Ruminant Protein Nutrition More appropriate: Rumen Nitrogen Metabolism

Protein Pathways in the Ruminant

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 Utilization Ruminant 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

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 8% CP 20% CP Measurement 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.

Rumen NH3 Following Protein Ingestion

Rumen VFA from Carbohydrate Sources

Matching Protein and Energy Sources

Protein Supplements for Beef Cows Type of feed used for beef cows? Would urea be utilized? Why is urea included in range pellets?

Range Pellets with NPN

Range Pellets – No NPN

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

Will urea meet the needs of steers at all weights? Diet 74% corn, 15% fescue hay, urea, molasses, minerals Weight (lb) 450 675 900 Daily intake (lb) 11 16.5 18 Daily gain (lb/day) 2.5 2.9 2.2 MP required (gm/day) 512 585 506 MP available (gm/day) 430 639 685 % of MP requirement available 89% 109% 135%

Feeding Urea - Dairy Dairy cows Upper limit ~1% of diet DM Palatability begins to limit intake

Urea Urea = 281% CP equivalent How can urea have >100% CP? 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

Energy pathways in the Ruminant