Potential for greater use of forage legumes in UK grassland systems for improved biodiversity, soil quality, and livestock nutrition and health
-for-
The Department for the Environment, Food and Rural Affairs
-by-
Rosemary P. Collins and Michael Fothergill
Institute of Biological, Environmental and Rural Sciences, Aberystwyth University, UK
Contents
Heading Page
Summary…………………………………………………………………. 3
List of abbreviations……………………………………………………. 4
Introduction………………………………………………………………. 5
Chapter 1. Biodiversity
Introduction…………………………………………… 10
(i) Plant diversity………………………………………… 11
(ii) Invertebrates and birds……………………………….. 20
(iii) Soil microbial diversity………………………………. 41
References……………………………………………. 43
Chapter 2. Soil Quality
Introduction………………………………………….. 52
(i) Nutrient content……………………………………… 53
(ii) Soil structure…………………………………………. 59
(iii) Soil organic matter…………………………………… 64
(iv) Microbial communities……………………………….. 66
(v) Soil fauna……………………………………………... 68
References…………………………………………….. 69
Chapter 3. Livestock Nutrition and Health
Introduction………………………………………….. 75
(i) Production of traceable home grown protein………… 76
(ii) Nutrition, utilisation and yield……………………….. 79
(iii) Protein protection by phenolic compounds ………….. 87
(iv) Effects of secondary metabolites ………………...…… 89
(v) Phytoestrogens……………………………………….. 90
(vi) Effect on parasitic diseases…………………………… 91
(vii) Bloat………………………………………………….. 92
References……………………………………………. .94
Chapter 4. Conclusions and Recommendations…………………... 101
References……………………………………………..106
SUMMARY
Grassland farming in the UK is experiencing a period of great change due to an array of economic, social and environmental pressures. These pressures, together with the continued strength of the organic sector, plus an increasing awareness of the flexibility of legumes for use in intensive systems have driven current interest in sustainable grassland agriculture based on swards containing forage legumes. Maintenance or improvement of grassland biodiversity and soil quality, combined with reductions in emissions from livestock production and improved animal health are potential benefits of increased use of legumes in UK grassland systems. This study evaluates the evidence for these benefits and suggests ways in which various forage legumes species could be incorporated successfully into multifunctional grasslands.
The overall conclusion of this study was that there would be considerable benefits from and scope for greater incorporation of forage legumes into UK grassland systems. Specific conclusions reached were:
· The incorporation of key plant functional groups into multispecies grassland assemblages will result in improved yield stability (i.e. permanence) through various mechanisms (eg. niche differentiation; facilitation etc.)
· Improving the perenniality and permanence of farmed grassland through the greater use of multispecies grass/legume mixtures will lead to improved environmental outcomes, whilst maintaining economically acceptable production levels
· Complementary patterns of resource use by components of multispecies mixtures will result in improved ecosystem functioning eg. nutrient retention; maintenance of high forage quality through reduced sward invasion by unsown species
· The inclusion of plants with a range of above-ground morphologies in grass/legume multispecies mixtures will promote an increase in the number and range of habitats suitable for invertebrates and birds via cascading effects of diversity up the food chain
· Soil microbial diversity will be enhanced in multispecies grass/legume mixtures through the creation of an array of root types and root exudates
· Biologically significant aspects of soil quality (eg. porosity) will be enhanced in legume-based mixtures
· The inclusion of forage legume species in animal diets has the potential to bring about measurable benefits in terms of improving the quality of meat and milk, increasing the efficiency of nutrient use and reducing the environmental footprint of grassland agriculture
· Delivery of condensed tannins to the diet through the inclusion of appropriate forage legume species has the potential to improve important aspects of the health and welfare of ruminant animals
List of abbreviations
AMF: arbuscular mycorrhizal fungi
ATP: adenosine triphosphate
BNF: biological nitrogen fixation
CAP: EU’s Common Agricultural Policy
CP: crude protein
CT: condensed tannin
DM: dry matter
GHG: greenhouse gases
Lotus: used generically to refer to Lotus corniculatus and L. pedunculatus
ME: metabolisable energy
N: nitrogen
NIAB: National Institute of Agricultural Botany (UK)
NVZ: nitrate vulnerable zone
OM: organic matter
P: phosphate
PPO: polyphenol oxidase
VCU: ‘Value for Cultivation and Use’ plant variety trials
VFA: volatile fatty acids
WSC: water soluble carbohydrate
INTRODUCTION
‘Multifunctionality’ gained formal political acceptance as a valid concept at the 1992 Rio Earth Summit and has gradually been incorporated into agricultural policy-making. The final recommendations of the Versailles Conference, held in December 2000 on ‘Agricultural Research in the European Research Area’ stated: ‘The multifunctionality of European agriculture is a key concept. Developing a multifunctional agriculture entails research towards a competitive agriculture which is environmentally benign, stimulates rural development, uses natural resources in a sustainable manner, while producing sufficient quantities of safe and affordable food and non-food products, high in quality and diversity and respectful of nature’ (Huyghe, 2002). Within the wider context of multifunctionality there is currently greater scientific emphasis on developing farming practices that maintain or improve the natural resource base of agriculture, which includes the quality of soil and the biodiversity of organisms living in and on it (McKenzie et al., 2006). The aim of this desk study is to evaluate the potential contribution of forage legume species to achieving this aim within the context of both conventional and organic grassland-based agriculture. In the latter, ‘grassland’ includes short-term leys, as much research has been carried out on various aspects of this phase of organic systems and the results are relevant for longer-term organic and conventional grassland. The capacity of forage legumes to improve the nutrition and health of livestock will also be considered, as this has implications for the economics of livestock production, the contribution of livestock production to various forms of pollution, and for animal welfare.
