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* Department of Dairy Science, University of Wisconsin
Agricultural Research Service, USDA US Dairy Forage Research Center, Madison, WI
Corresponding author:
G. A. Broderick; e-mail:
glenb{at}dfrc.wisc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYACKNOWLEDGEMENTSREFERENCES |
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Key Words: dairy cow • protein supplementation • in vivo degradation rate
Abbreviation key: BM = Blood meal, CGM = corn gluten meal, ESBM = expeller soybean meal, HMSC = rolled high-moisture shelled corn, IIV = inhibitor in vitro method, OTD = omasal true digesta, SSBM = solvent soybean meal, TAAN = total amino acid N
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYACKNOWLEDGEMENTSREFERENCES |
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An omasal canal sampling technique developed by Huhtanen et al. (1997) and modified by Ahvenjarvi et al. (2000) is a less invasive approach that has proven to be reliable for measuring nutrient flows out of the rumen. This technique, together with the continuous infusion of digesta flow markers and use of purines to quantify microbial protein, allows for the determination of rate and extent of ruminal protein degradation in vivo in cows fitted with only ruminal cannulas.
The objectives of this experiment were 1) to quantify the effects of feeding protein supplements with differing ruminal degradabilities on milk production, ruminal metabolites, and nutrient flows out of the rumen and 2) to estimate in vivo rates of ruminal protein degradation of four protein supplements commonly fed to dairy cows and to compare these results with those obtained using the IIV method.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYACKNOWLEDGEMENTSREFERENCES |
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) differed in the proportion and source of protein supplement in the concentrate: diet A (basal), (31% HMSC, no supplement); diet B [22% HMSC, 9% solvent soybean meal (SSBM)]; diet C [21% HMSC, 10% expeller soybean meal, (ESBM; SoyPlus, West Central Coop., Ralston, IA)]; diet D [25.5% HMSC, 5.5% ring-dried blood meal (BM)]; and diet E [24% HMSC, 7% corn gluten meal (CGM)]. All cows were injected with one-half dose of bST [250 mg of Posilac (Monsanto, St. Louis, MO)] beginning on d 8 of period one of the trial and then injected with that amount of bST once each week for the remainder of the trial. This was done to avoid the effect of having unequal numbers of biweekly bST injections during alternating 3-wk periods. Cows were housed in tie stalls and had free access to water throughout the trial. Care and handling of the experimental animals, including ruminal cannulation, was conducted as outlined in the guidelines of the University of Wisconsin institutional animal care and use committee.
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The markers used to estimate digesta flows at the omasal canal in the cannulated cows (France and Siddons, 1986) were indigestible NDF for the large particle phase (Huhtanen et al., 1994), YbCl3 for the small particle phase (Siddons et al., 1985), and Co-EDTA for the fluid phase (Uden et al., 1980). The YbCl3 and Co-EDTA were dissolved in distilled water and continuously infused into the rumen for 6 d. Digesta flowing out the rumen was collected from the omasal canal (Huhtanen et al., 1997; Ahvenjarvi et al., 2000) four times daily at 2-h intervals during 3 consecutive days to represent the 24-h feeding cycle. Each 400-ml digesta sample was divided into subsamples of 100 and 300 ml. Formaldehyde [1 ml of 40% (wt/vol)] was added to the 100-ml subsample, and both subsamples were stored at -20°C as they were collected and pooled over the sampling period to yield single 1.2- and 3.6-L composites from each cow in each period. Bacteria were isolated from the 1.2-L pooled composites by differential centrifugation. The 3.6-L pooled composites were thawed at room temperature and separated into the three digesta phases that were recombined in the correct proportions to reconstitute the omasal true digesta (OTD) flowing out of the rumen based on the triple-marker method of France and Siddons (1986). A sample of whole ruminal contents was taken at 6 h postfeeding and squeezed through two layers of cheesecloth to yield about 1000 ml of strained ruminal fluid for use in isolation of protozoa. At the time of the morning feeding on d 17 (first square) and d 20 (second square), marker infusions were stopped, and samples of whole ruminal contents were taken at 0 (prefeeding), 1, 2, 4, 6, 8, 12 and 24 h postfeeding. These samples were analyzed for DM, and Co, and Yb concentrations. Declines in Co and Yb concentrations were used to estimate ruminal passage rates of DM in, respectively, fluid and small particle phases. Other specific details regarding this approach, including marker preparation, infusion and analysis, digesta sampling and analysis, and bacterial and protozoal isolation and analysis are described in the companion paper (Reynal et al., 2002).
