S280 Science Matters
"Agriculture"
Summary
Oilseed Rape
Over the past 20 years oilseed rape has become an important feature of UK agriculture as a source of edible oil and protein feed for livestock
Three of the four oilseed speicies in the genus Brassica arose as interspecific hybrids; doubling of the diploid chromosome numbers in the hybrids ensured that they were fully fertile.
Plant oils are triglycerides, i.e. esters of glycerol and three fatty acids.
Fatty acids with one of more carbon-carbon double bonds are termed unsaturated; those without any such double bonds are termed saturated.
Erucic acid was present at high concentrations in older varieties of oilseed rape; since erucic acid is potentially harmful, the low-erucic-acid character found in Canadian spring rape was bred into European winter rape. The initial reduction in yeild, caused by the replacement fatty acids having fewer CH2 untis than erucic acid, was overcome by breeding for the production of more triglyceride molecules.
Different oilseed rape varieties have been bred according to the use to which their oil is to be put (e.g. margarine, cooking, engine lubrication).
Blucosinolates in rapeseed meal are broken down to give toxic products by the enzyme myrosinase released from crushed seed; this limited the use of rapeseed meal for animal feed despite its good balance of amino acids.
The genes responsible for low levels of glucosinolates in Polish spring rape were bred into high-yielding low-erucic-acid winter rape.
Most current oilseed rape vaieties are double-low: low in erucic acid and low in glucosinolates.
Lower levels of glucosinolates may have implications for the intereaction between oilseed rape and its insect pests.
Eggs and Bacon
Animal production may be viewed in terms of optimizing those biological processes responsible for the products or outputs required, e.g. ovulation (providing eggs) and growth (supplying meat).
Inputs to animal production systems include the animals themselves, their nutrition, their environment and their health.
Egg yolk colour is an example of a quality character that has little significance for nutrition or taste; its visual appeal has become linked with its perceived degree of 'wholesomeness'.
Egg yolk colour is determined by pigmenting agents in the chickens' diet; these may be natural, 'nature-identical' synthetic compounds or 'nature-related' synthetic compounds.
Egg-laying is often manipulated by artificially controlling the photoperiod to which chickens are exposed.
Optimum diets have been determined for pigs in terms of both nutrients and energy-yielding ingredients; some of the nutrients (e.g. lysine) are produced industrially, but are identical to their naturally occurring equivalents.
Pigs are kept in comparatively germ-free conditions, as a result of which they have little natural immunity to disease; therapeutic agents are therefore included in their diets. Care had to be taken that microbes do not build up resistance to antibiotics, particularly those used in human medicine.
Fat content is an important aspect of pig-meat quality. The amount of fat has been reduced considerably compared to a century or more ago. There is also now a greater emphasis on unsaturated fats in the human diet. These changes illustrate the often conflicting nature of quality considerations; lean meat is also less 'juicy'; a greater degree of unsaturation shortens shelf-life.
The fat of pig-meat is amenable to change as a result of breeding policy (selecting both for leanness and late maturation) and nutrition (avoiding large quantities of high-energy food and increasing the proportion of unsaturated fatty acids in the diet).
As a consequence of increasing affluence, product quality is assuming greater importance. Quality may have nutritional relevance, it may be important organoleptically or it may simply relate to visual appeal. Quality itself is often poorly defined and, in any case, it is often difficult to fulfil all quality objectives simultaneously.
The 'nitrate' problem
The evidence linking nitrate in clean water supplies to either methaemoglobinaemia or stomach cancer is fairly weak, except where nitrate concentrations are really excessive (>200 mg l-1). However, it does seem that excess nitrate in rivers and lakes contributes to eutrophication of teh water (in the presence of sufficient phosphate, which is largely derived from discharges from sewage works).
Nitrogen gas from the air is fixed into ammonium and ultimately nitrate by free-living algae and bacteria in the nodules of leguminous plants, as well as by the commercial manufacturing of fertilizers.
