Medicago sativa L.
Author: Dr John Frame
Lucerne, alfalfa, common purple lucerne, common lucerne, purple alfalfa, purple medick.
Erect or ascending, glabrous perennial, 30-90 cm, with alternate trifoliate leaves ; leaflets, 30 mm, narrowly obovate, toothed in upper third with a mucronate tip; stipules linear-lanceolate, usually serrate. Numerous stems originating from crown buds ; as the stems develop, axillary buds formed in lower leaf axils produce further stems which build up a crown of basal buds at their base. Crown is the main source of stems produced after defoliation ; axillary buds above ground develop into branches. Deep-rooted, 2-4 m, or more in deep, well-drained soils. About 60-70% of total root mass is in the upper l5 cm of soil profile (Heichel, l982). Inflorescences are compact racemes up to 40 mm, borne in axils of upper leaves ; purple florets, 8 mm, typically papilionacious. Cross-pollinated by various species of bee. Seed pods spirally coiled, glabrous or pubescent ; pods turn from green to brown as they mature, and contain 2-5 kidney-shaped, yellow or brown seeds. A proportion of the seeds are hard, probably inversely related to temperature during seed set (Fairey and Lefkovitch, l99l).
Modern cultivars of M. sativa usually contain varying proportions of M. sativa and M falcata genes in their make-up though the sativa gene is generally dominant. The common names of M. falcata are yellow lucerne or sickle lucerne. It is a decumbent, rhizomatous perennial, 30-60 cm, with alternate trifoliate leaves, leaflets being sharply pointed. Roots thinner and more branched than common lucerne. Inflorescences are compact racemes, 25 mm, with yellow florets. Seed pods straight to curved, 2-5 seeds per pod. Hybrids between M. sativa and M. falcata known as sand, hybrid or variegated lucerne (M. media) bear yellow, mauve or purple florets and can be back-crossed with both parents. Seed pods curved or spiral, 3-8 seeds per pod.
The following refers to common lucerne (M. sativa) with occasional reference to the other two species, as appropriate.
Widely distributed in temperate zones of the world, e.g. USA, southern Canada, Europe, China, southern Latin America, South Africa. In the USA, an estimated l0-ll million ha have been grown annually in recent years (Barnes and Sheaffer, l995).
Highest yielding forage legume. Requires deep, well-drained fertile soils to maximise potential. Perenniality and upright growth habit make it a highly suitable crop for conservation as hay, silage or in dehydrated form. Usually productive for 4-6 years. Range of cultivars with differing characteristics has extended its ability to grow in environments from very dry to very cold. Generally grown in monoculture. Has a higher tolerance of saline soils than many other forage species.
Season of growth.
Spring to autumn, but main flush of growth in late spring/early summer. Variable in winter dormancy. Winterhardy types suited to cold regions have a longer dormancy period and shorter growing season than the less winterhardy types suited to warm temperate regions.
Temperature for growth.
High air temperature of 270C optimum for seedling growth but optimum declines to 22oC as shoots develop (Fick et al., l988). Optimum temperatures for root growth, 2l-250C (Kendall et al., l99l). High temperatures reduce forage digestibility due to a decrease in total non-structural carbohydrates (Wilson et al., l99l). Plant hardening for cold resistance increases with declining temperatures from l00C downwards, completing the process at 1oC to 2 oC (McKenzie et al., l988). The most winterhardy cultivars harden more rapidly with decreasing temperatures and are slower to deharden in spring with rising temperatures than less winterhardy cultivars. During hardening the concentration of soluble sugars increases while starch concentration decreases (Castonguay et al., l995) . Lucerne is known to survive temperatures of 250C in Alaska and above 500C in California (Barnes and Sheaffer, l995).
Frost tolerance and regrowth.
There are varying degrees of frost tolerance among genotypes, the most tolerant being those with a high proportion of genes from yellow lucerne, e.g. Grimm in North America. The European Flamand (Flemish) and Provence types have only moderate frost tolerance because of lower proportions of yellow lucerne genes, the Provence types being less winter hardy than the Flamand types. In general, the most frost-tolerant genotypes are slower to start growth in spring and quicker to cease growth in winter and so have a shorter growing season. Winter hardiness is strongly associated with low rates of autumn forage production (Perry et al., l987).
