Standardized ileal ("true") digestibility is currently the best estimate of amino acid digestibility, but it does not measure bioavailability. Growth assays to determine amino acid bioavailability are expensive and laborious; thus, a rapid method is needed. Applying the principle of slope-ratio assay to the indicator amino acid oxidation (IAAO) method, we hypothesized that the reduction in indicator oxidation per gram of lysine in feedstuffs relative to that per gram of free lysine represented the bioavailability of lysine, here termed "metabolic availability." Indicator oxidation in pigs was linear over increasing lysine intakes (r = 0.90, P = 0.001) when the dietary lysine contents were 2 SD below the mean lysine requirement of the pigs. Peas were treated (raw, heated to reduce lysine availability, or heated with added lysine) to test the responsiveness of the IAAO to differing lysine availability. Free lysine reduced indicator oxidation by 3.16% of dose oxidized per gram added lysine, whereas the addition of protein lysine as raw (-2.81%) and heated peas (-1.73%) reduced oxidation to a lesser degree. Adding free lysine to heated peas decreased indicator oxidation, evidence that heating had worsened the utilization of pea protein for protein synthesis by reducing the bioavailability of lysine alone. Pea diets differed only in the availability of lysine; therefore IAAO detected differences in lysine bioavailability. Because the IAAO technique responds to lysine available at the sites of protein synthesis, the metabolic availability covers all losses during digestion, absorption, and utilization of lysine. This method can determine the metabolic availability of amino acids of a feedstuff within 2 wk.

Odor and greenhouse gas (GHG) emissions from stored pig (Sus scrofa) manure were monitored for response to changes in the crude protein level (168 or 139 g kg(-1), as-fed basis) and nonstarch polysaccharide (NSP) content [i.e., control, or modified with beet pulp (Beta vulgaris L.), cornstarch, or xylanase] of diets fed to pigs in a production setting. Each diet was fed to one of eight pens of pigs according to a 2 x 4, full-factorial design, replicated over three time blocks with different groups of animals and random assignment of diets. Manure from each treatment was characterized and stored in a separate, ventilated, 200-L vessel. Repeated measurements of odor, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions from the vessels were taken every two weeks for eight weeks. Manure from high-protein diets had higher sulfur concentration and pH (P < or = 0.05). High-NSP (beet pulp) diets resulted in lower manure nitrogen and ammonia concentrations and pH (P < or = 0.05). Odor level and hedonic tone of exhaust air from the storage vessel headspaces were unaffected by the dietary treatments. Mean CO2 and CH4 emissions (1400 and 42 g d(-1) m(-3) manure, respectively) increased with lower dietary protein (P < or = 0.05). The addition of xylanase to high-protein diets caused a decrease in manure CO2 emissions, but an increase when added to low-protein diets (P < or = 0.05). Nitrous oxide emissions were negligible. Contrary to other studies, these results do not support the use of dietary protein reduction to reduce emissions from stored swine manure.

There is disagreement about the adaptation time required when using the indicator amino acid oxidation (IAAO) technique. Our objective was to establish the adaptation time required to obtain a plateau in indicator (L-[1-(14)C]-phenylalanine) oxidation in response to a test diet using growing and adult pigs. Four barrows (20 kg) and 4 sows (240 kg) were surgically implanted with venous catheters for isotope infusion. Growing Pigs: After 7 d of adaptation to an adequate lysine intake of 8.8 g/d, phenylalanine oxidation in growing pigs was 9.38 +/- 1.25% of the infused dose. At 2, 3, 4, or 6 d after reducing lysine intake to 3.8 g/d, and then increasing it back to 8.8 g/d, phenylalanine oxidation was 16.94 +/- 0.84% (P < 0.05) and 9.70 +/- 0.80% (P < 0.05), respectively, with no significant effect of days of adaptation to diet. Adult Pigs: After 14 d of adaptation to an intake of 200% of the amino acid maintenance requirement, phenylalanine oxidation in sows was 4.23 +/- 0.45% of dose. Changing the intake to 100 and 50% of the maintenance requirement, increased (P < 0.05) phenylalanine oxidation to 5.95 +/- 0.26 and 7.90 +/- 0.26%, respectively, with no significant effect of time (1, 2, 5, 6, 9, and 10 d) after diet change. The CV for repeated phenylalanine oxidation measurements within pigs and diets was 13.5% for growing and 8.8% for adult pigs. This demonstrates that the IAAO requires <2 d of adaptation regardless of age, dietary challenge (individual amino acid or total protein) or direction (increase or decrease) of change, and that the measured oxidation rate (% of dose) is highly repeatable.

