Effects of sodium chloride intake on urea-N recycling and renal urea-N kinetics in lactating Holstein cows

The effects of high (2.5% of DM) versus normal dietary sodium chloride (NaCl) intake on renal urea-N kinetics and urea-N metabolism were investigated in 9 rumen-cannulated and multi-catheterized lactating dairy cows in a crossover design with 21-d periods. It was hypothesized that urinary urea-N excretion would be greater, and blood urea-N concentration lower in response to greater diuresis induced by high NaCl intake. Also, urea-N transport across ruminal and portal drained viscera (PDV) tissues was hypothesized to be affected by dietary sodium intake. A second experiment was conducted using 8 lactating cows in a crossover design with 14-d periods to test high NaCl (2.5% of DM) versus high KCl (3.2% of DM) intake on milk yield and milk urea-N concentrations. Experiment 1 showed that despite greater diuresis there was no effect of high NaCl intake on urinary urea-N excretion or blood urea-N concentration. The high NaCl intake did not affect rumen ammonia concentrations, total rumen VFA concentrations, ruminal venous - arterial concentration differences for ammonia, or ammonia absorption indicating that high NaCl did not adversely affect ruminal fermentation and microbial protein synthesis. High NaCl intake did not affect the total amount of urea-N transport from blood to gut, but ruminal venous - arterial concentration differences for urea-N were lower with high NaCl and ruminal extraction of arterial urea-N was numerically smaller, indicating that the ruminal epithelial urea-N transport was lower with high NaCl. Energy corrected milk yield was greater with high NaCl (3.2 ± 1.5 kg/d); however, milk urea-N concentrations were not affected by treatment. In experiment 2, ECM was greater with NaCl (1.4 ± 0.31 kg/d) compared with KCl (30.2 and 28.8 ± 0.91 kg ECM / d, respectively). Milk urea-N concentration was lower with KCl, suggesting a urea-N lowering effect in milk not evident with high NaCl intake. In conclusion, the present data show that dietary Na intake of 12-13 g/kg DM was followed by greater diuresis but did not impact urea-N excretion or blood urea-N concentration. High NaCl intake did not affect the total amount of urea-N transfer across PDV tissues. Energy corrected milk yield was greater with high NaCl compared with both control and feeding KCl, however, with KCl milk urea-N decreased.

Kidney function, but not nitrogen excretion differs between Brown Swiss and Holstein dairy cows

Brown Swiss (BS) cows have greater urea concentrations in milk and blood compared with Holstein (HO) cows. We tested the hypothesis that BS and HO cows differ in kidney function and nitrogen excretion. Blood, saliva, urine, and feces were sampled in 31 multiparous BS and 46 HO cows kept under identical feeding and management conditions. Samples were collected at different lactational stages after the monthly DHIA control test-day. To test the glomerular filtration rate (GFR) and urea excretion, concentrations of creatinine and urea were measured in serum, urine, and saliva. As an additional marker to estimate GFR, we determined symmetric dimethylarginine (SDMA) in serum. Feces were analyzed for dry matter content and nitrogen concentration. Data on milk urea and protein concentrations, and daily milk yield were obtained from the monthly DHIA test-day records. The effects of breed, time, and parity number on blood, saliva, urine, feces, and milk parameters were evaluated with the GLM procedure with breed, time, and parity number as fixed effects. Differences between BS and HO were assessed by the Tukey-corrected t-test at P < 0.05. Concentrations of urea, creatinine, and SDMA in serum, were greater in BS than in HO cows (P < 0.01): 5.46 ± 0.19 vs 4.72 ± 0.13 mmol/L (urea), 105.96 ± 2.23 vs 93.07 ± 1.50 mmol/l (creatinine), and 16.78 ± 0.69 vs 13.39 ± 0.44 µg/dL (SDMA). We observed a greater urea concentration in BS cows (25.8 ± 0.7 vs 21.8 ± 0.7 mg/dL) and protein content in milk (3.70 ± 0.08 vs 3.45 ± 0.07%) than in HO cows (P < 0.01). Urea and creatinine concentrations in urine and saliva did not differ among breeds. No differences between BS and HO were observed for milk yield, fecal DM, and fecal nitrogen content. Dry matter intake and body weight were similar in BS and HO cows (P > 0.05). Despite greater urea, creatinine, and SDMA concentrations in blood as well as a higher milk urea content in BS compared with HO, respective concentrations in urine did not differ between breeds. In conclusion, our results demonstrate a lower renal GFR in BS compared with HO cows, thereby contributing to the greater plasma urea concentration in BS cows. However, estimation of nitrogen excretion via milk, urine, and feces does not entirely reflect nitrogen turnover within the animal.

