Implications
Considerable advances have been made in improving the yield and quality of forage crops through selective breeding. This review addresses how collaboration between plant and animal science, use of emerging techniques and identification of novel targets for forage breeding can contribute to mitigating future challenges to the ruminant sector in terms of ensuring a sustainable feed supply while minimising negative outputs from livestock farming.
Introduction
The world population is predicted to increase by 50% between 2000 and 2050 to 9 billion people. This presents agricultural scientists with a significant challenge in delivering strategies to increase food production while not compromising quality or choice, and to achieve this in a sustainable, environmentally benign way (Tilman et al., Reference Tilman, Cassman, Matson, Naylor and Polasky2002). Animal production is a fundamental component of the food supply chain but developing sustainable systems of animal production that do not compete directly with mankind for food is imperative. Ruminants have a vital role to play in this scenario because they can convert plant material not suitable for human consumption into a high-quality protein product. Indeed global demand for ruminant product is expected to increase in line with population growth; by 2050 milk production is expected to rise from 580 to 1043 million tonnes (Food and Agriculture Organization (FAO), 2006) and meat from cattle and sheep is expected to increase from 70 to 127 million tonnes. Ruminant farming is conducted under both intensive (grain fed) and extensive (pasture-based) systems. We discuss below how our increased understanding of the nutritional requirements of ruminants has enabled significant progress to be made in terms of the quality of forage crops grown on improved pasture land, which is translated into improved animal performance. Continual advances are being made in our understanding of the ruminant animal, and in particular the operation of the complex ecosystem that is the rumen, from where many feed use inefficiencies arise. Indeed, it is likely that increased rumen efficiency will come from both increased understanding of the nutritional requirements of the functional rumen microbial ecosystem and identification of how improved varieties of different forage species can deliver those requirements. Historically, forage breeding has successfully delivered improved products for consumption by animals, and the rate of plant improvement is set to increase using new breeding technologies (Moose and Mumm, Reference Moose and Mumm2008). There is currently considerable opportunity for collaboration between plant and animal sciences to identify how the nutritional requirements of ruminants can be satisfied by high-quality forages, to identify key target traits for the future and also to address the next challenges, particularly the ability of plant breeding to manipulate traits relating to enhanced productivity while minimising environmental impact. This review focuses on conventional plant breeding methods, as opposed to genetic modification of crops for specific properties. The latter point has been covered previously by Ronald (Reference Ronald2011).
Pasture-based systems for nutrient delivery
Lamb, milk and beef production in the United Kingdom relies heavily on grazed home-grown forage grasses, legumes and other forages to support productivity, predominantly from mixtures of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) or red clover (Trifolium pratense L.). This brings with it challenges not encountered with feeding strategies based on conserved forages or concentrate feeds. For example, there is considerable potential to manipulate the formulation of a dairy cow ration based on conserved feeds and concentrate supplements in order to improve the efficiency of milk production. The relatively stable nature of such feeds means that analyses of chemical composition can be used to reliably formulate diets for a herd to optimise productivity and minimise losses. However, the composition of grazed forages changes considerably over a range of timescales, from within a day to across the growing season. Formulation of efficient diets that include grazed forages is much more difficult; unsupplemented grazing cows depend entirely on the forage sward for nutrient supply, which may not be optimally balanced for milk production, and supplementation with concentrate feeds is expensive and does not necessarily help balance nutrient supply at the within-day timescale. Therefore, for balanced nutrient supplies at grazing, choice of plant species and varieties within a sward is crucially important. Hoekstra et al. (Reference Hoekstra, Schulte, Struik and Lantinga2007) recently highlighted three ways of increasing the efficiency of grazing cattle: (1) matching protein supply to animal requirements, (2) balancing and synchronising carbohydrate and nitrogen supply in the rumen and (3) increasing the proportion of rumen undegradable protein. Balancing the supply of carbohydrates and proteins to improve the efficiency of rumen protein utilisation (2) can lead to the better matching of protein supply to animal requirements (1), and this, together with (3), can be achieved through appropriate choice and management of plant species (some of which contain secondary compounds that can help reduce rumen protein degradation) and varieties in a grazing sward.
Breeding for yield
Selective forage plant breeding has supported crop improvement for nearly a century. Until relatively recently, the primary objective of forage grass breeding programmes was increasing dry matter (DM) yield. This was largely successful, with yield gains of 4% to 5% per decade for perennial ryegrass (Wilkins and Humphreys, Reference Wilkins and Humphreys2003). Indeed, the rate of gain in DM yield achieved during the last 50 years by hybridisation and recurrent selection within elite breeding material is predicted to be maintained for at least another 10 generations, which equates to 30 years for perennial ryegrass (Wilkins and Humphreys, Reference Wilkins and Humphreys2003). For white clover, breeding programmes have focused on improving traits that ensure that there is a reasonable proportion of white clover in the sward (normally 30% over the growing season). It is predicted that yield will remain an important target for forage improvement, particularly in relation to underpinning our potential to achieve sustainable intensification of agriculture (Foresight, 2011), which will require more crops to be grown on a smaller land area.
