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1 Department of Animal Sciences and
2 Department of Veterinary Clinical Medicine, University of Illinois, Urbana 61801
Corresponding author: I. R. Ipharraguerre; e-mail: ipharrag{at}uiuc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: rumen-protected soy product • ruminal fermentation • intestinal supply of nutrients • dairy cow
Abbreviation key: ESBM = expeller soybean meal, EAA = essential amino acids, NANMN = nonammonia nonmicrobial nitrogen, NEAA = nonessential amino acids, NSBM = heated, xylose-treated soybean meal, SSBM = solvent-extracted soybean meal, WRSB = whole roasted soybeans.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Solvent-extracted soybean meal (SSBM) and raw soybeans are excellent sources of RDP that can be used to supply ruminal microorganisms with the forms of N required for driving their growth (Stern et al., 1985; Tice et al., 1993). Although soy proteins are a good source of digestible Lys and His (Santos et al., 1998; NRC, 2001), they are low in Met (1.44 to 1.47% of CP) and can be extensively degraded (
57.4 to 69.6%) by ruminal microbes (NRC, 2001), which may compromise their value for contributing quantitatively and (or) qualitatively to the supply of EAA in metabolizable protein. This limitation motivated the development of several methods for decreasing the ruminal degradability of proteins in SSBM and soybean seeds. Some of the most effective methods for this purpose that are approved for commercial use in the US involve the controlled administration of heat to soybeans (i.e., expeller processing and roasting of soybeans) or heat plus reducing sugars to SSBM [i.e., heated, xylose-treated soybean meal (NSBM); Cleale et al., 1987; Firkins and Fluharty, 2000; Grummer and Rabelo, 2000). Estimations by the NRC (2001) for dairy cows consuming DM at 4% of BW in a 1:1 forage to concentrate ratio indicate that these methods can decrease ruminal degradability of proteins in raw soybeans by about 11% (roasting of whole seeds) and of proteins in soybean meal by about 62% (expeller processing) and 86% (heated, xylose treatment) without negatively affecting (Harstad and Prestlokken, 2000), and sometimes even improving (Tice et al., 1993), their intestinal digestibility.
Numerous studies have examined the lactational response of dairy cows to the feeding of heat-treated soybean seeds or soybean meal. Surprisingly, much less attention has been given to the effects of these treatments on the intestinal supply of N and AA (Santos et al., 1998; Ipharraguerre and Clark, 2005). Indeed, only a few studies with dairy cows have been undertaken, and a summary of 9 comparisons from those experiments (Ipharraguerre and Clark, 2005) showed that replacing SSBM with soy protein supplements of reduced ruminal degradability increased (25.2%) the passage to the duodenum of nonammonia nonmicrobial N (NANMN), but only improved marginally (P > 0.05) that of EAA (4.7%), Lys (3.9%), and Met (0.6%). Additionally, to our knowledge, the impact of heat-treated soy protein supplements on the ruminal outflow of N and starch has not been compared in the same study with lactating dairy cows. Therefore, it is likely that the paucity of data from in vivo experiments limits the use of current protein systems for predicting accurately the supply of AA to dairy cows fed diets containing rumen-protected soy protein supplements.
The objectives of this study were to compare the effects of partially replacing SSBM with expeller soybean meal (ESBM), NSBM, or whole roasted soybeans (WRSB) in the diet of lactating dairy cows on ruminal fermentation, passage of nutrients to the small intestine, and nutrient digestibility.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The experimental design was a 4 x 4 Latin square with 14-d periods. The first 7 d of each period were used to adapt the cows to treatments, and the remaining 7 d were used to collect data. Each cow was randomly assigned to one of 4 treatment sequences in which a treatment never followed the same treatment for all sequences. The control diet contained 40.07% forage; 40.51% corn, minerals, and vitamins; and 19.42% SSBM (Table 1
). For the other 3 dietary treatments, SSBM was replaced with ESBM, NSBM, or WRSB to supply 10.20% of the dietary DM. All diets were formulated to contain 18% CP and to meet the NRC (2001) recommendations for all other nutrients. Diets were adjusted weekly to reflect changes in DM of forages and concentrate mixtures by drying each component overnight in an oven at 105°C.