Mammary gland, kidney and rumen urea and uric acid transporters of dairy cows differing in milk urea concentration

The milk urea concentration (MUC) serves as indicator of urinary nitrogen emissions, but at comparable crude protein (CP) intake, cows with high (HMU) and low (LMU) MUC excrete equal urea amounts. We hypothesized that urea and uric acid transporters and sizes of the kidney, mammary gland, and rumen account for these phenotypes. Eighteen HMU and 18 LMU Holstein dairy cows fed a low (LP) and normal (NP) CP diet were studied. Milk, plasma and urinary urea concentrations were greater with NP feeding, while plasma and urinary urea concentrations were comparable between phenotypes. Milk and plasma uric acid concentrations were higher with LP feeding but not affected by phenotype. The milk-urine uric acid ratio was greater in HMU cows. The mRNA expressions of the ruminal urea transporter SLC14A1 and AQP10, the mammary gland and rumen AQP3, and the mammary gland uric acid transporter ABCG2 were not affected by group or diet. Renal AQP10, but not AQP3, AQP7, and SLC14A2 expressions, and the kidney weights were lower in HMU cows. These data indicate that renal size and AQP10 limit the urea transfer from blood to urine, and that MUC determines if uric acid is more released with milk or urine.

Effect of Energy Provision Strategy on Rumen Fermentation Characteristics, Bacterial Diversity and Community Composition

This study was conducted to explore the rumen fermentation characteristics, bacterial diversity, and community composition of Hu sheep under four energy provision strategies. Ninety-six Hu sheep (body weight: 17.78 ± 1.24 kg) were equally assigned to four energy provision strategies: (1) low-energy diet for the whole finishing stage (LL); (2) high-energy diet for the whole finishing stage (HH); (3) low-energy diet in the early finishing stage and high-energy diet in the late finishing stage (LH); (4) high-energy diet in the early finishing stage and low-energy diet in the late finishing stage (HL). The results showed that the proportion of acetate was lower in the HH group than that in the HL group, whereas the opposite result was observed for the butyrate proportion (p < 0.05). The Chao 1, observed species, PD whole tree, and Shannon index of the rumen bacteria were higher in the LL group than that in the HH group (p < 0.05). The taxonomic annotations revealed that the Patescibacteria, Rikenellaceae RC9 gut group, Christensenellaceae R-7 group, and Anaeroplasma abundances were higher in the HL group than that in the HH group, and the opposite results were observed regarding the relative abundances of Selenomonas and Anaerovibrio (p < 0.05). The relative abundances of Spirochaetota and Treponema were higher in the LH group than that in the HH group (p < 0.05). Moreover, the analysis of similarity (ANOSIM) showed significant differences between groups (R = 0.6792 and p = 0.001). This study indicates that the energy provision strategy had little impact on the rumen fermentation characteristics, while it heavily affected the rumen bacterial diversity and community composition. This study may provide insight into the rumen fermentation characteristics and bacterial community under routine finishing models and contribute to the optimization of energy provision strategies of Hu sheep.

Evaluation of Molly model predictions of ruminal fermentation, nutrient digestion, and performance by dairy cows consuming ryegrass-based diets

Several studies have been conducted to improve grazing management and supplementation in pasture-based systems. However, it is necessary to develop tools that integrate the available information linking the representation of biological processes with animal performance for use in decision making. The objective of this study was to evaluate the precision and accuracy of the Molly cow model predictions of ruminal fermentation, nutrient digestion, and animal performance by cows consuming pasture-based diets to identify model strengths and weaknesses, and to derive new digestive parameters when relevant. Model modifications for adipose tissue, protein synthesis in lean body mass and viscera representation were included. Data used for model evaluations were collected from 25 publications containing 115 treatment means sourced from studies conducted with lactating dairy cattle. The inclusion criteria were that diets contained ≥45% perennial ryegrass (Lolium perenne L.), and that dry matter intake, dietary ingredient composition, and nutrient digestion observations were reported. Animal performance and N excretion variables were also included if they were reported. Model performance was assessed before and after model reparameterization of selected digestive parameters, global sensitivity analysis was conducted after reparameterization, and a 5-fold cross evaluation was performed. Although rumen fermentation predictions were not significantly improved, rumen volatile fatty acids absorption rates were recalculated, which improved the concordance correlation coefficient (CCC) for rumen propionate and ammonia concentration predictions but decreased CCC for acetate predictions. Similar degradation rates of crude protein were observed for grass and total mixed ration diets, but rumen-undegradable protein predictions seemed to be affected by the solubility of the protein source as was the intestinal digestibility coefficient. Ruminal fiber degradation was greater after reparameterization, driven primarily by hemicellulose degradation. Predictions of ruminal and fecal outflow of neutral detergent fiber and acid detergent fiber, as well as total fecal output predictions, improved significantly after reparameterization. Blood urea N and urinary N excretion predictions resulted in similar accuracy using both sets of model parameters, whereas fecal N excretion predictions were significantly improved after reparameterization. Body weight and body condition score predictions were greatly improved after model modifications and reparameterization. Before reparameterization, yield predictions for daily milk, milk fat, milk protein, and milk lactose were greatly overestimated (mean bias of 61.0, 58.7, 73.7, and 64.6% of mean squared error, respectively). Although this problem was partially addressed by model modifications and reparameterization (mean bias of 3.2, 1.1, 1.7, and 0.4% of mean squared error, respectively), CCC values were still small. The ability of the model to predict grass digestion and animal performance in dairy cows consuming pasture-based diets was improved, demonstrating the applicability of this model to these productive systems. However, the failure to predict grass digestion based on standard model inputs without reparameterization indicates there are still fundamental challenges in characterizing feeds for this model.

