Soybean meal (SBM) is the primary source of amino acids (AA) and energy in diets for swine throughout the world. However, less SBM is often used in diets to decrease nitrogen (N) excretion and environmental pollution. Nonetheless, it is demonstrated that lowering dietary protein does not consistently deliver the same performance of pigs as of pigs fed high protein diets, even though all indispensable AA meet the requirements of pigs. It is possible that soybean-derived bioactive compounds (i.e., soy isoflavones and saponins) in SBM act as immunomodulators, which improve the performance of pigs. It is also possible that low-protein diets contain N below the requirements for pigs to maximize growth. However, no data are available to prove this hypothesis. Therefore, the objective of this experiment was to test the hypothesis that N balance of pigs is not affected by reducing dietary protein if soy isoflavones or N source are supplemented in diets.
Experimental design
Six experimental diets based on corn and SBM were formulated. A high-protein diet contained corn and SBM as the sole sources of AA and did not contain any crystalline AA. A medium-protein diet contained less SBM than the high-protein diet and was supplemented with Lys, Met, and Thr. Four low-protein diets containing less SBM and six crystalline AA (i.e., Lys, Met, Ile, Thr, Trp, and Val) were prepared using a 2 Í 2 factorial arrangements with two levels of isoflavones (i.e., 0 vs. 0.4%) and two levels of N-source (i.e., 0 vs. 2% L-glutamic acid). All diets were formulated to meet or exceed the estimated requirements for standardized ileal digestible indispensable AA, vitamins, and minerals for 11 to 25 kg pigs (NRC, 2012).
Sixty growing pigs (initial body weight = 17.75 ± 0.91 kg) were allotted to one of six diets using a randomized complete block design. There were two blocks of 30 pigs, with five pigs per diet in each block for a total of 10 replicate pigs per diet. Pigs were housed individually in metabolism crates, and fecal and urine samples were collected quantitatively for four days after five days of adaptation. Pigs were limit-fed at 3.2 times the metabolizable energy requirement for maintenance (i.e., 197 kcal/kg × body weight0.60; NRC, 2012). Throughout the experiment, pigs had free access to water. At the conclusion of the experiment, urine samples were thawed and mixed within the animal and diet. Fecal samples were thawed and mixed within pig and diet and then dried in a forced-air oven at 50 °C prior to analysis. All samples were analyzed for dry matter (DM), and N, and crude protein was calculated as analyzed N × 6.25. Diet and ingredient samples were also analyzed for DM, ash, N, and AA. The apparent total tract digestibility (ATTD) of N and retention of N (% of intake and absorbed) were calculated using analyzed N in diets, feces, and urine and feed intake, dried fecal weight, and urine weight during the collection period.
Results
Pigs remained healthy during the experiment, and no feed refusal was observed.
Daily N intake, N excretions in feces and urine, absorbed N, and retained N (g/d) were linearly (P < 0.001) reduced, but the ATTD of N was not affected by dietary protein and N retention (% intake and % absorbed) was linearly (P < 0.001) increased by reducing dietary protein (Table 2). Fecal excretion was not influenced by the diet. Nitrogen excretion in feces (g/d) was greater (P < 0.05) in the high-protein diet compared with the low-protein diet without isoflavones and N-source. Nitrogen excretion in urine was greater (P < 0.05) in the high-protein diet compared with the other five diets. Absorbed N (g/d) was greater (P < 0.05) in pigs fed the high-protein diet compared with the other diets. Pigs fed the medium-protein diet and the low-protein diet containing soy isoflavones and N-source had greater (P < 0.05) absorbed N than pigs fed the low-protein diet containing isoflavones. Retained N was greater (P < 0.05) in the high-protein diet compared with the two low-protein diets without N-source, but no differences were observed in retained N among the medium-protein diet and the four low-protein diets. The ATTD of N was not affected by the dietary treatments. Retention of N (% of intake and % of absorbed) was lower (P < 0.05) in the high-protein diet compared with the medium- or low-protein diets, but there were no differences among the other five diets.
Key points
- Lowering dietary protein from 22% to 16% decreased daily retained N and absorbed (g/d) in growing pigs.
- The ATTD of N was not affected by the reduction of dietary protein in the diet.
- Nitrogen excretion in feces and urine can be reduced by reducing dietary protein.
Table 1. Analyzed nutrient composition of experimental diets, as-is basis
Table 2. Nitrogen balance of growing pigs fed the experimental diets, as fed basis1
a-cWithin a row, means without a common superscript differ (P < 0.05)
1Data are the least square means of 10 observations for all treatments.
2Linear effects of reducing dietary protein.
3Although the model was significant for the urine output (kg/d), the P-value of the pairwise multiple comparison was not significant.