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-amylase (Thermamyl 120L; Novo Nordisk Biochem, Franklinton, NC), sodium sulfite (Van Soest et al., 1991), and starch (Kartchner and Theurer, 1981). Except for Met, samples of feeds and TMR were prepared for AA determination, except Met, which was determined by acid hydrolysis with 6 N HCl (McCarthy et al., 1989). For Met determination, samples were first oxidized with performic acid, lyophilized after dilution with water to remove excess performic acid (Overton et al., 1995), and hydrolyzed with 6 N HCl as outlined by Moore (1963). Individual AA were then separated by ion exchange chromatography on a Beckman 126AA AA analyzer equipped with a Beckman 12-cm column (Beckman Instruments Inc., Palo Alto, CA). Samples of ruminal fluid and duodenal digesta were collected every 3 h during the last 3 d of each period. The sampling time was adjusted ahead 1 h daily so that a sample was obtained for each 1-h interval of the day (24 total samples). Ruminal fluid samples were taken from multiple sites in the rumen, and pH of ruminal fluid was measured immediately by glass electrode. After measurement of pH, a subsample of 50 mL was acidified to pH < 2 with 50% H2SO4 (vol/vol), centrifuged at 27,000 x g for 10 min at 4°C, and the supernatant was frozen at –20°C for later analyses. Following removal of the cap of the duodenal cannula, accumulated digesta was discarded, and when the flow appeared normal, 500 mL of duodenal contents were collected. Samples were pooled by cow and stored frozen in 20-L buckets at –20°C until analyses.
Ruminal fluid samples taken at 0, 1, 2, 3, 5, 7, 9, and 10 h after the morning feeding and at 0, 1, 2, 3, 5, 7, 9, 11, and 12 h after the afternoon feeding (17 total samples) were thawed, centrifuged at 27,000 x g for 20 min at 4°C, and an aliquot of 4 mL was diluted with 25% metaphosphoric acid (4:1). These subsamples were assayed with a gas chromatograph (model 5890 Series II; Hewlett-Packard, Avondale, PA) equipped with a 1.8-m glass column packed with 10% SP 1200/1% H3PO4 on 80/100 chromosorb W AW (Supelco, 1975) to determine the concentration of VFA. Nitrogen was the carrier gas, and the temperature of the injection port and column was 175°C and 125°C, respectively. Ruminal NH3N was determined according to the procedures out-lined by Chaney and Marbach (1962) as modified by Cotta and Russell (1982).
Duodenal samples were thawed and homogenized for 5 min using a propeller-type mixer set at high speed. During continuous stirring, a representative subsample (1000 mL) of digesta was collected by vacuum. Samples then were poured into shallow pans, lyophilized, ground through a 1-mm screen, and analyzed for DM, OM, Kjeldhal N, starch, and AA as described above. The concentration of NH3N in duodenal digesta was determined by steam distillation with MgO (Bremner and Keeney, 1965), and purines, used as a bacterial marker, were measured by the method of Zinn and Owens (1986).
Ruminal bacteria were isolated from samples (1000 mL) of whole ruminal contents obtained from the reticulum near the reticuloomasal orifice at 6 separate post-feeding times (0, 2, 4, 6, 8, and 10 h) during the last 3 d of each period. Ruminal contents were blended in a Waring blender (Waring Products Division, New Hartford, CT) for 1 min at low speed and strained through 6 layers of cheesecloth; the effluents were used to prepare a bacteria-rich sample by differential centrifugation (Overton et al., 1995). Bacterial samples were pooled by cow within period and frozen at –20°C. These samples were lyophilized and analyzed for DM, OM, Kjeldahl N, AA, and purines by the methods described above.
During the last 6 d of each period, fecal grab samples were collected twice daily at 12-h intervals. The sampling time was adjusted ahead 2 h daily so that a sample was obtained for each 2-h interval of the day (12 total samples). Samples were composited on an equal wet weight basis, dried at 55°C, ground through a 1-mm screen, and assayed for DM, OM, Kjeldahl N, and starch as described above.
Chromic oxide was used as an indigestible marker to assess the passage of digesta to the duodenum and fecal excretion by the cows. Gelatin capsules that contained 10 g of Cr2O3 powder were administered via the ruminal cannula at 0800 and 2000 h during the last 10 d of each period. Concentration of Cr in duodenal and fecal samples was quantified by atomic absorption spectroscopy (air plus acetylene flame; Perkin-Elmer, Norwalk, CT) after preparation of samples by the procedure of Williams et al. (1962).
Passage of microbial N and OM to the duodenum was calculated from the passage of DM and from the proportion of N or OM of bacterial origin, respectively. These proportions were estimated by dividing the N to purine ratio or the OM to purine ratio of isolated bacteria by the N to purine ratio or the OM to purine ratio of duodenal digesta. Passage of NANMN to the duodenum was calculated by subtracting passage of microbial N from passage of total NAN. Apparent digestibility of OM in the rumen was corrected for the passage of microbial OM to the duodenum to establish the amount of OM truly digested in the rumen.
