A meta-analysis of the relationship between milk protein production and absorbed amino acids and digested energy in dairy cattle

Milk protein production is the largest draw on AA supplies for lactating dairy cattle. Prior NRC predictions of milk protein production have been absorbed protein (MP)-based and utilized a first-limiting nutrient concept to integrate the effects of energy and protein, which yielded poor accuracy and precision (root mean squared error (RMSE) > 21%). Using a meta-data set gathered, various alternative equation forms considering MP, absorbed total essential AA (EAA), absorbed individual EAA, and digested energy (DE) supplies as additive drivers of production were evaluated, and all were found to be superior in statistical performance to the first limitation approach (RMSE = 14-15%). Inclusion of DE intake and a quadratic term for MP or absorbed EAA supplies were found to be necessary to achieve intercept estimates (non-productive protein use) that were similar to the factorial estimates of NASEM. The partial linear slope for MP was found to be 0.409, which is consistent with the observed slope bias of -0.34g/g when a slope of 0.67 was used for MP efficiency in a first-limiting nutrient system. Replacement of MP with the supplies of individual absorbed EAA expressed in g/d and a common quadratic across the EAA resulted in unbiased predictions with improved statistical performance as compared with MP-based models. Based on Akaike's Information Criterion (AIC) and biological consistency, the best equations included absorbed His, Ile, Lys, Met, Thr, the non-essential AA, and individual DE intakes from fatty acids, neutral detergent fiber, residual organic matter, and starch. Several also contained a term for absorbed Leu. These equations generally had RMSE of 14.3% and a concordance correlations (CCC) of 0.76. Based on the common quadratic and individual linear terms, milk protein response plateaus were predicted at approximately 320 g/d of absorbed His, Ile, and Lys; 395 g/d of absorbed Thr; 550 g/d of absorbed Met; and 70 g/d of absorbed Leu. Therefore, responses to each except Leu are almost linear throughout the normal in vivo range. De-aggregation of the quadratic term and parsing to individual absorbed EAA resulted in non-biological estimates for several EAA indicating over-parameterization. Expression of the EAA as g/100 g of total absorbed EAA or as ratios of DE intake and using linear and quadratic terms for each EAA resulted in similar statistical performance, but the solutions had identifiability problems and several non-biological parameter estimates. The use of ratios also introduced nonlinearity in the independent variables which violates linear regression assumptions. Further screening of the global model using absorbed EAA expressed as g/d with a common quadratic using an all-models approach, and exhaustive cross-evaluation indicated the parameter estimates for body weight, all 4 DE terms, His, Ile, Lys, Met, and the common quadratic term were stable, while estimates for Leu and Thr were known with less certainty. Use of independent and additive terms and a quadratic expression in the equation results in variable efficiencies of conversion. The additivity also provides partial substitution among the nutrients. Both of these prevent establishment of fixed nutrient requirements in support of milk protein production.

Ability of three dairy feed evaluation systems to predict postruminal outflows of amino acids in dairy cows: A meta-analysis

Adequate prediction of postruminal outflows of essential AA (EAA) is the starting point of balancing rations for EAA in dairy cows. The objective of this meta-analysis was to compare the performance of 3 dairy feed evaluation systems (National Research Council [NRC], Cornell Net Protein and Carbohydrate System version 6.5.5 [CNCPS], and National Academies of Sciences, Engineering and Medicine [NASEM]) to predict EAA outflows (Trp was not tested). The data set included a total of 354 treatment means from 70 duodenal and 24 omasal studies. To avoid Type I error, mean and linear biases were considered of concern if statistically significant and representing > 5.0% of the observed mean. Analyses were conducted on raw observed values and on observations adjusted for the random effect of study. The analysis on raw data indicates the ability of the feed evaluation system to predict absolute values whereas the analysis on adjusted values indicates its ability to predict responses of EAA outflows to dietary changes. For the prediction of absolute values (based on raw data), NRC underpredicted outflows of all EAA, from 5.3 to 8.6% of the observed mean (%obs.mean) except for Leu, Lys, and Val; NASEM overpredicted Lys (10.8%obs.mean); and CNCPS overpredicted Arg, His, Lys, Met, and Val (5.2 to 26.0%obs.mean). No EAA had a linear bias of concern with NASEM, followed by NRC for His (6.8%obs.mean), and CNCPS for all EAA (5.6 to 12.2%obs.mean) except Leu, Phe, and Thr. On the other hand, for the prediction of responses to dietary changes (based on adjusted data), NRC had 2 EAA presenting a linear bias of concern, followed by NASEM and CNCPS with 4 and 6 EAA, respectively. Predictions of His showed a linear bias of concern (5.3 to 9.6%obs.mean) with the 3 feed evaluation systems. Measured chemistry of crude protein and EAA were reported for 1 or more feed ingredients of the ration in 36% of the studies, and resulted in decreased linear biases in the 3 feed evaluation systems. The difference in mean biases of Met outflows was systematically positive when comparing omasal versus duodenal studies. Predictions of Met outflows with NRC had a higher concordance correlation coefficient in duodenal (used to develop NRC equations) versus omasal studies, whereas the opposite was observed with CNCPS, the latter showing the lowest mean bias for Met in omasal sampling studies. The 30% difference in Met mean biases between sampling sites appeared related to a similar difference found for observed Met versus NAN outflows between duodenal and omasal studies, which is independent of predictions. In conclusion, NRC and NASEM yielded accurate predictions of EAA outflows, with a small superiority of NASEM to predict absolute values, and slight superiority of NRC to predict the responses to dietary changes. In comparison, CNCPS may present mean and linear biases of concern for many EAA. Moreover, it remains to determine which sampling site is more representative of the true supply of EAA to the cows.

