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With the world population projected to be 9 billion by 2050 and doubling of the global food demand there is need for increased food production to feed the increasing mouths. In a bid to increase sustainable arable crop productivity three important tasks need to be done: (1) increasing crop yield per unit of land area, (2) increasing crop yield per unit of nutrients applied and (3) increasing crop yield per unit of water used. However, sustaining continuous arable crop productivity for future generations without compromising environmental integrity and public heath still remains uncertain due to fundamental shifts in politics, policies and incentives, institutions and technological advances (Tilman et al., 2002).
1.1 Sustainable arable crop production
Sustainable agriculture is defined “as practices that meet societal needs for food and fibre, ecosystem services for healthy lives by maximizing the net benefits to society when all costs and benefits of the practices are considered for both the present and future generations” (Tilman et al., 2002; Björklund et al., 2009). Sustainable arable crop production can be achieved by using ecologically sound management technologies to achieve long-term sustainable yields. It requires production economics aspects as well as considering vital issues of ecological stability and sustainability through restoration of agricultural diversity and landscape (Altieri, 1995). Sustainable arable crop production should aim at meeting the needs of the present without compromising the ability of the future generation to meet their needs (Chizari and Ommani, 2009).
In my own opinion, sustainable arable crop production involves the management, use and conservation of productive resources in a manner that ensures continuous provision for all needs of the present and future generation.
1.2Types of agriculture
Agricultural types influence the level of food production and impact on environmental sustainability. They are largely influenced by culture, soil type; international, regional or individual government polices as well as advances in science and technology. A wide range of agricultural practices are exercised under the different agricultural types. These include: use of agro-chemicals like synthetic fertilizer, herbicides, fungicides and nematicides, irrigation and use of genetically modified varieties.
(a) Organic agriculture
Organic (biological or biodynamic) farming is “an agricultural production system which seeks to avoid the direct and routine use of synthetically compounded chemicals like fertilizers, pesticides, herbicides, fungicides, nematicides, growth regulators and all biocides in order to minimise environmental degradation at both micro and macro levels” (Lampkin, 2002; Altieri, 1995; Hole et al., 2005; Ammann, 2008). The tools for organic farming include; crop rotation, tillage, time of sowing, varietal resistance and diversification and biological control. Organic farming is currently practiced in more than 150 countries and occupies 35 million ha of agricultural land globally (IFOAM, 2009).
(b) Conservational agriculture
Conservational agriculture is an integrated approach to crop management which minimises soil cultivation and utilises agrochemicals in a manner to reduce disruption of natural biological processes. Its key features include: minimum mechanical soil disturbance, permanent soil cover and diversified crop rotations. Conservational agriculture has been advocated for by the FAO in developing countries like Brazil, Burkina Faso and India and has reaped significant benefits like increased crop and livestock productivity and biodiversity conservation The agricultural practices under organic and conservational agriculture have been reported to significantly reduce the use of agrochemicals and tillage operations since they rely more on rotations, use of farmyard manure and crop residues to optimize productivity. This has resulted into a reduction in the emission of GHGs like CO2 and N2O (Chizari and Ommani, 2009; Brookes and Barfoot, 2008).
(c) Conventional agriculture
In conventional farming, farmers can use agrochemicals like synthetic fertilizers, pesticides, herbicides, fungicides, nematicides in their farming operations extensively without any restrictions (Altieri, 1995). Conventional farming heavily relies on the application of a range of modern management systems and external inputs to achieve high yields (Hole et al., 2005). The efficient use of nutrients by hybrid varieties achieved through precision agriculture practiced under large-scale intensive farming results into high yields compared to organic farming (Tilman et al., 2002)
Agroforestry denotes a sustainable land and crop management system; that strives to increase yields by combining production of woody forestry crops with arable crops and or animals simultaneously or sequentially on the same unit of land. It incorporates four main characteristics and these include: structure, sustainability, increased productivity and socioeconomic. Structurally agroforestry systems are grouped as: agrisilviculture, silvo-pastoral, agro-silvo-pastoral and multipurpose forest tree production (Farrell and Altieri, 1995).
