It is crucial to "Do the Math" and reevaluate how to choose protein sources for dairy cows. Normally, protein supplements are bought based on the protein they provide. For an example, soybean meal is 48% crude protein and canola meal is about 37% crude protein. Based on experience, we would think that % of crude protein, soybean meal provides a greater protein value pound for pound:
Canola meal value = 37/48 = 77 percent of the price of soybean meal.
However, one way to improve the math behind this is to reduce the value of each meal by its soluble protein. Assume soybean meal has 15% soluble protein and canola meal has 30%. This means that of the 48% protein in soybean meal, 85% is insoluble while for canola meal, it would be 70%. The calculation would then be:
Canola meal value = (0.7x37)/ (0.85x48) = 63 percent of the price of soybean meal
Many studies have shown that there is an error with the equation above. Recently, two feeding trials were reviewed:
In Table 1, soybean meal was replaced by canola meal in diets containing 17.3% crude protein. These diets were fed for 9 week and results showed that milk yield, fat yield and protein yield were all higher with canola meal diet. This shows that canola meal value, in protein basis, is as good as soybean meal.
From Table 2, soybean meal was again replaced with canola meal. This time, the diets only contained 16.5% crude protein.
There was significant improvement in milk yield and protein yield but not much difference in terms of fat yield. However, there was a significantly lower milk urea nitrogen (MUN) values with the canola meal diet. MUN values are often higher when the degraded portion of protein (RDP) is high. These results showed that less of the protein from canola meal was degraded. Canola meal may have higher soluble protein but less of the protein ends up wasted as MUN.
What's missing from the math is the rates at which both soluble and insoluble protein fractions are broken down in the rumen. We tend to underestimate the rumen-undegraded protein (RUP) each meal supplies.
Untreated soybean meal degrades rapidly while canola meal is slowly degraded. Table 3 shows that 47.4 % of soybean meal is not degraded while 60% of canola meal is not degraded. This shows that kg for kg, canola meal and soybean meal provide about the same amount of RUP. Even after taking into account the digestibility, the available RUP is still pretty respectable for canola meal. Hence, the calculation would now be:
Canola meal value = 18.9/21.2 = 89 percent of the price of soybean meal.