Ability of three dairy feed evaluation systems to predict postruminal outflows of nitrogenous compounds in dairy cows: A meta-analysis

Adequate prediction of postruminal outflow of protein fractions is the starting point for the determination of metabolizable protein supply in dairy cows. The objective of this meta-analysis was to compare the performance of 3 dairy feed evaluation systems (National Research Council [NRC], Cornell Net Protein and Carbohydrate System [CNCPS], and National Academies of Sciences, Engineering and Medicine [NASEM]) to predict outflows (g/d) of nonammonia nitrogren (NAN), microbial N (MiN), and nonammonia nonmicrobial N (NANMN). Predictions of rumen degradabilities (% of nutrient) of protein (RDP), NDF, and starch were also evaluated. The data set included 1,294 treatment means from 312 digesta flow studies. The 3 feed evaluation systems were compared using the concordance correlation coefficient (CCC), the ratio of root mean square prediction error (RMSPE) on standard deviation of observed values (RSR), and the slope between observed and predicted values. Mean and linear biases were deemed biologically relevant and are discussed if higher than a threshold of 5% of the mean of observed values. The comparisons were done on observed values adjusted or not for the study effect; the adjustment had a small effect on the mean bias but the linear bias reflected a response to a dietary change rather than absolute predictions. For the absolute predictions of NAN and MiN, CNCPS had the best-fit statistics (8% greater CCC; 6% lower RMSPE) without any bias; NRC and NASEM underpredicted NAN and MiN, and NASEM had an additional linear bias indicating that the underprediction of MiN increased at increased predictions. For NANMN, fit statistics were similar among the 3 feed evaluation systems with no mean bias; however, the linear bias with NRC and CNCPS indicated underprediction at low predictions and overprediction at elevated predictions. On average, the CCC were smaller and RSR ratios were greater for MiN versus NAN indicating increased prediction errors for MiN. For NAN responses to a dietary change, CNCPS also had the best predictions, although the mean bias with NASEM was not biologically relevant and the 3 feed evaluation systems did not present a linear bias. However, CNCPS, but not the 2 other feed evaluation systems, presented a linear bias for MiN, with responses being overpredicted at increased predictions. For NANMN, responses were overpredicted at increased predictions for the 3 feed evaluation systems, but to a lesser extent with NASEM. The site of sampling had an effect on the mean bias of MiN and NANMN in the 3 feed evaluation systems. The mean bias of MiN was higher in omasal than duodenal studies in the 3 feed evaluation systems (from 55 to 61 g/d) and this mean bias was twice as large when 15N labeling was used as a microbial marker compared with purines. Such a difference was not observed for duodenal studies. The reasons underlying these systematic differences are not clear as the type of measurements used in the current meta-analysis does not allow to delineate if one site or one microbial marker is yielding the "true" postruminal N outflows. Rumen degradabilities of protein was underpredicted with CNCPS, and RDP responses to a dietary change was underpredicted by the 3 feed evaluation systems with increased RDP predictions. Rumen degradability of NDF was underpredicted and had poor fit statistics for NASEM compared with CNCPS. Fit statistics were similar between CNCPS and NASEM for rumen degradability of starch, but with an underprediction of the response with NASEM and absolute values being overpredicted with CNCPS. Multivariate regression analyses showed that diet characteristics were correlated with prediction errors of N outflows in each feed evaluation system. Globally, compared with NAN and NANMN, residuals of MiN were correlated with several moderators in the 3 feed evaluation systems reflecting the complexity to measure and model this outflow. In addition, residuals of NANMN were correlated positively with RDP suggesting an overestimation of this parameter. In conclusion, although progress is still to be made to improve equations predicting postruminal N outflows, the current feed evaluation systems provide sufficient precision and accuracy to predict postruminal outflows of N fractions.

Review: How the efficiency of utilization of essential amino acids can be applied in dairy cow nutrition

How the efficiency of utilization of essential amino acids (EffUEAA) can be applied in dairy cow nutrition is presented in this review. The concept of EffUEAA proposed by the National Academies of Sciences, Engineering and Medicine (NASEM, 2021) is first detailed. It represents the proportion of the metabolisable essential amino acids (mEAA) supply used to support protein secretions and accretions (scurf, metabolic fecal, milk and growth). For these processes, the efficiency of each individual EAA is variable, and considered to vary similarly for all the protein secretions and accretions. The anabolic process of gestation is ascribed to a constant efficiency (33%), whereas the efficiency of endogenous urinary loss (EndoUri) is set at 100%. Therefore, the NASEM model EffUEAA was calculated as the sum of EAA in the true protein of secretions and accretions divided by the available EAA (mEAA - EndoUri - gestation net true protein/0.33). In this paper, the reliability of this mathematical calculation was tested through an example where the experimental efficiency of His was calculated assuming that liver removal represents catabolism. The NASEM model and experimental efficiencies were in the same range and varied in similar manner. Assuming that the NASEM model EffUEAA reflects EAA metabolism in the dairy cow, its different applications were examined. In NASEM, target efficiencies were determined for each EAA: 75, 71, 73, 72, 73, 60, 64, 86 and 74% for His, Ile, Leu, Lys, Met, Phe, Thr, Trp, and Val, respectively. From these, recommendations for mEAA supply can be calculated as: [(secretions + accretions)/(target EffUEAA × 0.01) + EndoUri + gestation/0.33], assuming energy supply is adequate. In addition to NASEM propositions, equations to predict EffUEAA with precision and accuracy are detailed, using the ratio of (mEAA-EndoUri) to digestible energy intake, in a quadratic model that includes days in milk. Moreover, milk true protein yield predictions from predicted EffUEAA or efficiency of utilization of metabolisable protein are better than those from the multivariate equation of NASEM (2021) and superior to those predicted with a fixed efficiency. Finally, either the NASEM model or the predicted EffUEAA can be used to assess the responsiveness of a ration to supplementation with a single EAA. If the EffUEAA of the EAA to supplement is higher than the target EffUEAA, while the EffUEAA of the other EAA are lower than the target value, this suggests a potential improvement in milk true protein yield to supplementation with this EAA.