Much of the world's soil-borne nitrogen is stored in relatively inert form in dead organic matter.
The biomass of the soil (in particular, soil microbes) releases nitrates from this organic matter. This happens on a large scale when old grassland is ploughed up. It happens particularly in the autumn, when the soil is still warm from summer but becoming more moist; thus it is very important that either a winter or a 'catch' crop is available to use this nitrate.
Farming based on animal husbandry is particularly inefficient in its utilization of nitrogen from fertilizer applied to grassland.
Untimely nitrate is nitrate present in the soil at a time when it cannot be utilized by a crop. Such nitrate is likely to be leached beyond reach of the crop's roots and end up in rivers, lakes and aquifers.
The Rothamsted 15N experiments suggest that much of the fertilizer nitrogen applied to winter wheat in spring ends up in the harvested parts of the crop. Most of the remainder ends up in organic material in the soil, where it is safe from leaching. Only 1-2% of the nitrogen is left as mineral nitrogen (amonium and nitrate) in the soil, and therefore vulnerable to leaching. On average 15% of the nitrogen was lost, one-third of which was probably leached and two-thirds converted into N2 and N2) through denitrification by soil microbes. Nitrogen fertilizer applied in the autumn was leached from the soil to a much greater extent than that applied in the spring.
Denitrification may be useful in water-treatment plants, where it can be controlled to minimize emission of envionmentally harmful N2O.
The Nitrate Sensitive Areas scheme are designed to protect water supplies that are vulnerable to pollution by nitrate for geological reasons; they are based on the scientific principles discussed in this chapter.
All individuals and organizations contribute to the nitrate problem; it is therefore incumbent upon us all to support strategies to combat the problem.
Bovine spongiform encephalopathy (BSE)
The new disease, BSE, was first diagnosed in cattle in early 1985. It belongs to a group of TSE diseasess of mammals which affect the central nervous system. These include CJD in humans and scrapie in sheep.
In March 1996, a possible link was established between BSE in cattle and CJD in humans, following the diagnosis, in 1994 and 1995 of a new variant of CJD.
Because of this possible link, consumers lost confidence in the safety of beef and for a time stopped buying it. The EU imposed a world-wide ban on the export of British beef and beef products.
Early epidemiological studies showed that exposure started in winter 1981/82 with no distinct geographic focus. Dairy calves had the greatest level of exposure and incubation times were long (2.5-8 years).
Investigation into the possible cause led to the formulation of the hypothesis that 'BSE came from consumption of concentrated feed by dairy calves which probably contained scrapie agents in MBM derived from infected sheep'.
From 1980, following changes in the regulations controlling the rendering process of waste materials from abattoirs, the hydrocarbon solvent extraction process was gradually phased out. The resulting single process is thought to be less effective at deactivating the scrapie agent.
In bioassays of tissues from infected cattle, infectivity has been detected in the brain, the spinal cord and the retina.
Measures were introduced to control animal health. Key ones were the ban on the use of ruminant protein in ruminant feed (July 1988) and payment of 100% compensation for the slaughter of animals suspected of having BSE (February 1990). It was thought that with these measures the disease would disappear.
However, cattle born after the ban continued to develop BSE. This has been attributed to the use of contaminated feed after the ban and a disregard of regulations for dealing with SBOs in abattoirs.
The existence of BSE in cattle born after the ban could also be explained if there was direct transmission from animal to animal in a herd and/or ... or from parent to offspring. Indirect evidence suggests that if direct transmission does occur it is at a low level.
The BSE agent, or prion, is thought to be an aberrant form of a natural protein, PrP, which is produced by mammals and birds. It is resistant to degradation by proteases and replicates itself by changing the form of the natural protein to that of its own.