Trends in breeding for quality
Quality traits have become increasingly important targets for forage breeding programmes (Stewart and Hayes, Reference Stewart and Hayes2011). Recent published data highlights improvements in in vitro DM digestibility of intermediate heading diploid perennial ryegrass varieties such as AberMagic and AberSweet in comparison with the control variety Talbot, and of late heading perennial ryegrass varieties such as AberChoice in comparison with S23 (Wilkins and Lovatt, Reference Wilkins and Lovatt2011). Although the basic targets of plant breeding programmes over the last few decades (yield, digestibility, persistency and disease resistance) have remained the same, recent advances in the understanding of the use of plant nutrients by the animal has led to improvements in the efficiency of use of grass nutrients for animal production. This increase in forage quality has accompanied improvements in animal production potential through the selection of high genetic merit herds and flocks; as animal genetics develop, so must the genetics of forage crops in order to match animal requirements.
There has been significant progress in recent years in the production of perennial ryegrass and white clover varieties with improved forage quality characteristics aimed at improving the nitrogen use efficiency in the rumen, including traits such as water soluble carbohydrate (WSC) concentration (Wilkins and Lovatt, Reference Wilkins and Lovatt2003; Abberton et al., Reference Abberton, Marshall, Humphreys, Macduff, Collins and Marley2008). To provide optimal nutrition for livestock from forage-based systems requires identification of appropriate plant varieties and species grown in combination that will produce nutritionally stable swards over the growing season under different management systems. Species complementarity and factors influencing sward stability are important (Helgadóttir et al., Reference Helgadóttir, Connolly, Collins, Fothergill, Kreuzer, Lüscher, Porqueddu, Sebastià, Wachendorf, Brophy, Finn, Kirwan and Nyfeler2008) particularly in response to different management systems, including under-grazing by livestock. To provide such information requires a thorough understanding of the quality characteristics of different forage species, an analysis of whether mixed swards containing varieties and species with different growth patterns can optimise the sward composition over the grazing season and whether inclusion of novel genetic material of grasses and legumes can be used to extend the grazing season. The stability of these species/variety combinations is also important in ensuring mixed swards provide the optimal composition over time and is a currently an important topic at the interface of agriculture and ecology (Humphreys et al., Reference Humphreys, Marshall, Collins and Abberton2011).
Limitations to rumen efficiency
Increasing ruminant production to feed the rising global human population involves finding ways of increasing the efficiency of animal feed use. Because of the way the rumen functions, this will have additional benefits of reducing emissions of nitrogen and methane. Although ruminants are efficient at extracting energy from plant cell walls compared with monogastric animals, it has been known for a long time that the production of methane as a product of rumen fermentation is an inefficiency in the use of feed energy (e.g. Kriss, Reference Kriss1930). However, methane generation is metabolically important as it is required to maintain equilibrium in the rumen by acting as a hydrogen sink. Livestock are generally considered to be responsible for ∼37% of the global anthropogenic methane production, most of which comes from ruminants (FAO, 2006 and 2009) as a direct consequence of the ability of the rumen microbial ecosystem to digest complex lingo-cellulosic materials through enteric fermentation. In contrast, incorporation of feed nitrogen into useful products (meat and milk) is extremely poor with about 70% losses, meaning that nitrogen excreted onto pasture land acts as a substrate for generation of nitrous oxide (N2O), itself a potent greenhouse gas. One quarter of the anthropogenic production of N2O is due to production and handling of manure-N, whereas nitrate pollution of land causes loss of biodiversity and nitrate in water has health issues for humans and aquatic populations (FAO, 2006 and 2009). With a global herd of ∼3.5 billion cattle, buffalo, sheep and goats (FAO, 2006 and 2009), increasing the efficiency of rumen function would have significant impact on both food security and reducing the contribution of agriculture to climate change.
Until relatively recently, increased efficiency of utilisation of feed nutrients was largely viewed as a way of increasing the output of animal products. In the last few years, the impact of farming on the environment has become much more apparent and a focus of national and international policy development (Abler, Reference Abler2004; Shortle et al., Reference Shortle, Ribaudo, Horan and Blandford2012). Thus, reduced excretion of nitrogen and energy-rich substances is a major goal not only because of the loss of productivity but also because of the release of greenhouse gases and other potential pollutants such as ammonia. Increased efficiency of nutrient utilisation to promote productivity and reduce excretion are mutually compatible objectives, and the improvement of grasses and other forages has a substantial role to play in these objectives.
