Abstract
Two experiments were conducted to evaluate the influence of drying method on the composition of ileal digesta and the standardized ileal digestibility (SID) of CP and AA in feed ingredients fed to pigs. The hypothesis was that oven drying of ileal digesta samples results in loss of N and AA, and therefore, in greater calculated values for SID of CP and AA compared with lyophilized samples. In Exp. 1, eight barrows (13.8 ± 0.5 kg BW) were equipped with a T-cannula in the distal ileum and randomly allotted to a replicated 4 × 4 Latin square design with four diets and four periods, for a total of eight replicate pigs per diet. Three diets containing three different sources of soybean meal as the sole source of AA and an N-free diet were used. In Exp. 2, 18 cannulated growing barrows (72.5 ± 9.2 kg BW) were allotted to a completely randomized design with three diets and six replicate pigs per diet. The three diets included two diets based on two sources of distillers dried grains with solubles and an N-free diet. In both experiments, ileal digesta samples were collected for 8 h on days 6 and 7 of the 7-d feeding period. At the conclusion, two representative sub-samples were collected from each ileal digesta sample. One sub-sample was lyophilized and the other was oven dried at 60 °C using a forced air oven. Results indicated that in both experiments, DM was greater (P < 0.05) in lyophilized samples than in oven dried samples. There was no difference in the SID of CP between the two drying methods. However, except for Ala and Val in Exp. 1, the concentration of AA (88% DM-basis) was greater (P < 0.05) in lyophilized samples than in oven dried samples, which resulted in reduced calculated values for basal endogenous losses of AA. Therefore, values for the SID of AA (except for Ala in Exp. 1 and Trp and Gly in Exp. 2) were greater (P < 0.05) when calculated from oven dried samples than from lyophilized samples. In conclusion, regardless of the diet, oven drying of ileal digesta samples does not result in loss of N, but appears to damage or convert AA to other N-containing compounds, which results in reduced estimates for basal endogenous losses and greater calculated values for SID of AA. Thus, freeze drying of ileal digesta samples is recommended when data for the SID of AA are calculated for feed ingredients fed to pigs.
Keywords: AA digestibility, drying method, freeze drying, ileal digesta, oven drying, pigs
INTRODUCTION
Ileal digestibility of AA is calculated by subtracting the AA output in ileal digesta from the quantity of AA ingested by the animal (Stein et al., 2007). An accurate estimation of the concentration of AA in the diet and in ileal digesta is, therefore, required in experiments that aim at estimating ileal digestibility of AA. Although it is easier and less expensive to oven-dry samples, freeze drying has usually been used as the drying method because of the assumption that some N compounds may evaporate during oven drying (Manoukas et al., 1964; Sibbald, 1979). Indeed, for poultry, an average of 5.5% and 8.8% N losses was reported if excreta were oven dried at 65 and 55 °C, respectively, compared with fresh samples (Manoukas et al., 1964; Ribeiro et al., 2001). However, there appears to be no differences in N losses between lyophilized and oven dried (60 °C) excreta from laying hens (Shannon and Brown, 1969). Concentrations of most AA in lyophilized excreta samples from broiler chickens were greater than in samples that were oven dried at 60 °C for 24 h (Wallis and Balnave, 1983), which concurs with data indicating that the true digestibility of Arg, Met, Lys, and Tyr determined in roosters was greater if samples were oven dried (60 °C) instead of lyophilized (Dale et al., 1985).
Apparent total tract digestibility (ATTD) of CP in pigs was not influenced by drying method of feces (Jørgensen et al., 1984), and there was no difference in the concentration of N in fresh, oven dried, or lyophilized feces from pigs (Jacobs et al., 2011). By contrast, N losses were observed in pig slurry in oven dried samples, but not in lyophilized samples (Mahimairaja et al., 1990). To our knowledge, there is, however, no information about the influence of drying method on nutrient composition of ileal digesta or the ileal digestibility of AA in diets fed to pigs. Therefore, the objective of this work was to test the hypothesis that oven drying of ileal digesta results in loss of N and AA, and therefore, in greater calculated values for the apparent ileal digestibility (AID) and standardized ileal digestibility (SID) of CP and AA compared with freeze drying of samples.
MATERIALS AND METHODS
Two experiments were conducted following protocols reviewed and approved by the Institutional Animal Care and Use Committee at the University of Illinois. Pigs used in both experiments were the offspring of Line 359 boars mated to Camborough sows (PIC, Hendersonville, TN).
Experiment 1: AA Digestibility in Soybean Meal
Eight growing barrows (initial BW: 13.82 ± 0.52 kg) were equipped with a T-cannula in the distal ileum and allotted to a replicated 4 × 4 Latin square design with four diets and four periods in each square. There were, therefore, eight replicate pigs per treatment. Pigs were housed in individual pens equipped with a feeder and a nipple drinker in an environmentally controlled room. Pens had smooth sides and fully slatted tribar floors.
Four diets were prepared (Table 1). Three sources of soybean meal (SBM) were included as the only source of AA in three diets, and an N-free diet that was used to measure basal endogenous losses of CP and AA was also used. Vitamins and minerals were included in all diets to meet or exceed current requirement estimates (NRC, 2012). All diets also contained 0.4% chromic oxide as an indigestible marker and all diets were provided in a meal form.
Table 1.
