RESUMEN
BACKGROUND: The crude protein (CP) of feedstuffs is important as an indicator of essential and non-essential amino acids for livestock. The protein (P) level needs to be known accurately, to minimize the feeding of excess nitrogen (N) and to reduce N pollution. Laboratory methods for determining N content report N from amino acids, but also N from ammonia and from non-amino acid sources. The determined CP based on 6.25 × N level typically overestimates the true protein of feedstuffs. RESULTS: Determined ingredient-specific N:P conversion factors k(A) , k(P) and k were not equal to the standard 6.25 factor. The k(A) had the highest value in all ingredients, which leads to the estimation of specific crude protein (SCP), which is closer to true protein (the summation of the total amino acid residues from amino acid analyses). The SCP(k(A) ) was lower than CP and true protein in all ingredients, demonstrating that CP might overestimate the actual protein in feedstuffs. CONCLUSION: Based on data from 677 feedstuff samples from 2009, it is concluded that the mean k(A) should be 5.68 for corn, 5.64 for soybean meal, 5.74 for corn DDGS, 5.45 for poultry by-product meal and 5.37 for meat and bone meal.