The energy supplied by a feed is a function of its digestibility, which has traditionally been a major target for plant breeding programmes (Kingston-Smith and Thomas, Reference Kingston-Smith and Thomas2003; Parsons et al., Reference Parsons, Edwards, Newton, Chapman, Caradus, Rasmussen and Rowarth2011a). Increasing the digestibility of feed DM is a good way of increasing its energy use efficiency, although other factors are increasingly important forage breeding targets (Casler and van Santen, Reference Casler and van Santen2010). As previously mentioned, feed DM digestibility is a major determinant of the efficiency of use of feed by, for example, dairy cows for milk production and in simple terms high yielding dairy cows require a good source of highly digestible feed in order to realise their milk production potential. However, the efficiency with which digestible feed nutrients are used for milk synthesis depends on a number of factors, probably the most important of which is rumen function.
The microbial population uses liberated forage protein breakdown products to synthesise more microbial protein, from which much of the meat and milk protein is ultimately derived. A source of energy is needed to drive this microbial synthesis, but when protein breakdown products are present in abundance the colonising microbial population is likely to be energy-limited due to the much slower rates of degradation of their main energy-yielding substrates, the structural carbohydrates, cellulose and hemicellulose (Kingston-Smith et al., Reference Kingston-Smith, Bollard, Thomas, Brooks and Theodorou2003b). When this happens, ammonia accumulates and, if it is not incorporated into microbial proteins, is absorbed from the rumen. It is subsequently converted to urea in the liver and is either recycled back to the rumen or excreted as waste nitrogen in urine (Moorby et al., Reference Moorby, Kingston-Smith, Abberton, Humphreys and Theodorou2009). Nitrogen in urine deposited by grazing animals contributes to N2O emissions to the atmosphere and nitrates leached into ground water (Oenema et al., Reference Oenema, Velthof, Yamulki and Jarvis1997; Wachendorf et al., Reference Wachendorf, Lampe, Taube and Dittert2008), but beyond the clearly important aspects of this potential pollution caused by nitrogen losses from the animal, excreted nitrogen represents a significant financial loss to the farmer. There are two possible strategies for increasing the efficiency of conversion of forage-N to microbial-N. One strategy is to increase the amount of readily available energy accessible during the early part of the fermentation to match quickly available nitrogen sources. The imbalance in the timing (or asynchrony) of nitrogen and energy sources in the rumen has been considered for a long time (Johnson, Reference Johnson1976) and improving the synchrony of availability of nitrogen and energy in the rumen should theoretically improve nitrogen utilisation and reduce its excretion. The second strategy is to provide a level of protection to the forage proteins, thereby reducing the rate at which their breakdown products are made available to the colonising microbial population.
Balancing nitrogen and energy
According to the nutritional synchrony concept, supplying appropriate quantities of protein and energy-yielding substrates to the rumen microbial population should improve the efficiency of diet nutrient use (Hall and Huntington, Reference Hall and Huntington2008). Typically, in fresh forages the rapid availability of protein degradation products is not balanced with an appropriate supply of readily available energy for microbial growth (Holden et al., Reference Holden, Muller, Varga and Hillard1994). As a consequence, ammonia is absorbed by the animal rather than being captured in the rumen through microbial protein synthesis. Understanding the mechanisms and impact of such processes has been aided by selective forage breeding, which has produced a wide range of germplasm with different forage quality characteristics and agronomic attributes (Abberton et al., Reference Abberton, Marshall, Humphreys, Macduff, Collins and Marley2008) for use in animal production studies. For example, the IBERS perennial ryegrass breeding programme has produced ryegrasses with increased DM digestibility and elevated concentrations of WSC (Wilkins and Lovatt, Reference Wilkins and Lovatt2011). A number of high WSC perennial ryegrass varieties have been released over the last few years, including AberMagic that was recommended in the United Kingdom in 2008 (Wilkins and Lovatt, Reference Wilkins and Lovatt2007 and Reference Wilkins and Lovatt2011) and in the Republic of Ireland in 2010 (Department of Agriculture, Food and the Marine (DAFM), 2012). It has been shown that a 9% varietal improvement in the WSC of perennial ryegrass reduced the proportion of dietary nitrogen excreted in the urine of dairy cows from 39% to 26% and that a 3% to 4% difference in WSC content improved milk yield in dairy cows by 2.7 l a day (Miller et al., Reference Miller, Moorby, Davies, Humphreys, Scollan, MacRae and Theodorou2001). However, production responses to ryegrass WSC concentrations have been very variable (Parsons et al., Reference Parsons, Rowarth and Rasmussen2011b), which has recently been explored in dairy cows using a modelling approach (Ellis et al., Reference Ellis, Dijkstra, Bannink, Parsons, Rasmussen, Edwards, Kebreab and France2011).
