Cow and herd variation in milk urea nitrogen concentrations in lactating dairy cattle

Effects of dietary starch and ruminally undegraded protein on glucogenic precursors in lactating dairy cows

Gluconeogenesis is a large contributor to the blood supply of glucose carbons. The impact of varying dietary starch and ruminally degraded protein (RDP) on glucose entry, and the contributions of propionate and lactate to total plasma glucose entry were evaluated. Six cannulated, lactating, Holstein cows were fed one of four treatment diets arranged as a 2 × 2 factorial within a 4 × 4 partially replicated Latin Square design: (1) 8% RDP (LRDP) and 16% starch (LSt), (2) LRDP and 30% starch (HSt), (3) 11% RDP (HRDP) and LSt, or (4) HRDP and HSt. On d 12 of each period, 2-[13C]-sodium propionate (0.15 g/h) was ruminally infused for 4 h; on d 13, 1,2-[13C2]-glucose (0.2 g/h) was infused into the jugular vein for 1 h followed by 1-[13C]-lactate (0.1 g/h) for 1 h. Blood samples were serially collected starting prior to the infusions, and analyzed for plasma glucose, propionate, and lactate isotopic ratios. A one-compartment, glucose carbon model with inputs from lactate, propionate, and other glucogenic precursors (Oth, primarily absorbed glucose plus amino acids) was fitted to the isotope ratio data to derive glucose entry rates and conversion of the precursors to glucose. Milk protein production additively increased when HSt and HRDP were fed (P = 0.05 and P = 0.02, respectively). Plasma glucose and propionate concentrations increased with HSt (P = 0.04 and P = 0.01, respectively) and LRDP (P = 0.02 and P < 0.01, respectively). Total glucose and Oth entry increased (P = 0.03 and P = 0.03, respectively) with HSt, indicating greater glucose absorption from the small intestine or conversion of amino acids to glucose in the liver. However, neither entry rate was affected by RDP. The lack of an RDP effect suggests the increase in microbial outflow in response to RDP did not significantly alter glucose precursor supplies. Entry rates of propionate and lactate carbon to glucose carbon were not affected by treatment suggesting that neither starch nor RDP significantly affected fermentation or lactate production. Derivation of absolute entry rates and contributions to glucose using isotopic tracers is complicated by single carbon removals in the pentose phosphate (PPP), tri-carboxylic acid (TCA), and gluconeogenic pathways, and label randomization with the PPP and TCA pathways. Multiple tracers must be used to avoid assumptions regarding the proportional entries. These results provide insights on glucose supply and contributors, and draw attention to significant label cycling when utilizing isotope techniques.

Determining the Effects of Pelleted Cranberry Vine Grains on the Ewe and Offspring during Pregnancy and Lactation

When creating any new anti-parasitic interventions, it is important to evaluate their effects across all life stages. This study had three objectives, which were to evaluate the effect of feeding cranberry vine pellet (CVP) on (1) ewes' body weights and BCS during late gestation and lactation; (2) ewes' milk quality during lactation; and (3) lambs' body weight and growth parameters from birth to 65 days of age. Across two years, 41 Dorset ewes were fed either a 50% CVP or a matching control pellet (CON) from 104 ± 1.60 days of gestation for 62.8 ± 0.68 days of lactation. Measurements were collected from ewes (BW, BCS, and milk) and lambs (BW and body size). Milk from CVP ewes exhibited reduced milk fat and solids (p < 0.01) and increased concentrations of milk urea nitrogen (p = 0.02) when evaluated for the treatment-time. There was no significant difference in the BCS, protein, lamb BW, or growth measurements for treatment-time (p ≥ 0.05). Additional research that targets blood biochemistry and metabolic assessments is needed to fully determine the impact of this pellet on ewes and lambs.

Relationship between milk urea content and important milk traits in Holstein cattle