Whole ruminal contents (100 to 200 ml) were obtained from all cannulated cows at 0, 1, 2, 4, 6, 8, 12, and 24 h after the morning feeding of d 20 of each period, strained through two layers of cheesecloth, and pH measured immediately. Two subsamples were taken from the resulting strained ruminal fluid: one 10-ml sample was preserved by addition of 0.2 ml of 50% (vol/vol) H2SO4 for later analysis of NH3 and total AA, and one 5-ml sample was preserved by the addition of 5 ml of formic acid for later analysis of VFA. Samples for VFA analysis were pooled by mixing equal volumes across times to yield one composite per cow per period. All ruminal samples were stored at -20°C. Just before analysis, samples were thawed, centrifuged (15,000 x g for 15 min at 4°C), and analyzed for NH3 and total free AA (Broderick and Kang, 1980) using flow-injection analysis (Dual-Channel QuikChem 8000 FIA, Lachat Instruments, Milwaukee, WI), and for VFA (Brotz and Schaefer, 1987). Blood samples were taken 4 h after feeding from the coccygeal artery or vein of each cow on d 17 of each period. Blood was heparinized, held at 4°C for 8 h until plasma was prepared by centrifuging (1,500 x g, 4°C, 15 min), deproteinized by mixing four volumes of plasma with one volume of 15% (wt/vol) 5-sulfosalicylic acid, and the supernatant (15,000 x g, 4°C, 15 min) stored at -20°C until analyzed for milk urea N by a colorimetric assay (Ekinci and Broderick, 1997).
The OTD samples and weekly composites of TMR and feed ingredients were analyzed for total N (Leco 2000, Leco Instruments, Inc., St. Joseph, MI), DM at 105°C, ash and OM (AOAC, 1980), and sequentially for NDF and ADF using heat stable 
-amylase and Na2SO3(Hintz et al., 1995). Extracts also were prepared from weekly composites of silages and TMR in distilled water (Muck, 1987). Samples of OTD, TMR extracts, protein supplements, and isolated bacteria were analyzed for NH3 and total free AA as described above for strained ruminal fluid. Extracts from TMR also were analyzed for NPN (Muck, 1987) using a combustion assay (Mitsubishi TN-05 Nitrogen Analyzer; Mitsubishi Chemical Corp., Tokyo). The OTD, bacterial, protozoal samples were analyzed for AA composition using ion-exchange chromatography with ninhydrin detection (Beckman 6300 Amino Acid Analyzer, Beckman Instruments, Inc., Palo Alto, CA) after hydrolysis in 6 N HCl (Nagel and Broderick, 1992). The methodology used to determine the microbial protein flows, using purines determined by HPLC and spectrophotometry and using linear programming based on AA composition of isolated bacteria and protozoa, is detailed in the companion paper (Reynal et al., 2002). Rate and extent of ruminal protein degradation of protein supplements (SBM, ESBM, BM, and CGM) were estimated using the inhibitor in vitro method (Broderick, 1987). Proportions (%) and amounts (g/d) of omasal total AA N (TAAN) and NAN flow originating from microbial protein and RUP on all diets were estimated using total purines (determined by both methods) as the microbial marker and computing RUP by difference (Reynal et al., 2002). Except for the protein supplement, the same basal ingredients were fed in basal diet A and other four diets. Therefore, the contribution to RUP flow from the basal ingredients in the protein supplemented diets could be computed based on intake of basal ingredients and applying the proportion of RUP from diet A. The specific details of this approach, including the equations used in these computations, are reported in the companion paper (Reynal et al., 2002).