Milk urea nitrogen variation explained by differences in urea transport into the gastrointestinal tract in lactating dairy cows

Milk urea nitrogen (MUN) and blood urea nitrogen are correlated with nitrogen balance and nitrogen excretion; however, there is also a genetic component to MUN concentrations that could be associated with differences in urea transport. It was hypothesized that a portion of the variation in MUN concentrations among cows is caused by variation in gastrointestinal and kidney urea clearance rates. Eight lactating cows with varying MUN concentrations while fed a common diet were infused with [¹⁵N¹⁵N]urea to determine urea N entry rate (UER), gastrointestinal entry rate, returned to ornithine cycle, urea N used for anabolism, urea N excretion in feces and urine. Urea clearance rates by the kidneys and gastrointestinal tract were calculated from isotopic enrichment of urea excretion in urine and gut entry rate, respectively, and plasma urea N concentrations (PUN). Over the course of the experiment, animals weighed an average of 506 ± 62 kg and produced 26.3 ± 4.39 kg of milk/d, with MUN concentrations ranging from 11.6 to 17.3 mg/dL (average of 14.9 ± 2.1 mg/dL). Plasma urea N was positively correlated with UER, urea N excretion in urine, and urea N used for anabolism. Plasma urea N and MUN were negatively correlated with gut clearance rates and ratio of gastrointestinal entry rate to UER. This relationship supports the hypothesis that differences in gut urea transport activity among animals causes variation in PUN and MUN concentrations, and that cows with high PUN and MUN are less efficient at recycling PUN to the gastrointestinal tract and thus may be more susceptible to ruminal N deficiencies when fed low RDP diets. Such biological variation in urea metabolism necessitates an adequate safety margin when setting regulations for maximal MUN levels as an indicator of herd N efficiency.

Differences between Holstein dairy cows in renal clearance rate of urea affect milk urea concentration and the relationship between milk urea and urinary nitrogen excretion

Urine and fecal excretions from cattle contribute to global nitrogen (N) emissions. The milk urea nitrogen (MUN) concentration in dairy cows is positively correlated with urinary urea N (UUN) emissions, and both decline with the reduction in crude protein intake. However, MUN concentration may differ between individual cows despite feeding the same ration. Thus, we hypothesized that due to differences in endogenous N utilization cows with high MUN concentration excrete more UUN than cows with a low MUN concentration. The objective of the present study was to elucidate N partitioning and urea metabolism in dairy cows with divergent MUN concentrations fed two planes of crude protein. Twenty Holstein dairy cows with high (HMU; n = 10) and low (LMU; n = 10) milk urea concentrations were fed two isocaloric diets with a low (LP) and normal (NP) crude protein level. Methane and ammonia emissions were recorded in respiration chambers. Feed intake, feces and urine excretions and milk yield were recorded for four days and subsamples were analyzed for total N and N-metabolites. A carbon-13 labeled urea bolus was administered intravenously followed by a series of plasma samplings. Total N and UUN excretions and ammonia emissions from excreta were lower on the LP diet, however, methane emissions, urinary N excretions and ammonia emissions were comparable between groups. Although plasma and salivary urea concentrations, urea pool size and urea turnover were higher, HMU cows had lower renal urea clearance rates. Additionally, HMU cows had lower renal clearance rates for creatinine, uric acid and creatine and excreted less uric acid (on the LP diet only) and creatine with urine. In conclusion, contrary to our hypothesis, HMU cows did not excrete more UUN than LMU cows. The lower urinary creatine excretion of HMU cows suggests that these animals have a lower environmental nitrogen footprint.