Omasal flow of nonstructural carbohydrates and nitrogenous compounds in lactating dairy cows fed diets containing timothy cut in the afternoon or morning

Shifting the cutting of grass from morning to afternoon has been shown to increase the concentration of nonstructural carbohydrates (NSC) in forages. We compared the effects of diets (66:34, forage:concentrate ratio) containing a mix (% of the diet dry matter) of baleages (46.5%) and silages (19.3%) harvested from timothy cut in the afternoon (p.m.-cut TIM diet) or morning (a.m.-cut TIM diet) on omasal flows of NSC and nitrogenous fractions, ruminal and total-tract digestibilities of nutrients, plasma concentration of AA, and milk yield and composition. Eight ruminally cannulated Holstein cows averaging (mean ± standard deviation) 31.4 ± 6.13 kg/d of milk, 136 ± 17.0 d in milk, and 611 ± 66.4 kg of body weight in the beginning of the experiment were used in a crossover design with 21-d periods (14 d for diet adaptation and 7 d for data and sample collection). Intake of total ethanol-soluble carbohydrates (TESC; +150 g/d), starch (+129 g/d), and total NSC (TESC plus starch = +278 g/d) was greater with feeding the p.m.- than the a.m.-cut TIM diet. Likewise, the apparent ruminal digestibilities of TESC (+149 g/d), starch (+167 g/d), and total NSC (+316 g/d) increased in the p.m.-cut TIM diet. Diets, however, had no effect on the omasal flows and apparent ruminal and total-tract digestibilities of dry matter and organic matter. Intake of N increased in cows fed the p.m.- versus the a.m.-cut TIM diet (562 and 528 g/d, respectively) despite no effect of diets on dry matter intake. Diets did not affect the omasal flows of total nonammonia N, total bacterial nonammonia N, nonammonia and nonbacterial N, and individual AA, and the efficiency of microbial protein synthesis in the rumen. Contrarily, supply of rumen-degradable protein increased (+9.2%) in cows fed the p.m.-cut TIM diet, with this response driven by the 6.4% increase in N intake. Plasma concentrations of essential and nonessential AA followed the omasal flow of AA and were not changed by diets. Feeding the p.m.- versus the a.m.-cut diet significantly increased yields of 4% fat-corrected milk and milk fat, and tended to increase energy-corrected milk, milk true protein, and milk lactose yields. Overall, feeding the p.m.-cut TIM diet to mid-lactation dairy cows did not improve microbial protein synthesis and omasal flow of AA, and these responses were in line with the lack of a treatment effect on dry matter intake.

Predicting ruminally undegraded and microbial protein flows from the rumen

The objectives of the present work were (1) to identify the cause of the linear bias in predictions of rumen-undegradable protein (RUP) content of feeds, and devise methods to remove the bias from prediction equations, and (2) to further explore the impact of rumen-degradable protein (RDP) on microbial N (MiN) outflow from the rumen. The kinetic model used by NRC (2001), which is based on protein fractionation and rates of degradation (Kd) and passage (Kp), displays considerable slope bias (-0.30 kg/kg), indicating parameter or structural problems. Regressing Kp by feed class and a static adjustment factor for the in situ-derived Kd on observed RUP flows completely resolved the slope bias problem, and the model performed significantly better than models using unadjusted Kd and marker-based Kp. The Kd adjustment was 3.82%/h, which represents approximately a 50% increase in rates of degradation over the in situ values, indicating that in situ analyses severely underestimate true rates of protein degradation. The Kp for concentrate-derived protein was 5.83%/h, which was slightly less than the marker-predicted rate of 6.69%/h. However, the derived forage protein rate was 0.49%/h, which was considerably less than the marker-based rate of 5.07%/h. Compartmental analysis of data from a single study corroborated the regression analysis, indicating that a 25% reduction in the overall passage rate and an 87% increase in the rate of degradation were required to align ruminal N pool sizes and the extent of protein degradation with the observed data. Therefore, one must conclude that both the in situ-derived degradation rates and the marker-based particle passage rates are biased relative to protein passage and cannot be used directly to predict RUP outflow from the rumen. The effects of RDP supply on microbial nitrogen (MiN) flow were apparent when intakes of individual nutrients were offered but not when DM intake and individual nutrient concentrations were offered, due to collinearity problems. Microbial N flow from the rumen was found to be linearly related to ruminally degraded starch, ruminally degraded neutral detergent fiber (NDF), RDP, and forage NDF intakes; and quadratically related to residual OM intake. More complicated models containing 2- and 3-way interactions among nutrients were also supported by the data. Independent MiN responses to RDP, ruminally degraded starch, and ruminally degraded NDF aligned with the expected responses to each of those nutrients. Nonlinear representations of MiN were found to be inferior to the linear models. Despite using unbiased predictions of RUP and MiN as drivers of AA flows, predictions of Arg, His, Ile, and Lys flow exhibited linear slope bias relative to the observed data, indicating that representations of the AA composition of the proteins may be biased or the observed data are biased. This is an improvement over the NRC (2001) predictions, where bias adjustments were required for all of the essential AA. Despite the bias for 4 AA flows, the revised prediction system was a substantial improvement over the prior work.

Predictions of ruminal outflow of essential amino acids in dairy cattle

The objective of this work was to update and evaluate predictions of essential AA (EAA) outflows from the rumen. The model was constructed based on previously derived equations for rumen-undegradable (RUP), microbial (MiCP), and endogenous (EndCP) protein outflows from the rumen, and revised estimates of ingredient composition and EAA composition of the protein fractions. Corrections were adopted to account for incomplete recovery of EAA during 24-h acid hydrolysis. The predicted ruminal protein and EAA outflows were evaluated against a data set of observed values from the literature. Initial evaluations indicated a minor mean bias for non-ammonia, non-microbial nitrogen flow ([RUP + EndCP]/6.25) of 16 g of N per day. Root mean squared errors (RMSE) of EAA predictions ranged from 26.8 to 40.6% of observed mean values. Concordance correlation coefficients (CCC) of EAA predictions ranged from 0.34 to 0.55. Except for Leu, all ruminal EAA outflows were overpredicted by 3.0 to 32 g/d. In addition, small but significant slope biases were present for Arg [2.2% mean squared error (MSE)] and Lys (3.2% MSE). The overpredictions may suggest that the mean recovery of AA from acid hydrolysis across laboratories was less than estimates encompassed in the recovery factors. To test this hypothesis, several regression approaches were undertaken to identify potential causes of the bias. These included regressions of (1) residual errors for predicted EAA flows on each of the 3 protein-driven EA flows, (2) observed EAA flows on each protein-driven EAA flow, including an intercept, (3) observed EAA flows on the protein-driven EAA flows, excluding an intercept term, and (4) observed EAA flows on RUP and MiCP. However, these equations were deemed unsatisfactory for bias adjustment, as they generated biologically unfeasible predictions for some entities. Future work should focus on identifying the cause of the observed prediction bias.

