Mixture Of Perennial Ryegrass Research Articles

Mitigation of nitrous oxide emissions in spray‐irrigated grazed grassland by treating the soil with dicyandiamide, a nitrification inhibitor

Abstract. Nitrous oxide (N2O) from animal excreta in grazed pasture systems makes up a significant component (c. 10%) of New Zealand's total greenhouse gas inventory. We report an effective method to decrease N2O emissions from animal urine patches by treating the soil with the nitrification inhibitor dicyandiamide (DCD), in a simulated grazed dairy pasture system under spray irrigation. The soil was a free‐draining Lismore stony silt loam (Udic Haplustept loamy skeletal) and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). By treating the soil with DCD, N2O emissions were decreased by 76% following urine application in the autumn, from 26.7 kg N2O‐N ha−1 without DCD to an average of 6.4 kg N2O‐N ha−1 with DCD over the 6‐month experimental period. N2O flux was decreased by 78% following urine application in the spring, from 18 kg N2O‐N ha−1 without DCD to 3.9 kg N2O‐N ha−1 with the application of DCD over the 3‐month period. A single application of DCD immediately after urine was sufficient to effectively mitigate N2O emissions from the urine. The results showed that repeated applications of DCD after urine application, or mixing DCD with urine, offered no advantage over a single application of DCD immediately after urine deposition.

The “critical” ESP value: Does it change with land application of dairy factory effluent?

Decisions about the tolerable loading of sodium during land application of dairy factory effluent (DFE) are often based on the risk of exceeding a “critical” exchangeable sodium percentage (ESP) in the soil. The concept of a “critical” ESP value is based on overseas research that investigated the effects of saline irrigation water on soil structure, rather than on experimental evidence based on research using DFE. The problem is that sodium may degrade the structural stability of the soil and eventually cause surface ponding and effluent run‐off. The aim of this investigation was to determine the effects of land treatment of DFE on key soil physical properties and to establish if a single “critical” ESP value is appropriate for these situations. Intact soil cores (200 mm diameter × 200 mm deep) were collected from three sites at the Clandeboye dairy factory in Canterbury covering two periods of application (4, 10 years) of land treatment of DFE, plus an adjacent control site that had not received DFE. The soil at the Clandeboye land treatment sites was a Lismore stony silt loam with a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The results show that the soil from DFE land treatment sites had greater wet aggregate stability than soil from the control site. Treatment of the soil cores in the laboratory with a sodium solution decreased the wet aggregate stability and the hydraulic conductivity of the soil. The greatest reduction in hydraulic conductivity occurred in the control soil with a smaller decrease in the hydraulic conductivity in the soil from the DFE treatment sites. A decrease of 50% in hydraulic conductivity occurred at an ESP value of 3 in soil from the control site, whereas in soil from the DFE sites this 50% reduction did not occur until an ESP of 13 was achieved. These results show that the “critical” ESP value may change following the application of DFE to soil, and that the use of a single ESP value in land‐use decision making is therefore too simplistic.

The use of a nitrification inhibitor, dicyandiamide (DCD), to decrease nitrate leaching and nitrous oxide emissions in a simulated grazed and irrigated grassland

Abstract. In grazed dairy pasture systems, a major source of NO3– leached and N2O emitted is the N returned in the urine from the grazing animal. The objective of this study was to use lysimeters to measure directly the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), in decreasing NO3– leaching and N2O emissions from urine patches in a grazed dairy pasture under irrigation. The soil was a free‐draining Lismore stony silt loam (Udic Haplustept loamy skeletal) and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The use of DCD decreased NO3–‐N leaching by 76% for the urine N applied in the autumn, and by 42% for urine N applied in the spring, giving an annual average reduction of 59%. This would reduce the NO3–‐N leaching loss in a grazed paddock from 118 to 46 kg N ha–1 yr–1. The NO3–‐N concentration in the drainage water would be reduced accordingly from 19.7 to 7.7 mg N L–1, with the latter being below the drinking water guideline of 11.3 mg N L–1. Total N2O emissions following two urine applications were reduced from 46 kg N2O‐N ha–1 without DCD to 8.5 kg N2O‐N with DCD, representing an 82% reduction. In addition to the environmental benefits, the use of DCD also increased herbage production by more than 30%, from 11 to 15 t ha–1 yr–1. The use of DCD therefore has the potential to make dairy farming more environmentally sustainable by reducing NO3– leaching and N2O emissions.