Statistical Analysis
Passage rates were estimated by regressing the ln of Co and Yb concentrations on time using a nonlinear method for determining slope (Littell et al., 1996; SAS, 1999); these slopes were equated to the rates of passage (/h) of the DM in the fluid phase (Co) and small particle phase (Yb). Data were analyzed using Proc Mixed in SAS (1999). The following model was fitted to all variables that did not have repeated measurements over time: Yijkl = µ + Si + Pj + Vk(i) + Tl + STil + Eijkl, where Yijkl = dependant variable, µ = overall mean, Si = effect of square i, Pj = effect of period j, Vk(i) = effect of cow k (within square i), Tl = effect of treatment l, STil = interaction between square i and treatment l, and Eijkl = residual error. All terms were considered fixed, except for Vk(i) and Eijkl, which were considered random. The interaction term STil was removed from the model when P > 0.25. Differences between least squares means were reported only if the F-test for treatment was significant at = 0.05. The following model was used for ruminal variables for which there were repeated measurements over time (pH, NH3, total free AA): Yijklm = µ + Si + Pj + Vk(i) + Tl + STil + E1ijkl + Zm + ZTml + E2ijklm, where Yijklm = dependant variable, µ = overall mean, Si = effect of square i, Pj = effect of period j, Vk(i) = effect of cow k (within square i), Tl = effect of treatment l, STil = interaction between square i and treatment l, and E1ijkl = whole plot error, Zm = effect of time m, ZTml = interaction between time m and treatment l, and E2ijklm = subplot error. The spatial covariance structure SP(POW) was used for estimating covariances and the subject of the repeated measurements were defined as cow(square * period * treatment). All terms were considered fixed, except for Vk(i), E1ijkl, and E2ijklm, which were considered random. The interaction STil was removed from the model when P > 0.25. Differences between least squares means were reported only if the F-test for treatment was significant at = 0.05.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYACKNOWLEDGEMENTSREFERENCES |
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). Although originally formulated to contain 17% CP in basal diet A and 21% CP in protein supplemented diets B, C, D, and E, the TMR actually fed in this experiment averaged 15.8, 19.1, 19.7, 20.3, and 19.3% CP for diets A (basal), B (SSBM), C (ESBM), D (BM), and E (CGM), respectively (Table 1). Because diet A contained less CP and had no added protein concentrate, it contained higher proportions of total N as NPN, NH3N, and total free AA N and lower concentrations of individual AA and TAAN, than the other four diets (Table 1). Diet A was formulated to contain large amounts of NPN to minimize the effects on microbial protein formation from RDP supplied by the supplemental proteins added to diets B to E.
Cows fed diet A had mean DMI that was 1.1 to 2.1 kg/d lower than cows fed the other diets (P < 0.05); DMI in cows fed diet C (ESBM) was higher (P < 0.05) than that in cows fed diet D (BM) but not different from cows fed diets B (SSBM) and E (CGM) (Table 2). Broderick et al. (2002) found that supplementing diets containing 60% hay-crop silage with ESBM increased DMI by 1.3 kg/d. Daily BW gain was not different among diets. However, cows fed basal diet A had the lowest yields of milk, FCM, fat, protein, and SNF, probably as a result of having the lowest DMI (Table 2). Cows fed diets C (ESBM) and E (CGM) produced 1.4 and 1.9 kg/d more milk than cows fed diet B (SSBM). Broderick et al. (1990) observed a 1-kg/d increase in milk yield in cows fed ESBM and similar milk yield on ESBM and CGM relative to feeding SSBM when alfalfa silage was the only forage and DMI averaged 24 kg/d. Berzaghi and Polan (1991) reported 0.9 kg/d greater milk yield when cows fed diets based on alfalfa silage were supplemented with CGM instead of SSBM. Milk yield of cows fed diet D, that supplemented with BM, was intermediate; however, DMI was numerically lowest on BM of the four protein-supplemented diets. Yields of fat, protein, and SNF were not different among diets B, C, D, and E. Although milk yield was affected by protein source, FCM yield was not. This may be explained by the numerical differences (P = 0.09) in milk fat concentration among diets B to E. Compared to cows fed diets C and E, cows fed diet B produced less milk but with numerically higher fat concentration, resulting in similar yields of FCM. Milk concentrations of fat, protein, and SNF were unaltered by diet.