Does blending canola meal with other protein sources improve production responses in lactating dairy cows? A multilevel mixed-effects meta-analysis

The objective of this meta-analysis was to evaluate the effect of blending canola meal (CM) with other protein sources on production responses in lactating dairy cows. To evaluate this effect, a data set was assembled containing 22 studies reporting at least 3 isonitrogenous dietary treatments (total of 74 treatment means). Each study needed to report 1 diet with CM <0.3 kg/d, 1 or more diets consisting of CM blended with another protein source, and 1 diet with CM as the main protein source in the protein supplement (>85%). The crude protein (CP) concentration of CM averaged 37.4 ± 3.09% (dry matter basis), and the predictor of interest was the intake of CP from CM, which averaged 0.46 ± 0.413 kg/d among studies. The maximal CP from CM ranged from 0.47 to 1.55 kg/d among studies. The quadratic relationship between CP from CM and responses in milk true protein concentration was significant, the maximum response (3.19%) being reached at 0.79 kg of CP from CM; the quadratic relationships were not significant for the other dependent variables. Responses in dry matter intake; yields of milk, energy-corrected milk, and milk true protein; and apparent N efficiency were related positively to CP from CM and negatively for responses in milk fat and milk urea N concentrations. Remembering that diets were isonitrogenous within studies, this indicates no nutritional benefit of blending CM with another protein source. Taken together, the results indicate that the whole-body N utilization efficiency by the dairy cow improved and that more dietary protein was used to synthesize milk protein when CM was used as the sole protein source in the protein supplement up to 1.55 kg/d.

Performance and nitrogen use efficiency in mid-lactation dairy cows fed timothy cut in the afternoon or morning

Shifting cutting from morning to afternoon has been shown to increase the concentration of nonstructural carbohydrates in forages. We hypothesized that, compared with a total mixed ration containing timothy baleage and silage cut in the morning (a.m.-cut TIM), a total mixed ration containing timothy baleage and silage cut in the afternoon (p.m.-cut TIM) would improve animal performance and N use efficiency in mid-lactation Holstein cows due to enhanced supply of ruminal fermentable energy. The objective of this study was to compare the effects of p.m.- versus a.m.-cut TIM on milk yield, concentrations and yields of milk components, ruminal metabolism, and plasma concentrations of AA in mid-lactation Holstein cows. Ten (6 ruminally cannulated) primiparous cows averaging 139±13 d in milk and 550±56 kg of body weight, and 6 (2 ruminally cannulated) multiparous cows averaging 128±11 d in milk and 632±57 kg of body weight at the beginning of the experiment, were used in a crossover design. Each period lasted 21 d with 14 d for diet adaptation and 7 d for data and sample collection. The concentration of nonstructural carbohydrates (water-soluble carbohydrates plus starch) was numerically greater in the p.m.- versus the a.m.-cut TIM and averaged 13.2±1.06% and 12.2±1.13%, respectively. Treatment × parity effects were observed for milk urea N, feed efficiency, and milk N efficiency, whereas parity effects were observed for nutrient intake, milk yield, and plasma concentration of several essential and nonessential AA. Intakes of dry matter (19.3 versus 18.6 kg/d) and nonstructural carbohydrates (2.56 versus 2.31 kg/d), and yields of 4% fat-corrected milk (23.1 versus 22.2 kg/d), energy-corrected milk (25.0 versus 24.1 kg/d), milk fat (0.91 versus 0.88 kg/d), and milk protein (0.77 versus 0.73 kg/d) were all greatest with feeding p.m.-cut TIM. Milk yield (23.5 versus 22.7 kg/d) tended to increase in cows fed p.m.-cut TIM. The ruminal fermentation profiles and plasma concentrations of AA were mostly unaffected by treatments. However, ruminal valerate (1.01 versus 1.17 mol/100 mol) and plasma Gly (172 versus 188 µM) were lowest with feeding p.m.-cut TIM. Overall, feeding mid-lactation dairy cows a total mixed ration that consisted of p.m.-cut timothy baleage and silage significantly increased dry matter intake and yields of milk, milk fat, and milk protein.

Effects of grain source, grain processing, and protein degradability on rumen kinetics and microbial protein synthesis in Boer kids

Microbial protein synthesis in the rumen would be optimized when dietary carbohydrates and proteins have synchronized rates and extent of degradation. The aim of this study was to evaluate the effect of varying ruminal degradation rate of energy and nitrogen sources on intake, nitrogen balance, microbial protein yield, and kinetics of nutrients in the rumen of growing kids. Eight Boer goats (38.2 ± 3.0 kg) were used. The treatments were arranged in a split-plot Latin square design with grain sources (barley or corn) forming the main plots (squares). Grain processing methods and levels of protein degradability formed the subplots in a 2 × 2 factorial arrangement for a total of 8 dietary treatments. The grain processing method was rolling for barley and cracking for corn. Levels of protein degradability were obtained by feeding untreated soybean meal (SBM) or heat-treated soybean meal (HSBM). Each experimental period lasted 21 d, consisting of a 10-d adaptation period, a 7-d digestibility determination period, and a 4-d rumen evacuation and sampling period. Kids fed with corn had higher purine derivatives (PD) excretion when coupled with SBM compared with HSBM and the opposite occurred with barley-fed kids ( ≤ 0.01). Unprocessed grain offered with SBM led to higher PD excretion than with HSBM whereas protein degradability had no effect when processed grain was fed ( ≤ 0.03). Results of the current experiment with high-concentrate diets showed that microbial N synthesis could be maximized in goat kids by combining slowly fermented grains (corn or unprocessed grains) with a highly degradable protein supplement (SBM). With barley, a more rapidly fermented grain, a greater microbial N synthesis was observed when supplementing a low-degradable protein (HSBM).

