요약:
- 농민들은 2013년 전체 작물 재배면적의 약 절반에 이르는 1억7000만 에이커에 유전자변형 작물 -주로 옥수수, 목화, 대두- 을 심었다.
- 해충 관리 특성은 유전자변형 작물로 설계된 주요 특징인데, 시간이 지남에 따라 해충에 대한 추가적 보호를 제공하는 특성과 몇 가지 특성이 복합된 종자를 농민들이 신속하게 도입했다.
- 연구결과에 의하면, Bt 종자는 수확량을 증가시키고 살충제 사용을 감소시켰으며, 제초제 저항성 종자는 농민들이 독한 대안 대신 덜 독한 제초제로 대체할 수 있도록 하고 보존경운을 도입하도록 촉진했다.
해충 관리 특성을 지닌 유전자변형(GE) 작물이 처음 상업적으로 이용된 건 1996년이다. 15년이 지난 현재 미국 농민들이 이러한 작물 품종을 널리 채택하고(2013년 1억7000만 에이커에 심음), 이 유전자변형 작물에서 유래된 많은 생산물 -콘밀, 기름, 당분- 이 가공식품에 일반적으로 활용되고 있다. 작물의 영양 성분과 작물학의 특성에 영향을 미치는 일부 유전자변형 종자가 이미 상업화되고, 여러 유전자변형 종자가 개발과 시험 단계에 있으며, 지금까지 미국 농민들에게 판매된 유전자변형 종자는 해충 관리를 위한 것(여기에서 해충은 작물의 생산을 방해하는 곤충과 잡초 및 여타의 다른 유기체를 포함하는 것이라 정의함)이다. 제초제 저항성(HT) 종자는 농민들이 작물에 해를 끼치지 않고 잡초를 통제할 수 있는 특정한 효과적인 제초제를 활용할 수 있도록 한다. 기타 유전자변형 종자는 토양의 박테리아인 Bacillus thuringiensis(Bt)가 자연적으로 생산하는 살충성 단백질을 이용하여, 식물은 보호하면서 특정한 곤충에게만 독성을 갖는다.
미국 농민들은 해충 저항성 종자보다 제초제 저항성 종자를 더 많이 활용하는 경향이 있는데, 이는 잡초가 만연한 문제이기 때문이다. 제초제 저항성의 채택은 특히 대두 농사에서 급속도로 빨랐는데, 미국 농민들은 2013년 전체 대두 재배면적의 93%를 제초제 저항성 대두로 재배했다. 2013년 제초제 저항성 목화는 전체 재배면적의 82%, 제초제 저항성 옥수수는 전체 재배면적의 85%를 차지했다. 해충의 만연은 잡초의 만연보다 더 지역화되는 경향이 있다. 농민들은 2013년 목화 재배면적의 75%에 Bt 목화(회색담배나방, 목화다래벌레, 솜벌레 같은 해충을 통제하려고 설계된)를 심었다. Bt 옥수수는 2013년 옥수수 재배면적의 76%에 심었다.
미국에서는 옥수수, 목화, 대두가 유전자변형 작물의 재배면적 가운데 대부분을 차지하는 한편, 상업적으로 재배되는 기타 유전자변형 작물에는 제초제 저항성 카놀라, 제초제 저항성 사탕무, 제초제 저항성 자주개자리, 바이러스 저항성 파파야, 바이러스 저항성 호박 등이 포함된다. 상업적으로 도입된 이후, 해충 관리 특성을 지닌 상업적으로 이용할 수 있는 유전자변형 종자의 품종은 더 넓은 범위의 곤충에 대한 저항성과 더 많은 제초제에 대한 저항성을 포함시킬 뿐만 아니라 제초제 저항성과 Bt 형질을 통합시키는 등 복잡해졌다. 이러한 혁신을 통해 유전자변형 종자의 가격은 명목 및 실질 가격이 모두 높아졌다. 농민들이 유전자변형 품종을 급속히 채택한 것은 유전자변형 종자가 사용할 만한 가치가 있을 정도로 성과를 개선시킨다거나 여타의 혜택을 제공한다는 믿음과 일치하지만, 연구결과는 무엇을 제시하는가?