Soluble carbohydrates in forages provide a source of rapidly available energy and over recent years we have demonstrated improvements in livestock growth (Lee et al., Reference Lee, Jones, Moorby, Humphreys, Theodorou, MacRae and Scollan2001) and milk production (Miller et al., Reference Miller, Moorby, Davies, Humphreys, Scollan, MacRae and Theodorou2001) when animals have been fed ryegrasses bred to express higher concentrations of WSC. Perhaps more importantly, reductions in the excretion of dietary nitrogen in urine of dairy cows have been seen in a number of studies (Miller et al., Reference Miller, Moorby, Davies, Humphreys, Scollan, MacRae and Theodorou2001; Taweel et al., Reference Taweel, Tas, Smit, Elgersma, Dijkstra and Tamminga2005; Moorby et al., Reference Moorby, Evans, Scollan, MacRae and Theodorou2006; Tas et al., Reference Tas, Taweel, Smit, Elgersma, Dijkstra and Tamminga2006a and Reference Tas, Taweel, Smit, Elgersma, Dijkstra and Tamminga2006b). Productivity improvements are most likely caused by improved feed intakes, whereas reductions in the partitioning of dietary nitrogen into urine result from reduced ammonia absorption and greater incorporation of feed nitrogen into microbial nitrogen in the rumen (Merry et al., Reference Merry, Lee, Davies, Dewhurst, Moorby, Scollan and Theodorou2006). Rumen ammonia concentrations are more constant during the day with increased WSC concentrations in the grass (Lee et al., Reference Lee, Harris, Moorby, Humphreys, Theodorou, MacRae and Scollan2002). In fresh grass-based diets, there is a curvilinear relationship between the dietary ratio of WSC to nitrogen and the proportional excretion of dietary nitrogen in urine (Parsons et al., Reference Parsons, Edwards, Newton, Chapman, Caradus, Rasmussen and Rowarth2011a). This relationship suggests that a high diet WSC : N ratio will increase the efficiency of nitrogen use by dairy cows, even if milk production is not affected. Ellis et al. (Reference Ellis, Dijkstra, Bannink, Parsons, Rasmussen, Edwards, Kebreab and France2011) found a similar relationship in a modelling study, and they concluded that the greatest improvement in nitrogen utilisation efficiency is expected when increased WSC occurs at the expense of grass CP, but this is likely to also lead to a reduction in milk yield. However, more detailed studies are required to confirm these relationships, and to define the point that optimises the partitioning of dietary nitrogen towards milk and away from excretion products.
Methane production by rumen micro-organisms can represent a significant loss of energy from the rumen (Van Nevel and Demeyer, Reference Van Nevel and Demeyer1996). Relatively little information is available on the effect of forage quality on enteric methane emissions from grazing ruminants, although a number of plant secondary products such as saponins and condensed tannins (CTs) have been shown to effectively reduce methane emissions (Hess et al., Reference Hess, Kreuzer, Diaz, Lascano, Carulla, Soliva and Machmuller2003; Carulla et al., Reference Carulla, Kreuzer, Machmuller and Hess2005; Ramírez-Restrepo and Barry, Reference Ramírez-Restrepo and Barry2005). However, not all CTs are the same – some result in little apparent benefit (Beauchemin et al., Reference Beauchemin, McGinn, Martinez and McAllister2007) – and care is therefore required to ensure that any breeding work with CT-containing species is appropriate. More generally, although enabling improved forage DM intake may decrease methane emissions per unit of feed intake, Buddle et al. (Reference Buddle, Denis, Attwood, Altermann, Janssen, Ronimus, Pinares-Patiño, Muetzel and Neil Wedlock2011) suggest that there is limited scope in reducing methane emissions through altering the composition of most commonly used forage species.
Protecting forage protein
Increased rumen efficiency could be achieved by altering the rumen degradability of forage protein to reduce its availability to rumen microbes, thereby better matching the availability of soluble carbohydrates present. The benefits of protecting plant proteins from degradation in the rumen have been known for some time. For example, the freezing of forage (which can reduce plant protein breakdown by up to 50%; MacRae et al., Reference MacRae, Campbell and Eadie1975) can increase the synthesis of microbial protein and increase the absorption of amino acids from the small intestine. A viable strategy for improving microbial efficiency in animals given forages would be to protect proteins in order to reduce the rate at which they become degraded. CTs in species such as birds-foot trefoil (Lotus corniculatus L.) can help to protect forage proteins from degradation and improve the efficiency of conversion of plant to animal protein (Barry and McNabb, Reference Barry and McNabb1999). They are not present in any significant quantity in white or red clover, and thus swards containing species that contain CTs are beneficial. However, levels are critical as they can also be anti-microbial (Min et al., Reference Min, Attwood, McNabb, Molan and Barry2005) and anti-nutritional, resulting in reduced feed intake and performance if CT levels are too high (McSweeney et al., Reference McSweeney, Palmer, McNeill and Krause2001). Considerable genetic variation in CT levels has been found within birds-foot trefoil (Marshall et al., Reference Marshall, Bryant, Latypova, Hauck, Olyott, Morris and Robbins2008) suggesting that breeding of improved varieties with appropriate levels is feasible. However, the value of species such as birds-foot trefoil is dependent upon the improved varieties also having the agronomic traits that will ensure they persist and produce a reasonable DM yield as monocultures (Marley et al., Reference Marley, Fychan and Jones2006) or in mixed swards.
