasshouse experiment was carried out to determine how an increasing P supply influences the growth and survival of white clover plants subjected to a range of defoliation frequencies. Treatments involved the factorial combination of P application rate (0, 30, 90, and 180 mg/pot) to a P-deficient Krasnozem soil and defoliation frequency (1, 2, or 4 defoliations over 36 days). The survival of P-deficient plants was threatened by the most frequent defoliation; their leaf area declined owing to a reduction in leaf number and individual leaf size with each successive defoliation. Increasing the P supply to 180 mg/pot reversed this downward trend as the high P plants were able to maintain leaf area by increasing leaf size and number. Increasing the frequency from 1 to 4 defoliations over the 36 days also changed the form of the leaf dry matter response to added P, from an asymptotic to a linear response. The P requirement of white clover for maximum leaf yield therefore increased under frequent defoliation. This effect was also apparent for a range of morphological measurements including stolon elongation rate, leaf area, root mass, leaf number, and stolon number, where the magnitude of the P response was consistently greater for frequently defoliated plants. Exceptions included stolon mass, which responded more to P addition under infrequent defoliation.

Agronomic effectiveness of a partially acidulated reactive phosphate rock fertiliser

Summary. The agronomic effectiveness of a partially acidulated phosphate rock (PAPR), produced by the 50% acidulation of North Carolina phosphate rock with sulfuric acid, was compared over 4 growing seasons with triple superphosphate (TSP) and the highly reactive North Carolina phosphate rock at 22 permanent pasture sites in the National Reactive Phosphate Rock Project. The performance of PAPR as a phosphorus (P) fertiliser for permanent pasture was determined by calculating the substitution value of TSP for PAPR at 50% of the maximum yield response for TSP from the fitted annual dry matter response curves. PAPR performance varied both between sites, and between years at individual sites. Annual yield responses with PAPR were larger than those with TSP at 1 high rainfall site where water-soluble P from TSP was thought to leach from the root zone. PAPR was superior to TSP at another site and generally similar in effectiveness to TSP at 4 sites with light-textured, low or medium P-sorbing soils with a moderate annual rainfall (500–750 mm). The mean substitution value over the 4 years for these sites was >0.9. PAPR performance at other sites where highly reactive phosphate rocks were effective in the short or medium term was variable: there were equivalent yield responses to TSP in some years but much smaller yield responses in other years. PAPR performed very poorly in a third group of sites where the soil had a high to very high P sorption capacity or where there was a very high demand for fertiliser P due to large legume responses on a P-deficient soil. Although generally inferior to TSP, the PAPR was more effective than North Carolina phosphate rock at the majority of sites during the 4-year study.

Effects of soil water deficit at different growth stages on rice growth and yield under upland conditions. 1. Growth during drought

Upland rice can be subject to drought at any time during growth, and responses of various growth processes to water stress may depend on the timing of drought in relation to crop growth stages. In two experiments, rice plants at three or four growth stages were subjected to drought periods of 23–35 days to examine the response of shoot dry matter (SDM) production and associated processes to water stress. Young plants, 33 days old at the commencement of stress, maintained high leaf water potential during the stress period because of the low demand for water. Radiation-use efficiency (i.e. SDM production per unit intercepted radiation) of the young plants in the drought treatment was similar to that in the irrigated control, but SDM production was reduced by 71% because of 68% reduction in radiation interception. The young plants extracted water more slowly than older plants immediately after stress was imposed, whereas the total water extraction during the stress period was not significantly different from that of the older plants. The young plants produced less SDM than the older plants (90 versus 300 g m −2), and hence had lower water-use efficiency (i.e. SDM production per unit water used 1.4 versus 4.2 g kg −1). During early growth stages, root length growth was very sensitive to soil water deficit. When stress developed in the older plants with the canopy intercepting greater than about 60% of full solar radiation, reduction in SDM production was mostly due to reduced radiation-use efficiency (1.2 versus 2.3 g MJ −1). The plants extracted water more rapidly and from deeper soils, and leaf water potential decreased rapidly with the large demand for transpiration water. The results therefore show that rice responds to drought in a manner depending on the stage of canopy development.

Orchardgrass response to different types, rates and application patterns of dairy manure

Manure management is a difficult task on many intensive dairy farms. Crops that can utilize large quantities of manure N, yield quality forage with larger rates of manure application, and allow manure spreading at different times in a year can simplify that task. A study was conducted in 1990 and 1991 on a Copake sandy loam soil (mixed mesic) in New Milford, Connecticut. The objectives were: (1) to measure and compare dry matter (DM) response of orchardgrass ( Dactylis glomerata L.) to different amounts and application times of N fertilizer and liquid and solid cattle manure; and (2) to determine crop uptake of fertilizer and manure N. Fertilizer and liquid and solid manure were applied to the soil surface annually in amounts of 150, 300 or 450 kg N ha −1 in one, two or four equal applications. Orchardgrass dry matter production increased over the entire range of N amounts from all sources. Yields varied from approximately 2500 kg DM ha −1 for control plots (0 kg N) to 10600 kg for plots receiving 450 kg N ha −1 either as fertilizer or liquid manure. Crop response to liquid manure application was greater in year one with abundant rainfall than in year two with dry conditions during most of the growing season, whereas crop response to solid manure application improved in the second year, due to the availability of residual organic N. Orchardgrass was more sensitive to the timing of fertilizer N application than to manure N application. Despite the large differences in weather patterns experienced during this study, analysis of application patterns indicated that manure could be applied throughout the growing season to crop stubble (post-harvest) with comparable rates of uptake overall. N uptake in control plots averaged 56 kg N ha −1 for both years, compared to 340 kg N ha −1 for fertilizer plots, 250 kg N ha −1 for liquid manure plots and 190 kg N ha −1 for solid manure plots receiving 450 kg total N ha −1.

