R&D Edition: Cover crops done right with Dr Kelvin Montagu

Vegalogue is a podcast from Australian vegetable peak industry body AUSVEG. Each episode we take a look at issues affecting the Australian vegetable, potato and onion sectors, unpacking levy-funded research and meeting some of the incredible people who make up the vegetable industry.

This episode, we look at the benefits of cover cropping, which are many and varied. Vegetable growers are increasingly working cover crops into their rotations, but getting a good return on the investment can get a little complex.

It’s important to be clear on what you want to get out of a cover crop, according to Dr Kelvin Montagu, a research scientist with Applied Horticultural Research.

Dr Montagu has done quite a bit of work in cover cropping, including a levy-funded project a few years ago called Optimising cover cropping for the Australian vegetable industry, which helped veg growers use cover cropping to boost soil health and improve productivity.

Learn more about the projects discussed in this episode:

• Optimising cover cropping for the Australian vegetable industry (VG16068)

Vegalogue is the podcast from AUSVEG, the peak body for Australia’s vegetable, potato, and onion industries, where we examine the pressing issues and latest developments in our sector.

Thanks for listening! You can find out more about AUSVEG and the Australian vegetable industry at ausveg.com.au. Subscribe to our newsletter, or follow us on Facebook, LinkedIn, YouTube, Instagram, or Tik Tok.

Transcript

And in three months, you could grow five or six hundred dollars worth of nitrogen.

And their ears prick up, and they go, Wow, how do I get into that?

Welcome back to the Vegalogue podcast, a dialogue about the Australian vegetable industry from Ausveg.

I'm Tom Bicknell.

The benefits of cover cropping are many and varied, and vegetable growers are increasingly working cover crops into their rotations.

Getting a good return on the investment can get a little complex, however, so it's important to be clear on what you want to get out of a cover crop.

That's according to Dr. Kelvin Montagu, a research scientist with Applied Horticultural Research.

Dr. Montagu has done quite a bit of work in cover cropping, including a levy-funded project a few years ago called Optimising Cover Cropping for the Australian Vegetable Industry, which helped veg growers use cover cropping to boost soil health and improve productivity.

Ausveg's Deborah Hill had a chat with Kelvin about the benefits growers can get from cover cropping done right.

Welcome to Ausveg, I'm Deborah Hill, and today we're having a chat about Brassicas as cover crops with Dr. Kelvin Montagu, who is a research scientist with Applied Horticultural Research.

So welcome, Kelvin, thank you for joining us today.

Thanks, Deborah, looking forward to having a chat.

Perhaps give us a quick overview of what a cover crop is and what it does.

Yeah, I mean, the real basics of cover is in the name, is a cover crop.

So really, the first thing you want your cover crop to be doing is to be protecting the soil from extremes, particularly heavy rain or, you know, strong winds.

So you're not losing your valuable topsoil, because when your topsoil blows away or washes away, it's taking the organic matter with it.

And that's where all the goodies are in terms of nutrients and biology and that.

So you really want to be covering the soil, protecting it.

And then there's a whole lot of other benefits that we'll talk about.

But that's sort of number one.

And if you're, you know, I guess, you know, like I was up in Bundaberg last week, you know, it's some pretty intense storms up there.

And really, their prime number one reason for growing a cover crop is to protect the soil from those heavy rains.

And I'm seeing that around the country. There's just more heavy rainfall events. Sometimes it's out of season too.

So keeping it covered on your soil is really important for keeping your soil there.

But then there's a whole lot of other things about keeping it green so that you're actually feeding the biology.

There's all sorts of other benefits.

Am I right in thinking that it was something that was done in our grandparents' time that you always had a cover crop and it was a more recent thing that we went to Fallow, and now it's coming full circle again?

Yeah, definitely. We tended to use cover crops a lot more historically.

My grandfather, I can remember my grandfather used to have a standard cover crop. It was a lupin, a cereal, and a brassica in it. That was pretty standard over winter that would go in.

