Sally A. Flis, Ph.D. – Feed and Crop Support Specialist, Dairy One

The next nutrient in the series is Phosphorus (P). Phosphorus is a key nutrient for plants and animals, but when it appears in excess in our fresh water systems, it can cause some environmental issues fostering algal blooms and fish kills.

Soil – Phosphorus in the soil can be in three different forms – 1. as part of organic matter (living or dead), 2. P in solution (H2PO4 and HPO4-2), and 3. mineral associated. In the soil system, P does not easily leach and is tied up by other minerals like Fe, Al, and Ca (all positively charged) and in most plants is available between pH of 5.5 and 7.5.

Crop Use – Crops need P for energy storage and transfer (Adenosine di- and triphosphate, ADP and ATP). Plant roots take up P from the soil as orthophosphate (H2PO4 and HPO4-2). Soil pH, soil content of Fe, Al, and Ca, and moisture will all influence the ability of the plant to take up P.

Feed Analysis – Phosphorus in feeds is measured as a percent of dry matter (P, % DM). This is measured by a mineral digestion and ranges between 0.1 and 0.5 %. However, when we feed byproduct feeds, P can be high, e.g., 1.06 ± 0.17 % DM for dry distillers grains.

Animal Nutrition – Animals have a requirement for P. Phosphorus is used primarily for bones and teeth and as in plants, for energy in ADP and ATP. It is also important for blood buffering and is a component of cell membranes, phospholipids, phosphoproteins, and nucleic acids. In dairy cows and other ruminants, microbes in the rumen require P to digest cellulose and create microbial protein. Only about 70% of the P in the diet is absorbed and used in the animal, the other 30% is excreted in milk and manure.

Nutrient Management – Phosphorus losses in runoff and erosion can cause increased P levels in fresh water systems, resulting in eutrophication. It is important to consider all the potential sources of P when fertilizing to meet crop needs (available soil P concentration, animal manures, and commercial fertilizers). Increaseing P fertilization can increase the plant concentration of P, but it is risky to over fertilize P because of the chance of loss and further environmental contamination.

There is a lot more detail in each part of the system, but the important point to remember is that the P we are applying to the crop is creating energy for growth in the plant, that is then used by the rumen microbes to digest cellulose and create microbial protein, and absorbed by the animal for use in bones, energy cycles, and cell maintenance or excreted in manure and milk.

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