Milk weights were recorded at each milking during the last 7 d of each period. Milk samples were taken at each milking during the last 7 d of each period, preserved with 2-bromo-2-nitropropane-1,3-diol, and stored at 4°C. Samples were sent to Dairy One Cooperative Inc. (Ithaca, NY) for analyses of fat, CP, true protein, TS, and urea N by infrared procedures (AOAC, 1990; Foss 4000; Foss North America, Eden Prairie, MN).
Statistical Analyses
Data were analyzed as a 4 x 4 Latin square using PROC MIXED of SAS (2000), with cow treated as a random variable and according to the following model:
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where
| Yijk | = | dependent variable,
| µ | = | overall mean,
| Ci | = | effect of cow i (i = 1, 2, 3, 4),
| Pj | = | effect of period j (j = 1, 2, 3, 4),
| Tk | = | effect of the treatment k (k = 1, 2, 3, 4), and
|  ijk | = | residual error.
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A multiple comparison test (protected Fisher’s least square difference) was used to separate least squares means into significant main effects.
Ruminal VFA, NH3 N, and pH data were analyzed as repeated measures in time using PROC MIXED of SAS (Littell et al., 1996) and according to the following model:
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where
| Yijkl | = | dependent variable,
| µ | = | overall mean,
| Ci | = | effect of cow i (i = 1, 2, 3, 4),
| Pj | = | effect of period j (j = 1, 2, 3, 4),
| Tk | = | effect of the treatment k (k = 1, 2, 3, 4),
| ijk | = | whole plot error,
| Hl | = | effect of the sampling time l (l = 1, 2,..., 17),
| THkl | = | interaction between treatment k and sampling time l, and
|  ijkl | = | split plot error.
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The smallest value for the Akaike’s information criterion was used to select the most appropriate covariance structure (Littell et al., 1996). Because the treatment x hour interaction was not significant (P > 0.05) for the ruminal parameters measured, treatment effects were compared across treatment sampling times using the procedure described above.
Differences among treatments were considered to be significant when P < 0.05, whereas when P > 0.05 but < 0.10 differences were considered to indicate a trend toward a significant effect.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). The retrospective evaluation of the diets using the NRC (2001) model as outlined in Table 1 indicated that replacing SSBM with ESBM or NSBM, but not with WRSB, increased the percentage of dietary DM and CP supplied as RUP. As expected, variation in the percentage of NDF (SD = 1.30), ADF (SD = 0.40), and starch (SD = 1.25) among diets was small.
The chemical composition of the protein supplements and forages (Table 2) was within the range reported in the literature (Miller and Hoover, 1998; NRC, 2001). With the exception of Met, ESBM and NSBM contained less EAA per unit of CP than SSBM and WRSB, which suggests that a larger proportion of EAA, especially Lys, might have been irreversibly bound to carbohydrates in ESBM and NSBM than in SSBM and WRSB, impeding their release during the acid hydrolysis performed prior to analysis (Firkins and Fluharty, 2000). Alternatively, differences in the AA composition among sources of soybeans used to manufacture the soy products evaluated in this experiment might have contributed to these findings. Other researchers have reported similar variability for the concentration of EAA between SSBM and ESBM (Maiga et al., 1996) or NSBM (Harstad and Prestlokken, 2000).
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). Other researchers also reported that replacing SSBM with ESBM (Reynal and Broderick, 2003), NSBM (Mansfield and Stern, 1994), or WRSB (Knapp et al., 1991) to provide varying proportions of the dietary DM did not affect DMI of lactating cows. In some experiments, however, feeding WRSB depressed DMI when compared with SSBM (Mohamed et al., 1988; Pires et al., 1996).
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Intake, apparent ruminal and total tract digestibility (amount and percentage), flow to duodenum, and the apparent postruminal digestibility (amounts and proportions) of starch were not significantly influenced by treatments (Table 4). In relation to numerical trends, feeding WRSB tended to decrease the quantity of starch that was apparently digested in the total digestive tract compared with ESBM and NSBM. Because the percentage of starch that was digested in the total tract did not follow the same trend, this response can be attributed to the numerically smaller amount of starch consumed by cows fed WRSB, which probably arose from small variations in both DMI and starch content of the diet (smaller proportion of corn grain in the WRSB diet).
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Ruminal Fermentation
The source of soy protein did not affect the mean ruminal pH, which averaged 5.81 for all treatments (Table 5). Similar results were obtained in other studies with dairy cows (Knapp et al., 1991; Reynal and Broderick, 2003) or steers (Ludden and Cecava, 1995; Orias et al., 2002) in which heat-treated soy products were compared with SSBM or raw soybeans.