Other compounds found in a number of forage plants are the polyphenol oxidase (PPO) enzymes. These catalyse the aerobic conversion of monophenols to o-phenols and finally to o-quinones (Macheix et al., Reference Macheix, Sapis and Fleurit1991) and are responsible for the browning reaction in food such as apples and bananas (Martinez and Whitaker, Reference Martinez and Whitaker1995), and are also present in significant quantities in red clover (Jones et al., Reference Jones, Hatfield and Muck1995a). Until relatively recently, no nutritional or environmental significance had been attributed to PPO in forage crops. High levels of the enzyme are present and active in red clover leaves when cut or crushed and exposed to air (Jones et al.,Reference Jones, Muck and Hatfield1995b; Winters et al., Reference Winters, Minchin, Merry and Morris2003) and the chemical process results in the formation of a cross-linked protein–phenol complex when appropriate enzyme substrates are present (Gerdemann et al., Reference Gerdemann, Eicken and Krebs2002; Parveen et al., Reference Parveen, Threadgill, Moorby and Winters2010). It has been demonstrated that such complexes are resistant to enzymatic digestion by proteases and the PPO mechanism may effectively reduce protein breakdown in silage (Sullivan and Hatfield, Reference Sullivan and Hatfield2006) and result in over 80% of the protein in red clover silage being retained as ‘true protein’ providing an improved nitrogen use efficiency and product quality when fed to cattle (Broderick et al., Reference Broderick, Walgenbach and Maignan2001; Winters and Minchin, Reference Winters and Minchin2002). Another recently discovered source of PPO activity is that found in forage grasses such as cocksfoot (Dactylis glomerata L.), albeit at rates substantially lower than in red clover (Lee et al., Reference Lee, Olmos Colmenero, Winters, Scollan and Minchin2006). Nevertheless, protein complexing, reduced proteolysis and reduced lipolysis can be demonstrated with grass leaf extracts (Lee et al., Reference Lee, Olmos Colmenero, Winters, Scollan and Minchin2006). The inhibitory effect of PPO activity on proteolysis, and also lipolysis (Lee et al., Reference Lee, Olmos Colmenero, Winters, Scollan and Minchin2006), adds support for a strategy to breed improved forage varieties with increased PPO activity. Despite cocksfoot's limited agricultural importance in the United Kingdom, transfer of PPO activity between cocksfoot and other more productive grasses presents significant challenges but may provide a protein protection mechanism that can be exploited. The opportunity of transferring the cocksfoot PPO system to a high WSC-containing perennial ryegrass is an intriguing possibility.
In a grazing situation, proteins contained in fresh forage are generally rapidly broken down in the rumen following ingestion through a combination of the action of plant and microbial enzymes (Zhu et al., Reference Zhu, Kingston-Smith, Davies, Troncoso, Merry, Pichard, Thomas and Theodorou1999; Kingston-Smith and Theodorou, Reference Kingston-Smith and Theodorou2000; Kingston-Smith et al., Reference Kingston-Smith, Bollard, Humphreys and Theodorou2002, Reference Kingston-Smith, Bollard, Humphreys and Theodorou2003a and Reference Kingston-Smith, Merry, Leemans, Thomas and Theodorou2005). Animals grazing at pasture consume large portions of leafy tissue containing numerous intact cells. These cells are subject to a number of simultaneous stresses including elevated temperature, oxygen deficiency, constant darkness and the presence of an invasive microbial population. Plants contain a wide array of proteases in significant quantities in mature leaves and are further induced during stress and senescence (i.e. during cell death). Leaf cells therefore possess the intrinsic capacity to degrade their own proteins before invasion by cellulolytic and other rumen micro-organisms and it is reasonable to expect that plant cell death, in response to ruminal stresses, influences protein turnover rate. Studies have shown that plant proteins break down when plants are subjected to rumen-like conditions, even in the absence of rumen microbes (Zhu et al., Reference Zhu, Kingston-Smith, Davies, Troncoso, Merry, Pichard, Thomas and Theodorou1999; Beha et al., Reference Beha, Theodorou, Thomas and Kingston-Smith2002; Kingston-Smith et al., Reference Kingston-Smith, Bollard, Armstead, Thomas and Theodorou2003a). Understanding the processes causing this may help to reduce the amount of protein degradation occurring in grass after ingestion and thus increase the efficiency of protein use in the ruminant animal. For instance, senescence in plants is usually accompanied by yellowing as chlorophyll is degraded. Many crop plants exhibit variation in or absence of normal senescence processes and are often termed ‘stay-green’ because they retain the chlorophyll (Thomas and Smart, Reference Thomas and Smart1993). Retention of chlorophyll in senescing leaves of a stay-green mutant of meadow fescue is associated with the persistence of several chlorophyll-binding proteins (Vicentini et al., Reference Vicentini, Hortensteiner, Schellenberg, Thomas and Matile1995; Hörtensteiner, Reference Hörtensteiner2009). Through conventional breeding, the gene responsible for this has been transferred to forage and amenity grasses to produce stay-green varieties of perennial ryegrass (Thomas et al., Reference Thomas, Evans, Thomas, Humphreys, Morgan, Hauck and