Stella E. C. Nhanala 03-2016
- IntroductionAgriculture is necessary to feed the world. However, even though this is the main economic activity in most of the developing countries, farmers still face issues to get improved and satisfactory outputs. It has been shown that agriculture modernization is important in order to get better outcomes (Arnon, 1981, Bahiigwa, 2005, Feder et al., 1985, Juma, 2010, Juma et al., 2011). Agricultural biotechnology, especially genetic engineering, is one of the ways/tools that should be considered to improve agricultural outputs (Juma et al., 2011). However, the use of genetically modified (GM) crops is still controversial (Kumar, 2015, Qaim, 2009). Regarding the socio-economics of the GM crops, one of the topics that has been most discussed is coexistence between genetically modified and non-genetically modified crops.
Coexistence is the ability to successfully produce and market products from both Genetically Modified (GM) and non-GM crops within the same agricultural system. It is primarily an issue when there is a demand for a crop to be grown without the use of GM technology. Therefore, it is necessary to create a system that is sustainable for all stakeholders and that reduces the likelihood of mixing crops that are grown by transgenic, conventional, organic or subsistence agriculture (Brookes, 2004, Demont et al., 2007). In this way each farmer can decide which kind of crops he/she wants to cultivate without interfering with the needs of others.
There is a need to understand the process of GM and non-GM crops, because the production of GM crops has steadily increased in the past decade (James, 2011) making it common to have GM crops in markets worldwide. In countries that produce GM crops, not all farmers want to grow these crops. In addition, some farmers and consumers wish to maintain separation between GM and non-GM crops and products. This makes it necessary to establish systems to allow for coexistence of GM and non-GM crops.
It should be noted that the issue of coexistence of GM with non-GM crops is not a safety issue. All GM products in the market have already passed health and environmental safety reviews and regulations (James, 2011). It is an economic issue that is market driven. Successful establishment of coexistence practices enables choice by consumers and agricultural producers (Devos et al., 2009). This article describes how gene movement can be managed to meet threshold requirements of adventitious mixing (i.e. the unintended introduction of seed or crop material between GM derived material and non-GM crops).
- Development of guidelines that are needed to achieve coexistence
Coexistence is achieved by applying appropriate guidelines and regulations (Eastham & Sweet, 2002, Qaim, 2009). Many studies have been made to establish co-existence regulations
For different cropping systems. The objective is to minimize the possibility of adventitious mixing.
Because zero mixing is not achievable in agricultural systems, it is important to set a threshold that is legally accepted for such mixing. The legal threshold for adventitious products varies with different countries. For most countries the legal tolerance threshold for GM in non-GM product is 0.9% (Brookes, 2004, Devos et al., 2009, Ferment et al., 2009). Some examples of the threshold in different countries are shown on Table 1. The recommended threshold set by African Model Law on Biosafety is 0.9% (ACB, 2011).
The main principles that make coexistence possible are: context, consistency, proportionality, fairness and practicality (Timpo, 2012, Brookes, 2004). These principles are important to identify the best way to regulate the different types of systems, as they can vary according to different work environments (Timpo, 2012). The principles are also important to evaluate if the principles must be based on science and legally minimize inconsistency in the market (Brookes, 2004).
Table 1– Examples of threshold for acceptable level of adventitious presence in different countries where GMs are cultivated
Country Adventitious Presence Threshold Australia1,6 0.9 % Brazil2 1.0% European Union (EU) 1,2,3,4,6 0.9% Japan1,2,6 5.0% South Africa 5 0.9% * USA1,2,6 No threshold
*recommended threshold set by the African Model Law on Biosafety
1Australian Government – Department of Agriculture: Fisheries and Forestry (2011) 2Viljoen, et al., 2006
3Scholderer & Verbeke , 2012
5African Centre for Biosafety (ACB), 2011
6Žel et al., 2012
- Factors affecting coexistence in terms of environmental safety In order to establish appropriate guidelines and regulations it is important to know which factors can affect co-existence. The primary sources of adventitious presence are pollen flow and seed or product mixing (Christey & Woodfield, 2001, Devos et al., 2009). Establishing practices that minimize adventitious presence of GM materials requires an understanding of how to prevent mixing of seed and how to minimize gene movement through pollen flow between GM and non-GM crops in agricultural systems.
In this context it is important to understand the concept of gene movement. Gene movement is the exchange of genes between organisms or populations (Burczyk et al., 2004, Glover, 2002, Jaramillo, 2009, Quist 2010). Gene movement is a natural process that can occur through pollen transfer or seed dispersal (Brookes, 2004, Chandler & Dunwell, 2008).
- What conditions are necessary for gene flow to occur via cross pollination between GM and non-GM crops?Pollination mechanisms refer to the process by which pollen produced by the anthers is transferred to the stigma to allow for fertilization (Fehr, 1987, Poehlman & Sleper, 1995). Depending on the mechanism of pollination, plants can be predominantly self-pollinated or cross pollinated. The mode of pollination of key crops for which future GM varieties are under development in Africa is shown in Table 2.