Relation of net portal flux of nitrogen compounds with dietary characteristics in ruminants: a meta-analysis approach

Decrease of N intake (NI) with the aim of increasing efficiency of N utilization and decreasing the negative environmental effects of animal production requires assessment of the forms in which N is absorbed. A meta-analysis was conducted on 68 publications (90 experiments and 215 treatments) to study the effect of NI on net portal appearance (NPA) of nitrogenous nutrients [amino acids (AA), ammonia, and urea] in ruminants. In addition, the effect of several dietary energy and protein factors on this relationship was investigated. These factors were: dry matter intake; proportion of concentrate; diet concentrations and intakes of nonfiber carbohydrates and neutral detergent fiber (NDF); diet concentrations of total digestible nutrients (TDN) and crude protein; rumen-degradable protein and rumen-undegradable protein, as percent dry matter or percent crude protein. The effect of species and physiological stage was also investigated. Within-experiment analyses revealed that the NPA of AA-N and ammonia-N increased linearly, whereas the NPA of urea-N decreased (or recycling of urea-N increased) linearly with NI. Besides NI, many significant covariates could be introduced in each NPA model. However, only TDN and neutral detergent fiber intake (NDFi) were common significant covariates of NI in each NPA model. In this database, ruminants converted 60% of incremental NI into NPA of AA-N with no species effect on that slope. However, at similar NI, TDN, and NDFi, sheep absorbed more AA-N than did cattle and dairy cows. On the other hand, species tended to affect the slope of the relationship between NPA of ammonia-N and NI, which varied from 0.19 for the sheep to 0.38 for dairy cows. On average, the equivalent of 11% of incremental NI was recycled as urea-N to the gut through the portal-drained viscera, which excludes salivary contribution, and no species difference was detected. Overall, at similar TDN and NDFi, sheep and cattle increased their NPA of AA-N relative to NI increment by a similar magnitude. The higher absorption of AA-N observed in sheep compared with cattle, at similar NI, TDN, and NDFi, might result from lower losses of AA through portal-drained viscera metabolism.

Effects of lasalocid or monensin on in situ biohydrogenation of flaxseed and sunflower seed unsaturated fatty acids

The objective of this study was to compare the effects of two ionophores, lasalocid and monensin (24 mg kg-1 dry matter), on in situ biohydrogenation of C18:2 and C18:3 from whole sunflower seed and flaxseed, respectively. There were no treatment effects on biohydrogenation of C18:2. Ionophore supplementation did not affect the lag time, but monensin decreased the rate of biohydrogenation of C18:3 compared with control and lasalocid. Key words: Ionophore, monensin, lasalocid, biohydrogenation (in situ), sunflower seed, flaxseed

Effects of the Method of Conservation of Timothy on Nitrogen Metabolism in Lactating Dairy Cows

Six ruminally and duodenally cannulated lactating primiparous Holstein cows were used to study the effects of different methods of conservation of timothy on N metabolism. Cows were assigned randomly to 2 replicated 3 x 3 Latin squares (35-d periods). Because of missing data from 2 cows, data were analyzed as a 3 x 4 Youden square. Diets contained a similar concentrate (44% of total ration on a dry matter basis) plus first-cut timothy conserved as hay, or as restrictively (formic) or extensively fermented silage (inoc). Crude protein contents were 10.4, 13.6, and 14.8% for hay, formic, and inoc, respectively. Hay and formic had a high soluble carbohydrate content (> or =8.0% of dry matter) and formic and inoc had a high soluble protein content (> or =8.0% of dry matter). Haying and restricting fermentation resulted in increased efficiency of partition to milk N (30.9, 28.2, 24.7% of N intake for hay, formic, and inoc, respectively). Despite a 14% lower N intake with hay, no effects of treatments were detected on microbial protein synthesis and apparent intestinal digestion of essential AA. Haying reduced feed protein degradation in the rumen, whereas this effect was not observed when restricting fermentation in the silage. Haying and restricting fermentation induced a lipogenic fermentation pattern in the rumen (4.55, 4.23, and 3.78 ratio of acetate to propionate for hay, formic, and inoc), but no effects on milk fat yield and plasma glucose were observed. Whole-body protein metabolism was unaffected by treatments.

Left ventricular end-diastolic pressure is a predictor of mortality in cardiac surgery independently of left ventricular ejection fraction

BACKGROUND

Several risk factors have been shown to increase mortality in cardiac surgery. However, the importance of left ventricular end-diastolic pressure (LVEDP) as an independent risk factor before cardiac surgery is unclear. Method. This observational study investigated 3024 consecutive adult patients who underwent cardiac surgical procedures at the Montreal Heart Institute from 1996 to 2000. The primary outcome was in-hospital mortality with 99 deaths (3.3%) among these patients.

RESULTS

Of the 35 variables subjected to univariate analysis, 23 demonstrated a significant association with mortality. Stepwise multivariate logistic regression identified LVEDP as an independent predictor of mortality after cardiac surgery. The area under the receiver operating characteristic curve of the model predicting mortality was 0.85.

CONCLUSIONS

Elevated LVEDP is an independent predictor of mortality in cardiac surgery. This variable is independent of left ventricular ejection fraction.

In situ degradation of timothy conserved as restrictively or extensively fermented silage or as hay

The objective of this study was to determine the effects of three modes of preserving timothy (Phleum pratense L.) on in situ degradation of dry matter (DM), organic matter, crude protein (CP), neutral detergent fiber and acid detergent fiber. First-cut timothy was conserved as hay, as restrictively fermented silage (formic acid 85% applied at 6 L t-1 of fresh crop), or as extensively fermented silage (inoculation with a mixture of Lactobacillus plantarum and Pediococcus cerevisiae yielding 1.25 × 1011 total CFU t-1). Six mid-lactating Holstein cows, previously adapted to the tested forage, were used to study the in situ degradation of each forage type (two cows per forage type). Triplicate bags were filled with 3.5 g DM equivalent of chopped wet forage and incubated in the rumen during 2, 4, 8, 12, 24, 48, or 72 h. Zero-hour bags were not inserted in the rumen. The conservation method affected in situ degradation of CP. Haying decreased (P ≤0.03) soluble protein (fraction a), degradation rate (c), lag time and effective degradability of CP in the rumen, and increased (P < 0.01) potentially degradable protein (fraction b) when compared with ensiling. Restricting fermentation in silage decreased (P < 0.01) fraction a and increased (P = 0.01) CP fraction b when compared with extensively fermented silage. Results indicate that conserving timothy as hay significantly decreases solubility, rate and extent of protein degradation in the rumen when compared with ensiling. Results also show that solubilization of protein in silage can be reduced by restricting fermentation with formic acid as compared with promoting fermentation with a bacterial inoculant, without a concurrent decrease in the extent of protein degradation in the rumen. Key words: In situ degradation, timothy, hay, restrictively fermented silage, extensively fermented silage, formic acid, inoculation

