Prediction of ammonia emission from dairy cattle manure based on milk urea nitrogen: relation of milk urea nitrogen to urine urea nitrogen excretion

The objectives of this study were to assess the relationship between urinary urea N (UUN) excretion (g/d) and milk urea N (MUN; mg/dL) and to test whether the relationship was affected by stage of lactation and the dietary crude protein (CP) content. Twelve lactating multiparous Holstein cows were randomly selected and blocked into 3 groups of 4 cows intended to represent early [123 ± 26 d in milk (DIM); mean ± standard deviation], mid (175 ± 3 DIM), and late (221 ± 12 DIM) lactation stages. Cows within each stage of lactation were randomly assigned to a treatment sequence within a split-plot Latin square balanced for carryover effects. Stage of lactation formed the main plots (squares) and dietary CP levels (15, 17, 19, and 21% of diet dry matter) formed the subplots. Graded amounts of urea were added to the basal total mixed ration to linearly increase dietary CP content while maintaining similar concentrations of all other nutrients among treatments. The experimental periods lasted 7 d, with d 1 to 6 used for adjustment to diets and d 7 used for total collection of urine as well as milk and blood sample collection. Dry matter intake and yields of milk, fat, protein, and lactose declined progressively with lactation stage and were unaffected by dietary CP content. Milk and plasma urea-N as well as UUN concentration and excretion increased in response to dietary CP content. Milk and urine urea-N concentration rose at increasing and decreasing rates, respectively, as a function of plasma urea-N. The renal urea-N clearance rate differed among lactation stages and dietary CP contents. The relationship between UUN excretion and MUN differed among lactation stages and diverged from linearity for cows in early and late lactation. However, these differences were restricted to very high MUN concentrations. Milk urea N may be a useful tool to predict the UUN excretion and ultimately NH₃ emission from dairy cattle manure.     

Interaction between dietary content of protein and sodium chloride on milk urea concentration,urinary urea excretion,renal recycling of urea,and urea transfer to the gastrointestinal tract in dairy cows

Dietary protein and salt affect the concentration of milk urea nitrogen (MUN; mg of N/dL) and the relationship between MUN and excretion of urea nitrogen in urine (UUN; g of N/d) of dairy cattle. The aim of the present study was to examine the effects of dietary protein and sodium chloride (NaCl) intake separately, and their interaction, on MUN and UUN, on the relationship between UUN and MUN, on renal recycling of urea, and on urea transfer to the gastrointestinal tract. Twelve second-parity cows (body weight of 645 ± 37 kg, 146 ± 29 d in milk, and a milk production of 34.0 ± 3.28 kg/d), of which 8 were previously fitted with a rumen cannula, were fitted with catheters in the urine bladder and jugular vein. The experiment had a split-plot arrangement with dietary crude protein (CP) content as the main plot factor [116 and 154 g of CP/kg of dry matter (DM)] and dietary NaCl content as the subplot factor (3.1 and 13.5 g of Na/kg of DM). Cows were fed at 95% of the average ad libitum feed intake of cows receiving the low protein diets. Average MUN and UUN were, respectively, 3.90 mg of N/dL and 45 g of N/d higher for the high protein diets compared with the low protein diets. Compared with the low NaCl diets, MUN was, on average, 1.74 mg of N/dL lower for the high NaCl diets, whereas UUN was unaffected. We found no interaction between dietary content of protein and NaCl on performance characteristics or on MUN, UUN, urine production, and renal clearance characteristics. The creatinine clearance rate was not affected by dietary content of protein and NaCl. Urea transfer to the gastrointestinal tract, expressed as a fraction of plasma urea entry rate, was negatively related to dietary protein, whereas it was not affected by dietary NaCl content. We found no interaction between dietary protein and NaCl content on plasma urea entry rate and gastrointestinal urea entry rate or their ratio. The relationship between MUN and UUN was significantly affected by the class variable dietary NaCl content: UUN = −17.7 ± 7.24 + 10.09 ± 1.016 × MUN + 2.26 ± 0.729 × MUN (for high NaCl); R² = 0.85. Removal of the MUN × NaCl interaction term lowered the coefficient of determination from 0.85 to 0.77. In conclusion, dietary protein content is positively related to MUN and UUN, whereas dietary NaCl content is negatively correlated to MUN but NaCl content is not related to UUN. We found no interaction between dietary protein and NaCl content on performance, MUN, UUN, or renal urea recycling, nor on plasma urea entry rate and urea transfer to the gastrointestinal tract. For a proper interpretation of the relationship between MUN and UUN, the effect of dietary NaCl should be taken into account, but we found no evidence that the effect of dietary NaCl on MUN is dependent on dietary protein content.     

