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Department of Animal Science, Michigan State University, East Lansing, MI
4 To whom correspondence should be addressed. Email: allenm{at}msu.edu.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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KEY WORDS: • rumen • biohyrogenation • fatty acid • kinetics
Dietary fat serves a number of physiological functions in lactating dairy cows and has a large effect on the milk fatty acid (FA)5 profile. Intermediates of FA biohydrogenation are biologically active and modify reproductive efficiency (1), milk fat synthesis (2), and metabolism (3) of cows. In addition, the transfer of these FA to milk fat confers functional food properties on milk that may improve human health (4). Ruminal microorganisms modify the dietary FA profile through isomerization and biohydrogenation of unsaturated FA. The ability to design an absorbed FA profile depends on an understanding of ruminal FA metabolism. Commercially available FA supplements differ in FA profile and ruminal metabolism. Fatty acids bound to calcium ions are assumed to be unavailable for bacterial uptake, but FA become available by dissociation of the calcium ion. Unsaturated FA available for uptake are partially biohydrogenated in the rumen, leading to the production of trans-FA isomers and saturated FA (5).
The extent of biohydrogenation is determined by the characteristics of the fat source, retention time in the rumen, and characteristics of the microbial population (6). Ruminal biohydrogenation may be simply described as a function of the available FA pool size, ruminal retention time, and bacterial hydrogenation capacity. Bacterial hydrogenation capacity depends on the species and concentration of the microbial population, and the ruminal environment. Microbial biohydrogenation is a multistep process, one whose kinetics are not well documented. Beam et al. (7) presented a schematic of lipid metabolism in the rumen that included lipolysis, isomerization, and hydrogenation, resulting in the formation of saturated FA. Hydrogenation of linolenic and linoleic acid results in the formation of trans-monounsaturated FA after the formation of trans-diene intermediates that are rapidly metabolized (5). Biohydrogenation of oleic acid also includes formation of a number of trans-18:1 intermediates (8). Trans-18:1 can be hydrogenated to stearic acid or passed to the duodenum. Numerous in vitro studies have demonstrated rapid biohydrogenation of 18:2 and 18:3, but have noted a lower capacity for trans-18:1 biohydrogenation, especially with increased PUFA (7), decreased pH (9), and decreased dietary fiber (5).
The fatty acid profile reaching the duodenum is determined by the dietary FA profile and metabolism in the rumen. The profile of absorbed FA can be altered to maximize animal efficiency and increase the value of animal products by manipulating FA metabolism in the rumen. An understanding of ruminal FA kinetics is essential to a mechanistic understanding of ruminal FA metabolism. Observing the kinetics of ruminal biohydrogenation is complicated by the movement of FA among pools within the rumen. A descriptive research model of rumen FA metabolism was created to allow calculation of ruminal FA kinetics. The objective of this experiment was to determine the effects of FA supplements differing in FA saturation on ruminal biohydrogenation and duodenal FA flow and provide a more mechanistic description of the process of biohydrogenation.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Cows and treatments. Early lactation (77 ± 8.7 d in milk; mean ± SD) ruminally and duodenally cannulated multiparous Holstein cows (n = 8) from the Michigan State University Dairy Cattle Teaching and Research Center were assigned randomly to replicated 4 x 4 Latin squares in a dose-response arrangement of treatments plus a control. Treatments were diets containing control treatment mix (CON) with no added FA or 25 g/kg dry matter (DM) FA from FS as saturated (SAT; prilled hydrogenated free FA, Energy Booster 100®, MS Specialty Nutrition), an intermediate mixture of saturated and unsaturated (INT), or partially unsaturated FA (UNS; Ca Soaps of LCFA, Megalac-R®, Church and Dwight). Treatment periods were 21 d with the final 11 d used for sample and data collection. Cows were ruminally and duodenally cannulated before calving and assigned randomly to treatment sequence. Duodenal cannulas were a soft gutter type made of Tygon and vinyl tubing (12). For each cow, the duodenum was fistulated distal to the pylorus region before the pancreatic duct, and the cannula was placed between the 10th and 11th ribs as described by Robinson et al. (13). Surgery was performed at the Department of Large Animal Clinical Science, College of Veterinary Medicine, Michigan State University. Immediately before the initiation of the experiment, empty body weight (ruminal digesta removed) was 516 ± 33 kg.
