What a great question! It even caused me to put down my breakfast of coffee and mixed GMO and non-GMO grain cereal. Note in passing that, worldwide, coffee is being devastated by coffee rust disease: resistance has been discovered, but one way to protect our crops from disease in the future will be biotechnology. An example of this is GMO blight-resistant chestnut, where a gene from wheat destroys the plant-damaging toxic chemical produced by the fungus, so rendering the tree disease-resistant.
But I digress. This is difficult to calculate perfectly, but we can do some back-of-the-napkin estimations from available data:
- In the commercial Bt for application (www.cdms.net/ldat/ld4KK005.pdf), the bacteria is 54 percent, and the application rate (from the label) is about 1 lb. per acre, with repeat applications every 3–14 days; this is then about one-half lb. per acre per application. Let’s say every seven days for a crop life of four months; then the amount would be 8 lb. of Bt bacteria per acre.
Now to a Bt crop, using corn (maize) as the example:
- EPA gives the level of Bt in the grain of corn (MON-810) as 0.3ng/g of grain(1 ng is one-billionth of a gram, and a gram is 1/28 of an ounce), or 0.00003 percent by weight in the grain.
- The USDA reports average U.S. corn yields as 160 bu/acre in 2013 (a bushel of corn is 56lb, so the weight of corn grain is 8,960 lb./acre).
- At 0.00003 percent, the amount of Bt in the grain is 0.2588 lb. per acre.
- But of course that is not the whole story: what about the green parts of the plant? The stover yield is 3–4.5 (6,000–9,000 lb.) dry tons per acre.
- And EPA lists the Bt in the leaf as 10ng/g, or 0.001 percent, so the Bt in the leaves would be 6–9 lb. per acre.
This is interesting! It means that the Bt in Bt-containing crops contains about the same level of Bt as is applied by organic farmers when they spray the bacteria. Now, of course, the bacteria is not 100 percent Bt protein, so I need to check with colleagues on that and amend this answer when I have that information.
However, note that EPA also reported that plant-produced Bt biodegraded over 42 days and had a half-life of 4.5 days in test soils. As Bacillus thuringiensis is a natural soil bacterium, there is an additional amount of Bt in soils from this natural source.