I Know the Protein Content of a Dry Forage. How Do I Calculate What It Would Be When I Feed It Green?

Mark Hare in Nicaragua was trying to determine how much protein was being produced in a given area of land with a given forage. He found the protein content for the forages on a dry weight basis in some books, but green material is what farmers feed to the animals.

ECHO turned to Dr. Ivan Barineau, a veterinarian with Christian Veterinary Mission, for the following answer to Mark’s questions.

“Yes, there is a direct mathematical relationship between the protein content of a forage plant expressed as green vs. dry. In practical terms, one (the researcher or the farmer) might be interested in the amount of protein contained in a plant as (1) pasture or green chop or (2) ensiled or (3) dried into hay or (4) other. Nutritional analysis is often available in books or journals. The protein content is calculated for the parts of the plant that are most often used for feed. It is expressed as the percent protein in a plant that has been dried to essentially 0% moisture.

"Sometimes the analysis is based on the wet plant material 'as fed.’ This might be (1) ‘pasture’ or 'green chop’ or 'fresh’ (2) air dried, i.e. hay or (3) ensiled.

"100% dry (0% moisture) is used as a way to directly relate protein contents between species. For example, on a dry weight basis the protein content of Cajanus cajan, arial part, fresh midbloom is 38.3% and Gliricidia sepium nicaraguan, aerial part, fresh midbloom is19.9%. One can not use these percentages to compare the protein content (or other nutrient content) of these two forage species if they are fed fresh because the difference may be partially due to natural differences in the amount of water in the plant. For example fresh C. cajan contains 63.5% water while G. sepium contains 73.7% water. However, comparisons could be made between different forms (e.g. fresh vs. hay) of the same plant.

"So to convert values between fresh, hay, ensiled, etc. forms of a grass, one needs to know the percent moisture. This is not difficult to ascertain. It usually has been worked out already and can be found in publications dealing with the nutrition content of forages.

"If you can not readily locate the value you can determine it yourself with a scale and sunshine. Weigh the fresh sample, then air dry it until it reaches a steady weight. This usually requires 1-3 days, depending on humidity and stem size. This 'air dried’ form of the plant averages 90% dry matter or 10% moisture. In rainy season where drying might have to be done under shelter the 'air dry’ moisture might be closer to 15-20%. Use 90% as your average under all but the rainiest conditions for 'air dry’ value.

"Then, if you know the 'dry’ (0% moisture) content of protein from a publication, you can mathematically convert to 'as fed'–fresh or hay or ensiled. If you don’t know anything about the protein content of at least one form of the forage (i.e. from a book or other reference), you will need to send samples to a forage-testing lab. This is not a bad idea anyway since soil fertility, fertilization type or rate and environmental conditions may alter the analysis from what you find in a book.

"However, for estimating use the following formula:4 Known protein % ÷ Known DM % = X ÷ known DM% (Dry matter = DM; Unknown protein % = X)

"Any form of the plant will do as long as the numerator and denominator are relative to the same form.”

“For example, if we wish to know the protein content of C. cajan hay (90% DM) when we only know the percentage for 'as fed’ (fresh) or 100% dry. Use the value for 'as fed’ (26.3% DM, 5.2% protein) and plug them into the formula.”