(e) Polyculture cropping system
Polyculture involves the growing of crops in mixtures or intercrops; annual crops with annuals, annuals with perennials or perennials with perennials grown in a spatial and temporal manner (Liebman, 1995). However, mixed agriculture is inhibited by the land tenure system and design of farm machinery.
1.3 Impact of agricultural types on sustainable arable crop production
(a) Organic farming
Sustainable arable crop productivity requires proper management of soil fertility and biodiversity conservation. The productivity of organically grown food has increased annually by 15 percent with a global market worth US$50 billion per year mainly in developed countries. Organic farming has well established practices that simultaneously mitigate climate change, build resilient farming systems, soil structure and fertility and increase biodiversity which builds resistance to storms and increased pest and disease pressure (IFOAM, 2009). Organic farming is reported to increase species richness with an average of 30% higher than conventional farming systems. However, other studies have indicated a 16% negative effect on species diversity due to organic farming. Literature shows that some organisms are more abundant in an organic farming system for instance; weeds, predators like carabid, beetles and spiders. In addition, non-predatory species were more abundant under the conventional farming system thus natural enemies are negatively affected by conventional management. Organic farming practices are believed to be more environmental friendly than intensive agriculture which is dependent on the routine use of herbicides, pesticides and inorganic nutrient applications in the production of crops and animals (Bengtsson et al., 2005).
However, practicing organic farming requires careful planning, management and decision making in order to establish a viable system and maintain farm income. The rotational design, crop types and varieties and timely soil management are important in maximising crop productivity in organic farming (HGCA, 2008). Below are some principle agronomic practices used in organic farming!
(i) Push and pull approach
The push and pull technology has significantly resulted into increased arable productivity in many parts of the world. In East Africa (EA), the technology has been exploited to maintain soil fertility, control pests and parasitic weed; Striga. Lepidopteran stem borers like:Chilo partellus, Eldanasaccharina, Busseola fusca and Sesamia calamistis cause 50% yield losses tomaize, sorghum and sugarcane. This technology integrated with other crop management approaches has contributed to the sustainable production of maize in EA by increasing farmers' yields from 1 t/ha to 3.5t/ha with minimal inputs and currently used by 25,000 small holder farmers.
The technology involves intercropping maize with a repellent plant (Desmodium) and planting an attractive trap plant (Napier grass) as a border crop around the intercrop. Gravid stem borer females are repelled from the target crop by stimuli and simultaneously attracted to the trap crop, leaving the target crop protected. Desmodium produces some root exudates which stimulate the germination of Striga seeds and others inhibit their growth after germination. This combination provides a novel means of in situ reduction of the Striga seed bank. Desmodium also acts perennial cover crop and able to exert its Striga control effect and together with Napier grass protect fragile soils from erosion. Desmoduim fixes nitrogen, conserves soil moisture, enhances arthropod abundance and diversity and improves soil organic matter thereby enabling cereal cropping systems to be more resilient and adaptable to climate change while providing essential ecosystem services and making farming systems more robust and sustainable (Cook et al., 2007).
Table 1:Push and pull strategies used in arable crop production
Population regulation used
Oviposition deterrent: neem
Trap crop: maize with sugar bait
Pollen beetle (Meligethes aeneus)
Oilseed rape (Brassica napus)
Non-hostile repellents: lavender
Trap crop: turnip rape
Insectides&Biopesticide: (Metarhizium anipsoliae)
Adopted from (Cook et al., 2007)
(ii) Biological control
Biological control involves the use of parasites, predators or pathogens to maintain another organisms population at a lower average than would occur in a naturally in their absence. The aim is to reduce and achieve long term stabilization of weeds and pests. The technique has been utilised to control the Japanese knotweed, Fallopia japonica using Aphalara itadori(http://www.cabi.org/japaneseknotweedallaiance/). Most biological control studies have been largely successful on a small scale and have reduced the reliance on pesticides, insecticides and herbicides thus conserving biodiversity.
Despite of its potential for biodiversity conservation, organic farming has been reported to have limited output and productivity. This could be attributed to the limited use of synthetic fertilizers which results into unavailability of important soil nutrients, increased perennial weeds, pests and diseases. As a result, the organic fields are abandoned in the long run or farmers adopt conventional farming practices. All the different types of agriculture should be managed in a precautionary and responsible manner to protect the health and well-being of current and future generations and the environment (Ammann, 2008).