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). However, ruminal NH3 concentrations were substantially different. Basal diet A and diet E (CGM) gave rise to the lowest, and diet B (SSBM) the highest, NH3 in the rumen; ruminal NH3 was intermediate on diets C (ESBM) and D (BM). Lower CP and higher HMSC, which could have enhanced microbial NPN utilization, may account for reduced NH3 concentrations on diet A. Blood urea concentration also was affected by diet (P < 0.01) with that on diet A being lowest. However, ruminal NH3 was not well correlated with blood urea across diets. The relationship of ruminal NH3 to protein degradation will be discussed below. Cows fed diets B and D had higher ruminal concentrations of total VFA than cows fed diets A and E (P < 0.05), with cows fed diet C being intermediate (Table 2). Ruminal VFA concentrations are the net result of production and absorption and do not necessarily reflect fermentation rates (Dijkstra et al., 1993). Moreover, the range of total VFA concentrations was narrow from only 110 to 119 mM, and these small changes may not have been biologically important. Acetate concentration was lower in cows fed diet A than cows fed diets B, C, and D but not different from that on diet E. Ruminal propionate was higher on diet D than on all other diets except B. However, dietary effects on ruminal acetate and propionate concentrations were small, and there were no significant differences in acetate:propionate ratios. Although ruminal butyrate, isovalerate plus 2-methyl butyrate, and valerate were not affected by diet, cows fed diet B (SSBM) had higher isobutyrate than cows fed basal diet A and diets D (BM) and E (CGM); isobutyrate was intermediate on diet C (ESBM). The branched-chain VFA, isobutyrate, isovalerate, and 2-methyl butyrate originate from ruminal catabolism of branched-chain AA and are important growth factors for cellulolytic bacteria (Hoover, 1986). Lower isobutyrate concentrations on diets A and D may have reflected smaller amounts of true protein degradation in the rumen on the basal diet and from BM. However, there were no dietary effects on isovalerate plus 2-methyl butyrate, which represented about twice the proportion of degraded AA as isobutyrate, suggesting that branched-chain VFA were not a sensitive indicator of ruminal protein degradation in this experiment.
Neither ruminal OM and NDF digestibilities nor ruminal passage rates of the large particle and small particle pools were altered by diet (Table 3). However, the similarity of the passage rates for the fluid (Co-EDTA mean = 0.14/h) and small particle phases (Yb = 0.13/h) was surprising. Hristov and Broderick (1996) reported mean liquid and particle passage rates of 0.16 and 0.05/h in cows with DMI of 17 kg/d, but they estimated particulate passage rates using Yb adsorbed onto feed material retained on a screen with 1.15-mm pores. Infusing Yb directly into the rumen probably results in preferential attachment onto small particles that may pass from the rumen more rapidly. Siddons et al. (1985) reported that in sheep continuously infused with Yb-acetate into the rumen >90% of the Yb in rumen, duodenal, and ileal digesta was associated with particulate matter; however, the Yb was not uniformly distributed and Yb concentration per unit DM increased as particle size decreased. Smaller feed particles have shorter mean retention times than larger particles, although small particles still do not pass out of the rumen as rapidly as the liquid phase (Cherney et al., 1991). Generally, apparent total tract digestibility was greatest on the diet B (SSBM): digestibility of DM, NDF, and ADF on this diet was higher than on diets A (basal) and C (ESBM). Also, digestibility of DM was higher on diet B than diet D, and digestibility of both NDF and ADF was higher on diet B than on diet E. This suggests that replacing HMSC with DM, and especially nonforage fiber, from SSBM increased total tract digestibility.
