Perennial Swards Research Articles

Due to symbiotic N2 fixation, five years of elevated atmospheric pCO2 had no effect on the N concentration of plant litter in fertile, mixed grassland

Experimental findings indicate that, in terrestrial ecosystems, nitrogen cycling changes under elevated partial pressure of atmospheric CO2 (pCO2). It was suggested that the concentration of N in plant litter as well as the amount of litter are responsible for these changes. However, for grassland ecosystems, there have been no relevant data available to support this hypothesis. Data from five years of the Swiss FACE experiment show that, under fertile soil conditions in a binary plant community consisting of Lolium perenne L. and Trifolium repens L., the concentration of litter N does not change under elevated atmospheric pCO2; this applies to harvest losses, stubble, stolons and roots as the sources of litter. This is in strong contrast to the CO2 response of L. perenne swards without associated legumes; in this case the above-ground concentration of biomass N decreased substantially. Increased symbiotic N2 fixation in T. repens nodules and a greater proportion of the N-rich T. repens in the community are regarded as the main mechanisms that buffer the increased C introduction into the ecosystem under elevated atmospheric pCO2. Our data also suggest that elevated atmospheric pCO2 results in greater amounts of litter, mainly due to increased root biomass production. This study indicates that, in a fertile grassland ecosystem with legumes, the concentration of N in plant litter is not affected by elevated atmospheric pCO2 and, thus, cannot explain CO2-induced changes in the cycling of N.

LINGRA‐CC: a sink–source model to simulate the impact of climate change and management on grassland productivity

A simulation model for the prediction of grassland (Lolium perenne) productivity under conditions of climate change is described and validated for grass growing in the Wageningen Rhizolab, Wageningen, The Netherlands. In this work the model was used to study the impact of different management strategies on the productivity of grassland under present and increased atmospheric CO2 concentrations. In LINGRA‐CC simulated key processes are light utilization, leaf formation, leaf elongation, tillering and carbon partitioning. The daily growth rate is determined by the minimum of a sink and a source term. As in a previous model (LINGRA), the potential growth of the sink depends on the mean daily temperature, and can be modified by the effects of the availability of assimilates on tillering. The growth of roots is calculated from the amount of carbohydrates the shoot is unable to utilize when the number or activity of the sinks is small (overflow hypothesis). The main difference between LINGRA and LINGRA‐CC is the way the source of assimilates for growth is calculated. Assimilate production depends on intercepted radiation, and a photosynthetic light‐use efficiency (LUE) calculated as a function of CO2, temperature, light intensity and the Rubisco concentration of upper leaves. Other differences are that in LINGRA‐CC, the specific shoot area for new growth depends on the level of reserves. Data from two independent experiments with L. perenne swards, grown in enclosures at two levels of CO2 during 1994 and 1995, were used to calibrate and validate the model, respectively. The model predicted well the observed amounts of harvested biomass, and the dynamics of the leaf area index, tiller number and specific shoot area. LINGRA‐CC was used to study the effects of different combinations of cutting interval and cutting height on biomass production, at ambient (350 μmol mol−1 CO2) and double (700 μmol mol−1 CO2 ) CO2 conditions. Under both ambient and doubled CO2, maximum biomass was produced with cuttings of leaf area index >1, and at cutting intervals of 20 and 17 d for ambient and increased CO2 environments, respectively. Under high CO2 conditions the cutting interval for maximum yield was 15% shorter than at ambient CO2. However, the gain in harvested biomass obtained by reducing the cutting interval by 3 d under high CO2 conditions was negligible.