Effect of sodium chloride intake on urine volume,urinary urea excretion,and milk urea concentration in lactating dairy cattle

Milk urea nitrogen (MUN; mg of N/dL) has been shown to be related to excretion of urinary urea N (UUN; g of N/d) and total excretion of urinary N (UN; g of N/d) in dairy cows. In the present experiment, it was hypothesized that MUN and the relationship between MUN and UUN or UN is affected by urine volume as a result of dietary sodium chloride intake. Twelve lactating Holstein-Friesian dairy cows (mean ± SD: milk production 28.1 ± 3.23 kg/d and 190 ± 41 d in milk), of which 4 were fitted with catheters in the urine bladder and jugular vein, were randomly assigned to 4 dietary levels of sodium chloride (3, 9, 14, and 19 g of Na/kg of DM) according to a triple 4 × 4 Latin square design. Cows were fed at 95% of ad libitum intake, excluding salt addition. Milk was analyzed for MUN and protein content; urine was analyzed for total N, urea, and creatinine content; feces were analyzed for total N and DM content; and blood plasma was analyzed for urea and creatinine content. Creatinine clearance rate (CCR; L/min) and renal urea reabsorption ratio were estimated based on plasma concentrations of urea and creatinine, and total excretion of urea and creatinine in urine. Intake of DM and N, milk production, and milk protein content were (mean ± SD), on average, 21.4 ± 1.24 kg/d, 522 ± 32.0 g/d, 25.4 ± 2.53 kg/d, and 3.64 ± 0.186%, respectively. A linear relationship was found between Na intake and urine production [urine (kg/d; mean ± SE) = 7.5 ± 4.33 + 0.136 ± 0.0143 × Na intake (g/d)] and between Na intake and MUN [MUN (mg/dL; mean ± SE) = 13.5 ± 0.35 − 0.0068 ± 0.00104 × Na intake (g/d)]. Despite the decrease in MUN with increased Na intake, UN excretion increased linearly with Na intake. Excretion of UUN was not affected by dietary Na content. A linear plateau relationship was observed between CCR and renal urea reabsorption. An increase in CCR coincided with an increase in calculated renal urea reabsorption until a CCR breakpoint value (mean ± SD) of 1.56 ± 0.063 L/min was reached. We conclude that Na intake is negatively related to MUN, whereas UUN is not affected. Variation in mineral intake levels that affect urine volume should, therefore, be taken into account when using MUN as an indicator of UUN in dairy cattle.     

Short communication: Analytical method and amount of preservative added to milk samples may alter milk urea nitrogen measurements

Milk urea N (MUN) is used by dairy nutritionists and producers to monitor dietary protein intake and is indicative of N utilization in lactating dairy cows. Two experiments were conducted to explore discrepancies in MUN results provided by 3 milk processing laboratories using different methods. An additional experiment was conducted to evaluate the effect of 2-bromo-2-nitropropane-1, 3-diol (bronopol) on MUN analysis. In experiment 1, 10 replicates of bulk tank milk samples, collected from the Pennsylvania State University's Dairy Center over 5 consecutive days, were sent to 3 milk processing laboratories in Pennsylvania. Average MUN differed between laboratory A (14.9 ± 0.40 mg/dL; analyzed on MilkoScan 4000; Foss, Hillerød, Denmark), laboratory B (6.5 ± 0.17 mg/dL; MilkoScan FT + 6000), and laboratory C (7.4 ± 0.36 mg/dL; MilkoScan 6000). In experiment 2, milk samples were spiked with urea at 0 (7.3 to 15.0 mg/dL, depending on the laboratory analyzing the samples), 17.2, 34.2, and 51.5 mg/dL of milk. Two 35-mL samples from each urea level were sent to the 3 laboratories used in experiment 1. Average analyzed MUN was greater than predicted (calculated for each laboratory based on the control; 0 mg of added urea): for laboratory A (23.2 vs. 21.0 mg/dL), laboratory B (18.0 vs. 13.3 mg/dL), and laboratory C (20.6 vs. 15.2 mg/dL). In experiment 3, replicated milk samples were preserved with 0 to 1.35 mg of bronopol/mL of milk and submitted to one milk processing laboratory that analyzed MUN using 2 different methods. Milk samples with increasing amounts of bronopol ranged in MUN concentration from 7.7 to 11.9 mg/dL and from 9.0 to 9.3 mg/dL when analyzed on MilkoScan 4000 or CL 10 (EuroChem, Moscow, Russia), respectively. In conclusion, measured MUN concentrations varied due to analytical procedure used by milk processing laboratories and were affected by the amount of bronopol used to preserve milk sample, when milk was analyzed using a mid-infrared analyzer. Thus, it is important to maintain consistency in milk sample preservation and analysis to ensure precision of MUN results.     