Fate of nitrogen in dairy factory effluent irrigated onto land

It is important to ensure that the irrigation of dairy factory effluent (DFE) onto farmland does not cause adverse effects on the wider environment, e.g., nitrate leaching and groundwater contamination, or significant emissions of nitrous oxide (N 2 O), which is a greenhouse and ozone‐depleting gas. The aim of this project was to determine the fate of nitrogen (N) from DFE applied on land. Nitrogen removals or losses by plant uptake, leaching, denitrification, volatilisation, and immobilisation were quantified on undisturbed soil lysimeters and in field studies. The soil was Templeton fine sandy loam (Typic Haplustept) and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The results showed that when DFE was applied at 300 or 600 kg N ha–1 yr–1, 3.7–51.4% was taken up by the pasture, 0.2–0.3% was leached, 8.4–12.2% was lost by denitrification, 51.6–63.6% was recovered in the soil organic N, and <1% was lost by volatilisation. The high amount of N immobilised was probably due to the high soluble organic carbon content of the DFE (i.e., lactose). Immobilisation was probably also responsible for the relatively low nitrate leaching losses. However, the leaching losses were higher under grazed than under cut and carry systems. There is a need to study the long‐term fate of N from DFE that is immobilised in the soil.

Advanced analysis of dry‐weight‐rank data to discriminate direct and indirect interactions between white clover and grasses in a multi‐species pasture under a range of management strategies

AbstractInfestations of pastures by species, such as creeping bentgrass (Agrostis stolonifera), may compromise the white clover (Trifolium repens) content in perennial ryegrass (Lolium perenne) mixtures. However, the interactions between white clover and species other than perennial ryegrass are not well understood. Strategies to prevent creeping bentgrass infestations require an understanding of its interactions with white clover, as the exclusion of white clover from infested pastures could be the result of either direct interaction or niche‐differentiation in response to management. A methodology is presented which enables the segregation of the effects of direct interaction and niche‐differentiation, based on existing dry‐weight‐rank measurements of a number of experimental pastures, subjected to a range of management strategies.Only between the two management extremities, i.e. permanent cutting of silage and lax grazing for long periods, did niche‐differentiation occur between white clover and creeping bentgrass. The white clover content was enhanced under the cutting regime, whereas lax grazing for long periods stimulated the content of creeping bentgrass. White clover was actively excluded from creeping‐bentgrass‐dominated patches by direct interaction, whereas it showed a high compatibility with perennial ryegrass. This direct interaction presents challenges to the prevention of creeping bentgrass by management, as creeping bentgrass and white clover showed nearly identical requirements in terms of environmental conditions and grassland management.

Nitrate leaching and pasture production from different nitrogen sources on a shallow stoney soil under flood-irrigated dairy pasture

The leaching of nitrate (NO3–) in intensive agricultural production systems, e.g. dairy pastures, is a major environmental concern in many countries. In this lysimeter study we determined the amount of NO3– leached following the application of urea, dairy effluent, urine returns, and pasture renovation to a freedraining Lismore stony silt loam (Udic Haplustept loamy skeletal) growing a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture. The study showed that NO3–-N leaching losses ranged from 112 to 162 kg N&amp;sol;ha per year, depending on the amount and forms of N applied and pasture conditions. Nitrate leaching under the urine patches was the main contributor to the N leaching loss in a grazed paddock. Nitrate leaching losses were lower for urine applied in the spring (29&amp;percnt; of N applied) than for urine applied in the autumn (38–58&amp;percnt;). The application of urea or dairy effluent only contributed a small proportion to the total NO3– leaching loss in a grazed paddock. Pasture renovation by direct-drilling may also have caused an increase in NO3– leaching (c. 31 kg N&amp;sol;ha) in the first year. Modelled annual average NO3–-N concentrations in the mixed recharge water in the acquifer were significantly lower than those measured under the rooting zone due to dilution effects by recharge water from other sources (3.9 v. 13–27 mg N&amp;sol;L). Herbage nitrogen offtake and dry matter yield were higher in the urine treatments than in the non-urine treatments. groundwater, denitrification, mineralisation, grazing, forage.