Nitrogen capture by plants grown in N-rich organic patches of contrasting size and strength

This study investigated the response of root proliferation, plant N capture and soil microbial activity in a Lolium perenne sward to l-lysine (15N/13C dual-labelled) patches of contrasting size and strength. Three patch treatments provided the same quantity (c. 5.6 mg) of N, but at different concentrations (i.e. 200 mM × 1 ml; 100 mM × 2 ml; 40 mM × 5 ml), while the other two treatments provided five times (1 M × 1 ml) or 0.2 (40 mM × 1 ml) as much N. Control patches were added as 1 ml H2O. Root production and root mortality in the patch addition zone were measured in situ using mini-rhizotron tubes and were found to differ between the various treatments. Instantaneous root birth rates were significantly (P<0.05) greater in the highest N added patch (1 M × 1 ml) at day 14 only, indicating a more marked proliferation response in this treatment compared to those containing less N. However, of the total N available, roots in the 1 M × 1 ml l-lysine patch captured significantly (P < 0.05) less (29%) than in the other four treatments (40–47%), which did not differ significantly from each other. Microbial activity was significantly (P < 0.05) greater in the highest N added patch (1 M × 1 ml) than in all other treatments. The response of roots and micro-organisms in relation to N capture is discussed.

Quantifying different causes of leaf and tiller death in grazed perennial ryegrass swards

Death of plant material in grazed perennial swards results from a number of processes, particularly leaf senescence, tiller death, and grazing. These processes must be understood in detail to manage supply of high quality pasture for, especially, dairy cattle through the reproductive season. Although measurements have been made under a variety of pasture conditions, a generalised model has not yet been established. This paper reviews experimental evidence for the rates of leaf and tiller death resulting from five independent physical and physiological mechanisms operating in a grazed ryegrass‐dominant pasture. First‐order (linear) models are formulated to predict the rates of leaf and stem death on vegetative and reproductive tillers in such a pasture. Leaf death rate accounts for 90% of dead matter production in the non‐reproductive season, and was found to be linearly correlated to soil temperature. During reproduction, decapitation of reproductive stems, usually prior to ear emergence, is the dominant source of dead material. These models provide an essential component of the detailed modelling of grazed pasture tissue dynamics.

Woody species colonisation in relation to habitat productivity

A study was conducted to analyse the effect of habitat productivity on woody species colonisation. Three soil types were distinguished: a relatively poor sandy soil type (1), a somewhat richer sandy type (2) and a relatively rich sandy loamy type (3). Chronosequences were established on these three soil types of 38 (type 1), 20 (type 2) and 54 years (type 3) after abandonment. In total 117 vegetation relevées were used to analyse life form change and species responses during old field succession via regression models. On the rich soil type the colonisation rate of woody species was slower than on the poor soil type. This can be explained by higher abundances of perennial species during the first 20 years after abandonment on the rich soil type in contrast to the poor soil type. Perennial species may delay the woody species colonisation. First they close the bare ground which inhibits germination and next they compete with woody seedlings for light, water and nutrients. The effect of habitat productivity on woody species colonisation can only be determined appropriately by taking life history traits into account. Early successional 'pioneer' woody species dispersed by wind have less difficulties colonising old fields than late successional 'forest' species; they colonise old fields prior to the development of a dense perennial sward. Forest species depend on animals to be dispersed which are attracted by vegetation structure. In ± 30 years on the poor soil type and in ± 45 years on the rich soil type woody species become dominant relative to other life forms. Forest species like Quercus robur L. invaded relatively early (<5 years) in contrast to other studies which probably coincides with the distance to seed sources (forest edges).

Long-term responsiveness to free air CO2 enrichment of functional types, species and genotypes of plants from fertile permanent grassland.

