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

Sitting in some recent meetings, my mind kept going back to the theme of my earlier blog post “Bringing it all Together”, and how it applies to nutrients depending upon if we are talking about soil, crop nutrition, forage analysis, animal nutrition, or nutrient management. I realized that we often talk about a nutrient within one of these groups in a certain context and not thinking about how it fits into the big picture.  For instance, the nitrogen fertilizer we recommend and apply eventually becomes the crude protein we feed to our livestock.  So, I am starting a series of blogs to connect the nutrients through the system and try to help bring it all together.

Let’s start with Nitrogen (N). Nitrogen is one of the most dynamic nutrients and can be hard to follow through the system.

Soil – Nitrogen in the soil can be in four different forms – 1. As part of organic matter (living or dead), 2. Ammonium (NH4+), 3. Nitrites (NO2), and 4. Nitrates (NO3). In the soil system, N can also be converted into gasses and lost from the soil as N2 and N2O. Additionally the negative charge of the soil and the organic matter can lead to losses of the nitrites and nitrates by leaching. Unlike other nutrients that have a positive charge and are attracted to soil particles, nitrite and nitrate are negatively charged and repel from the negative charge on soil making it easier for them to move as water moves in the soil and be lost to leaching.

 Crop Use – Crops need N to make amino acids, protein and nucleic acids. Plant roots take up N from the soil as Ammonium (NH4+) or Nitrates (NO3). Additionally, legumes can fix N from the atmosphere and bacteria in the nodules on the roots of the plant convert it to plant usable forms. Soil pH, temperature and moisture will all influence the ability of the plant to take up N.

 Feed Analysis – In general, when we talk about crude protein in feed analysis, we are really talking about N. Crude Protein is really the measure of total N in the feed multiplied by 6.25.  This is based on the idea that “all protein” is made up of the same amount of N, namely 16% (6.25 = 100/16). Since CP is based on the total N in the sample, it includes N from protein and non-protein (NPN) sources.  For ruminants, based on ruminal availability, the protein is analyzed in several fractions including soluble protein, rumen degradable protein, ADICP (acid detergent insoluble CP), NDICP (neutral detergent insoluble protein), individual amino acids, nitrates, urea, and ammonia.

 Animal Nutrition – Animals do not have requirements for CP, but for N and amino acids. Amino acids are absorbed and used as the building blocks for protein.  In dairy cows and other ruminants, microbes in the rumen convert NPN in to microbial protein.  The microbes, in turn, pass out of the rumen and are used as a protein source for the animal. Proteins are used for maintenance, growth, reproduction, and milk protein synthesis in dairy cows.

Nutrient Management – Nutrient management generally deals with the soil, crop, manure and fertilizer nutrients in the system. It is important to consider all the potential sources of N when fertilizing to meet crop needs. How N fertilization and crop harvest is managed will influence the CP concentration of the plant. Applications of N from a commercial fertilizer or a manure can increase CP concentrations of crops. Additionally as forages mature in the field CP concentrations decrease as fiber and starch concentrations increase.

There is a lot more detail in each part of the system, but the important point to remember is that the N we are applying to the crop is making the protein and amino acids in the plant that contribute to meeting the amino acid requirements of the animal and N needs of the microbes in the rumen that are going to make the meat and milk protein.

cow in pasture

Grazing cows bring it all together in the field!

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