What are the speakers mainly discussing? Why does the professor discuss the minimum till farming method?

Listen to part of a lecture in a plant ecology class. (P = Professor, M = Male Student, F = Female Student)
P: Last class we began talking about a herbicide, a weed killer that’s been important for farmers. It was developed in the 1970s and is the most widely used method by farmers for killing weeds in the United States. I’m talking of course about glyphosate. Now, remember, we said glyphosate works by interfering with the synthesis of certain amino acids in weeds, and when it was first introduced, glyphosate was considered a miracle chemical. That’s because it was a wide spectrum herbicide, meaning it killed a wide range, a large variety of weeds. It was safe for farmers to work with it and it wasn’t toxic to cows or other farm animals. And because it broke down quickly after it was applied, when it decomposed, its environmental cost was pretty low. Researchers have developed crops, genetically engineered crops that aren’t damaged by glyphosate, while the herbicide kills the surrounding weeds. But what do you think happens with these weeds after a while? Jim?
M: Well, I mean weeds evolved so they must eventually become resistant to the weed killer.
P: Yeah. And that’s what’s happening with glyphosate. In over 20 states in the United States we have unwanted plants, weeds that can now tolerate many herbicides including glyphosate. They are known as super weeds and they are real threat to agriculture. For one thing they threaten to make minimum till farming impossible. Minimum till farming is a common plowing method that doesn’t affect deeper layers of soil. Just upper layers. It’s an environmentally conscious method because it helps conserve organic matter in soil and cuts back on soil erosion. But one disadvantage of this method is that it tends to allow grass, weeds, to germinate more easily. So without an effective herbicide, minimum till farmers end up spending a lot of time trying to get rid of super weeds, sometimes even by going in and manually pulling the weeds out of their fields.
F: Huh, weeding like that might work in a small garden, but we must be talking about millions of acres of land here.
P: Exactly. It’s highly inefficient. So finding an alternative to glyphosate is really an urgent issue. And a recent research study shows a promising potential way to do this. The researcher’s goal, rather than developing new herbicides, was to find new ways to engineer crops that are resistant to existing herbicides, so the crops grow unharmed while herbicides get rid of super weeds. The researcher’s breakthrough came when they identified two bacterial enzymes that can help protect some plants from some herbicides. Well, more specifically they figured out how to use these two bacterial enzymes to engineer corn and soybean plants so that they are resistant to a herbicide called 2,4-D.
F: 2,4-D. It is the name of the herbicide?
P: Yes. An unusual name, right? 2,4-D is a very commonly used weed killer worldwide. It’s used frequently to control dandelions. Now I mentioned earlier that glyphosate targets amino acid synthesis. We don’t need to get into this specific of these processes, but 2,4-D is a hormone regulator so it works differently. And because 2,4-D works in a different way from glyphosate, it’d be an ideal herbicide to use on super weeds that are now resistant to glyphosate. So when researchers pinpointed the bacterial enzymes that resist 2,4-D, they went ahead and engineered the corn plant to produce one of those enzymes and the soybean plant to produce the other.
M: How?
P: Well, they use pretty sophisticated techniques involving plant seeds. They manipulate the seeds with the enzymes, so the plants will eventually grow to be resistant to 2,4-D. So after growing these corn and soybean plants, the researchers tested them by treating them with 2,4-D both in greenhouses and in fields. They’re done that no matter what the condition, the plants were highly resistant to the herbicide. None of the usual effects of treatment were exhibited, malformation or lower yield, meaning lower production of corn or soybeans. This is promising because if we can get the right crop plants to be sort of immune to 2,4-D, that means that 2,4-D could be used as a weed killer to get rid of the super weeds that would otherwise try to grow around them, which would solve the super weed problem for now, anyway. But let’s not forget what Jim pointed out before.
M: Weeds evolve?
P: Exactly.