Assessment of Cattle Preference for Perennial Ryegrass Varieties in Association with White Clover. Does White Clover Content Interfere?

There is a need to evaluate grass varieties under grazing management to allow selection of varieties to increase animal performance. However, the evaluation of animal performance under grazing with each variety is arduous and time-consuming. An alternative is to describe the preference by grazing animals when given a choice of different varieties. Indeed, there is some indications that preference is related to variety characteristics such as leaf content or digestibility, characteristics having also an impact on the level of intake of this variety. However, preference for grass varieties, when sown alone, may not reflect agricultural practice as grasses often occur in mixed swards, associated with white clover. So, the aim of the present work was to determine whether, under grazing management, cattle expressed selections when given a choice of mixed swards in which different perennial ryegrass varieties were grown in association with a common white clover variety at two levels of fertilizer application (60 or 160 kg N ha⁻¹ annum⁻¹). The preference for perennial ryegrass variety per se was then assessed after taking white clover content into consideration. The results show that there were significant differences in preference for perennial ryegrass varieties, per se, even for varieties with the same heading date and ploidy. The effect of white clover cover on sward selection was more marked in swards which received the higher level of nitrogen fertilizer application which may have been related to a more variable white clover content between swards. This suggests that, under low nitrogen fertilizer application, differences in perennial ryegrass varieties preference can be determined in the same experiments as are used to determine ryegrass varieties persistence in association to white clover. Nevertheless, before such ‘variety preference’ tests can be applied in practice, a validation of the possible correlation between ‘variety preference’ and ‘variety intake’ will be necessary.     

Effects of Competition upon the Productivity of White Clover-Perennial Ryegrass Mixtures. Analysis of and Interrelations between Characters

A diallel crossing programme amongst tour contrasting white clover varieties was used to generate material for studying the interrelationships amongst those characters which affect the productivity of white clover- perennial ryegrass mixtures under a cutting-only management regime. Such a regime favoured clover growth. All the clover characters exhibited strong positive correlations with mixture yield. However, this combination of clover characters did not promote growth of the grass companion. For this a low growing variety with a dense stolon network appeared preferable. Path analysis indicated that differences existed between the two companion grass varieties used for this experiment in their upon clover characters.     

Contribution of White Clover Varieties in High-Productivity Systems under Grazing and Cutting

Two medium leaf size varieties of white clover (Trifolium repens L.) were grown in mixture with a range of grass species and varieties in a field experiment conducted over 4 years. The clovers were grown singly or as a blend in swards with a tetraploid perennial ryegrass (Lolium perenne L.), a diploid Italian ryegrass (Lolium multiflorum L.) or tetraploid hybrids between these two species (Lolium x boucheanum Kunth). These grasses were also grown in monoculture. Swards were managed in a regime involving two periods of sheep grazing, two periods of cattle grazing and a silage cut in the same year for 4 years. The objectives of this experiment were 3‐fold: (1) to examine the yield and persistency of white clover in highly productive systems with modern perennial, hybrid and Italian ryegrasses; (2) to compare the performance of white clover varieties grown singly in mixture with grasses with their performance together as a blend, and (3) to examine the persistency of new Italian ryegrass varieties, exemplified by AberComo. The clover content of the sward remained high throughout the experiment, with a mean of 31 % overall. Differences in clover yield between plots were largely attributable to the effects of companion grasses. Total yields were very similar in plots containing different clover varieties and in plots where these varieties had been grown singly or as a blend. Grass yields in mixture were greater than those in monoculture but showed similar relative rankings. The diploid Italian ryegrass variety AberComo showed an unexpectedly high yield in the 4th year of the experiment. The implications of these results for strategies for productive, sustainable agriculture using modern white clover and ryegrass varieties are discussed.     

