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Ruminal Degradability and Lysine Bioavailability of Soybean Meals and Effects on Performance of Dairy Cows¹

Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-1600

² Corresponding author: etitgeme{at}ksu.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Evaluations of 4 soybean meal (SBM) products were conducted in 3 experiments. The 4 products were 1) solvent SBM (SSBM), 2) SSBM treated with 0.05% baker’s yeast and toasted at 100°C (YSBM), 3) expeller SBM (ESBM), and 4) lignosulfonate-treated SBM (LSBM). Multiparous Holstein cows (n = 32; 152 ± 63 d in milk; body weight = 708 ± 77 kg; producing 41 ± 7 kg/d of milk at the beginning of the study) were used in a 4 x 4 Latin square design with 28-d periods to investigate cow responsiveness to supplemental ruminally undegradable protein (RUP) from the SBM products. Dietary treatments were formulated by substituting all of the SSBM and part of the ground corn with YSBM, ESBM, or LSBM to yield isonitrogenous diets. Diets were formulated to provide adequate ruminally degradable protein, but deficient RUP and metabolizable protein supplies. No differences among dietary treatments were observed for dry matter intake, body weight gain, milk and component yields, or efficiency of milk production. The lack of response to changes in SBM source was likely due to an adequate RUP and metabolizable protein supply by all the diets. In situ ruminal degradations of YSBM and LSBM were slower than those of SSBM or ESBM; thus, RUP contents of YSBM and LSBM were greater than those of SSBM or ESBM. The RUP of all SBM products had similar small intestinal digestibility. Available Lys contents, estimated chemically or by using a chick growth assay, were less for YSBM and LSBM than for SSBM or ESBM, suggesting deleterious effects of processing on Lys availability in YSBM and LSBM.

Key Words: dairy cow • soybean meal • lysine bioavailability

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Metabolizable protein requirements of dairy cows are met by microbial protein and by dietary protein that escapes the rumen. In early lactation, microbial protein is not able to support milk production in high-producing dairy cows. Thus, supplementing RUP is warranted. Increasing the RUP supply from plant and animal products improves the performance (yield of milk and protein) of dairy cows (NRC, 2001). Whole soybeans and soybean meal (SBM) are the most commonly used supplemental plant proteins in the United States (Lin and Kung, 1999). Soybean products are characterized by high palatability and well-balanced and available essential AA (EAA) contents. Among the 11 protein sources evaluated, the EAA index of soybean bypass protein is the second best after microbial protein (Chandler, 1989), but extensive ruminal degradability (74% for whole soybeans and 71% for SBM; NRC, 2001) limits the utilization of these soybean products by ruminants as sources of RUP. Various methods have been used to treat soybean products to alter their ruminal degradability and consequently increase their escape protein content, but "overprotection" can impair the protein quality of SBM by altering the nutritional availability of AA, particularly Lys (Faldet et al., 1992). The objectives of our study were to compare the nutritive value of 4 SBM products in terms of the performance of dairy cows (experiment 1), ruminal degradation kinetics and intestinal digestibility (experiment 2), and Lys bioavailability (experiment 3).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The use of animals in all experiments was approved by the Kansas State University Institutional Animal Care and Use Committee. In 3 experiments, a comprehensive evaluation of 4 SBM products was conducted. The 4 products were 1) solvent SBM (SSBM), 2) SSBM treated with 0.05% baker’s yeast (Saccharomyces cerevisiae) and steeped for 10 min at 30°C before toasting at 100°C (YSBM), 3) expeller SBM (ESBM), and 4) lignosulfonate-treated SBM (LSBM).

Experiment 1: Production Study
Thirty-two multiparous Holstein cows averaging 152 ± 63 DIM, 708 ± 77 kg of BW, and 41 ± 7 kg/d of milk at the beginning of the study were blocked by DIM, BW, milk production, and lactation number, and were assigned to 1 of 4 free-stall pens in a 4 x 4 Latin square balanced for carryover effects. The lactation number of cows ranged between 2 and 7, averaging 3.1. The experimental units were pens of 8 cows. The 4 dietary treatments (Table 1) were formulated to be isonitrogenous by substituting all SSBM and part of the ground corn with YSBM, ESBM, or LSBM. The control (SSBM) diet was formulated to supply adequate amounts of RDP, but deficient amounts of RUP, and consequently to be deficient in MP supply (NRC, 2001) for the described cows. In addition, diets were formulated to be more limited by Lys supply than by Met supply by adding Met from MetaSmart (Adisseo, Alpharetta, GA) to all diets; this was to investigate the responsiveness of cows to RUP and Lys supplies from the SBM products. Cows were injected throughout the study with recombinant bST (Posilac, Monsanto, St. Louis, MO) at 14-d intervals. Each period was 28 d with a 14-d adaptation and 14 d for data collection. Only data from the last 14 d for each period were used for statistical analyses.