Ingredient composition and nutrient analysis of experimental diets containing three sources of soybean meal (SBM), as-fed basis, Exp. 1
| Diet | ||||
|---|---|---|---|---|
| Item | SBM-1 | SBM-2 | SBM-3 | N-free |
| SBM-1, % | 38.00 | — | — | — |
| SBM-2, % | — | 38.00 | — | — |
| SBM-3, % | — | — | 46.00 | — |
| Soybean oil, % | 3.00 | 3.00 | 3.00 | 4.00 |
| Ground limestone, % | 0.75 | 0.75 | 0.75 | 0.45 |
| Dicalcium phosphate, % | 1.10 | 1.10 | 1.10 | 2.15 |
| Lactose, % | 20.00 | 20.00 | 20.00 | 20.00 |
| Cornstarch, % | 36.05 | 36.05 | 28.05 | 67.80 |
| Solca flok1, % | — | — | — | 4.00 |
| Magnesium oxide, % | — | — | — | 0.10 |
| Potassium carbonate, % | — | — | — | 0.40 |
| Sodium chloride, % | 0.40 | 0.40 | 0.40 | 0.40 |
| Chromic oxide, % | 0.40 | 0.40 | 0.40 | 0.40 |
| Vitamin–mineral premix2, % | 0.30 | 0.30 | 0.30 | 0.30 |
| Total, % | 100.00 | 100.00 | 100.00 | 100.00 |
| Analyzed composition | ||||
| DM, % | 94.83 | 95.51 | 92.40 | 95.24 |
| CP, % | 21.32 | 21.59 | 21.47 | 0.19 |
| Indispensable AA, % | ||||
| Arg | 1.24 | 1.37 | 1.56 | 0.01 |
| His | 0.56 | 0.53 | 0.58 | 0.00 |
| Ile | 0.97 | 0.97 | 1.02 | 0.02 |
| Leu | 1.56 | 1.60 | 1.71 | 0.02 |
| Lys | 1.23 | 1.08 | 1.43 | 0.02 |
| Met | 0.28 | 0.28 | 0.31 | 0.01 |
| Phe | 1.00 | 1.05 | 1.11 | 0.01 |
| Thr | 0.74 | 0.80 | 0.85 | 0.01 |
| Trp | 0.30 | 0.31 | 0.33 | 0.02 |
| Val | 1.01 | 1.03 | 1.09 | 0.01 |
| Dispensable AA, % | ||||
| Ala | 0.88 | 0.91 | 0.96 | 0.01 |
| Asp | 2.20 | 2.30 | 2.46 | 0.02 |
| Cys | 0.26 | 0.27 | 0.30 | 0.01 |
| Glu | 3.50 | 3.69 | 3.91 | 0.02 |
| Gly | 0.87 | 0.88 | 0.93 | 0.01 |
| Pro | 0.91 | 1.05 | 1.11 | 0.03 |
| Ser | 0.74 | 0.89 | 0.95 | 0.01 |
| Tyr | 0.53 | 0.65 | 0.70 | 0.01 |
| Total AA, % | 18.78 | 19.66 | 21.31 | 0.25 |
1Fiber Sales and Development Corp., Urbana, OH.
2Provided the following quantities of vitamins and micro minerals per kilogram of complete diet: vitamin A as retinyl acetate, 11,136 IU; vitamin D3 as cholecalciferol, 2,208 IU; vitamin E as dl-α tocopheryl acetate, 66 IU; vitamin K as menadione dimethylprimidinol bisulfite, 1.42 mg; thiamin as thiamine mononitrate, 0.24 mg; riboflavin, 6.59 mg; pyridoxine as pyridoxine hydrochloride, 0.24 mg; vitamin B12, 0.03 mg; d-pantothenic acid as d-calcium pantothenate, 23.5 mg; niacin, 44.1 mg; folic acid, 1.59 mg; biotin, 0.44 mg; Cu, 20 mg as copper sulfate and copper chloride; Fe, 126 mg as ferrous sulfate; I, 1.26 mg as ethylenediamine dihydriodide; Mn, 60.2 mg as manganese sulfate; Se, 0.3 mg as sodium selenite and selenium yeast; and Zn, 125.1 mg as zinc sulfate.
Pigs were fed their assigned diets on an ad libitum basis (Chastanet et al., 2007) and water was available at all times. Each period lasted 7 d. The initial 5 d of each period were considered an adaptation period to the diet, but ileal digesta were collected for 8 h on days 6 and 7 by attaching a plastic bag to the barrel of the opened cannula. Bags were removed whenever they were filled with digesta or every 30 min, and stored at –20 °C.
Experiment 2: AA Digestibility in Distillers Dried Grains with Solubles
Eighteen growing barrows with an approximate initial BW of 72.5 ± 9.2 kg were equipped with a T-cannula in the distal ileum and allotted to a completely randomized design with three diets and six replicate pigs per diet. Pigs were housed as described for Exp. 1.
Two sources of distillers dried grains with solubles (DDGS) were procured, and each source was used in one diet as the sole source of AA and CP (Table 2). An N-free diet was also formulated. Pigs were fed at a level of three times the maintenance energy requirement (i.e., 197 kcal ME per kg0.60; NRC, 2012). The daily allotment of feed was divided into two equal meals and provided at 0800 and 1600 hours, and water was available at all times. Ileal digesta collection followed procedures described for Exp. 1.
Table 2.