Breeding cattle with low nitrogen emissions has been proposed as a countermeasure against eutrophication due to dairy production. Milk urea content (MU) could potentially serve as a new readily measured indicator trait for nitrogen emissions by cows. Therefore, we estimated genetic parameters related to MU and its relationship with other milk traits. We analysed 4 178 735 milk samples collected between January 2008 and June 2019 from 261 866 German Holstein dairy cows during their first, second, and third lactations. Restricted maximum likelihood estimation was conducted using univariate and bivariate random regression sire models in WOMBAT. We obtained moderate average daily heritability estimates for the daily MU of 0.24 in first lactation cows, 0.23 in second lactation cows, and 0.21 in third lactation cows with average daily genetic SDs of 25.16 mg/kg, 24.93 mg/kg, and 23.75 mg/kg, respectively. Averaged over days in milk, the repeatability estimates were low at 0.41 in first, second, and third lactation cows. A strong positive genetic correlation was found between MU and milk urea yield (MUY; 0.72 on average). In addition, 305-day heritabilities were estimated as 0.50, 0.52, and 0.50 in first, second, and third lactation cows, respectively, with genetic correlations of 0.94 or higher for MU in different lactations. By contrast, the averaged estimates of the genetic correlations between MU and other milk traits were low (-0.07 to 0.15). Moderate heritability estimates clearly allow the possible selection for MU, and the near-zero estimates of genetic correlations indicate no risk of undesired correlated selection responses in other milk traits. However, a relationship still needs to be established between MU as an indicator trait and the target trait, defined as total individual nitrogen emissions.

Evaluation of Urine Nitrogen Excretion as the Measure of the Environmental Load and the Efficiency of Nitrogen Utilization

The aim of this study was to evaluate the relationship between nutrition, the concentration of milk urea nitrogen (MUN) and the urinary nitrogen excretion in farm conditions and to use the MUN concentration as a tool to control protein nutrition and environmental load in dairy farming. Urinary N excretion was evaluated by an empirical model according to the intake and metabolic transformation of N to milk protein in selected farms was on average 208.8 ± 34.8 g with a range of values from 127.7 to 277.8 g N.day−1. The evaluated proportion of excreted N in relation to crude protein (CP) intake in the total mix ration (TMR) was statistically significant (R2 = 0.504; P < 0.0001). Urinary N excretion, evaluated according to the analysis of the MUN content, using selected regression equations, was on average 211.8 ± 24.3 g.day−1 with an individual variation of 157.2‒274.7 g.day−1 with a significantly higher positive correlation to the received CP in the TMR (R2 = 0.693; P < 0.0001). The evaluated effect of CP concentration in the TMR on urine N excretion confirmed the higher nitrogen excretion in the urine by 25.6 g per day with an increased CP in the TMR by 1 %. The proportion of urea nitrogen in the total N excreted in the urine was on average 80.5 %. The validation of the models for the prediction of nitrogen excretion, according to the MUN for the practical application on farms, was determined the best equation by Kaufmann a St-Pierre, which used available data from routine analysis of milk composition by the Breeding service of Slovakia. The MUN analysis offered a simple and non-invasive approach to the evaluation of the urinary N excretion, as well as, the efficiency of N utilization from feed to milk.

Fermented soybean meal modified the rumen microbiome to enhance the yield of milk components in Holstein cows

The study was conducted to evaluate the rumen microbiota as well as the milk composition and milk component yields of Holstein cows supplemented with fermented soybean meal (FSBM). Eighteen Holstein cows in their 2nd parity with 54.38 ± 11.12 SD days in milking (DIM) were divided into two dietary groups (CON and TRT) of nine cows per group. The cows in the TRT group received 300 g of FSBM per cow per day in addition to the conventional diet, while each cow in the CON group was supplemented with 350 g of soybean meal (SBM) in their diet daily throughout the 28-day feeding trial. Rumen bacterial composition was detected via 16S rRNA sequencing, and the functional profiles of bacterial communities were predicted. Milk composition, milk yield, as well as rumen fermentation parameters, and serum biochemistry were also recorded. The inclusion of FSBM into the diets of Holstein cows increased the milk urea nitrogen (MUN), milk protein yield, fat corrected milk (FCM), and milk fat yield while the milk somatic cell count (SCC) was decreased. In the rumen, the relative abundances of Fibrobacterota, and Spirochaetota phyla were increased in the TRT group, while the percentage of Proteobacteria was lower. In addition, the supplementation of FSBM to Holstein cows increased the acetate percentage, rumen pH, and acetate to propionate ratio, while the proportion of propionate and propionate % was observed to decrease in the TRT group. The KEGG pathway and functional prediction revealed an upregulation in the functional genes associated with the biosynthesis of amino acids in the TRT group. This enrichment in functional genes resulted in an improved synthesis of several essential amino acids including lysine, methionine, and branch chain amino acids (BCAA) which might be responsible for the increased milk protein yield. Future studies should employ shotgun metagenomics, transcriptomics, and metabolomics technology to investigate the effects of FSBM on other rumen microbiomes and milk protein synthesis in the mammary gland in Holstein cows. KEY POINTS: • The supplementation of fermented soybean meal (FSBM) to Holstein cows modified the proportion of rumen bacteria. • Predicted metabolic pathways and functional genes of rumen bacteria revealed an enrichment in pathway and genes associated with biosynthesis of amino acids in the group fed FSBM. • The cows supplemented with FSBM record an improved rumen fermentation. • Cows supplemented with FSBM recorded an increased yield of milk protein and milk fat.