Black cutworm (Lepidoptera: Noctuidae) larval emigration and biomass in mixtures of endophytic perennial ryegrass and Kentucky bluegrass.

Studies examined the possibility that mixtures of endophytic perennial ryegrass and Kentucky bluegrass provide resistance against larvae of the black cutworm, Agrotis ipsilon (Hufnagel). We hypothesized that resistance against A. ipsilon in such stands would stem from the influence of Kentucky bluegrass on A. ipsilon growth and behavior rather than the influence of endophytic perennial rvegrass. In replicated greenhouse experiments, black cutworm larvae initially emigrated more quickly from pots containing monocultures of endophytic perennial ryegrass than from Kentucky bluegrass monocultures or polycultures of Kentucky bluegrass and endophytic perennial ryegrass. However, biomass of emigrating larvae decreased linearly as the proportion of Kentucky bluegrass increased. Turfgrass mixtures containing endophytic perennial ryegrass and Kentucky bluegrass may provide resistance against A. ipsilon mainly through the physiological effects of Kentucky bluegrass on A. ipsilon growth and development, but possibly through the influence of endophytic perennial ryegrass on A. ipsilon movement and foraging behavior as well.

How does the vertical and horizontal structure of a perennial ryegrass and white clover sward influence grazing?

Mixtures of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) sown in alternate rows or in a thoroughly mixed matrix were grazed by sheep, either continuously or during short grazing tests, and were used to investigate the influence of the vertical and horizontal components of the sward structure on defoliation by sheep.In an experiment under continuous grazing, the defoliation intensity was greater for white clover compared with perennial ryegrass leaves (0·80 and 0·58 respectively). In spring, perennial ryegrass leaves were more defoliated than white clover leaves, whereas the reverse was observed in summer. The ratio of the proportion of white clover to perennial ryegrass leaves grazed was negatively correlated with the difference between the surface height of the perennial ryegrass and white clover rows in spring. In both spring and summer, white clover leaves of the same extended leaf length had a higher proportion of them grazed than perennial ryegrass leaves.In another experiment, during short grazing tests with perennial ryegrass–white clover swards that were grazed at the same sward surface height and at the same white clover content as in the previous experiment, there were no significant differences in the proportion of white clover and perennial ryegrass leaves grazed between strips of the two species and thoroughly mixed structures. The proportion of white clover leaves grazed was higher than that of perennial ryegrass leaves.These results show that the differential defoliation by sheep of perennial ryegrass and white clover leaves varies according to their vertical distribution in the mixed canopy, but is little affected by their horizontal distribution. Even small differences in sward surface height between mixed perennial ryegrass and white clover can affect diet selection by sheep to a rather large extent.

N budget and NH3 exchange of a grass/clover crop at two levels of N application

Atmospheric ammonia (NH3) exchange during a single growing season was measured over two grass/clover fields managed by cutting and treated with different rates of mineral nitrogen (N) fertilizer. The aim was to quantify the total NH3 exchange of the two systems in relation to their N budget, the latter was split into N derived from symbiotic fixation, from fertilization, and from the soil. The experimental site was located in an intensively managed agricultural area on the Swiss plateau. Two adjacent fields with mixtures of perennial ryegrass (Lolium perenne L.), cocks foot (Dactylis glomerata L.), white clover (Trifolium repens L.) and red clover (Trifolium pratense L.) were used. These were treated with either 80 or 160 kg N ha−1 applied as NH4NO3 fertilizer in equal portions after each of four cuts. Continuous NH3 flux measurements were carried out by micrometeorological techniques. To determine the contribution of each species to the overall NH3 canopy compensation point, stomatal NH3 compensation points of the individual plant species were determined on the basis of NH4+ + NH3 (NHx) concentrations and pH in the apoplast. Symbiotic N2 fixation was measured by the 15N dilution method.