Ingredient composition and nutrient analysis of experimental diets containing two sources of distillers dried grains with solubles (DDGS), as-fed basis, Exp. 2
| Diet | |||
|---|---|---|---|
| Item | DDGS-1 | DDGS-2 | N-free |
| DDGS-1, % | 37.00 | — | — |
| DDGS-2, % | — | 50.00 | — |
| Soybean oil, % | 2.00 | 2.00 | 4.00 |
| Ground limestone, % | 1.00 | 1.25 | 0.50 |
| Monocalcium phosphate, % | 0.80 | 0.15 | 1.75 |
| Sucrose, % | — | — | 20.00 |
| Cornstarch, % | 58.10 | 47.50 | 68.15 |
| Solca flok1, % | — | — | 4.00 |
| Magnesium oxide, % | — | — | 0.10 |
| Potassium carbonate, % | — | — | 0.40 |
| Sodium chloride, % | 0.40 | 0.40 | 0.40 |
| Chromic oxide, % | 0.40 | 0.40 | 0.40 |
| Vitamin–mineral premix2, % | 0.30 | 0.30 | 0.30 |
| Total | 100.00 | 100.00 | 100.00 |
| Analyzed composition | |||
| DM, % | 91.85 | 91.85 | 94.97 |
| CP, % | 14.08 | 14.14 | 0.27 |
| Indispensable AA, % | |||
| Arg | 0.65 | 0.71 | 0.01 |
| His | 0.38 | 0.43 | 0.00 |
| Ile | 0.63 | 0.66 | 0.02 |
| Leu | 1.72 | 1.87 | 0.03 |
| Lys | 0.58 | 0.58 | 0.02 |
| Met | 0.27 | 0.29 | 0.00 |
| Phe | 0.77 | 0.85 | 0.02 |
| Thr | 0.56 | 0.61 | 0.01 |
| Trp | 0.13 | 0.13 | 0.02 |
| Val | 0.81 | 0.87 | 0.02 |
| Dispensable AA, % | |||
| Ala | 1.05 | 1.14 | 0.02 |
| Asp | 1.02 | 1.06 | 0.02 |
| Cys | 0.28 | 0.31 | 0.02 |
| Glu | 2.35 | 2.41 | 0.03 |
| Gly | 0.61 | 0.67 | 0.01 |
| Ser | 0.59 | 0.63 | 0.01 |
| Tyr | 0.53 | 0.57 | 0.01 |
| Total AA, % | 14.10 | 15.11 | 0.40 |
1Fiber Sales and Development Corp., Urbana, OH.
2Provided the following quantities of vitamins and micro minerals per kilogram of complete diet: vitamin A as retinyl acetate, 11,136 IU; vitamin D3 as cholecalciferol, 2,208 IU; vitamin E as dl-alpha tocopheryl acetate, 66 IU; vitamin K as menadione dimethylprimidinol bisulfite, 1.42 mg; thiamin as thiamine mononitrate, 0.24 mg; riboflavin, 6.59 mg; pyridoxine as pyridoxine hydrochloride, 0.24 mg; vitamin B12, 0.03 mg; d-pantothenic acid as d-calcium pantothenate, 23.5 mg; niacin, 44.1 mg; folic acid, 1.59 mg; biotin, 0.44 mg; Cu, 20 mg as copper sulfate and copper chloride; Fe, 126 mg as ferrous sulfate; I, 1.26 mg as ethylenediamine dihydriodide; Mn, 60.2 mg as manganese sulfate; Se, 0.3 mg as sodium selenite and selenium yeast; and Zn, 125.1 mg as zinc sulfate.
Chemical Analysis and Data Processing
At the conclusion of Exp. 1, ileal digesta samples were thawed and mixed within pig and period, and for each sample, two representative sub-samples were collected for analysis. One of the sub-samples was lyophilized in a freeze dryer (Gamma 1-16 LSCplus, IMA Life, Tonawanda, NY) for 8 d and the other was oven dried at 60 °C for 6 d in a forced-air oven (METALAB, Equipment Corp., Hicksville, NY). All dried samples were ground through a 1 mm screen (Wiley Mill Model 4; Thomas Scientific; Swedesboro, NJ). Diets, ingredients, and ileal digesta samples were analyzed in duplicate for DM by oven drying at 135 °C for 2 h (Method 930.15; AOAC Int., 2007). CP was analyzed by combustion (Method 999.03; AOAC Int., 2007) using a Rapid N cube apparatus (Elementar Americas Inc., Mt. Laurel, NJ), and for AA [Method 982.30 E (a, b, c); AOAC Int., 2007]. Diets and ileal digesta samples were also analyzed for chromium (Method 990.08; AOAC Int., 2007).
Data for the composition of ileal digesta were adjusted to 88% DM. Basal endogenous losses were calculated from pigs fed the N-free diet and values for AID and SID of CP and AA were calculated for the three diets containing SBM for both oven dried and lyophilized ileal digesta samples (Stein et al., 2007). Because SBM was the only AA-containing ingredient in the diets, values calculated for each diet also were representative for the three sources of SBM. Normality of data was verified using the UNIVARIATE procedure (SAS Inst. Inc., Cary, NC). Data for AA composition of ileal digesta as well as data for the SID of CP and AA were analyzed using the PROC MIXED of SAS with pig as the experimental unit. The statistical model only included the fixed effect of diet and drying method because the interaction between diet and drying method was not significant. The model also included the random effects of period and pig. Treatment means were calculated using the LSMEANS statement and LSmeans were separated using the PDIFF option of SAS. Statistical significance and tendency were considered at P ≤ 0.05 and 0.05 ≤ P < 0.10, respectively.
At the conclusion of Exp. 2, ileal digesta were thawed, mixed, and two sub-samples were collected. One sub-sample was lyophilized, and the other sub-sample was oven dried as described for Exp. 1. After grinding, diets, ingredients, and ileal digesta samples were analyzed in duplicate for DM, CP, AA, and chromium.
