Guys - it's not rocket science. It's biology 101: All plants allocate the resources available to them, with various degrees of efficiency, to perform the various functions of being a plant. They do all kinds of stuff with them - for one thing, they grow and establish and defend the mechanical means by which they obtain the resources in the first place. But each thing they do, each expression of code, each function performed and structure grown, costs them some of their limited resources. There is no free lunch. A certain amount of the work put into Task A involves energy lost, some of the resources devoted to Task A are permanently sequestered in physical structure and no longer available for other Tasks. Nothing can be in two places at the same time, no machine can be perfectly efficient. Nitrogen unrecoverably put into stems and leaves is nitrogen no longer available to put into corn kernels. Neither is at least some of the energy used to put it there, or recover whatever could be. Resources - energy, nutrients - diverted and devoted to the structures and functions of Bt expression, or glyphosate poisoning resistance, or any other damn thing, is thereby made unavailable to put into corn kernels - or stems and leaves, for that matter. From our pov, a yield hit. This is widely known as the 2nd Law of Thermodynamics. If you divert some of a plant's available resources into new structures and new functions, on top of the old ones, not present before and in addition to and all else being equal and so forth, the old functions are left with less in the way of available resources. This is physical reality. Bt expression and glyphosate resistance are two such structures and functions. So are all of the other currently marketed GMs, as far as I know. A yield hit from this is inevitable - no yield hit would violate the 2nd Law. The only question is how big it is, and whether the other advantages (which might even result in net yield gains, in certain circumstances) make up for it. One of the many costs of the current GMO development and marketing is the spread and imposition of the bad side of industrial agriculture incorporated within it - the ecological, economic, and political damage done. This is a common area in which GMO proponents seem oblivious to the objections of critics, and end up accusing them of being anti-science or irrationally against "GMOs". Sure you can. It's a whole shelf in the Blaine Walmart. Or so I'm told, by apologists for the current GMO deployments. Are you pretending to not understand the argument, or honestly oblivious? Let's try an example: One documented method of suicide among farmers ruined by the economic damage of the currently marketed GMOs, is by drinking the chemicals necessarily on hand in current GMO farming - especially (because they work better) the more poisonous and more expensive ones that were not supposed to be needed any more, but turned out to be essential and used in large quantities. Does that count as "killed by GMOs"? The risks to humans are ecological, economic, and political, not just medical. And they are quite serious, large, scary risks. They are larger and more serious even than they need to be, because the commercial promulgators of GMOs seem to be oblivious to them - or worse, dismissive. That little fairy tale is nothing like what actually happened, of course. And it has little to do with GMOs. You do know that? There was a blight, for example, the Irish Potato Famine, that occurred long after the disease-resistant crops had been bred. There were many varieties of potato available, from centuries of farming and breeding and so forth in the northern Andes mountains, some of them blight resistant, but they were unsuited to the needs of the early manifestations of industrial agriculture taking hold in the British Isles at the time. Also, the basic research had not been done, so the threat was unknown. When the imprudently rapid - commercially/politically/economically pushed and promoted - expansion of an unfamiliar mode of agriculture crossed paths with one of its (several) key vulnerabilities, all of them unknown, the population of the region that had adopted (been pressured and forced to adopt, actually) this single, dependent, highly productive mode of food production, dropped by half within about five years. Millions starved. Millions emigrated, desperately poor. Fortunately, they had somewhere to emigrate to without war - an unusual, almost unique circumstance in human history. It could have been much worse than mere mass starvation. Other examples? The Dust Bowl, Yellow River, and Mexico City region ecological disasters of history, the financial or credit crunch famines of places like Haiti and other post-colonial degradations. There's a fairly long list of these. Every known risk - medical, ecological, economic, political - of the current deployment of the currently deployed GMOs has caused disaster in the past. That's why we call them "known risks". The issue we face with them is largely the new scale - from islands to continents. The issue we face with the unknown is larger yet.