Crop Production in the East of Scotland - SASA

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Inputs and Sustainability5. INPUTS AND SUSTAINABILITY(e.g. genetic modification to incorporatenitrogen-fixing genes), in plant/microbialrelations (e.g. more effective mycorrhizalassociations) or in potential yield on lowergrade land under “organic” systems, themaintenance of existing yields willnecessitate similar applications of N and Kfor the foreseeable future. The exception isthe potato crop which could be managedwith much lower inputs of nitrogen (Hay &Walker, 1989), if the aim were dry matter,rather than fresh weight, yield.Timothy (Phleum pratense) grown for hay and seed,carselands west of StirlingIn evaluating the sustainability of croppingsystems in the East of Scotland, it isimportant to start with the inputs requiredto sustain existing systems and yields (e.g.Tables 1.1, 4.3 & 4.4).Inputs: Mineral NutritionThe 1997 survey of fertiliser use in Scotland(MAFF, 1998) showed that the (mean)application of industrially-synthesisednitrogen fertiliser to arable crops andmanaged grassland has remained above100 kg N ha -1 for the last twenty years, withno sign of significant reduction. Levels ofphosphorus and potassium application havealso remained steady (around 70 kg ha -1phosphate and potash on arable crops; 30 –40 kg of each on grassland), even thoughthe reserves of available phosphate in manyScottish soils would justify reducedapplications. Unless there are marked andrapid changes in the physiology of varietiesThe continued availability of these mineralresources depends upon world supplies andprices of fossil energy, global security, andthe size of global potash and phosphatedeposits, although there could be somesubstitution by increased reclamation ofagricultural waste. There are seriousdoubts about the capacity of knownphosphate deposits to meet world demand,although, in the UK, the “mining” ofexisting soil reserves could delay any crisis;and there is at least the potential forutilising domestic sewage, if problems ofcontamination by heavy metals andmicrobial pathogens are overcome. Soilsin humid areas such as Scotland will requireregular and heavy liming at least once perdecade for the foreseeable future tomaintain yield capacity; there are veryextensive deposits of useable limestone inthe UK and elsewhere, but their exploitationcontributes to climate change anddegradation of the environment. Thefrequency of liming will depend upon thetypes of nitrogen fertiliser used(ammonium-based products accelerateacidification) and levels of atmosphericpollution (acid rain).One important consequence of these heavyinputs, and the additional inputs to39
Crop Production in the East of Scotlandlivestock production in the form ofimported feed, is that there is a serious riskof pollution of non-agricultural areas, byvarious forms of nitrogen (most seriouslygaseous ammonia, and nitrate), phosphatein solution, and effluents with a highbiological oxygen demand. Theconsequences of such pollution, both acuteand chronic, can be the sterilisation oreutrophication of water bodies, thepollution of drinking water, and theartificial fertilisation of terrestrial plantcommunities. These effects are in additionto those resulting from the relevantmanufacturing activities.Inputs: Crop ProtectionThe protection of crops growing in Scotlandis much more complex than mineralfertilisation, as the incidence andimportance of pests and diseases varieswith crop species, variety, location and withtime. Crop protection is an unremittingstruggle against a wide range of rapidlyevolvingpathogens (viruses, bacteria, fungi,nematodes, molluscs, insects and otherarthropods), affecting all stages from theseed, through vegetative and reproductivephases, to harvest and storage. Thefollowing case histories for the majorScottish crop, barley, are necessarilyillustrative rather than comprehensive.The struggle is typified by seed-bornefungal diseases of barley. As resistance toroutinely-applied organo-mercuryfungicides built up in the 1980s, there wasan epidemic of barley leaf stripe(Pyrenophora graminea), a disease whichformerly caused serious yield losses, butwhich most Scottish farmers of the presentgeneration could not recognise from itssymptoms in the field. By 1990, the funguswas found in more than 80% of seed stocksof spring barley, and was a major threat toScottish agriculture. Yet, within two years,the epidemic had been brought undercontrol by ensuring that all seed with morethan 4% infection was treated with aneffective fungicide before planting(Cockerell et al., 1995).Crop protection must change with theevolution of systems of crop management.For example, before the introduction ofwinter barley, the spring crop was sown inMarch or April and harvested in latesummer; for six months there was nogreen crop. The move to winter cropping inthe early 1980s created a “green bridge” ofcrop plants or volunteers throughout thecalendar year, which could act as reservoirsof barley diseases. This has led to anincrease in the incidence of traditionalfungal diseases of green tissues, (powderymildew Erisiphe graminis; net blotchPyrenophora teres; and rhynchosporiumRhynchosporium secalis) but also lessfamiliar diseases such as snow rot (Typhulaincarnata) which can develop on winterbarley during the winter months. Thus, inthe pursuit of higher yield potential(greater interception of solar radiation),farmers have encountered increaseddisease pressure.There can be important interactionsbetween varietal characters and disease. Inthe 1990s, spring barley varieties (e.g.Chariot and Derkado) were introducedwhich were resistant to powdery mildew.As this was the most common foliar diseaseat that time, it was hoped that fungicideusage would fall sharply, but these mildewresistantvarieties turned out to besusceptible to rhynchosporium: the solutionof one disease problem permitted anotherto take the dominant position. Control ofrhynchosporium is now causing concern, asno current fungicide is effective alone, andresistance to some groups of fungicide isdeveloping.Fungal diseases predominate in cereals (see40
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Crop Production in the East of Scotland - SASA

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