Values for AID, endogenous losses, and SID of CP and AA were calculated for the oven dried as well as for the lyophilized samples in the two diets containing DDGS (Stein et al., 2007). Data were analyzed as described for Exp. 1, with the exception that the model only included the random effect of pig.
RESULTS
Experiment 1: AA Digestibility in Soybean Meal
The concentration of DM, Cr, CP, and most AA in ileal digesta samples was different (P < 0.05) between the three diets containing SBM and the N-free diet (Table 3). The concentration of DM was greater (P < 0.05) in lyophilized samples than in oven dried samples, but there was no difference in the concentration of CP or Cr (88% DM basis) between the two drying methods. Likewise, the concentration of Val (88% DM basis) in ileal digesta samples was not different between drying methods. The concentration of Ala was less (P < 0.05) in lyophilized samples compared with oven dried samples. However, for all other AA (88% DM basis), the concentration of AA was greater (P ≤ 0.05) in lyophilized samples than in oven dried samples.
Table 3.
Effect of diet and drying method on the concentration of DM, Cr, CP, and AA in ileal digesta from pigs fed diets containing three sources of soybean meal (SBM) or an N-free diet, Exp. 11
| Diet | Drying method | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Item | SBM-1 | SBM-2 | SBM-3 | N-free | SEM | P-value | Freeze dried | Oven dried | SEM | P-value |
| DM, % | 87.78b | 87.06b | 87.70b | 90.58a | 0.63 | <0.001 | 90.12 | 86.44 | 0.53 | <0.001 |
| Cr, % | 1.13a | 1.04a | 1.04a | 1.82b | 0.08 | <0.001 | 1.21 | 1.30 | 0.06 | 0.234 |
| CP, % | 21.64b | 25.01a | 19.50bc | 18.66c | 1.35 | <0.001 | 20.80 | 21.60 | 1.27 | 0.244 |
| Indispensable AA, % | ||||||||||
| Arg | 0.42b | 0.55a | 0.41b | 0.45ab | 0.05 | 0.010 | 0.64 | 0.28 | 0.05 | <0.001 |
| His | 0.29b | 0.37a | 0.24bc | 0.20c | 0.02 | <0.001 | 0.32 | 0.22 | 0.01 | <0.001 |
| Ile | 0.66ab | 0.70a | 0.59b | 0.41c | 0.03 | <0.001 | 0.61 | 0.57 | 0.03 | 0.031 |
| Leu | 1.04ab | 1.12a | 0.96b | 0.64c | 0.06 | <0.001 | 1.01 | 0.88 | 0.05 | 0.002 |
| Lys | 1.02b | 1.43a | 0.72c | 0.39d | 0.08 | <0.001 | 1.07 | 0.71 | 0.07 | <0.001 |
| Met | 0.15a | 0.15a | 0.13a | 0.10b | 0.01 | <0.001 | 0.15 | 0.11 | 0.01 | <0.001 |
| Phe | 0.62ab | 0.69a | 0.60b | 0.40c | 0.03 | <0.001 | 0.63 | 0.52 | 0.03 | <0.001 |
| Thr | 0.82 | 0.80 | 0.77 | 0.82 | 0.06 | 0.751 | 0.84 | 0.76 | 0.05 | 0.050 |
| Trp | 0.13 | 0.13 | 0.13 | 0.12 | 0.01 | 0.935 | 0.18 | 0.08 | 0.01 | <0.001 |
| Val | 0.80a | 0.85a | 0.73a | 0.59b | 0.05 | <0.001 | 0.75 | 0.73 | 0.04 | 0.635 |
| Total | 5.94b | 6.80a | 5.27b | 4.13c | 0.35 | <0.001 | 6.19 | 4.87 | 0.32 | <0.001 |
| Dispensable AA, % | ||||||||||
| Ala | 1.04a | 1.01a | 0.88b | 0.67c | 0.06 | <0.001 | 0.82 | 0.98 | 0.06 | <0.001 |
| Asp | 1.76a | 1.93a | 1.46b | 0.87c | 0.10 | <0.001 | 1.74 | 1.27 | 0.09 | <0.001 |
| Cys | 0.37ab | 0.39a | 0.33ab | 0.29b | 0.03 | 0.018 | 0.40 | 0.29 | 0.02 | <0.001 |
| Glu | 2.40a | 2.89a | 1.71b | 0.94c | 0.19 | <0.001 | 2.37 | 1.60 | 0.16 | <0.001 |
| Gly | 1.44a | 1.44a | 1.17b | 1.65a | 0.16 | <0.001 | 1.50 | 1.36 | 0.15 | 0.054 |
| Ser | 0.64 | 0.63 | 0.58 | 0.59 | 0.05 | 0.623 | 0.69 | 0.53 | 0.04 | <0.001 |
| Tyr | 0.35a | 0.35a | 0.38a | 0.26b | 0.02 | <0.001 | 0.42 | 0.25 | 0.02 | <0.001 |
| Total | 7.99a | 8.65a | 6.51b | 5.27c | 0.51 | <0.001 | 7.93 | 6.28 | 0.46 | <0.001 |
| All AA | 13.93a | 15.44a | 11.77b | 9.40c | 0.84 | <0.001 | 14.12 | 11.15 | 0.77 | <0.001 |
a–cMeans within a row lacking a common superscript letter are different (P ≤ 0.05).
1Values for Cr, CP, and AA were adjusted to 88% (DM-basis).
The SID of CP and most AA was different (P ≤ 0.05) among the three experimental diets containing SBM (Table 4). The SID of CP and Ala was not different between drying methods, but the SID of CP and Ala was not different between drying methods. The SID of all other AA was greater (P < 0.05) in oven dried samples than in lyophilized samples.
