CHOOSING BETWEEN CRUDE PROTEIN AND METABOLIZABLE PROTEIN FOR YOUR COWS
You often hear people ask about the crude protein of your herd’s diet, and in response some number ranging between 15 and 18% is given. However, have you ever stopped to think what crude protein is and why we put so much weight behind this number? The dairy industry has been using crude protein as a benchmark value for years with little concern that to the cow, crude protein means nothing. Instead, the cow cares about metabolizable protein, a number that few of us can rattle off as quickly as dietary crude protein but more importantly one that’s of concern when formulating diets.
Crude protein vs. metabolizable protein
The difference between crude protein and metabolizable protein is critical to understanding why it’s important that your cows are being fed adequate protein. Crude protein is a value derived from laboratory analysis in which the nitrogen content of a feed is determined and the resulting value multiplied by 6.25 to arrive at a crude protein value, as protein on average contains 16% nitrogen. The reason the dairy industry has been using crude protein for years as a benchmark for adequate protein is that it’s a quick and relatively inexpensive analysis. On the other hand, metabolizable protein is the protein that is available to the cow. It’s the combination of bacterial protein, feed protein, and endogenous protein that passes from the rumen into the small intestine.
Is there a relationship between crude protein and metabolizable protein?
There’s no direct relationship between crude protein and metabolizable protein. Two diets containing the same crude protein content can differ in the amount of metabolizable protein they supply. This is partly due to the fact that as milk yield and milk protein yield increase so does the need for metabolizable protein. Another reason is that not all crude protein is the same, nor is the ruminal digestibility the same between protein sources. Crude protein for ruminants has three portions. First is the rumen-degradable protein, that available to the rumen microbes to use for protein synthesis.
The second is rumen undegradable protein, which passes through the rumen to the small intestine and is available for absorption across the intestinal lumen if the cow. The third is insoluble protein that will be excreted. Doepel and Lapierre (2006) illustrated the difference between two diets with exactly the same crude protein, but which differed in metabolizable protein due to the quality of the protein used. This shows that the diet with greater quality protein sources has a greater supply of metabolizable protein which could result in greater milk yield.
What to use when formulating rations for your lactating cows
Preferably metabolizable protein should be evaluated in your lactating dairy cow and the ration formulated to provide adequate metabolizable protein. Several trials have shown the on-farm benefits of providing sufficient metabolizable protein. Haque et al. (2012) increased metabolizable protein by approximately 500 g/day, which significantly increased milk yield by 9 lbs/day as well as increasing milk true protein percent by 0.15% (163 g/day). Another trial by Lee et al (2012) found increased total tract digestibility of several key nutrients including dry matter, organic matter, NDF, ADF, and crude protein. These studies illustrate the whole cow benefits that can be achieved through knowing the metabolizable protein content of the diet rather than just the crude protein content.
* Selected References:
Doepel, L. and H. Lapierre. 2006. Challenges in protein nutrition for dairy cows. WCDS Adv. Dairy Tech. 18:57-67.
Haque, M. N., H. Rulquin, A. Andrade, P. Faverdin, J. L. Peyraud, and S. Lemosquet. 2012. Milk protein synthesis in response to the provision of an “ideal” amino acid profile at 2 levels of metabolizable protein supply in dairy cows. J Dairy Sci. 95:5876-5887.
Lee, C., A. N. Histrov, T. W. Cassidy, K. S. Heyler, H. Lapierre, G. A. Varga, M. J. de Veth, R. A. Patton, and C. Parys. 2012. Rumen-protected lysine, methionine, and histidine increase milk protein yield in dairy cows fed a metabolizable protein deficient diet. J. Dairy Sci. 95:6042-6056.
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