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). However, omasal flows of total N and NAN were greater on the four protein-supplemented diets than on diet A. Although both fractions are representative of true protein, there were greater differences due to diet in omasal TAAN flow than in NAN flow: again diet A was lowest, but TAAN flow was higher for diet D (BM) than for diet B (SSBM), with TAAN flows for diets C (ESBM) and E (CGM) being intermediate. Waltz et al. (1989) reported that cows supplemented with BM had lower TAAN intake but similar TAAN flow to the duodenum when compared to cows supplemented with SSBM. Had DMI been equal in the Waltz et al. (1989) trial, BM may have increased the supply of TAAN to the small intestine. The flow of NAN was lowest for diet A but was not significantly different among diets B to E. In a literature review, Santos et al. (1998) summarized 24 trials where high RUP sources replaced SSBM in the diet and found that NAN flows to the small intestine were increased in only five of these trials. They also reported that the lack of an increase in total NAN flow when high RUP sources replaced SSBM occurred because decreased microbial N flow approximately compensated for increased flow of dietary NAN. In our experiment, although not significantly different (P = 0.08), bacterial NAN flows to the omasum were numerically lower for BM and CGM than for SSBM and ESBM using estimates based on linear programming (Reynal et al., 2002). Lower microbial NAN flow could have counterbalanced any increased dietary NAN flow in cows fed BM and CGM. Interestingly, milk protein yield—which increased with protein supplementation but was not different among the four protein containing diets (Table 2)—reflected the pattern of omasal NAN flow rather than that of TAAN flow. That greater omasal TAAN flow due to greater ruminal BM escape did not give rise to greater protein yield than when SSBM was fed may have been related to differences between these two proteins in RUP digestibility. The NRC (2001) assigned intestinal digestibilities of 93 and 80% for the RUP fractions of SSBM and BM, respectively.
Rates of Protein Degradation
Estimates of extent of ruminal escape of the four protein supplements based on omasal TAAN or NAN flows were computed using results from both spectrophotometric and HPLC methods for purine determination (Reynal et al., 2002). Mean ruminal escapes for SSBM, ESBM, BM, and CGM were, respectively, 27, 45, 60, and 73% (Table 4). A major objective of our study was to determine in vivo ruminal degradation rates of the four supplemental proteins. Extent of protein degradation is the resultant of the rates of passage and degradation, and both must be known to estimate the amount of protein escaping rumen degradation for a given feeding situation. In our study, rates of passage of fluid and small particle phases were estimated using Co and Yb, respectively. Particle size of the protein concentrates was such that they would have been expected to pass with the small particle phase; however, passage rates estimated using Yb and Co were nearly identical (Table 3). The NPN (fraction A) and ADIN (fraction C) contents were small, and the proportions of degradable protein (fraction B) ranged from 92.6 to 98.5% for the four proteins (Table 4). In vivo degradation rates for the fractions B were computed using the passage rates observed for small particles (mean Yb rate = 0.14/h), and assuming a passage rate of 0.06/h, based on TAAN and NAN flows from both purine assays. A discount for the small amount of fraction C in each protein supplement also was included in the computation. Mean rates of ruminal protein degradation for SSBM, ESBM, BM, and CGM were, respectively, 0.417, 0.179, 0.098, and 0.051/h (small particle passage rates), and 0.179, 0.077, 0.042, and 0.026/h (passage rate = 0.06/h). Rates computed using TAAN data tended to be more rapid than those computed using NAN flows (Table 4), and these differences were substantially influenced by the proportions of RUP estimated for basal ingredients from flow of TAAN (30%) and NAN (20%) on basal diet A (Reynal et al., 2002). Values based on TAAN flow likely were less affected by the behavior of non-AA N that may escape the rumen but would not contribute to the AA nutrition of the animal.
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) ranged from 2.5-times (SSBM) to about 10-times (BM) more rapid than those determined using the IIV method (Broderick, 1987). The degradation rate computed for SSBM using the 0.06/h passage rate was more similar to that determined by IIV. However, although slower, degradation rates computed for ESBM, BM, and CGM, using the 0.06/h passage rate, were still two, four, and three times more rapid than those determined by IIV. Previously, we saw very slow IIV degradation rates in all but two of 81 BM samples; the two exceptions were BM that were completely buffer soluble and appeared to be processed without heating (G. A. Broderick, unpublished data). Earlier, we observed an IIV degradation rate of 0.02/h for a sample of CGM (Broderick et al., 1990). That milk yields were not different with feeding of ESBM, BM, or CGM (Table 2) suggested that the IIV degradation rates were too slow for both BM and CGM. The relative in vivo degradation rates, estimated using either small particle passage rates or an assumed rate of 0.06/h, should make these proteins useful standards for comparison with in situ results and for development of in vitro methods. |
SUMMARY |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Received for publication June 14, 2002. Accepted for publication October 14, 2002.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYACKNOWLEDGEMENTSREFERENCES |
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