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). The molar proportion of acetate was higher for ESBM and NSBM than for SSBM and WRSB, whereas the opposite was observed for the molar percentage of propionate. The percentage of butyrate was highest for ESBM and WRSB, intermediate for NSBM, and lowest for SSBM. Although not always significant, similar changes in the molar proportion of acetate and (or) propionate have been reported in a number of studies in which SSBM was compared with ESBM (Ludden and Cecava, 1995; Reynal and Broderick, 2003) or NSBM (Windschitl and Stern, 1988). Similarly, results reported here agree with data from other experiments, indicating that the replacement of SSBM with roasted (Mohamed et al., 1988; Knapp et al., 1991) or extruded (Stern et al., 1985) soybeans has generally failed to alter the relative concentration of these VFA in ruminal fluid. Windschitl and Stern (1988) explained these changes in the molar proportions of acetate and propionate by suggesting that increasing the availability of true protein in the rumen by feeding more degradable protein sources stimulates the production of propionate more than acetate. Because the intake and ruminal fermentation of OM and starch were similar among treatments, this response might also be explained by a more rapid rate of degradation of OM and protein in SSBM than in ESBM and NSBM as well as by the lower fat content in the SSBM diet compared with the WRSB diet. To some extent, these observations are supported by data from experiments showing that the in situ rate of protein degradation of SSBM was significantly faster than that of ESBM (Maiga et al., 1996) or NSBM (Harstad and Prestlokken, 2000) and by the well-documented effects of lipids on the molar proportion of acetate and propionate (Jenkins, 1993). The molar concentrations of isobutyrate and valerate were highest for SSBM and did not differ among the rumen-protected soy supplements. The percentage of isovalerate was highest for SSBM and ESBM, intermediate for NSBM, and lowest for WRSB. Because these VFA are the end products of the oxidative deamination and decarboxylation of branched-chain AA (Allison, 1978), it is likely that less feed protein was degraded in the rumen when SSBM was replaced with the other soy protein supplements in the diet. Some support for this hypothesis is provided by the finding that the NH3N concentration in the rumen was highest for SSBM and WRSB and lowest for ESBM and NSBM. Previous work also showed that the replacement of SSBM with NSBM (from 13.8 to 17% of the dietary DM) decreased the molar proportion of branched-chain VFA and the concentration of NH3N in ruminal fluid (Windschitl and Stern, 1988; Mansfield and Stern, 1994). Also, in agreement with our results, the substitution of WRSB for SSBM to supply 12 to 24% of the dietary DM decreased the percentage of isobutyrate (Mohamed et al., 1988) or valerate (Knapp et al., 1991) without altering the ruminal NH3N concentration (Mohamed et al., 1988). However, changes in the concentration of isoacids and (or) NH3N in response to the replacement of SSBM or raw soybeans with heat-treated soy supplements have not always been observed (Ludden and Cecava, 1995; Reynal and Broderick, 2003).
Intake and Ruminal Outflow of Nitrogen
The intake of N ranged from 567 to 622 g/d, but differences among treatments were not significant (Table 6). The amount of total N that passed to the duodenum tended to be greater for NSBM than for SSBM and WRSB. This trend, however, was not evident when passage of N was expressed as a percentage of N intake. Compared with SSBM, the ruminal outflow of NAN was higher for NSBM (+20.8%), tended (P < 0.09) to be higher for ESBM (+10.1%), and was similar for WRSB. These increases in ruminal outflow of NAN were the results of the intestinal supply of NANMN being higher for NSBM (+44.5%) and WRSB (+30.2%) and tending (P < 0.08) to be higher for ESBM (+27.5%). Among rumen-protected soy products, ruminal escape of NAN was higher for NSBM than for WRSB. Conversely, the source of soy protein did not affect the passage of microbial N to the small intestine (expressed as g/d or percentage of NAN flow) or the efficiency of microbial growth.
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Apparent N digestion in the intestines and total tract was unaltered by treatments (Table 6). Similar to results from this experiment, other researchers have reported no change in intestinal and (or) total tract digestibility of N when heated SSBM (Mabjeesh et al., 1996), NSBM (Cleale et al., 1987; Mansfield and Stern, 1994), or ESBM (Ludden and Cecava, 1995) were compared with SSBM.
