Monoculture crops unsuited to intercropping
Modern intensive agriculture usually involves the mass cultivation of singel crop species, like a field of wheat for instance. But plants didn't evolve to grow in a monoculture, and forcing them in that direction might mean wwwe're missing a trick in terms of yields and environmental sustainability. So thinking is shifting towards to use of mixtures of crops - this is called intercropping - where multiple species are grown side by side and where certain characteristics of one plant can benefit another. But the seeds we're using to plant these mixtures have often been bred specifically to grow as monoculture crops, so might not be the pick of the bunch. Speaking with Chris Smith, Laura Stefan, who's at Agroscope in Switzerland, wanted to explore this, and find out whether intercropped species might evolve to become better bedfellows again...
Laura - We were working on intercropping. So when we grow more than two species together, and we were thinking that usually intercropping is done with commercial seeds, but that have been bred for optimal performance in monocultures, they have been selected for decades so that they would yield better in a monoculture. And so we thought maybe these particular seeds are not the best for mixtures because the plants would have to behave differently. So we wanted to see if the crop species had this possibility to adapt.
Chris - Before we explore that. Mm-Hmm. <Affirmative>, what's the reason that scientists, agronomists, farmers, producers, do into cropping? Why do that at all?
Laura - So it's a way to increase the diversity of agricultural systems and to get a bit out of this intensive very low diverse systems which are not very good for the environment. Intercropping can be also positive in terms of finding species that are complementary to each other, and so that will a bit help each other to grow together. And this can have benefits for productivity.
Chris - Would a simple way of looking at this be then to say, well, you've got two different species, so they're not competing with each other directly for all the same things. There will be some competition in some respects, but one might help the other. For instance, by putting something back into the soil that boosts the other one mm-hmm. <Affirmative> or one's a bit bigger so it gives a bit of protection to the one that likes a bit of shade. Is, is that the kind of relationships that can be garnered from these sorts of mixtures?
Laura - Yes. One very famous example is when you mix a cereal with a legume because the legume can fix nitrogen from the air and so the cereal will have more access to this nitrogen.
Chris - It stands to reason I suppose, because naturally plants evolve to grow in communities, not just a monoculture. And so what we do with agriculture really is to force them into a situation that they aren't that evolved for. And then we select for ones that can tolerate it and you are saying, well, well actually might there be a better way where in fact we, we don't do that and we look for plants that are better at growing in communities and and also whether there is an evolution as they do so.
Laura - Yes, that's exactly what we were trying to do. We started with an intercropping experiments where we had six species growing in monocultures but also in mixtures of two and four species. And then we took these seeds at the end of the first year and the second year we planted again monocultures and mixtures, but each time with seeds that were either coming from the monocultures or coming from the mixtures.
Chris - And what were you measuring? What was the outcome? Was this yield or other growth parameters?
Laura - Yeah, we measured productivity, so biomass and yield. And we also measured some classic plant characteristics along the way.
Chris - So what you'll get at there is whether or not having a monoculture and then putting that into a community is less good than having a community, getting the seeds from that and then growing that again in a community. (Yes.) And seeing if there's some kind of evolution or development or, or benefit that comes from that.
Laura - Yes. We looked at a plant interaction index, which basically allows you to tell whether the plants in a community are more experiencing competition or facilitation. And we did see that when the seeds were coming from a mixtures, the plants were competing less.
Chris - How were they achieving that? Is that just because basically the ones that thrive and therefore the ones that set more seed and therefore the seeds you are more likely to plant are the ones that got on well in that situation, they were already there in the population? Or did that evolve and enrich with time?
Laura - The mechanisms are a bit hard to disentangle. So we thought over time the species would adapt so that they would use the resources better. We saw some indications of that the plants coming from a mixtures were, for instance, intercepting the light better than when they had a monoculture history, which sort of indicates that they learn to use the resources better over generations.
Chris - Fascinating findings. But does it translate into significant differences in yield and plant behaviour? Because if it's, if it's just lost in the noise in terms of what sort of a difference it makes, then we could probably say, well, it's interesting, but, but not of commercial benefit. But is it a big difference that you get
Laura - Practically so far, we did not show that there was a significant increase in yield, which does not mean that there couldn't be, because our experiment was quite short. It was only three years, so we only had two generations. We know that evolutionary processes are very long-term, so this might be that it didn't have enough time to happen properly. So of course this effect should be researched further.