Treatment mixes included limestone and rice hulls to balance for calcium and FA concentration and ground corn as a carrier (Table 1). The base ration was formulated to provide 25 g/kg DM rumen available FA from cottonseed, as would be expected in commercial diets, and treatments were formulated to provide 25 g/kg DM FA from FS. Diets differed slightly in neutral detergent fiber (NDF) concentration due to inclusion of rice hulls in balancing diets. All diets were fed as total mixed rations.
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Ruminal contents were evacuated manually through the ruminal cannula at 1350 (4.5 h after feeding) on d 20, and at 0700 (2 h before feeding) on d 21 of each period. Total ruminal content mass and volume were determined. During evacuation, 10% aliquots of digesta were separated to allow accurate sampling. Aliquots were squeezed through a nylon screen (1-mm pore size) to separate into primarily solid and liquid phases. All samples were frozen immediately at –20°C.
Sample and statistical analysis. Forages and orts were ground with dry ice in a Wiley® mill (6-mm screen; Arthur H. Thomas), subsampled, and lyophilized (Tri-PhilizerTM MP, FTS Systems) for analysis of DM concentration. Dried forage, orts, and FS were reground in a Wiley mill with a 1-mm screen, and dry ice was ground with the FS to prevent fat from melting in the grinder. Concentrates were ground in a cyclone mill with a 2-mm screen (Udy Mill, Seedburo Equipment). Rumen liquid and solid subsamples were lyophilized, ground in a Wiley mill with a 1-mm screen, and recombined according to the original ratio of solid and liquid DM. Duodenal samples were thawed, combined, and filtered into primarily solid and liquid phases using nylon mesh (1-mm pore size) to minimize sampling errors due to segregation of samples into solid and liquid phases. Both phases were weighed, and subsamples were taken from each phase. Liquid and solid subsamples were lyophilized, ground in a Wiley mill with a 1-mm screen, and recombined by weight according to the original ratio of solid and liquid DM. A portion of all samples was placed in a Whirl Pac bag (NASCO) flushed with nitrogen gas and frozen for FA analysis.
Concentrations of all nutrients except DM were expressed as percentages of DM determined by drying at 105°C in a forced-air oven for >8 h. Indigestible NDF was estimated as NDF residue after 240 h of in vitro fermentation (14). Rumen fluid for the in vitro incubations was collected from a nonpregnant dry cow fed only alfalfa hay. Feeds and rumen and duodenal digesta FA were extracted according to Sukhija and Palmquist (15). Ruminal pool sizes (kg) of nutrients were determined by multiplying the concentration of each component by the ruminal digesta DM mass (kg).
Fractional rates of FA biohydrogenation and passage from the rumen were calculated utilizing a model that accounts for transfer of FA among ruminal pools (Fig. 1). The model assumes that unsaturated FA are not oxidized, but are hydrogenated and appear in a less saturated pool, that pool sizes are representative of steady-state conditions, and that biohydrogenation follows first-order kinetics. Ruminal turnover, fractional passage rate (kp) and fractional biohydrogenation rate (kb) for each FA (FAi) pool were calculated using the following equations:
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This equation for extent of biohydrogenation is analogous to those used to calculate ruminal extent of digestion of carbohydrate and protein fractions (16)
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where µ = overall mean, Ci = random effect of cow (i = 1–8), Pj = fixed effect of period (j = 1–4), Tk = fixed effect of treatment (k = 1–4), eijk = residual error.
