Will I Really Benefit from Grazing My Cover Crop?

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God designed the world to function as a perpetual motion machine. If done right, it should always be building fertility, not extracting it.  But you have to understand the principles of soil health, and the four ecosystem processes, in order to achieve that outcome.

 While many individuals are content with simply “growing a cover crop” and “reducing tillage” as their contribution to soil health, many experienced soil health practitioners agree that the single most beneficial aspect of the soil health principles is the addition of animals ON the land.  What lies at the root of this commonly unifying realization?

Grazing animals as biological/nutrient ‘force multipliers’

It revolves around the nutrients tied up in the plants of your cover crop, and the conversion of those nutrients back into a plant RE-useable form.  It’s important to understand that this process is ALWAYS and ONLY accomplished biologically.

Table 1. Concentrations of minerals in cereal rye forage at several developmental stages when harvested at a 4-inch height. Results from several locations in Iowa (Dahlke, 2019).

Maturity stage N Ca P Mg K S
% % % % % %
3-4 leaf stage 4.0 0.5 0.5 0.3 3.1 0.3
Boot stage 3.8 0.5 0.3 0.3 3.8 0.3
Full head 2.2 0.3 0.3 0.2 2.9 0.2

N= nitrogen; Ca=calcium; P=phosphorus; Mg=magnesium; K=potassium; S=sulfur

Nutrient scavenging

Winter rye is noted for its ability to remove nitrogen from the soil that remains after an annual crop. Nutrient removal is a function of not only nutrient concentration but also forage yield. Using data shown in Table 1, each ton of forage harvested at the boot stage removes 76, 6, and 76 lb/acre of N, P, and K, respectively. Winter rye forage yield and nutritive value (umn.edu)

An easy rule of thumb to use for consumption is that the average bovine consumes roughly 3% of its body weight in forages, on a “dry matter” basis.  Let’s assume that the information in Table 1 regarding the amount of N, P, and K, etc. are on a per ton of dry matter basis for the forage.

This would mean that the average 1300# beef cow will need to consume approximately 39# of forage (dry matter basis) per day.  This then means that she will “consume” approximately 2% of that “ton” of forage per day, so one 1500# bale would be able to supply her with her daily forage needs for about 38 days.  (I typically assume one bale per month per cow).  So, that would mean that she would “consume” or “remove” from the field approximately 1.52# of N, .12# of P, and 1.52# of K, per day.

The table below illustrates the nutrient content of manure (urine and feces) from a finishing animal (I didn’t find applicable information for a lactating beef cow/calf).  Notice what it states about the N and P excretion from the animals as the protein content of the feed they are eating goes up (final column).  The protein content that a grazing animal would be eating if put on 12-inch-tall winter rye in early spring would potentially be higher than that seen in the bottom row (18.7%), and therefore, the P and N excretion would also be higher.  But assuming the numbers in the bottom row are “relatively accurate” for a grazing cow/calf pair (in the @168 days to market assumption), the “application” of N, P, and K would be .93, .09, and .93# per day, respectively.  If you notice how much the protein content of the forage affects the N and P levels (in the bottom two columns), you will realize that if that protein content of the forage is, say… 22%, the level of N excreted and returned back to the soil will also be significantly higher.


Understanding Whole Farm Nutrient Balance (lpelc.org)

The bottom line then is that while the cow will “consume” or take in slightly more nutrients in the vegetation she eats than she will excrete out her “back door” (something has to be left over for her to grow and maintain herself and her calf), so do “soil microbes”.  The full return of the nutrients can only happen upon the death of the critter (or the microbe).  The cow is simply another biological entity in the nutrient cycle process of breaking down plant materials into re-useable plant available nutrients.