Whole‐Plant Physiological and Yield Responses of Maize to Plant Density

AbstractModern 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 ✕ plant density interactions were observed for DM and grain yields. The more prolific hybrid 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.

Coupling of dry matter and nutrient accumulation in forage grass

Abstract The logistic model has proven very useful in relating dry matter production of forage grasses to applied nitrogen. A recent extension of the model coupled dry matter and plant ? accumulation through a common response coefficient c. The objective of this analysis was to establish the validity of the extended model for each of the three major nutrients (?, ?, ?), with a common coefficient c between dry matter and each applied nutrient. Analysis of variance established the validity of this hypothesis. The model accurately described response of dry matter, plant nutrient removal, and plant nutrient concentration to applied nutrient, with overall correlation coefficients of 0.9928, 0.9972, and 0.9975 for applied N, P, and K, respectively. Furthermore, the model closely described the relationship between yield and plant nutrient removal, as well as between plant nutrient concentration and plant nutrient removal, for each nutrient. This work confirmed earlier results for applied ? with various grasses a...

Crop Sequence Effects on Response of Corn and Soil Inorganic Nitrogen to Fertilizer and Manure Nitrogen

AbstractFertilizer N and manure frequently are applied to corn (Zea mays L.) grown after alfalfa (Medicago sativa L.) at rates similar to those applied to continuous corn, although corn following alfalfa typically requires little additional N to attain maximum dry matter. Consequently, similar amounts of applied N may affect soil NO3 differently in rotational than in continuous corn. There is little information evaluating crop sequence effects on residual soil NO3 derived from fertilizer N and manure. In two 2‐yr experiments at two locations in Minnesota, we evaluated the effect of crop sequence on response of corn grain dry matter, grain N, and stover N, and of soil inorganic N (NO3‐N and NH4‐N) to fertilizer N and dairy manure. Grain dry matter of first‐year corn following alfalfa did not respond to applied N at Rosemount and typically was less responsive than continuous corn at Waseca. Crop sequence effects on soil NO3 response to manure and fertilizer N were similar. Preplant fertilizer N application of 157 kg ha−1 increased residual soil NO3‐N the following spring an average of 45 kg ha−1 more in first‐year corn following alfalfa than in continuous corn, except when excessive precipitation caused apparent high losses of applied N. Grain N content and soil NO3 responded similarly to fertilizer N in both second‐year corn following alfalfa and continuous corn. Efforts to reduce the buildup and potential loss of soil NO3 in the corn portion of alfalfa‐corn rotations should focus on reducing N application to first‐year corn following alfalfa. These N applications have little agronomic value and can dramatically increase residual soil NO3.

Rational basis for the logistic model for forage grasses

Abstract The logistic model has proven very useful in relating dry matter yields and plant N uptake of forage grasses to applied N. In the past the model has been treated simply as a regression model. This article provides a more rational mathematical foundation for the model. Differential response of dry matter to applied N was related to the product of filled and unfilled dry matter capacity of the system. Characteristics of the logistic and gaussian distributions were compared and refection symmetry of both noted. Symmetry in the logistic model was related to conservation of potential dry matter yield. At lower yields the logistic model approximates exponential behavior, while at higher yields it approximates the Mitscherlich model. Dry matter was coupled to plant N uptake through a hyperbolic equation. As a consequence, plant N uptake was also shown to follow logistic response to applied N with the same N response coefficient as for dry matter. In addition, plant N concentration exhibited linear dependence on plant N uptake. Field data were used to confirm all of these characteristics of the logistic model and to affirm its continued use to model forage grasses.

The influence of fertilizer nitrogen and season on the carbon-13 abundance of wheat straw

Carbon-13 abundance, expressed as δ13C in ‰, was measured in wheat straw grown between 1984 and 1989 on the Broadbalk Continuous Wheat Experiment at Rothamsted. In all six years, straw grown without fertilizer N contained less carbon-13 (i.e.δ13C was more negative) than straw grown with fertilizer, although the magnitude of this difference varied with year. In a dry year, when dry matter response to fertilizer N was relatively small, there was a large difference between the δ13C of straw grown with and without N. Conversely, in a wet year, when there was a marked response to N, there was little difference in the isotopic composition of N-fertilized and unfertilized straw. Over the six years, the difference between the δ13C value of straw grown with and without nitrogen (D13C, in ‰) was related to drought, measured as the calculated soil water deficit on 15 July (Wj, in mm), by the equation D13C=−0.299+0.01034 Wj (r=0.87). H Lambers Section editor

Coupling of dry matter and nitrogen accumulation in ryegrass

The logistic model has proven very useful in relating dry matter production of warm season perennial forage grasses to applied nitrogen. A recent extension of the model coupled dry matter and plant N accumulation through a common response coefficient c. The objective of this analysis was to apply the extended logistic model to cool season Gulf annual ryegrass (Lolium multiflorum Lam.) and to establish a common response coefficient c between accumulation of dry matter and plant N. Analysis of variance established the validity of this hypothesis. The model accurately described response of dry matter, plant N removal, and plant N concentration to applied N, with an overall correlation coefficient of 0.9954. Furthermore, the model closely described the relationship between yield and plant N removal, as well as between plant N concentration and plant N removal. The logistic equation is well-behaved and simple to use on a pocket calculator. It can be used to estimate yields and plant N removal in evaluation of agricultural production and environmental quality.