Table 4.
Effect of diet and drying method on the standardized ileal digestibility (%) of CP and AA in three sources of soybean meal (SBM) fed to growing pigs, Exp. 11
| Diet | Drying method | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Item | SBM-1 | SBM-2 | SBM-3 | SEM | P-value | Freeze dried | Oven dried | SEM | P-value |
| CP, % | 89.5a | 82.7b | 87.9a | 1.88 | <0.001 | 85.7 | 87.7 | 1.80 | 0.063 |
| Indispensable AA, % | |||||||||
| Arg | 97.7a | 94.5b | 97.2a | 1.03 | 0.007 | 93.7 | 99.2 | 0.94 | <0.001 |
| His | 93.8a | 88.1b | 94.1a | 0.87 | <0.001 | 90.0 | 94.0 | 0.81 | <0.001 |
| Ile | 91.3a | 88.7b | 90.8a | 0.86 | 0.002 | 89.4 | 91.1 | 0.81 | 0.007 |
| Leu | 91.3a | 88.9b | 90.9a | 0.89 | 0.004 | 89.1 | 91.6 | 0.85 | <0.001 |
| Lys | 87.3b | 74.6c | 91.1a | 1.96 | <0.001 | 80.6 | 88.1 | 1.86 | <0.001 |
| Met | 93.4ab | 92.3b | 93.8a | 0.78 | 0.047 | 91.8 | 94.7 | 0.74 | <0.001 |
| Phe | 92.1a | 89.7b | 91.3a | 0.80 | 0.002 | 89.3 | 92.7 | 0.76 | <0.001 |
| Thr | 89.3 | 88.4 | 88.6 | 1.34 | 0.754 | 86.9 | 90.7 | 1.24 | 0.001 |
| Trp | 96.0 | 95.2 | 95.1 | 0.82 | 0.517 | 92.2 | 98.7 | 0.75 | <0.001 |
| Val | 90.5a | 87.1b | 89.9a | 1.07 | 0.001 | 88.1 | 90.2 | 1.01 | 0.004 |
| Mean | 91.8a | 87.6b | 92.0a | 1.02 | <0.001 | 88.5 | 92.5 | 0.97 | <0.001 |
| Dispensable AA, % | |||||||||
| Ala | 84.9ab | 82.4b | 86.0a | 1.85 | 0.042 | 84.6 | 84.3 | 1.76 | 0.811 |
| Asp | 88.6a | 84.9b | 90.1a | 1.22 | <0.001 | 84.4 | 91.3 | 1.12 | <0.001 |
| Cys | 82.7ab | 78.2b | 83.4a | 1.68 | 0.051 | 77.8 | 85.0 | 1.42 | <0.001 |
| Glu | 89.4a | 85.5b | 92.3a | 1.56 | 0.001 | 85.4 | 92.8 | 1.41 | <0.001 |
| Gly | 89.5 | 85.9 | 90.6 | 4.20 | 0.191 | 85.5 | 91.9 | 4.06 | 0.005 |
| Ser | 91.3 | 91.5 | 92.0 | 1.14 | 0.786 | 89.4 | 93.8 | 1.06 | <0.001 |
| Tyr | 92.3 | 92.0 | 91.6 | 0.82 | 0.768 | 88.5 | 95.4 | 0.73 | <0.001 |
| Mean | 88.9ab | 86.3b | 90.7a | 1.54 | 0.008 | 85.6 | 91.7 | 1.44 | <0.001 |
| All AA | 90.4a | 86.4b | 91.4a | 1.27 | <0.001 | 86.8 | 91.9 | 1.20 | <0.001 |
a–eMeans within a row lacking a common superscript letter are different (P ≤ 0.05).
1Values for SID were calculated by correcting values for AID for basal ileal endogenous losses. Basal ileal endogenous losses were determined (g/kg of DMI) as CP, 21.85; Arg, 0.72; His, 0.29; Ile, 0.53; Leu, 0.83; Lys, 0.61; Met, 0.13; Phe, 0.52; Thr, 0.96; Trp, 0.19; Val, 0.68; Ala, 0.71; Asp, 1.16; Cys, 0.38; Glu, 1.28; Gly, 2.15; Ser, 0.75; and Tyr, 0.37, for lyophilized ileal digesta samples; and as CP, 22.65; Arg, 0.49; His, 0.21; Ile, 0.46; Leu, 0.73; Lys, 0.44; Met, 0.10; Phe, 0.46; Thr, 0.86; Trp, 0.12; Val, 0.69; Ala, 0.86; Asp, 1.11; Cys, 0.24; Glu, 1.32; Gly, 2.09; Ser, 0.61; and Tyr, 0.30 for oven dried ileal digesta samples.
Experiment 2: AA Digestibility in Distillers Dried Grains with Solubles
The concentration of DM and most AA (88% DM basis) in ileal digesta samples was greater (P ≤ 0.05) in the samples from the two sources of DDGS than in samples from pigs fed the N-free diet (Table 5). There was no difference in the concentration of CP among diets, but the concentration of Cr was greater (P < 0.05) in the ileal digesta from pigs fed the N-free diet than from pigs fed the DDGS diets. The concentration of DM and all AA (88% DM basis) was greater (P < 0.05) in lyophilized samples compared with oven dried samples, and there was a tendency (P = 0.092) for the concentration of CP to be greater in the lyophilized samples compared with oven dried samples. However, the concentration of Cr (88% DM basis) was greater (P < 0.05) in oven dried samples than in lyophilized samples.