Hybridization, which results from cross-pollination, can only occur between members of the same species or closely related species (Eastham & Sweet, 2002, Fehr, 1987, Hancock, 2012). For cross-pollination to occur between GM and non-GM crops they must be of the same species or closely related species and grown in close enough proximity to allow for pollen transfer (Eastham & Sweet, 2002). For instance, cross-pollination can occur between GM and non-GM maize if they are planted in close proximity and flower at the same time. But no cross pollination is possible between GM cotton and rice, for example.
Self-pollinated species have the ability to pollinate themselves, i.e., the transfer of pollen grains from the anthers to the stigma occur in the same flower or between flowers of same plant.
Cross-pollinated species are those where the pollen is transferred from a flower of one plant to another plant. Gene transfer between adjacent fields planted with same crop is more likely to occur in cross-pollinated then self-pollinated species.
Mechanisms that promote out crossing in cross pollinated species include: differential timing of development of pistils and stamens; self-incompability; male sterility; and separation of the sexes into different flowers (e.g. monoecy or dioecy) (Fehr, 1987).
- Factors that influence gene flow through pollen transfer
The primary factor influencing pollen transfer is the pollination mechanism of the plant, i.e., is it self-pollinated or cross pollinated? If it is cross pollinated, primary factors influencing the frequency of pollen transfer include the agents of pollen transfer (such as insects or wind), the time of flowering of donor and recipient, amount and viability of pollen produced, and the distance between crops.
- How is pollen flow between plants facilitated?
The primary agents of cross-pollination are wind and animals (primarily insects, although a few species are pollinated by birds or bats). The distance that the pollinator flies, the distance between the GM and non–GM crop and the number of open flowers per plant may influence ability to attract pollinators and can influence the rate of cross pollination (Naterro et al., 2011).
In general pollen is carried further by insects than by wind (Australian Government, 2002). There are exceptions, however, such as bent grass, for which pollen can travel kilometers (Watrud et al., 2004). The extent of both insect and wind-mediated cross pollination is also affected by how long pollen remains viable after removal from the anther.
- What are the factors that can limit pollen flow?Limiting pollen flow can facilitate coexistence between GM and non-GM crops. Measures that can be adopted to avoid cross pollination between GM and non-GM crops include: spatial isolation (physical distance between fields growing GM and non-GM crops), temporal isolation (separating flowering time) and reproductive isolation barriers (e.g. male sterility) (Chandler & Dunwell, 2008; Dick et al., 2008 Hancock, 2012). It should be noted that pollen transfer is not a concern for crops that do not produce viable pollen, such as banana. For crops that do produce flowers and seeds, the primary mechanism to prevent pollen flow is isolation distance.
- The influence of isolation distance in pollen flow
Isolation distance is the distance maintained between fields of crop plants to help minimize cross-fertilization by pollen flow. Isolation distance has been long used by plant breeders and seed producers to produce pure seed lines for certified seed (Glover, 2002, McCormack, 2004). Spatial isolation is important for coexistence because it can greatly restrict pollen flow between GM and non-GM crops (McCormack, 2004).
Spatial isolation is based on the principle that pollen concentration levels decrease with increasing distance from the pollen source (Devos et al., 2009). A greater isolation distance will result in reduced pollen exchange (Czarnak-Klos & Rodriguez-Cerez, 2010).
Appropriate isolation distance is determined according to the tolerance threshold, i.e., how much potential mixing is acceptable. For certified seed the tolerance is 0.9% (Brookes, 2004, Devos et al., 2009, Ferment et al., 2009, GMO Compass, 2005).
The minimum isolation distance depends on many factors, including the crossing mechanism of the species (selfing or cross-pollinated crop) and the agent of pollination (wind or insect).This distance can vary in different environments, and because of that, it may be necessary to establish isolation distances for specific environments (Devos et al., 2009). Factors to take into account include: field (such as sizes and distribution), crop type and crop breeding system, flowering time and environmental conditions (Andersson & Vicente, 2009).
- How does gene movement occur by seed dispersal?
Seed dispersal is the movement of seeds from one place to another leading to establishment of plants in a different location. Seed dispersal can occur by wind, water, fire, animals or humans (Christey & Woodfield, 2001, Dick et al., 2008, Howe & Smallwood, 1982). Seed dispersal by humans occurs intentionally when transporting or processing seed and accidentally by unintended mixtures or inadvertent transport along with clothing, possessions, or vehicles (Andersson & Vicente, 2009).
- What are natural agents of seed dispersal?
The potential of a seed to be dispersed by a given agent in nature depends on its size, shape, weight and whether it has dispersal mechanisms. Seed dispersal through wind usually occurs for seeds that are very small and lightweight. If the agent of seed dispersal is an animal, it can occur by two ways: when the animal eats seeds or fruits with seeds they can be deposited in another location through the excrement; the second is when the seeds cling to the animal´s body. In both cases the distance moved depends on the behavior of the animal (e.g. migrating birds versus rodents). Seed dispersal can also occur by water flow from one place to another (e.g. via rivers).
