Kenneth Bruerton PhD
11 August 2011

Breaks down
◦ Protein
 Digested to amino acids by rumen micro-organisms and
resynthesized into microbial protein.
 This is digested in the small intestine to amino acids which are
absorbed and used for growth (enzymes, immune proteins, muscle
protein).
 Some amino acids can be used to make glucose for energy
production in specialized tissues such as brain and formation of
rapid energy stores (liver and muscle glycogen).
◦ Fibre (cellulose, hemicellulose, lignin).
 Only digested in the rumen by certain micro-organisms. What is
undigested is excreted in the faeces.
 Producing volatile fatty acids (VFAs) such as propionate and
butyrate which are absorbed in the small intestine and used for
energy production and fat synthesis.
◦ Starch, sugars
 Digested rapidly in the rumen to form lactate or acetate. Excess
fermentation of starch can cause acidosis, particularly in lotfed
cattle receiving grain.

Urea
 Supplies Nitrogen (N).
 Contrary to common belief Urea is not protein. Urea is a
simple N compound. Urea is ~45% N = 281% crude protein
equivalent.
 Rumen microorganisms can convert N to ammonia which
is converted to amino acids and microbial protein.
 Urea can supply up to 1/3 of the total protein requirement
of cattle. However at these levels sufficient fermentable
energy must be supplied to allow utilization of available N
Urea
CO2
Carbohydrates
Keto Acids
Microbial Urease
Microbial Enzymes
Keto Acids + NH3
Amino Acids
Amino Acids
Microbial Protein
Microbial Protein
Free Amino Acids
NH3 +
FA +
Microbial Enzymes
Microbial Enzymes
Small Intestine Enzyme
Free Amino Acids Absorbed
Toxic dose generally regarded as 0.5grams/kg LWT in a single
feeding episode.

Sulphate of Ammonia (NH4)2SO4
 Supply N as with urea. Contains approx ½ the level
of N as urea
 Important source of Sulphur (S)
 Sulphur required as a precursor to the production
amino acids
 N:S ratio should be in the range of 12-15:1
 SO4 often used to control intake (more bitter than
urea )
 More expensive source of N than urea
Elemental Sulphur
 Source of S.
 Less available source of S than SO4.







Protein Meal
Source of Rumen Escape Protein
Source of Energy to aid utilization of Urea.
Used to manipulate intake.
Commonly used meals include
 Cotton Seed Meal 43%
 Copra Meal 20%
 Canola Meal 35%
 Soybean meal 48%
 PKE 15%
Very important to calculate the input cost of the protein.