Table 5.
Effect of diet and drying method on the concentration of DM, Cr, CP, and AA in ileal digesta from pigs fed diets containing two sources of distillers dried grains with solubles (DDGS) or an N-free diet, Exp. 21
| Diet | Drying method | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Item | DDGS-1 | DDGS-2 | N-free | SEM | P-value | Freeze dried | Oven dried | SEM | P-value |
| DM, % | 93.54a | 93.12a | 89.90b | 0.96 | 0.039 | 94.26 | 90.12 | 0.61 | <0.001 |
| Cr, % | 1.07b | 0.83b | 2.43a | 0.11 | <0.001 | 1.36 | 1.52 | 0.07 | 0.018 |
| CP, % | 18.38 | 16.99 | 14.28 | 2.32 | 0.472 | 17.26 | 15.84 | 1.40 | 0.092 |
| Indispensable AA, % | |||||||||
| Arg | 0.46 | 0.38 | 0.60 | 0.08 | 0.201 | 0.72 | 0.24 | 0.05 | <0.001 |
| His | 0.29a | 0.29a | 0.14b | 0.02 | <0.001 | 0.33 | 0.14 | 0.01 | <0.001 |
| Ile | 0.57a | 0.61a | 0.29b | 0.04 | <0.001 | 0.55 | 0.43 | 0.03 | <0.001 |
| Leu | 0.92a | 1.16a | 0.49b | 0.09 | <0.001 | 0.98 | 0.73 | 0.05 | <0.001 |
| Lys | 0.64a | 0.65a | 0.32b | 0.05 | 0.001 | 0.67 | 0.41 | 0.04 | <0.001 |
| Met | 0.16a | 0.19a | 0.06b | 0.01 | <0.001 | 0.16 | 0.12 | 0.01 | <0.001 |
| Phe | 0.53a | 0.57a | 0.28b | 0.04 | 0.001 | 0.57 | 0.35 | 0.03 | <0.001 |
| Thr | 0.72a | 0.68a | 0.40b | 0.04 | <0.001 | 0.71 | 0.49 | 0.03 | <0.001 |
| Trp | 0.15a | 0.13a | 0.09b | 0.01 | <0.001 | 0.13 | 0.11 | 0.01 | 0.012 |
| Val | 0.80a | 0.86a | 0.41b | 0.06 | <0.001 | 0.74 | 0.64 | 0.04 | 0.005 |
| Total | 5.23a | 5.51a | 3.10b | 0.39 | 0.001 | 5.56 | 3.66 | 0.25 | <0.001 |
| Dispensable AA, % | |||||||||
| Ala | 0.92ab | 0.99a | 0.63b | 0.10 | 0.046 | 0.92 | 0.77 | 0.06 | 0.010 |
| Asp | 1.30a | 1.14a | 0.62b | 0.08 | <0.001 | 1.27 | 0.77 | 0.06 | <0.001 |
| Cys | 0.32a | 0.32a | 0.14b | 0.02 | <0.001 | 0.32 | 0.20 | 0.01 | <0.001 |
| Glu | 1.50a | 1.68a | 0.86b | 0.15 | 0.006 | 1.56 | 1.14 | 0.10 | <0.001 |
| Gly | 1.35 | 1.11 | 1.75 | 0.21 | 0.140 | 1.64 | 1.17 | 0.14 | 0.001 |
| Ser | 0.56a | 0.50a | 0.36b | 0.03 | 0.002 | 0.59 | 0.35 | 0.02 | <0.001 |
| Tyr | 0.30a | 0.32a | 0.19b | 0.03 | 0.028 | 0.38 | 0.17 | 0.02 | <0.001 |
| Total | 6.26 | 6.0 | 4.56a | 0.56 | 0.108 | 6.68 | 4.57 | 0.36 | <0.001 |
| All AA | 11.48a | 11.58a | 7.66b | 0.94 | 0.017 | 12.24 | 8.24 | 0.60 | <0.001 |
a–eMeans within a row lacking a common superscript letter are different (P ≤ 0.05).
1Values for Cr, CP, and AA were adjusted to 88% DM.
With the exception of Leu and Met, no differences in the SID of CP and AA between the two sources of DDGS were observed (Table 6). The SID of CP was not different between drying methods, but the SID of all AA except Trp and Gly was greater (P < 0.05) in samples that were oven dried compared with samples that were lyophilized.
Table 6.
