Different types of plants allocate sugars in different ways — to truly build soil organic matter, you have to prioritize forbs
A group of dairy and beef producers approached me recently and said, “John, we have a problem. We want to improve our soil health, but despite implementing the best intensive rotational grazing practices for 20 or 30 years, we’ve made no progress in improving our soil organic matter. In fact, we’ve gone backwards.”
I started asking around, and it turns out this is a common phenomenon. There are hundreds of operations that have this challenge. So, I started digging into grazing management systems more deeply than I have in the past, and I came to the conclusion that we have a foundational failure in understanding how different types of plants move sugars around and how that should translate into effective management.
This conversation is focused on forage production, but it is equally true with how we’re growing annual vegetable crops or annual grain crops, or even tree fruit. We need to really understand how different plants move sugars around at different lifestages because that determines the plant’s optimal expression.
Take Half, Leave Half?
The prevailing idea in grazing management is that to build soil, we need to take half of the forage and leave half of the forage, while occasionally — but not on a regular basis — grazing aggressively and removing the majority of the forage. But there isn’t a lot of foundational guidance to describe why to do this. Why, and when, should we graze more aggressively? Why, and when, should we leave half of the forage behind? To help answer these questions, we need to understand how different types of plants partition carbohydrates.
First, consider a grass plant. A clump of grass has a very large, fibrous root system. These large root systems are what grasses are celebrated for — the tremendous amount of root biomass they have. A single rye plant — an annual grass — can have a root system that is more than a mile and length.
But when a grass plant produces sugar through photosynthesis, where does it send those sugars? (The numbers I’m going to present are variable — based on different soil conditions, grass species and field conditions they will of course differ. They are not hard and fast numbers, but directionally they’re correct.) Where the plant sends sugars changes based on the stage of plant growth. I’m going to focus on the stage of plant growth that is just before the bloom stage — at the end of the vegetative growth stage and before the reproductive stage. During the reproductive stage, all plants devote most of the sugar they produce toward seed production.
There are four different sinks for the sugars produced during photosynthesis. Two of these are aboveground: vegetative biomass and reproduction —seeds, blossoms, flowers, etc. And two are belowground: root biomass and root exudates. This applies to any type of plant.
For a grass plant that is just approaching the bloom period, the proportion of sugars the plant sends to produce aboveground vegetative biomass is roughly 40 percent, and the amount allocated toward reproduction is roughly 10 percent. About 45 percent of the total sugar energy will go to root biomass, and about 5 percent will be dedicated to root exudates.
The important point here is that a grass plant stores its surplus energy primarily in its roots — not in root exudates. This is why these grass plants have such huge root systems.
Now, we’ve been told that the root system gets pruned proportionally to the top of the plant being removed — that grazing or cutting 50 percent off of the plant causes 50 percent of the plant’s roots to be sluffed off. The problem is that this isn’t true. It’s dead wrong. There’s a lot of literature to describe this phenomenon. Fifty percent of forage removal in fact produces zero root removal. Root dieback only begins at 50 percent removal of aboveground biomass. At 100 percent of forage removal, there is 25 percent root dieback. Those outer boundaries are fairly well established in the literature. Between 50 and 100 percent of grazing or cutting, somewhere between 0 and 25 percent of the roots die back.
What this means is that grasses don’t root build a lot of soil organic matter or soil organic carbon until we take them all the way to the ground. You have to graze them into the dirt. This is consistent across warm season and cool season grasses.
Building Long-Term Organic Matter: Through Exudates
Christine Jones has some very interesting research on how much of the sugar energy in each of the four sugar sinks is held stable in the soil as organic matter for the long term. Reproductive carbohydrates are typically removed as grain, but she discovered that only 10 percent of the carbon that is contained in vegetative biomass remains stable in the soil for the long term — long term meaning 24 to 36 months. In other words, it cycles very quickly. It gets released as carbon dioxide very quickly. So, 90 percent of the carbon contained in vegetative biomass, whether it’s a cover crop on the soil surface, or even if it’s incorporated into soil, tends to cycle very quickly and is released as carbon dioxide back into the atmosphere in a very rapid manner.
Of the root biomass, 60 percent of its carbon remains in the soil in the long term. When you have root dieback or root pruning of 25 percent of the root biomass, that dead root biomass gets converted into organic matter through decomposition and microbial digestion.
But the important aspect of this is root exudates. Christine Jones says that greater than 90 percent of all of the carbon sent out as root exudates remains stable in the soil. These exudates are complex humic substances that have a half-life of decades. So, if you want to build soil organic matter in the long term, you have to build lots of root exudates in the soil profile.
Forbs: Extraordinary Exudaters
Forbes, or broadleaf plants, are plants that are not grasses and are not legumes — e.g., dandelion, chicory, plantain, thistles, etc. What does the root architecture of these plants look like? They have large taproots with small lateral roots. They don’t not have a fibrous root system like grasses.