Although AA requirements for the mean in a population of growing pigs are well established, there are no direct estimates of their variability within the population. The indicator AA oxidation method allows repeated measurements in a short period of time so that the AA requirement can be determined for individual pigs. The objective was to determine the Lys requirement in individual pigs to derive a first estimate of the population mean requirement and its variability. Nine individually housed barrows (15 to 18 kg) were surgically implanted with venous catheters for isotope infusion. Pigs were offered, in random order, isonitrogenous and isoenergetic diets with one of seven Lys concentrations (4.8 to 15.5 g of Lys/ kg diet, as-fed basis). The pigs were fed twice daily, except for study days when they received one-half of the daily allowance in eight equal hourly meals. After a validated minimum adaptation period, indicator (Phe) oxidation was determined for each dietary Lys level during a 4-h primed, constant infusion of L-[1-14 C]Phe

Inevitable catabolism contributes to the inefficiency of using dietary lysine intake for body protein deposition (PD). This study was conducted to determine the effects of true ileal digestible (TID) lysine intake, body weight (BW), and growth potential on lysine catabolism in growing pigs. Starting at 15 kg BW, 16 female Yorkshire pigs were offered a purified diet providing all nutrients in excess of requirements for maximum protein deposition (PDmax). At ϳ25 kg BW, the pigs' PDmax was determined using the N-balance method. Thereafter, 4 pigs were allocated to each of 4 diets, first-limiting in lysine, providing lysine intakes corresponding to 60, 70, 80, and 90% of estimated requirements for PDmax. The pigs were surgically fitted with catheters in the jugular and femoral veins. Lysine catabolism was determined at 2 BW (40 -45 kg, low; 70 -75 kg, high) either directly (oxidation) using a primed, constant infusion of L-[1-14 C]-lysine or indirectly (disappearance) using the N-balance method. There was no effect of BW on the rate (g/d) or fraction of TID lysine intake catabolized. Lysine catabolism decreased with increasing growth potential. Lysine disappearance and lysine oxidation (% of TID lysine intake) were independent of lysine intake, except for the lowest lysine intake level, where they were lower. When lysine catabolism was independent of intake, lysine oxidation based on plasma free lysine specific radioactivity (SRA) was lower (9.9% of TID intake) than lysine disappearance (17.4% of TID intake) or lysine oxidation based on liver free lysine SRA (13.4% of TID intake).

Tryptophan is the precursor for several neurotransmitters and metabolic regulators, which, although quantitatively of little importance in determining the dietary requirement, have major importance for interpreting symptoms of dietary tryptophan deficiency and excess. The quantitative dietary tryptophan requirement appears to vary widely across species, so intakes relative to requirements are more appropriate expressions for comparison of adverse effects across species than daily intake or diet concentration. Symptoms of tryptophan deficiency may occur at intakes as little as 25% below the requirement. Symptoms include reduced feed intake and reduced growth rate but also impaired skeletal development and aberrant behavior. Older animals appear less susceptible than younger animals to tryptophan deficiency and females less than males. Symptoms of excess tryptophan intake include reduced food intake and growth rate. In growing animals, it appears that tryptophan intakes of >10 times the requirement are necessary before there are detrimental effects on growth performance. At still greater intakes, fatty liver and fibrotic changes in muscles, lung, and pancreas and the serotonin syndrome may develop. In pigs, tryptophan intake of 60 times the daily requirement did not cause mortality. The maximal tryptophan oxidation rate, measured in vivo using (13)C universally labeled tryptophan, may be a possible marker of the intake above which increasing intake increases the risk of adverse effects. The advantage of the oxidation technique is that it does not necessarily rely on but still allows the identification and measurement of amino acid metabolites and is therefore simpler and more universally applicable.

