Soybean meal has traditionally been considered an excellent protein source for pigs because of its well-balanced amino acid profile and its competitive cost compared to other protein sources. Inclusion levels of soybean meal in swine diets has decreased over the years because of the availability of crystalline amino acids, increased use of distillers’ dried grains with solubles (DDGS), and the price reduction of tryptophan; the latter causing dietary soybean meal content to dramatically decrease. For example, the average diets in the year 2000 (no DDGS and 3 lbs./ton of lysine), 2008 (12 lbs./ton of lysine because tryptophan became affordable), 2010 (DDGS included at 20%), 2017 (aggressive DDGS use at 35%) would contain about 464, 346, 172, and 52 lbs. per ton of soybean meal, respectively. Thus, the inclusion of soybean meal in pig diets has become largely dependent on pricing and does not account for any potential value of soybean meal beyond its contribution as a source of amino acids. However, soybean meal contains a significant amount of health promoting compounds, such as isoflavones, saponins, and phenolic antioxidants that have been demonstrated to modulate immune responses and improve growth performance of pigs exposed to viral disease challenges (see review by Smith and Dilger, 2018. J. Anim. Sci. 96:1288-1304). With the inclusion of DDGS and high levels of crystalline amino acids, the levels of dietary soybean meal have been driven to very low levels, especially in late finishing diets, largely eliminating the growth and potential health promoting benefits that soybean meal may provide.
With crystalline tryptophan being reliably cost effective, and with the availability of valine and even isoleucine at improved affordability, there is much greater ability to drive diet soybean meal content even lower. In a study that was conducted more than 20 years ago, it was shown that as soybean meal content declined from 375 lbs./ton to near zero (in 110-pound pigs), pigs became fatter and less efficient despite having restored the original dietary balance of all indispensable and dispensable amino acids. Potassium level and dietary electrolyte balance was also maintained equal or above a safe minimum (Johnson, Usry and Boyd, 1999. J. Anim. Sci. 77 (Suppl. 1): 69). We now believe that this is not a crude protein limitation, but rather is related to growth and health promoting components in soybean meal that is not common to alternatives. Further, there may be a soybean meal level below which growth and feed conversion are compromised; this could differ by growth phase and with high or low health status (respiratory).
We conducted a study, funded by the soy checkoff, to determine the impact of displacing soybean meal with DDGS and high levels of crystalline amino acids on production performance of growing pigs. A total of 512 pigs weighing an average of 84.9 ± 0.3 lbs. were blocked by BW and sex and placed in 64 pens (4 gilts and 4 barrows per pen). Pens were assigned to 8 dietary treatments (8 blocks per treatment) arranged as a 2 × 4 factorial. Factors included DDGS supplementation (either at 0 or 25%) and crystalline amino acid supplementation (L-lysine-HCl added at 0, 4, 8, or 12 lbs per ton, with DL-methionine, L-threonine, L-tryptophan, L-valine, and L-isoleucine supplemented to maintain appropriate standardized ileal digestible (SID) amino acid ratios). Dietary treatments were fed in two phases, contained 1.00% (Phase 1, 21 days) and 0.90% (Phase 2, 18 days) SID lysine, and were balanced for ideal protein and net energy. Ratios of SID methionine plus cystine, threonine, tryptophan, valine, and isoleucine relative to SID lysine were set at a minimum of 0.58, 0.63, 0.19, 0.67, and 0.56 for Phase 1 diets and 0.59, 0.64, 0.19, 0.67, and 0.56 for Phase 2 diets. Dietary SBM inclusion declined with the inclusion of crystalline amino acids from 32.06 to 13.14% (Phase 1) and 28.25 to 9.40% (Phase 2) for control diets without DDGS. Inclusion of SBM declined from 27.85 to 8.89% (Phase 1) and 24.05 to 5.10% (Phase 2) with increasing crystalline amino acid inclusion for DDGS diets.
Replacing a portion of soybean meal with DDGS (P = 0.024 and 0.032, respectively) and crystalline amino acids (P < 0.001) reduced average daily gain and final body weight (Figure 1). Pigs fed the DDGS diet had a 2.46 lb. lower body weight at the end of the study, whereas replacing soybean meal with 12 lbs./ton of lysine·HCl (plus other crystalline amino acids) reduced final body weight by 5.71 lbs. The response to supplemental crystalline amino acids was dependent on DDGS inclusion; supplementation of crystalline amino acids decreased growth rate and final body weight linearly when DDGS were not included in the diet, whereas the response was quadratic in DDGS supplemented diets. Similarly, feed intake was linearly decreased with crystalline amino acid supplementation within control diets, but responded in a quadratic manner when supplemented to DDGS containing diets (interaction, P = 0.004; Figure 2). Feed:gain was 3.2% higher for pigs fed diets with DDGS compared to control-fed pigs, consuming 0.08 lbs. more feed per lb. of gain. Feeding diets with supplemental crystalline amino acids (12 lbs./ton of lysine·HCl) resulted in a 4.2% higher feed:gain or 0.10 lbs. more feed per pound of gain.
Displacement of soybean meal with DDGS reduced growth rate and increased feed:gain, whereas supplementation with crystalline amino acids negatively affected growth and feed efficiency for soybean meal control diets, but did not cause further reductions in DDGS containing diets. Diets were carefully formulated to meet requirements for standardized ileal digestible lysine, methionine plus cystine, threonine, tryptophan, valine and isoleucine, yet growth performance was compromised with high inclusion of crystalline amino acids.
Thus, increasing the amount of soybean meal in the diet was positively related to improved total weight gain and feed efficiency (Figure 3). The response of total weight gain to soybean meal inclusion was more evident within the control diets (blue circles) compared to the DDGS containing diets (red circles). On the other hand, feed efficiency linearly improved with increasing inclusion of soybean meal regardless of DDGS inclusion. It should be noted that pigs in the present study were extremely healthy and that the effects of soybean meal are more pronounced under respiratory health challenges. These impacts would likely be of greater importance especially during the summer when average daily gain is at a premium to achieve market weights.