Lucerne is a long-day plant though the minimum photoperiod required to initiate flowering varies among cultivars (Major et al., l99l). Flower number and floral development are also promoted by long photoperiods. Shortening autumn daylengths initiate cold hardening, and in conjunction with low temperatures induce winter dormancy. These effects, together with prior cessation of stem elongation, occur earliest in autumn with the most cold-tolerant types (Hesterman and Teuber, l98l). During periods of rapid growth in the growing season, competition for light within the canopy leads to the dominance of shoots emanating from the crown buds over those initiated from axillary stems (Gosse et al., l988).
More drought tolerant than other forage legumes such as red clover or birdsfoot trefoil (Peterson et al., l992). Deep-rooting ability is an important factor in drought tolerance and any adverse soil physical or chemical conditions which restrict root growth will reduce drought tolerance. During severe drought, plants become dormant but resume growth when moisture becomes available (Hall et al., l988).
Compared with many forage species lucerne is an efficient user of water supply largely as a result of its deep tap-root system. A linear relationship between available water capacity and yield has been shown (Douglas, l986). While water use efficiency varies with season, it is normally in the range of 5 to 9 cm evapotranspiration/ha per tonne of forage DM produced (Sheaffer et al., l988). The less winterhardy genotypes suited to warm temperate regions are more efficient water users than the cold-tolerant types. Water deficit adversely affects shoot production and hence efficiency of light interception by the canopy thus leading to reduced yield (Durand et al., l989). Many of the physiological functions, such as photosynthesis, respiration and nitrogen fixation, are also adversely affected by moisture stess. In comparison with irrigated lucerne, moisture-stressed lucerne had lower mineral contents in the forage though forage digestibility was higher due to delay in maturity (Kidambi et al., l990).
Tolerance of flooding.
Intolerant of prolonged flooding which adversely affects root development, forage yield and plant persistence. Root and crown rots, e.g. Phytophthora root rot (Phytophthora megasperma), may develop and thus reduce plant populations (Sheaffer et al., l988). Well-drained soils are necessary in areas with high rainfall or for irrigated crops so as to avoid soil saturation.
Needs well-drained, deep friable soils with high levels of soil fertility, including a high soil pH, 6.0-6.5, for optimal performance. Soils which are compacted, have indurated layers or are inherently shallow adversely affect lucerne growth and development. Wheel tracking during crop harvesting, or during application of fertilizers or slurry for example, reduce plant persistence and growth vigour partly through soil compaction and partly by plant damage (Sheesley et al., l974).
Rhizobial N-fixation in the nodules is by strains of Rhizobium meliloti., distinct strains being required for different lucerne genotypes. Nodules are concentrated on the fibrous roots in the upper soil layer. There is also scope to breed cultivars receptive to a wide range of Rhizobium strains (Barran and Bromfield, l997). Seed inoculation is essential when introducing lucerne to land without a recent history of growing lucerne. Lack of available Ca at low soil pH reduces Rhizobium activity and root nodulation, and the natural occurrence of R. meliloti is markedly reduced below a soil pH of 6.0 (Graham, l992). Lime pelleting of inoculated seed increased nodulation of branch roots at the plant crown in seedlings, thereby increasing nitrogen concentration and dry weight of the shoots (Pijnenborg, et al., l99l).
Ability to spread naturally.
Some winterhardy cultivars develop stem-producing rhizomes from the plant crown, while some are creeping-rooted, i.e. they produce adventitious stems from primordia which arise from roots (Teuber and Brick, l988). Winterhardy yellow lucerne has a creeping, rhizomatous habit as have the Spanish mielga lucerne types which are persistent under grazing.
Land preparation for establishment.
Well-cultivated, uniform and firm seed bed required for good results.