A prion from one species can, under some circumstances, break the speices barrier and infec another species. Because of an incompatibility of the prion PrP (from one species) with the host PrP (from another species), this process is unpredictable and characterised by long incubation times. Once the species barrier has been broken and the prion PrP is from the same species as the host, infection is predictable and incubation times are shorter.
In November 1989, SBOs were banned from inclusion in the human food-chain in order to protect humans from the risk of contracting CJD from products of BSE-infected animals. Following the announcement of a possible link between BSE and CJD in March 1996, carcasses of all cattle over 30 months old wee banned from the human food-chain.
The greatest danger to humans of contracting CJD from contaminated meat came from eating beef before November 1989, but because of the long incubation time of TSE diseases the true risk is still unknown.
Current research into the transmission of BSE to humans uses monkeys, which are closely related to humans, and mice genetically engineered with the human PrP gene as models.
Pesticides
Agricultural monocultures are less stable than natural ecosystems and can be exploited by organisms (pests) which can significantly reduce the quality and yield of a crop.
If the use of pesticides were stopped, the price of agricultural commodities would rise and their quality would deteriorate.
The development of modern pesticides is very expensive. It involves careful evaluation of hazards, especially those which have been revealed for other pesticides in the past. Governments approve the use of the new pesticides after examining the safety and other data dubmited by the manufacturers and other agencies.
Most insecticides are nerve poisons; their diversity relates more to how they reach the insesct (routes to target) than to a variety of biochemical modes of action. The diversity of fungicides and herbicides is largely based on a variety of biochemical interactions with the target organisms.
Modern synthetic pesticides (and their breakdown products) are much less persistent in the environment and less dangerous to humans than many of the early ones, particularly organochlorides like DDT.
The application of pesticides by spraying is a very wasteful process because a wide range of drop sizes is produced. Larger drops account for much of the pesticide applied, but are less likely than drops of intermediate size to be deposited on plants effectively. Very small drops are likely to be carried away from the target by air movement, etc.
The spinning cage, the spinning cup and the electrostatic sprayer are among the devices that have been developed in attempts to improve the efficiency of pesticide application; all present new problems which limit their usefulness in practice. The spinning cage is probably as far as we can go at present in narrowing the spectrum of drop sizes.
The environment is remarkably resilient to pesticides; physical and chemical processes seem able to inactivate or degrade the quantities of pesticide that currently enter it.
Target organisms are well adapted to withstand pesticides; it is easy for resistance to pesticides to reach levels which limit their ability to control pest populations.
Pesticides can destroy the natural enemies of pests (making them an even greter problem) or the natural enemies of relatively harmless species (which then become pests).
Integrated farming systems
Since the Second World War, agricultural policies of the UK government, and later the European Union, have encouraged increases in agricultural production and ensured that farmers receive a reasonable income.
In the 1980s, increasing concern over agricultural surpluses led to reforms of the CAP which weakened the link between production and payments of subsidies. On the completion of the Uruguay round of the GATT talks, the EU was bound, through the CAP, to comply with measures to liberalize world trade in agricultural products.
The reforms in the CAP, GATT and committments to sustainable development, have all encouraged the movement to less costly, more efficient and more environmentally friendly farming systems.
Integrated farming systems occupy the middle ground between high-input conventional agriculture and minimal input organic farming.
Integrated techniques aim to reduce applications of agrochemicals and incorporate natural control systems, good husbandry methods and knowledge of ecology. There is a substitution of labour and knowledge for external inputs. Currently, a substantial research effort is bein gdirected at developing techniques which are less harmful to the environment.
When integrated farming systems are compared with conventionaly high-input systems, yields of crops are usually lower but the costs of inputs are also lower. As a result, the overall gross margins are similar or a little higher. From year to year, the stability of yields is generally greater in integrated systems.
Information on agricultural practices which minimize harm to the environment is being passed on to farmers by MAFF and independent bodies such as LEAF. Large retailers have collaborated with the NFU to develop protocols for crop production based on integrated techniques.
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