Forage & Pasture Management: Class 4

This is the fourth of a ten part series of posts chronicling core concepts from a class we are taking. Further details and links to walk through the series can be found on the class 1 post.

Class started with a reminder that the 6th class would be held at the annual meeting of the Oregon Forage and Grassland Council at Benton County Fairgrounds on April 7th. The OFGC is a new group that brings together local representatives of the university extension, seed companies, forage producers, and ranchers for mutually beneficial discussion. Anyone who attends the Wednesday session (or shows up after 5pm for the trade show) can attend the 6th class. Follow the link above for details.

Manure and soil

One should not underestimate the value of animal manure as a fertilizer. Although less concentrated than chemicals, they are applied in larger volumes. As a fertilizer source, manures are generally examined on a “dry basis” to remove the effects of water on the values. Cow manure is 86% water, so a ton of cow manure contains 280 lbs of dry matter (DM). Of this, 11 lbs are nitrogen (N), 3 lbs are phosphorus oxide (P2O5), and 10 lbs are potash (K2O). This works out to a fertilizer value of 4-1-4 (on a dry matter basis). Unlike purchased fertilizers, most the N, P, and K will actually be tied up in organic forms that must be broken down by microorganisms before they can be used by plants. This makes it “slow release”. Manure also adds organic matter to the soil, further boosting fertility. We also considered sheep manure, which is dryer at only 68% water. The fertilizer value when worked out would be 3-2-1. Sulfur and other trace minerals will also be found in manure. A common factor across manures is the relatively low N-P-K numbers (under 5%), with N generally being the most plentiful.

Soil science often concerns itself with soil types, studying the quantities of sand, silt, and clay in the soil structure and determining the effects of soil type upon productivity, water permeability, supported animal life, and so forth. You can find this information for your property on the USDA soil site. Our focus in pasture management is on soil fertility, so we can largely ignore the soil type for a simple reason: proper management can increase the land’s productivity by 4x making the effects of soil type largely negligible.

Handouts covered taking soil samples properly. We are going to turn in samples of our soil for the 7th class, to be available about a week later. Robin and I plan to also sample her mom’s garden soil, which has had high levels of horse manure applied to it for over 20 years. We want to test a theory about nutrient concentration in overly manured/composted soils.

Here’s an interesting note about soil tests. Soil tests are performed by first checking the pH, and then mixing the soil into liquid with the same pH. The nutrient levels reported are those which can enter the solution at its current pH. Even though there are certainly much higher total levels of various nutrients present in the sample, the only ones we are interested in are those that are immediately available to plants. This differs significantly from forage tests, which take a sample of vegetation, break it down, and report the exact amounts of various elements present in the sample.

Ruminant digestion

One third of all plant material on earth is cellulose, and none of it can be digested directly by any mammal. Among monogastric species (those having only one stomach: humans, cats, dogs, etc), cellulose is often referred to as “dietary fiber” as it passes through their digestive tract unchanged. Since cellulose is composed of tightly linked glucose molecules, any animal that could break it down for energy would obviously have a competitive advantage. Unsurprisingly, most common grazing species are such animals: cattle, goats, sheep, alpacas, llamas, bison, deer, and giraffes. These are collectively called “ruminants”.

Ruminants have multiple stomach-like organs and can regurgitate partially digested food and chew it further to improve digestion. Most ruminants have four organs responsible for initial digestion: the reticulum, the rumen, the omasum, and the abomasum. The rumen is the key to cellulose digestion, being a large fluid filled sac with a headspace of gas. Within this organ, large colonies of beneficial bacteria, protozoa, and fungi drive anaerobic fermentation of consumed vegetable matter. Enzymes produced by these microorganisms can tear apart the chains of cellulose, extracting maximum energy from fiber. The reticulum is a small pouch off the rumen that works in conjunction with it during rumination (cud chewing). The omasum receives the fermented food that leaves the rumen, and is a poorly understood organ with baffles involved in the absorption of water, certain minerals, and fatty acids. From the omasum, digesta (food being digested) travels to the abomasum which is a true stomach as found in other mammals. Here a highly acidic environment breaks down proteins (in both plant material and bacteria from the rumen), preparing it for the small intestines where amino acids and additional compounds will be extracted. Thus, ruminants achieve extremely efficient digestion both by absorbing the byproducts of bacterial fermentation and by digesting bacteria lost in the process.

Horses eat grass, so are they ruminants? No. Horses have a normal stomach and small intestines, but a greatly enlarged large intestines where bacterial fermentation occurs. This allows them to derive energy from cellulose in forage, but they do so less efficiently as they cannot chew their cud nor can they further digest the bacteria involved in the fermentation (it simply exits). This also makes horses more susceptible to toxins in their food (such as molds), because they do not have the initial fermentation stage which neutralizes many harmful compounds.