유전자변형 종자와 해충으로 인한 수확량 손실
해충이 없으면 상업적으로 이용할 수 있는 유전자변형 종자는 작물 수확량의 최대치를 증가시키지 못한다. 그러나 특정 해충으로부터 식물을 보호함으로써 유전자변형 작물은 해충으로 인한 수확량 손실을 막을 수 있기에, 식물이 그 수확량 잠재력에 도달하도록 할 수 있다. Bt 작물은 수확량 손실을 완화시키는 데 특히 효과적이다.
Bt 옥수수의 평균 수확량은 새로운 해충 저항성 특성이 종자에 통합되고, 복합적 특성을 이용할 수 있게 되면서 증가했다. 1996년, Bt 옥수수는 조명나방이란 한 가지 유형의 해충에만 저항성을 지녔다. 그 이후 옥수수뿌리벌레(2003년)와 큰담배나방(2010년)에 저항성을 지닌 Bt 옥수수가 도입되었다. 대부분의 실험 농지와 농장 조사에서는 기존 작물보다 Bt 작물의 평균 수확량이 더 높은 것으로 밝혀졌다. 미국 농무부의 Agricultural Resource Management Survey(ARMS)에서도 2005년에는 기존 옥수수 수확량보다 에이커당 17부셀이, 2010년에는 약 26부셀이 더 높은 것으로 밝혀졌다. 또한, ERS 연구진은 다른 요인들을 통제하는 계량경제모델을 활용하여 Bt옥수수를 채택한 비율이 10% 증가한 것이 2005년에 수확량이 1.7% 높아지고, 2010년에 2.3% 높아진 것과 연관이 있다는 사실을 발견했다. 또 연구진은 1997년 Bt 목화의 채택이 10% 증가한 것이수확량에서 2.1% 높아진 것과 연관이 있다는 사실도 발견했다.
한편, 대두와 옥수수, 목화의 수확량에 제초제 저항성 종자가 미친 영향에 대한 증거는 복합적이다. 일부 연구진은 제초제 저항성 작물을 채택한 사람과 그렇지 않은 사람 사이에 뚜렷한 차이가 없다고 하며, 다른 연구진은 제초제 저항성을 채택한 사람들의 수확량이 더 높았다고 한다.
농업자원관리조사의 옥수수 자료를 분석한 결과, 여러 유전자변형 특성을 지닌 종자가 기존 종자나 한 가지 유전자변형 특성을 지닌 종자보다 수확량이 더 높았다고 나타났다. 예를 들어 2010년 농업자원관리조사 자료에서는 기존 옥수수 종자가 1에이커당 134부셀의 평균 수확량을 올렸다고 나오는 반면, 두 유형의 제초제 저항성(글리포세이트와 글루포시네이트)과 세 가지 유형의 해충 저항성(조명나방, 옥수수뿌리벌레, 큰담배나방)을 지닌 종자는 에이커당 171부셀의 평균 수확량을 올렸다고 한다.
당연하게도 여러 특성을 지닌 종자의 품종을 채택하는 비율이 빠르게 증가했다. 여러 특성을 지닌 옥수수 종자의 사용은 2000년 재배면적의 1%에서 2013년에는 71%로 증가했다. 이제 3-4가지 특성을 지닌 유전자변형 품종이 일반적이다.
Bt seeds, net returns, and household income
The market price of seed incorporates the costs associated with seed development, production, marketing, and distribution. The price of GE soybean and corn seeds grew by about 50 percent in real terms (adjusted for inflation) between 2001 and 2010. The price of GE cotton seed grew even faster. The increase in GE seed prices can be attributed in part to increasing price premiums over conventional seeds associated with the rising share of GE seeds with multiple (stacked) traits and /or more than one mode of action for particular target pests Another factor contributing to the increase in seed prices is the improvement in seed genetics (germplasm).