Each SBM product was analyzed for DM (105°C in a forced-air oven for 24 h), NDF, ADF, ether extract (AOAC, 1984), CP, and AA profile by using HPLC with postcolumn derivitization with o-phthalaldehyde after acid hydrolysis (6 N HCl at 105°C for 24 h; Table 2). Before NDF and ADF analyses, SBM samples were extracted with acetone for 12 h. Residuals after NDF and ADF extraction were analyzed for N (Kjeldahl; AOAC, 1984) to determine neutral detergent insoluble nitrogen (NDIN) and ADIN.

Immediately after the morning milking, cows were weighed and scored for body condition (scale of 1 to 5) at the beginning of the study and at the end of each period. At the same times, blood samples were collected from the coccygeal vein into vacuum tubes (Becton Dickinson, Franklin Lakes, NJ) containing sodium heparin, immediately chilled on ice, centrifuged for 20 min at 1,000 x g to obtain plasma, and stored (–20°C) for later analysis of glucose (Gochman and Schmitz, 1972) and urea (Marsh et al., 1965) of individual samples. Plasma samples were composited by pen for each period and stored (–20°C) for later analysis of AA, after deproteinization with sulfosalicylic acid, by using HPLC with postcolumn derivitization with o-phthalaldehyde.

Statistical Analyses.
Data from 1 cow for the last 2 periods were missing because she was removed from the study for reasons not related to treatment. Six weekly milk-production averages for individual cows were not used for reasons unrelated to treatments (mastitis, lameness, and the cow that was removed from the study), leaving 250 weekly averages that were statistically analyzed. To account for differences in the production levels of cows with missing observations, weekly pen means for milk and component yields, SCC, and MUN were generated by using PROC MIXED of SAS (SAS System for Windows, Release 8.1, SAS Inst. Inc., Cary, NC) with a model including terms for pen x period x treatment x week and cow (pen). Daily DMI for each pen was adjusted for the sick cows (mastitis or lameness) in that pen according to level of milk production [adjusted DMI per cow = actual pen DMI x (milk production for healthy cows/total pen milk production)/number of healthy cows]. Data were analyzed statistically by using PROC MIXED of SAS.

For data collected daily (milk production and DMI) and weekly (milk composition), weekly averages were analyzed as a split plot, with Latin square as the main plot and week as the subplot. The model contained the effects of pen, period, treatment, week, and week x treatment. The pen x period x treatment interaction was included as a random variable and served as the main-plot error term. Data collected once per period (BW and BCS changes and plasma metabolites) were analyzed by using PROC MIXED of SAS according to a Latin square design. The model contained the effect of pen, period, and treatment. In experiment 1, as well as for all other experiments, treatment means were computed by using the LSMEANS option, and F-test protected pairwise t-tests among means were used for treatment separation.

Experiment 2: In Situ Protein Degradability and Intestinal Digestibility of Ruminally Undegraded Nitrogen
Two ruminally cannulated Jersey steers (average BW = 518 kg) were fed a typical dairy diet containing (DM basis) 15% alfalfa hay, 21% wet corn gluten feed, 22% corn silage, 6% ESBM, and 36% corn-based concentrate for ad libitum consumption. Daily DMI of steers averaged 9.0 kg. To estimate in situ protein degradability, 1.5 to 2.0 g of ground (2-mm screen, Wiley mill, Arthur H. Thomas Co., Philadelphia, PA) samples of the 4 SBM products were weighed in duplicate for each incubation time (0, 2, 4, 8, 16, 24, and 48 h) into polyester bags (5 x 10 cm, 53 ± 10 µm pore size, Ankom Technology Corp.). The sample weight to surface area ratio was approximately 17.5 mg/cm², which is within the appropriate range (Nocek, 1988). Bags were heat-sealed, presoaked in cold tap water, placed into a weighted mesh bag (36 x 42 cm), and placed into the ventral sac of the rumen at different time points; all bags were then simultaneously removed from the rumen. Upon removal, bags (including 0 h) were washed with cold water in a commercial top-loading washing machine (Kenmore Ultra Fabric Care, Sears, Roebuck and Co., Chicago, IL) with 1-min agitation at the delicate setting and 2-min spin per rinse as described by Coblentz et al. (1997), for 5 cycles, as recommended by Vanzant et al. (1998). After rinsing, bags were dried (55°C in a forced-air oven for 24 h), air-equilibrated, and weighed to determine residual DM. Bags plus residuals were analyzed for Kjeldahl N to determine residual N.