The seed is normally drilled after conventional seed-bed cultivations but seed can also be broadcast. When growing lucerne with a companion grass there is no advantage in sowing the two components in alternate drills (Fairey and Lefkovitch, l994). Sowing lucerne under a cover crop, e.g. wheat, barley, reduces weed invasion and the cover crop provides a cash crop. However, there is a greater risk of poor establishment compared with direct sowing due to competition from the cover crop and so its seed rate and N fertilization should be reduced. The risk is greatest in dryland conditions where moisture can be limiting. Lucerne can be direct drilled (sod seeded) into an existing grass sward or cereal stubble, but this method is not common since it is more risky than conventional seeding methods. Direct drilling is most sucessful on swards with low-density vegetation when there is adequate moisture for germination and seedling development (Brash, l983). Dense grass swards can be thinned out by severe grazing, cutting for conservation or by partial chemical desiccation. Guidelines for successful establishment include : control of perennial weeds before sowing ; adequate soil pH and fertilization with water-soluble phosphate ; sufficient soil moisture. Control of pests, e.g. slugs, may also be necessary (Byers and Templeton, l988). Autotoxicity militates against sod seeding lucerne into a thin lucerne stand or reseeding lucerne after a previous lucerne crop (Hegde and Miller, l990) but disease and/or pest prevalence may also be a factor.
Sowing depth and cover.
The optimum depth is l0-l5 mm with a light but firm soil cover to promote seed-soil contact.
Sowing time and rate.
Spring is normally the best time since temperature and moisture conditions are usually satisfactory for good seed germination and efficiency of Rhizobium action. Lucerne can also be sown conventionally or drilled into the stubble of a cereal crop in autumn, provided there is sufficient time for the plants to develop enough to withstand winter cold and possible frost heaving of the soil. An autumn deficit of soil moisture can be overcome by irrigation if autumn sowing is preferred to spring sowing (Janson, 1975)
Seed rates between 6 and 20 kg/ha are commonly used. Much higher rates of l7-34 kg/ha are used in southern USA due to the occurrence of seedling diseases compared with l0-l3 kg/ha used in northern USA and southern Canada (McKersie, l997). An initial plant population of l35-270 plants per m2 is suggested as a reasonable target (Barnes and Sheaffer, l995). When sown with a companion grass, non-aggressive species are used, e.g. 2-4 kg/ha of smooth brome grass (Bromus inermis) or meadow fescue (Festuca pratense). Other grasses used include cocksfoot (Dactylis glomerata) or reed canary grass (Phalaris arundinacea) (Sheaffer et al., l990). Lucerne sown at 3-5 kg/ha in 30-70 cm-wide drills produced high seed yields (Askarian et al., l995).
Number of seed per kg.
Circa 500 000.
Percentage hard seed.
A high proportion of hard seed, 30-40 % can be present in seed lots.
Seed treatment before sowing.
Seed is not normally treated to reduce the hard seed content since these may germinate over time following abrasion of the seed coat by fluctuating soil conditions after sowing.
The optimal soil pH for plant development is in the range 6.0-6.5 and so liming is often a key requirement ; Mn and Al toxicity are problems in acid soils especially if available Ca levels are low. Adequate P in the seed bed is necessary for seedling development, and in the USA, phosphate is often band-applied below the sown lucerne seed. In soils with a low soil N status a small starter N dressing of 25-50 kg/ha at establishment will encourage seedling development but thereafter nodulation and rhizobially-fixed N will satisfy plant needs (Hannaway and Shuter, l993). Because of the high rate of removal of nutrients in conservation crops, replenishment is necessary to maintain production. In particular, large amounts of K are removed and need to be replaced ; K fertilization improves lucerne tolerance to intense cutting management and winter injury (Sheaffer et al., l986). However, the major nutrients, P, S and sometimes Mg, require replacement too. B and Mo may need to be applied ; deficiency of B adversely affects young tissue growth and stem elongation while Mo deficiency reduces the functioning of the nitrogenase enzyme in the N-fixation process. Critical levels of nutrients, i.e. the minimum concentrations within the plant required to produce 90-95% optimum growth, are shown in Table 4. The interpretation of herbage nutrient analyses is clouded by nutrient interactions. For example, increasing availability of soil P increases uptake of Ca, Mg, Na and S but reduces the uptake of K and some trace elements, while increasing availability of soil K reduces the uptake of Mn, Zn, Ca and B (James et al., l994). The problem can be partly overcome by the use of nutrient ratios, e.g. the diagnosis and recommendation integrated system (DRIS) (Russelle and Sheaffer, l986).
Table 4 Critical concentrations of nutrients in lucerne (from Frame et al., l998)
Compatibility with grasses and other legumes.
