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 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 nitrogen 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 reducing the dietary protein by decreasing the SBM and increasing crystalline AA in diets may have influence growth performance, carcass composition, and meat quality and that soy isoflavones act as immunomodulators, which may improve the immune system and thus results in better performance of pigs fed low-protein 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 glutamate as extra nitrogen source (i.e., 0 vs. 2% _L-glutamic acid). Diets were fed in four phases, with phase 1 for 21 d (nursery phase; 11 to 25kg), phase 2 for 28 d (early grower phase; 25 to 50 kg), phase 3 for 28 d (late grower phase; 50 to 75 kg), and phase 4 for 56 d (finisher phase; 75 to 125 kg). All diets were formulated to meet or exceed the estimated requirements for standardized ileal digestible indispensable AA, vitamins, and minerals for pigs in each phase (NRC, 2012).

A total of 240 pigs with an initial body weight (BW) of 9.65 ± 0.87 kg were allotted to one of six dietary treatments using a randomized complete block design with two blocks of 24 pens. Therefore, a total of 48 pens were used, with four pigs per pen and eight replicate pens per treatment group. The weaning group was the blocking factor. Diets were provided to pigs on an ad libitum basis. Individual pig weights were recorded at the beginning and at the conclusion of the experiment. Feed additions were recorded daily, and the weight of feed left in the feeder was recorded at the end of each phase. Data were summarized to calculate average daily feed intake (ADFI), average daily gain (ADG), and gain:feed ratio (G:F) within each pen and treatment group.

At the conclusion of phase 1, blood samples were collected of one pig per pen for plasma urea nitrogen (PUN), total protein, and cytokines [e.g., interferon-gamma (IFN-γ), interleukin- (IL-) 1α, IL-1β, IL-1 receptor antagonist, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12, IL-18, and tumor necrosis factor-α (TNF-α)]. Ileal-jejunal tissue samples were collected from the same pigs that were bled for analyzing gene abundance of gut-protective proteins (i.e., occludin, claudin-1, zonula occludens-1, thyroid transcription factor-1, and mucin-2). Because one pig per pen was euthanized at the end of phase 1, only four pigs per pen were housed from phases 2 to 4. Consequently, growth performance data from the pigs that were euthanized at the end of phase 1 were excluded from the final analysis using the partitioning method (Lee et al., 2016). At the end of phase 4, a pig in each pen with a BW closest to the pen average was harvested at the Meats Science Laboratory at the University of Illinois. Standard carcass measurements (i.e., hot carcass weight, dressing percentage, backfat thickness, longissimus dorsi depth, longissimus dorsi area, drip loss, loin eye area, and fat and loin color scores; CIE, 1978) were determined after slaughter (NPPC, 1991; 1999).

Results

The final BW and the ADG of pigs were not affected by the treatments in all phases (Table 1). In phase 3, ADFI was linearly (P = 0.039) reduced as dietary protein was reduced, but ADFI of pigs was not different among the dietary treatments in other phases. Gain to feed was also not affected by dietary treatments except that G:F of pigs fed the low-protein diet containing glutamate in phase 1 was greater (P < 0.05) than the high-protein diet, but no difference was observed among other diets.

Dietary treatments did not affect % carcass yield, drip loss, most loin quality traits except for lightness and yellowness, and backfat color (Table 2). Hot carcass weight was greater (P < 0.05) in pigs fed the medium-protein diet compared with the low-protein diets containing soy isoflavones and glutamate, but no differences were observed among the other diets. Back fat thickness tended to increase linearly (P = 0.075) as protein decreased in the diets. Loin eye area was greater (P < 0.05) in pigs fed the high- and medium-protein diets compared with pigs fed the low-protein diet containing soy isoflavones and glutamate. Loin eye area tended to be linearly (P = 0.058) reduced as dietary protein decreased. The loin color was lighter (L*; P < 0.05) when pigs were fed the low-protein diets compared with the medium protein diet, but there were no differences among the other diets. The loin lightness (L*) tended to increase (linear, P = 0.090) as dietary protein was reduced. Loin of pigs fed the four low-protein diets had more yellow color (b*; P < 0.05) compared with the medium-protein diet. Serum concentrations of cytokines, TNF-α, and abundance of gut protective genes were not different among treatments (data not shown).

Key points

• The overall growth performance of pigs was not affected by reducing dietary protein, supplementing isoflavones, or adding extra nitrogen.
• Carcass leanness decreased and backfat thickness increased in pigs fed diets containing lower protein levels compared with pigs fed the high-protein diet.
• The immune responses of pigs were not affected by dietary protein, soy isoflavones, or extra nitrogen.

Appreciation

Funding for this research form the United Soybean Board (St. Louis, MO) is greatly appreciated.

Table 1. Growth performance and carcass characteristics and loin quality of growing pigs fed experimental diets¹

^a-bWithin a row, means without a common superscript differ (P < 0.05).

¹Least square means for dietary treatments represent 8 observations.

²Linear effects of reducing dietary protein.

Table 2. Carcass characteristics and loin quality of pigs fed experimental diets¹

^a-bWithin a row, means without a common superscript differ (P < 0.05)

¹Least square means for dietary treatments represents 8 observations.

²Linear effects of reducing dietary protein.