To test inter- and intraspecific variability in the responsiveness to elevated CO2, 9-14 different genotypes of each of 12 perennial species from fertile permanent grassland were grown in Lolium perenne swards under ambient (35 Pa) and elevated (60 Pa) atmospheric partial pressure of CO2 (pCO2) for 3 years in a free air carbon dioxide enrichment (FACE) experiment. The plant species were grouped according to their functional types: grasses (L. perenne, L. multiflorum, Arrhenatherum elatius, Dactylis glomerata, Festuca pratensis, Holcus lanatus, Trisetum flavescens), non-legume dicots (Rumex obtusifolius, R. acetosa, Ranunculus friesianus), and legumes (Trifolium repens, T. pratense). Yield (above a cutting height of 4.5 cm) was measured three times per year. The results were as follow. (1) There were highly significant differences in the responsiveness to elevated pCO2 between the three functional types; legumes showed the strongest and grasses the weakest yield increase at elevated pCO2. (2) There were differences in the temporal development of responsiveness to elevated pCO2 among the functional types. The responsiveness of the legumes declined from the first to the second year, while the responsiveness of the non-legume dicots increased over the 3 years. During the growing season, the grasses and the non-legume dicots showed the strongest response to elevated pCO2 during reproductive growth in the spring. (3) There were no significant genotypic differences in responsiveness to elevated pCO2. Our results suggest that, due to interspecific differences in the responsiveness to elevated pCO2, the species proportion within fertile temperate grassland may change if the increase in pCO2 continues. Due to the temporal differences in the responsiveness to elevated pCO2 among species, complex effects of elevated pCO2 on competitive interactions in mixed swards must be expected. The existence of genotypic variability in the responsiveness to elevated pCO2, on which selection could act, was not found under our experimental conditions.

Herbage intake, ingestive behaviour and diet selection, and effects of condensed tannins upon body and wool growth in lambs grazing Lolium perenne and Holcus lanatus swards in summer

A comparative study was undertaken in New Zealand to investigate the effects of low concentrations of condensed tannins (CT) on diet selection, herbage intake, and performance in lambs grazing on four 0.2 ha paddocks each of perennial ryegrass (Lolium perenne) or Yorkshire fog (Holcus lanatus) with less than 10% white clover (Trifolium repens), continuously grazed at a constant height of approximately 6 cm from December 1992 to March 1993. The effects of CT on rumen metabolism and animal production were assessed by twice daily oral administration of polyethylene glycol (PEG; MW 4000) to half of the lambs on each sward. Bite weight was greater for Yorkshire fog swards than for ryegrass swards during January (111 vs 85 ± 5 mg OM/bite, P < 0.01), but not in December. In both December and January, grazing, ruminating, and idling times were similar between swards, while in January, the rate of biting in ryegrass swards was higher than in Yorkshire fog swards (64 vs 61 ± 1 bites/min, P < 0.05). The organic matter digestibility (OMD) of the herbage selected was higher in ryegrass swards in December (81 vs 78 ± 0.04 %, P < 0.01), but not in January (80 vs 79 ± 0.05 %). The herbage intake achieved by lambs grazing ryegrass swards was 23% higher than that achieved on Yorkshire fog swards in December (990 vs 800 ± 36 g OM/lamb/day, P < 0.05) whereas, in January, herbage intakes did not differ significantly. Lambs grazing on ryegrass swards had higher clean wool growth (1147 vs 1085 ± 15 ng/cm2 per day, P < 0.10) and carcass weight (17.5 vs 16.3 ± 0.22 kg, P < 0.05) than lambs grazing on Yorkshire fog swards. The stocking rate maintained on ryegrass plots was 25% greater than on Yorkshire fog plots. Low concentrations of CT were recorded in the diets of sheep on ryegrass and Yorkshire fog swards (< 0.2% on a DM basis). The low levels of CT had no significant effects on diet selection, herbage intake, grazing behaviour patterns, or lamb performance. However, the lambs grazing on Yorkshire fog swards showed small and non‐persistent responses to CT in terms of wool growth and liveweight gain. The results of this study indicate that: (i) under high fertility conditions and intensive management, perennial ryegrass/white clover swards appear to have higher feeding value than Yorkshire fog/white clover swards for lamb production and (ii) the low CT concentrations (< 0.2% in the DM) observed in both swards did not influence lamb performance significantly.

Does nitrogen nutrition restrict the CO2 response of fertile grassland lacking legumes?