Effects of the forage to concentrate ratio on B-vitamin concentrations in different ruminal fractions of dairy cows

Two studies were undertaken to verify the effect of the forage to concentrate ratio of the diet on B-vitamin concentrations in ruminal contents. In Study 1, eight primiparous and eight multiparous cows were used in a cross-over design, and concentrations of biotin, folates and vitamin B12 were determined in ruminal fluid and plasma of cows fed a high-forage (HF; 58:42 forage to concentrate ratio; DM basis) or a low-forage (LF; 37:63 forage to concentrate ratio; DM basis) diet. In Study 2, six ruminally cannulated lactating cows were used in a cross-over design to evaluate the effects of forage to concentrate ratio (HF = 60:40; LF = 40:60; DM basis) on concentrations of seven B-vitamins in the particle-free fluid and in both liquid- and solid-associated bacteria. Results showed that B-vitamins were present mainly in the bacterial fractions of the ruminal content, while only limited amounts were found in the surrounding fluid. A change in the forage to concentrate ratio had a greater effect on vitamin concentration in the bacteria associated with the solid fraction than in those present in the liquid portion of the rumen. The most noticeable effects of a low forage diet were an increase in riboflavin, but a decrease in true vitamin B12 concentrations in solid-associated bacteria as well as a decrease in biotin concentration in particle-free fluid. In conclusion, it appears that ruminal B-vitamin concentration is altered by changes in the forage to concentrate ratio, which suggests that the supply of vitamins to dairy cows is influenced by diet composition. Key words: B-vitamins, dairy cow, ruminal fractions, forage to concentrate ratio

Effects of Feeding Micronized and Extruded Flaxseed on Ruminal Fermentation and Nutrient Utilization by Dairy Cows

Four lactating Holstein cows with ruminal and duo-denal cannulas were used in a 4 x 4 latin square design to determine the effects of feeding heat-treated flaxseed on ruminal fermentation and site and extent of nutrient utilization. Four diets were formulated: a control diet with no flaxseed, a raw flaxseed diet (RF), a micronized flaxseed diet (MF), and an extruded flaxseed diet (EF). Flaxseed diets contained 12.6% flaxseed (dry matter [DM] basis). Ruminal pH, NH3 N, and total concentration of volatile fatty acids were not affected by dietary treatments. However, feeding flaxseed decreased the molar proportion of acetate and increased that of propionate. Flaxseed supplementation had no effect on ruminal digestion of DM, organic matter (OM), neutral detergent fiber (NDF), crude protein (CP), fatty acids (FA), and gross energy. However, ruminal digestion of acid detergent fiber (ADF) was lower for cows fed the flaxseed diets than for cows fed the control diet. Feeding flaxseed tended to increase post-ruminal and total tract digestibilities of DM, OM, NDF, and gross energy. Feeding heat-treated flaxseed diets relative to RF had no effect on ruminal, post-ruminal, and total tract nutrient digestibilities. Cows fed EF had higher ruminal and lower post-ruminal digestibilities of DM, OM, ADF, CP, and FA than cows fed MF. However, total tract digestibilities were similar for the 2 heat treatments. It was concluded that flaxseed supplementation improved total tract nutrient utilization with no adverse effects on ruminal fermentation. Extrusion failed to protect flaxseed from ruminal digestion. However, micronization can be used to increase the ruminal undegraded protein value of flaxseed.

A phase II dose-response study of hemoglobin raffimer (Hemolink) in elective coronary artery bypass surgery

BACKGROUND

We performed this study to determine the dose-response of hemoglobin raffimer administered in conjunction with intraoperative autologous donation in patients undergoing coronary artery bypass grafting surgery. A secondary objective was to evaluate hemoglobin raffimer for reducing the incidence of allogeneic red blood cell transfusions.

METHODS

This was a phase II, single-blind, multicenter, placebo-controlled, open-label study. Patients undergoing coronary artery bypass grafting with cardiopulmonary bypass and intraoperative autologous donation were randomized to receive a single dose of hemoglobin raffimer or control (10% pentastarch). Patients were sequentially enrolled in a dose block of 250, 500, 750, and 1000 mL.

RESULTS

Sixty patients received hemoglobin raffimer (n = 30) or control (n = 30). Hemoglobin raffimer was well tolerated. Most (98%) adverse events were mild or moderate in severity. There was an expected dose-dependent increase in the incidence of blood pressure increases and jaundice in hemoglobin raffimer-treated patients. In a dose-pooled analysis of hemoglobin raffimer versus control, increased blood pressure (43% vs 17%), nausea (37% vs 33%), and atrial fibrillation (37% vs 17%) were the most frequently reported adverse events. All serious adverse events were considered unrelated or unlikely to be related to study drug. No hemoglobin raffimer-treated patient required an intraoperative allogeneic red blood cell transfusion, compared with 5 (17%) pentastarch-treated patients (P =.052). This advantage of hemoglobin raffimer was maintained at 24 hours after surgery (7% vs 37%; P =.010) and up to 5 days after surgery (10% vs 47%; P =.0034).

CONCLUSIONS

Hemoglobin raffimer was not associated with any serious adverse events in patients undergoing primary coronary artery bypass grafting with cardiopulmonary bypass and intraoperative autologous donation in a dose-response study up to 1000 mL. Hemoglobin raffimer was effective in facilitating decreased exposure or avoidance of allogeneic red blood cell transfusions when used in conjunction with intraoperative autologous donation.

[Beating heart surgery and left ventricular dysfunction]

We report our experience with systematic coronary revascularization on the beating heart among patients with left ventricular dysfunction as defined by a left ventricular ejection fraction < or = 40%. Between September 1996 and April 2000, 500 off-pump (OPCAB) revascularizations were performed (95% of all revascularizations for the same time frame, single surgeon). Among them, 76 patients qualified as left ventricular dysfunction and were compared to a similar cohort of 237 patients operated on with cardiopulmonary bypass (CPB) during the same time frame. Age and sex distribution, average preoperative left ventricular ejection fraction and incidence of preoperative unstable angina were the same for both groups. On average, 3.04 +/- 0.89 and 2.97 +/- 0.69 grafts/patient were made in the OPCAB and CPB groups respectively (p = NS). Complete revascularization was achieved in 95% of the OPCAB group. Incidence of preoperative intra-aortic balloon assistance were higher in OPCAB (22% versus 9%, p = 0.005) whereas postoperative need for new intra-aortic balloon assistance was higher in CPB (8% versus 0%, p = 0.02). Incidence of postoperative myocardial infarction was comparable in both groups (6.5% versus 5.5%). Maximal creatinine phosphate of myocardial origin were lower in OPCAB group (beating heart: 32 +/- 52%, cardiopulmonary bypass: 45 +/- 51%, p = 0.055). Operative mortality was lower in OPCAB group although it did not reach statistical significance (beating heart: 2.6% versus cardiopulmonary bypass: 4.6%, p = 0.3). Complete coronary revascularization on the beating heart can be achieved in patients with left ventricular dysfunction with excellent outcome and low operative mortality.