Performance, digestibility, nitrogen balance and ingestive behaviour of goat kids fed diets supplemented with condensed tannins from Acacia mearnsii extract

Context Tannins are phenolic compounds derived from secondary plant metabolism that at moderate levels can limit the excessive degradation of the protein in the rumen and provide greater amino acids availability in the small intestine. Aims Two experiments were performed to determine the best level of inclusion of condensed tannins (CT) from Acacia mearnsii extract in the diet of goat kids. An experiment was performed with 32 castrated crossbreed Boer goat kids, with a mean age of 4 months and mean bodyweight (BW) of 19.7 ± 2.05 kg, to measure nutrient intake and performance, and another experiment was developed with 20 Boer goat kids, at 5 months of age and 29.0 ± 2.45 kg BW, in which the digestibility, nitrogen (N) balance, microbial protein synthesis, ingestive behaviour and blood urea N were determined. In both experiments, a completely randomised design was used, including CT levels at 0, 16, 32 and 48 g/kg DM. Results DM; crude protein; ether extract; neutral detergent fibre (NDF); non-fibrous carbohydrates and total digestible nutrients intakes, ether extract digestibility; time spent on ruminating, idling or chewing; number of chews per bolus; eating and ruminating rate of DM and NDF; N intake and retained, blood urea N and loin eye area were not affected (P &gt; 0.05) by the addition of CT from A. mearnsii extract in the goat kid diets. However, the NDF g/kg BW and BW0.75 intakes and N faecal excretion showed a linear increase (P &lt; 0.05). There was a linear increase trend (P &lt; 0.1) in time spent eating, and a linear decrease on N urinary excretion by CT from A. mearnsii extract inclusion. The digestibility of DM, crude protein, NDF, non-fibrous carbohydrates and total digestible nutrients exhibited a linear decrease (P &lt; 0.05) by CT from A. mearnsii extract inclusion. There was a quadratic increase (P &lt; 0.05) in synthesis and microbial efficiency, final BW, average daily gain, total gain, gain:feed ratio, and hot and cold carcass weights and yields. Conclusion The inclusion of CT from A. mearnsii extract up to 17.7 g/kg DM is recommended, as it improves the synthesis and efficiency of microbial production, increasing the gain:feed ratio and performance of goat kids. Implications The use of condensed tannins from A. mearnsii does not affect the nutrient intakes on goat kids’ diet and also improves performance and carcass trait of these animals.

Performance, digestibility, microbial protein synthesis, and body composition of Brangus x Zebu steers on tropical pasture receiving supplementation

The use of additional supplement can affect ruminant performance by increasing the animal weight gain and maximizing profits from the activity. Thus, the objective of this study was to evaluate the influence of protein-energy supplementation on microbial synthesis, animal performance, nutrient digestibility, and body composition of Brangus x Zebu steers on pasture. The experiment lasted 160 days and included 36 animals divided into two groups; 18 steers received protein-energy supplementation (PES), and the other 18 received non-supplementation (NPES). Individual pasture intake and nutrient digestibility were estimated using the double indicator technique-chromium oxide and lignin in potassium permanganate. Spot urine samples were collected from 36 animals to determine creatinine, allantoin, and uric acid concentrations. All animals were slaughtered at the end of the experiment to evaluate body composition. There was increase in intake (P < 0.001) and dry matter digestibility (P = 0.01); it resulted in higher animal weight gain (P < 0.001) receiving supplementation. However, there was no difference (P > 0.05) in pasture nutrient intake between treatments. Supplementation increased microbial nitrogen (P < 0.001). For body composition, the model identity test that was applied showed no difference (P > 0.05) between the models, so it was adopted a common equation for both treatments. There was no difference (P > 0.05) for body composition between treatments. Therefore, the use of protein-energy supplementation for steers on pasture allowed higher microbial protein synthesis and better utilization of nutrients, which resulted in better animal performance. The use of protein-energy supplementation at 0.6% BW did not alter the carcass composition of Brangus x Zebu steers. However, it recommends protein-energy supplementation in pasture systems during the rainy period or using an irrigation system.