Compatible with non-aggressive grasses. Different grass species favoured by different countries e.g. smooth brome grass, cocksfoot and reed canary grass are the species most commonly used in mixtures in northern USA (Sheaffer et al., l990) where mixed stands rather than monocultures are sown although the latter two species are considered to be aggressive towards lucerne. In Europe, timothy (Phleum pratense) and meadow fescue, cocksfoot, and tall fescue (Festuca arundinacea) are among the species used.
Ability to compete with weeds.
Relatively low at early establishment phase but improves with the development of the canopy. Sowing a companion grass deters weed ingress though herbicides to control grass weeds cannot then be used. Once established and agronomically well managed, vigorously-growing, dense lucerne stands prevent severe weed invasion.
Tolerance of herbicides.
Tolerates clover-safe herbicides such as 2,4-DB, MCPB, benazolin and bentazone types but not less-selective types. In established monocultures, winter application of propyzamide will control grass weeds.
Initially poor on account of small seed size.
Vigour of growth and growth rhythm.
Once established, capable of vigorous growth provided there is high soil fertility, adequate water supply, whether rainfed or by irrigation, and good crop management. Growth is slow in early spring peaking in late spring-early summer. Autumn yield is related to genotype and is lowest for cold-tolerant, winter-dormant cultivars.
Estimates of annual rates range from 85 to 360 kg N/ha with a wide variation among sites (Witty et al., l983 ; Heichel and Henjum, l99l). Plant nutrient deficiencies in the soil, excessive soil acidity, a high soil N status, or applied fertilizer N all limit N-fixation due to the sensitivity of the nitrogenase enzyme system to the soil environment. Nitrogen-fixation is markedly reduced though not stopped by high levels of fertilizer N application (Lamb et al., l995). Defoliation causes a marked decline in N-fixation due to the removal of photosynthate supply to the nodules (Cralle and Heichel, l98l) but also to feedback inhibition since nitrogenous products accumulate in the nodules (Hartwig and Nösberger, l994) ; however, nodule growth and Nfixation increase again with new vegetative regrowth. Lucerne requires half the energy of that needed by sainfoin for N-fixation (Witty et al., l983). In lucerne, l0 mol of carbon dioxide (CO2) is lost in respiration per mol of fixed N (Sheehy et al., l984). The main route of N transfer from lucerne to companion grasses is by death and decomposition of below-ground roots and nodules and subsequent mineralisation of this N from the organic matter (Dubach and Russelle, l994). However the proportion of total N transferred is generally lower than from red or white clover (Heichel and Henjum, l99l). Stress conditions, defoliation in particular, causes senescence of root material.
Response to defoliation.
Best suited to an infrequent defoliation regime in which cutting for hay or silage is the main objective. This management allows nutrient reserves within the plant to build up between defoliations and so maintain yield and persistence. Nitrogen reserves in the roots are important in determining the rate of growth after defoliation, with regrowth drawing mainly from the vegetative-storage protein pool (Barber et al., 1996). In cold regions with relatively short growing seasons, e.g. the northern states of the USA, three or four harvest are taken annually as a rule (Sheaffer et al., l988) but in warmer, drier regions at least double this number can be taken under irrigation (Barnes and Sheaffer, l995). Within a growing season, increasing the frequency of cutting reduces yield but increases forage nutritive value. The timing of harvests is best linked to stage of development of the primary growth and regrowths, and an index system based on ten developmental stages from early vegetative through to ripe seed pod has been produced (Table 3).
Table 3 Summary of stages of development of lucerne plants ( Kalu and Fick, l98l)
In autumn a rest interval is needed before growth ceases so that carbohydrate and nitrogen reserves can build up to enhance winter-cold tolerance. The dormant forage can then be utilised after onset of winter. If reserves are depleted by late-autumn defoliation the number of crown buds, from which early-season growth is initiated, is reduced. This also has repercussions later in the season since yield becomes dependent on stem axillary buds ; these buds are encouraged by tall (l0-l4 cm) rather than shorter stubble heights at defoliation. Leaving tall stubble at the final defoliation in cold regions can ameliorate frost effects on the soil and retain snow cover which protects crown buds against frost (Barnes and Sheaffer, l995). Timing of the autumn rest interval is less critical in milder climates, the Mediterranean region ; for example, although first-cut yields the following year were reduced by cutting before the first frosts, annual yields were similar to those from stands cut near or after the first autumn frost (Marble et al., l989 ; Lloveras et al., l998).