Prediction of ammonia emission from dairy cattle manure based on milk urea nitrogen: Relation of milk urea nitrogen to ammonia emissions

The main objectives of this study were to assess the relationship between ammonia emissions from dairy cattle manure and milk urea N (MUN; mg/dL) and to test whether the relationship was affected by stage of lactation and the dietary crude protein (CP) concentration. Twelve lactating multiparous Holstein cows were randomly selected and blocked into 3 groups of 4 cows intended to represent early [123 ± 26 d in milk (DIM)], mid (175 ± 3 DIM), and late (221 ± 12 DIM) lactation stages. Cows within each stage of lactation were randomly assigned to a treatment sequence within a split-plot Latin square design balanced for carryover effects. Stage of lactation formed the main plots (squares) and dietary CP levels (15, 17, 19, and 21% of diet dry matter) formed the subplots. The experimental periods lasted 7 d, with d 1 to 6 used for adjustment to diets and d 7 used for total collection of feces and urine as well as milk sample collection. The feces and urine from each cow were mixed in the proportions in which they were excreted to make slurry that was used to measure ammonia emissions at 22.5°C over 24 h using flux chambers. Samples of manure slurry were taken before and after ammonia emission measurements. The amount of slurry increased by 22% as dietary CP concentration increased from 15 to 21%, largely because of a greater urine volume (25.3 to 37.1 kg/d). Initial urea N concentration increased linearly with dietary CP from 153.5 to 465.2 mg/dL in manure slurries from cows fed 15 to 21% CP diets. Despite the large initial differences, the final concentration of urea N in manure slurries was less than 10.86 mg/dL for all dietary treatments. The final total ammoniacal N concentration in manure slurries increased linearly from 228.2 to 508.7 mg/dL as dietary CP content increased from 15 to 21%. Ammonia emissions from manure slurries ranged between 57 and 149 g of N/d per cow and increased linearly with dietary CP content, but were unaffected by stage of lactation. Ammonia emission expressed as a proportion of N intake increased with percentage CP in the diet from about 12 to 20%, whereas ammonia emission as a proportion of urinary urea N excretion decreased from 67 to 47%. There was a strong relationship between ammonia emission and MUN [ammonia emission (g/d per cow) = 25.0 (±6.72) + 5.03 (±0.373) × MUN (mg/dL); R² = 0.85], which was not different among lactation stages. Milk urea N concentration is one of several factors that allows prediction of ammonia emissions from dairy cattle manure.     

The association between milk urea nitrogen and DHI production variables in western commercial dairy herds

A retrospective observational study was conducted using data from Dairy Herd Improvement monthly tests to investigate the association between milk urea nitrogen (MUN) concentration and milk yield, milk protein, milk fat percentage, SCC, and parity for commercial Holstein and Jersey herds in Utah, Idaho, and Montana. Mean MUN for Holstein cows was 15.5 mg/ dl (5.5 mmol/L) MUN and 14.1 mg/dl (5.0 mmol/L) for Jersey cows. Mean MUN, categorized by 30-d increments of days in milk (DIM), paralleled changes in milk values and followed a curvilinear shape. For Holstein cows, concentrations of MUN were different among lactation groups 1, 2, and 3+ for the first 90 DIM for Holsteins. Overall, concentrations of MUN were lower during for the first 30 DIM compared with all other DIM categories for both Holstein and Jersey cows. Multivariate regression models of MUN by milk protein showed that as the milk protein percentage increased, MUN concentration decreased; however, models for Jersey cows showed that MUN did not decrease significantly until above 3.4% milk protein. Milk fat percentage also decreased as MUN increased, but by only 1 mg/dl MUN over the range of 2.2 to 5.8% milk fat. Somatic cell count showed a negative relationship with MUN. Holstein cows with milk protein percentage >3.2% had lower MUN compared with cows having milk protein <3.2% for milk yields from 27.3 to 54.5 kg/d and lower than cows having a milk protein <3.0% for milk yield of 54.5 to 63.6 kg/d. In Jersey cows, MUN concentrations were not different among milk protein percentage categorized by milk yield. This study found that MUN was inversely associated with milk protein percentage and paralleled change in milk yield over time.     

