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How to Determine Alfalfa & Forage Quality

By Dr. Harold Willis

What is quality? Is a high quality hay crop one that produces the largest number of bales or tons per acre? Or one that produces the great­est amount of meat, milk, or wool per pound and that fosters good animal health and reproduction? I think you would agree that mere bulk does not ensure high quality. After all, unless you are in the seed business, the reason you are growing a forage is to feed animals, and the results shown by the animals are what count.

But how is the quality of animal feed measured? There are many ways of measuring, or attempting to measure feed quality, and we do not need to go into detail about them here. Briefly, some methods attempt to measure the total energy value of feeds, based on actual feeding trials with animals or by measuring the number of calories contained in the food. Other meth­ods measure only the protein content of the feed as an index of quality. In one such method, one of the most commonly used, only the nitrogen content of the feed is measured and the result is multiplied by 6.25 to give an estimate of protein content. This quantity, called crude protein, is a very inadequate and often misleading protein measure, for in poor quality sick plants, there is often an excess of nitrates and other non-protein forms of nitrogen. A better measure of protein quality would be an analysis of the amino acid content, since a certain balance of amino acids is needed for high quality protein, but such tests are very expensive.

Besides the protein and amino acid content of food, the mineral con­tent is also vital in promoting good animal and human health, for many serious health problems and diseases can be cured with a diet containing sufficient minerals, in biologically useful forms. “Well-defined patho­logical symptoms appear in livestock deprived of certain needed ele­ments, and thus there is abundant proof that the health and well-being of animals is directly dependent on the mineral content of the soil on which their food is grown.” (F. A. Gilbert, Mineral Nutrition and the Balance of Life, 1957, p. 6.)

How good is your alfalfa? One very simple and inexpensive method being used by a growing number of farmers is a refractometer.

Other methods try to measure the digestibility of food, correlated with feeding trials and the percent of fiber in the plants, since in forages, as the amount of cellulose and fiber increases, the protein content decreases.

Of course, the most important measure of food quality is how your animals respond and what your expenses and income are—not only the animals’ production of food or fiber, but also their health and repro­ductive success. “Thus, the total advantage of high-quality alfalfa goes beyond that indicated by digestible nutrient content and is compounded by a potential for being consumed at higher levels, a faster rate of digest­ibility, and perhaps a more efficient conversion of digested energy to produce energy.” (R. F. Barnes & C. H. Gordon, p. 603 in Alfalfa Science and Technology, 1972.)


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Measuring with Refractometers

But how can you measure quality while the crop is growing without sending in samples for an expensive and hard-to-interpret lab test?

One very simple and inexpensive method being used by a growing number of farmers is a refractometer, an optical instru­ment that measures percent sugars in the sap of a plant, which is cor­related with the plant’s food-producing efficiency (photosynthesis) and with ultimate food quality. Refractometers are routinely used in the food industry, by canneries, wineries, and breweries for example, to measure the quality of the fruits and vegetables they buy from farmers or of the foods and drinks they manufacture.

Using a refractometer to check the quality of your crops is easy. Simply squeeze a few drops of juice from the stems or leaves of the plants onto the glass prism of the refractometer, close the “lid,” and look through the eyepiece. The sugar content is read on a numbered scale in units called Brix (same as percent). By comparing with standard levels (see table) or past readings that you have made, you can see how your crops measure up that day.

Because of the difficulty in measuring feed usability and protein quality, there may not be a correlation between refractometer sugar readings and protein test figures from a feed analysis lab. Not all “high protein” feeds are truly nutritious for your animals. The best indicator to go by is the refractometer reading. A feed with a high protein figure but a low sugar reading will not be of high quality for your animals. It would be better to have a lower protein figure and high sugar.

Growing High Quality Forages

Now that you have seen the impor­tance of high quality feeds, how do you go about growing them yourself? What is next?

Balanced Fertility

The most important factor that determines the quality and yield of a crop is the soil. We have already covered the importance of loose, well-aerated soil with plenty of humus and benefi­cial microorganisms. The other aspect of good soil is to have the right fertility—the proper amounts and balance of soil nutrients — what the plants need, when they need them.

The actively growing forage plants need those plus the others that crop plants need. According to a typical study (which may not have been made with balanced fertility), 10 tons of alfalfa removes from the soil 500 lb./acre of nitrogen, 50 pounds of phosphorus (120 pounds of P2O5), 500 pounds of potassium (600 pounds of K2O), 350 pounds of calcium, 60 pounds of magnesium, and 50 pounds of sulfur. (C. L. Rhykerd & C. J. Overdahl, p. 438 in Alfalfa Science and Technology, 1972.)

Also needed are 11 oz./acre of boron, 10 oz. of zinc, 1.1 lb. of manganese, 1.5 lb. of iron, and 3 oz. of copper. (D. Ankerman & R. Large, Soil and Plant Analysis, undated, p. 62.)  The iron and copper are also needed by nitrogen-fixing bacteria inside root nodules, plus they also need molybdenum and cobalt in very small amounts.

