Ruminant Protein Nutrition More appropriate: Rumen Nitrogen Metabolism
General Information • No proteases in saliva • No rumen secretions • Microorganisms responsible for protein digestion in rumen (and reticulum) • Bacteria • Protozoa
Sources of Rumen Nitrogen • Feed • Protein nitrogen • Protein supplements (SBM, CSM, grains, forages, silages... • Nonprotein nitrogen (NPN) • Usually means urea • However, from 5% of N in grains to 50% of N in silage and immature forages can be NPN • Endogenous (recycled) N • Saliva • Rumen wall
Ruminal Protein Degradation • Fermentative digestion – enzymes of microbial origin • MO proteases & peptidases cleave peptide bonds and release AA • AA deaminated by microbes, releasing NH3 and C-skeleton • MO’s use NH3, C-skeleton and energy to synthesize their own AA • Energy primarily from CHO’s (starch, cellulose) • Formation of NH3 rapid...very few free AA in rumen
NPN Utilization • Urea (and most sources of NPN) rapidly degraded to NH3 • MO’s don’t care where NH3 comes from
Limitations of Microbial Protein Synthesis • Two most likely limitations • Energy available • NH3 available • These need to be synchronized • For diets containing urea, may also need • Sulfur (for S-containing AA) • Branched-chain C-skeletons • MO cannot make branched-chain C-chains • These normally not a problem
Overflow Ammonia • Shortage of energy relative to available NH3 • Liver: NH3 Urea • Urea recycled or excreted, depending on animal needs • Saliva • Rumen wall
Protein Leaving Rumen • Microbial protein • Escape protein (also called “bypass” protein) • Enter abomasum & small intestine • Digested by proteolytic enzymes similar to nonruminants • Escape vs Bypass protein • Technically not “bypass” • Reticular groove
Protein UtilizationRuminant vs Nonruminant Similarities and Dissimilarities
Ruminant vs Nonruminant - Similarities • At tissue level – Metabolic pathways similar • Ruminant tissues can synthesize dispensable AA • Cannot synthesize indispensable AA • Essential AA must be provided from digestive tract • Tissue proteins constantly undergoing turnover • AA not stored • Constant supply of AA required
Ruminant vs Nonruminant - Dissimilarities • Microbial population has profound effect on AA reaching S.I. • AA profile at S.I. different from diet • Up-grades low quality dietary protein • Down-grades high quality dietary protein • Enables ruminants to use NPN efficiently • Ruminants can be productive without a source of dietary true protein • Animal can survive on low amounts of dietary protein by recycling N (as urea) back to rumen
Ruminant vs Nonruminant - Dissimilarities • Microbial population has profound effect on AA reaching S.I. (cont.) • Why we say nitrogen metabolism (vs protein metab.) • Microbial intervention • NH3 formation • Disadvantage: more protein can be destroyed in the rumen than is synthesized Result = Net loss of protein Advantage: can have more protein leaving rumen than is in the diet Result = Net gain of protein
Measurement 20% CP 8% CP Example: More Protein Leaving Rumen than was in Diet • Weston & Hogan (Australia) first to show this • Fed sheep 2 diets containing 20% and 8% CP • 20% Lucerne (alfalfa), corn, PNM • 8% Wheaten hay, corn • Diets supported identical wool growth N fed (gm/day) 13.8 5.5 AA-N entering S.I. (gm/day) 8.8 8.1 N entering S.I. vs diet Net loss Net gain
Ruminant vs Nonruminant - Dissimilarities • In ruminant nutrition – generally not concerned with AA composition of dietary protein • Type of feed does not affect AA comp. of bacteria and protozoa leaving rumen • AA comp. of MO’s reaching duodenum strikingly similar when measured in labs around the world • Biological value (BV) of microbial protein ~80%
Matching Available Energy with Rates of Protein Degradation To maximize efficiency of microbial protein synthesis from ammonia, available energy must be present.
Protein Supplements for Beef Cows • Type of feed used for beef cows? • Would urea be utilized? • Why is urea included in range pellets?
Feeding Urea - Beef • Feedlot cattle (fed grain or silage diets) • Up to 650-750 lb, use natural protein (SBM, CSM) • Can’t consume enough for MO’s to meet protein needs • >650-700 lb, urea = natural protein as N source • Above 0.75% urea in diet DM, start observing palatability problems ( intake) • General recommendation... • don’t exceed 1% urea in diet
Feeding Urea - Dairy • Dairy cows • Upper limit ~1% of diet DM • Palatability begins to limit intake
Urea • Urea = 281% CP equivalent • N = 45% of urea • 45%N x 6.25 = 281% CP • How can urea have >100% CP? • Does this mean anything practical or is it just academic?
Urea Toxicity (NH3 Toxicity) • Mechanism • Rumen [NH3] Rumen pH • As pH , shift from NH4+ to NH3 • NH3 absorbed faster than NH4+ • Liver capacity to convert NH3 to urea is exceeded • NH3 goes to blood • 2 mg NH3/100 ml plasma is toxic
Urea Toxicity (NH3 Toxicity) • Signs of toxicity • Appear 20-30 min after urea ingestion • Rapid and labored breathing • Tremors • Incoordination • Inability to stand & tetany increasingly apparent
Urea Toxicity (NH3 Toxicity) • Treatment • Orally dose with 5% acetic acid (~1 gal. for 1,000 lb cow) • Shift equilibrium from NH3 to NH4+ • rate of absn • Drench with cold water • rumen temp. which rate of urea hydrolysis • Dilutes NH3 concentration • Takes 6-12 gal.; not practical when several sick
Urea Toxicity (NH3 Toxicity) • Prevention • Mix feeds well • Don’t switch rapidly from natural protein to urea • Always have feed available • Don’t allow hungry animals access to highly palatable, high urea diet, feed, or supplement (including lick tanks) • Don’t use urea with low-energy feeds