The period x treatment interaction was evaluated, but was removed from the statistical model when it was not significant (P > 0.10). Period x treatment was not significant for any reported variables. Data points with Studentized Residuals >3.0 were considered outliers and excluded from analysis. Very few observations were excluded and rarely more than 1 per response variable. Preplanned contrasts included the effect of the addition of FS (CON vs. SAT, INT and UNS), the linear effect of increasing the concentration of unsaturated fat [L (SAT vs. UNS)], and the quadratic effect of increasing the concentration of unsaturated fat [Q (INT vs. SAT and UNS)]. The preplanned contrasts do not allow individual comparison of each fat treatment to the control. Protected Least Significant Difference was used for mean separation when treatment was significant. Treatment effects, linear and quadratic responses, and correlations were declared significant at P < 0.05, and tendencies were declared at P < 0.10. One cow developed clinical mastitis during period 3. Period 3 and period 4 data from this cow were excluded because of incomplete recovery of intake.
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RESULTS AND DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Dietary FA are isomerized and biohydrogenated in the rumen by bacteria, and animal response is highly dependent on the resulting FA profile reaching the duodenum. Dietary treatments were selected to maximize the difference in duodenal unsaturated FA flow, especially PUFA, using commercially available products. Saturated FS increased FA flow by increasing dietary FA density with little effect on DMI, whereas UNS did not increase duodenal FA flow compared with CON because DM intake was depressed and dietary FA concentration was slightly lower (11). Within FS, increasing saturated FS linearly increased duodenal FA flow. Duodenal flow of 16:0 and 18:0 was increased by SAT compared with CON, but was not affected by UNS. Increasing saturated FS linearly increased 18:0 flow, with SAT delivering nearly twice the flux of 18:0 to the duodenum as UNS. In contrast, duodenal flow of cis-18:1 did not differ between SAT and CON, but flows were increased by INT and UNS. Increasing unsaturated FS quadratically affected duodenal flow of trans-18:1 FA and linearly increased cis-18:1 FA. Although treatments were expected to drastically change duodenal PUFA flow, treatment did not affect the flow of 18:2 FA reaching the duodenum, and 18:3 FA was linearly decreased by increasing unsaturated FS. Increasing unsaturated FS did not increase the duodenal flow of 18:2 and 18:3 FA because of intake depression and incomplete protection of unsaturated FA. The addition of FS increased the duodenal flow of saturated FA for the SAT treatment and monounsaturated FA for the UNS treatment in this experiment.
Fat supplements increased duodenal 16:0 concentration, and increasing unsaturated FS linearly increased 16:0 (Table 3). Increasing unsaturated FS linearly decreased duodenal 18:0 concentration and linearly increased cis- and trans-18:1, 18:2, and 18:3 concentrations. Using mean separation, SAT did not change, but UNS increased trans-18:1 and 18:2 concentrations relative to CON. Saturated FS decreased the concentration of 18:3, but UNS did not differ from control.
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9-desaturase enzyme is the major contributor to total milk CLA secretion (7.2–9.1 g/d CLA), making rumen contribution of total CLA small compared with de novo synthesis (19). Duodenal flow was calculated using indigestible NDF as described elsewhere (11). Ruminal digestion of diet macronutrient components were within expected ranges and consistent with total tract digestion, further supporting the observed duodenal FA flows (11).
Ruminal pool and turnover. Ruminal FA pool size was determined as the mean of before and after feeding, and is assumed to represent the largest and smallest pools experienced during a day (Table 4). FS increased ruminal pool size of total and 16:0 FA. Saturated FA linearly increased the 18:0 pool relative to UNS, whereas it was not different for UNS compared with CON. The ruminal trans-18:1 pool was linearly increased by UNS, but treatment did not affect the cis-18:1, 18:2, or 18:3 pool size. We were unable to find previous research reporting FA pool sizes in the rumen. However, Abughazaleh et al. (21,22) reported 26–87 g/kg DM trans-11 18:1, 0.9–2.6 g/kg DM cis-9,trans-11 CLA, and up to 1.3 g/kg DM trans-10, cis-11 CLA as a fraction of total FA in rumen grab samples. The fatty acid profile of grab samples from the rumen may not represent the true FA profile of rumen contents because FA may associate differently with liquid and solid fractions of the rumen, biasing the sample. In the current study, trans-18:1 FA ranged from 54 to 84 g/kg of total FA.
