Soybean meal (SBM) is often included in cereal-based diets for growing pigs because it provides a well-balanced profile of digestible amino acids (AA), which maximizes growth performance and protein synthesis. However, in recent years, SBM has often been partially replaced by crystalline AA or alternative protein sources such as corn distillers dried grains with solubles (DDGS) or corn protein. These co-products are by-products of the ethanol and vegetable oil industries and are commonly used in swine diets due to their availability and lower cost. Although they provide some indispensable AA, they have a less balanced AA profile and greater fiber concentrations than SBM, which may result in reduced AA digestibility, nitrogen retention, and energy utilization in pigs. However, pigs have become leaner and have greater requirements for dietary AA to support protein synthesis. In addition, it is often assumed that replacing SBM with corn and crystalline AA increases the energy of the diet, but recent data indicate that SBM may provide as much or more digestible energy (DE) and metabolizable energy (ME) than previously estimated. Therefore, it is possible that replacing SBM with corn co-products and crystalline AA may result in a reduction of nitrogen and energy utilization and increase nitrogen excretion in manure. Therefore, the objective of this work was to test the hypothesis that feeding intact protein from SBM to growing pigs, instead of combinations of SBM with crystalline AA or co-products such as DDGS or corn protein, results in greater nitrogen retention and greater DE in the diet without affecting ME.
Experimental design
Six experimental diets that met the requirements for digestible indispensable AA were formulated. The control diet contained corn and 31.2% SBM as only sources of protein and AA. The second diet was a low SBM diet containing corn, 21% SBM, and L-Lys×HCl, DL-Met, and L-Thr. The third (10% SBM) and fourth (5% SBM) diets contained corn, SBM, and DDGS or high-protein DDGS (HP-DDGS). Two additional diets were similar to the previous diets, but supplemented with additional crystalline AA (i.e., 0.05% L-Trp, 0.10% L-Val, and 0.10% L-Ile) to ameliorate the negative effects of excessive Leu in corn co-products.
Animals, housing, feeding, and sample collection
A total of 48 pigs with an initial body weight of 39.91 ± 3.9 kg were allotted to the six dietary treatments using a randomized complete block design. There were two blocks of 24 pigs with four pigs per diet for a total of eight replicate pigs per diet. The blocking factor was the weaning group. Pigs were housed individually in metabolism crates that allow for the separate but total collection of feces and urine. Pigs were fed limited to 3.2 times the ME requirement for maintenance (i.e., 197 kcal/kg × body weight0.60; NRC, 2012). Water was available during the whole experiment.
Fecal samples were collected according to marker-to-maker procedure (Adeola, 2001), and urine samples were collected using the time-to-time procedure for 4 days after 7 days of adaptation. All samples were immediately stored at –20 °C after collection. At the conclusion of the experiment, urine samples were thawed and mixed within the animal and diet. Fecal samples were thawed and mixed within the pig and diet and then dried in a forced-air oven at 50 °C before analysis. All samples were analyzed for dry matter (DM), gross energy (GE), and nitrogen. Crude protein was calculated as analyzed nitrogen × 6.25. Diet and ingredient samples were also analyzed for DM, ash, nitrogen, GE, and AA. The apparent total tract digestibility (ATTD) of DM, GE, and nitrogen, retention of nitrogen, and biological value of nitrogen were calculated using analyzed nitrogen in diets, feces, and urine, and feed intake, dried fecal weight, and urine weight during the collection period. The DE and ME for each diet were calculated as well.
Results
There were no effects of the treatment on feed intake, gross energy intake, or urine energy output (Table 1). However, nitrogen intake of pigs was reduced (P < 0.05) in pigs fed the low SBM diet than pigs fed the high SBM diet, the two DDGS diets, and the HP-DDGS diet containing no additional AA and nitrogen intake was greater (P < 0.05) in pigs fed the high SBM diet compared with the DDGS diet without additional AA and the two HP-DDGS diets. Daily fecal output, fecal GE, and fecal nitrogen excretion were greater (P < 0.05) in pigs fed the two DDGS diets compared with other diets. Pigs fed the two SBM diets tended to excrete more (P < 0.10) nitrogen in urine than pigs fed the other diets. The ATTD of DM, GE, and nitrogen were greater (P < 0.05) in pigs fed the two SBM diets than in those fed DDGS or HP-DDGS diets, regardless of the AA supplementation. Nitrogen absorption and retention (g/day) were also greater (P < 0.05) in pigs fed the SBM diet with no AA. However, absorbed and retained nitrogen (g/day) were reduced from pigs fed the low SBM diet compared with high SBM diet. No differences in nitrogen retention (g/day) were observed between pigs fed the high SBM diet and those fed the two DDGS diets. The biological value was greater (P < 0.05) in pigs fed the DDGS diet with additional AA compared with those fed the high SBM diet, but no differences were observed among the remaining treatments. The DE and ME of the diets were greater (P < 0.05) in diets containing SBM or HP-DDGS compared with the DDGS diet, regardless of the addition of crystalline AA. The ME to DE ratio tended to be greater (P < 0.10) in pigs fed the HP-DDGS diet with AA supplementation compared with those fed the DDGS with no AA supplementation. The ME to GE ratio was greater (P < 0.05) in pigs fed the low SBM diet compared with pigs fed the two DDGS diet and the HP-DDGS diet without AA addition.
Key points
- Retained nitrogen (g/day) of pigs was greater for the high SBM diet.
- Addition of crystalline Trp, Val, and Ile did not improve the nitrogen retention of pigs fed the DDGS and HP-DDGS diets.
- Diets containing SBM and HP-DDGS had greater DE and ME compared with DDGS diets, regardless of AA supplementation.
- The addition of crystalline AA to the low SBM diet reduced nitrogen retention.
Table 1. The ATTD of DM and GE and nitrogen balance in experimental diets fed to growing pigs1,2
a-dWithin a row, means without a common superscript differ (P < 0.05).
1Least square means represent eight observations for all treatments.