Effect of diet and drying method on the standardized ileal digestibility of CP and AA in two sources of distillers dried grains with solubles (DDGS) fed to growing pigs1
| Diet | Drying method | |||||||
|---|---|---|---|---|---|---|---|---|
| Item | DDGS-1 | DDGS-2 | SEM | P-value | Freeze dried | Oven dried | SEM | P-value |
| CP, % | 81.5 | 79.8 | 2.83 | 0.685 | 80.3 | 81.0 | 2.08 | 0.593 |
| Indispensable AA, % | ||||||||
| Arg | 90.9 | 92.3 | 1.93 | 0.621 | 88.4 | 94.8 | 1.50 | <0.001 |
| His | 86.8 | 85.7 | 1.14 | 0.506 | 80.9 | 91.5 | 0.87 | <0.001 |
| Ile | 84.5 | 80.9 | 1.66 | 0.163 | 80.2 | 85.2 | 1.28 | 0.001 |
| Leu | 90.8 | 86.8 | 1.16 | 0.033 | 86.9 | 90.6 | 0.88 | <0.001 |
| Lys | 80.7 | 76.4 | 2.14 | 0.185 | 72.9 | 84.3 | 1.69 | <0.001 |
| Met | 89.4 | 85.3 | 1.26 | 0.045 | 85.3 | 89.5 | 0.95 | <0.001 |
| Phe | 88.3 | 86.0 | 1.32 | 0.230 | 83.6 | 90.7 | 1.04 | <0.001 |
| Thr | 78.3 | 77.8 | 1.65 | 0.834 | 74.3 | 81.8 | 1.34 | <0.001 |
| Trp | 80.8 | 80.7 | 1.29 | 0.975 | 79.8 | 81.6 | 1.18 | 0.258 |
| Val | 82.9 | 79.4 | 1.61 | 0.158 | 79.3 | 83.0 | 1.24 | 0.004 |
| Mean | 86.5 | 83.9 | 1.34 | 0.206 | 82.2 | 88.1 | 1.01 | <0.001 |
| Dispensable AA, % | ||||||||
| Ala | 85.6 | 82.6 | 1.93 | 0.290 | 83.1 | 85.1 | 1.44 | 0.059 |
| Asp | 77.6 | 78.0 | 1.99 | 0.901 | 72.3 | 83.2 | 1.62 | <0.001 |
| Cys | 79.7 | 78.6 | 1.24 | 0.568 | 74.1 | 84.2 | 1.04 | <0.001 |
| Glu | 89.2 | 85.5 | 1.59 | 0.127 | 85.3 | 89.5 | 1.22 | 0.001 |
| Gly | 82.0 | 77.7 | 5.26 | 0.580 | 78.8 | 80.8 | 3.83 | 0.310 |
| Ser | 84.5 | 85.0 | 1.21 | 0.809 | 81.4 | 88.2 | 0.94 | <0.001 |
| Tyr | 90.5 | 88.4 | 1.45 | 0.341 | 85.2 | 93.7 | 1.14 | <0.001 |
| Mean | 84.5 | 82.9 | 2.05 | 0.605 | 81.0 | 86.5 | 1.53 | <0.001 |
| All AA | 85.5 | 83.4 | 1.66 | 0.402 | 81.6 | 87.3 | 1.24 | <0.001 |
1Values for SID were calculated by correcting values for AID for basal ileal endogenous losses. Basal ileal endogenous losses were determined (g/kg of DMI) as CP, 16.69; Arg, 0.76; His, 0.18; Ile, 0.29; Leu, 0.49; Lys, 0.34; Met, 0.06; Phe, 0.30; Thr, 0.43; Trp, 0.09; Val, 0.39; Ala, 0.61; Asp, 0.70; Cys, 0.16; Glu, 0.88; Gly, 1.81; Ser, 0.42; and Tyr, 0.23, for lyophilized ileal digesta samples; and as CP, 10.81; Arg, 0.11; His, 0.05; Ile, 0.18; Leu, 0.28; Lys, 0.17; Met, 0.04; Phe, 0.17; Thr, 0.22; Trp, 0.05; Val, 0.25; Ala, 0.32; Asp, 0.31; Cys, 0.08; Glu, 0.48; Gly, 0.87; Ser, 0.16; and Tyr, 0.08, for oven dried ileal digesta samples.
Effect of Drying Method on Calculated Values for Endogenous Losses of CP and AA
In Exp. 1, there was no difference in the concentration of endogenous losses of CP and AA between the two drying methods with the exception that the endogenous loss of Arg was greater (P < 0.05) if calculated based on lyophilized samples than from oven dried samples (Table 7). In Exp. 2, there was no difference in the concentration of endogenous losses of CP between the two drying methods, but for most AA, greater (P < 0.05) endogenous losses were calculated from lyophilized ileal digesta samples than from oven dried samples.
Table 7.
Effect of drying method on basal ileal endogenous losses (g/kg DMI) of CP and AA calculated from pigs fed N-free diets
| Exp. 1 | Exp. 2 | |||||||
|---|---|---|---|---|---|---|---|---|
| Item | Freeze dried | Oven dried | SEM | P-value | Freeze dried | Oven dried | SEM | P-value |
| CP | 21.85 | 22.36 | 2.64 | 0.833 | 16.69 | 10.98 | 2.26 | 0.149 |
| Indispensable AA | ||||||||
| Arg | 0.72 | 0.46 | 0.10 | 0.037 | 0.76 | 0.12 | 0.12 | 0.006 |
| His | 0.29 | 0.21 | 0.04 | 0.171 | 0.18 | 0.05 | 0.02 | 0.003 |
| Ile | 0.54 | 0.48 | 0.08 | 0.494 | 0.29 | 0.19 | 0.04 | 0.111 |
| Leu | 0.83 | 0.75 | 0.13 | 0.582 | 0.49 | 0.29 | 0.08 | 0.087 |
| Lys | 0.61 | 0.45 | 0.10 | 0.236 | 0.34 | 0.20 | 0.05 | 0.085 |
| Met | 0.13 | 0.11 | 0.02 | 0.348 | 0.06 | 0.04 | 0.01 | 0.031 |
| Phe | 0.52 | 0.48 | 0.09 | 0.674 | 0.30 | 0.17 | 0.04 | 0.060 |
| Thr | 0.96 | 0.95 | 0.17 | 0.947 | 0.43 | 0.22 | 0.04 | 0.004 |
| Trp | 0.19 | 0.13 | 0.04 | 0.211 | 0.09 | 0.05 | 0.01 | 0.012 |
| Val | 0.68 | 0.70 | 0.12 | 0.889 | 0.39 | 0.26 | 0.06 | 0.167 |
| Total | 5.46 | 4.74 | 0.83 | 0.419 | 3.35 | 1.51 | 0.40 | 0.013 |
| Dispensable AA | ||||||||
| Ala | 0.71 | 0.86 | 0.12 | 0.333 | 0.61 | 0.33 | 0.10 | 0.130 |
| Asp | 1.16 | 1.14 | 0.19 | 0.934 | 0.70 | 0.31 | 0.08 | 0.024 |
| Cys | 0.38 | 0.31 | 0.06 | 0.145 | 0.16 | 0.08 | 0.02 | 0.009 |
| Glu | 1.28 | 1.33 | 0.21 | 0.853 | 0.88 | 0.51 | 0.14 | 0.079 |
| Gly | 2.15 | 2.03 | 0.46 | 0.811 | 1.81 | 0.87 | 0.19 | 0.009 |
| Ser | 0.75 | 0.66 | 0.11 | 0.442 | 0.42 | 0.16 | 0.05 | 0.006 |
| Tyr | 0.37 | 0.31 | 0.06 | 0.311 | 0.23 | 0.08 | 0.03 | 0.020 |
| Total | 6.80 | 6.62 | 1.09 | 0.883 | 4.81 | 2.31 | 0.55 | 0.012 |
| All AA | 12.26 | 11.34 | 1.89 | 0.665 | 8.15 | 3.82 | 0.94 | 0.012 |
DISCUSSION