Effects of contrasting seasonal growth patterns on composition and persistence of mixed grass‐legume pastures over 5 years in a semi‐arid Australian cropping environment

Perennial grass‐based pastures are uncommon in phased cropping rotations in south‐eastern Australia, where rainfall often limits production and persistence. The high reliance on pure legume‐based pastures limits overall pasture productivity and has adverse effects on environmental parameters such as weed incursion and ground cover. A field experiment was monitored over 5 years to examine the relative productivity of the temperate perennial grasses, phalaris (Phalaris aquatica) and cocksfoot (Dactylis glomerata) and determine whether contrasting seasonal activity enhances their persistence when grown in mixtures with lucerne (Medicago sativa) and annual legumes. Phalaris swards were shown to increase cumulative aboveground biomass by up to 96% and 32%, and in year 5 reduce annual grass emergence by between sevenfold and threefold compared to annual legume and lucerne/annual legume mixed swards, respectively. Only swards that included phalaris maintained ground cover above 70% in each autumn of the experimental period. Swards based on the highly summer dormant cocksfoot cultivar, Kasbah, were generally less productive over all, despite the cocksfoot showing a high level of persistence and good recovery following drought. Grass‐based swards that included lucerne were observed to produce ~35% greater legume biomass than a sward sown only to annual legumes. Cumulative legume biomass was greatest in lucerne swards in the absence of perennial grasses. The experiment was unable to demonstrate a consistent benefit in production and persistence associated with contrasting seasonal growth patterns. However, mixtures containing a range of functional types were consistently shown to enhance productivity over the duration of the experiment compared to less diverse swards. The potential to substantially increase forage productivity and ground cover, while reducing incursion by annual grass weeds, by including well‐adapted perennial grasses in mixtures with lucerne and annual legumes should be utilized.     

行间生草对秦岭北麓猕猴桃园土壤养分及pH的影响

[目的]通过对秦岭地区猕猴桃园进行行间生草,来探讨多年生草对土壤pH、有机质及养分的影响。[方法]在西安猕猴桃试验站4年生的海沃德种植园,设置间作黑麦草、三叶草和林间组合,以自然生草为对照,于种植后第1、第2和第3年萌芽期分别测定果园不同土层的土壤pH、有机质、全氮、全磷、全钾、速效磷、速效钾含量的变化,并采用方差分析对数据进行分析。[结果]随着生草年限的递增,通过生不同的草种,土壤中全氮、有机质含量整体呈递增趋势;当生草年限达到3年时,随着土壤深度的增加,土壤中速效磷、速效钾含量逐渐降低;随着生草年限的递增,不同土层中自然生草的pH先增加后降低,三叶草的pH先降低后增加,黑麦草和林间组合的pH逐渐降低。[结论]在猕猴桃园生草能改良土壤养分状况,提高土壤有机质含量调节土壤pH。不同草种对土壤pH的影响不同,行间生林间组合和黑麦草能够改良北方的盐碱地。     

六种稻田土壤冬季种植黑麦草功能效应研究

选取我国亚热带地区6种主要成土母质发育的水稻土,通过定位试验研究了黑麦草产量、碳氮环境效应和土壤微生物量变化。结果表明,6种稻田土壤冬季均适合生长黑麦草,黑麦草地上部和根系干物质产量,在河沙泥田表现最好,分别为11 324.8 kg hm^-2和8 227.3 kg hm^-2。6种稻田土壤黑麦草地上部和根系碳蓄积量均存在显著差异(P＜0.05),地上部分碳蓄积量在河沙泥田最高,为4 495.3 kg hm^-2;根系碳蓄积量在河沙泥田和麻沙泥田表现最好,分别为2 799.6 kg hm^-2和2 711.8 kg hm^-2;黄泥田最低,为1 852.9 kg hm^-2。而黑麦草氮蓄积量,在河沙泥田最高,地上氮蓄积量为238.1 kg hm^-2,地下氮蓄积量为60.1 kg hm^-2。6种稻田土壤微生物量碳和土壤微生物量氮,在种草区和冬闲田间差异均达到了显著水平(P＜0.05),除灰泥田外种草区均大于冬闲田,冬季种植黑麦草增加了土壤微生物商。     

Productivity under contrasting cutting regimes of perennial ryegrass selected for short and long leaves

The productivity of short-leaved and long-leaved perennial ryegrass was evaluated in 5 different sites. In each site, half-sib families, derived from the first cycle of divergent selection on leaf length, were managed both under infrequent cutting and under frequent cutting simulating grazing. Despite the importance of the effects of the site and the cutting frequency, the dry matter yield was dependent on the interaction between the morphogenetic type resulting from the selection and the cutting frequency imposed by the management. Thus, under infrequent cutting, the long-leaved families on average out-yielded the short-leaved families. Conversely, under frequent cutting the short-leaved families were on average the most productive. The use of morphogenetic traits as selection criteria in breeding for management-dependent yield is briefly discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Improved seed yield in perennial ryegrass (Lolium perenne L.) from two generations of phenotypic selection