Estimating legume N 2 fixation in grass-clover mixtures of a grazed organic cropping system using two [formula omitted] methods

The input of Nitrogen (N) through symbiotic N 2 fixation (SNF) in grass-clover mixtures was determined in an organic cropping system for grazing during 3 years. The mixture of perennial ryegrass ( Lolium perenne L.) and white clover ( Trifolium repens L.) was established by undersowing in spring barley ( Hordeum vulgare L.) and maintained subsequently for two production years. Dinitrogen fixation was determined using the 15 N isotope dilution techniques and two labelling procedures. Using either pre-labelling of the soil with immobilisation of the 15N by addition of a carbon source before establishment of the grass-clover mixture or adding the 15N labelled ammonium to the established crop, resulted in similar estimates of the proportion of N derived from the atmosphere (pNdfa). However, the average coefficient of variance of the pNdfa values was significantly lower with the pre-labelling immobilisation method. The proportion of fixed N in the clover varied between 0.70 and 0.99 with the lowest values in the beginning and at the end of the growing season. The total amount of N fixed in the above-ground plant parts of the grass-clover mixture varied between 10.0 and 23.5 g N m −2 per year. The total input from N 2 fixation in the rotation was between 30 and 50 g N m −2 per year (above-ground plant parts only). This variation could be ascribed to differences in climatic conditions during the 3 years. The amount of N in grass-clover residues incorporated in the soil after the second production year ranged between 19.6 and 29.6 g N m −2, of which 52–66% was estimated to be derived from fixation. The amount of atmospherically derived N actually entering the soil in the grazed cropping system is discussed.

Interactions between perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) under contrasting nitrogen availability: productivity, seasonal patterns of species composition, N2 fixation, N transfer and N recovery.

Nitrogen (N) fertiliser and clover cultivar choice affect competition and productivity in grass-clover mixtures. Pure stands and mixtures of perennial ryegrass and white clover cultivars with contrasting growth habits were examined. The aim of this work was to study the effect of repetitive nitrogen (N) application and cultivar combination on competition and productivity, N yield in the harvested herbage, N2 fixation in mixtures and pure stands, and transfer of N from clover to the companion grass. Large-leaved white clover cultivar Alice and small-leaved cv. Gwenda and perennial ryegrass cvs. Barlet (erect) and Heraut (prostrate) were sown in pure stands and as four binary grass-clover mixtures on a sandy soil in 1995. In the mixtures, two levels of N fertiliser were applied: 0 (-N) and 150 and 180 kg ha-1 y-1 N (+N) in 1996 and 1997, respectively, while the grass monocultures received three N levels (0, 140/180 and 280/360 kg ha-1) in 1996 and 1997, respectively. No N was applied to pure clover. The plots were cut five times during 1996 and six times during 1997. Fertiliser N was applied in early spring and after every harvest. The treatments were continued until the summer of 1999. In pure grass, the applied N was effectively recovered. In mixtures, N application affected competition by enhancing grass growth and the overall effect of N application was 17 kg DM per kg N applied in 1996. However, there was no yield response to N fertilizer in 1997, because this was compensated for by a higher clover production in unfertilised mixtures. In 1997, -N mixtures yielded more N than +N mixtures, owing to the higher clover content and N2 fixation. Large-leaved clover cv. Alice was better able to withstand the negative effect of repetitive N application on clover production in mixtures and increased its proportion during the growing season of the second harvest year. In 1997, mixtures with Alice yielded more N than mixtures with Gwenda, but in pure clover swards, there was no cultivar effect on N yield. Also, during the autumn of 1998 and the spring of 1999, the clover content was

Modelling and measuring vertical light absorption within grass–clover mixtures

The vertical profile of leaf area, leaf dispersion, and light partitioning and absorption were studied during two regrowth periods in mixtures of perennial ryegrass and two white clover cultivars differing in leaf size under cutting management without fertiliser nitrogen. A triangular leaf area density function with height gave a good description of the vertical leaf area profile for both species. Leaf dispersion was studied by analysing inclined point quadrat data. Calculated leaf dispersion factors of both species were linearly correlated with downward cumulative LAI of the mixture and appeared to be the principal cause for variation in the extinction coefficient ( k) with canopy height. This relationship was negative for clover and positive for grass, corresponding with a shift from regular leaf dispersion in the top layers to a clumped dispersion in the bottom layers for clover and the reverse pattern for grass. Measured light profiles could be exactly mimicked with a modified version of a general multi-layer light competition model by incorporating for both species leaf dispersion as a function of downward cumulative LAI in combination with fitted dispersion-free values of k which only reflect the leaf-angle distribution. Competitive success of clover over grass for light absorption in this study was, next to its greater contribution to total LAI and a more planofile leaf-angle distribution, related to its higher position in the mixture where maximum leaf area density occurred and regular leaf dispersion in the top layers of the canopy. These last two characteristics were especially manifest in the large-leaved clover.