Outcome with high blood lactate levels during cardiopulmonary bypass in adult cardiac operation

BACKGROUND

High blood lactate levels during cardiopulmonary bypass (CPB) are associated with tissue hypoperfusion and may contribute to postoperative complications or death. The objective of this study was to determine an association between blood lactate levels during CPB and perioperative morbidity and mortality.

METHODS

We reviewed 1,376 patients who underwent cardiac operation with CPB. Patients with abnormal preoperative blood lactate levels were excluded (n = 101). Blood lactate concentration during CPB, clinical data, and perioperative events were recorded.

RESULTS

Peak blood lactate levels of 4.0 mmol/L or higher during CPB were present in 227 patients (18.0%). Postoperative mortality was higher in this group than in the patients who had peak blood lactate levels of less than 4.0 mmol/L during CPB (11.0% versus 1.4%; p < 0.001, relative risk [RR] = 9.0). Postoperative hemodynamic instability occurred in 29.5% of patients with elevated levels of lactate during CPB compared with 10.9% of patients with lower lactate levels (p < 0.001, RR = 3.4). Overall, major postoperative complications occurred in 43.2% and 21.8% of patients in each group, respectively (p < 0.001, RR = 2.7). Logistic regression analysis revealed that peak blood lactate levels of 4.0 mmol/L or higher during CPB were strongly associated with postoperative mortality (p = 0.0001) and morbidity (p = 0.013).

CONCLUSIONS

Blood lactate concentration of 4.0 mmol/L or higher during CPB identifies a subgroup of patients with increased risk of postoperative morbidity and mortality.

Vertical and horizontal transmission of Neospora caninum in dairy herds in Québec

Neospora caninum is an important cause of abortion in dairy cattle. The objective of this observational study was to estimate the rate of vertical transmission of N. caninum in dairy herds in Québec and to investigate horizontal transmission in the same herds. The genealogy of cows from 23 dairy herds were examined. Prevalence of seropositive animals in herds studied varied from 4.3% to 61.8% (average, 21.9%). The overall rate of vertical transmission was estimated to be 44.4%, varying from 0% to 85.7%. Seven cases of horizontal transmission were identified in 6 of the 23 herds studied. Estimated vertical transmission rate varied from herd to herd, but appeared to be higher in herds with a high prevalence of seropositive animals. Although horizontal transmission was identified in 6 herds, it does not appear to be the major route of infection for N. caninum.

Left main coronary artery stenosis: early experience with surgical revascularization without cardiopulmonary bypass

BACKGROUND

To determine the safety of surgical revascularization without cardiopulmonary bypass (CPB) in left main stem (LMS) coronary stenosis.

METHODS AND RESULTS

Between October 1996 and April 1998, 67 consecutive patients with a > or =50% LMS stenosis underwent coronary revascularization without bypass (BH) and were compared to a contemporary group of 160 patients revascularized with conventional bypass (CPB). Mean ages in both groups were similar: 63.1 and 64.5 years in BH and CPB groups respectively (p=0.91). Significant triple vessel disease occurred in 40 (80%) and 75 (47.5%) patients in BH and CPB groups respectively (p=NS). Average grafts per patient was numbered 3.1 in BH group and 2.9 in CPB group (p=NS). The perioperative infarction rate (defined arbitrarily as a CK-MB >100 U/l) was 4% (2 patients, excluding 1 preoperative infarct) and 3.1% (5 patients, excluding 2 preoperative infarcts) in groups BH and CPB respectively (p=0.28). Postoperative blood transfusion requirements were less in BH group (19 patients, 38%) compared to CPB group (103 patients, 64.4%), (p=0.04). Postoperative inotropic requirements were similar in both groups; BH group (15 patients, 30%) and CPB group (72 patients, 45%). Mean hospital stay was 6.4 and 7.6 days in BH and CPB groups respectively (p=0.49). The hospital mortality rate was 0% and 3.8% (6 patients) in BH and CPB groups respectively (p=NS).

CONCLUSIONS

Our early experience suggests that non-CPB surgical revascularization in LMS stenosis is a feasible and safe alternative to conventional cardiopulmonary bypass.

Systematic off-pump coronary artery revascularization in multivessel disease: experience of three hundred cases

OBJECTIVE

We sought to report our recent experience with off-pump coronary artery revascularization in multivessel disease.

METHODS

Between October 1996 and December 1998, 300 off-pump beating heart operations were performed at the Montreal Heart Institute by a single surgeon, representing 94% of all procedures undertaken during this same time frame (97% for 1998). This cohort of patients was compared with 1870 patients operated on with cardiopulmonary bypass from 1995 to 1996.

RESULTS

Mean age, sex distribution, and preoperative risk factors were comparable for the two groups. On average, 2.92 +/- 0.8 and 2.84 +/- 0.6 grafts per patient were completed in the beating heart and cardiopulmonary bypass groups, respectively. A majority of patients (70%) had either a triple or quadruple bypass. Coronary anastomoses were achieved with myocardial mechanical stabilization and heart "verticalization." Ischemic time was shorter in the beating heart group (29.8 +/- 0.9 vs 45 +/- 0.4 minutes, P <.05). Similarly, the need for transfusion was significantly less in the beating heart group (beating heart operations, 34%; cardiopulmonary bypass, 66%; P <.005). Reduced use of postoperative intra-aortic counterpulsation, as well as a lower rise in creatine kinase MB isoenzyme, was observed in the beating heart group. Operative mortality rates (beating heart operations, 1. 3%; cardiopulmonary bypass, 2%) and perioperative myocardial infarction (beating heart operations, 3.6%; cardiopulmonary bypass, 4.2%) were comparable for the two groups.