Sources of variation in milk urea nitrogen in Ohio dairy herds

The purpose of this study was to estimate the amount of variation in milk urea nitrogen (MUN) concentrations attributable to test-day, individual cow, and herd effects and to describe factors associated with MUN measurements in Ohio dairy herds. The data came from 24 Holstein herds, half of which were classified as low producing (LP) [rolling herd average (RHA) milk production < 7,258 kg] and half as high producing (HP) herds (RHA production > 10,433 kg). MUN concentration was measured from cow's monthly test-day milk samples. The data were analyzed using multilevel modeling technique in MLwiN, separately for LP and HP herds. The unadjusted mean MUN was 13.9 mg/dl for the HP herds and 11.3 mg/dl for the LP herds. The variance structure was different between the two groups. Most of the variability was found at test-day level in the LP herds, but at herd level in HP herds. MUN was lowest during the first month of lactation, and also season was associated with MUN in both groups. Test-day milk yield, milk fat percentage, and SCC were associated with MUN in the HP herds. With significant explanatory variables in the model, proportionally more of the variation was explained at herd level and less at test day level in both groups. Lower variability in MUN between test days in the HP herds may indicate more consistent day-to-day feeding and management within a herd. The great variability between test days should be considered when interpreting MUN and samples should be collected at the same time of the day to minimize day-to-day variability.     

Statistical evaluation of factors and interactions affecting dairy herd improvement milk urea nitrogen in commercial midwest dairy herds

In this study, 400,729 Dairy Herd Improvement (DHI) records collected on 77,178 cows in 692 Midwest herds over 29 mo (January 1999 to May 2001) were used to analyze milk urea nitrogen (MUN) as collected the day of the test in 6 breeds. Records of Holsteins, Jerseys, and Brown Swiss were subjected to stepwise backward elimination analysis with a model including parity (primiparous vs. multiparous cows), sample type (morning vs. evening), milking frequency (2× vs. 3× [Holstein only]), season (winter, spring, summer, and fall), yield of fat-corrected milk (FCM) classified into 1 of 3 FCM categories (FCMc) and all possible higher-order interactions. Results indicated that FCMc contributed to test-day MUN variation in multiparous, but not primiparous, Holsteins. Sample type and season were significant in both parity groups; milking frequency was not significant, but milking frequency × season and milking frequency × FCMc were significant in both parity groups. The nature of these interactions differed for each parity group. For Jersey and Brown Swiss data analyzed by sample type separately, parity was not significant but tended to interact with FCMc, whereas season, FCMc, and season × FCMc were generally significant. Mean test-day MUN was 12.7, 14.6, and 14.4 mg/dL, with 24, 45, and 42% of records above 14.5 mg/dL in Holsteins, Jerseys, and Brown Swiss in single-breed herds, respectively. In Holsteins, MUN peaked at 7 to 10 d in milk (DIM), declined until 28 to 35 DIM, and rose again thereafter. In primiparous Holsteins, MUN did not change with FCM ≤42 kg/d, but for higher FCM yield, MUN declined linearly by 0.05 mg/dL per kilogram of FCM. In multiparous Holsteins, MUN increased by 0.06 and 0.03 mg/dL per kilogram of FCM as FCM yield increased from 5 to 29 and from 30 to 59 kg/d, respectively, but decreased by 0.06 mg/dL as FCM yield increased from 60 to 85 kg/d. The use of adjustment coefficients may facilitate interpretation of test-day MUN on commercial herds. Research should focus on the biological significance of the pattern of change in MUN the first few weeks postpartum and the drop in MUN in unusually high-producing cows.     

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.     