Two generations of recurrent phenotypic selection for seed yield per plant under controlled pollination were carried out to combine improved seed yield with improved forage grass performance in perennial ryegrass (Lolium perenne L.). Selected and unselected varieties (AberDartand AberElan respectively) and control varieties were grown for seed in pots in a glasshouse experiment and in two field plot experiments over 5 harvest years. Seed yield components and seed yield were measured. Significant improvements in seed yield of the selected AberDart over the unselected AberElan were observed both in individual plants grown in the glasshouse and in field plots. Detailed observations of the individual components of seed yield showed that the increased seed yield of AberDart can be attributed to a higher proportion of ovules forming seeds (% seedset), greater seed number per tiller and more reproductive tillers per plant. This improvement in seed yield of AberDart was also observed when it was grown in field plots with AberElan, experimental selection lines and commercial varieties. AberDart produced more seeds per tiller than the other commercial varieties and selection lines studied. The implication of this approach for the breeding of perennial ryegrass varieties that combine good agronomic performance and acceptable seed yields is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Late heading of perennial ryegrass caused by introducing an Arabidopsis homeobox gene

Perennial ryegrass (Lolium perenne L.) is the most important temperate forage grass species. Unfortunately, the nutritional value of perennial ryegrass declines as maturity progresses, mainly because of a high concentration of poorly digestible compounds in inflorescences. Therefore, the development of forage‐type ryegrass varieties with extended vegetative growth is of interest for agriculture. To delay floral transition in perennial ryegrass the Arabidopsis ATH1 gene driven by the maize ubiquitin promoter, the rice actin promoter or the rice OSH1 promoter, respectively was introduced. In ATH1‐expressing plants heading was delayed, and in a number of cases the plants never flowered at all. Such non‐ or late‐heading was accompanied by the outgrowth of normally quiescent lateral meristems into extra leaves, resulting in a leafy growth habit. When eventually heading, these plants generally produced a reduced number of inflorescences. These observations suggest that ATH1‐mediated delay of heading may be useful to improve fodder quality of perennial ryegrass.     

Effects of Competition upon the Productivity of White Clover-Perennial Ryegrass Mixtures. Seasonal Trends

Seasonal trends among those characters, affecting productivity in white clover-perennial ryegrass mixtures were investigated m a set of 20 mixtures using clover material derived from a. diallel cross between four contrasting white clover varieties. The productivity and associated vegetative characters showed the familiar pattern of high grass production in spring, and early summer, with a secondary peak of growth in autumn, whilst clover achieved its; highest productivity levels in mid-summer. A significant linear relationship was established between clover canopy height and ryegrass canopy height for all 20 mixtures. Although no significant heterogeneity was detected among the fitted regression lines, there was an indication that the linear regressions provided a better fit to the data in those mixtures having ‘Skomer’ as the grass companion. An analysis of the differences in canopy height between ryegrass and clover indicated that the ‘Menna ‘/’S. 23’ mixture had a reduced seasonal range compared to the other mixtures. Consequently, during the early par', of the growing season, when the tallest canopies occurred and. when shading of clover by the grass was most intense, the difference in canopy height between ‘S. 23’ and ‘Menna’ was relatively low. There was a pronounced effect of grass companion upon canopy height difference, due entirely to the taller growth habit of the ‘Skomer’ ryegrass variety, These interrelationships are discussed briefly in the context of improving the productivity of white clover-perennial ryegrass mixtures.     

The Persistence of Tetraploid Perennial Ryegrass in a Mixture with Diploid Perennial Ryegrass

An investigation was carried out into the survival of two tetraploid varieties of perennial ryegrass ( Lolium perenne L.), sown in a mixture with a diploid and very intensively grazed for over five years.
Mixture M1 consisted of the late heading varieties Meltra and Vigor , and mixture M2 of the early heading varieties Merlinda and Melino.
The ploidy was determined by microscopic analysis.
The share of the tetraploid component decreased in both mixtures. The decrease was more pronounced in the early heading mixture. Information was also sought on the technique of sampling such swards.     