Major Elements

As can be seen from the above paragraph, plants need more of some elements than others, so those are called the major elements. Nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca) are the big four, with magnesium (Mg) and sulfur (S) sometimes called secondary elements.

Legume Nodules

Nitrogen is definitely needed by legumes, but if things are working right, they can get most of their needs from nitrogen taken from the air (which is 78% nitrogen) by bacteria in root nodules. How­ever, if soil conditions do not allow the nodules to form or if they are not functioning well, legumes will take nitrogen from the soil, just as any other plant that has no nodules. Healthy, active nodules will be pink or red inside due to a red pigment, leghemoglobin; a greenish color inside indicates a sick or dying nodule. Nodule bacteria also require oxygen and freedom from toxic chemicals. Many alfalfa fields in this coun­try have few or no nodules, mainly from poorly aerated soil and toxic substances.

Effective nodules can fix from 75 to 240 pounds of nitrogen per acre per year, which isn’t bad for free fertilizer! If your alfalfa doesn’t have nodules, you had better find out what’s wrong.

Calcium and Legumes

Legumes are large consumers of calcium, and to obtain high quality forage that promotes animal health and productivity, large amounts of calcium are essential. “Calcium serves so many important roles in the soil medium that it seems extremely doubtful that alfalfa culture can be very successful without it.” Calcium is vital for cell division and healthy cell walls, for root growth and root hair forma­tion, for enzyme activation, and protein production. A deficiency of calcium has been found to decrease resistance to insect pests.10 Calci­um also stimulates growth of beneficial soil microorganisms, including nitrogen-fixing bacteria, and helps counteract toxins in the soil and in the plant.

An alfalfa field in full bloom.

In general, soils in the western U.S. are well supplied with calcium, but those in the East require additional calcium. High calcium lime (cal-cite) is preferred over dolomite lime, partly because of its more rapid availability and also because of its lower magnesium content, even in areas with magnesium-deficient soil. The fertilizer material sul-po-mag is a better source of magnesium, plus potassium and sulfur. You should have your soil tested to be sure, because there are localized areas in the East with excess calcium and calcium-deficient soils in the West.

If your soil needs it, calcium should be applied even though the pH test results show no need for lime to “correct” soil acidity, because the plants need the calcium no matter what the pH. High pH can be caused by other elements than calcium.

“Calcium deficiency in the soil has come to be plant-nutrient problem number one in agricultural production . . . and calcium is now considered to be far more important in the production of food high in nutritive value than was formerly believed.”

Phosphorus and Legumes

Phosphorus is the major element most often overlooked in crop fertilization programs. Even though the soil has large amounts of phosphorus in its minerals, it is nearly all chemically locked up and unavailable to the plant at any one time. If soil conditions are favora­ble, the beneficial soil microorganisms will slowly break down mineral phosphorus and make it available, but unfortunately, most of today’s soils are in such poor shape that plants cannot obtain their phosphorous needs.

Soil expert and author William McKibben, The Art of Balancing Soil Nutrients, from the 2009 Eco-Ag Conference & Trade Show. (1 hour, 8 minutes). Listen in as McKibben talks about the steps you can take after you receive your soil test results to help balance your soil.

Phosphorus is essential for quality crops. “The lack of avail­able phosphorus is usually reflected in low yields, poor quality, and delayed maturity …” Experiments with alfalfa have shown that adding phosphorus fertilizer can increase protein content. Phosphorus (and calcium) deficiency in alfalfa decreases milk production.15 Phosphorus is needed for all cell activities by transferring energy within the cell, it is part of the cell’s genes and membranes, and is important in growing roots and stem tips. Phosphorus is especially needed by young plants; as much as 75% of a plant’s supply of phosphorus may be absorbed by the time it has produced 25% of its dry weight. Phosphorus also directly or indirectly increases drought and disease resistance and nitrogen fixation, and decreases maturity time, especially in cool temperatures.

Although superphosphate fertilizers are sometimes recommended for forages, nearly all (80-90%) of the soluble phosphorus they contain “in the bag” reverts back in a matter of hours to insoluble forms that are unavailable to the plant. Better forms of phosphorus are the rock phosphates, mainly soft rock phosphate, or colloidal phosphate, which is already in a form available to plants (hard rock phosphate is not avail­able until soil life acts on it, a slow process). Basic slag can also be used as a less desirable phosphorus source.

Potassium and Alfalfa

The main problem with growing alfalfa today is that traditional recommendations are for too much potassium in proportion to the other elements, and you cannot grow really top quality forage when the soil’s available potassium exceeds available phosphorus or calcium. Now don’t get me wrong, you can grow lots of alfalfa with high potassium—and pretty good-looking alfalfa, too. Scores of research experiments prove that.  But it is not the highest quality animal food. It is what is called carbonaceous — too much cellulose. Plenty of stiff stems. What animals need is proteinaceous feed, with an increased proportion of high quality protein, and that kind of forage can only be grown when the soil has plenty of calcium and phosphorus and not too much potassium (see box).