Donnison1997). Although there is little evidence of an advantage of the stay-green trait in terms of protein concentrations or rate of protein degradation in the fresh crop or when the crop was cut for hay (Kingston-Smith et al., Reference Kingston-Smith, Bollard, Humphreys and Theodorou2002), an advantage could be gained from the stay-green trait in field situations where a standing crop of largely senescing leaf material is used as a forage source for livestock (Dove et al., Reference Dove, Wood, Simpson, Leury, Ciavarella, Gatford and Siever-Kelly1999; Gatford et al., Reference Gatford, Simpson, Siever-Kelly, Leury, Dove and Ciavarella1999; Leury et al., Reference Leury, Siever-Kelly, Gatford, Simpson and Dove1999). This could be an important consideration for development and application of improved forages for delivering consistent production under climate change providing that the yield penalty incurred from introgressing the stay-green gene can be overcome by further selection (Wilkins et al., Reference Wilkins, Lovatt and Hayes2007).
Meeting future challenges
Matching animal production to meet human food demand, within the confines of sustainability is a major challenge of the future (Vergé et al., Reference Vergé, Worth, Desjardins, McConkey and Dyer2012). One aspect of this is the need to minimise inputs for forage production without negatively impacting on our ability to produce high-quality animal feed. Sustainable agriculture is encouraging the use of minimal fertiliser and strategic (limited) concentrate feeding. Therefore, the emerging challenge for forage plant breeding is to identify and improve traits that when incorporated into the appropriate agronomic background will increase the efficiency of incorporation of forage proteins into meat and milk and thereby reduce environmental-N pollution. The use of chemical fertilisers has improved productivity of pastures but application rates must be appropriate (Potter et al., Reference Potter, Ramankutty, Bennett and Donner2010). Because of the rumen microbial requirements for protein and carbohydrate and the inverse relationship between them in terms of partitioning of assimilated nutrients by the plant, decreasing forage protein through decreased nitrogen fertilisation could result in decreased nitrogen deposition to the land (Peyraud and Astigarra, Reference Peyraud and Astigarraga1998). Furthermore, optimum ryegrass growth occurs when nitrogen concentrations are ∼40 g N/kg DM (Woledge and Pearse, Reference Woledge and Pearse1985), which equates to about 250 g CP/kg DM. However, continual increase in nitrogen application does not result in corresponding increase in forage protein, and accumulation of nitrate that is potentially poisonous to ruminants (Peyraud and Astigarra, Reference Peyraud and Astigarraga1998). In addition, at high concentrations of plant protein, the amount of soluble carbohydrates required to achieve a ratio for optimum utilisation efficiency is probably unachievable in practice. Therefore, in order to manipulate the WSC : N ratio that is available to the rumen microbial population, a number of options can be considered. These include technologies to aid in strategic application of nitrogen fertiliser (e.g. Neményi et al., Reference Neményi, Mesterházi, Pecze and Stépán2003) coupled with forage improvement programmes to increase the efficiency by which the plant uses fertiliser nitrogen. Further development of agronomically excellent grass varieties with high WSC levels can be coupled to the development of white clover varieties with a lower CP content. This opens the possibility of supplying a grass/clover mixture that delivers an optimised ratio of WSC : N in the grazing sward without compromising the performance of individual plant species.
Target traits for forage crop improvement can be categorised under three general headings concerned with (a) improving forage quality characteristics such as protein quality and stability, fermentable carbohydrate content and cell wall digestibility, (b) plant performance factors such as resistance to abiotic and biotic stresses and (c) plant development processes. In addition, recent demands for more sustainable production systems have required high yielding, high-quality forages that enable efficient animal production with minimal environmental impact. Achieving this involves considering the entire farm system and identifying opportunities for maximising nutrient use efficiency in both forage and animal components (Tilman et al., Reference Tilman, Cassman, Matson, Naylor and Polasky2002). An increasingly important objective for forage breeders is to increase the efficiency by which plants assimilate and partition resources (nutrients and water) available to them. An emerging target is the need to breed for increased phosphorus use efficiency (Stewart and Hayes, Reference Stewart and Hayes2011). Phosphorus is a key component of current fertiliser mixes, and is especially important in symbiotic nitrogen fixation as the process consumes large amounts of energy (Schulze et al., Reference Schulze, Adgo and Merbach1999) and energy-generating metabolism depends upon the availability of phosphorus (Plaxton, Reference Plaxton2004). However, supply of phosphorus is finite and diminishing; it has been estimated that peak phosphorus use will occur around 2035, after which there will be insufficient supply to meet demands of agriculture and industry (Cordell et al., Reference Cordell, Drangert and White2009).