And as we got more intense and as we went into the 80s and 90s, our inputs became larger, our tractors became larger, our farms became larger, and we got more and more intense, and we tended to drop out some of those rotational aspects, and cover crops was one of those.

And then over the last two or three decades, we've started to realize that going bigger with inputs and tractor size, and that has its limitation.

And so what we're seeing is that the cost of inputs are rising because we're trying to get bigger, trying to do it with more and more ag chem, but our yields are not going up, our input costs are going up, and our gross margins are getting squeezed.

So growers are really looking for options, management options to try and grow that gross margin again so that they've got a little bit more profit in their system.

And cover crops are one way of investing, a long-term investment in your system.

In southern climates, it tends to be done more in the winter, isn't it? Is there options for up north as well as down south for cover crops?

Yeah, so it really depends on when your cash crop season is. So that's your critical point. That's when you make your money. So you've got to work around that.

So, you know, like down southern parts of the states, you know, Victoria and the likes, it tends to be a winter activity.

And then up around where I was in Bundaberg, it's definitely a summer.

They're not cropping so much up there, and they're just preparing and keeping their soil covered.

But there's also plenty of other opportunities. You know, so like in Bundaberg, they've got four or five months worth sitting there. So it's an awful long time for a cover crop.

But, you know, if you've got four or six weeks, you can do something pretty useful with a cover crop.

You know, in an extreme, say down in the South Australia, I've worked with some of the covered crop, the protected cropping people down there by putting a short-term cover crop in within their polyhouses before they go back into an eggplant or a cucumber or something.

So, you know, there are opportunities all the time.

And I guess one of the, probably the key principle, I'd say, is if you've got any fallow within your system, any bare fallow, is there an opportunity to put a cover crop in there instead?

Because that's when you see some of the biggest benefits from when you take a part of your rotation that's currently fallowed and start putting a cover crop in and you keep it green over that time.

Typically, what sort of plants are used for cover crops, then?

A lot of it depends on what your cash crops are. So what you're trying to do is have a break.

So if you can think of the simplest rotation is our, you know, arable cousins out west.

You know, they grow wheat, and they use a brassica as a break crop, and that's really effective.

If you're a broccoli grower, putting a brassica cover crop, it is not a break crop.

So if you all wanted to try and keep your rotation, so you're trying to drive down disease in between growing your cash crop, then obviously brassica cover crop is not suitable, and you might look to a grass.

So the grasses are really good break crop and veggies because apart from corn, we don't really grow any other grasses.

So they're quite distant in terms of what disease they'll host. So they're a really good break crop. And so my rule of thumb is if in doubt, put in an oat crop. And that's a really good place to start.

If you've got no idea what else to put in, pretty much an oat crop will do a lot of benefit, and there's not a lot of risk in terms of disease carryover from that.

So my understanding is that you talked about grasses, which would be the cereals like oats and barley and so on.

Then there's the legumes as well, and then the brassicas.

So each one, if you're doing a mix, for example, would have some benefits, you know, assuming that you're not going brassica on brassica with your rotation.

I'll just go back to the legume, because we've got a lot more interests from growers in legumes, and that's just simply the price of nitrogen fertilizer being so high.

And so again, you know, they're hearing me saying, oh, you could, in three months, you could grow five or $600 worth of nitrogen.

And their ears prick up, and they go, wow, how do I get into that? And so, you know, that's happening.

And also, growers are looking more at organic sources of nitrogen.

And once you go into that world of organic nitrogen, there's a whole lot of biology that's happening in your soil that you have to understand and manage so that you get the nitrogen available to your crop.

And that gets a little bit more complicated than adding it out of a bag.

And so the other sort of things that we're concentrating on at the moment is when you grow a legume or when you put on organic forms of nitrogen in terms of compost or amendments, how does that nitrogen become available to the following crop?

So that might be in a corn crop or a potato crop or just a normal brassica or whatever you're growing, because you're trying to match that profile, that release profile, from those organic sources to your cash crop.