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Methods for modeling ruminal carbohydrate digestion (27) can be applied to ruminal FA metabolism. The pool and flux method determines first-order ruminal passage and digestion rates using duodenal flow and ruminal pool size (16). Measurement of FA ruminal pool size and duodenal flux allows calculation of passage rates from the rumen and enables determination of the fractional rate of FA biohydrogenation with the assumption that FA disappearance is from biohydrogenation and not from oxidation or absorption. The model also allows calculation of the extent of biohydrogenation of individual FA calculated from passage and biohydrogenation rates.
Determination of fractional passage and biohydrogenation rates is complicated by heterogeneous pools (e.g., free in rumen, adsorbed to feed, or associated with metal ion), and FA entry rate for some pools is from biohydrogenation in addition to intake. Two simplified models of ruminal FA metabolism were developed to account for the appearance of FA from dietary intake as well as from biohydrogenation and isomerization of other ruminal FA (Fig. 1). Model A (Fig. 1A) assumes biohydrogenation of 18:3 and 18:2 directly to 18:1, and biohydrogenation of 18:1 to 18:0. Model B (Fig. 1B) partitions 18:1 to cis- and trans-18:1 FA pools and assumes biohydrogenation of 18:3, 18:2, and cis-18:1 to trans-18:1, and biohydrogenation of trans-18:1 to 18:0. The 18:3 and 18:2 pools are cis isomer pools only, representing cis-9,cis-12 18:2 and cis-9,cis-12,cis-15 18:3; in the remainder of the paper, 18:3 and 18:2 refer to these FA.
Harfoot and Hazlewood (5) provide a detailed description of FA biohydrogenation, including the production of trans-diene intermediates during biohydrogenation of 18:3 and 18:2. Others have reported low ruminal concentrations of trans-diene FA as previously discussed (21,22). These intermediates have extremely high rates of biohydrogenation as indicated by the large flux of 18:2 and 18:3 biohydrogenation products entering this pool compared with its very small size. In addition, the passage of FA through these intermediate pools cannot be determined without the use of isotope labeling because there are multiple intermediates that may be formed from biohydrogenation of PUFA. The production of trans-diene intermediates and more complex pathways are recognized, but are not feasible within the current experimental procedures.
Biohydrogenation of cis-18:1 to 18:0 was traditionally believed to occur by direct biohydrogenation without formation of intermediates (5). However, increasing ruminal available oleic acid was shown to increase trans-18:1 FA concentration in ruminal digesta (21,28), duodenal flow, (28) and milk (21,28,29). Mosley et al. (8) observed the production of a number of trans-18:1 intermediates during in vitro biohydrogenation of cis-9 18:1. The in vivo observation of trans-18:1 formation from cis-18:1 FA may be from cis-18:1 inhibition of biohydrogenation of PUFA. In vitro production of trans-18:1 from cis-18:1 provides strong evidence for the formation of trans-18:1 intermediates. Verification of direct formation of 18:0 from cis-18:1 requires a kinetic approach because both paths result in the formation of 18:0. The presence of the direct path and the partitioning of biohydrogenation between the direct and indirect routes is not known and may vary with bacterial population and rumen environment. In addition, Proell et al. (30) observed 15% formation of cis-18:1 from labeled trans-9 18:1 in an in vitro batch culture, although it appeared to be a slow reaction, resulting in only a slight enrichment of the cis-18:1 pool after a 48-h incubation. Trans-9 18:1 is expected to be a very small pool in the rumen; combined with the slow rate of synthesis, this may limit appreciable formation of cis-18:1 in vivo. Data concerning the formation of cis-18:1 from trans-18:1 FA present in the rumen in higher concentrations are not available. Reverse reactions that result in very small fluxes have little consequence because of dilution by the large forward flux and are ignored in the current experiment. Model B assumes that all biohydrogenated cis-18:1 enters the trans-18:1 pool.