Some form of rotational grazing is required to sustain plant persistence and production. The rest intervals following defoliation replenish the root and plant crown reserves of carbohydrates and nitrogen which are needed for regrowth. Short grazing periods ensure young regrowths are not grazed (Janson, l982). The duration of the rest intervals depends on the growing conditions which prevail but are likely to be in the 5- to 7-week range. If continuously grazed, defoliation should be lax to prevent over-severe defoliaton and damage to plant crowns. In mixed swards, a highly acceptable grass companion is needed and stocking rate and grazing intensity controlled so as to prevent selective overgrazing of the lucerne (Leach, l983). Some degree of success has been achieved in the development of lucerne cultivars with improved tolerance to grazing, including continuous grazing (Brummer and Bouton, l99l).
Lucerne is cross-pollinated by several types of bee honey, short-tongued cutter, alkali bumble bees. The chromosome number is 2n=4x=32 for tetraploid common lucerne and 2n=2x=l6 for diploid yellow lucerne.
These include : improved yield, persistence, multiple pest resistance, disease resistance, N-fixation, cold hardiness, winter growth (for warm temperate regions), nutritive value, grazing tolerance. For example, from l965 to l990, Canadian breeding programmes increased first-, second- and third-year yields by 0.l6, 0.27 and l.07% per year, respectively (McKersie, l997).
Dry matter yields.
In the USA DM yields up to 20 t/ha have been achieved in experiments (Sheaffer et al., l988). Yields from 9.4 to l7.6 t/ha have been reported from the UK (Aldrich, l984 ; Frame and Harkess, l987) and from l4.5 to l9.0 t/ha in France (Guy, l993) over a range of sites. On-farm yields are likely to be less than experimental yields because of less-precise management control. At fixed cutting intervals and with no moisture stress, yields of successive harvests decreased over the growing season (Corletto et al., l994). In general, annual yields of lucerne decline with age of stand, the decline being accelerated by factors such as winter damage, pests and diseases, and mismanagement.
Suitability for hay and silage.
The inherent growth characteristics and good yield response to infrequent cutting make lucerne a highly suitable species for conservation as hay or silage. A succession of cut crops firstly at l0% bloom thereafter and at 5- to 7-week intervals maximises yield, gives satisfactory nutritive value and aids stand longevity (Sheaffer et al., l988). When making hay or wilting crops cut for silage, it is important to save the nutritious leaf fraction as much as possible during handling since leaf shatter and loss is major hazard during drying. In spite of lucernes high protein content, low sugar content and high buffering capacity against acidification during silage fermentation, compared with grass crops, high-quality silage can be made using the techniques of wilting, short chopping and the application of an effective additive. Artificial dehydration is also used in order to produce high-quality lucerne cubes, pellets or meal.
Value as standover or deferred feed.
Not a common method of utilization but the forage accumulated in the late-autumn rest period required to build up root carbohdyates and nitrogen reserves can be utilized soon after winter dormancy has set in.
Rich in protein, minerals and vitamins. Feeding value largely determined by stage of growth at the time of utilization since nutritive value falls with advancing maturity and associated increase in stem :leaf ratio. Cell wall constituents (cellulose, hemicellulose, lignin) increased by 0.l6% of dry matter per day with advancing maturity (Keftassa and Tuvesson, l993) and this decreases forage digestibility. The decrease in digestibility is more marked in the lower than upper stem fractions and is least in leaves (Buxton et al., l985). Increasing the number of crops in a season by cutting at earlier stages of growth improves forage digestibility and crude protein content but at the expense of yield (Brink and Marten, l989 ; Hesterman et al., l993). In comparison with grass at similar stages of growth lucerne has lower cell wall and digestible fibre contents but higher digestible cell and crude protein contents (Campling, l984). It also has higher contents of most minerals of nutritional importance to livestock. Table 5 shows the mineral composition of lucerne. Protein-rich lucerne and energy-rich maize make a valuable combination for dairy cow feeding.
Table 4 Mineral composition of lucerne (from Spedding and Diekmahns, l972)
Highly acceptable forage to livestock whether as hay, silage, dried pellets, cubes or meal or when grazed at a leafy growth stage. Thus, has high voluntary intake characteristics.
