The value of protein is usually expressed as crude protein (CP) or digestible crude protein (DCP). The DCP and/or CP values are indicated:
Sometimes as %/gram CP/DCP per kg food on a wet/fresh basis,
or sometimes as the same values on a DM basis!
Care should be taken! In this paper calculations will only use the system of gram DCP per kg DM of a food. If one feed is compared with another the same system should be applied, otherwise the results will be distorted!
Note: the Polish system is using the term Bia_ko Trawione w Jelicie (BTJ), which means intestinal digestible protein. For more information, it is referred to the book “_ywienie Prze_uwaczy”, Omnitech Press – Warsaw.
As a rough rule;
A 600 kg cow producing 15 litre milk per day (4% fat) requires in total 1335 gr DCP, for
– maintenance ————- 390 gr
– production of every 1 kg milk 63 gr—- 15 * 63 = 945 gr
The CP value is measured by determining the amount of N in a foodstuff. As all proteins contain 16% N, the CP content is determined by: 100/16 = 6.25 * N. For good milk production a certain amount of undegradable protein is required. See appendix 1 for the degradability of the proteins in various feedstuffs. Urea is the main NPN compound used in animal nutrition. Its CP content is very high. Urea contains 46.6% N (= 466 gr/kg), which is equivalent to a CP content of 466 gr * 6.25 = 2,913 gr/kg (all degradable).
Generally speaking, DCP values are estimated to be 60-70% of the CP values. However, variations are considerable and this estimation might not be accurate enough. DCP values vary from food to food (and from quality to quality) and should be separately determined by digestibility trials. Evaluation of large food numbers to determine DCP by digestibility trials as routine is impracticable. In concentrates the DCP is usually calculated with the CP value multiplied by the available digestibility coefficients. In roughage the approach is different. Due to greater variability and regression, equations are used to calculate DCP from CP.
A typical equation widely used for grass, hay and silage is:
DCP (gr/kg DM) = CP (gr/kg DM) * 0.91 – 36.7
Some examples of digestibility of CP for certain feedstuffs are:
– Good hay/silage (young material) 60% – 70%
– Average hay/silage 50% – 60%
– Mature crop (hay/silage) 30% – 50%
– Mature crop, mouldy/dusty 20% – 40%