Synthetic amino acids (AA) are often included in diets fed to pigs at the expense of SBM or other protein sources to provide a portion of the indispensable AA needed by pigs. Substituting SBM for synthetic AA may support pig growth performance, but daily N retention by pigs may be reduced if diets containing synthetic AA instead of some of the SBM are used. This indicates that there may either be an AA deficiency in diets with synthetic AA, or there are factors other than AA in SBM that are needed to maximize protein synthesis in pigs.
It has been speculated that synthetic AA are absorbed more rapidly than AA from intact protein, which may impair protein synthesis. It is, therefore, possible that if protein from SBM is replaced by synthetic AA, some of the synthetic AA will not be used for protein synthesis, which will result in reduced nitrogen retention by the pigs. Results of recent research also indicate that the DE in SBM is greater than in corn, and therefore, dietary DE may be reduced if SBM is reduced in the diet. However, at this time, no research has been conducted to confirm this assumption. Therefore, the objective of this experiment was to test the hypothesis that use of synthetic AA instead of some of the intact protein from SBM reduces N retention and also reduces DE of the diet if fed to growing pigs.
Experimental design
A control diet was formulated based on corn and SBM without synthetic AA. This diet met all nutrient requirements for growing pigs from 25 to 50 kg (NRC, 2012). Three additional diets were formulated by reducing the inclusion rate of SBM and adding 3 synthetic AA (i.e., Lys, Met, Thr); 4 synthetic AA (i.e., Lys, Met, Thr, Trp); or 5 synthetic AA (i.e., Lys, Met, Thr, Trp, Val) to the diet. Therefore, a total of four diets were used (Tables 1 and 2). Concentrations of standardized ileal digestible indispensable AA were at or above requirements for growing pigs (NRC, 2012) in all diets, but the concentration of crude protein (CP) was reduced from 20.0% to 16.4, 15.4, or 13.4% by including 3, 4, or 5 synthetic AA in the diets.
Animals, housing, feeding and sample collection
Forty growing pigs (average initial body weight: 20.5 ± 2.4 kg) were allotted to a randomized complete block design with four diets and 10 replicate pigs per diet. There were two blocks of 20 pigs, with five pigs per diet in each block. Pigs were housed individually in metabolism crates that were equipped with a self-feeder, a nipple waterer, and a slatted floor. A screen and a urine pan were placed under the slatted floor to allow for the total, but separate, collection of urine and fecal materials. Pigs were limit fed at 3.2 times the ME requirement for maintenance (i.e., 197 kcal/kg × body weight0.60; NRC, 2012). Throughout the experiment, pigs had free access to water. Pigs were fed experimental diets for 12 days. The initial five days were considered the adaptation period to the diet, whereas urine and fecal material were collected from the feed provided during the following four days. Urine was collected in urine buckets over a preservative of 50 mL of hydrochloric acid. Fecal samples and 10% of the collected urine were stored at −20 °C immediately after collection.
At the conclusion of the experiment, urine samples were thawed and mixed within animal and diet, and a sub-sample was lyophilized before analysis. Fecal samples were thawed and mixed within pig and diet, and then dried in a forced-air oven at 65 °C and ground prior to analysis.
Diet, ingredient, and fecal samples were analyzed for dry matter and for N. Crude protein in diet and fecal samples was calculated as analyzed N × 6.25. Thawed urine samples were filtered and analyzed for N. Diet, ingredient, fecal, and lyophilized urine samples were analyzed for gross energy (GE). Other proximate analysis were determined in diets and ingredients.
The apparent total tract digestibility (ATTD) of dry matter (DM), N, and GE, retention of N, and biological value were calculated. Digestible energy and ME for each diet were calculated as well. The statistical model included diet as the fixed effect and block as the random effect. Least square means were calculated, and contrast coefficients were used to determine linear and quadratic effects of reducing dietary protein. Statistical significance and tendencies were considered at P < 0.05 and 0.05 ≤ P < 0.10, respectively.
Results
Pigs remained healthy during the experiment and no feed refusals were observed.
Daily feed intake and daily GE intake were not affected by dietary protein (Table 3). Daily N intake linearly (P < 0.001) decreased with decreasing CP in diets. Daily dry feces output and daily fecal GE output quadratically (P < 0.05) increased as CP decreased in diets, but daily N in feces tended to decrease (quadratic, P = 0.082) as CP decreased in diets. Daily urine output tended (linear, P = 0.096) to decrease as CP was reduced in diets. Daily GE in urine and daily N excretion in urine were reduced (linear, P < 0.05) as CP decreased in diets. The ATTD of DM decreased (quadratic, P = 0.027), and ATTD of GE tended to decrease (quadratic, P = 0.076) as CP was reduced in diets.
Absorbed N, retained N (g/day), and the ATTD of N decreased (linear, P < 0.001) as CP decreased in diets, but retention of N, calculated as percent of intake and percent of absorbed N, increased (linear, P < 0.001) as CP concentration was reduced in diets. The DE was reduced (linear, P = 0.007) as CP decreased in diets, but dietary protein concentration had no effects on ME in diets and ME to GE ratio, but ME to DE increased (linear, P = 0.008) as dietary protein was reduced.
Key points
- Reducing SBM inclusion while supplementing with synthetic AA reduced digestibility of gross energy and also reduced DE, but did not affect ME in diets fed to growing pigs.
- Reducing SBM inclusion while supplementing with synthetic AA in diets reduced digestibility of N and the daily N retention (g/d) whereas the biological value increased as diet CP decreased.
- Amino acids from SBM appear to be better utilized than synthetic AA, and diets supplemented with synthetic AA may result in reduced carcass value due to reduced protein synthesis.
- Nitrogen retention obtained in this experiment was in line with some other recent experiments but much greater than what was observed in older genotypes of pigs.
Table 1. Ingredient composition of experimental diets
1The vitamin-mineral premix provided the following quantities of vitamins and micro-minerals per kilogram of complete diet: Vitamin A as retinyl acetate, 11,150 IU; vitamin D3 as cholecalciferol, 2,210 IU; vitamin E as DL-alpha tocopheryl acetate, 66 IU; vitamin K as menadione nicotinamide bisulfate, 1.42 mg; thiamin as thiamine mononitrate, 1.10 mg; riboflavin, 6.59 mg; pyridoxine as pyridoxine hydrochloride, 1.00 mg; vitamin B12, 0.03 mg; D-pantothenic acid as D-calcium pantothenate, 23.6 mg; niacin, 44.1 mg; folic acid, 1.59 mg; biotin,
0.44 mg; Cu, 20 mg as copper chloride; Fe, 125 mg as iron sulfate; I, 1.26 mg as ethylenediamine dihydriodide; Mn, 60.2 mg as manganese hydroxychloride; Se, 0.30 mg as sodium selenite and selenium yeast; and Zn, 125.1 mg as zinc hydroxychloride.
Table 2. Nutrient composition of ingredients and experimental diets, as-fed basis
Table 3. Apparent total tract digestibility (ATTD) of energy, nitrogen balance, and concentrations of digestible energy (DE) and metabolizable energy (ME) in experimental diets fed to growing pigs, as-fed basis1
1Data are least square means of 10 observations for all treatments.