The models assume that the disappearance of FA is because of biohydrogenation and not absorption or oxidation. Loss of FA from the rumen through absorption and oxidation is often overlooked or considered minimal. Ruminal infusion of radioactively labeled linoleic acid and diversion of duodenal flow showed minimal plasma recovery of label (31). However, ruminal loss of FA is commonly observed in digestion studies; Jenkins (31) reported FA loss in 15 of 47 published studies. Regression analysis predicted an 8% loss of FA intake, and up to a 30% FA loss was reported in the dataset (31). Ruminal loss of FA is associated with diets containing higher FA concentrations (31). Ferlay et al. (32) reported a 14% increase in FA flow with control diet and 36.7 and 21.3% ruminal FA loss with rapeseed FA fed as calcium salts and triglycerides, respectively. Flow marker bias does not explain these occurrences because digestion of other dietary components did not differ and was within expected ranges. Doreau and Chillard (33) discussed evidence supporting FA oxidation and absorption in the rumen, citing in vitro oxidative capacity of ruminal epithelium and possible oxygen transfer across the epithelium to bacteria capable of oxidative metabolism. Observed ruminal loss of FA may also represent methodological limitations because some modified FA and FA present at low concentrations are not detected by current FA analysis procedures. Ruminal digestion or absorption of 18:3, 18:2, and cis-18:1 would inflate the observed biohydrogenation rate of these FA and trans-18:1. Wu et al. (24) proposed the calculation of biohydrogenation as a proportion of total 18-carbon FA to correct for ruminal loss of FA. The same approach could be taken for the current kinetic models by setting 18-carbon FA intake equal to duodenal 18 carbon FA flow by calculating individual 18-carbon FA intake as duodenal total 18 carbon FA flow multiplied by the 18-carbon proportion of the FA fed. However, it is not known from which 18-carbon FA pool the FA are being lost, making it impossible to correct without additional bias. Observed intake and duodenal flow values were used in model parameterization in the present study, and the biohydrogenation rates presented represent the rate of FA loss from the rumen through biohydrogenation and oxidation. Fractional ruminal disappearance of total and 16-carbon FA was not changed by FS saturation, but the disappearance rate of 18-carbon FA tended (P = 0.07) to increase with increasing unsaturated FS (Table 5). Apparent ruminal digestion of 16-carbon FA was increased with FS, and increasing unsaturated FS tended to increase apparent digestion of total and 18-carbon FA (11).
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The rumen FA supplement did not affect fractional passage rate of total FA, 18:2, or 18:3 from the rumen (Table 5). The fractional passage rate of total 18-carbon FA was linearly increased by increasing saturated FS, but did not differ for UNS compared with CON. Increasing unsaturated FS linearly decreased 16:0, 18:0, and total 18-carbon fractional passage rate; it did not affect fractional passage rate of cis-18:1 and affected the fractional passage of trans-18:1 quadratically with the highest value for INT. Differences in FA passage rates within FS may represent different FA pool types, association with different fractions, or associative effects on ruminal passage of other nutrient pools; the fractional passage rate of indigestible NDF from the rumen increased linearly with increasing saturated FS (11).
The fractional rate of biohydrogenation of individual FA in the rumen is an important mechanistic measure because it reports FA biohydrogenation as a proportion of the available FA. Increasing unsaturated FS had no effect on 18:2 biohydrogenation but linearly increased the fractional biohydrogenation rate of 18:3 (Table 5). The increased fractional rate of 18:3 biohydrogenation with UNS identified poor rumen protection by calcium salts. Dissociation of the calcium ion, which makes the FA available for biohydrogenation, is affected by pH and the binding affinity of the FA, with greater dissociation at lower pH, especially with more highly unsaturated FA (34). More highly unsaturated FA are expected to have a higher pKa (34), increasing dissociation of the calcium FA complex as pH decreases. However, ruminal pH in this experiment was moderate and not affected by treatment (data not shown).