2.0 SCIENTIFIC ADVANCES
Scientific advances have significantly contributed to the sustainable production of arable crops to feed the increasing global population. These range from the discovery of the DNA structure and genetics by Craig and Watson and Gregor Mendel respectively to high throughput genomics-based approaches. They have been exploited by plant breeders to generate genetic diversity among crop species by crossing varieties with desired characteristics, use of mutations; chemical mutagens like gamma, x- and β-rays. Other scientific advances that have contributed to sustainable arable crop production include; discovery of agrochemicals like fertilizers, herbicides, pesticides, insecticides, fungicides and nematicides; agricultural machinery and equipments like; tractors, planters, harvesters, ploughs, sprayers and irrigation equipments which facilitate precision agriculture.
2.1 Impact of scientific advances on sustainable arable crop productivity
Many factors impact on arable crop productivity and these include: land, climate change, production economics, legislation, breeding, labour and management and agronomic practices. Scientific advances in plant breeding are reported to have a significant impact on sustainable arable crop productivity (Tong et al., 2003).
(a) Plant breeding
(i) Conventional breeding
Advances through conventional breeding and genetic transformation have provided genetic change to crops like wheat, rice, maize, cotton, soybean resulting into sustainable increased yields. These approaches have been used to exploit heterosis and feed the increasing world population; circumnavigate the effects of climate change by producing drought tolerant and disease and pest resistant varieties. For instance, the production of new rice for Africa (NERICA) a cross between Oryza sativa indigenous to India and West African native Oryza glaberrimathrough inter-specific hybridization. The drought and weed resistant variety is widely adopted and cultivated under the rainfed systems in SSA (Dingkuhna et al., 1999). Its cultivation has resulted into a reduction in paddy rice growing which emits GHGs: N2O and CH4 thus mitigating the adverse effects of global warming through destruction of the Ozone layer. In addition, world wheat production increased significantly due to the adoption of hybrid dwarf wheat varieties and use of agro chemicals. The hybrids have a short stature, resistant to lodging and early maturing (Peng et al., 1999). In the UK wheat yields were reported to be increasing with an average of 110kgha-1 annually though further increase in sustainable wheat productivity can be achieved through by breeding resource use efficient varieties (Austin, 1999).
(ii) Genetic Transformation
Globally, transgenic crops are cultivated in 23 countries and occupy 114.3 million ha of land with an average increase of more than 12% annually. The number of transgenic crop traits and hectares planted are predicted to double by 2015. The use of rDNA technology in plant breeding has resulted into sustainable arable crop production through biodiversity conservation. This has been achieved by reducing pesticides and herbicides use and adoption of reduced tillage practices (Craig et al., 2008; Bitista and Oliveira, 2009; Hillocks, 2009).
The development of insect resistant transgenic cultivars like Bt cotton transformed with Bacillus thuringiensis (Bt) gene which controls the production and expression an endotoxin within the plant to control Lepidopteran tobacco budworm, cotton bollworm and pink bollworm pests does not require pesticides. Bacillus thuringiensis has been used as an organic pesticide for decades and poses no threat to biodiversity which makes crops transformed with the Bt gene acceptable on the organic market and have an adoption rate of over 66% and 85% in China and South Africa respectively (Hillocks, 2009).
Despite of its potential in ensuring sustainable arable crop production; GM technology has been criticised by anti GM activists as a threat to biodiversity. Concerns of horizontal transfer of antibiotic resistance markers (ARMs), allergenicity and toxicity of the new GM food products have been reported as possible threats to biodiversity (Craig et al., 2008; Bitista and Oliveira, 2009).
The use of insect and herbicide resistant varieties has resulted into emergence of new pests and herbicides resistant weeds due to selection pressure which results into secondary pests becoming major pests.
GMOs contain ARM genes which are used in the selection process during transformation. The ARM commonly used is Neomycin phosphotranferase II (nptII) which inactivates the aminoglycoside antibiotics neomycin and kanamycin. There is concern that ARMs when introduced into food or the environment could drive the evolution of drug-resistant bacteria by functioning as sources of antibiotic resistance (Craig et al., 2008).