Physiological parameter values for physiologically based pharmacokinetic models in food-producing animals. Part I: Cattle and swine

Physiologically based pharmacokinetic (PBPK) models for chemicals in food animals are a useful tool in estimating chemical tissue residues and withdrawal intervals. Physiological parameters such as organ weights and blood flows are an important component of a PBPK model. The objective of this study was to compile PBPK‐related physiological parameter data in food animals, including cattle and swine. Comprehensive literature searches were performed in PubMed, Google Scholar, ScienceDirect, and ProQuest. Relevant literature was reviewed and tables of relevant parameters such as relative organ weights (% of body weight) and relative blood flows (% of cardiac output) were compiled for different production classes of cattle and swine. The mean and standard deviation of each parameter were calculated to characterize their variability and uncertainty and to allow investigators to conduct population PBPK analysis via Monte Carlo simulations. Regression equations using weight or age were created for parameters having sufficient data. These compiled data provide a comprehensive physiological parameter database for developing PBPK models of chemicals in cattle and swine to support animal‐derived food safety assessment. This work also provides a basis to compile data in other food animal species, including goats, sheep, chickens, and turkeys.

High-density diet improves growth performance and beef yield but affects negatively on serum metabolism and visceral morphology of Holstein steers

The objective of this study was to evaluate the effect of different dietary densities on growth performance, carcass characteristics, meat quality, serum metabolism, ruminal papillae morphology and liver injuries of steers. For this purpose, a total of eighteen Holstein steers were randomly fed one of the three diets: high energy and protein diet (H), standard energy and protein diet (C), and low energy and protein diet (L) for 11 months fattening with three-step finishing strategy. Steers fed with H diet had higher (p < .05) average daily gain, feed efficiency, hot carcass weight, serum aspartate aminotransferase to alanine aminotransferase ratio, and monounsaturated fatty acids along with continuous low ruminal pH value, severer hepatic steatosis and ruminal papillae parakeratosis. Meanwhile, steers fed L diet increased the proportion of C20:0, C22:6n-3, saturated fatty acids and n-3 polyunsaturated fatty acids along with lower n-6 to n-3 ratio in longissimus dorsi muscle as compared to that of steers fed H diet. Dietary densities did not influence (p > .10) proximate nutrients and sensory characteristics of beef. The present study indicates that Holstein steers could achieve better growth and carcass performance under high-density diet, whereas they are under threat of visceral injuries and metabolic disorders. This study gives comprehensive relationship between productivity and animal health and suggests that a proper diet should be adopted for fattening Holstein steers in consideration of both beef quality and quantity and animal health.

Invited review: Nitrogen in ruminant nutrition: A review of measurement techniques

Nitrogen is a component of essential nutrients critical for the productivity of ruminants. If excreted in excess, N is also an important environmental pollutant contributing to acid deposition, eutrophication, human respiratory problems, and climate change. The complex microbial metabolic activity in the rumen and the effect on subsequent processes in the intestines and body tissues make the study of N metabolism in ruminants challenging compared with nonruminants. Therefore, using accurate and precise measurement techniques is imperative for obtaining reliable experimental results on N utilization by ruminants and evaluating the environmental impacts of N emission mitigation techniques. Changeover design experiments are as suitable as continuous ones for studying protein metabolism in ruminant animals, except when changes in body weight or carryover effects due to treatment are expected. Adaptation following a dietary change should be allowed for at least 2 (preferably 3) wk, and extended adaptation periods may be required if body pools can temporarily supply the nutrients studied. Dietary protein degradability in the rumen and intestines are feed characteristics determining the primary AA available to the host animal. They can be estimated using in situ, in vitro, or in vivo techniques with each having inherent advantages and disadvantages. Accurate, precise, and inexpensive laboratory assays for feed protein availability are still needed. Techniques used for direct determination of rumen microbial protein synthesis are laborious and expensive, and data variability can be unacceptably large; indirect approaches have not shown the level of accuracy required for widespread adoption. Techniques for studying postruminal digestion and absorption of nitrogenous compounds, urea recycling, and mammary AA metabolism are also laborious, expensive (especially the methods that use isotopes), and results can be variable, especially the methods based on measurements of digesta or blood flow. Volatile loss of N from feces and particularly urine can be substantial during collection, processing, and analysis of excreta, compromising the accuracy of measurements of total-tract N digestion and body N balance. In studying ruminant N metabolism, nutritionists should consider the longer term fate of manure N as well. Various techniques used to determine the effects of animal nutrition on total N, ammonia- or nitrous oxide-emitting potentials, as well as plant fertilizer value, of manure are available. Overall, methods to study ruminant N metabolism have been developed over 150 yr of animal nutrition research, but many of them are laborious and impractical for application on a large number of animals. The increasing environmental concerns associated with livestock production systems necessitate more accurate and reliable methods to determine manure N emissions in the context of feed composition and ruminant N metabolism.