Nitrogen Uptake by Italian Ryegrass after Destruction of Non-fertilized Set-aside Covers at Different Times in Autumn and Winter

The introduction of the European set-aside into the rotation can modify the nitrogen availability for subsequent crops. With a view to evaluating this effect, three non-fertilized covers were sown in May 1995. These covers were ploughed in at three different times: October 1995, December 1995 and March 1996. The ploughed-in nitrogen quantities did not vary greatly with destruction time. The greatest quantities were incorporated into red clover plots and the smallest quantities into perennial ryegrass plots. The C/N of perennial ryegrass was on average 24 and that of red clover 14.
The soil cover by clover just before ploughing is significantly related to the ploughed-in nitrogen quantities.
An Italian ryegrass was sown as the subsequent crop to the set-aside. Overall, during the whole year 1996 and 1996–1997, there was no difference between the yields for different destruction times except for the first cut of 1996.
The highest production was obtained after red clover and the lowest after perennial ryegrass. The relationship between the nitrogen amount or C/N of the ploughed-in material and the nitrogen export for the annual harvest of Italian ryegrass showed highly significant correlations in 1996 as well as for the 2-year harvest. Two multiple regressions were calculated in order to take into consideration the characteristics of the ploughed-in material of the set-aside.
The measured percentage of soil cover by clover before ploughing and the annual nitrogen export of Italian ryegrass were also significantly related in 1996 and for both years.     

Productivity and quality,competition and facilitation of chicory in ryegrass/legume-based pastures under various nitrogen supply levels

Traditional perennial ryegrass–white clover mixtures have limitations in combined productivity and quality that herbs like chicory may alleviate. This study examined the consequences on productivity and quality of as well as competition and facilitation after introducing chicory into varies ryegrass–legume-based pastures in a field study over three consecutive growing seasons. A cultivar of chicory, suitable for grazing, in pure stand was found to out-yield a pure stand ryegrass in terms of dry matter and nitrogen (N) accumulation but was found to yield similar to mixtures of chicory and ryegrass. The inclusion of chicory, increased N accumulation per area unit irrespective of associated leguminous species but had no effect (P > 0.05) on the combined dry matter yield of these mixtures as compared to the chicory–ryegrass mixture. Chicory was not found to co-exist well with associated fodder legumes but it co-existed well with perennial ryegrass. Determined by a direct ¹⁵N plant labelling technique, chicory transferred little N to associated legumes and under moderate soil N conditions it almost out-competed the white clover whereas lucerne was able to withstand the competition with birdsfoot trefoil as intermediate. Chicory and ryegrass did exchange N amounting to less than 5% of the receiver plants’ N economy whereas the N transfer from the N-rich lucerne constituted 15% of the associated ryegrass’ N economy but less (P < 0.05) of the chicory's N economy. These differences are ascribed to the species’ root morphology and root zonation. Chicory accumulated large amounts of calcium, potassium, sodium and zinc but significant less of magnesium and manganese, irrespective of the N supply. In the case of sodium it was a short-term effect whereas calcium and possibly also sulphur, copper and zinc accumulation increased over time. It is concluded that chicory may improve the management of intensive dairy farms with a large N surplus because of the increase in productivity per unit area and N uptake efficiency and add significant improvements of the quality of the forage.     

Biological nitrogen fixation in three long-term organic and conventional arable crop rotation experiments in Denmark

Biological nitrogen (N) fixation (BNF) by legumes in organic cropping systems has been perceived as a strategy to substitute N import from conventional sources. However, the N contribution by legumes varies considerably depending on legumes species, as well as local soil and climatic conditions. There is a lack of knowledge on whether the N contribution of legumes estimated using short-term experiments reflects the long-term effects in organic systems varying in fertility building measures. There is also limited information on how fertilizer management practices in organic crop rotations affect BNF of legumes. Therefore, this study aimed to estimate BNF in long-term experiments with a range of organic and conventional arable crop rotations at three sites in Denmark varying in climate and soils (coarse sand, loamy sand and sandy loam) and to identify possible causes of differences in the amount of BNF. The experiment included 4-year crop rotations with three treatment factors in a factorial design: (i) rotations, i.e. organic with a year of grass-clover (OGC), organic with a year of grain legumes (OGL), and conventional with a year of grain legumes (CGL), (ii) with (+CC) and without (−CC) cover crops, and (iii) with (+M) and without (−M) animal manure in OGC and OGL, and with (+F) mineral fertilizer in CGL. Cover crops consisted of a mixture of perennial ryegrass and clover (at the sites with coarse sand and sandy loam soils) or winter rye, fodder radish and vetch (at the site with loamy sand soil) in OGC and OGL, and only perennial ryegrass in CGL at all sites. The BNF was measured using the N difference method. The proportion of N derived from the atmosphere (%Ndfa) in aboveground biomass of clover grown for an entire year in a mixture with perennial ryegrass and harvested three times during the growing season in OGC was close to 100% at all three sites. The Ndfa of grain legumes in both OGL and CGL rotations ranged between 61% and 95% depending on location with mostly no significant difference in Ndfa between treatments. Cover crops had more than 92% Ndfa at all sites. The total BNF per rotation cycle was higher in OGC than in OGL and CGL, mostly irrespective of manure/fertilizer or cover crop treatments. There was no significant difference in total BNF between OGL and CGL rotations, but large differences were observed between sites. The lowest cumulated BNF by all the legume species over the 4-year rotation cycle was obtained at the location with sandy loam soil, i.e. 224–244, 96–128, and 144–156 kg N ha⁻¹ in OGC, OGL and CGL, respectively, whereas it was higher at the locations with coarse sand and loamy sand soil, i.e. 320–376, 168–264, and 200–220 kg N ha⁻¹ in OGC, OGL and CGL, respectively. The study shows that legumes in organic crop rotations can maintain N₂ fixation without being significantly affected by long-term fertilizer regimes or fertility building measures.     