Grassland covers some 5.6 million ha of agricultural land in the UK (Mortimer et al., 2006). The term ‘grassland’ encompasses a wide range of managements: it can be cut for silage or grazed by livestock; it may be part of a ley-arable rotation or a more permanent grassland system (Davies et al., 2001). All these managements may be carried out in accordance with organic farming regulations and certified by the UK Soil Association, or may be carried out in a ‘conventional’, non-organic manner. In the non-organic grassland sector there is considerable diversity in the identity of nitrogen (N) sources for crop growth, including animal manures, high inputs of synthetic fertiliser N or the use of legumes plus ‘tactical’ applications of smaller amounts of fertiliser N eg. to boost sward growth in spring. In the organic sector, forage legumes play an important role in the fertility-building phase of arable rotations, into which livestock production is frequently integrated in order to provide an economic return. Nutrient inputs in these mixed organic systems are augmented by animal manures (Oehl et al., 2004). Stockless organic rotations are common in continental Europe and in the drier, eastern regions of the UK. In such systems the emphasis is on fertility-building strategies such as the use of green manures and grain legumes. Longer term grassland is also acceptable within organic regulations, with an emphasis on the maintenance of soil fertility through nutrient recycling and minimal external inputs (Watson et al., 2002). Forage legumes play a pivotal role in these systems. Over 80% of organically managed land in the UK is devoted to grassland – either rough grazing or permanent pasture – with a further 9% used as temporary leys within arable crop rotations (Defra, 2002). However, the organic grassland sector itself is small, comprising just 4% (in terms of area) of UK grassland agriculture as a whole.
Legumes have traditionally been important components of both grassland and arable farming systems (as a break crop) because they form a symbiotic association with Rhizobium bacteria in the soil enabling them to fix atmospheric nitrogen into forms that are chemically available to plants (‘biological nitrogen fixation’ – BNF). They therefore supply ‘free’ N not only to themselves but also to the surrounding soil and hence to companion plant species. Forage legumes have also been highly regarded in agriculture because they possess superior feeding value, a function of both nutritive value and voluntary intake, in relation to grasses (Frame et al., 1998). It is now well-established that greater utilisation of forage legumes would capitalise on the attributes of these species to deliver improved animal performance and ‘healthier’ fatty acid profiles in meat and milk (Davies and Hopkins, 1996; Dewhurst et al., 2006). In addition, the correctly managed incorporation of legumes has the potential to reduce, or even eliminate, requirements for expensive synthetic fertilisers and concentrates and thus contribute positively to the economics of livestock production from grassland (Doyle and Bevan, 1996). Another factor that should encourage the greater uptake of forage legumes by grassland farmers has been the significant improvement in their agronomic performance (including grazing tolerance, winter hardiness, spring growth, drought tolerance, persistence and tolerance of applied N) and seed yield brought about by plant breeding efforts around the world in the last 20 years or so (eg. Rhodes et al., 1994; Woodfield and Caradus, 1994; Jahufer et al., 2002; Abberton and Marshall, 2005). Nevertheless, despite all the putative advantages of forage legumes, non-organic grassland in the UK is currently predominantly based on perennial ryegrass (Lolium perenne) receiving substantial inputs of synthetic nitrogenous fertilisers (Frame et al., 1998). Whilst legumes have always played a pivotal role in organic systems, in many regions of the world, including the UK, the use of legumes in non-organic agriculture declined dramatically in the latter half of the twentieth century due to the availability of cheap synthetic nitrogenous fertilisers (eg. urea; ammonium nitrate). Data from the International Fertiliser Industry Association illustrate the rapid adoption of synthetic fertilisers by farmers: global consumption of nitrogenous fertilisers was estimated as 20 million tonnes per year in 1960 and 140 million tonnes per year in 2000 (http://www.fertilizer.org/statistics). However, in recent years sharp upturns in the price of fossil fuel, upon which the production of fertilisers is very dependent, combined with increasing concern about