by Larry Hawkins, PAS
The benefits of alfalfa roots are well known and understood. This article will be about the benefits of the roots of both cool and warm season grasses. With the emergence of Regenerative Agriculture, grasses play an important and almost necessary role. We will look at grasses and their root systems and especially their ability to increase and improve organic matter
I have been told for a number of years (by the owner of a grass-seed producing-company) that his cover crop grass generated 40 tons of roots under an acre of this grass. Obviously, this would be at the as-sampled moisture, not dry matter (DM). It still seemed like an incredible number, but I guess I just let it fly over my head. In preparing for this article, I wanted to know where the research was done to come up with this number. I still do not know this answer for sure, but I did find out a lot of great information. Of course, weighing the roots under an acre of grass sounds like a lot of work!
Also, I have heard for years that many cool season grasses produced one pound of below-ground growth (roots) for every pound of growth above-ground (forage). Obviously above-ground growth is easier to measure! The question then becomes, if that is true (and I confirmed it from the Oregon Ryegrass Commission1), how much below-ground growth do other crops produce such as corn for corn silage and alfalfa?
Measuring Root Mass
Another grass seed professional, Chad Hale (you will see his columns in Progressive Forage and Progressive Dairyman), in searching through all his “stuff” found some interesting research. So, somebody did weigh the roots! In a 5-year trial at Iowa State University2 compared the above and below ground production, of two crops: corn and prairie grass. The grasses were multispecies prairie grasses used for biofuel production. Workers measured the roots of both crops down to 39” (or 1 meter) deep by using a core sampler. The field size was 20 acres, and four replications were sampled. The crops were all no-tilled.
The root biomass was measured once each fall after harvest in the years 2009 to 2013. The corn root mass was approximately one ton of dry matter (DM) per acre at the most and often much less. The prairie grasses (I assume grasses such as switchgrass) produced up to 4.9 tons DM per acre. This is a huge difference!
The Bonus with Cool Season Grasses
There is one important difference in the growing habits of cool season grass (CSG) roots especially the ones we use for forages at Forage Innovations. This would include improved European grasses such as fescues and ryegrasses. Each time these grasses are harvested, roots are sloughed off, as in Figure 1.
As the grass regrows, the roots regrow. Grasses are shedding organic matter (carbon) that is quickly incorporated into the soil. In the ISU work, only one cutting was made. By measuring the grass roots only after a single harvest, much of the advantage of cool season grasses were lost and still the CS grass difference was truly remarkable.
40 Tons of Roots?
Oh, and the 40 tons of roots per acre? I did some math. 40 tons of roots (bottom growth), at say, 20% DM is 8 tons of DM. If this is from a field of Italian Ryegrass or tall fescue that can easily produce 8 tons/acre of DM forage (top growth). Doing the math, 8 tons dry matter oi root would work out the 40 tons, at the as- sampled moisture. Maybe, that 40 tons of roots is true! Whatever the case, with grasses there is a huge supply of fibrous roots to add carbon to your soil.
So, what does all this have to do with organic matter? Organic matter (OM) is measured in a soil-testing lab by measuring the particulate (easily available) carbon in a soil sample. A soil sample is heated to over 700ºF. the particulate carbon is driven off in the form of CO2. The sample is then reweighed and the difference from the beginning weight to the final weight represents the carbon (or organic matter) that was driven off.
Carbon gets in your soil from both living and dying plants and roots, soil microbes, carbon from the atmosphere which the plant photosynthesizes to make sugar, and manure.
One of the largest and most effective contributor to raising organic matter in your soils comes from planting cool season grasses (mixed with some legume and warm season grass for good measure!) Corn, for example, at only one ton DM in roots, and when much of the trash from some fully-stacked corn hybrids may take several years to degrade and become organic matter cannot match a grass! Grasses which provide tons and tons of roots are the clear winner. Again, grasses slough off a portion of their roots with every cutting and then regrow them again until the next cutting. Perennials and many of the annuals will also have living root systems throughout the winter keeping your soils alive 24/7and 365.
Other Aspects of Soil Quality
An abundance of roots is the main contributor to soil quality goals like improved soil structure and a higher water-holding capacity. Grass roots are also much better than monoculture alfalfa in holding your soils in place to prevent wind and water erosion. Many deep-rooted forage grasses are also great at scavenging otherwise lost nutrients that previous corn crops have left in the field.
One thing that we always hear after farmers start using grasses after a long absence is that earthworms have returned to the soil. I am not sure if fishing worms being present in your soils is as good as a soil test, but they do indicate there is life back in your soil. You still might not have time to go fishing, but the worms are a reassuring sight!
What About Warm Season Grasses?
Warm season grasses also have deep, substantial roots. I have not found someone who has weighed them, but we always hear after a sorghum crop (including BMR sorghum x sudan and BMR hybrid sudangrass) is that the following corn crop does amazing. The deep sorghum roots provide channels for the corn roots to use to deepen their reach.
Another reason is that the sorghum roots have obliterated the rootworms. A substance called dhirrin is exudated by the sorghum roots (and not Pearl millet) and becomes a biofumigant against the root worms.
There is one caveat for sorghum graziers and that is, of course, dhirrin is the agent that allows the formation of prussic acid when the crop either is drought stressed or undergoes freezing temperatures. Farmers who store all their sorghum forage before feeding will avoid the prussic acid problem.
Purdue University has just developed a dhirrin-free sorgum x sudan that may be a good idea for graziers, who need to graze through the above-mentioned events, but to those who harvest and store their sorghum can better take advantage of the effect has dhirrin on rootworms.
And What About Alfalfa?
I am also still looking for someone to dig up an acre of alfalfa of alfalfa roots and report it back to me. However, I do know when the subject is preventing soil erosion from both wind and water, there is no comparison when you match pure alfalfa to pure grass or grass/legume mixes. Yes, alfalfa brings nitrogen to the soil (and so does clover!) which is important, but we also need to retain the soil to have a medium to add the N to!
Grass Roots and Regenerative Agriculture
The main goal of regenerative agriculture is to increase organic matter in your soils. There is no better crop to do this than warm and cool season grasses either alone or in a mix.
1 Dan Towery, Oregon Ryegrass Commission, Personal communications
2 Iowa State University, 2009-13, Liebman, et al)