Effect of Post-Ruminal Casein Infusion on Milk Yield, Milk Composition, and Efficiency of Nitrogen Use in Dairy Cows

Adequate supply of amino acids can improve the efficiency of nitrogen use. Casein is the predominant milk protein, and its supplementation can improve milk protein synthesis and nitrogen efficiency. We evaluated the effects of post-ruminal supplementation of casein on milk yield and composition and whole-body protein deposition. Two ruminally cannulated Holstein dairy cows (599 kg) were used in a switch-back design, and treatments were an abomasal infusion of 0 or 400 g/day casein. Cows were fed a diet consisting of corn silage, alfalfa hay, wet corn gluten feed, whole cottonseed, and grain mix, and they received 320 g/day dextrose via abomasal infusion to increase energy:metabolizable protein. The experiment used three 8-day periods. Milk, urine, and feces samples were collected to evaluate milk production, milk composition, and nitrogen retention. Abomasal casein infusion increased (p < 0.01) milk protein percentage and milk urea nitrogen. Nitrogen retention (p = 0.03) and urinary N excretion (p < 0.001) were increased and fecal N excretion (p < 0.001) was decreased by casein infusion. Results suggest casein stimulated protein deposition and altered nitrogen use in lactating dairy cattle. Adaptation periods of 4 days were appropriate for evaluating responses to casein supplementation. Our data provide elements that can aid the design of future experiments.

Assessment of dietary protein supplementation on milk productivity of commercial organic dairy farms during the grazing season

Variability of protein and energy supply from pasture during the grazing season is a primary factor that can influence milk production of grazing organic dairy herds in the Northeast United States. This study evaluated the effects of altering the crude protein (CP) content of dietary supplements included in dairy rations fed to grazing organic dairy herds, on milk production and composition. Six commercial organic farms participated in a 6-wk trial, consisting of a 2-wk baseline period and 4-wk experimental period. Farms were paired by their summer 2017 milk urea nitrogen profile, and farms within each pair were assigned by restricted randomization to (1) continuation of their regular supplements (n = 3, control group, CON), or (2) supplement with altered CP as percentage of dry matter, formulated using an organic barley and roasted soybean mix (n = 3, treatment group, TRT). Throughout the 6-wk trial, individual milk samples were collected at 2 consecutive milkings weekly, while pasture and supplement samples, pasture measurements, and management information were collected twice weekly per farm. Data were statistically analyzed using the MIXED procedure of SAS (version 9.4, SAS Institute Inc.) for all parameters, and effects of treatment, week, and their interaction (treatment × week) were determined. The supplement CP (percentage of dry matter) during the baseline period was 13.5% for CON and 15.3% for TRT and 14.8% for CON and 19.3% for TRT during the experimental period. Milk production was 21% higher during the experimental period for TRT compared with CON (24.1 vs. 19.9 kg of milk per day, respectively). Milk production decreased for CON from wk 1 to wk 6 (23.6 vs. 20.4 kg of milk per day), whereas TRT maintained milked production from wk 1 to wk 6 (22.8 vs. 22.7 kg of milk per day). Milk composition was different between groups, with CON having higher fat percent (4.21 vs. 3.73%, respectively) and protein percent (3.15 vs. 3.05%, respectively) compared with TRT for the 6 wk. The milk urea nitrogen concentrations were similar between TRT and CON for the baseline period (11.9 vs. 12.1 mg/dL) and the final week of the experimental period (14.5 vs. 14.2 mg/dL). Although the effects of different diet CP fractions, particularly rumen undegradable protein and soluble protein, must be further delineated, these results indicate that altering the CP content of dietary supplements fed to grazing organic dairy cattle during the summer period in the Northeast US could be a useful mechanism to maintain milk production.

