Features — 08 March 2013 — by Lisa Carne

And, what is Bt-corn?

GMO refers to any genetically modified organism; meaning a piece of DNA from one species, often from a bacterium, is spliced into the DNA of another species. This discussion focuses on a specific genetically engineered (GE) crop species, Bt-corn, with a background on the difference between traditional, conventional and sustainable cropping systems and an emphasis on environmental effects, real and potential, of Bt-crops.

Traditional cropping systems often grow multiple crops, having different canopy heights and growing seasons, and may include crop rotation and livestock versus conventional cropping systems that are often mono-cultures with short, repetitive growing seasons and high energy input (machinery, fertilizers, herbicides, pesticides).

Conventional cropping systems are profit-driven (versus subsistence) so they may or may not rotate crops or use fallow periods. Sustainable cropping systems cause minimal land degradation or reduction in natural capital/resources, maintain ecological integrity, yet are still profitable to the farmer.

Many researchers evaluating these farming techniques point out that using universal indicators to assess cropping systems is necessary; not just yields but soil quality, nutrient balance, organic matter, water holding capacity, biodiversity, etc., all need to be considered to evaluate total sustainability of the system over only yields (Altieri 2002, Seufert et al 2012).

The first GE approved crop in the USA was the FlavrSavr tomato in 1994 that was altered to extend shelf life. Since then GE crops have expanded over 60 fold (1996-2006) globally to over 102 million hectares in 22 countries, with the USA leading in global production of GE crops by >50% (Icotz and Stotsky 2008). The most common GE crops are the glycophosphate (herbicide) resistant commercial crops, so that farmers may apply RoundUp to kill weeds without harming their crops.

The next most common GE crops are Bt-crops. This refers to using pieces of DNA from a bacteria that is a natural pesticide to pests that eat corn, cotton, tobacco, soybean, potatoes, etc., such that the GE plant produces its own insecticide. There are numerous strains of this bacteria related to the different pest species for each crop, and it is important to understand that most GE crops now have what’s called “stacked genes”.

This means they can be engineered to be both herbicide tolerant and/or produce up to 60 different pesticide proteins, called Cry proteins. This complication is just one of many in experiments designed to evaluate yields and environmental effects of GE crops versus sustainable (organic crops).

Higher yields with less input (money/time/amount of land) is the most cited benefit of GE crops, thus the reminder above that yields alone cannot be the single indicator of a successful cropping system, but also soil quality, long term environmental effects, etc. Studies attempting to compare organic crops with GE crops are crippled by a number of variable factors: crop types (fruits, oilseeds, cereals, vegetables, etc.), geographic location (temperature, rainfall, etc.), management practices (irrigation versus rainfall, tilling versus non-tilling, rotation with non-food crops, etc.) and soil quality.

Soil has too many different classifications to describe here, but crucial to agriculture is the water-holding capacity of soil, and its microbial (bacteria, fungi, etc.) community, because they play the important role of nutrient (carbon, nitrogen, phosphorous) cycling, decomposition of organic matter, etc. Furthermore, soil quality and function is affected by pH, temperature and other physical factors.

So, just what is a Bt-crop? These GE crops produce insect-killing proteins in every cell: roots, stems, leaves, fruits, pollen, seeds, etc. An important comparison is the older method of applying insecticides, by spraying.
While this method has its own associated environmental effects, it is a topical application, thus subject to biodegradation by sunlight and rainfall, such that by harvest time most traces are gone.

Not so with Bt-crops: because the many and varied Cry proteins are produced in all the plant cells, they enter the soil through the roots and fallen plant matter.
Difficulties in studying the effects of these Cry proteins in soil are many: lack of full understanding of soil microbial activity/function, short term-laboratory studies versus long-term field studies, the varied parameters mentioned above including that >60 Cry proteins exist and teasing out effects on soil community structure and function is nearly impossible. With that said, no study disputes these facts:

-When these Cry proteins are absorbed/bound to clay particles, they become more stable and less subject to biodegradation.

-Such that these insecticide proteins are found in all soil samples from Bt grown crops,

-And downstream in rivers, potentially altering fresh water ecosystems,

-And in the intestinal tracts and feces of cows, and

-In the feces of certain insects.

This also means that we humans are ingesting these Cry proteins without knowing any short or long term repercussion and, shows, significantly, that these Cry proteins have a persistence and accumulation in both soil and aquatic ecosystems, as well as in any animal that eats them.

Important to note: these genetic modifications are carried in the pollen/seed of the plants, and the next topic to consider is the effect of cross-pollination with GE and local/wild plants.


Altieri, M. (2002) Agroecology: the science of natural resources management for poor farmers in marginal environments. Agriculture, Ecosystems and Environment 1971:1-24.

Benbrook, C. M. (2012) Impacts of genetically engineered crops on pesticide use in the U.S.-the first sixteen years. Environmental Sciences Europe Vol 24(24):1-13.

Flores, S., Saxena, D. and Stotzky, G. (2005) Transgenic Bt plants decompose less in soil than non-Bt plants. Soil Biology and Biochemistry Vol 37: 1073-1082.
Icoz, I. and Stotzky, G. (2008) Fate and effects of insect-resistant Bt crop in soil ecosystems. Soil Biology and Biochemistry Vol 40: 559-586.

Seufert, V., Ramankutty, N. and Foley, J. A. (2012) Comparing the yields of organic and conventional agriculture. Nature Vol 485:229-234.

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