the impact of the large quantities of greenhouse gases (GHG) emitted as a consequence of the manufacture of synthetic fertilisers (eg. the industrial production of ammonium nitrate from atmospheric N using the Haber-Bosch process emits CO2 and NO3) have led to a reappraisal of the role of legumes in temperate agriculture. It has been estimated that the production of synthetic fertilisers is currently responsible for 1.2% of the UK’s total GHG emissions (Defra, 2008). As a result of all the above factors annual UK seed sales of forage legumes have tended to increase in recent years. For example, red clover (Trifolium pratense) seed sales have increased sharply from 55 tonnes in 1997 to 134 tonnes in 2004 (Defra, 2004). Sales of white clover (Trifolium repens) seed in 2004 were 345 tonnes, whilst Lotus and lucerne (Medicago sativa) tonnages were 13 and 18 respectively (Defra, 2004).
Grassland agriculture in the UK has become more responsive to political, economic, social and environmental pressures. Following major reform of the EU’s Common Agricultural Policy the emphasis on quality produce from UK grassland-based agriculture has become an increasingly important aspect of competitiveness. The reduction in payment subsidies for production to EU farmers has reduced the economic optima for various inputs, including N fertiliser, making farming systems based on BNF from legumes more attractive (Ledgard, 2001). Whilst it has been predicted that a greater proportion of grassland farming in the EU in the future will be carried out at low-to-moderate intensities (Frankow-Lindberg and Frame, 1996), it is also necessary for food security that a core of high-input, intensive grassland production continues to exist. Against this economic background there is a growing need for grassland agriculture not only to deliver high value products efficiently, but to do so with reduced impact on the environment. In particular, decreases in diffuse pollution of water and air, including GHG, are necessary if livestock production from grassland is to play its part in meeting UK government objectives with respect to climate change and water quality. International agreements require the UK to reduce the level of diffuse pollutants from agriculture eg. EU Nitrate Directive (91/676/EEC), which the UK has implemented, and the EU Water Framework Directive (2000/60/EC), which requires all inland and coastal waters to reach ‘good status’ by 2015 (www.defra.gov.uk). The former directive has led to the establishment of Nitrate Vulnerable Zones (NVZs) in the UK in which the amount of organic manure and inorganic fertiliser that can be applied to pastures is limited, with the concomitant effect of bringing about reductions in productivity (Scholefield, 2003). An alternative approach to maintaining soil N levels involves increased reliance on BNF in pastures, with one or more legume species growing in association with one or more grasses. The viability of this strategy in terms of maintaining grassland productivity whilst reducing its environmental impact was recently examined for UK conditions by Andrews et al. (2007), who concluded that within the stocking rate restrictions imposed on NVZs white clover could provide the N required by a pasture at a lower financial cost than that incurred by the application of N fertiliser. In addition, GHG associated with fertiliser manufacture and application would be avoided by using the white clover/perennial ryegrass association. A comparison of the fossil-energy expenditure in a grass/N fertiliser system and a grass/clover system reliant on BNF, both receiving 200 kg N/ha, showed that the latter expended only 5% of the fossil energy of the grass/N fertiliser system (Wood, 1996). In temperate areas with well developed grassland-based livestock production systems appropriate forage legumes are heavily relied upon eg. New Zealand (white clover), Australia (subterranean clover) and the USA (alfalfa/lucerne). In the UK, however, the contribution of forage legumes to grassland agriculture is comparatively low. For example, annual BNF carried out by legumes has been estimated as producing 1.57 million tonnes of N in New Zealand (Caradus et al., 1996) and only 80,000 tonnes of N the UK (Ball and Crush, 1985), despite the two countries having similar areas of grassland. There is considerable scope, then, for greater incorporation of legumes into UK grassland systems, if only to take full advantage of the benefits of BNF. The potential additional benefits of legumes in terms of enhancing grassland biodiversity, soil quality, and animal nutrition and health greatly strengthen the case for a re-evaluation of their role.