And because it's biologically driven, it's affected by a whole lot of factors and it's a little bit more complicated.

But hey, it's complicated, but so is putting it on as nitrophoska. You can lose it in one storm, you can lose it. So it's a dynamic thing.

What are the typical plants then for legumes?

So a lot of it will depend whether you're growing a winter or a summer one.

So the standard ones for winter would be your field peas, so your Morgan field peas, your vetches, your faba beans. So they're the sort of typical winter ones.

For summer, you've got cow peas, so you typically be up more north.

So cow peas, lab lab, sunn hemp are ones that again, a lot of the times, one of the complications of legumes is they do host a lot of soil borne diseases.

So you have to be careful about which one you choose.

For example, cow pea will host rhodanodinotide, whereas a sunn hemp won't.

And so you're trying to, you know, depending on what you're trying to do with your cover crop and what your constraints are, you're trying to select those.

So quite often cover crops are done as the mix of those three, but can you use brassicas on its own?

Yeah, I'll talk about mixes in a minute, but brassicas is their own.

You're perfectly fine to use them just on their own.

And if we look at a specialist use of a cover crop, the bifumigant cover crop, you really do have to use them as a straight bifumigant cover crop.

And I'll just tell a little story.

A grower, a lettuce grower, rung me up a few years ago, and he's been having a bit of problem with sclerotinia.

He tried some bifumigant over winter and then planting his lettuce into that, and he found some good success of reducing the sclerotinia load, and he was getting better or less disease in his lettuce crop.

So he did that for a few years. That was working well.

He heard all of the hype around mixes, and so he thought he'd try a mix. And so he put in a mix of a caliente bifumigant. It had a pea and a cereal, and it was oats.

The following crop, the following lettuce crop, was one of the worst he's grown for sclerotinia.

And so he rang me up and said, what the hell happened there?

And I said, well, what you've done, you've done two things.

You've complicated by adding in your legume, which sclerotinia will infect, and so you're building up your inoculum.

So you've not broken the disease cycle. You've actually carried it over.

And the second thing you've done is you've diluted your biofumigant effect.

So by growing less of your biofumigant, your caliente, when you incorporate that, you've got effectively less active ingredients.

A bit like if you're using 600 liters per hectare of your plant protection product, you suddenly put 20 liters on or 30 liters on instead.

So you reduce your active ingredient. So just be really careful about your mixes.

And I guess one of the key things around cover crops is be really clear about what your purpose of that cover crop is.

And I've talked about a couple of them. One is covering your soil, so you want something that's going to give you cover.

If you're after nitrogen, then obviously legumes.

If you've got soil borne diseases and you're trying to break the cycle, then make sure you get a good one that does not host your disease at the very best, and at the least doesn't host.

And if you can get something that actually is antagonistic to that disease, that's even better.

So be really clear about what your objective for your cover crop is.

You talked about in the case of where the lettuce grower effectively diluted his effectiveness of the biofumigants.

Are some plants more effective than others?

Yeah, so biofumigants are sort of a real specialized cover crop.

And when we had the cover crop research project a number of years ago, we did some specific work on biofumigants, and that was led by John Duffett up in Queensland there.

And he produced a really good summary of that work. So that's a really useful resource.

And there's a whole range of commercial biofumigants out there. The main one that gets used is caliente, and that tends to be...

And then all these biofumigant brassicas have been selected.

So they have been selected for the chemical compound, the glucosinthalate, which is expressed. So basically it's produced a lot by the plant.

So it's a case of actually getting specialized with some of the biofumigants or some of the cover crops.

And I guess that's a trend we're seeing more and more in cover crops, rather than just grabbing the cheapest seed that the broadacre guys use and using that.

There's more and more effort into breeding cover crops for their specific traits.

And another brassica example is a tillage raddish, is a specifically bred cover crop as well.

So what are the best results then for a cover crop?

Do you still need to put some fertilizers inputs on, or one of them would be water to get that organic matter flourishing well, I would imagine?