Estimating the energetic cost of feeding excess dietary nitrogen to dairy cows

Feeding N in excess of requirement could require the use of additional energy to metabolize excess protein, and to synthesize and excrete urea; however, the amount and fate of this energy is unknown. Little progress has been made on this topic in recent decades, so an extension of work published in 1970 was conducted to quantify the effect of excess N on ruminant energetics. In part 1 of this study, the results of previous work were replicated using a simple linear regression to estimate the effect of excess N on energy balance. In part 2, mixed model methodology and a larger data set were used to improve upon the previously reported linear regression methods. In part 3, heat production, retained energy, and milk energy replaced the composite energy balance variable previously proposed as the dependent variable to narrow the effect of excess N. In addition, rumen degradable and undegradable protein intakes were estimated using table values and included as covariates in part 3. Excess N had opposite and approximately equal effects on heat production (+4.1 to +7.6 kcal/g of excess N) and retained energy (-4.2 to -6.6 kcal/g of excess N) but had a larger negative effect on milk gross energy (-52 to -68 kcal/g of excess N). The results suggest that feeding excess N increases heat production, but more investigation is required to determine why excess N has such a large effect on milk gross energy production.

Prediction of portal and hepatic blood flow from intake level data in cattle

Interest is growing in developing integrated postabsorptive metabolism models for dairy cattle. An integral part of linking a multi-organ postabsorptive model is the prediction of nutrient fluxes between organs, and thus blood flow. The purpose of this paper was to use a multivariate meta-analysis approach to model portal blood flow (PORBF) and hepatic venous blood flow (HEPBF) simultaneously, with evaluation of hepatic arterial blood flow (ARTBF; ARTBF=HEPBF - PORBF) and PORBF/HEPBF (%) as calculated values. The database used to develop equations consisted of 296 individual animal observations (lactating and dry dairy cows and beef cattle) and 55 treatments from 17 studies, and a separate evaluation database consisted of 34 treatment means (lactating dairy cows and beef cattle) from 9 studies obtained from the literature. Both databases had information on dry matter intake (DMI), metabolizable energy intake (MEI), body weight, and a basic description of the diet including crude protein intake and forage proportion of the diet (FP; %). Blood flow (L/h or L/kg of BW^0.75/h) and either DMI or MEI (g or MJ/d or g or MJ/kg of BW^0.75/d) were examined with linear and quadratic fits. Equations were developed using cow within experiment and experiment as random effects, and blood flow location as a repeated effect. Upon evaluation with the evaluation database, equations based on DMI typically resulted in lower root mean square prediction errors, expressed as a % of the observed mean (rMSPE%) and higher concordance correlation coefficient (CCC) values than equations based on MEI. Quadratic equation terms were frequently nonsignificant, and the quadratic equations did not outperform their linear counterparts. The best performing blood flow equations were PORBF (L/h)=202 (±45.6) + 83.6 (±3.11) × DMI (kg/d) and HEPBF (L/h)=186 (±45.4) + 103.8 (±3.10) × DMI (kg/d), with rMSPE% values of 17.5 and 16.6 and CCC values of 0.93 and 0.94, respectively. The residuals (predicted - observed) for PORBF/HEPBF were significantly related to the forage % of the diet, and thus equations for PORBF and HEPBF based on forage and concentrate DMI were developed: PORBF (L/h)=210 (±51.0) + 82.9 (±6.43) × forage (kg of DM/d) + 82.9 (±6.04) × concentrate (kg of DM/d), and HEPBF (L/h)=184 (±50.6) + 92.6 (±6.28) × forage (kg of DM/d) + 114.2 (±5.88) × concentrate (kg of DM/d), where rMSPE% values were 17.5 and 17.6 and CCC values were 0.93 and 0.94, respectively. Division of DMI into forage and concentrate fractions improved the joint Bayesian information criterion value for PORBF and HEPBF (Bayesian information criterion=6,512 vs. 7,303), as well as slightly improved the rMSPE and CCC for ARTBF and PORBF/HEPBF. This was despite minimal changes in PORBF and HEPBF predictions. Developed equations predicted blood flow well and can easily be used within a postabsorptive model of nutrient metabolism. Results also suggest different sensitivity of PORBF and HEPBF to the composition of DMI, and accounting for this difference resulted in improved ARTBF predictions.