Protein fractions (A, B, and C) and the degradation rate of fraction B (k_B) were estimated for each steer by using PROC NONLIN of SAS with the following model:

where B is the slowly degraded protein fraction, C is the completely undegradable protein fraction, t is incubation time, and k_B is the degradation rate of fraction B. Fraction A was calculated by difference: A = 1 – (B + C).

Digestibility of ruminally undegraded nitrogen (RUN) was determined according to the 3-step procedure described by Calsamiglia and Stern (1995) using the in situ residues after 16 h of ruminal incubation. Ten bags for each SBM product were incubated to obtain enough residue to measure pepsin-pancreatin digestibility. After 16-h incubations, bags were removed and washed with cold water in a washing machine as described above. Intestinal digestibility of RUN was calculated as N in the supernatant as a percentage of added N (16-h bag residuals). Digestible N in the original SBM (before ruminal incubation) was calculated as RUN multiplied by intestinal digestibility of RUN.

Statistical Analyses.
Data for residual N at each time point were analyzed statistically by using PROC MIXED of SAS (SAS System for Windows, Release 8.1, SAS Inst. Inc.) as a randomized-block design. The model contained the effect of steer, time, treatment, and treatment x time. Pool sizes (A, B, and C) and k_B for each SBM were analyzed statistically by using the PROC MIXED of SAS as a randomized-block design with a model containing the effect of treatment and steer. Data for intestinal digestibility of RUN were analyzed statistically by using PROC MIXED of SAS. The model contained the effects of steer and treatment.

Experiment 3: Lys Bioavailability
Extensive heating of SBM may impair Lys availability and consequently result in deleterious effects on overall protein quality. Available Lys contents of SBM products were investigated by using a chick growth assay and by chemical availability.

Broiler chicks (n = 480, 1 d old) were used in a randomized complete-block design to compare the relative bioavailability of Lys in YSBM, ESBM, and LSBM with that of SSBM. Blocks were 4 temperature-regulated starter batteries (Petersime Incubator Co., Gettysburg, OH) with 12 pens in each battery (total of 48 pens). Pens in each battery were randomly assigned to 1 of the 12 treatments (Table 3). Upon arrival from a commercial source, 10 birds were randomly selected, group-weighed, and placed into each of the 48 pens. Birds had free access to feed and water. At the conclusion of the study (9 d), each pen of birds was weighed to calculate ADG, and unconsumed feed was weighed to allow calculation of DMI.

Dry matter (105°C in a forced-air oven for 24 h) of dietary ingredients (SBM, ground corn, and cornstarch) was measured. The SBM and ruminal residuals of each SBM were analyzed for CP (Leco FP 2000 Nitrogen Analyzer) and AA profile (HPLC). Ruminal residuals were obtained by using 4 ruminally cannulated Jersey steers (average BW = 535 kg) consuming a typical dairy diet, described in experiment 2, for ad libitum consumption. Average DMI of steers was 9.2 kg/d. Approximately 200 g of SSBM, YSBM, ESBM, and LSBM was weighed into polyester bags (45 x 30 cm, 53 ± 10 µm pore size, Ankom Technology Corp.), with a ratio of sample weight to surface area of approximately 74 mg/cm². Bags were heat-sealed, individually placed into a weighted mesh bag (36 x 42 cm), and placed in the ventral sac of the rumen in each steer (1 bag of each SBM product, total of 4 bags per steer for each incubation). After 12-h incubations, bags were removed and washed in cold water with a washing machine as described for experiment 2. After rinsing, bag residues were freeze-dried and weighed to determine residual DM, which averaged 48, 62, 50, and 60% for SSBM, YSBM, ESBM, and LSBM, respectively.

