OK, sorry to spam here, but I found another relevant link:
https://www.geneticliteracyproject.org/2015/12/14/gmo-patent-controversy-terminator-genes-seed-piracy-forcing-farmers-buy-seeds/

That confirms that "terminator seeds" are a technology that does exist -- in practice as well as theory -- but isn't commercially sold or otherwise available.

More importantly, it mentions that there is a legal sort of "license agreement" that farmers sign when they buy the GM seed stating that they won't replant harvested stuff in the future, and that they can inspect your storage areas etc. if they suspect you are doing so. That would explain why my family worked hard to clean out storage areas that had grain resulting from GM seed. I erroneously thought that was because that grain was sterile.

Those agreements are for a single crop / harvest, so it is definitely possible to use a GM seed one year and then go back to non-GM seed in other years, as my family does. I guess that we just have to keep good records to show that we're not holding any back for future planting / sale.

Sorry for the several LONG posts, and thanks to @Hastur for asking the question that got me to figure out the misconception I had!

**EDIT**
I'm finding other sources that say that sterile "terminator seeds" are a patented technique, but that Monsanto has promised not to use it. Straight from the horse's mouth:
http://www.monsanto.com/newsviews/pages/terminator-seeds.aspx

So it appears that my info below is wrong. I will try to talk with my family and get the full story. That being said, I'll leave my original comment and the followup below unaltered.
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My firsthand knowledge of this stuff was from more than 10 years ago, and also when I was pretty young (early 20's). So I did some web searching to try to get updated since your question is a very interesting one:

http://web.mit.edu/demoscience/Monsanto/about.html

According to that, Monsanto is the company behind "Roundup Ready", and their corn (and other crops in the line) do use sterile "terminator seeds". It also mentions that farmers "must purchase the most recent strain of seed from Monsanto" each year.

I was never in the decision-making structure of my family farm, but I did remember that we couldn't just buy the Roundup Ready seed *once* and then hold a small amount back as seed for the next year and continue to get the benefits.

I'm not 100% sure exactly how the modification for sterility works -- I don't know if the plant will sprout if you plant the sterile seeds and just fail to produce any ears / fruit, or if it just won't germinate at all. I do remember that we had to be quite careful to fully clean out the corn grown from the GM seeds from our storage bins, and better yet to store our non-GM corn to be used for future seed in entirely different bins. That was done to make sure that we didn't end up planting any of the sterile stuff.

I'm sure that the seed dealers that sell the GM stuff really push farmers to buy and plant it every year, as hinted to in that link. But you certainly don't *have* to. On the other hand, if you go back to non-GM seed for a year or two or more, you can't use a strong herbicide like Roundup if you have an unexpected outbreak of weeds or other pest plants -- the Roundup would kill the non-GM crop along with everything else.

Basically, I don't specifically begrudge companies like Monsanto for their practices concerning these GM crops. The "terminator seeds" are controversial, but don't seem like a big deal to me. If you could buy GM seeds once and then just hold back some of your harvest for next season's seed, they'd only get your money once AND we'd probably lose the original strains. So I see that as kinda win-win, especially if you don't 100% buy into their sales department urging you to use GM seed every single year.

I don't want to sound like a shill for Monsanto -- some of their other practices are pretty shady, particularly political lobbying. But from the perspective of my family farm, the GM corn that we use was/is a real beneficial thing. Significantly less pesticide/herbicide use over time, and it allows for expanded low/no till farming. Before herbicides, tilling was one of the only ways to kill off pest plants. But, it also makes the fields lose some moisture and nutrients. Expanded farming and ubiquitous tilling was largely the cause of the "dust bowl" dirty 30's. Anyway, I'd say that a lot of good has come out of modernized techniques and technology like GM crops.

I think many people don't realize how GMOs have made farmers' lives so much easier.

I'm surprised to read what you said about your family's GM seeds being modified to be sterile though; the video states that terminator seeds were never commercialized. Since you're talking about corn, maybe it was just hybrid?

Some additional notes based on growing up in a wheat / corn farming family:

My family uses GMO herbicide/pesticide-resistant corn seed (Roundup Ready). It's a tradeoff, because:

1) Roundup Ready seed is somewhat expensive, especially compared to just holding on to a small amount of your own harvested crop as next year's seed.

2) Like the video mentioned, the GM seeds we used have been modified to be sterile, so the grain they produce can't be replanted. Part of the justification for that is not wanting the GM version to intermingle with unmodified strains. But, most is pure profit motivation -- they want you to be forced to buy that GM seed. I don't really see that as nefarious, just business -- but opinions differ.