Contributions to nitrogen leaching and pasture uptake by autumn-applied dairy effluent and ammonium fertilizer labeled with 15 N isotope

The objective of this study was to compare the N leaching loss and pasture N uptake from autumn-applied dairy shed effluent and ammonium fertilizer (NH4Cl) labeled with 15N, using intact soil lysimeters (80 cm diameter, 120 cm depth). The soil used was a sandy loam, and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The DSE and NH4Cl were applied twice annually in autumn (May) and late spring (November), each at 200 kg N ha-1. The N applied in May 1996 was labeled with 15N. The lysimeters were either spray or flood irrigated during the summer. The autumn-applied DSE resulted in lower N leaching losses compared with NH4Cl. However, the N applied in the autumn had a higher potential for leaching than N applied in late spring. Between 4.5–8.1% of the 15N-labeled mineral N in the DSE and 15.1–18.8% of the 15N-labeled NH4Cl applied in the autumn were leached within a year of application. Of the annual N leaching losses in the DSE treatments (16.0–26.9 kg N ha-1), a fifth (20.3–22.9%) was from the mineral N fraction of the DSE applied in the autumn, with the remaining larger proportion from the organic fraction of the DSE, soil N and N applied in spring. In the NH4Cl treatments, more than half (53.8–64.8%) of the annual N leaching loss (55.9–57.6 kg N ha-1) was derived from the autumn-applied NH4Cl. DSE was as effective as NH4Cl in stimulating pasture production. Since only 4.4–4.5% of the annual herbage N uptake in the DSE treatment and 12.3–13.3% in the NH4Cl treatment were derived from the autumn-applied mineral N, large proportions of the annual herbage N uptake must have been derived from the N applied in spring, the organic N fraction in the DSE, soil N and N fixed by clover. The recoveries of 15N in the herbage were similar between the DSE and the NH4Cl treatments, but those in the leachate were over 50% less from the DSE than from the NH4Cl treatment. The lower leaching loss of 15N in the DSE treatment was attributed to the stimulated microbial activities and increased immobilization following the application of DSE.

Effects of cultivar and cutting frequency on dynamics of stolon growth and leaf appearance in white clover grown in mixed swards

AbstractThe hypothesis that dynamics of growth, branching of stolons and appearance of leaves are important for the persistence of white clover (Trifolium repens) in mixed swards was tested. The effect of cutting frequency and white clover cultivar on stolon and leaf dynamics was studied throughout the growing season in a field experiment. Mixtures of perennial ryegrass (Lolium perenne) and white clover cultivars with different leaf sizes, cvs. Alice, Gwenda and Retor, were evaluated for white clover persistence at two cutting frequencies. Stolon dynamics, stolon survival, leaf and node appearance rate, branching, flowering and stolon elongation rate were analysed and related to white clover content and yield.There were clear seasonal fluctuations in stolon and leaf characteristics. Stolon elongation rate and appearance rates of nodes, leaves and branches declined in autumn. Temperature and irradiation explained a major part of the variation of stolon elongation rate and leaf appearance rate and 25% of the variation in stolon branching rate.Significant and consistent differences in yield and botanical composition were found between mixtures. Mixtures with cv. Alice had the highest total and white clover yield and the highest white clover content, whereas mixtures with cv. Retor had the lowest yield and the lowest white clover content. Stolon elongation rate and stolon internode length were significantly higher in cv. Alice than in the other white clover cultivars, whereas cv. Retor had a significantly higher percentage of leaves with damage caused by insects and slugs than the other cultivars. Little support was found for the hypothesis that branching characteristics are a key component of competitive success in mixed pasture, as most measured branching characteristics were not associated with differences in white clover yield between cultivars and cutting regimes.