In a preliminary study with large (45 x 30 cm) polyester bags, we measured the 12-h ruminal degradation by using polyester bags containing SSBM with sample weight to surface area ratios ranging from 20 to 221 mg/cm². For incubations in our large bags, ratios of sample weight to surface area up to 111 mg/cm² were not significantly different from those with 20 mg/cm² (data not shown).

Chemically available Lys in the original SBM and residues after 12 h of ruminal incubation were measured in duplicate according to the 1-fluoro-2,4-dinitrobenzene (FDNB) procedure described by Carpenter (1960). Available Lys is defined as units whose -amino groups are free and undergo a Sanger reaction with FDNB to form dinitrophenyl-Lys (Carpenter, 1960). Units whose -amino groups are bound to other groups will not react with FDNB and are therefore considered to be nutritionally unavailable. A standard curve was established by using 0, 0.053, 0.105, and 0.210 mg of dinitrophenyl-Lys (mono-6-N-dinitrophenyl-Lys·H-Cl·H₂O; Sigma, St. Louis, MO). A linear standard curve was used to calculate available Lys in the sample, which was corrected for loss during acid hydrolysis. To correct for acid-hydrolysis loss, the recovery of a known amount (4.15 mg) of the standard added to samples before acid hydrolysis was determined. The recovery was measured in 4 samples (duplicates of original and ruminal residuals of YSBM) and averaged 83%.

Statistical Analyses.
Data from the standard chick growth study were analyzed statistically by using PROC MIXED of SAS (SAS System for Windows, Release 8.1, SAS Inst. Inc.) as a randomized complete-block design. The model contained the effects of treatment and block. Linear and quadratic effects of SSBM treatments (20, 23, 26, and 29%) were tested by using single-degree-of-freedom contrasts. Original SBM were compared with ruminal residuals of SBM by using t-tests. Within-group (original and in situ residues) pairwise t-tests among all 4 products were used to separate means.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Experiment 1: Production Study
The nutrient composition of dietary ingredients is presented in Table 4. The CP, NDF, and ADF contents (DM basis) of alfalfa, corn silage, and cottonseed were comparable to those reported by the NRC (2001). Diets contained an average of 16.9% CP (DM basis) and were formulated to be isonitrogenous (Table 1).

The AA profiles (Table 2) of SSBM, ESBM, and LSBM were comparable to those reported by the NRC (2001). The Lys contents, as a percentage of CP, were greater for our SBM products than the NRC (2001) values (7.6 vs. 6.3% for SSBM, 7.6 vs. 6.3% for ESBM, and 6.7 vs. 5.8% for LSBM). Individual AA contents of the 4 SBM products were very similar, except for Lys. Lysine contents in SSBM and ESBM (7.6%) were greater than those in YSBM (6.2%) or LSBM (6.7%), likely due to more cross-linked Lys in YSBM and LSBM that was not measured in our assay.

The effects of dietary treatments on cow performance are presented in Table 5. There were no significant effects of dietary treatments on DMI, BW gain, milk and component yields, and efficiency of milk production. We purposefully formulated the SSBM and ESBM diets to be sufficient in RDP, but deficient in RUP supply, and MP supply was designed to be inadequate to support production of 40.8 kg/d of milk (Table 6), on the basis of evaluation with the NRC (2001) guidelines. For these evaluations, we used our measured values for protein fractions (Table 7). The NRC (2001) predicted, based on milk production at the beginning of the study (40.8 kg/d) and the predicted DMI (25.5 kg/d), that the SSBM and ESBM diets were 181 and 169 g/d deficient in RUP supply and 145 and 134 g/d deficient in MP supply, respectively (Table 6). Thus, increasing the RUP supply by replacing SSBM with YSBM or LSBM could improve cow performance. At the end of the study, diets were reevaluated by using the actual DMI (average of 27.3 kg/d) and milk production (average of 33.6 kg/d), and all diets (including SSBM and ESBM) were found to be adequate in RUP and MP supply to support the actual amount of milk produced (Table 6). This might explain the lack of response to changes in RUP supply.

Olmos Colmenero and Broderick (2006) demonstrated that 16.5% CP in diets based on corn silage and alfalfa silage (50% of DM; 1:1) was adequate to support maximal milk and protein yields in midlactation dairy cows producing approximately 40 kg/d milk. They did not observe an advantage from increasing the CP of the diet to 17.9 or 19.4% by increasing dietary SSBM. When the diets of Olmos Colmenero and Broderick (2006) were evaluated by using NRC (2001) values, the data suggested no advantage of increasing the RDP and RUP supply above 11.0 and 5.5% of DM, respectively. Our control diet contained 11.3 and 5.5% of DM as RDP and RUP, respectively. Other researchers also did not observe improvements in milk and protein productions when dietary CP increased from 16.1 to 18.9% (Leonardi et al., 2003), from 16.5 to 18.5% (Cunningham et al., 1996), or from 16.7 to 18.4% (Broderick, 2003).

