Maintenance Rations Week 4 Lecture 12 Clair Thunes, PhD Animal Science 126 Equine Nutrition Goals Understand that in reality that horses have an amino acid requirement not a CP requirement That there are essential and non-essential AA s Understand the concept of limiting amino acids and why having an adequate AA pool is important Understand why dietary protein quality is important for the horse and what defines protein quality Know about total tract and preceal protein digestibility why this is important. Goals Know what effects protein digestibility Know what available protein is Know how NRC came up with maintenance protein requirement and some of the issues/assumptions/errors Know how maintenance lysine requirement is estimated and the associated errors 1
Protein. Animal Science 126 Equine Nutrition Protein Protein a major component of all tissues in the body Also hormones, enzymes, antibodies etc Made up of amino acids (AA s) Requirement is actually for AA s but individual amino acid requirements are unknown In non-ruminant species there are 10 essential AA s presumed essential for the horse Arginine, histodone, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, valine and tryptophan. Need to provide adequate protein to insure enough circulating amino acid in blood. Protein Digestion/Absorption Dietary protein digested mainly in the foregut Enzymatic digestion in the stomach and SI In stomach via pepsin Pepsin has specificity to L-amino acids such as phenylalanine and tryptophan Pancreatic proteases secreted into the SI further digestion and allow absorption of AA s and dipeptides Dipeptides hydrolyzed in gut wall to AA s and are absorbed into blood. Non-protein nitrogen like urea also absorbed in SI 2
Protein Digestion/Absorption Any non-protein nitrogen or protein escaping absorption pass into the hindgut where they are degraded by microbes and turned into microbial protein. No evidence of absorption of AA s from microbial synthesis in any great quantity Studies have shown high correlation between dietary concentration of AA s and blood AA s but no correlation between cecal AA concentration and blood AA concentration Quality of dietary protein source therefore very important to the horse. Non-Protein Nitrogen NPN not useful to horses in most cases Ammonia toxicity can cause death Inclusion of urea in the diet found to be useful when dietary protein deficient but not when quality protein sources provided. Inclusion of urea increases blood and urine urea suggesting it contributes to excess nitrogen in the body. This leads to increased excretion of urea in urine. Protein Quality Protein quality a function of amino acid profile and digestibility of protein source The greater the foregut digestibility the higher the AA absorption and the more it contributes to the AA pool. Digestibility of N and CO related to the DM intake as well as CP concentration in diet Digestibility of protein increases as DM intake and CP concentration increase. Total tract apparent digestibility depends on; protein source components in the diet. Ratio of forage to concentrate in diet 3
Total tract digestibility of alfalfa was estimated to be 73-83% with apparent pre-cecal digestibility 28.5%. For bermuda grass hay this was 57-64% and 16.8% respectively. Endogenous fecal losses estimated to be 5.8 mg N/g DM intake True prececal digestibility of protein from forages 37%. Feeding high grain diets with hay altered results Corn, oats or sorghum with bermuda grass hay in a 3:1 concentrate to forage ratio resulted in apparent total tract digestibility of 88, 82.8,84.6% respectively. Apparent prececal digestibility was 38.5, 45.8 and 56.1% respectively. Digestibility for the grains alone was determined to be apparent prececal 48.1, 53.9, and 70.7% and apparent total tract 98.1, 88.5 and 93.1%. Relative preceal digestion of protein appears to be 25-30% of the dietary protein for all forage diets Approaches 70-75% when protein supplements like soy added to the ration. Using 16 studies that gave nitrogen intake as well as fecal losses were used by NRC 2007 to estimate apparent nitrogen digestibility of 79% for the total tract using linear regression (r 2 = 0.94) Whether the diets were 100% forage or a mix of forage and concentrate there was no significant difference (77% for forage only NRC 2007). 4
NRC used 4 studies of prececal digestibility that reported nitrogen intake and foregut N disappearance. Prececal digestibility was estimated to be 51% (r 2 = 0.83) Only 1 study was 100% forage and it was slightly lower at 42%. The mixed feed studies reviewed used soy, cotton seed meal, linseed meal, brewers dried grains, fishmeal, milk byproducts and corn gluten meal. Overall protein digestibility may be different if different protein sources used. Nitrogen balance studies give varied results of the amount of nitrogen needed to maintain a zero nitrogen balance. Due to difference in digestibility and amino acid profile Understanding digestibility allows for requirement to be set as digestible protein (DP) instead of CP This allows diets to be balanced in terms of DP if data available French have Matieres Azotees Digestibles Cheval (MADC) which accounts for differences in digestibility of various protein sources and their availability in the foregut and hindgut Provides insight into available protein for the horse Needs more data 5
Protein Bioavailability Protein digested in the foregut are potentially available and of higher quality Quality further determined by AA composition Therefore different amounts of protein can be needed for zero N retention when different sources are fed Not all CP is available to the horse adjusting for this could allow estimation of digestible or available protein (AP) AP = CP NPN ADIN ADIN = acid detergent insoluble protein bound protein Protein Bioavailability AP is therefore calculated estimate of protein that is available DP is based on whole tract digestibility studies that measured the amount of protein apparently digested. Moving to AP could be the next step in moving away from CP towards DP. Maintenance Protein & Lysine Requirements Animal Science 126 Equine Nutrition 6
Maintenance Protein When protein needs are evaluated energy intake must be adequate When CP is deficient weight loss results however when energy is deficient despite CP being adequate weight loss still results. NRC modeled research data and found minimum DP intake for maintenance should be >620 mg DP/kg BW/d This was fitted to another model resulting in a CP of 1.26 g/kg BW/d for an average horse Maintenance Protein Using 95% CI for the data gave a range of 1.08 g CP/kg BW/d to 1.44 g CP/kg BW/d This 3 levels results similar to energy with the assumption that more active horses have more lean muscle mass and therefore higher protein need. Minimum = BW x 1.08 g CP/kg BW/d Average = BW x 1.26 g CP/kg BW/d Elevated = BW x 1.44 g CP/kg BW/d No studies have been done in adult sedentary horses for lysine requirement Lysine Requirement No studies have been done in adult sedentary horses for lysine requirement Requirement based on average content of protein and looking at how much protein has been fed in various feeding trails Analysis by the NRC of 7 trials gave a minimum intake requirement of 0.036 g lysine/kg BW/d for a zero N retention N retention hit a plateau at 0.054 g lysine/kg BW/d which is assumed to be the optimal lysine intake 7
Lysine Requirement Using these equations for a 500kg horse gives a minimum lysine requirement of 18g and an optimal requirement of 27g. CP requirement for the same horse is 630g (check requirement handout for 500kg horse with table 16-3 page 298) 27g is 4.3% of 630g Lysine requirement in g/d is therefore calculated as = CP requirement x 4.3% You can see that this contains a lot of assumptions and potential sources of error!!!!! 8