Forage crops of the future must be able to utilise limited resources (water and nutrients) to maximise production on a limited land area (Foley et al., Reference Foley, DeFries, Asner, Barford, Bonan, Carpenter, Chapin, Coe, Daily, Gibbs, Helkowski, Holloway, Howard, Kucharik, Monfreda, Patz, Prentice, Ramankutty and Snyder2005). It is also essential that we consider predicted climate change scenarios that may open the possibility for inclusion of species not hitherto considered suitable for livestock production systems, but when grown in mixtures may be more robust under predicted climate change scenarios. Major advances are being made in the development of improved varieties of forage grasses and legumes that are better adapted to likely climate change scenarios such as increased soil moisture deficit (Humphreys et al., Reference Humphreys, Yadav, Cairns, Turner, Humphreys and Skot2006 and Reference Humphreys, Marshall, Collins and Abberton2011). To this end novel drought-tolerant forage varieties are currently under development by transferring drought resistance from fescue species into perennial ryegrass (Humphreys et al., Reference Humphreys, Yadav, Cairns, Turner, Humphreys and Skot2006; Durand et al., Reference Durand, Bariac, Ghesquiere, Brion, Richard, Humphreys and Zwierzykowski2007) and into white clover from closely related Trifolium ambiguum while maintaining forage quality (Marshall et al., Reference Marshall, Williams, Abberton, Michaelson-Yeates, Olyott and Powell2004; Abberton and Marshall, Reference Abberton and Marshall2005). An alternative approach is to use currently available varieties of species such as tall fescue, cocksfoot and timothy that are more drought tolerant than perennial ryegrass and can produce high forage yields but are currently generally of inferior quality. Indeed, future climate change scenarios may also require us to consider alternative plant species to those currently favoured. Forage legumes such as birds-foot trefoil, sainfoin and lucerne are consequently attracting renewed interest in the United Kingdom (e.g. Marley et al., Reference Marley, Fychan, Fraser, Winters and Jones2003 and Reference Marley, Fychan and Jones2006; Liu et al., Reference Liu, Lane and Davies2008). A changing climate requires crops that can adapt to the changing conditions as well as mitigate the effects of global warming and anthropogenic CO2 emissions. Given the time required to develop new varieties of forage crops (up to 12 years from first crossing to commercial availability), the challenge for plant breeders is to determine whether their emphasis should be on selection and breeding of currently grown species and to develop improved varieties better adapted to possible climate change scenarios, or whether to focus on breeding species not currently widely grown within the United Kingdom. This requires information on (1) the potential impact of climate change on the growing environment, (2) the extent of genetic variation within gene pools of currently grown species to enable selection of forage varieties for climate change scenarios and (3) the potential of introducing genetic material from different environments into current plant breeding programmes while maintaining improvements in forage quality.