The extent of the response of plant growth to atmospheric CO2 enrichment depends on the availability of resources other than CO2. An important growth-limiting resource under field conditions is nitrogen (N). N may, therefore, influence the CO2 response of plants. The effect of elevated CO2 (60 Pa) partial pressure (pCO2) on the N nutrition of field-grown Lolium perenne swards, cultivated alone or in association with Trifolium repens, was investigated using free air carbon dioxide enrichment (FACE) technology over 3 years. The established grassland ecosystems were treated with two N fertilization levels and were defoliated at two frequencies. Under elevated pCO2, the above-ground plant material of the L. perenne monoculture showed a consistent and significant decline in N concentration which, in general, led to a lower total annual N yield. Despite the decline in the critical N concentration (minimum N concentration required for non-N-limited biomass production) under elevated pCO2, the index of N nutrition (ratio of actual N concentration and critical N concentration) was lower under elevated pCO2 than under ambient pCO2 in frequently defoliated L. perenne monocultures. Thus, we suggest that reduced N yield under elevated pCO2 was evoked indirectly by a reduction of plant-available N. For L. perenne grown in association with T. repens and exposed to elevated pCO2, there was an increase in the contribution of symbiotically fixed N to the total N yield of the grass. This can be explained by an increased apparent transfer of N from the associated N2-fixing legume species to the non-fixing grass. The total annual N yield of the mixed grass/legume swards increased under elevated pCO2. All the additional N yielded was due to symbiotically fixed N. Through the presence of an N2-fixing plant species more symbiotically fixed N was introduced into the system and consequently helped to overcome N limitation under elevated pCO2.

Dietary preferences of domestic ruminants

To develop sustainable grazing systems, an understanding of the complex interactions between competing plant species and grazing herbivores is needed. An understanding of dietary preference is a prerequisite to predicting how much and from which plant species animals select. This work initially concentrated on dietary preferences of sheep grazing perennial ryegrass and white clover swards and was then extended to other animal species to test the generality of our findings.

Interspecific and intraspecific variability in the response of grasses and legumes to free air CO 2 enrichment

Nine to fourteen genotypes of seven grass and two legume species from permanent grassland were grown at two levels of atmospheric CO 2 concentration in gaps of established Lolium perenne swards in a Free Air Carbon dioxide Enrichment (FACE) experiment. Cumulative biomass of individual plants was determined for two growing seasons. In the first year, elevated CO 2 increased biomass production in all species. The CO 2-induced increase in the biomass of Trifolium repens and T. pratense (159%) was much greater than the increase in the grass species (27%). In the second year the response to elevated CO 2 was weaker in grasses (2%, ns) and legumes (73%). However, interspecific differences in the response to CO 2 remained significant. Interspecific differences in the response to elevated CO 2 occurred between the two functional groups of grasses and legumes, while within these groups no significant interspecific differences were found. In contrast to the interspecific variability in the response to CO 2, no significant intraspecific variability in the response to CO 2 was detected. Our results suggest that significant interspecific differences in the response to CO 2 occur. Intraspecific differences in the response to elevated CO 2 were, however, not detected. Thus, it seems unlikely that evolutionary adaptation of the species' response to elevated CO 2 will level out the interspecific differences in the response to CO 2.

Vegetation establishment on chalk marl spoil: the role of nurse grass species and fertiliser application

Abstract. The Channel Tunnel workings on the UK side have yielded nearly 4 million m3 of chalk‐marl spoil which now forms a 36 ha landscaped reclamation platform. To establish vegetation of amenity and conservation interest on the spoil, seed mixtures of native wild flowers and grasses were sown with Lolium perenne (perennial rye grass) as a nurse species. Potentially, L. perenne is a suitable nurse species for grassland creation on infertile substrates as it provides rapid initial cover and stability, but it is non‐persistent and declines in vigour with time, allowing wild flower species sown alongside to expand their cover and spread in the longer term. On very low fertility substrates like chalk marl, an initial application of fertilizer is needed to encourage plant growth. Results are reported of a fertilizer experiment on Channel Tunnel spoil to determine appropriate levels of fertilizer for establishment of species‐rich grassland vegetation. An area hydroseeded with L. perenne and wild flowers in autumn 1992 was subjected to factorial treatment of four levels each of N and P in spring 1993. The results the following summer showed significant positive effects of nitrogen and phosphorus on L. perenne biomass and a negative impact of nitrogen on densities of wild flower species, especially legumes, establishing in the L. perenne sward. In general, low fertilizer applications encouraged low productivity and maximal species richness in the vegetation. Conversely high applications encouraged high productivity and competitive exclusion of sown wild flower species. Fertilizer applications must therefore balance encouragement of the stabilising nurse grass sward, while preventing competitive exclusion of wild flowers by the nurse grass.