“The luxuriance of a plant is not necessar­ily a criterion of its mineral content, and the usual commercial fertilizer may or may not add to a deficient soil all that is needed.”

Gary Zimmer, Gaining a Working Knowledge of Calcium, from the 2002 Eco-Ag Conference & Trade Show. Listen to Gary Zimmer talk about how calcium drives the flow of other minerals and nutrients through the soil and plant, and how to measure and balance calcium levels in your fields.

For growing top quality forages, the soil’s available calcium should be higher than available phosphorus, and phosphorus should be higher than available potassium. Another advantage of maintaining a high phosphorus — low potassium ratio is that weed problems will be greatly reduced or eliminated. Also, because the plants will be healthier, disease and pest problems will be minimal.

By “available,” we mean readily available to the plant, or water-soluble. Unfortunately, most soil testing labs use methods that give too high readings for most elements and thus not enough fertilizers may be recommended (on most soil tests, however, the P test does approximately equal the amount of phosphorus readily available to the plant).

Because of over-application of potassium fertilizers (potash) in the past and the large supply already present in most soils (except sand), most fields need little if any additional potassium to grow high quality forages. If after several years some is needed, manures or crop residues should supply plenty. If even then some is needed, be sure to use potassium sulfate rather than the commonly sold potassium chloride (muriate of potash, kalium potash), since chloride is detrimental to crop quality and soil, especially at high rates. It has been shown to “burn” new seedings of alfalfa.

Secondary Elements

Smaller amounts of magnesium and sulfur (in the sulfate form) are needed than of the previous four elements. Mag­nesium is needed as a part of the plant’s chlorophyll and is required for many enzyme and metabolic activities within the cell. Sulfur is a neces­sary part of some of the essential amino acids (cysteine and methionine) as well as having several other metabolic functions in the plant.

In most soils, magnesium and sulfur are not in short supply; in fact, “acid rain” delivers a more than adequate supply of sulfur (50 pounds/ acre or more) in many areas. Nevertheless, some regions may have soil deficiencies in these elements, so a soil test is important to be sure. If magnesium is needed, dolomitic lime is a cheap source, but not a good source, since it is very slow to break down. If sulfur is needed, be sure to use a sulfate form, such as calcium sulfate (gypsum), rather than elemental sulfur (flowers of sulfur), which has harmful effects on the soil and is slow to become available to the plant. Sul-po-mag supplies sulfur and magnesium, as well as potassium.

Trace Elements

The remaining essential nutrients are only needed in tiny or trace amounts, but they are just as important as the major and secondary elements. The ones most needed by alfalfa are boron, zinc, manganese, iron, and copper, plus molybdenum and cobalt for nitrogen-fixing bacteria. Most soils have adequate supplies of all of them with the possible exception of boron, which is often deficient in U. S. soils in the entire eastern one-half and in the Northwest and California. A boron shortage can seriously reduce alfalfa yield. Boron is needed for cell divi­sion, normal growth and maturity, for protein and seed production, and other metabolic cell activities. Boron deficiency problems in alfalfa can be worsened by high potassium and high pH soils (neutral or above). Boron can most cheaply be supplied by applying borax, at whatever rate your soil requires (have a soil test run).

If a soil test shows any of the other trace elements to be deficient, apply recommended rates, but first be sure that the major elements are at adequate levels, especially calcium and phosphorus. In fact, once those elements are adequate, trace element “shortages” may disappear, because calcium stimulates soil microorganisms to release trace elements, and phosphorus is needed by the plant to transport them.

Mineral Balance

No single element is more important than another for op­timal plant growth; all are necessary, but in different amounts or propor­tions. Therefore, the proper balance of nutrients is the important thing. And that means the nutrients available to the plant, not the total present (but tied up) in the soil. That’s the only way to grow top quality crops.

One of the keys to keeping the right balance of nutrients is to have well aerated soil with plenty of humus, for humus and its associated microorganisms tend to provide balanced nutrition to plants. But that’s not all. They also provide many other valuable pluses. They produce growth-stimulating substances, improve the general vigor and health of the plant, and combat plant diseases and pests. Properly fertilized alfalfa stands can live 30 or 40 years.

Experiments have shown that unbalanced fertility may result in increased yields (bulk), but also give crops of lower nutritional value to animals. For example, alfalfa was fed to Guinea pigs. Moderately ferti­lized alfalfa increased the animals’ weight gain, while alfalfa fertilized with too much phosphorus and potassium decreased weight gains. (W. F. Wedin, A. W. Burger, & H. L. Ahlgren, Agronomy Journal, vol. 48, p. 147-152, 1956.)

Source: How to Grow Great Alfalfa, Chapter 3


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