Harvest Frequency and Cultivar Influence on Yield and Protein of Alfalfa‐Ryegrass Mixtures

AbstractBinary mixtures of alfalfa (Medicago sativa L.) and perennial ryegrass (Lolium perenne L.) are commonly grown for forage in the U.S. Northeast. The competitiveness of ryegrass with alfalfa as modified by management practices has not been extensively studied. Two experiments were conducted to measure the influence of harvest frequency and ryegrass ploidy level on production, botanical composition, and crude protein concentration of alfalfa‐ryegrass mixtures. The first experiment used six replicates of a factorial treatment structure (three ryegrass cultivars × three harvest cycles) in a split‐plot design. The second experiment used three replicates of three cultivars × three harvest treatments. Increasing the length of the harvest cycle from 20 to 40 d increased alfalfa and total dry matter production (10.1 to 14.9 Mg ha−1) while decreasing ryegrass production and proportion (49 to 18%) of grass in the stand. Ryegrass was nearly eliminated from the stand at the 40‐d harvest cycle, but when the cutting cycle was changed to 20 d the next year, it became productive and contributed one‐fourth to one‐half of the total forage yield. Alfalfa, which had low productivity (7.3 to 8.4 Mg ha−1) in a 20‐d cycle, became very productive (16.4 to 17.4 Mg ha−1) when the cycle was changed to 40 d the next year. Crude protein concentration in alfalfa decreased from 248 to 200 g kg−1 as harvest cycle increased from 20 to 40 d. Ryegrass crude protein increased from 163 to 184 g kg−1 as harvest cycle increased. Frances and Diplomat perennial turfgrass ryegrasses were as productive and persistent as Citadel perennial forage ryegrass in binary mixtures with alfalfa.

The effects of elevated CO2 on symbiotic N2 fixation: a link between the carbon and nitrogen cycles in grassland ecosystems

The response of plants to elevated CO2 is dependent on the availability of nutrients, especially nitrogen. It is generally accepted that an increase in the atmospheric CO2 concentration increases the C:N ratio of plant residues and exudates. This promotes temporary N-immobilization which might, in turn, reduce the availability of soil nitrogen. In addition, both a CO2 stimulated increase in plant growth (thus requiring more nitrogen) and an increased N demand for the decomposition of soil residues with a large C:N will result under elevated CO2 in a larger N-sink of the whole grassland ecosystem. One way to maintain the balance between the C and N cycles in elevated CO2 would be to increase N-import to the grassland ecosystem through symbiotic N2 fixation. Whether this might happen in the context of temperate ecosystems is discussed, by assessing the following hypothesis: i) symbiotic N2 fixation in legumes will be enhanced under elevated CO2, ii) this enhancement of N2 fixation will result in a larger N-input to the grassland ecosystem, and iii) a larger N-input will allow the sequestration of additional carbon, either above or below-ground, into the ecosystem. Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO2 under free-air or field-like conditions, supports the first two hypothesis, since: i) both the percentage and the amount of fixed N increases in white clover grown under elevated CO2, ii) the contribution of fixed N to the nitrogen nutrition of the mixed grass also increases in elevated CO2. Concerning the third hypothesis, an increased nitrogen input to the grassland ecosystem from N2 fixation usually promotes shoot growth (above-ground C storage) in elevated CO2. However, the consequences of this larger N input under elevated CO2 on the below-ground carbon fluxes are not fully understood. On one hand, the positive effect of elevated CO2 on the quantity of plant residues might be overwhelming and lead to an increased long-term below-ground C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over and, hence, limit the storage of below-ground carbon.

Ozone exposure-response relationships for mixtures of perennial ryegrass and white clover depend on ozone exposure patterns

The effect of different ozone (O 3) exposure regimes on yield and species composition of frequently cut mixtures of ryegrass ( Lolium perenne L.) and white clover ( Trifolium repens L.) was studied in order to assess the importance of intermittent periods with low O 3 and to evaluate the need to use an exposure index that emphasizes peak concentrations. Exposure-response relationships were used to test the performance of different indices characterising the accumulated exposure. Potted plant mixtures were exposed in open-top chambers and cut repeatedly after four-week growth periods (GP). Yield data were related to O 3 exposure expressed as AOT40 (accumulated exposure above 40 ppb during daylight hours) for the GP immediately preceding the harvest, or for all GPs combined. Data for relative clover yield showed that the effect determined after several GPs represented the combined effect of all preceding GPs, and significant correlations were found between clover yield and AOT40 calculated over all GPs. During GPs with low O 3, clover recovered to some extent from injury produced during previous exposures to high O 3. By comparing the effects of treatments with similar AOT40 values but different peak concentrations, i.e. a treatment with a more constant O 3 exposure vs a treatment with episodic O 3 stress, no difference in the effect on clover yield was found, but a stronger effect on total forage yield by the episodic treatment. The strength of the correlation between total yield and AOT increased with increasing threshold concentration suggesting that peak concentrations are more effective in eliciting the yield response than are lower concentrations. It is concluded that AOT40 calculated over all GPs is a suitable exposure index to describe the effect of O 3 on the species composition of this type of mixture, but to describe the effect on total forage yield, peak concentrations of > 110 ppb must be emphasized by using an AOT with higher threshold concentrations. A comparison with Swiss O 3 monitoring data revealed, however, that in ambient air such high concentrations rarely occur.