Yeah, look, input-wise, there are probably three things that spring to mind.

One, you want to grow a good cover crop, and it will come back a bit to what you're trying to do as well.

If you're just trying to get a cover on the soil that's low input, then you don't have to be too fussy about it.

You can just broadcast the seed, roughly tickle it in, and off it goes if there's a bit of moisture around.

Most growers will just give a starter irrigation to it, if need be, just to get it up. But it will depend.

So if you're growing a biofumigant crop, then you are going to have to treat that like a crop.

You're going to feed it, water it, and manage it like a crop so that you get a maximum biomass, because essentially that's your active ingredient.

The more biomass you get, the more active ingredient you get.

So you want to grow it as well as you can.

And I'll say the same thing for legumes. If you want to maximize your nitrogen fixation by your legume, you've got to grow biomass.

For every ton of dry matter we grow in a legume, you get 20 to 25 kgs of nitrogen.

So if you can grow 8 to 10 tons of your legume, then you're getting 200 to 250, plus what's in the root system, of nitrogen fixed by that legume.

Another example is if you're growing, if weed control is a major issue, if you want to out-compete weeds, then you have to think about what inputs you need for that crop.

And that might be not fertilizer, but just making sure it's watered so it gets up quickly, but increasing the seeding rate so that you get a very quick cover and you're shading out there.

Now brassicas are very good at that because they germinate quickly and they come up with a roar and they'll shade out weeds, so it is a very useful emulsion.

So now that the cover crop is up and lush and looking good and it's doing its thing with preserving the soil and enriching the soil and so on, termination is the next big thing, isn't it?

Yeah, I would say, Deborah, that termination is the most critical step of all of it.

Growers often focus on sowing it and getting it up, but the success and failure of the whole system is usually around the termination time.

If you don't get it right, then the transition back into your cash crop can be problematic.

So you either create complications around how you transplant or sow your cash crop because you've got too much stubble or the soil's not quite right.

So you need to really...

And I say to growers, I start with the termination phase.

That's where I start, and I'll just work back from then and go, I want to terminate it this way, I want it to do this, and then I'll work back to when I sow.

And so that way, you work back to when you sow and you work forward to when you want to actually establish your cash crop in there.

So it's a really important one, and you've got to think about what equipment you've got to terminate.

Are you going to mulch and cultivate in? Are you going to roll it and leave it as a brown mulch on the soil surface for a bit longer, and then prepare it?

And there's some great options, particularly for brassica growers.

Strip tiller to your cover crop works fantastic.

And we've had a lot of success with growers around the country strip tilling cover crops.

And you can virtually go in green, roll it, strip till it, and be planting in brassicas the same day. So the transition can be really quick.

The other thing about termination is you can use that to manage your soil moisture.

So if you're coming out of, say, a dry winter into spring, it's warming up, you're worried about your water, then make sure you terminate your cover crop early so that you're not drying down your subsoil.

And if you terminate it early, you've got six, eight weeks to go before you're actually going to plant into it, think about just rolling it.

So you leave basically a mulch on the surface, and so you essentially stop the cover crop using moisture.

But if it rains, the surface is still protected, and you've still got really good infiltration rates so that any rainfall you can effectively use.

And in broad-acre terms, it's called fallow efficiency, and they are, particularly in the northern parts, they are driven by that because they crop in winter, so they try and get as much of that summer rain stored in the soil, and that's the sort of techniques that they use as they grow a short-term cover crop.

Roll it and leave it over summer so there's a nice cover.

And we can do the same thing in veg if you're trying to bank some of that water and not dry the subsoil out.

And as we go back into the next drought cycle, that's a really useful management practice you can do.

There's also lots of other benefits of rolling and leaving on the surface for a while.

Such as?

It's really good from a biological point of view. You're maintaining a moist soil.

You're also favoring the decomposition of your cover crop by fungi. So the fungi actually grow up out of the soil in mycelium. They excrete a lot of proteins and mucous cells, which act like a glue.