Total Digestible Nutrients (TDN)

The study of ruminant nutrition is really the study of how animals derive energy, protein, vitamins, minerals, and water from forage. The first of these is energy, and the unit most commonly used to measure the energy level in feeds is TDN. This is an old metric, which roughly corresponds to the percentage of food by weight that “disappears” into the animal during digestion. If an animal eats 100 lbs of food and produces 28 lbs of manure, then the food has a TDN value of 72%. This is not how it’s calculated of course. In fact, it’s best not to think of it as a percentage. It’s just a score of relative nutrient value. Since TDN is based on the energy value of proteins and carbohydrates, and fats have 2.25 as much energy per gram, it’s possible to have TDN values well above 100. Pure vegetable oil, for example, would have a TDN value of 225. Here is a table of feeds and their TDN value:

TDN   Feed
100 Table sugar
90 Corn, wheat, barley grain
72 Oats (lower due to fibrous outer coating)
72 Grasses in Oregon pastures during March / April
63 Great hay
60 Good hay, alfalfa
50 Average hay
40 Straw
0 Car license plates (yes, this applies to goats too)

From this table we can see that Oregon pastures in March/April are an extremely high quality feed. Pastures at this time of year can be hugely productive: cattle can gain 3 lbs a day and lambs can gain 3/4 lb per day. Finally, note that TDN is intended to report the actual energy that a ruminant can extract from the food. This is different from “gross energy”, which is the theoretical value of the feed as it is consumed (which, incidentally, is all that human nutrition measures). TDN applies to ruminants. Horses will have a different TDN score for the same feed.


Protein content is often listed for hay and other animal feeds. Measuring the exact protein content is difficult as there are literally 1000’s of different proteins present in most living things. Instead, a shortcut is used. Since all proteins are about 16% nitrogen (N) by weight, it is much simpler to just measure the N content of a substance and multiply by 6.25 (which is the same as dividing by 0.16). (Side note: Many seed meals intended as animal feed can be used as an organic fertilizer to supply nitrogen. Just divide the listed protein content by 6.25. A 30% protein animal feed is about 5% N.) Protein levels determined by measuring N instead of actual protein are known as “crude protein”. Here’s a table of feeds and their percentage protein:

%Protein   Feed
75 Animal by-products: fish meal, blood meal
50 Soybean meal
42 Cottonseed meal
30 Linseed meal
25 Grasses in Oregon pastures during March / April
21 Alfalfa (average, generally between 16 and 26)
16 16% hay
12 Barley grain
10 Corn, wheat, oats grain
9 Hay from Oregon pastures in June
4 Straw

Looking at this table raises the question: if the grass is 25% protein in April, why is it 9% protein when we’re making hay in June?

Pasture nutrition vs time

All of the above leads us to one of the most important diagrams in forage management. I’ve recreated it here to save myself from trying to describe it in words:

As the season progresses, forages convert more and more of their structure into lignin, which is indigestible even to ruminants. There is a knee point as the plants start to work toward setting seed. After this point (shown here in May), nutritional value drops precipitously at a rate of approximately 0.5 TDN per day. Protein levels drop at a similarly increased rate. So, if you experience an equipment failure in early June and have to postpone hay making by 3 weeks, the nutritional value of your crop falls by 10.5 TDN. That’s a huge decrease in value.

So, how do we work with this? There are many ways.

Cut the grass. The biggest impact can be had by cutting the grass (either by mowing, making hay, or grazing). Notice from the graph that the regrowth loses value much more slowly, by about 0.1 TDN per day. The regrowth produces more leaves and converts to lignin more slowly. So, that same 3 week equipment failure on the regrowth only causes a nutritional drop of 3 TDN. Much less significant. You often hear people make distinctions between first, second, and third cuttings. The cuttings are not intrinsically different, but often are from a practical standpoint. Late spring rains and rapid TDN/protein loss conspire to make the first cutting lower on both metrics. Similarly, drier weather and slower loss of TDN/protein means someone has to go out of their way to make bad hay on a second cutting. The third cutting behaves similarly.

Improve fertility/genetics. Another way to preserve nutrition is to delay the knee point. Improving the soil fertility reduces plant stress and can delay when they set seed. A more effective way is to select improved pasture species which go to seed much later. Paying for improved genetics is almost always worth the cost.

Grow more legumes. Increasing clover and alfalfa populations will improve late nutrition. The graph has a dotted line for legumes. Nitrogen fixing crops have a late season advantage and will generally not fall below 11-12% protein.

Grow a vernalized annual. Planting an annual in the spring such as Italian Ryegrass lets you break the rules for the first season. As it has no immediate plans to set seed, the TDN value of Italian Ryegrass remains constant for the first year. It will still dry out over the summer without irrigation, but will bounce back from dormancy in the fall at the same nutritional level.

Make balage. Making a fermented stored forage such as balage in May allows you to capture the higher TDN values available earlier in the season without having to wait for dry conditions to work.


Wow, that was an exhausting summary to write and I still have one more pending! The reading for next week is taken from Greener Pastures on Your Side of the Fence: Chapter 2 and Chapter 4.

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2 Responses to Forage & Pasture Management: Class 4

  1. Benita says:

    Wow! I’m sure my parents and grandparents were aware of this, though not in such technical tersm, but it is all new and interesting to me.

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