Effects of milk urea nitrogen and other factors on probability of conception of dairy cows

The objective of this study was to evaluate the relationships between milk urea nitrogen (MUN) and other factors and the probability of conception in dairy cows. Data were retrieved from the Lancaster Dairy Herd Improvement Association (DHIA). A total of 713 dairy herds and 10,271 dairy cows were included in the study. Logistic regression was used to determine the within-herd effects of MUN, milk production, lactation number, and breeding season on the probability of conception for each of 3 services. Within herds, MUN displayed a slight negative association with probability of conception at first service. For example, there was a 2- to 4-percentage unit decrease in conception rate at first service with a 10-mg/dL increase in MUN. In among-herd regression analysis, there was no effect of MUN on probability of conception. These results suggest that MUN may be related to conditions affecting reproduction of individual cows within a herd. Diet formulation usually would affect MUN equally among all cows at a similar stage of lactation in a herd. Because there was no effect of MUN among herds, diet formulation did not appear to affect conception rate.     

Effects of dietary crude protein concentration on late-lactation dairy cow performance and indicators of nitrogen utilization

The objectives of this study were to measure performance responses and to evaluate indictors of N utilization in late-lactation cows fed diets with incremental reductions in crude protein (CP) concentration. Holstein cows (n = 128; 224 ± 54 d in milk) were stratified by parity and days pregnant (86 ± 25 d) and randomly assigned to 1 of 16 pens in a randomized complete block design. For 3 wk, all cows received a covariate diet containing 16.9% CP [dry matter (DM) basis]. For the subsequent 12 wk, pens were randomly assigned to 1 of 4 treatments that contained 16.2, 14.4, 13.1, or 11.8% CP (DM basis). Diets were offered once daily and contained 32.5% corn silage, 32.5% alfalfa silage, 13.5% high-moisture corn, and 21.5% concentrate mix. A reduction in dietary CP was achieved by replacing soybean meal with soy hulls in the concentrate mix (DM basis). Dry matter intake, milk urea N (MUN; mg/dL), and the yield of milk urea N (g/d) decreased linearly with dietary CP. Compared with a 16.2% CP diet, a 14.4% CP diet did not alter milk yield throughout the study, but the 13.1 and 11.8% CP diets reduced milk yield after 4 and 1 wk, respectively. Furthermore, milk protein percentage was reduced for all dietary CP less than 16.2%, but this negative effect was temporary and disappeared after 7 wk for the 14.4% CP diet. In contrast, MUN adjusted to a new steady state within 1 wk for all dietary treatments. Modeling quadratic responses with a plateau led to predictions of no reduction in fat- and protein-corrected milk (32.6 kg/d) and yields of fat (1.31 kg/d), lactose (1.49 kg/d), and true protein (1.12 kg/d) until dietary CP decreased below 15.5, 15.3, 15.9, and 16.2%, respectively. In this study, MUN and the yield of MUN were highly correlated with N intake, milk protein yield, and fat- and protein-corrected milk. Surprisingly, N use efficiency (milk protein N/intake N) was not correlated with any variables related to N utilization and reached an apparent upper limit of approximately 30%. Although this observation may be associated with feeding diets deficient in metabolizable protein, late-lactation cows in this study adjusted to low dietary CP concentration better than anticipated as milk production was 2.6, 3.6, 6.4, and 8.0 kg/d higher than National Research Council (2001)-predicted metabolizable protein-allowable milk for dietary CP of 16.2, 14.4, 13.1, and 11.8%, respectively.     

A randomized herd-level field study of dietary interactions with monensin on milk fat percentage in dairy cows

This field trial evaluated the effects of dietary supplementation with 16 mg/kg (based on total dry matter intake) of monensin sodium on bulk tank milk fat percentage (MFP) of commercial dairy herds. Interactions between monensin and nutritional factors on MFP were studied. The trial was conducted in 47 Holstein dairy herds in Québec, Canada, between November 2005 and May 2006. The herd was the unit of interest. Enrolled herds were followed for a 7-mo period. Monensin treatment was randomly allocated in a crossover design where monensin was supplemented to the lactating dairy cow diet for a consecutive 12-wk period. Twenty-four herds were allocated to monensin treatment for the first period of trial, and 23 herds were allocated for the second period. Diet composition and ration physically effective particle level were collected every 8 wk. Milk fat percentage data were retrieved from weekly bulk tank measures. Data were analyzed in linear mixed models using repeated measures within herd where MFP was considered the outcome variable. In addition to the main effect of monensin treatment, the following covariates were forced a priori into all statistical models: treatment period, weekly herd mean parity, and weekly herd mean days in milk. The majority of herds were fed a total mixed ration (n = 29) and were housed in tie-stalls (n = 42). Monensin significantly decreased bulk tank MFP by 0.12 percentage points. The reduction of MFP associated with monensin was larger for herds having a diet high (>39.7%) in nonfiber carbohydrates, having a low level of physically effective particles in ration (>45.0%; ≥8 mm), and not feeding dry hay as first meal in the morning. Significant interactions between monensin and nutritional factors on bulk tank MFP were related to nonfiber carbohydrate and fiber concentrations in the diet.     