And what they do is they stabilize the soil surface, the structure on the soil surface, and they really improve the water infiltration rate dramatically.

After one cover crop, you can see it. And so that's a really important one.

In our very intense systems, if we're rotary harrowing, what you see is typically the soil, it's all fluffy and nice the first for the day after, but as soon as it rains heavily or put a good irrigation on it, it'll potentially cap, it'll slump.

The water infiltration rate goes down, particularly if your soil is getting a bit tired.

And so having that time where the biology, particularly the fungi, can improve that soil is really useful.

And so that's another one of thinking about when you terminate.

Look, just the other side of that incorporation versus leaving on the surface is if you mulch it and rotary harrow it in or disc it in, you'll get very good, very rapid decomposition because what you've done is chopped it up really fine, and then the bacteria in the soil, it's just party time for them.

So suddenly they get all this carbon, all this food coming in, and a lot of bacteria can double in population in a matter of hours.

So they suddenly explode.

There's all this activity, and then they sort of tail off after three or four days as that carbon source starts to dwindle, and then nutrients start to become available.

The plus side, the really good side is that it gets rid of 8, 10 ton of biomass in a quick time.

So if you want to get rid of it, so if you're a baby leaf grower, then that's definitely the sort of thing you want to be trying to do.

You want to be with your cover crop, not letting it get big and buffy and woody.

You want to keep it green and fairly small, so you're terminating earlier.

You're growing shorter term ones, and you're mulching and incorporating in so that it's decomposing.

You're not having a cover crop residue you're seeing on the surface interfering when you come to harvest, and you get residues in your harvest, which is a no-no.

So growers that are just starting down the path of cover crops, their yield might be going down, the soil's not quite as healthy as they'd like, even just increasing their carbon content in the soil.

How many years or what sort of time frame before they can see genuine benefit?

So if you're just starting out, the first thing I'd say is keep it really simple.

Don't plant a mix. Just pick one cover crop, look around the region.

If you're down south in winter, grow an oat. It's a nice simple one. It won't host anything, and it's fairly easy to terminate and manage.

So that's the first thing.

How long will it take?

You can see changes in the soil after one cover crop. So I did a little video for another group where I took soil from the cultivated area that was under capsicums, put it in a jar of water, and just watched it dissolve.

And all the water became cloudy. I took soil from under one cover crop and put it in the same water, and it just sat there.

So after growing one cover crop, the stability of your soil is improved. And you can generally see that.

The first thing you'll see as a grail, the first thing you'll notice, and you can see it after one cover crop, is your infiltration rate.

So that stability of the soil means that when it rains, it doesn't cap.

The pores stay open, and the water gets into the soil.

So that's a real big benefit from a grower's point of view, because a lot of times, you'll see water running off beds down the wheel tracks and out.

And that's one of the early signs that you can see an improvement.

You'll start to see less run with your irrigation and rainfall events. So if you use your eyes, that's what you should be looking for. So that's generally the first thing you'll see.

The second thing you'll see is your horsepower suddenly starts to drop off. You don't need to use as much horsepower when you're cultivating. The ground's softer. It's easy to work. You'll see that.

Probably the last thing you'll see is a change in your soil carbon or your soil organic matter. That is a long-term thing.

And look, in the vegetable production systems that we run, it's very hard to actually increase your soil organic matter.

And the reason for that is, even if you're growing cover crops in there, you've still got an awful lot of time where you've got bare soil, because you might be seeding, and then it takes six to eight weeks before you get a good cover on it, and then you're harvesting, and then there might be a couple of weeks before you get a cover crop in.

There's a lot of time in that system where there's nothing growing, or there's not a lot growing.

And secondly, we disturb it a lot. So we are cultivating it a lot.

And what that does is it is quite hostile to soil fungi, and fungi are better converters of plant material into soil organic matter than bacteria.

So as soon as we hammer down the fungi, we get less conversion of our plant material.

Our 10 tonnes that we grow as a cover crop, less of that will go through into organic matter.