Testing perennial ryegrass (Lolium perenne L.) as spaced plants in swards

Summary A method of testing spaced plants in swards of another grass crop is described. The procedure provides information on sward performance at each stage of the breeding scheme. A large number of entries may be included, together with whatever cutting frequency.When used for perennial ryegrass, the method favours the selection of productive and persistent plants.     

多年生黑麦草分子标记应用及基因组研究进展

多年生黑麦草(Lolium perenne L.)是一种重要的栽培牧草,具有多年生、植株繁茂、叶多质嫩、可消化物质产量高等特征,适宜用作饲草及草坪草,具有经济价值和生态效益。本综述总结概括了多年生黑麦草分子标记的开发与应用进展,重点介绍分子标记在多年生黑麦草遗传多样性分析、遗传连锁图谱构建、QTL定位分析中的应用情况,并简述了多年生黑麦草基因组研究进展。旨在为进一步发掘多年生黑麦草基因组结构和功能特点,选育优良黑麦草品种提供理论依据。     

Summer drought decreases Leymus chinensis productivity through constraining the bud,tiller and shoot production

Extreme drought events can directly decrease productivity in perennial grasslands. However, for rhizomatous perennial grasses it remains unknown how drought events influence the belowground bud bank which determines future productivity. Ninety‐day‐long drought events imposed on Leymus chinensis, a rhizomatous perennial grass, caused a 41% decrease in the aboveground biomass and a 28% decrease in belowground biomass. Aboveground biomass decreased due to decrease in both the parent and the daughter shoot biomass. The decreases in daughter shoot biomass were due to reductions in both the shoot number and each individual shoot weight. Most importantly, drought decreased the bud bank density by 56%. In addition, drought induced a bud allocation change that decreased by 41% the proportion of buds that developed into shoots and a 41% increase in the buds that developed into rhizomes. Above results were supported by our field experiment with watering treatments. Thus, a 90‐day‐long summer drought event decreases not only current productivity but also future productivity, because the drought reduces the absolute bud number. However, plasticity in plant development does partly compensate for this reduction in bud number by increasing bud development into rhizomes, which increases the relative allocation of buds into future shoots, at the cost of a decrease in current shoots.     

生草栽培对柑桔丛枝菌根形成及果实品质的影响

试验以枳砧锦橙(CitrussinensisOsbeck.)初结果树为试材。试验设4个处理,分别为柑桔园间作百喜草(Bahiagrass)(PaspalumnotaturnFlugge)、白三叶草(Whiteclover)(TrifoliumrepensL.)、紫花苜蓿(Alfalfa)(MedicagosativaL.)和多年生黑麦草(Perennialryegrass)(LoliumperenneL.),以清耕为对照。单行小区,随机区组排列,3次重复。结果表明,在干旱季节,所有生草处理区都显著改善了根际土壤水分和温度状况,而百喜草和白三叶草生草区土壤丛枝菌根真菌孢子数和柑桔根系菌根侵染率显著高于清耕区和其它生草区。表明在干旱季节,百喜草和白三叶草生草栽培促进了果园土壤丛枝菌根真菌的生长和柑桔根系菌根的形成,从而促进了柑桔根系对水分和磷素营养的吸收利用。试验结果表明果园管理中宜选择菌根侵染率高的草种如百喜草、白三叶草在柑桔园进行生草栽培,促进柑桔根系菌根形成,改善植株生理代谢,提高果品质量。     