Productivity and nitrogen uptake of ageing and newly sown swards of perennial ryegrass (Lolium perenne L.) at different sites and with different nitrogen fertilizer treatments

A field trial was conducted on 8 sites for 3 years to compare the productivity and nitrogen (N) uptake of ageing sown leys (5–12 years old) with newly sown diploid perennial ryegrass at a range of N-fertilizer rates (0, 125, 250, 375 and 500 kg N ha−1) and with a diploid-tetraploid mixture of perennial ryegrasses (at 250 kg N ha−1 only). All plots were cut at 6-weekly intervals. Mean annual DM yields at 500 kg N ha−1 were 12.3 and 13.2 t ha−1 for ageing and newly sown swards, respectively.There were differences between sites in the relative productivity of sward types : on some sites, particularly at low rates of N fertilizer, ageing leys gave greater production than newly sown swards. Three-year mean DM production varied by over 200 per cent between sites, and the 8-site mean was ca. 45 per cent greater in year 1 than year 3, for both sward types, due to seasonal differences in weather. Swards resown with a diploid-tetraploid perennial ryegrass mixture had greater early season production than the diploid (cv. Vigor) sown alone, although not at all sites.Mean N yield of herbage, at 0 fertilizer N, was 20 per cent higher from ageing leys than resown swards. Apparent N recovery was higher from resown swards and highest in year 1. Results are discussed in relation to previous multi-site trials, and relationships between sward type, DM yield, N uptake and N supply are presented for sites with different response patterns. Implications for grassland management are considered.

Effects of cutting height on the growth of leaves and stolons in perennial ryegrass–white clover swards

SUMMARYAll combinations of five cutting heights (2, 4, 6, 8 and 10 cm) were compared, for 3 years (1986–88), on field plots sown with a mixture of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) in Wales. The plots were cut at 4-week intervals from April to October each year.A reduction in cutting height reduced the length, width and weight of clover leaflets, petiole length, stolon diameter, the length of stolon internodes and the height of stolons above ground level, but increased the length of clover stolon/m2, the number of stolon nodes/m2 and the proportion of stolon nodes which produced branches. The latter factors out-weighed the former, leading to the inverse relationship between cutting height and the proportion of clover in the sward previously reported (Acuñia &amp; Wilman 1993).A reduction in cutting height reduced the following attributes of ryegrass: the height of the shoot apex, the length and width of leaf blades, sheath length, leaf extension rate, the death rate of leaves and the net gain in blade length.Cutting height had little effect on either the thickness of clover leaflets or the rates of leaf emergence in clover and ryegrass.

Effects of cutting height on the productivity and composition of perennial ryegrass–white clover swards

SUMMARYAll combinations of five cutting heights (2, 4, 6, 8 and 10 cm), two rates of P application (0 and 100 kg P/ha per year) and two irrigation treatments (0 and maximum soil water deficit 35 mm) were compared on field plots sown with a mixture of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) in Wales. The cutting and P treatments were applied for 4½ years (1986–90) and the irrigation treatments for 2 years (1987 and 1988). The plots were cut at 4-week intervals from April to October each year.Applied P and irrigation had only a small effect on the productivity and composition of the swards. The proportion of white clover in the sward was inversely related to the height of cutting, so that clover was almost eliminated by 3 years of 10 cm cutting. This reduction in clover content led to a 50% reduction in herbage yield. Close cutting increased the tiller density of ryegrass in May and June of the first harvest year, but this effect was subsequently reversed as clover competed more strongly with ryegrass under close cutting. The proportion of unsown species, predominantly Agrostis tenuis, was much higher in the third and fourth than in the first and second harvest years and was positively related to the height of cutting. Close cutting depleted soil K and soil water.