And that's why I really like the combo of cover crops and strip till, because what we're doing is we're minimizing the amount of disturbance, but we're doing enough disturbance to make planting easy, but you've still got those areas of strips where your cover crop is still sitting there, the fungi are doing well, they're converting into soil organic matter, they're protecting the soil, they're doing all the good things.

And if you look at some of the, there's a really good co-study we did out at Cowra with Ed Fagan with cucumbers, where we strip tilled into a cereal rye.

And yeah, they just work to treat.

And we've done stuff over in WA with Jake Ryan and brassicas, with Val Mikhailov here in Sydney, again, cover crops into brassicas, lots of options there.

And certainly with brassicas, it's the best fit. Cover crops straight into your brassicas, using strip till.

One thing I would say is, if you are thinking about taking a fodder cut, and I see a lot of newbie, what I'd call newbie cover croppers doing this, they're saying, oh, I'll spend $250 per hectare on seed.

I want a return. I want to get some money out of it.

And so they'll ring up the local contractor and they'll rumble in after three months, and they will compact crap out of your paddock.

And so you've got to be really careful and really focused.

And I know most growers that I work with, they'll do it for a couple of years, and they'll scratch the head and go, hmm, I think I'm losing more money than I'm making growing that cover crop.

Just from the compaction and then the cultivation you have to do to get it back to a situation that you can actually do something about.

Would that be the same case then if you put in livestock? Because I know forage kale, for example, is often used as a...you're grimacing, so perhaps not.

I mean, it depends on your rotation.

You can graze it, but yeah, it depends on how wet it is when you graze and what sort of soil you've got, how compactable hooves will make it.

If you're going to put a herd of dairy cows and graze it in the middle of winter and in a wet winter, it's going to be plugged down, it's going to be a mess.

So what growers quickly realize is that the income that they get from these forage harvesting or grazing, compared to what the gross margin is on their vegetable crop, is just miniscule.

And so they work out, look, if I can prove the soil a bit and I get that to flow over into my cash crop, I'm miles ahead, and I don't worry about the $200 of seed it costs me.

That tends to be the thinking pattern that I see many growers go down.

Thinking they can make a quick buck out of their cover crop when actually, you know, let it be.

Yeah, look, it's perfectly understandable. Land's expensive. You want a return on it.

And then you've got something sitting there doing nothing for three months. And you're going, well, I need to get some money out of it. And so you look around thinking about it.

When it actual fact, it's not doing nothing. It's feeding your soil ready for next time.

Absolutely. You're aiming to improve it.

So, you know, we talked about adding oxygen. We talked about reducing disease pressure. We talked about making cultivation easier.

You know, and that might help you harvest.

You're keeping your soil in place. You know, all those things.

So your cover crop is worth a lot. And I think we've got a...

We do have a fact sheet of what's your cover crop worth. Because you can quickly calculate how much it costs you. But getting the value of it is always more problematic.

And what I ask is saying, well, look at the value of what it costs you to put the...

Or what it costs to put the cover crop in, and then compare to that and saying how much improvement in yield or packout or reduce inputs do I need to generate from that cover crop?

And normally it's pretty minuscule. You don't have to see much of an improvement to get a payback on that investment in your cover crop.

And sometimes it can come around in different ways.

It can be there's less work in the packing shed.

You know, your workers are happier. Less diesel to till the soil and all those things.

Yeah.

So Kelvin, we'll leave it there, and I thank you very much for your time today and your wisdom as the cover crop guru with AHR.

Thank you very much for your time today.

Thanks, Deborah, for the opportunity.

And I guess I'd like to thank all the growers too, because every time I go on farm and talk to growers and do stuff with them, I learn so much.

And if they didn't let me on the farm, I'd be stuffed.

Well, thank you very much again, Kelvin, for your time today.

It's much appreciated.

Thanks a lot.

This project is funded by Hort Innovation using the Vegetable Industry Research and Development Levy and contributions from the Australian Government under project code VG16068.

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