Our objective was to determine the metabolic availability (MA) of sulfur amino acids in dietary proteins using the indicator amino acid oxidation (IAAO) technique. Five to seven men received graded levels (20, 40, 60, and 70%) of the mean total sulfur amino acid (TSAA) requirement of 13 mg x kg(-1) x d(-1) as a crystalline AA mixture, casein, and soy protein isolate (SPI) (40, 50, 60, and 70%), respectively. Five of these subjects received 40% of TSAA requirement from SPI supplemented with methionine to the level of 40% of requirement. These 5 subjects also repeated the level of 60% TSAA requirements from both casein and SPI to assess repeatability. The mean MA of TSAA from SPI (71.8 +/- 3.6%) was lower than from casein (87.4 +/- 3.8%, P < 0.05). Supplementation of SPI with methionine decreased the IAAO (11.5 +/- 0.3% administered dose) compared with unsupplemented SPI (12.8 +/- 0.5% administered dose, P < 0.05). IAAO was similar for repeated measurements of casein and SPI, respectively, at the 60% TSAA intake level (10.8 +/- 1.0 vs. 10.7 +/- 1.2% for casein; 12.7 +/- 1.3 vs. 12.9 +/- 2.6% for SPI). In conclusion, the IAAO technique can be used to determine the MA of AA for protein synthesis in test proteins for humans.

Current AA recommendations for sows are to provide a fixed amount of AA intake throughout gestation; however, the demand for nutrients changes from maternal lean tissue in early gestation (EG) to fetal and mammary growth in late gestation (LG). The objective of this study was to determine the Lys requirement in EG (d 24 to 45) and LG (d 86 to 110) using the indicator AA oxidation method with simultaneous determination of heat production. Each of 7 Large White × Landrace sows received 6 diets in random order in both EG and LG. Three semisynthetic diets (14.0 MJ ME/kg) based on corn were formulated and mixed to produce a basal diet (60% of 1998 NRC Lys requirement) and high diets for EG and LG (150% and 185% of 1998 NRC Lys requirements, respectively). The 6 test diets provided Lys intakes of 7.5 to 19.3 g/d in EG and 8.1 to 23.7 g/d in LG. Sows were placed in respiration chambers, and expired air and blood were collected every 30 min for 5.5 h. The tracer AA, l-[1-(13)C]Phe, was given orally at a rate of 2 mg/(kg BW ⋅ h) over the last 4 h, divided into 8, 0.5-h meals. Expired air was measured for (13)CO(2) enrichment, and plasma was measured for l-[1-(13)C]Phe enrichment and free Lys concentration. Background (13)CO(2) was subtracted from plateau (13)CO(2) enrichment. Requirements were determined using a 2-phase nonlinear model. Mean maternal BW gain in gestation (43.7 kg; pooled SE, 1.2 kg), litter size (14.6 total born piglets; pooled SE, 0.8), and litter weight (19.4 kg; pooled SE, 0.9 kg) did not differ between parities. Sow weight gain and BW was greater (P = 0.001) in LG than EG. Lysine requirement was 9.4 and 17.4 g/d in EG and LG, respectively. Phenylalanine retention in LG was maximized at a Lys intake of 17.7 g/d. Heat production was more (P = 0.069) and energy retention less (P = 0.019) in LG than EG. Energy retention in LG was not different from 0. Quantitative Phe kinetics in EG were not affected by Lys intake. In LG, Phe retention increased with Lys intake (P = 0.004), whereas Phe oxidation decreased (P = 0.005). The Lys requirement was determined to be less than current recommendations in EG and more than current recommendations in LG. To meet the change in requirements, diets with increased lysine content are needed in LG. Increasing the feed allowance in LG is necessary to maintain a positive energy balance throughout gestation.