Absorption and intermediary metabolism of purines and pyrimidines in lactating dairy cows

About 20 % of ruminal microbial N in dairy cows derives from purines and pyrimidines; however, their intermediary metabolism and contribution to the overall N metabolism has sparsely been described. In the present study, the postprandial patterns of net portal-drained viscera (PDV) and hepatic metabolism were assessed to evaluate purine and pyrimidine N in dairy cows. Blood was sampled simultaneously from four veins with eight hourly samples from four multi-catheterised Holstein cows. Quantification of twenty purines and pyrimidines was performed with HPLC–MS/MS, and net fluxes were estimated across the PDV, hepatic tissue and total splanchnic tissue (TSP). Concentration differences between veins of fifteen purine and pyrimidine nucleosides (NS), bases (BS) and degradation products (DP) were different from zero (P≤ 0·05), resulting in the net PDV releases of purine NS (0·33–1·3 mmol/h), purine BS (0·0023–0·018 mmol/h), purine DP (7·0–7·8 mmol/h), pyrimidine NS (0·30–2·8 mmol/h) and pyrimidine DP (0·047–0·77 mmol/h). The hepatic removal of purine and pyrimidine was almost equivalent to the net PDV release, resulting in no net TSP release. One exception was uric acid (7·9 mmol/h) from which a large net TSP release originated from the degradation of purine NS and BS. A small net TSP release of the pyrimidine DP β-alanine and β-aminoisobutyric acid ( − 0·032 to 0·37 mmol/h) demonstrated an outlet of N into the circulating N pool. No effect of time relative to feeding was observed (P>0·05). These data indicate that considerable amounts of N are lost in the dairy cow due to prominent intermediary degradation of purines, but that pyrimidine N is reusable to a larger extent.

Simultaneous quantification of purine and pyrimidine bases, nucleosides and their degradation products in bovine blood plasma by high performance liquid chromatography tandem mass spectrometry

Improved nitrogen utilization in cattle is important in order to secure a sustainable cattle production. As purines and pyrimidines (PP) constitute an appreciable part of rumen nitrogen, an improved understanding of the absorption and intermediary metabolism of PP is essential. The present work describes the development and validation of a sensitive and specific method for simultaneous determination of 20 purines (adenine, guanine, guanosine, inosine, 2'-deoxyguanosine, 2'-deoxyinosine, xanthine, hypoxanthine), pyrimidines (cytosine, thymine, uracil, cytidine, uridine, thymidine, 2'-deoxyuridine), and their degradation products (uric acid, allantoin, β-alanine, β-ureidopropionic acid, β-aminoisobutyric acid) in blood plasma of dairy cows. The high performance liquid chromatography-based technique coupled to electrospray ionization tandem mass spectrometry (LC-MS/MS) was combined with individual matrix-matched calibration standards and stable isotopically labelled reference compounds. The quantitative analysis was preceded by a novel pre-treatment procedure consisting of ethanol precipitation, filtration, evaporation and reconstitution. Parameters for separation and detection during the LC-MS/MS analysis were investigated. It was confirmed that using a log-calibration model rather than a linear calibration model resulted in lower CV% and a lack of fit test demonstrated a satisfying linear regression. The method covers concentration ranges for each metabolite according to that in actual samples, e.g. guanine: 0.10-5.0 μmol/L, and allantoin: 120-500 μmol/L. The CV% for the chosen quantification ranges were below 25%. The method has good repeatability (CV%≤25%) and intermediate precision (CV%≤25%) and excellent recoveries (91-107%). All metabolites demonstrated good long-term stability and good stability within-runs (CV%≤10%). Different degrees of absolute matrix effects were observed in plasma, urine and milk. The determination of relative matrix effects revealed that the method was suitable for almost all examined PP metabolites in plasma drawn from an artery and the portal hepatic, hepatic and gastrosplenic veins and, with a few exceptions, also for other species such as chicken, pig, mink, human and rat.

Short communication: urea transporter protein UT-B in the bovine parotid gland

Ruminant nutrition relies upon the symbiotic relationship that exists with microbial populations in the rumen. Urea transported across the ruminal epithelia and secreted by the salivary glands is a key source of nitrogen for microbial growth in the rumen. As ruminal urea transport can be mediated by specific UT-B urea transporters, this study investigated whether UT-B urea transporters were also present in the bovine salivary gland. Western blotting experiments detected only small amounts of UT-B protein in whole-cell lysate from the bovine parotid gland. In contrast, strong 32 to 34 and 40 kDa UT-B proteins were detected in parotid plasma membrane-enriched protein, showing the importance of using enriched samples. These signals were also detected in rumen and correspond to bovine UT-B1 and UT-B2 urea transporters, respectively. Further immunolocalization studies identified that these proteins were located in the ductal system of the parotid gland. This study, therefore, confirmed the presence of UT-B urea transporter protein in the bovine parotid salivary gland.