Annual variation in the balance between winter fodder production and requirements in beef cow systems

Herbage intake by beef cows is highly correlated with sward height and therefore intake can be managed in a predictable manner by controlling sward height. For beef cows this requires adjustment of the grazing area and conservation of herbage which is not grazed, with the silage aftermath subsequently grazed later in the summer. Year-to-year variation in herbage growth rate will result in variation in the amounts of silage produced. Most grazing experiments do not consider this year-to-year variation, but it constitutes one of the major features which has to be accommodated in the operational management of beef cow systems which are based on pastoral resources.During three consecutive summers, 40 spring-calving Hereford x Friesian beef cows and their Charolais-cross calves grazed perennial ryegrass-dominated swards receiving 200 kg N/ha at one of two annual stocking rates (2.0 or 2.5 cows/ha) at one of two sward heights (4 to 5 (short) or 7 to 8 cm (tall)) in a 2x2 factorial experiment, replicated twice.

The Effects of Competition on Growth and Form in Glechoma hederacea

Morphological responses to competition were analysed in Glechoma hederacea, a clonal herb with plagiotropic shoots. Ramets of clones were grown either in isolation or in competition with cut or uncut Lolium perenne swards. G. hederacea produced the same above ground biomass, but different morphologies when grown with short and tall grass. Despite considerable reductions in biomass and stolon branching when grown with competition rather than in isolation, the capacity for lateral extension of primary stolons was maintained. All other measured variables were reduced by competition, except for stolon internode length and petiole length and mass. These variables reached their greatest values in the uncut grass treatments. These responses could promote avoidance of competition by escape in both the horizontal and vertical planes; growth through vegetated sites would be rapid, and sites where competitors are absent would be occupied more densely. Reductions in PAR and in the R/FR ratio incident on the plant are both implicated in inducing these morphological changes. Differences in relative humidity, caused by the proximity of neighbours, also appeared to exert a considerable influence upon growth and morphology of G. hederacea. Connected parts of G. hederacea clones grown with and without competition developed different morphologies; each primary stolon behaved as an integrated physiological unit, developing a morphology appropriate for the conditions in which it was growing.

Response of permanent and reseeded grassland to fertilizer nitrogen. 2. Effects on concentrations of Ca, Mg, K, Na, S, P, Mn, Zn, Cu, Co and Mo in herbage at a range of sites

AbstractThe concentration of mineral elements in herbage from permanent swards of mixed species composition (predominantly Agrostis spp., Holcus lanatus, Festuca rubra, Lolium perenne, Poa spp., etc.) was compared with that from sown perennial ryegrass (Lolium perenne) at two fertilizer rates (0 and 300kg N ha−1 year−1, both with P and K) and with L. perennel Trifolium repens (at 0 N with P and K). The investigation was conducted over four years at sixteen contrasting sites in England and Wales. Herbage samples for mineral analysis were taken at first and fourth harvests (May and August) each year from plots under 4‐weekly cutting. There was a wide degree of variation between sites, particularly for concentrations of Ca, Mg, Na and all trace elements analysed. Herbage from L. perenne reseeds had significantly higher concentrations of Ca, Mg, Na, Co and S, but K, Mo, Zn, Cu and P were generally lower than in the permanent swards. L. perennel T. repens swards had the highest concentration of major cations, but trace element levels were generally intermediate between those for 0 N permanent and L. perenne swards. Fertilizer N reduced concentrations of Ca, Mn, Mo and S, and increased Mg, Na and Zn, with no consistent effects on K, Co and Cu. Herbage at harvest 4, compared with harvest 1, had higher Ca, Mg, Na, Mn, Zn and S. Permanent and L. perenne swards responded similarly to fertilizer N and between harvest dates. Results are discussed in relation to previous reports of botanical and fertilizer effects on mineral concentrations, and the mineral requirements of livestock. Changes in mineral concentration resulting from reseeding permanent swards are considered unlikely to increase mineral‐related livestock disorders, and in many cases could be beneficial, though problems might arise on sites where some elements, e.g. Cu and Mg, are low.

