AMINO ACID REQUIREMENTS IN RUMINANTS
Considering that each protein synthesized by the animal organism is genetically determined and possesses a unique and unchanging amino acid (AA) composition, the requirement varies for each AA. Therefore, maintaining a balance of AAs in the diet of ruminants is essential (SCHWAB; WHITEHOUSE, 2021).
Out of the 20 AAs present in proteins, 9 are classified as nutritionally essential (indispensable), and one (arginine [Arg]) is considered conditionally essential. Essential AAs cannot be synthesized in the body or are inadequately produced endogenously and must be obtained through the diet. The essential AAs include: 1) histidine (His); 2) isoleucine (Ile); 3) leucine (Leu); 4) lysine (Lys); 5) methionine (Met); 6) phenylalanine (Phe); 7) threonine (Thr); 8) tryptophan (Trp); and 9) valine (Val) (NASEM 2021).
In the case of Arg, even though there is a significant level of de novo synthesis, for high-producing cows, synthesis is insufficient to meet the demand. Consequently, Arg is traditionally considered an essential AA for ruminants. The other 10 AAs are classified as nutritionally non-essential due to their ability to be synthesized de novo from nitrogen sources in sufficient quantities, either from excess essential AAs or from other non-essential AAs (NASEM, 2021; SCHWAB; WHITEHOUSE, 2021).
For ruminants, AA absorption occurs through three main pathways:
Microbial Proteins (MicP) synthesized in the rumen,
Undegraded Dietary Protein in the Rumen (UDR), and, to a lesser extent,
Endogenous proteins (SCHWAB; WHITEHOUSE, 2021).
Scientific evidence suggests that microbial protein generally accounts for over 50% of the flow of crude protein (CP) to the small intestine (SCHWAB; BRODERICK, 2017).
For a long time, it was assumed that MicP (synthesized in the rumen) combined with a portion of UDR (dietary protein escaping ruminal degradation) would fulfill all AA requirements. This assumption was likely accurate for low-producing cows used in initial experiments (considering approximately 2,300 kg/year average milk production in the United States). However, with the increase in average production, this assumption needed reassessment (SCHWAB; BRODERICK, 2017).
In a classic experiment using 15N-labeled ammonium sulfate for lactating cows, it was observed that 15 hours after feeding, all amino acids in milk proteins (separated by fractionation after hydrolysis) were labeled. This demonstrated that MicP c...
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