Expression of urea transporters is affected by dietary nitrogen restriction in goat kidney

Ruminants are known to be able to very effectively recycle urinary urea and reuse it as a source of N for ruminal microbes. It is presumed that urea recycling is accomplished by specialized urea transporters (UT) which are localized in the kidney. This could be especially important in times of increased N requirement, such as during growth or during reduced dietary N intake. The aim of our study was to characterize and to localize UT in the goat (capra hircus) kidney and to investigate its response to reduced dietary N intake in growing goats. Therefore, 12 growing, male goats were fed either a diet containing high (17% CP in complete diet) or low (9% CP in complete diet) N content for 6 wk. After harvesting, blood and kidney samples were taken and analyzed. The mRNA of the different UT isoforms, UT-A1, UT-A2 and UT-B, were detected semiquantitatively in renal tissue by Northern blot analysis. For UT-A2 and UT-B, no statistically significant effect of dietary N restriction on renal mRNA expression could be detected (UT-A2: P = 0.26, UT-B: P = 0.07). However, renal mRNA abundance of UT-A1 significantly increased in the kidney of low-N-fed goats (P = 0.01). Furthermore, protein amounts of UT-B were verified by western blotting; and the localization of UT-A2 and UT-B protein was demonstrated by immunohistochemistry. No significant differences in protein amounts of UT-B could be observed comparing the 2 feeding groups (P = 0.78). The UT-B was localized in renal medulla and papilla, whereas UT-A2 was only found in renal medulla. In addition, comparison of UT-A and UT-BAA sequences of monogastric animals and ruminants showed a high degree of homology, indicating a similar function of the transporters among these species. In summary, we conclude that in ruminants, urea reabsorption in the kidney is most likely increased in response to a low-N diet via an upregulation of UT-A1 mRNA expression. Hypothetically, the reabsorbed urea can then be returned to the rumen via the bloodstream and thus be reused as a source of N for protein synthesis of ruminal microbial community.

Effect of abomasal infusion of oligofructose on portal-drained visceral ammonia and urea-nitrogen fluxes in lactating Holstein cows

The effects of abomasal infusion of oligofructose in lactating dairy cows on the relationship between hindgut fermentation and N metabolism, and its effects on NH(3) absorption and transfer of blood urea-N across the portal-drained viscera versus ruminal epithelia were investigated. Nine lactating Holstein cows fitted with ruminal cannulas and permanent indwelling catheters in major splanchnic blood vessels were used in an unbalanced crossover design with 14-d periods. Treatments were continuous abomasal infusion of water or 1,500 g/d of oligofructose. The same basal diet was fed with both treatments. Eight sample sets of arterial, portal, hepatic, and ruminal vein blood, ruminal fluid, and urine were obtained at 0.5h before the morning feeding and at 0.5, 1.5, 2.5, 3.5, 4.5, 5.5, and 6.5 h after feeding. It was hypothesized that an increased supply of fermentable substrate to the hindgut would increase the uptake of urea-N from blood to the hindgut at the expense of urea-N uptake to the forestomach. The study showed that abomasal oligofructose infusion decreased the total amount of urea-N transferred from the blood to the gut, NH(3) absorption, and arterial blood urea-N concentration. Subsequently, hepatic NH(3) uptake and urea-N production also decreased with oligofructose infusion. Additionally, urea-N concentration in milk and urinary N excretion decreased with oligofructose treatment. The oligofructose infusion did not affect ruminal NH(3) concentrations or any other ruminal variables, nor did it affect ruminal venous - arterial concentration differences for urea-N and NH(3). The oligofructose treatment did not affect milk yield, but did decrease apparent digestibility of OM, N, and starch. Nitrogen excreted in the feces was greater with the oligofructose infusion. In conclusion, the present data suggest that increased hindgut fermentation did not upregulate urea-N transfer to the hindgut at the expense of urea-N uptake by the rumen, and the observed reduction in arterial blood urea-N concentration appeared not to be due to increased urea-N transport, but rather could be explained by reduced NH(3) input to hepatic urea-N synthesis caused by increased sequestration of NH(3) in the hindgut and excretion in feces. Increasing the hindgut fermentation in lactating dairy cows by abomasal infusion of 1,500 g/d of oligofructose shifted some N excretion from the urine to feces and possibly reduced manure NH(3) volatilization without impairing rumen fermentation.