The effect of the energy amd protein content of a strawmix supplement on the milk production and bloat incidence of cows grazing a high clover pasture

A mixed perennial ryegrass and white clover sward must be grazed to a short height to encourage clover growth. If high milk output is expected a supplement must be offered. A straw and concentrate mix (strawmix) utilizes surplus straw and enables nutrient intake to be controlled. Forage supplements also reduce the risk of bloat. In this experiment strawmix supplements of varying energy and protein content were compared.Twenty Friesian dairy cows were allocated to a Latin Square design with five treatments: no supplement N, or pasture supplements of high or low energy (E and e) and high or low protein (P or p) in a factorial design (Table 1). Energy and protein contents were manipulated by changing the concentration of molasses and soyabean meal respectively.

Dry matter yield and response to nitrogen of an Agrostis stolonifera‐dominant sward

AbstractA small‐plot cutting trial was carried out 1983‐85 at North Wyke Research Station, Devon to compare the dry matter (DM) production and response to nitrogen (N) of a naturally occurring Agrostis stolonifera‐dominant sward, developed under low fertility conditions on a poorly structured soil. and a sward of Lotium perenne cv. Melle, direct‐drilled in 1982 at the same site. All plots received standard inputs of P. and graded inputs of K, related to the four annual N treatments (0, 200, 400 and 800 kg ha−1, in seven equal applications). All plots were cut six times each year, except in 1983; DM yield, herbage digestibility, herbage N concentration, response to applied N, apparent recovery of applied N. and botanical composition of the swards were recorded.On average, A. stolonifera gave 69% of the DM yield of L. perenne. A. stolonifera did not out‐yield L. perenne at the low N levels (0 or 200 kg N ha−1 a−1), or in mid‐season. and yielded significantly (P&lt;0‐001) less in drought conditions in 1984 (4‐4 t DM ha−1 and 6‐9 t DM ha−1, s.e.d.±0‐22). Average digestibility of A. stolonifera was 3 units of D‐value lower (P&lt;0‐001) than L. perenne, but N concentrations were higher (3‐01% and 2‐53%, s.e.d. ±0‐074). However, total yields of N from A. stolonifera were lower. Responses in DM production per kg N applied were lower in A. Stolonifera than in L. perenne for all N increments, except the highest, (400‐800 kg N ha−1). Fitted response curves allowed interpolation of the values of DM yield (Y10) and N input (N10) where the response was 10 kg DM (kg N)−1. Values of Y10 for A. stolonifera and L. perenne averaged 7‐1 t DM ha−1 and 12‐9 t DM ha−1respectively. and values of Mo averaged 460 kg N ha−1 and 570 kg N ha−1, which further indicated the lower responsiveness of A. stolonifera to applied N. Apparent recovery of applied N was generally low, with significantly higher values (P&lt;0‐001) for L. perenne in 1983 and 1984, but not in 1985. The botanical composition of L. perenne swards with no applied N showed a steady and rapid decline in ground cover by the sown species. AH L. perenne swards showed an abrupt decrease of L. perenne cover in late 1985, with invasion by A. stolonifera at all but the highest N level (800 kg N ha−1). A. stolonifera treatments receiving 400 or 800 kg N ha−1 showed initial expansion of their native L. perenne populations; however, these declined after the drought in 1984. Only low levels of L. perenne cover remained in the A. stolonifera swards at the end of the trial; these were unaffected by N level. It was concluded that under the conditions of this trial, A. stolonifera had lower DM productivity and efficiency of N use than L. perenne.