Let’s compare forbs to grasses in terms of where the sugars the plant produces are allocated. Again, we’re talking about the bloom stage, just prior to the reproductive period. The vegetative biomass aboveground looks very similar to that of grass: a 40/10 split between biomass and reproductive elements. But belowground it looks dramatically different: only about 15 percent of the sugars go toward root biomass, compared to 45 percent for grasses, and about 35 percent is devoted to root exudates, compared to a mere 5 percent for grasses.
We see that the sugar sinks for these two types of plants are incredibly different. A broadleaf plant does not store surplus energy in the root system. It stores its surplus energy outside the root system, as root exudates. And these forb plants are known for having exceptionally strong relationships with the soil microbiome — with mycorrhizal fungi, with bacteria, with algae, with protozoa, etc. Forbs are not wasting the energy they’re sending out as root exudates. They will be able to extract those nutrients when they need them.
So, if root exudates are how you build long-term soil organic matter, you can do so much more effectively with forbs than with grasses. Grasses hold onto their carbon; they store it in the root biomass. They don’t store it in root exudates. Forbs store their extra carbon as root exudates, and this helps build soil in the long term.
For completeness, let’s also consider legumes. As with grasses are forbs, the aboveground distribution of sugars during the bloom period goes approximately 10 percent toward reproduction and 40 percent toward biomass. The belowground distribution for legumes is somewhere between that of grasses and forbs — about 35 percent root biomass and 15 percent root exudates.
This belowground ratio is variable for all types of plants, depending on a number of factors, but especially so for legumes. The reason for the variation is the amount of sugar energy that legumes expend for nitrogen fixation, which will be greater in poorer soils. In very sandy soils, or soils with very low organic matter and/or very low microbial activity, legumes will spend a lot more sugar energy on nitrogen fixation, and therefore more sugars going into exudates, than into biomass.
Pasture Management in Light of This
In talking with many pasture managers who have practiced take-half, leave-half for years — decades even — in many different types of environments, from the Dakotas to Florida and everything in between, their experience has been that if they consistently practice take-half, leave-half, their pasture becomes dominated by grasses and they have almost no forbs. The forbs do not survive. This is because when you take half and leave half, the grass has so much of its energy stored in the root system that it grows back very quickly. Forbs have less energy stored in their root systems, so they’re outcompeted. And, as we’ve just discussed, more grasses and less forbs means long-term soil organic carbon is not built.
The solution — and this is what happens in natural ecosystems — is occasionally stressing the grass very hard. I’m not suggesting doing this every year; it could be every couple of years. But, occasionally, you need to take grasses all the way to the ground. Perhaps you take the grasses all the way to the ground, and you even continue to graze on that ground for two weeks or so. You have to deplete the grass root systems.
When you do that — when you graze very intensively — for the remainder of that season, and maybe the next year, depending on which part of the year you do this, you will get a proliferation of forbs. I’ve talked with dozens of graziers who have tried this. The common theme is that in the year after they graze their grasses very hard, there appears to be less forage biomass there. And there is less forage biomass, there’s less yield per acre, and there’s less meat production per acre. But the following year — year two, and thereafter, for a three- to a five-year window — the meat production per acre is significantly higher than it was before the intensive grazing — in spite of the appearance of less forage biomass.
When I talk about rangeland being dominated by forbs, I’m not suggesting that the Forbes are the most visible plants. Forbs have a small amount of visible biomass compared to grasses. The pasture will still look like it is dominated by grasses in terms of the amount of biomass that’s out there. But when you start doing a count of the number of plants per square meter, you’ll see that you actually have a greater number of forb plants than you do of grass plants, even though the vegetative biomass for the forbs is less.
But the common experience has been that when we have rangelands that are dominated by forbs, the conversion from forage to meat becomes much more efficient. Forbs are more nutrient dense than grasses. They have higher protein levels, higher mineral concentrations, higher energy levels overall, and they get result in more meat production per acre, even though there doesn’t visually appear to be biomass there to support it.
Native Ecosystems
I had the privilege of speaking on this topic at Joel Salatin’s farm this summer for a Stockman Grass Farmer event, and we had a pretty fun conversation. Joel proposed changing the magazine’s name to Stockman Forb Farmer, so I guess I was persuasive enough!
Joel shared a conversation with an agronomist who described how they had evaluated several native prairies — which of course probably hadn’t been managed in terms of grazing pressure the way they were prior to a couple hundred years ago — but they still identified 1,600 species of plants in the native prairie, of which only 60 were grasses. I.e., the vast majority of the native species were forbs, along with some legumes. Species diversity comes from having forbs, and species diversity means diverse exudates, which means a diverse soil microbial community and long-term soil organic matter.
When I was describing these different plant movements of sugars, Joel shared a story of Mountain Man Jim Bridger’s — how he described in his diary crossing the western plains on horseback and riding through a herd of bison that it took seven days to ride across. If they hadn’t packed feed for their horses, the horses would’ve starved because there was no grass. The bison had taken the grass down into the dirt.
Does that sound like take half, leave half?
John Kempf is the founder of Advancing Eco Agriculture and the executive editor of Acres U.S.A. magazine. This is an edited transcription of his seminar at the 2024 Acres U.S.A. conference. John will be hosting a seminar again at the 2025 conference in Madison, Wisconsin, 1-4 December.