Milk production and nitrogen excretion of dairy cows fed different amounts of protein and varying proportions of alfalfa and corn silage

Four trials were conducted to determine the effect of dietary protein amount on lactation performance and N utilization. Each trial used one of the following alfalfa-to-corn-silage ratios for the forage part of the diet: 100:0, 75:25, 50:50, and 25:75. All trials utilized 16 mid-lactation Holstein cows (days in milk averages ranging from 80 to 140 among trials) in a replicated 4 × 4 Latin square design with 3-wk periods, including 2 wk for adaptation and 1 wk for data collection. Diets consisted of 50% forage and 50% concentrate (dry matter basis) and were formulated to contain 15.00, 16.25, 17.50, or 18.75% protein in each trial. The analyzed protein content of the diets was 15.7, 16.9, 18.0, and 19.2% when averaged across trials. Milk yield was similar among dietary protein levels in each trial, ranging from 35.2 to 36.1 kg/d when data were combined across trials. Changes in milk fat and protein due to the protein content of the diet were small and inconsistent. Both milk urea nitrogen and blood urea nitrogen concentrations increased linearly as the protein content of the diet was increased, ranging from 9.9 to 13.1 and from 9.9 to 13.8 mg/dL, respectively, across trials. As dietary protein was increased from the lowest to the highest concentrations when data were combined and analyzed, mean fecal N concentration increased from 2.8 to 3.0%, and urinary N from 5.8 to 7.3 g/L. At the same time, mean total N excretion increased from 484 to 571 g/d, and conversion of intake N to milk N decreased from 0.27 to 0.22, resulting in an average change of 18%. Of the N excreted, urinary N accounted for an increasing proportion, ranging from 41 to 48%, as dietary protein was increased. Overall, based on N utilization as well as milk production, 17% protein in diets utilizing various proportions of alfalfa and corn silage as the forage source appeared sufficient for cows producing 38 kg/d of milk in this study.     

Analysis of milk urea nitrogen and lactose and their effect on longevity in Canadian dairy cattle

The aim of this study was to assess the phenotypic level of lactose and milk urea nitrogen concentration (MUN) and the association of these traits with functional survival of Canadian dairy cattle using a Weibull proportional hazards model. A total of 1,568,952 test-day records from 283,958 multiparous Holstein cows from 4,758 herds, and 79,036 test-day records from 26,784 multiparous Ayrshire cows from 384 herds, calving from 2001 to 2004, were used for the phenotypic analysis. The overall average lactose percentage and MUN for Ayrshires were 4.49% and 12.20 mg/dL, respectively. The corresponding figures for Holsteins were 4.58% and 11.11 mg/dL. Concentration of MUN increased with parity number, whereas lactose percentage decreased in later parities. Data for survival analysis consisted of 39,536 first-lactation cows from 1,619 herds from 2,755 sires for Holsteins and 2,093 cows in 228 herds from 157 sires for Ayrshires. Test-day lactose percentage and MUN were averaged within first lactation. Average lactose percentage and MUN were grouped into 5 classes (low, medium-low, medium, medium-high, and high) based on mean and standard deviation values. The statistical model included the effects of stage of lactation, season of production, the annual change in herd size, type of milk-recording supervision, age at first calving, effects of milk, fat, and protein yields calculated as within herd-year-parity deviations, herd-year-season of calving, lactose percentage and MUN classes, and sire. The relative culling rate was calculated for animals in each class after accounting for the remaining effects included in the model. Results showed that there was a statistically significant association between lactose percentage and MUN in first lactation with functional survival in both breeds. Ayrshire cows with high and low concentration of MUN tended to be culled at a higher than average rate. Instead, Holstein cows had a linear association, with decreasing relative risk of culling with increasing levels of MUN concentration. The relationship between lactose percentage and survival was similar across breeds, with higher risk of culling at low level of lactose, and lower risk of culling at high level of lactose percentage.     