The profitability of GE seeds for individual farmers depends largely on the value of the yield losses mitigated and the pesticide and seed costs, which vary by crop and technology. Most studies show that adoption of Bt cotton and Bt corn is associated with increased net returns/variable profits. However, some studies of Bt corn show that profitability is strongly dependent on pest infestation levels (adoption of Bt cotton and Bt corn was associated with increased returns when the pest pressure was high).
The evidence on the impact of HT seeds on net returns is less consistent. Several researchers found that the adoption of herbicide-tolerant cotton had a positive impact on net returns. However, other researchers found no significant difference between the net returns of adopters and non-adopters of HT soybeans, and others found that HT soybean farmers are less profitable than their conventional counterparts. Overall, the empirical evidence on the impact of adopting herbicide-tolerant soybeans on net returns is inconclusive.
The fact that adoption of HT crops has been continuously rising, even though several researchers found no significant differences between the net returns of adopters and non-adopters, suggests that adopters derive other benefits. In particular, weed control for HT soybeans may be simpler and more flexible (e.g., HT seed-based production programs allow growers to use one product to control a wide range of both broadleaf and grass weeds instead of using several herbicides to achieve adequate weed control), freeing up valuable management time for leisure, or to generate enterprise growth or off-farm income.
ERS research shows that HT adoption is associated with increased off-farm household income for U.S. soybean farmers, most likely because time savings associated with HT crops were used in off-farm employment. More recently, other researchers confirmed that GE crops led to household labor savings and that farmers adopting GE crops derived value from the convenience, flexibility, and increased worker safety associated with growing HT crops that enable them to use fewer toxic herbicides.
Adoption and pesticide use
Studies based on field tests and farm surveys have examined the extent to which GE crop adoption affects pesticide (insecticide and herbicide) use, and most results show a reduction in pesticide use. A 2010 National Research Council study concurred that GE crops lead to reduced pesticide use and /or to use of pesticides with lower toxicity compared to those used on conventional crops.
Generally, Bt adoption is associated with lower levels of insecticide use. Pounds of insecticide (per planted acre) applied to corn and cotton crops have decreased steadily over the last 10 years (except for cotton in 1999-2001, when application levels were distorted during the boll weevil eradication program).
Insecticide use trends suggest that insect infestation levels on corn and cotton farms were lower in 2010 than in earlier years and are consistent with the fact that European corn borer populations have steadily declined over the last decade. In addition, several researchers have shown that areawide suppression of certain insects such as the European corn borer and the pink bollworm are associated with Bt corn and Bt cotton use, respectively. This suggests that Bt seeds have benefited not only adopters but non-adopters as well.
Herbicide use on corn, cotton and soybean acres (measured in pounds per planted acre) declined slightly in the first years following introduction of HT seeds in 1996, but increased modestly in later years. Despite the relatively minor effect HT crop adoption has had on overall herbicide usage, HT crop adoption has enabled farmers to substitute glyphosate (which many HT crops are designed to tolerate) for more traditional herbicides. Because glyphosate is significantly less toxic and less persistent than traditional herbicides, the net impact of HT crop adoption is an improvement in environmental quality and a reduction in health risks.
HT crops and conservation tillage
Conservation tillage (including no-till, ridge-till, and mulch-till) is known to provide environmental benefits and is facilitated by use of HT crops. By leaving at least 30 percent of crop residue covering the soil surface after all the tillage and planting operations, conservation tillage reduces soil erosion by wind and water, increases water retention, and reduces soil degradation and water/chemical runoff. In addition, conservation tillage reduces the carbon footprint of agriculture.
By 2006, approximately 86 percent of HT soybean planted acres were under conservation tillage, compared to only 36 percent of conventional soybean acres. Differences in the use of no-till were just as pronounced. While approximately 45 percent of HT soybean acres were cultivated using no-till technologies in 2006, only 5 percent of the acres planted with conventional seeds were cultivated using no-till techniques, which are often considered the most effective of all conservation tillage systems. Cotton and corn data exhibit similar though less pronounced patterns.