Soybean meal and DDGS are the two ingredients used in the United States that provides most AA to diets for pigs. To test the hypothesis that drying method influences AID and SID of AA, we therefore focused on these two ingredients. We included lactose in the diets for the weanling pigs and not in the diets for the finishing pigs and thereby provided differences in the carbohydrate sources in the diets.
Freeze drying has been used as the preferred method to dry ileal digesta from pigs for at least three decades (Moughan et al., 1987) because data from poultry demonstrated that oven drying of excreta results in loss of N (Manoukas et al., 1964; Shannon and Brown, 1969). The observation that the concentration of DM is greater in lyophilized ileal digesta samples than in oven dried samples indicates that the freeze dryer under the conditions of these experiments removed more water than the drying oven. However, AA concentration of ileal digesta as presented in this manuscript is not influenced by differences in DM because all values were adjusted to 88% DM.
The concentration of N in feces and the ATTD of CP is not affected by drying method (Jørgensen et al., 1984; Jacobs et al., 2011), which raises the question if freeze drying is needed for ileal digesta from pigs. To our knowledge, there are no data demonstrating that it is necessary to freeze dry ileal digesta samples from pigs and, if there is no loss of AA or N from ileal digesta during oven drying, it was speculated that it may be possible to use oven drying instead of freeze drying.
The observation that there is no difference in the concentration of CP or the SID of CP between lyophilized and oven dried ileal digesta samples concurs with the data for ATTD of N in feces and indicates that there is no loss of N in ileal digesta from pigs as a result of the drying method. However, the present data clearly indicated that the concentration of AA decreased if ileal digesta samples were oven dried, indicating that oven drying may damage AA in the ileal digesta or convert AA to non-AA compounds. This may be a consequence of the greater temperature used during oven drying compared with freeze drying, which may result in Maillard reactions, which bind the free NH2 group of proteins or AA to the carbonyl group of a reducing sugar (Pahm et al., 2008). In this work, the source of reducing sugars may be the lactose supplied in the diets in Exp. 1, and the resistant starch from DDGS, which may be up to 18% (Li et al., 2014), in the DDGS diets in Exp. 2. The AA that is most susceptible to Maillard reaction is Lys because of the exposed NH2 group (González-Vega et al., 2011). Therefore, the reduction in Lys concentration in oven dried ileal digesta samples observed in these experiments supports the hypothesis that Maillard reaction took place. However, because oven drying resulted in reduction in the concentration of almost all AA, it is likely that other forms of AA damage also occurred.
The observation that oven drying reduced the concentration of most AA in ileal digesta from pigs fed diets containing SBM, DDGS, or an N-free diet indicate that the effect of drying method on the composition of ileal digesta is not related to a specific type of diet. Values for the ileal digestibility of AA are calculated by subtracting the AA concentration in ileal digesta from the amount ingested by the pig (Stein et al., 2007). Therefore, the reduced concentration of AA in oven dried samples resulted in greater calculated values for SID of AA in samples that were oven dried compared with samples that were lyophilized, which has the consequence that oven drying will result in an overestimation of the SID of AA.
The increase in the concentration of Cr in oven dried ileal digesta samples in Exp. 2 compared with lyophilized samples indicates that volatile compounds disappeared from the samples during oven drying. It is, therefore, possible that the digestibility of energy or other nutrients also are affected by drying method, but additional research is needed to address this hypothesis.
The observation that in Exp. 2 there was reduced endogenous losses of AA from oven dried ileal digesta samples compared with lyophilized samples further indicates that AA disappeared or were changed during oven drying. As a result, the increased SID values that were calculated for oven dried samples in both experiments is a result of the combined effects of fewer AA in the ileal digesta from pigs fed the protein-containing ingredients and reduced endogenous losses calculated for oven dried samples.
CONCLUSION
Oven drying of ileal digesta samples from pigs does not result in losses of N, but reduces the concentration of AA, which results in reduced estimates for basal endogenous losses and greater calculated values for the SID of AA. This effect is not diet specific, and therefore, for AA digestibility experiments, ileal digesta samples from pigs need to be lyophilized prior to analysis rather than oven dried.
Conflict of interest statement. None declared.
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