Experimental design

Non-heat-treated soybean expellers (L-1) and soybean expellers conditioned for 60 s at 90 ºC followed by expansion at 110 ºC (L-2) were used (Table 1). Two additional sources of soybean expellers (L-3 and L-4) were processed as L-2 with the exception that the initial conditioning was followed by long-term conditioning for 12 or 48 min at 100 ºC before expansion. The analyzed trypsin inhibitor activity in L-1, L-2, L-3, and L-4 was 34.0, 23.1, 4.2, and 2.4 mg/g, respectively.

Experiment 1: Amino acid digestibility

Ten growing pigs (initial BW: 54.2 ± 4.5 kg) were equipped with a T-cannula in the distal ileum and allotted to a replicated 5 × 4 Youden square design with 5 diets and 4 periods. Two pigs were assigned to each diet within each period; therefore, there were 8 replicate pigs per diet. Each source of soybean expellers was included in 1 diet as the sole source of AA, and an N-free diet was formulated to measure basal endogenous losses of AA; thus, a total of 5 diets were prepared. Each experimental period lasted 7 d. Ileal digesta were collected on d 6 and 7 of each period, and ileal digesta were analyzed for AA to calculate SID of all AA.

Experiment 2: Energy measurements

A corn-based diet, plus 4 diets containing corn and each source of soybean expellers were formulated; thus, a total of 5 diets were prepared. Forty barrows (initial BW: 17.5 ± 1.6 kg) were housed individually in metabolism crates and allotted to the 5 diets with 8 replicate pigs per diet. Feces and urine samples were collected using the marker-to-marker approach with 5-d adaptation and 4-d collection periods. Samples were analyzed for gross energy (GE) to calculate for the apparent total tract digestibility (ATTD) of GE, as well as concentrations of DE and ME in each diet. The DE and ME values of each source of soybean expellers were calculated using the difference procedure.

Results

Experiment 1: Amino acid digestibility

The apparent ileal digestibility (AID) and SID values for crude protein and all AA in L-1 soybean expellers were less than in L-2, L-3, or L-4 soybean expellers (Table 2). The AID and SID of crude protein and all AA in L-2 soybean expellers were also less than in L-3 and L-4 soybean expellers but no differences between L-3 and L-4 were observed. It therefore appeared that only short term conditioning prior to expansion was not sufficient to inactivate all trypsin inhibitors, but if the longer term conditioning was used, trypsin inhibitor concentration was sufficiently reduced. As a consequence, the SID of AA was only maximized after the long term conditioning was applied.

Experiment 2: Energy measurements

The apparent total tract digestibility (ATTD) of GE in L-1 soybean expellers was less than in corn, L-2, L-3, or L-4 soybean expellers (Table 3). The concentrations of DE and ME (as-fed and dry matter basis) in corn and L-1 soybean expellers were less than in L-2, L-3, or L-4 soybean expellers. However, there were no differences among L-2, L-3, and L-4 soybean expellers. It therefore appears that ME of soybean expellers is only maximized if trypsin inhibitors have been reduced by heating, but ME seems to be less sensitive to trypsin inhibitors than SID of AA. The observation that the ME in the heat-treated soybean expellers was greater than in corn is likely a consequence of the greater fat concentration in soybean expellers.

Key points

• The SID of AA in non-heat-treated soybean expellers were less than in heat-treated soybean expellers.
• The SID of AA in L-2 soybean expellers was less than in L-3 or L-4 soybean expellers indicating that short-term conditioning (60 s) at 90 ºC is not enough to inactivate trypsin inhibitors in soybean expellers.
• Long-term conditioning must be applied to sufficiently inactivate trypsin inhibitors and subsequently increase digestibility of AA in soybean expellers.
• There was no difference in digestibility of AA or energy between soybean expellers having 4.2 or 2.4 mg/g trypsin inhibitor activity, indicating that if trypsin inhibitor activity is below 4.2 mg/g, digestibility of AA and energy is maximized.
• Soybean expellers also need some heat treatment to maximize ME.
• The ME in soybean expellers is greater than in corn.

Table 1. Analyzed nutrient composition of 4 sources of soybean expellers

¹TIA = trypsin inhibitor activity; ND = not detected.

Table 2. Apparent ileal digestibility (AID) and standardized ileal digestibility (SID) of crude protein (CP) and amino acids (AA) in 4 sources of soybean expellers^1,2

¹Data are least squares means of 7 to 8 observations per treatment.

Table 3. Apparent total tract digestibility (ATTD) of gross energy (GE) and energy values in corn and in 4 sources of soybean expellers fed to pigs¹

¹Data are least squares means of 7 to 8 observations per treatment.

²DE = digestible energy; ME = metabolizable energy.

^a-cMeans within a row lacking a common letter are different (P < 0.01).