Biology as ‘nutrient cycle accelerant’

The cow biologically speeds up the nutrient cycling process, and the nutrients of the consumed plant material become available for new plant growth more quickly, if “processed” through the “biological gut” of the cow.  Much of the nutrient content she has processed becomes readily plant available upon deposition (specifically, nitrogen, in the form of ammonium, primarily found in the cow’s urine).  The remaining components are also much more “available” to the soil biology, having been “pre-processed” biologically through the cow’s gut, which then prepares the plant material into a more palatable “food” for the soil biology, which in turn consumes and processes this “food” again, thereby making the original plant material nutrients available to growing plants.

The manure, urine, saliva, etc. that the grazing animal will apply also have an IMMEDIATE positive value to the soil biology and thereby the plant community.  This is why you can typically see an immediate response to an application of manure, particularly following a rain.  This is in CONTRAST to biologically decomposing plant residues (such as a terminated winter rye crop without the livestock component), which can take quite a long period of time to break down and made “plant available” again.  In other words, the cow accelerates the nutrient return cycle into the soil system.

It is commonly recognized that 80-90% of P and K in manures are available for plant use almost immediately upon application of the manure by the grazing animal. So, the bite the cow eats today, deposits P and K today which becomes available almost immediately.  Nitrogen in manure however is found in two forms: organic (which must be broken down further by soil microbes to be made available to plants) and inorganic (typically in the form of ammonium, which is directly available to plants for use).  Because inorganic N is generally in the form of ammonium, it is susceptible to “volitization”… i.e.: It easily evaporates off into the atmosphere as a gas.

The unconfined animal-nutrient advantage

In situations where animals are confined, their “stored manure” is composed of both feces and urine.  The ammonium in manure pits is the reason why they must be “ventilated,” and is the reason for the strong odor in a barn.  These pit fans force the ammonium away from the confinement building, where it would otherwise become concentrated, creating health concerns, and out into the atmosphere, where it dissipates.

When blended feces and urine (all stored manures) are “spread on top” of the soil, the urine is exposed to the atmosphere, and much if not most of the ammonium volatizes off as well.  However, with a grazing animal, the feces is kept “separate” from the urine inside the cow right up to the point of “application.”  When she delivers the urine, it generally will immediately penetrate down into the soil (particularly in “healthy soil” that has a high water infiltration capacity), thereby capturing nearly all of the nitrogen/ammonium that the cow produces in the soil.

The feces, having been kept separate from the urine, when deposited by grazing animals, results in “cow pies”, which will typically very quickly develop a dry crust over the top surface, thereby preserving the “fresh manure” and all of its valuable nutrients inside.  This creates a “water-resistant” shelter and a perfect habitat for soil biology, which then comes up into the “cow pie” from the soil below, consuming it from within, all the while in a “protected environment” because of that crust.

Heavy rains won’t “wash away” the manure, because it has been protected by that “roof.”  These biological soil critters then move up and down freely through the cow pie and the soil, consuming and incorporating the manure and nutrients as they work, creating “channels” in the soil profile, improving water infiltration.  Almost none of the nutrient value contained in the manure or urine as it is left behind by the animal is lost, and because it is a biologically PRE-decomposed product (by the cow’s gut), it is much more readily “plant available” than if the same plant residues are simply terminated and left to decompose over time by soil biology alone.

The ‘animal-as-applicator’ advantage

  • While simply “applying animal manure” is ALWAYS a better choice biologically for improving fertility than synthetic fertilizers, it’s important to understand these differences, and to remember that:  There is a whole, biologically driven world of difference between manure that comes out the back end of a manure spreader, and manure that comes out the back end of a grazing animal.
  • And, the animal also adds another level of biological diversity to the entire environmental system, making it healthier and more resilient.  These additional components will have LONG TERM EFFECTS… lasting for many years, because it is a biological SYSTEM enhancement.

So, this is why, if you ask experienced soil health practitioners who actually incorporate ALL of the principles of soil health, they will generally be quite willing to share which practices they’ve found helped them make the largest advancements, the most quickly-- and in most cases, that one practice will be the addition of grazing animals ON the land.

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