Protein level for alfalfa and corn silage-based diets: I. Lactational response and milk urea nitrogen

This study was designed to evaluate lactational responses of cows fed corn silage (CS) or alfalfa silage (AS) as primary forage source when the diet was balanced for recommended (RP) or excessive (HP) amounts of rumen degradable protein (RDP) and undegradable protein (RUP) according to the recommendations of the National Research Council (NRC). A second objective was to evaluate different sources of variations in milk urea N (MUN). The total mixed rations included 55% forage on a dry matter (DM) basis as either 14% CS and 41% AS or 14% AS and 41% CS. Diets were offered to 48 multiparous Holstein cows (body weight = 652 kg) that were assigned randomly to treatments arranged as a 2 x 2 factorial in 12 complete blocks based on calving date. Data collected during wk 4 to 12 of lactation were adjusted to those obtained from a pretreatment diet fed during wk 1 to 3. Crude protein (CP) averaged 16.5, 18.0, 16.2, and 17.1% of DM in the AS-RP; AS-HP; CS-RP; and CS-HP diets, respectively. Overall DM intake (DMI) was 1.5 kg/d lower than predicted by NRC (24.6 vs. 26.1 kg/d), but 3.5% fat-corrected milk (FCM) was higher than expected (46.1 vs. 45.0 kg/d). The responses to a reduction in dietary protein were independent of primary forage source, except for milk true protein (TP) percentage. Primary forage source did not influence DMI, 3.5% FCM, TP yield, or MUN. However, compared with the AS-based diets, cows fed CS-based diets produced more milk (49.0 vs. 46.4 kg/d), less fat (3.07% vs. 3.54% and 1500 vs. 1651 g/d), and tended to gain more body weight. There were no benefits to feeding diets above NRC protein recommendations, regardless of forage source. Reducing CP from 17.5 to 16.4% of diet DM did not alter milk yield (47.7 kg/d) or milk TP yield (1293 g/d), but lowered N intake by 65 g/d (700 vs. 635 g/d) and lowered MUN by 1 unit (12.7 vs. 11.7 mg/dL). A positive correlation between MUN and production efficiency (3.5% FCM/DMI) on wk 3 of lactation suggested that body protein mobilization might impact MUN in early lactation. The correlation between MUN and DMI tended to be negative in wk 3, but was positive in wk 6 to 12 of lactation. The same was true for the correlation between MUN and somatic cell score. Regression analysis of the postpeak lactation data of this study indicated that the expected MUN was essentially 12 mg/dL when NRC-predicted RDP and RUP balances were 0 g/d, with a linear deviation of 0.1 and 0.03 mg/dL per 10 g of change in RDP and RUP balance, respectively.     

Short communication: Variability in milk urea nitrogen and dairy total mixed ration composition in the northeastern United States

The main objective of this survey was to examine variability in milk urea nitrogen (MUN) for Dairy Herd Improvement Association (DHIA) herds in the northeastern United States (the Northeast), examine trends in dairy cow diet composition, and determine potential relationships for MUN and diet composition. Trends in milk fat and protein concentrations, milk yield, days in milk on test day, and lactation number of the cows were also evaluated. The data set for the survey included 10,839,461 DHIA dairy cow records from 2004 to 2015 for 13 states (CT, DE, MA, MD, ME, NH, NJ, NY, PA, RI, VA, VT, and WV) and was retrieved from Dairy Records Management Systems (Raleigh, NC). Average (across states and years) milk yield, milk fat, and milk protein were 31.6 ± 0.24 kg/d, 3.85 ± 0.021%, and 3.13 ± 0.013%, respectively. No obvious trends were observed for milk fat or protein content, but milk yield steadily increased during the survey period. Milk urea N concentration averaged 13.3 ± 0.13 mg/dL, with no obvious or consistent trends. Examination of variability in dairy feed cost and all milk price for the Northeast indicated that high MUN generally coincided with high feed cost and high milk price. For the diet composition survey, 9,707 records of total mixed ration (TMR) analyses, unrelated to the milk composition data set, from the Cumberland Valley Analytical Service (Maugansville, MD) database were examined. Concentration of TMR crude protein (CP) decreased from 17.1% in 2007 to 16.4% in 2015, but there was not an obvious trend in soluble protein concentration. Concentration of TMR neutral detergent fiber (NDF) and 24-h in vitro NDF degradability declined steadily during the survey period and was accompanied by a steady increase in TMR starch concentration. Examination of these unrelated data sets revealed lack of correlation between MUN and diet chemical composition. Thus, we conclude that individual cow MUN in Northeast dairy herds fluctuated between 2004 and 2015. It appeared that MUN followed variability in feed cost; however, ration feed ingredient data were not available to better define the reasons for the variations in MUN.     