Beef production from silages produced from Lolium multiflorum, Lolium perenne and permanent grassland swards

Grass forms a major component of the total ruminant feed in both Ireland and the UK. Much of the grassland consists of permanent swards of diverse botanical composition with many of the grass species present considered ‘secondary’ or ‘undesirable’ (Frame and Tiley, 1987). The objective of this study was to compare reseeded Lolium multiflorum and Lolium perenne swards with an old permanent grassland sward managed under intensive silage conservation systems, and to determine the annual production of grass silage and subsequent beef carcass output per hectare when the silages were offered to finishing cattle.

Response of permanent and reseeded grassland to fertilizer nitrogen. 1. Herbage production and herbage quality

AbstractThe productivity of permanent swards of mixed species composition was compared with that of sown Lolium perenne, cv. Melle at five fertilizer‐N rates (0, 150, 300, 450 and 900 kg N ha−1) and with L. perenne/Trifolium repens, cv. Grasslands Huia at 0 kg N ha−1. The investigation was conducted under two cutting frequencies at sixteen sites in England and Wales, representing a range of grassland environments.Annual total herbage dry matter (DM) production from both permanent and reseeded swards increased with successive increments of fertilizer‐N up to 450 kg N ha−1. Herbage DM production from reseeded swards in the first year after sowing was signficantly higher than from the permanent swards, at all fertilizer‐N treatments. In subsequent years the production advantage of the L. perenne reseeds was maintained only at the higher N rates, though sown L. perenne/T. repens was the most productive sward type at 0 N. Average differences in modified acid‐detergent fibre suggested small advantages in herbage quality to the reseeded swards.It is concluded that, while reseeded swards are more productive in the year after sowing, many permanent swards are capable of high levels of production and that reseeding to a L. perenne sward cannot always be justified, particularly for grassland receiving low or moderate inputs of fertilizer‐N.

Ewe and lamb performance on perennial ryegrass and perennial ryegrass plus white clover swards under controlled sward heights

Lamb performance at grass falls in July/August on upland farms. Allowing sward height to rise has been recognised as having an amoreilating effect. The objective of this trial was to determine the influence of both sward quantity (sward height) and sward quality (sward type) on ewe and lamb performance from mid April to September. An associated objective was the evaluation of the use of clover to replace nitrogen fertiliser.

Management options for increasing the productivity of tropical savanna pastures. IV. Population biology of introduced Stylosanthes spp

In a large-scale grazing experiment in the Monsoon Tallgrass savannas of the Northern Territory, Townsville stylo (Stylosanthes humilis), Caribbean stylo (S. hamata) and perennial stylo (S. scabra + S. viscosa) were sown into native pasture and grazed at three stocking rates, under two fertilizer rates and with trees killed and unkilled. These treatments, together with the occurrence of anthracnose, led to the rapid decline in both yield and numbers of plants in those pastures sown to Townsville stylo. In contrast, yields of Caribbean stylo and Perennial stylo stabilized after three years in both fertilized and unfertilized swards. While total legume yield of these latter two treatments was similar, the dominant contribution in Caribbean stylo was made by annual plants, while in Perennial stylo swards individuals more than 18 months old contributed 80% of the yield.There were marked differences between all species in their population biology. Townsville stylo declined in numbers with death of individuals prior to seed set and the rapid depletion of the soil seed pool which had little long-lived seed. In contrast, in swards of Caribbean stylo a large amount of hard seed rapidly built up in the soil seed pool and ensured continuity of productivity even if seed was not set in some years. In the perennial stylos, seed softened quickly, and the soil seed pool remained small and short-lived. In these latter Stylosanthes spp., long-term stability depended on the survival of perennating plants rather than seed.Pasture management must take into account these different patterns of population dynamics. In particular, while fire is a potentially useful tool in renovating Caribbean stylo swards with their large soil seed pool, it can be extremely detrimental to perennial stylo swards where individuals are susceptible to burning and there is little soil seed available to re-establish the sward.