Seed dispersal can also occur through dehiscent pods that open and release seeds to the environment or by seeds that remain in the soil after the harvest. In these cases the seeds are not likely to move long distances (Andersson & Vincente, 2009).
- The influence of seed production and distribution system to adventitious mixing
African farmers utilize two types of seed systems: formal and informal. The formal system is well organized and usually used by commercial farmers, while the informal system is used by approximately 80% of farmers in Africa who are small or subsistence farmers (Van Mele et al., 2011, Martens et al., 2012, Tansey, 2011). The formal seed system, has certified seeds, which are multiplied and distributed by registered enterprises (Van Mele et al. , 2011). For the formal systems procedures should be in place to ensure that admixture between GM and non-GM does not occur.
In an informal seed system, the seeds are saved by farmers, and distributed by registered or unregistered traders and vendors (Van Mele et al., 2011). Seed systems in African countries
Often do not have an efficient production chains, due to the economical needs of farmers and unstable local markets. Some seed companies are involved in seed trade, but they are usually small companies, as most of the seed purchase is by small farmers (Martens et al., 2012, Tansey, 2011). In an informal system, seed mixing and adventitious presence is more difficult to control than in the formal systems and is more likely to occur.
- How can adventitious mixing be minimized?
Adventitious mixing can occur during sowing, harvest, transportation or storage. There are measures that can be taken to minimize adventitious mixing, but none can completely prevent mixing. For this reason in many countries, legal thresholds have been established. The exact threshold can vary depending on the country (Ceddia, et al., 2011, Consmüller et al., 2009, Devos et al., 2009). Examples of threshold levels are shown in table 1.
Steps that can be taken to minimize adventitious mixing include:
- Cleaning of all machinery or equipment to remove residual seeds to prevent moving seed from one field to another (Ceddia, et al., 2011, Consmüller et al., 2009);
- Sealing of all containers or packages with seed during transport to prevent loss when moving from one place to another;
- Storing GM separated from non GM seed;
- Labeling of containers carrying GM seed to prevent unintended mixing (Gru`ere et al., 2008);
- GM growers should also inform the neighbors so that farmers of the neighboring fields can take actions, such as spatial (isolation distance) if needed.
- Examples of coexistence between GM and non-GM crops
Many countries are successfully growing both GM and non-GM crops. Some examples of developing countries producing these crops include:
- South Africa. South Africa is the major African country planting GM crops. This country began growing GM crops in 1997, and has occupied the 9th position (James, 2014) with respect to worldwide production of GM crops since 2008. GM crops grown in South Africa include: maize cotton and soybean, (James, 2008, James, 2012, IFPRI, 2013, South Africa Department of Agriculture, Forestry and Fisheries, 2009).
- Burkina Faso. The first field tests of Bt cotton were successfully performed in Burkina Faso in 2003, leading to the adoption of this crop in 2008 (James, 2008) and its commercial production (ABNE, 2009). As of 2012, Burkina Faso was in the 14th position among the 28 countries worldwide producing GM crops (James, 2012, James 2014). Farmers in Burkina Faso freely choose to grow conventional, organic or Bt cotton and thus successfully achieve coexistence of these three types of non GM and GM crops.
- Sudan. Is another African country which has been growing GM crops (ABNE, 2009, James, 2014), where it occupies the 19th position worldwide (James, 2014).
- India. India is another developing country that has been cultivating GM cotton. Since adopting cotton in 2002, India is now in the 4th position for worldwide production of GM crops (James, 2012, James, 2014). Economic gains for the Indian farmers are highlighted as the main reasons for this quick adoption (James, 2012, Singla et al., 2013, VIB, 2013).
Table 2 – GM crops that are under development for future production in Africa
Mode of Propagation
Mode of pollination
Agents of pollination
Agents of seed dispersal
Recommended Isolation distance (m) for pure seed production (1%)
Banana (Musa spp. )
underground stem (corm)
cultivated bananas are sterile
insects (but cultivated bananas are sterile)
the flowers are sterile
Cassava, (Manihot esculenta Crantz)
predominately cross-pollination, limited self-pollination in varieties with many branches
insects and wind
insects and animals
Cotton (Gossypium hirsutum L. )
wind, animals, and water
100 – 200 1,3
Cowpea (Vigna unguiculata (L.) Walp.)
Dehiscent pods, ants, birds, and rodents
Maize (Zea mays L. )
Potato (Solanum tuberosum L. )
self- and cross-pollination
insects and wind
buried in soil, water and animals
Asian rice (Oryza sativa L.)
insect and wind
buried in soil, water and animals
Sorghum (Sorghum bicolor (L.) Merr.)
wind, water and animals
Sweet potato (Ipomoea batatas (L.) Lam.)
predominately cross-pollination but rarely flower
Birds and water
No report of pollen studies 1,7
*based on Oryza sativa L. data.
Isolation distance of GM crops that are under development for future production in Africa
Table 2 – GM crops that are under development for future production in Africa
1Andersson et al., (2010)
2Halsey et al., M.E., (2008)
4Conner & Dale, (1996)
6Messeguer et al. (2001)
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