Bloat is a hazard when grazing lush lucerne stands but conventional preventative methods are available e.g. provison of anti-foaming agent such as poloxalene. Lucerne contains oestrogens which reduce conception rates in cattle and sheep if grazed or fed lucerne prior to mating. The oestrogen content differs among genotypes but may be increased in the leaves by pest and fungal attack which is often prevalent in the autumn (White, l982). Saponin content in the forage has a dual effect of causing adverse haemolytic effects in stock, but also conferring plant resistance to pests (Tava et al., l993).
Seed harvesting methods.
Seed crops are usually direct combined following spraying with a crop desiccant to speed up drying. Correct timing of harvesting is necessary, i.e. when 65-75% of the seed pods are dark brown, to avoid loss of seed by shedding. Guidelines for successful seed production are available (Dunbier et al., l983) ; these include a spring hay cut or grazing before crop closure, irrigation as required up to the seed stage, the use of bee colonies for pollination, spraying with fungicides against fungal diseases or with pesticides against insect pests as necessary.
Yields average 750 kg/ha in the main seed-producing areas of the USA, i.e. the western states where the majority of crops are grown under irrigation, but in the midwestern states yields of 50 kg/ha are more typical (Barnes and Sheaffer, l995). In Europe, an average seed yield is about 400 kg/ha.
Seed quality standards.
Using the Fodder Plant Seeds Regulations for the United Kingdom as an example, certified seed requires a minimum germination of 80% and a permissible maximum hard seed content of 40% by number of pure seeds in the sample. Required analytical purity is 97% by weight for the minimum standard and 98% for the higher voluntary standard. The maximum permissible content of seeds of other species is l.5% by weight for both the minimum and higher voluntary standards.
There is a wide range of genotypes with differing characteristics available, many being the result of mixed ancestry from sativa and falcata types and their hybrids. Winterhardy cultivars, e.g. the Grimm types, are used in northern USA, south-east Canada and other winter-cold regions because of winter dormancy and cold hardiness whereas Mediterranean types more suited to warmer climes have a longer growing season and shorter winter-dormancy period. The basic lucerne germplasm used in present North American cultivars represents a spectrum of winter dormancy due to its derivation from nine world regions (Fairey et al., l996).
In recent decades there have been notable successes in the development of cultivars with resistance to a number of pests and diseases e.g. cv. Ranger (bacterial wilt), cv. Moapa (spotted alfalfa aphid), cv. Agate (Phytophthora root rot), cvs Vertus and Vernema (Verticillium wilt). There has been a marked increase in lucerne breeding in the USA in recent decades, 440 cultivars having been released beteen l962 and l992, with most of the releases in the later years resulting from privately- rather than publicly-funded research programmes (Barnes and Sheaffer, l995). Because of the numerous cultivars on the market in the USA, cultivar databases such as ALFALFA CATALOG have been compiled (Townsend et al., l994). This gives general information on the dormancy index and dormancy reference population, flower colour composition and origin together with the resistance levels to the most common pests and diseases. Some examples of lucerne cultivars are Apollo II, Boreal, Trumpetor, Admiral, Vernal (USA) Angus, Beaver, Drylander, Rambler (Canada), Estival, Europe, Euver, Resis, Sitel, Vela (Europe), Hunter River, Grasslands Otaio (Australasia).
Innovative breeding techniques using different biotechnologies are being increasingly used to develop new cultivars of lucerne and other forage legumes with improved N-fixation, nutritive value, and tolerance or resistance to biotic and abiotic stresses McKersie and Browne, 1997)
Many diseases can attack lucerne from the seedling to the seeding stages and both foliage and roots can be affected. The diseases may strike individually or in combination and sometimes complexes with other pathogens and pests can occur. Reviews of the major diseases, over 20, of lucerne in the USA are available (Leath et al., l988 ; Stuteville and Erwin, l996). Table 6 lists 22 major foliar and stem diseases and the fungal or bacterial agents.