New tools are becoming available for plant breeders and geneticists to transfer key (and complex) traits into improved plant varieties (Mohan et al., Reference Mohan, Nair, Bhagwat, Krishna, Yano, Bhatia and Sasaki1997; Moose and Mumm, Reference Moose and Mumm2008). Conventional plant breeding relies on the two fundamental principles of creating variation in a breeding population with traits that are agriculturally important, and the selection of progeny that combine the best traits with the fewest shortcomings. Modern breeding techniques such as maker-assisted selection allied with high throughput phenotyping can increase the speed and precision with which traits can be introduced and selected into and from breeding populations (Xu and Crouch, Reference Xu and Crouch2008). This is based on detection of DNA variation among individuals (Bowley, Reference Bowley1997; Henry, Reference Henry2001; Spangenberg et al., Reference Spangenberg, Kalla, Lidgett, Sawbridge, Ong and John2001) and quantitative trait locus (QTL) analysis for dissecting complex traits and assessing gene effects. Molecular-based techniques offer considerable potential advantages, especially in speeding up breeding of forage crops that are typically outbreeding perennial species where evaluation of advanced lines in plots takes several years and where studies involving animals or impacts on the environment are time consuming and expensive. Associations of QTL with valuable traits in crop species have been identified (McCouch and Doerge, Reference McCouch and Doerge1995; Mohan et al., Reference Mohan, Nair, Bhagwat, Krishna, Yano, Bhatia and Sasaki1997; Duncan and Carrow, Reference Duncan and Carrow1999; Cahill and Schmidt, Reference Cahill and Schmidt2004; Alm et al., Reference Alm, Busso, Ergon, Rudi, Larsen, Humphreys and Rognoli2011). Genetic linkage maps have been used to rapidly progress development of the high WSC-containing varieties of L. perenne (Turner et al., Reference Turner, Cairns, Armstead, Ashton, Skot, Whittaker and Humphreys2006 and Reference Turner, Cairns, Armstead, Thomas, Humphreys and Humphreys2008). However, in general the promised advantages of marker-assisted selection in variety development have been slow to materialise (Kearsey and Farquhar, Reference Kearsey and Farquhar1998; Roldán-Ruiz and Kölliker, Reference Roldán-Ruiz and Kölliker2010) and over recent years research has shifted towards the use of association mapping strategies (Skøt et al., Reference Skøt, Humphreys, Humphreys, Thorogood, Gallagher, Sanderson, Armstead and Thomas2007) that eliminate the need to develop dedicated mapping populations and allow the identification of QTL within breeding germplasm. Marker-assisted selection approaches that are not based on the use of QTL information, such as genomic selection, are also increasingly being considered to be aided by the increasing number of DNA markers spread over crop genomes and the decrease in cost per marker-data-point (Roldán-Ruiz and Kölliker, Reference Roldán-Ruiz and Kölliker2010). In addition to these approaches, to meet the challenges of enhanced forage supply and quality, our abilities to make selective improvements by natural breeding (vertical gene transfer) could need supplementing by horizontal gene transfer. Genetically improved crops are commonplace in some countries, notably soya produced in the Americas. Other targets currently being pursued at a research level include opportunities to use genetic modification to fortify forage to deliver essential amino acids to livestock, as well as enhancing yield, pathogen and disease resistance of the crop (as discussed by Ronald, Reference Ronald2011). Whereas conventional plant breeding is mainly concerned with the transfer of large numbers of genes, the characteristics of which are known through phenotyping, genetic manipulation is an approach that is concerned with the transfer of small numbers of genes with high precision. Successful genetic manipulation is therefore dependent upon a detailed understanding of the physiological, morphological and biochemical basis of action of individually important genes as well as their identification and cloning. The pace of future development in this area is therefore dependent upon significant improvements not just in the technology for transformation but also in the detailed understanding of mechanisms that govern agronomically important trait characteristics. New breeding targets will be identified as the interactions between plants and the animals that consume them become better understood. This will ensure that available resources are targeted at delivering maximum benefits to the animal through enhanced transformation efficiency. The ruminant animal has evolved to maximise the digestion of roughage through its symbiotic relationship with the rumen microbial population (Stevens and Hume, Reference Stevens and Hume1998). The key to understanding how to improve efficiency of forage utilisation by ruminants is through obtaining a more detailed understanding of the processes of rumen fermentation and how these can be manipulated, providing breeding targets for novel forage varieties. Systems approaches and the advance of ‘omics technologies have considerable potential to reveal more detail of the biotransformations occurring in the rumen (e.g. Hess et al., Reference Hess, Sczyrba, Egan, Kim, Chokhawala, Schroth, Luo, Clark, Chen, Zhang, Mackie, Pennacchio, Tringe, Visel, Woyke, Wang and Rubin2011). This knowledge will allow us to identify points of inefficiency and forage-based routes to manipulation.
Conclusions
Forage plant breeding has made significant progress in developing improved varieties to meet the nutritional demands of livestock. Increasing emphasis on environmentally and nutritionally focused priorities such as sustainable agriculture, climate change mitigation and increased biodiversity means that new breeding objectives are emerging. Given the time required to translate trait identification to the release of commercially available varieties, action is required now to conduct the fundamental research in plant and animal science that will identify key target traits and their incorporation into improved varieties. Forage plant breeding and development is a continual process, and as we gain a better understanding of animal and rumen microbial processes we can tailor the nutritional characteristics of grasses and other forage species to match them. A more complete understanding of the metabolic transformations required for efficient rumen function will inform the improved matching of nutrient delivery by forage substrate (by genetic improvement) with the requirements of the microbial population. As forage quality affects nutrient use efficiency of the animals, a consequence is that the whole farm cycle must be considered in consolidated efficiency models. Perhaps more controversially, although forage improvement has been proven to deliver results in terms of improved animal production we also need to consider how far we can progress with conventional approaches. To achieve a future of sustainable intensification, more radical feed strategies could be required to meet our objectives. For many parts of the word it is important that we continue to see pasture-based ruminant agriculture widely defined by free-grazing animals.
Acknowledgements
The authors thank BBSRC (A. K.-S) and the Department for Environment, Food and Rural Affairs and the European Community (J. M., A. M.) for funding.
