Protein fractions are constantly being evaluated for use in ration balancing. Soluble protein is actually a relatively old measurement, and is defined as the portion of crude protein that goes into solution when mixed in a buffered solution in laboratory conditions. If 30% of the protein goes into solution, by definition, 30% of the crude protein is soluble.

Limitations with Soluble Protein
Researchers first thought that the soluble protein fraction was totally rumen degradable, and therefore would be a helpful estimate of nitrogen available in the rumen. This information was originally used to help develop the rumen degradable or Degradable Intake Protein (DIP) fraction that we use in ration balancing today. As more research was completed, it was determined that not all of the soluble protein was degraded in the rumen. Some of it was actually Undegradable Protein (UIP) or bypass protein, and was instead digested in the lower gut. In fact, due to enzyme action, some insoluble proteins are degraded in the rumen quicker than some of the so called "soluble" proteins!

In work reported by Satter (JDS vol. 69, no. 10) and Mahadevan, et al (JAS vol. 48), soluble proteins tend to be degraded more rapidly or completely than insoluble proteins, but they differ greatly in the rate they are hydrolyzed. It was suggested that the differences in the rates of microbial hydrolysis of some proteins are caused by structural, and not solubility differences. Satter quotes, "Protein solubility as a measure of protein degradation can lead to serious error when applied across a variety of feeds."

Since the degradability of the soluble protein was variable, especially between various feedstuffs, many nutritionists switched to the UIP/DIP system as it was a more accurate indicator of protein utilization.

Factors affecting protein degradation
There are many factors that affect protein degradation or solubility of a feedstuff. In forages, these include stage of maturity, moisture, mechanical processing, heat damage, and total protein. Haylages are generally in the range of 40-60% soluble protein, hay 20%, dry corn 12%, high moisture corn 40%, corn silage 55%, soybean meal 20%, and roasted beans 15%. Proper formulas need to be utilized to make soluble protein even usable. Lab testing is too variable to use as a meaningful tool.

How do we use soluble protein?
In ration balancing, the recommendation is that 50% of the degradable protein (DIP) be soluble protein. If the ration is 64% DIP (36% UIP), the guideline is that we need at least 32% soluble protein. The only time soluble protein may become useful is in very high corn silage diets, which may include some dry hay, with soybean meal as the primary protein source. In this case soluble protein could be short, but this is easily corrected by the addition of 2-3 ounces of urea, which is 100% soluble, and 100% rumen degradable.

Conversely, if feeding a diet of wet haylage, and high moisture corn, the soluble protein may be higher than desired. This is especially true of production levels below 70-80 pounds of milk, as less supplemental protein is needed. In this situation using a protein blend that is higher in bypass and lower in degradable (and soluble) protein would be beneficial.

At ANC, we have had soluble protein in the system for some time, and have checked rations accordingly. Even in corn silage rations, it has been our practice to include urea where necessary. We will print the soluble fraction, as a percent of crude protein, on the rations starting shortly.

Summary
This is one more reason why corn silage/haylage rations work better than feeding either of these feeds alone. The use of soluble protein as a ration balancing tool is limited because solubility does not always equate with digestibility rates in the rumen or lower digestive tract. Be wary if too much emphasis is put on soluble protein by your consultant or feed dealer, as it may be just a sales tool!

There is a lot more to balancing rations today, than 20 years ago. We at ANC will continue to utilize the necessary technology and ideas to make your rations perform even better.

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