Several transgenic plants constructed to be resistant to herbicides, insecticides or diseases are transformed with genetic material containing genes coding for compounds like antimicrobial agents. These could affect non target microbiota such as nitrogen-fixing bacteria, mycorrhizal fungi and other beneficial soil microorganisms. For instance; a reduction in the colonisation potential of mycorrhizal fungus Glomus mosseae has been attributed to the production of anti-fungal pathogenesis-related proteins from a transgenic tobacco plant containing β-1, 3-glucanase (Araujo and Azevedo, 2003).
Using insect resistant transgenic canola increases fitness in oilseed rape varieties expressing the Bt gene. However, pollen flow between canola cultivars with different herbicide resistance traits resulted into gene stacking causing genetic contamination of seed. In addition, gene flow can lead to development volunteer oil seed rape with multiple tolerance to several herbicides due pollination between adjacent crops (Natarajan and Turna, 2007). Controlling these super weeds requires toxic herbicides like 2, 4-D and paraquat which are hazardous to man and the environment.
The use of agrochemicals like inorganic fertilizers (NPK) during crop production has increased world crop productivity and reduced the rate of encroachment on natural ecosystem like forests and virgin land (Tilman et al., 2002). The increased use of fertilizers has been a major contributing factor to the increase in yield growth in developing countries since the Green Revolution. In addition, globally fertilizer use has plateaued due to a decline in its use in industrial countries and Soviet Union countries after joining the market economy (Fischer et al., 2009). In China increase in cereal production has been attributed to introduction of agrochemicals; yields increased from 1.21 t/ha in 1961 to 4.83 t/ha in 1998 (Tong et al., 2003). It is presumed that the historical decline in crop yields is due to the genetic ceiling for maximal yield potential being reached.
Despite their potential increased use of agrochemicals has resulted into environmental pollution, eutrophication of water bodies and global warming due to its emission of GHGs and stratospheric ozone depletion (Tong et al., 2003). Environmental pollution is due to nitrate leaching into ground water causing nitrate toxicity due to elevated nitrate levels in drinking water; chlorofluorocarbons (CFC's) released from fertilizers applied as aerosols. Eutrophication results into death of aquatic fauna and flora hence threatening biodiversity.
Advances in scientific technology like agricultural engineering and development of modern farm equipments and machinery has boosted the agricultural sector and is one of the factors that will ensure sustainable arable crop productivity. The growth of irrigated area has expanded steadily over the last decade at 0.6 percent annually in developing countries and irrigation technology accounts for 0.2 percent in overall cereal yields from 1991-2007 (Fischer et al., 2009). Irrigation equipments like sprinklers have significantly contributed to the continued cultivation of arable crops throughout the year and under drought conditions. Farm machinery like combine harvesters, sprayer, planters, ploughs among others have helped in the efficient management of farm operations. However, the use of fuels by farm machinery during farm operations has been cited as a source of GHG, CO2 leading to an increase in atmospheric CO2 levels (Brookes and Barfoot, 2008).
2.2Impact of science and policies on sustainable crop production
The government land laws and policies, product prices and social economic factors like population migration, urbanization and world trade play a significant role in ensuring sustainable arable crop production.
Organic farming is viewed as a solution to biodiversity conservation and has received substantial support in form of subsidy payments through EU and national government legislation. As a result the certified organic and in-conversion area within the EU increased from 0.7 to 3.3 million ha from 1993 to1999 accounting for 24.1% of global organic land area (Hole et al., 2005). In 2007, 3% of the total UK land area is managed organically (HGCA, 2008). This direct support and intervention through fixation of ceiling and floor prices protects farmers from exploitation at both the domestic and international market hence producing more crops under organic farming whilst conserving the environment. However, enacting of the healthy check on CAP will remove restriction on farmers thus helping them to respond to new market signals, opportunities and challenges
In SSA the cultivation of GM crops has been hindered due to lack of a biosafety legislation supporting the biotechnology policy that can allow them acquire GM crops from the biotechnology companies that hold the intellectual property rights (Hillocks, 2009).
To ensure sustainable arable crop productivity; agronomic, breeding and institutional or infrastructural factors like increased investment into research and development of appropriate tool and technologies need to be considered.
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