Grass-legume mixtures sustain strong yield advantage over monocultures under cool maritime growing conditions over a period of 5 years.

Background and Aims: Grassland-based livestock systems in cool maritime regions are commonly dominated by grass monocultures receiving relatively high levels of fertilizer. The current study investigated whether grass-legume mixtures can improve the productivity, resource efficiency and robustness of yield persistence of cultivated grassland under extreme growing conditions over a period of 5 years. Methods: Monocultures and mixtures of two grasses (Phleum pratense and Festuca pratensis) and two legumes (Trifolium pratense and Trifolium repens), one of which was fast establishing and the other temporally persistent, were sown in a field trial. Relative abundance of the four species in the mixtures was systematically varied at sowing. The plots were maintained under three N levels (20, 70 and 220 kg N ha-1 year-1) and harvested twice a year for five consecutive years. Yields of individual species and interactions between all species present were modelled to estimate the species diversity effects. Key Results: Significant positive diversity effects in all individual years and averaged across the 5 years were observed. Across years, the four-species equi-proportional mixture was 71 % (N20: 20 kg N ha-1 year-1) and 51 % (N70: 70 kg N ha-1 year-1) more productive than the average of monocultures, and the highest yielding mixture was 36 % (N20) and 39 % (N70) more productive than the highest yielding monoculture. Importantly, diversity effects were also evident at low relative abundances of either species group, grasses or legumes in the mixture. Mixtures suppressed weeds significantly better than monocultures consistently during the course of the experiment at all N levels. Conclusions: The results show that even in the less productive agricultural systems in the cool maritime regions grass-legume mixtures can contribute substantially and persistently to a more sustainable agriculture. Positive grass-legume interactions suggest that symbiotic N2 fixation is maintained even under these marginal conditions, provided that adapted species and cultivars are used.

Temporal changes in population genetic diversity and structure in red and white clover grown in three contrasting environments in northern Europe.

UNLABELLED: BACKGOUND AND AIMS: Extending the cultivation of forage legume species into regions where they are close to the margin of their natural distribution requires knowledge of population responses to environmental stresses. This study was conducted at three north European sites (Iceland, Sweden and the UK) using AFLP markers to analyse changes in genetic structure over time in two population types of red and white clover (Trifolium pratense and T. repens, respectively): (1) standard commercial varieties; (2) wide genetic base (WGB) composite populations constructed from many commercial varieties plus unselected material obtained from germplasm collections. METHODS: At each site populations were grown in field plots, then randomly sampled after 3-5 years to obtain survivor populations. AFLP markers were used to calculate genetic differentiation within and between original and survivor populations. KEY RESULTS: No consistent changes in average genetic diversity were observed between original and survivor populations. In both species the original varieties were always genetically distinct from each other. Significant genetic shift was observed in the white clover 'Ramona' grown in Sweden. The WGB original populations were more genetically similar. However, genetic differentiation occurred between original and survivor WGB germplasm in both species, particularly in Sweden. Regression of climatic data with genetic differentiation showed that low autumn temperature was the best predictor. Within the set of cold sites the highest level of genetic shift in populations was observed in Sweden. CONCLUSIONS: The results suggest that changes in population structure can occur within a short time span in forage legumes, resulting in the rapid formation of distinct survivor populations in environmentally challenging sites.

Combining winter hardiness and forage yield in white clover (Trifolium repens) cultivated in northern environments.

BACKGROUND AND AIMS: White clover (Trifolium repens) is an important component of sustainable livestock systems around the world. Its exploitation for agriculture in the northern, marginal areas is, however, currently limited by the lack of cultivars that combine persistence and high production potential. The aims are to investigate whether it is feasible to create breeding material of white clover for these areas by combining winter hardiness of northerly populations with good yielding ability of more southerly cultivars. METHODS: A total of 166 crosses of 14 different parental combinations between winter-hardy, low-yielding populations of northern origin and high-yielding commercial cultivars of more southerly origin were tested under field conditions in Iceland and Norway and the parental populations were compared in Norway. Spaced plants were transplanted into a smooth meadow grass (Poa pratensis) sward. Dry matter yield was estimated for 2 years after planting in Norway and morphological characters associated with yielding capacity were measured at both sites. KEY RESULTS: The results showed that southerly cultivars had larger leaves and higher yielding potential than northern types but suffered more winter damage. Significant variation was found between full-sib families within the different parental combinations for all morphological characteristics measured in all three trials. However, it was difficult to detect any consistent morphological patterns between progeny groups across trial sites. No significant correlations were found between leaflet area and survival. CONCLUSIONS: The present study has confirmed that it should be possible to simultaneously select for good winter survival and larger leaves and, hence, higher yielding ability under marginal conditions.