Evaluation of milk urea nitrogen as a diagnostic of protein feeding

An evaluation of milk urea nitrogen (MUN) as a diagnostic of protein feeding in dairy cows was performed using mean treatment data (n = 306) from 50 production trials conducted in Finland (n = 48) and Sweden (n = 2). Data were used to assess the effects of diet composition and certain animal characteristics on MUN and to derive relationships between MUN and the efficiency of N utilization for milk production and urinary N excretion. Relationships were developed using regression analysis based on either models of fixed factors or using mixed models that account for between-experiment variations. Dietary crude protein (CP) content was the best single predictor of MUN and accounted for proportionately 0.778 of total variance [MUN (mg/dL) = -14.2 + 0.17 x dietary CP content (g/kg dry matter)]. The proportion of variation explained by this relationship increased to 0.952 when a mixed model including the random effects of study was used, but both the intercept and slope remained unchanged. Use of rumen degradable CP concentration in excess of predicted requirements, or the ratio of dietary CP to metabolizable energy as single predictors, did not explain more of the variation in MUN (R(2) = 0.767 or 0.778, respectively) than dietary CP content. Inclusion of other dietary factors with dietary CP content in bivariate models resulted in only marginally better predictions of MUN (R(2) = 0.785 to 0.804). Closer relationships existed between MUN and dietary factors when nutrients (CP to metabolizable energy) were expressed as concentrations in the diet, rather than absolute intakes. Furthermore, both MUN and MUN secretion (g/d) provided more accurate predictions of urinary N excretion (R(2) = 0.787 and 0.835, respectively) than measurements of the efficiency of N utilization for milk production (R(2) = 0.769). It is concluded that dietary CP content is the most important nutritional factor influencing MUN, and that measurements of MUN can be utilized as a diagnostic of protein feeding in the dairy cow and used to predict urinary N excretion.     

Association between milk urea nitrogen and fertility in Ohio dairy cows

The purpose of this study was to evaluate the association between milk urea nitrogen (MUN) and fertility of dairy cows using field data. The data came from 24 dairy herds belonging to Ohio Dairy Herd Improvement Cooperative Inc. Reproductive data and MUN measurements from cows that calved between June 1998 and May 1999 and that had been bred at least once were included in the study. Survival analysis, using the Cox proportional hazards model, was performed and days from calving to conception or to the end of the study was used as the outcome. Cows that had not been reported pregnant during the study were considered censored. The mean of monthly MUN values of cows before conception (or the end of the study for censored cows) was used to reflect the MUN status of a cow. Animals were categorized into quartiles based on MUN values in these data. Parity, calving season, peak milk yield, number of services, and herd were included in the models as fixed effects. Cows with MUN levels below 10.0 were 2.4 times more likely and cows with MUN levels between 10.0 and 12.7 mg/dl were 1.4 times more likely to be confirmed pregnant than cows with MUN values above 15.4 mg/dl. Our results indicate that increasing MUN levels appear to be negatively related to dairy cow fertility and are associated with a lower risk of detectable pregnancy at herd checks. They also suggest that the levels of MUN that are adversely associated with fertility might be lower than reported earlier.     

Short communication: Evaluation of milk urea nitrogen as a management tool to reduce ammonia emissions from dairy farms

The purpose of this study was to compile and evaluate relationships between feed nitrogen (N) intake, milk urea N (MUN), urinary urea N (UUN), and ammonia (NH₃) emissions from dairy farms to aid policy development. Regression relationships between MUN, UUN, and NH₃ emissions were compiled from studies conducted in Wisconsin, California, and the Netherlands. Relative reductions in NH₃ emissions were calculated as percentage decreases in NH₃ emissions associated with a baseline MUN level of 14 mg/dL (prevailing industry average). For 3 studies with cows in stanchion barns, relative NH₃ emission reductions of 10.3 to 28.2% were obtained when MUN declined from 14 to 10 mg/dL. Similarly, analyses of 2 freestall studies provided relative NH₃ emission reductions of 10.5 to 33.7% when MUN levels declined from 14 to 10 mg/dL. The relative reductions in NH₃ emissions from both stanchion and freestall barns can be associated directly with reductions in UUN excretion, which can be determined using MUN. The results of this study may help create new awareness, and perhaps eventual industry-based incentives, for management practices that enhance feed N use efficiency and reduce MUN, UUN, and NH₃ emissions from dairy farms.