Armentano et al. (1993), Grummer et al. (1996), and Santos et al. (1998) suggested that better-producing cows (>30 kg/d, Armentano et al., 1993, and Santos et al., 1998; >40 kg/d, Grummer et al., 1996) are more likely to respond to supplemental RUP. In contrast, Dunlap et al. (2000) observed no interaction between production level and response to supplemental RUP. Dunlap et al. (2000) suggested that their diets may have been limited by energy supply for the high-producing cows in their study, which may have led to the lack of response to supplemental RUP. In our study, statistical analysis of data from only the cows with the greatest milk production failed to detect differences among treatments, suggesting that the lack of treatment response cannot be attributed solely to the modest production level of the cows.

Santos et al. (1998) suggested that the lack of response to increased RUP supply could be a result of depressed microbial protein synthesis as a consequence of a deficiency in the supply of RDP, because RUP sources were often added at the expense of RDP. All of our diets contained an adequate RDP supply to support cow requirements (Table 6).

High DMI by our cows (27 kg/d) may have supported microbial protein supplies to the small intestine, resulting in little need for supplemental RUP. Replacing SSBM with heated whole soybeans did not improve performance in early-lactating dairy cows (Pires et al., 1996); the authors partly explained their observations as being a result of high DMI (24 kg/d). High DMI (24 to 25 kg/d) also has been used as an explanation for a lack of response to supplemental RUP (Broderick et al., 1990; Polan et al., 1997). Broderick et al. (1990) demonstrated that the increase in production of milk and milk components with supplemental RUP (ESBM) was greater at low DMI (20 kg/d) than at intermediate DMI (22 kg/d), with no increase at high DMI (24 kg/d). The authors attributed the lack of response to RUP supplementation at high DMI to the large supply of microbial protein.

Replacing SSBM with ESBM increased milk production in early-lactation dairy cows fed diets with alfalfa silage as the only forage source (Broderick et al., 1990). Dairy cows fed diets based on alfalfa silage were more likely to benefit from supplemental RUP than those fed diets based on corn silage (Voss et al., 1988). Replacing SSBM with heated whole soybeans also improved milk production in cows fed diets based on alfalfa silage (Faldet and Satter, 1991; Knapp et al., 1991), but not in diets based on corn silage (Bernard, 1990). Our forage sources (corn silage and alfalfa hay) likely would provide more RUP than alfalfa silage.

The effects of dietary treatments on plasma metabolites are presented in Table 8. There were no effects (P 0.30) of dietary treatment on plasma glucose or urea concentrations. Because diets were isonitrogenous, the similarity among treatments for plasma urea was not surprising, and agrees well with the similarity among treatments for MUN (Table 5). In general, total AA, EAA, nonessential AA, branched-chain AA, individual AA concentrations (except Leu), and individual AA as a percentage of total AA (except Leu; data not shown) were not affected (P 0.20) by dietary treatments. The lack of effects of dietary treatments on plasma AA could be explained by only modest differences among treatments in the supply of absorbable AA to the small intestine. Our production data (Table 5) indicated that AA requirements were met by all of our diets, so increases in plasma AA could be expected in response to increases in their supplies. Although YSBM and LSBM contained more RUP than did SSBM and ESBM (experiment 2), they likely failed to greatly increase the supply of absorbed AA.

Experiment 2: In Situ Protein Degradability and Intestinal Digestibility of RUN
Residual N remaining after ruminal incubation (RUN) of SBM products for different incubation times is presented in Table 9. At each time point, RUN was greater (P < 0.05) for YSBM and LSBM than for SSBM or ESBM, with no differences between YSBM and LSBM or between SSBM and ESBM, except at 0 h, at which point SSBM and YSBM did not differ, and at 24 h, at which point RUN for ESBM was greater than that for SSBM. The RUN declined more rapidly over time for SSBM and ESBM than for YSBM and LSBM (treatment x time, P < 0.05).