Weed suppression greatly increased by plant diversity in intensively managed grasslands: A continental-scale experiment.

Grassland diversity can support sustainable intensification of grassland production through increased yields, reduced inputs and limited weed invasion. We report the effects of diversity on weed suppression from 3 years of a 31-site continental-scale field experiment.At each site, 15 grassland communities comprising four monocultures and 11 four-species mixtures based on a wide range of species' proportions were sown at two densities and managed by cutting. Forage species were selected according to two crossed functional traits, "method of nitrogen acquisition" and "pattern of temporal development".Across sites, years and sown densities, annual weed biomass in mixtures and monocultures was 0.5 and 2.0 t  DM ha-1 (7% and 33% of total biomass respectively). Over 95% of mixtures had weed biomass lower than the average of monocultures, and in two-thirds of cases, lower than in the most suppressive monoculture (transgressive suppression). Suppression was significantly transgressive for 58% of site-years. Transgressive suppression by mixtures was maintained across years, independent of site productivity.Based on models, average weed biomass in mixture over the whole experiment was 52% less (95% confidence interval: 30%-75%) than in the most suppressive monoculture. Transgressive suppression of weed biomass was significant at each year across all mixtures and for each mixture.Weed biomass was consistently low across all mixtures and years and was in some cases significantly but not largely different from that in the equiproportional mixture. The average variability (standard deviation) of annual weed biomass within a site was much lower for mixtures (0.42) than for monocultures (1.77). Synthesis and applications. Weed invasion can be diminished through a combination of forage species selected for complementarity and persistence traits in systems designed to reduce reliance on fertiliser nitrogen. In this study, effects of diversity on weed suppression were consistently strong across mixtures varying widely in species' proportions and over time. The level of weed biomass did not vary greatly across mixtures varying widely in proportions of sown species. These diversity benefits in intensively managed grasslands are relevant for the sustainable intensification of agriculture and, importantly, are achievable through practical farm-scale actions.

Variation amongst survivor populations of white clover collected from sites across Europe: growth attributes and physiological responses to low temperature.

Experiments were carried out at IGER, Aberystwyth, UK to investigate traits of direct relevance to the processes of overwintering and spring growth in white clover (Trifolium repens L.). The plant material used was derived from baseline populations of the cultivar AberHerald and survivor populations generated after 2-3 years' growth in Germany (Kiel), Sweden (Uppsala) and Switzerland (Zürich). The aims of the experiments were to measure the level of genetic shift that had occurred in certain traits due to selection in the survivor populations by comparing these with the baseline population. The adaptive significance of traits was assessed by determining the extent to which stabilizing selection had operated to reduce levels of intra-population variation. Significant differences were found in the responses of leaf production to two temperature treatments in the survivor populations from Germany and Sweden compared with the Swiss and baseline material. Plants of the former two populations produced much more leaf than the others at the higher temperature, but leaf production rates at the lower temperature did not differ. As this experiment used cloned genotypes in the two treatments, the result suggests that a higher degree of phenotypic plasticity for this trait had been selected for in the German and Swedish populations. These populations also showed greater rates of regrowth of leaves from terminal buds exposed to sub-zero temperatures, but there were no differences between populations in levels of freezing tolerance, or in stolon carbohydrate content. Genetic shift occurred in the degree of unsaturation of stolon lipids, with all three survivor populations possessing higher proportions of unsaturated fatty acids than the baseline. Stabilizing selection also operated on this trait in the survivor populations, suggesting that it is of adaptive significance in cool climates.