The addition of SAT did not affect the rate of biohydrogenation of total 18:1, trans-18:1, and cis-18:1 compared with CON (Table 5). However, increasing unsaturated FS linearly decreased the rate of biohydrogenation of 18:1 and trans-18:1 and tended (P = 0.08) to increase the rate of cis-18:1 biohydrogenation. A potential bias of increased ruminal FA loss with increasing unsaturated FS may have increased the observed fractional rate of 18:1 biohydrogenation. Biohydrogenation of trans-18:1 is considered to be the rate-limiting step of ruminal biohydrogenation (5), allowing trans-18:1 to accumulate in the rumen. Biohydrogenation of trans-18:1 cannot be determined by previous measurements of ruminal FA metabolism, but the model developed determines ruminal production of trans-18:1, allowing calculation of 18:1 biohydrogenation. Increased duodenal flow of trans-18:1 for UNS is from greater intake of unsaturated FA (particularly cis-18:1), increasing in-flow to the trans-18:1 pool from biohydrogenation, and a decreased rate of biohydrogenation and increased passage rate of trans-18:1 from the rumen.
Extent of biohydrogenation. The extent of biohydrogenation of 18:1 and more specifically trans-18:1 was linearly decreased by increasing concentration of unsaturated FS (Table 6). There was no difference in the extent of biohydrogenation of cis-18:1, but biohydrogenation of 18:2 and 18:3 was increased by increasing unsaturated FA. The biohydrogenation index of individual PUFA showed that biohydrogenation of 18:3 tended (P = 0.07) to increase linearly and 18:2 tended (P < 0.10) to be affected quadradically with increasing unsaturated FS.
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The model described increased duodenal trans-18:1 FA flow as the result of increased ruminal production of trans-18:1 and decreased trans-18:1 biohydrogenation. Future ability to predict duodenal FA profile depends on observation and analysis of individual steps of FA metabolism. Modeling biological systems requires assumptions and simplifications of unknown or undeterminable events. Assumptions for this model must be tested in the future. It is our hope that this model will be challenged and improved with new experimental data. More complex models may be developed in the future to model production of individual trans isomers, although flux through such pathways requires isotope labeling.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Supported in part by MS Specialty Nutrition, Dundee IL.
3 Present address: Cornell University, Department of Animal Science, 223 Morrison Hall, Ithaca, NY 14850.
5 Abbreviations used: CLA, conjugated linoleic acid; CON, control treatment; DM, dry matter; DMI, dry matter intake; FA, fatty acids; FS, fat supplement; INT, intermediate treatment; kb, fractional rate of biohydrogenation; kp, fractional rate of passage; L, linear effect of treatment; NDF, neutral detergent fiber; Q, quadratic effect of treatment; SAT, saturated FA treatment; UNS, unsaturated FA treatment.
Manuscript received 8 June 2005. Initial review completed 9 August 2005. Revision accepted 18 November 2005.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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6. Allen MS. Effects of diet on short-term regulation of feed intake by lactating dairy cattle. J Dairy Sci. 2000;83:1598–624.[Abstract]
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8. Mosley EE, Powell GL, Riley MB, Jenkins TC. Microbial biohydrogenation of oleic acid to trans isomers in vitro. J Lipid Res. 2002;43:290–6.
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J. A. Voelker Linton and M. S. Allen Nutrient Demand Affects Ruminal Digestion Responses to a Change in Dietary Forage Concentration J Dairy Sci, October 1, 2007; 90(10): 4770 - 4779. [Abstract] [Full Text] [PDF]
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E. Castaneda-Gutierrez, M. J. de Veth, A. L. Lock, D. A. Dwyer, K. D. Murphy, and D. E. Bauman Effect of Supplementation with Calcium Salts of Fish Oil on n-3 Fatty Acids in Milk Fat J Dairy Sci, September 1, 2007; 90(9): 4149 - 4156. [Abstract] [Full Text] [PDF]
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J. L. Firkins, Z. Yu, and M. Morrison Ruminal Nitrogen Metabolism: Perspectives for Integration of Microbiology and Nutrition for Dairy J Dairy Sci, June 1, 2007; 90(13_suppl): E1 - E16. [Abstract] [Full Text] [PDF]
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A. E. Relling and C. K. Reynolds Feeding Rumen-Inert Fats Differing in Their Degree of Saturation Decreases Intake and Increases Plasma Concentrations of Gut Peptides in Lactating Dairy Cows J Dairy Sci, March 1, 2007; 90(3): 1506 - 1515. [Abstract] [Full Text] [PDF]
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