609 The Fit for Cover Crops

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#RealisticRegenAg | Cover crops aren’t required in every situation. This is a key theme throughout my book “Practical Regeneration”. In this episode I’m going to talk about the development of no-till and how it might give us some clues on how to think through where cover crops work best.

Welcome to the sixth season of Plants Dig Soil, a podcast about #RealisticRegenAg. I’m your host, Scott Gillespie, and I’m an author and independent agronomist from the Western Canadian Prairies specializing in climate-smart agriculture. I discuss scientifically proven practices that benefit the planet and, just as importantly, farmers' economic sustainability. Be sure to visit my website, www.plantsdigsoil.com, to learn about my book, Practical Regeneration, and for the services that I offer for farmers and agribusiness.

Articles mentioned:
https://www.covercropstrategies.com/articles/3025-podcast-can-we-count-the-nitrogen-credits-of-cover-crops-for-the-following-cash-crop
https://fieldcropnews.com/2023/06/when-do-cover-crops-with-and-without-manure-applied-release-nitrogen-that-can-be-used-for-my-corn-crop/
https://acsess.onlinelibrary.wiley.com/doi/10.1002/crso.20334
https://www.plantsdigsoil.com/podcast/the-27-inch-precipitation-threshold

Transcript is available:
https://www.plantsdigsoil.com/podcast/the-fit-for-cover-crops

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Let’s imagine we’re on the space station orbiting the earth once every ninety minutes. We’ll focus on North America because I know it best. If you were able to watch the landscape change through the middle area – the border between Canada and the United States – you’d see a stark contrast west to east.

The Prairies of Canada and the Plains of the United States are vast areas east of the Rocky Mountains and generally referred to as The West. There’s the West Coast of course, but let’s focus on The West as that’s where I work and live. For the majority of the year, you’d see it either white with snow or brown. Only in a short time – June & July – would you see it green. In wetter years you’d see the green a little earlier and later. In the past few years of drought, you might not have even seen it green up.

The greening comes as crops emerge and the grasslands come to life. The crops would be the greenest in June, but without rain they really wouldn’t show much. I noticed the grassland in 2023 hardly greened up at all. It’s hard to overstate how dry it was. Our normal rain is about 350mm or 14”. Last year we received 160mm or just over 6”. The biggest precipitation event was the equivalent of 30mm that came as snow in mid-October. Most of our rain came a few millimetres at a time. This hardly gets the vegetation wet and does not sink in far. It is evaporated or taken up by plants so quickly it is gone in days.

This year we seem to have broken the drought. We had a long, slow rain in May that gave us 3” or about 75mm. The rivers hardly rose. It mostly all soaked in. That’s a lot of water but that will only soak in about 21” or just over 50cm. That’s half the rooting depth of a wheat crop. In the peak of the growing season that is only a two-week supply.

Focusing on the eastern part of the continent around the Great Lakes, you’d see something very different. I grew up in Ontario right in the southwest tip. I hadn’t really noticed how much it rained until I got looking at the numbers. Typical precipitation is around 1000 mm or 40” in a year. It’s also a much milder climate with winters rarely seeing temperatures below -15C. More typically there is rain through most of the fall and most of the end of winter and early spring. The song about dreaming of white Christmas applies here: It was a treat to have snow on the ground at that time.

Growing up on a farm, it was also typical that farming activities could go well into November or possibly into December. Some farmers even had corn standing in January. From the space station you’d see a green landscape from April to October. You might even see green almost for the entire winter where farmers had planted winter wheat. This grows in the fall, goes mostly dormant over the winter, and comes back to life in March. You might even see cover crops, though you probably wouldn’t be able to tell what was a cover crop and what was not. It would just look green.

So, here’s where I’m going with this. In the eastern areas where there is amble or excess rainfall, cover crops make a lot of sense. Recall that I said about 3” of water is needed to fill the soil profile to the half the rooting depth. If you get that amount of water, or more, when there are no crops growing, you have amble water to grow a cover crop. It might even help you at seeding time by using up the excess. It will very likely hold onto nitrogen and other nutrients that are typically lost to depth or tile drainage.

Shifting back to The West, where 3” of water is one-quarter of our typical annual rainfall, we need to capture and hold as much of that as we can. An irrigated crop can use 20” of water in a year. A dryland crop will adapt to less water. We very rarely have excess water to deal with. Can you see why cover crops may not be a good fit? If your cover crop uses 3” of water that’s a lot of potential water for the cash crop that you may lose.

For the West, the best option is to keep the landscape brown as much as possible. When there is water to spare or you can run water through a plant rather than have it evaporate off, you are ahead. For most of the rest of the time it is best to mimic the prairie – keep it brown and wait for the appropriate time to grow. No-till agriculture serves this purpose. It keeps a cover of dead plant material on the surface as much as possible. This protects the soil from erosion and helps to hold more moisture in. A cover crop could benefit you if the soil is bare and you need to build more armor.

A long-term study conducted in Washington demonstrated this possibility. The region experiences dryland conditions with 16 inches (400 mm) of normal annual rainfall. The study compared a fallow / winter wheat rotation with a winter pea / winter wheat rotation. The winter pea was terminated after flowering but before pod formation, which balanced water usage and biomass production. Initially they saw small gains yield.  The real effect showed up in time. At the ten-year mark they were able to use half the nitrogen to attain the same yield as a fallow system.

I’ve previously talked about the 27” rule for cover crops. You can go back to my episode on this to dive deeper, but the main point was that researchers taking the “space station view” on cover crop studies found that in order to make money using cover crop you needed more than 27” of annual precipitation. Below this they found you lost money.

The difference that I was able to see is that the winter pea gave ground cover to the wheat crop. In the fallow year it appears most of the residue was broken down by the soil critters and microbes. This soil cover led to lower soil temperatures, and it helped to trap more water. This, combined with the added nitrogen from the legume, added up to a win for cover crops.

Some might argue you could take the pea crop to a cash crop and make more money. However, if it doesn’t leave the cover that a terminated crop does, leaves less water for the wheat crops, and doesn’t leave as much nitrogen behind, it could hurt the winter wheat more than the gain made with the pea crop.

Let’s finish in the east. I’ve said cover crops have a fit here. There is a long period where most of our cash crops do not grow well. The temperatures are low and there is often excess precipitation. Does no-till work here? Not as well. It’s still good to keep the surface covered to protect from erosion but the problem is that it holds onto water too well. The soils get saturated and can turn into a soupy mess. In this case cover crops instead of no-till, or along with no-till, serve the goals of keeping the soil at a moderate moisture and easily trafficable.

If you want to dive deeper into this concept, and other regenerative practices that need a little more critical thought, why not check out my new book? As announced previously it is out now and available wherever you buy books, print or for e-readers. Particularly relevant will be chapters 9-12 that go through the species, the mixes, and how to implement them.

Thanks for listening. If you’re not already subscribed, please be sure to do so. There are also links in the description for my monthly newsletter delivered through email and LinkedIn. I also have a community on WhatsApp where I post updates a few times a month plus interesting things as I see them. Talk to you again next time.