The RUN contents of SBM products estimated by using the predicted model, based on estimated A, B, C, and k_B values in Table 7, were comparable to those measured by using the single 16-h time point. Our measured value for RUN (16-h residue) for ESBM was smaller than the NRC (2001) value (56 vs. 69%), but that for LSBM was comparable to the NRC (2001) value (81 vs. 79%). Assuming a passage rate of 7%/h, our measured RUN content for SSBM (50%) was somewhat more than the 43% value reported by the NRC (2001). This likely contributed to our SSBM diet providing adequate RUP to the cows, thereby preventing a response to supplemental RUP from the other SBM products.

Intestinal digestibilities of RUN from SBM products are presented in Table 10. There were no differences (P = 0.22) among SBM products for intestinal digestibility of RUN, which averaged 82%. Because there was more RUN in YSBM (83%) and LSBM (81%) than in SSBM (48%) and ESBM (55%), the proportions of ingredient N that were available postruminally were greater for YSBM (66%) and LSBM (64%) than for SSBM (39%) or ESBM (48%). The NRC (2001) has assigned a single value of 93% for intestinal digestibility for SSBM, ESBM, and LSBM.

Experiment 3: Lys Bioavailability
The performance of chicks fed different SBM products is presented in Table 11 and Figure 1. Chick performance [DMI, ADG, and gain:feed ratio (G:F)] was linearly improved (P < 0.05) with increasing protein supply from SSBM (Figure 1), indicating that chick performance was, as intended, limited by protein supply. In addition, supplementing Lys to the diet containing 26% SSBM resulted in improved (P < 0.05) performance (DMI, ADG, and G:F), compared with the 26% SSBM treatment. This indicates that chick performance was indeed limited by Lys supply, and our study provided a comparison of the relative Lys bioavailabilities in SBM products.

View larger version (9K): [in this window] [in a new window]   Figure 1. A) Dry matter intake, B) ADG, and C) gain:feed ratio for the solvent soybean meal standard curve (•) and for the diet with 0.15% added Lys·HCl (; experiment 3).

Feeding in situ residues of SSBM, ESBM, or LSBM resulted in similar performance (DMI, ADG, and G:F) but 39% more DMI, 66% faster ADG, and 20% better G:F compared with feeding in situ residues of YSBM. If a prediction of performance was based on total Lys content alone (Table 12), feeding in situ residues of LSBM would be expected to result in worse chick performance than feeding SSBM or ESBM, simply because LSBM (5.8% of CP) had less total Lys content than did SSBM (6.7% of CP) or ESBM (7.0% of CP).

Data from experiment 2, that YSBM and LSBM had greater RUP contents than SSBM or ESBM, suggests that YSBM or LSBM could support higher performance of cattle compared with SSBM or ESBM if the total RUP supply was limiting. Data from this experiment, however, indicated that there were likely differences in Lys availability. We purposefully formulated the control diet offered to cows in experiment 1 to be limiting in RUP, specifically in Lys supply. When the Lys supply available to cows was estimated, by multiplying the available Lys content as a percentage of CP of the in situ residues (Table 13) by the 16-h RUP value (Table 7), available Lys supplied by the SSBM (2.1%) was not very different from the amounts supplied by ESBM (1.9%), YSBM (2.4%), or LSBM (2.7%). The fact that diets were not very different in available Lys supply could explain the lack of treatment differences in experiment 1. It is also possible that our control diet might not have been limited by Lys supply.

The chemically available Lys contents in original SBM and in situ residues of SBM products are presented in Table 13. Chemically available Lys as a percentage of CP was greater for original SSBM (5.5%) and ESBM (5.3%) than for YSBM (4.1%) or LSBM (4.3%). In addition, when expressed as a percentage of total Lys, the chemically available Lys contents of SSBM (73%) and ESBM (70%) were greater than those of YSBM (66%) or LSBM (63%); these percentages are likely overestimates for YSBM and LSBM, in which total Lys content is perhaps an underestimate of the Lys present in those products before processing.

The chemically available Lys contents for in situ residues of SSBM (4.0% of DM), ESBM (3.5%), and LSBM (3.7%) were comparable but were greater than that in YSBM (2.9%), which agreed with chick performance data. When expressed as a percentage of CP, in situ residues of SSBM (5.2%), ESBM (5.3%), and LSBM (5.1%) were almost identical but were greater than that in YSBM (3.9%). Because total Lys contents in in situ residues for YSBM (4.9% of CP; Table 12) and LSBM (5.8%) were less than that in residual SSBM (6.7%) or ESBM (7.0%), chemically available Lys as a percentage of total Lys was greater in in situ residues of YSBM (81% of total Lys) and LSBM (87%) than in those for SSBM (78%) and ESBM (76%).