Whether subclinical or clinical, these diseases impair lucerne performance ; for example, reduced or failed establishment by fungal attack from Pythium spp. ; lack of plant persistence and reduced yield by Sclerotinia crown and stem rot. Fusarium spp. are often secondary invaders following damage by other organisms, either fungal or pest (Keld et al., l994). Lucerne wilt diseases, e.g. Fusarium wilt, invade through the roots but the adverse effects are expressed in the shoots. In some cases, fungicidal control is possible and economic (Douglas, l986) but in other cases it is neither possible nor economic and breeding resistant cultivars is the only solution.. Notable advances have been made in breeding resistant cultivars to bacterial wilt or Verticillium wilt and other major diseases. Major exceptions where breeding progress has been less successful include resistance to Sclerotinia crown and root rot, Rhizoctonia crown and bud rot, and spring black stem (Barnes, l992). Various management strategies can be adopted to avoid or minimise disease occurrence and effects. These strategies include : maintenance of high soil pH and fertility needed for vigorous stand growth and development ; optimal cutting frequency, intensity and timing with respect to the region and utlization aims ; weed disease and pest control as appropriate ; avoidance of physiological and biotic stresses as much as possible ; rotation with arable crops.
Table 6 Principal diseases of lucerne and causal agents (from Frame et al., l998)
A number of viruses can infect lucerne and cause varying degrees of injury up to serious loss of yield and persistence (Forster et al., l997) ; for example, alfalfa mosaic alfavirus has caused yield losses of 24-67%. The main remedial measures are the control of insect vectors and use of resistant cultivars where available.
Lucerne is prey to many pests whose activities are often covert and insidious but can also be highly visible and devastating. Damage can occur in the stand at all stages of growth and development from seedling to seeding and plant death may ensue. Damaged plant parts can also be the focus for fungal or bacterial attack and some pests are vectors of plant viruses. The principal pests have been reviewed insects and mites (Manglitz and Ratcliffe, l988) and nematodes (Leath et al., l988) but mainly from a North American perspective though the principles and practicalities of combating the pests are universal. In their review of the lucerne crop, Frame et al., (l998) listed 20 major pests (Table 7).
Table 7 Pests of lucerne (from Frame et al., l998)
These can be broadly classed into foliage-, root crown/rot- and seed pod-damaging groups. Apart from direct damage to the foliage by the larvae of alfalfa weevils, cutworms and caterpillars, for example, the foliage growth can be indirectly and severely affected by crown/root pests such as various species of nematodes. Some leaf-eating pests, e.g. caterpillars, may be controlled by natural predators but others require the use of insecticides, e.g. autumn-applied against adult alfalfa weevils or spring-applied against their larvae. Other pests such as aphids can be controlled to some extent by varying the timing of harvests from the normal pattern but this may influence yield (Latheef et al., l988). The stem nematode is a significant pest of lucerne and though there are resistant cultivars, they are not resistant to all nematode biotypes ; rotation with arable crops, which is effective against some pests and diseases, is not totally effective against nematodes because of their long-term viability but is effective against the larvae of the snout beetle. Seed production is affected indirectly by pests which attack foliage and roots but also directly by sap-sucking mirids in the inflorescences or by the larvae of alfalfa seed chalcids feeding on seed pods. It is necessary to harness the various methods of pest control, either individual, or in combination through integrated pest control (Fleming, l988). These methods include the use of resistant cultivars, cultural control such as manipulation of cutting dates, insecticide use as required and if cost effective, and rotation with arable crops.
Highly productive protein- and mineral-rich legume adapted to a wide range of environmental conditions. Its erect growth habit makes it suitable for hay, silage and artificially dehydrated forage. Drought resistant. Has high voluntary intake characteristics (Conrad and Klopfenstein, l988). Valuable break crop in arable and organic systems on account of N-fixation ability and as source of oganic matter. Major source of honey production.
Lacks long-term persistence. Unsuited to intensive grazing. Susceptible to many pests and diseases. May cause bloat in ruminants. Oestrogens in the plant can reduce fertility of breeding ewes if they are grazed on lucerne during pre-mating and mating periods. Plant saponins may interact with rumen bacteria and cause haemolysis in animals.
High nutritive value allied to high intake characteristics of conserved or grazed lucerne combine to give improved individual animal performance from different classes of animals compared with equivalent grass forage (Conrad et al., l983 ; Thomson et al., l99l ; Tyrell et al., l992) though on occasion milk yields from dairy cows have been similar from grazed lucerne and grazed grass (Sanchez and Campling l982).