CONCLUSION

In a majority of patients, off-pump complete coronary artery revascularization is an acceptable alternative to conventional operations, yielding good results given progressive experience, rigorous technique, and adequate coronary artery stabilization.

Low-dose aprotinin is ineffective to treat excessive bleeding after cardiopulmonary bypass

BACKGROUND

Uncontrolled clinical experience at our institution suggested that low-dose aprotinin could control excessive bleeding after cardiopulmonary bypass (CPB). A randomized clinical trial was conducted to determine the efficacy of low-dose aprotinin in the treatment of hemorrhage after cardiac surgery.

METHODS

One hundred seventy-one patients undergoing cardiac surgery with CPB were included. Forty-four patients (26%) bled significantly in the intensive care unit (>100 mL/h) and received either aprotinin (200,000 KIU bolus + 100,000 KIU/h for 8 hours) or placebo in addition to our standard management of excessive bleeding.

RESULTS

Median bleeding before study drug administration was not different between aprotinin (200 mL) and placebo (212.5 mL) groups. Bleeding decreased significantly with time and similarly in both groups. Ninety-five percent of patients required transfusions in both groups. Median blood products transfused were 13 and 8 units per patient in the aprotinin and placebo groups respectively (p = NS).

CONCLUSIONS

Routine administration of low-dose aprotinin as part of the treatment protocol to control hemorrhage after CPB does not reduce bleeding or transfusion requirements and, therefore, cannot be recommended.

[Systemic coronary surgery in the beating heart. Experience in 250 cases]

OBJECT

To report our recent experience with off-pump coronary artery revascularization in multi-vessel disease.

METHODS

Between October 1996 and August 1998, 250 off-pump (OP) procedures were completed at the Montreal Heart Institute, representing more than 90% of all procedures done during the same time frame (97% for 1998). These patients have been compared to 1870 patients operated upon under cardiopulmonary bypass during the years 1995-1996 (CPB).

RESULTS

Mean age, sexe distribution, and preoperative risk factors were comparable for both groups. On average 2.89 +/- 0.8 and 2.84 +/- 0.6 grafts/patient were completed in OP and CPB groups respectively. A majority (70%) of patients had either a triple or quadruple bypass. Coronary anastomoses were achieved with myocardial mechanical stabilization and heart "verticalization". Ischemic time was shorter in the OP group (29.8 +/- 0.9 vs 45 +/- 0.4 min, p < 0.05). Similarly, need for transfusion was significantly less (OP: 34 vs CPB: 66%, p < 0.005). Use of postoperative intra-aortic counterpulsation as well as the raise of CK-MB were lesser in the OP group. Operative mortality (OP: 1.6%, vs CPB: 2%, p = ns) and perioperative myocardial infarction rate (OP: 3.6% vs CPB: 4.2) were comparable for both groups.

CONCLUSION

Off-pump complete coronary artery revascularization is an acceptable alternative to conventional surgery in a majority of patients with good results given progressive experience, rigorous technique, and adequate coronary artery stabilization.

[Left main coronary artery stenosis and revascularization in the beating heart. Short- and long-term experience]

OBJECT

To determine the safety of surgical revascularization without cardiopulmonary bypass in left main coronary artery stenosis.

METHODS

Between October 1996 and October 1998, 67 patients with a left main stem stenosis (LMS) (> 50%) underwent revascularization on beating heart surgery (BHS) and were compared to a cohort of 192 patients with LMS disease that were operated on under cardiopulmonary bypass (CPB) during 1996.

RESULTS

Mean age and sex distribution and prevalence of preoperative risk factors were the same in both groups as well as the average number of grafts per patient was 3.1 +/- 0.7 and 2.9 +/- 0.7 in BHS and CPB groups respectively. Perioperative infarction rate (defined arbitrarily as a CK-MB > 100 IUL) was 2.9% in BHS group and 3.1% in CPB group. Postoperative blood transfusion requirements were less in BHS group (38%) compared to CPB group (64%), p < 0.05. Inotropic requirements postoperatively were similar in both groups. Hospital stay was shorter in BHS group (6.8 days) compared to CPB group (7.6 days) although not significant. There was no operative mortality in BHS group whereas 4.7% died postoperatively in CPB group.

CONCLUSION

Our experience suggests that non-bypass surgical revascularization is a feasible and safe alternative to conventional cardiopulmonary bypass.

[Peri-operative benefits of beating-heart coronary revascularization in patients with triple vessel disease]

OBJECTIVES

To compare the perioperative benefits of the offpump coronary artery surgery to conventional surgery with cardiopulmonary bypass (CPB) assistance.

METHOD

Retrospective study comparing 50 consecutive patients with triple vessel disease (TVD) operated upon with CPB assistance to 50 consecutive patients with TVD operated on beating heart (BH) by the same surgeon between January 1996 and August 1997 at the Montreal Heart Institute. Matching criteria between the 2 groups were: first time operation, normal left ventricular ejection function (LVEF), and coronary TVD.

RESULTS

Demographic data and risk factors were comparable in both groups. Unstable angina was the most common surgical indication (CBP: 72%, BH: 62%). The average number of grafts were comparable in both groups (CBP: 3.5 vs 0.6 vs BH: 3.3 +/- 0.4, p = 0.06) as well as the preoperative LVEF (CEC: 60 +/- 1% vs CB: 62 +/- 1.2%, p = 0.38). Arterial lactate count was lower in the BH group during the perioperative period (3.0 +/- 1.0 vs 3.9 +/- 1.7 mM) as well as the maximal CK-MB level (BH: 20 +/- 6.4 vs CBP: 40 +/- 3.5 IU/L, p = 0.003), transfusional needs (CB: 34% vs CEC: 66%, p = 0.003), and postoperative creatinin level (BH: 100 +/- 4.2 vs CBP: 120 +/- 7.1 mM, p = 0.001). Postoperative HB level was also higher in the BH group (110 +/- 14 vs 104 +/- 12, p = 0.001).

CONCLUSION

Our data confirm potential benefits from BH surgery on patients affected with triple vessel disease relatively to transfusional needs, and myocardial as well as renal protection.