3) My family discovered that for corn, we could us the GM Roundup Ready seed roughly once every 5 years while still benefiting from drastically reduced insect / plant pests. If corn is within pollination range of another less known crop plant called milo, the plants can hybridize and produce a plant called shattercane. Shattercane is essentially worthless as a food crop, but is very hardy, and can spread and in many cases outcompete the corn or milo that you really want.

Getting rid of it was a very difficult and intensive process -- until the GM seed came along. Now if we see shattercane starting to make incursions, we can plant the GM seeds the next year and then hit the field with a herbicide that kills the shattercane. It works so well that the field remains clear of the pest plants / insects for several years after that without having to use much if any herbicides / pesticides.

4) In our situation, we found that we used way less herbicide / pesticide per year on average once we started rotating in the GM seeds once every several years. That would be close to a wash, but still likely a net savings even if we used the GM seeds every year (seed companies will try to sell it to you every year). Factor in increased crop yields because of the reduced/eliminated pests, and it is a clear win.

5) I'm sort of worried about the potential for a "superbug" effect, similar to overusing / misusing antibiotics. If farmers buy into the GM seed thing 100% and use it every year, I think it will increase the chances / rate of the pests becoming resistant to the pesticides / herbicides used. That's a long-term concern, and in my opinion doesn't even come close to outweighing the "pro" side of the GM argument (at least from the perspective of my family's farm), but it is something to think about.

better bugs with gm seeds and pesticides.

In reply to this comment by Gibletses:
This is the cost of maintaining almost 7 billion humans. It's the cost. Pay it or perish.

When the US incorporated (1776), there were only one tenth the number of people there are now. Eliminate the factory farms and destructive ag then watch five billion people eat YOU.

For the veganoids; destructive ag (deep-till, fertilize, gm seed, irrigate, harvest, repeat) -- the process used to produce your tasty soy curd -- kills more animals than all of the factory meat production combined. Mostly through destruction of habitat. Most species extinction results from loss of habitat to farming so that you have food that makes you smug and makes other people assume you're feeble minded. (Guess what...)

In order for you to be smug behind a steamy pile of curd, thousands of animals are killed outright or die from habitat loss. The great plains now produce wheat, soy, and corn (for now...until the topsoil is gone). Do you realize how many elk, bison, wolves, foxes, voles, skunks, etc. were killed to do this? Do you know what the run-off from those crops does to streams, rivers, and the ocean? Death on a massive scale, that's what. Vegans are the worst hypocrites of them all.

I'm sorry, but your argument makes no sense. The bulk of current soy and corn crops right now are used for animal feed, in a process that only produces 1 calorie of meat for every 7 calories of vegetables used. Reducing, or removing, meat production could effectively halve the amount of farmland which needs to be sowed and maintained.

Livestock is the most ineffective way to produce food and nutrition. By reducing it you could effectively support larger populations of humans with fewer crops grown.

Just to hammer the point in, a lifestyle focused on less meat and more vegetables would require LESS farmland and agriculture, not MORE.

This is the cost of maintaining almost 7 billion humans. It's the cost. Pay it or perish.

When the US incorporated (1776), there were only one tenth the number of people there are now. Eliminate the factory farms and destructive ag then watch five billion people eat YOU.

For the veganoids; destructive ag (deep-till, fertilize, gm seed, irrigate, harvest, repeat) -- the process used to produce your tasty soy curd -- kills more animals than all of the factory meat production combined. Mostly through destruction of habitat. Most species extinction results from loss of habitat to farming so that you have food that makes you smug and makes other people assume you're feeble minded. (Guess what...)

In order for you to be smug behind a steamy pile of curd, thousands of animals are killed outright or die from habitat loss. The great plains now produce wheat, soy, and corn (for now...until the topsoil is gone). Do you realize how many elk, bison, wolves, foxes, voles, skunks, etc. were killed to do this? Do you know what the run-off from those crops does to streams, rivers, and the ocean? Death on a massive scale, that's what. Vegans are the worst hypocrites of them all.

I'll trust a person who likes their chick pulp wrapped in bacon, deep fried, smothered in cheese, and served in a bowl of gravy before I'll waste spit on a hypocritical vegan retard.

I, as another poster here, grow my own food in a sustainable manner (no-till method) and raise my own livestock in a sustainable manner. Where does YOUR food come from?