Intake and Ruminal Outflow of AA
With the exception of Met, Thr, Ala, Pro, Ser, and total nonessential AA (NEAA), replacing SSBM with ESBM increased the intake of individual EAA and NEAA, total EAA (P < 0.07), and total AA (Table 7). These differences were less pronounced when NSBM or WRSB were compared with SSBM. That is, NSBM increased the intake of Ile and Val and decreased the intake of Ser; WRSB enhanced the intake of Arg but depressed that of Thr and Ser. Among rumen-protected soy supplements, the intakes of His, Lys, and total EAA (P < 0.07) were largest for ESBM and those of Leu, Phe, Thr, Val, and NEAA (i.e., individual and total) were lowest for WRSB. Because the source of supplemental CP was essentially the only ingredient that varied in the diets, these results can be attributed to variations in DMI (Table 3) and in the AA composition of the soy protein supplements (Table 2). In other trials, the replacement of SSBM with heated SSBM (Mabjeesh et al., 1996), NSBM (Windschitl and Stern, 1988), or extruded soybeans (Stern et al., 1985) also altered AA intake.
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), but affected markedly the ruminal escape of AA in the NANMN (Table 9) and NAN fractions (Table 10). With the exception of Met, Ser, and Tyr, partially replacing SSBM with NSBM improved or tended to improve (i.e., Ile, Lys, Val, and Gly) the intestinal supply of all other EAA and NEAA (Table 10). Similar improvements in the amount of AA that reached the small intestines of lactating cows were observed when heated and unheated SSBM were compared (Mabjeesh et al., 1996). However, Windschitl and Stern (1988) reported that only the passage to the duodenum of Glu and Pro increased when SSBM was replaced with xylose- or lignosulfonate-treated SSBM in the diet of dairy cows. In another experiment (Mansfield and Stern, 1994), the amount of AA delivered to the lower digestive tract of dairy cows did not change when SSBM was totally replaced with lignosulfonate-treated SSBM. In comparison with SSBM, supplementing the diet with WRSB increased the passage to duodenum of Arg, His, Ile (P < 0.06), Leu, Lys (P < 0.06), Phe, Val (P < 0.06), Asp, Glu, Gly (P < 0.07), Pro, total NEAA, and total AA (Table 10). Previous work showed that the postruminal supply of most EAA and NEAA was not enhanced when extruded soybeans were compared with SSBM (Stern et al., 1985). Apart from Glu and Pro, the partial replacement of SSBM with ESBM did not alter the amount of AA that passed to the lower digestive tract (Table 10). Other researchers also found that diets supplemented with either SSBM or ESBM resulted in similar amounts of AA delivered to the small intestine of dairy cows (Reynal et al., 2003) and steers (Ludden and Cecava, 1995). Among the rumen-protected soy products, the ruminal outflow of Leu, Thr, Ala, Asp, Glu, Pro, and total AA was highest for NSBM (Table 10). In comparison with ESBM, feeding NSBM and WRSB tended to increase postruminal supplies of Arg (P < 0.05), Ile, Lys, Val, and Gly. None of these RUP sources enhanced the amount of Met that reached the small intestine when compared with each other or with SSBM.
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), it is reasonable to expect that significant improvements in the intestinal supply of EAA, particularly of Lys and Met, would be achieved only if the contribution of NANMN to the passage of NAN to the duodenum is remarkably increased. As previously indicated, the proportion of the ruminal outflow of NAN contributed by NANMN was not affected significantly by treatments, which provides an explanation for the lack of significant responses in the postruminal supply of Lys and Met in response to the addition of ESBM, NSBM, or WRSB to the SSBM-based diet.
Production of Milk and Milk Composition
Yields of milk and 3.5% FCM were highest for NSBM, intermediate for ESBM and WRSB, and lowest for SSBM, although differences in milk production were significant only between SSBM and NSBM (Table 11). The concentration of fat, CP, true protein, TS, and urea N were not affected by the source of soy protein. The yield of milk components, however, paralleled changes in milk production. That is, the yield of fat, CP, true protein, and TS were highest for NSBM, intermediate for ESBM and WRSB, and lowest for SSBM. However, differences in the daily output of true protein and TS were not significant among SSBM, ESBM, and WRSB.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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These results suggest that replacing a portion of SSBM with ESBM, NSBM, or WRSB may increase the quantity of feed protein that reaches the small intestines of dairy cows. However, significant improvements in the supply of previously reported limiting AA for milk production, particularly of Met, should not be expected. To accomplish this goal, it seems that these rumen-protected soy products must be combined with other RUP sources and (or) rumen-protected AA that complement their EAA profile.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication October 20, 2004. Accepted for publication April 5, 2005.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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