Chemically available Lys contents as a percentage of DM were greater in in situ residues, compared with original SBM, likely as a result of greater CP contents in in situ residues than in original SBM. As a percentage of CP, however, chemically available Lys contents in in situ residues were almost identical to the original SBM for the same product, except for LSBM. It is possible that some of the unavailable Lys in the original LSBM became available after ruminal incubation, resulting in a greater content of available Lys (5.1 vs. 4.3%).

It seems that application of heat to SBM during LSBM and YSBM manufacturing might decrease Lys availability. This would appear to be more of a problem during YSBM manufacturing than during LSBM manufacturing. It should be noted that SSBM and YSBM were from the same source, but LSBM was a commercial product and the SSBM used to produce it was unknown.

Amino acid composition of the in situ residues used to feed chicks is presented in Table 12. Although no statistical tests could be performed, concentrations of individual AA were similar among the 4 SBM products, except for Lys. Lysine concentrations in YSBM (4.9% of CP) and LSBM (5.8%) were less than those in SSBM (6.7%) and ESBM (7.0%), likely because of the greater cross-linked Lys in YSBM and LSBM. Individual AA were generally degraded within each SBM product to a similar extent during in situ incubation, except for Lys. As discussed for N, however, there were clear differences among SBM products. Others (Kerry et al., 1993; O’Mara et al., 1997; Tamminga, 1979) have reported differences in ruminal degradation among individual AA. For example, Met (Tamminga, 1979), Leu, and Ile (O’Mara et al., 1997) were the least degradable AA, and Glu (O’Mara et al., 1997) was the most degradable AA.

Across SBM products, Lys was the most degraded AA. O’Mara et al. (1997) and Lykos and Varga (1995) reported that Lys was one of the most degradable AA in SBM. Erasmus et al. (1994) also showed decreased Lys contents in SBM after exposure to in situ ruminal degradation. All AA in YSBM and LSBM were degraded less upon in situ incubation than were AA in SSBM or ESBM as a result of protection from ruminal degradation of YSBM and LSBM products. Kerry et al. (1993) demonstrated that replacing SSBM with heated SBM resulted in less degradation of dietary Arg, Ile, Phe, and Thr in steers.

Borucki Castro et al. (2007) extended the 3-step procedure of Calsamiglia and Stern (1995) for assessing intestinal digestion of CP in vitro to evaluate intestinal AA digestibility. They observed that Lys digestibility in in situ residues of various SBM products was greater than that of other AA, although the 3-step procedure has not been validated with regard to its ability to accurately assess intestinal digestion of individual AA. Our work with the 3-step procedure demonstrated no differences among SBM products in intestinal CP digestion, but we observed significantly impaired in vivo Lys availability in several of the products; this suggests that the 3-step procedure may not be appropriate for assessing availability of individual AA, particularly of Lys.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Treating SBM with lignosulfonate or yeast followed by heating, to induce nonenzymatic browning, was successful in protecting LSBM and YSBM from ruminal degradation. Thus, replacing SSBM with YSBM or LSBM increased the RUP supply. Dairy cows producing approximately 33 kg of milk/d and fed dietary RUP concentrations of 5.5% of DM did not benefit when SSBM was replaced by less degradable sources of SBM, likely because of the adequate RUP supply in the control diet or a lack of change in the supply of absorbable Lys (the AA predicted to be the most limiting in our diets). Processing of YSBM and LSBM seemed to decrease Lys bioavailability. The chemically available (FDNB) Lys procedure and a chick growth assay gave comparable results regarding Lys availability in SBM products, but using original SBM or SBM residues after in situ incubation in these procedures yielded different results, especially for LSBM. Thus, using in situ residues of SBM products in the chick growth assay to assess the impact of treating SBM on Lys bioavailability would be recommended, rather than using the original SBM.

	FOOTNOTES

 
¹ Contribution number 06-330-J from the Kansas Agricultural Experiment Station, Manhattan, Kansas. Research was funded by Carr’s Milling Industries PLC and AFGRI Operations Limited.

Received for publication March 19, 2007. Accepted for publication June 18, 2007.

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