These trends suggest that HT crop adoption facilitates the use of conservation tillage practices. In addition, a review of several econometric studies points to a two-way causal relationship between the adoption of HT crops and conservation tillage. Thus, in addition to its direct effects on herbicide usage, adoption of herbicide-tolerant crops indirectly benefits the environment by encouraging the use of conservation tillage.
Future trends
The acceptance of GE crops by farmers has been due, in large part, to the pest management traits incorporated into GE seeds. Farmers were willing to adopt GE seeds because their benefits exceeded their costs, while domestic consumers were largely indifferent to these traits. But how long can farmers expect to benefit from the pest management traits engineered into the seeds currently commercially available? And, what other traits might be engineered into seeds that would attract farmer and consumer interest?
As with other efforts to control agricultural pests, pests will inevitably develop resistance to the pest management traits incorporated in GE seeds. Prior to the commercial introduction of Bt crops, entomologists and other scientists persuasively argued that mandatory minimum refuge requirements (planting sufficient acres of the non-Bt crop near the Bt crop) were needed to reduce the rate at which targeted insect pests evolved resistance. Analysis of more than a decade of monitoring data suggests that minimum refuge requirements and natural refuges have indeed helped delay the evolution of Bt resistance in some insect pests. However, Bt resistance in western corn rootworm, cotton bollworm, and fall armyworm populations leading to reduced efficacy of Bt corn and Bt cotton has been recently documented in some U.S. crop fields.
Likewise, an overreliance on glyphosate and a reduction in the diversity of weed management practices by HT crop producers contributed to the evolution of glyphosate resistance in 14 weed species in the United States. Because no new major classes of herbicides have been made commercially available in the last 20 years, and because few new ones are expected to be available soon, growing resistance to glyphosate is expected to reduce the benefit farmers derive from using the most widely available HT seed varieties. Furthermore, the weed management practices needed to slow the spread of glyphosate-resistant weeds may themselves reduce the short-term benefits of planting glyphosate-tolerant (i.e., HT) seeds. As a result, their benefits may erode over time in the absence of further developments affecting HT seeds and their associated herbicides and/or improvements in weed management practices. one such development is the introduction of crops tolerant to the herbicides dicamba and 2, 4-D if used in the context of a diversified approach to weed management.
While relatively few GE traits are currently commercially available, the number of field releases to test GE varieties approved by USDA’s Animal and Plant Health Inspection Service indicates continued GE-related R&D activities since field testing is a critical part of seed development. The number of field releases grew from 4 in 1985 to 1,194 in 2002 and has since averaged around 800 per year. Other measures suggest that GE-related R&D activity has increased dramatically since 2005.
Field releases approved for GE varieties continue to focus heavily on herbicide tolerance and insect resistance, but other traits are being developed and tested in large numbers as well. These include traits that provide favorable agronomic properties (resistance to cold/drought/frost/salinity, more efficient use of nitrogen, increased yield); enhanced product quality, such as delayed ripening, flavor and texture (fruits and vegetables); increased protein or carbohydrate content, fatty acid content, or micronutrient content; modified starch, color (cotton, flowers), fiber properties (cotton), or gluten content (wheat); naturally decaffeinated (coffee); and nutraceuticals (added vitamins, iron, antioxidants such as beta-carotene).
New HT and insect resistance traits may give farmers more pest management options and slow the spread of pesticide resistance among pest populations. Approval of other “first generation” traits that improve yields or reduce yield losses could result in further adoption of GE varieties. Farmer response to the approval of “second generation” traits that alter end product quality may be more cautious. Farmers can expect to benefit from the adoption of these GE traits only if consumer acceptance is assured. In short, the future of GE seed use depends on the ability of farmers to adopt best management practices, the ability of biotech companies to develop new GE varieties, and consumer acceptance of products from GE sources.
This article is drawn from...
Genetically Engineered Crops in the United States, by Jorge Fernandez-Cornejo, Seth James Wechsler, Michael Livingston, and Lorraine Mitchell, USDA, Economic Research Service, February 2014
http://www.ers.usda.gov/amber-waves/2014-march/adoption-of-genetically-engineered-crops-by-us-farmers-has-increased-steadily-for-over-15-years.aspx#.U-FCb41_vEh