Management of Italian and Perennial Ryegrasses for Seed and Forage Production in Crop Rotations

Italian ryegrass (Lolium multiflorum Lam.) and perennial ryegrass (L. perenne L.) can be grown for seed and forage in cold winter regions provided the stand persists well over winter. Seed yield and plant characteristics during primary growth, and forage yield during regrowth, were determined for two Italian and one perennial ryegrass cultivars in Atlantic Canada. Establishment methods and dates included sowing ryegrass in cultivated soil alone or with barley in mid‐May and, after harvesting the barley crop, by sowing ryegrass following conventional or reduced cultivation and by no‐till drilling into barley stubble in mid‐August and early September. Despite some winterkill, particularly in Italian ryegrass, seed and forage yields were adequate in post‐establishment growing seasons. Seed yield for Italian ryegrass was greatest (1270 kg ha^?1) when it was sown into cultivated soil in mid‐August and least (890 kg ha^?1) when sown alone in May. Italian ryegrass yielded 15–17 % more seed when plots were established in mid‐August rather than in mid‐May or early September. Italian ryegrass cv. Lemtal had a greater density of fertile tillers (1030 m^?2) in the sward than cv. Ajax (860 m^?2) and its tiller density was greater when seeded into cultivated soil in September than in mid‐August. There were fewer spikelets per seed head for sowing Italian ryegrass with barley in May than for the other methods of establishment. Forage yield in regrowth was greater for Italian ryegrass cv. Ajax (2770 kg ha^?1) than for cv. Lemtal (2480 kg ha^?1). Seed yield of perennial ryegrass was greater when seeded in mid‐May than in mid‐August or early September. The seed yield of perennial ryegrass was greater when it was sown with barley in May and harvested for grain, than when it was sown alone or with barley harvested at late milk stage. The establishment methods for mid‐August and early September sowing had little effect on seed yield. However, the no‐till and reduced tillage methods resulted in a greater tiller density than sowing into the cultivated seedbed. Fertile tillers tended to be denser under reduced cultivation for sowing in August. Forage yield of perennial ryegrass regrowth was not influenced by the sowing method and timing. In conclusion, Italian and perennial ryegrasses produce adequate seed and forage regrowth under different establishment methods and timing. However, the poor persistence of Italian ryegrass may limit commercial production after the establishment year in Atlantic Canada.     

Symbiotic Nitrogen Fixation of White Clover in a Mixed Sward is not Limited by Height of Repeated Cutting

The aim of this study was to investigate whether symbiotic nitrogen fixation in white clover nodules limits nitrogen supply and hence clover growth by repeated defoliation at two cutting heights. Other possible factors governing symbiotic nitrogen fixation in the field were also elucidated. Using ¹⁵N, a 2-year field experiment including white clover ( Trifolium repens L. cv. Ladino) and perennial ryegrass ( Loliumperenne L. cv. Bastion) in monocultures and in mixtures was conducted in Eschikon, Switzerland. The effect of two cutting heights (4 cm and 10 cm above ground level) on the performance of symbiotic nitrogen fixation of white clover in the different sward-types was investigated. After each harvest, the plots were fertilized with 3 g N m^-2(equivalent to 30 kg N ha^-1 cut^-1 or 210 kg N ha^-1 year^-1). In both years, white clover grown in a mixture with grass received a significantly higher percentage of nitrogen from symbiotic fixation compared with clover grown in monoculture. This phenomenon is attributed to the strong competitiveness of ryegrass in soil nitrogen uptake. Consequently, white clover in the clover-ryegrass mixture was more dependent on symbiotic nitrogen fixation than that grown in monoculture. The cutting height did not preferentially influence symbiotic nitrogen fixation, as opposed to the uptake of mineral nitrogen from the soil. From this finding it is suggested that symbiotic nitrogen fixation did not limit the supply of nitrogen to clover and hence its growth. It is proposed that symbiotic nitrogen fixation in white clover is regulated by the demand for nitrogen rather than by the availability of carbohydrate reserves in the stolons. Symbiotic nitrogen fixation should thus be looked upon as an integrated plant growth factor and not as an isolated phenomenon.