Grazed rain-fed small-grain cereals as a forage option for small-scale dairy systems in central Mexico

Small-scale dairy systems face reduced availability of water for irrigation of pastures and disruption in the amount or pattern of rains due to climate change, so research on alternative short-cycle rain-fed forages is needed. Grazing reduces feeding costs and small-grain cereals may be an option. The objective was to assess on-farm the performance of dairy cows grazing 6 h/day of three small-grain cereals: rye (RYE), wheat (WHT), and triticale (TRT), and supplemented 4.5 kg dry matter (DM)/cow/day of concentrate. Twelve Holstein cows were used in repeated 3 × 3 Latin squares with 14-day experimental periods. Pasture variables were analysed with a split-plot design, and economic analysis was performed with partial budgets. Sampling of forage (sward height, net herbage accumulation, botanical and chemical composition of herbage) and animal variables (milk yield and composition, live weight, and body condition score) were at the end of each period. The RYE showed a trend (P > 0.05) for higher net herbage accumulation (NHA) with highly significant differences (P < 0.001) among periods. The RYE had higher DM, a lower crude protein (CP) content (P < 0.05), and no differences for other chemical components between treatments (P > 0.05). The RYE proportion in pasture was consistently higher, whilst the lowest cereal proportion was in WHT. There were no differences (P > 0.05) for any animal variable. The RYE and TRT treatments proved useful as rain-fed forage alternatives under grazing. WHT showed lower crop and economic performance. However, given the higher cereal component in pastures, regrowth potential, and post-grazing herbage mass, as well as better economic performance, the RYE treatment was better ranked by the participating farmer.

Milk Urea Concentration to the Evaluation of Nitrogen Efficiency Transformation on Dairy Farms

The aim of this study was to evaluate the effects of nutrition on the milk urea nitrogen (MUN) concentration; on the transformation of N in the farm’s conditions; and there-by allow the milk urea nitrogen concentration to serve as a tool to maximize the protein nutrition and the metabolism of the cows. The relations evaluated by linear or multiple regression confirmed that the highest nutritional effects of the crude protein (CP) on the MUN concentration, which represented a 69.3 % variation in the MUN content. According to the CP content in the total mix ration (TMR) and MUN content (3150 milk samples) under farm conditions, a regression relationship was determined for the estimated of MUN (mg.dl–1) = –13.2 + 0.16 × CP (g.kg–1 dry matter). For multiple regression, the rate of variation expressed by this relationship increased to 72 for nutrient content and 78.3 % for nutrient intake in the TMR. The efficiency of nitrogen utilization (ENU) determined by calculations based on the MUN content according to the regression equations represented a negative correlation (P < 0.0001; R2 = 0.854) with respect to the CP content in the TMR and that the increased CP content by 1 % in the range of 14 to 18 % in the TMR decreased the ENU by 1.48 %. Validation of the models for prediction of nitrogen transformation (ENU) for practical application on the farms determined the best equation, which used the available data from the routine analysis of Breeding services of Slovakia. After taking into consideration of our breeding conditions, it was confirmed that the equation of ENU had taken into account the MUN, in addition to the amount of the milk produced.

Nitrogen Use Efficiency and Excretion in Grazing Cows with High and Low Milk Urea Nitrogen Breeding Values

Milk urea nitrogen content is moderately heritable and is phenotypically related to urine nitrogen (UN). Based on this relationship, it has been suggested that genetic selection for lower milk urea nitrogen in grazing dairy cows could decrease UN concentration thereby reducing nitrogen excretions into the ground. The objective of this study was to compare the nitrogen use efficiency (NUE) and excretion in grazing cows with high and low milk urea nitrogen breeding values (MUNBV) in two farms of contrasting farming intensity. On the high-intensity farm (HIF) 68 and 70 cows with low and high MUNBV, respectively, were fed higher levels of supplementation and milked twice-daily, while on the low-intensity farm (LIF) 82 and 86 cows with low and high MUNBV, respectively, were fed lower levels of supplementation milked once-daily. Nitrogen use efficiency (g/g) was calculated as the ratio of daily milk N to daily N intake. Daily N intake (g/day) was derived from feed intake estimates based on energy requirements. The UN (g/day) was estimated by back-calculation from dietary N and subtracting milk N, faecal N, and N retained in body tissues. Irrespective of farm, cows with low MUNBV had significantly lower MY and milk urea nitrogen (p < 0.001) but this was not linked to significantly less UN. In the LIF, cows with low MUNBV had lower NUE (p < 0.001) than cows with high MUNBV, and this was explained by the reduced protein yield (p < 0.001). Selecting cows for low MUNBV was not an effective tool to reduce N losses and to increase the NUE in two dairy farms of contrasting farming intensity.

Identification of Genomic Regions Influencing N-Metabolism and N-Excretion in Lactating Holstein- Friesians

Excreted nitrogen (N) of dairy cows contribute to environmental eutrophication. The main N-excretory metabolite of dairy cows is urea, which is synthesized as a result of N-metabolization in the liver and is excreted via milk and urine. Genetic variation in milk urea (MU) has been postulated but the complex physiology behind the trait as well as the tremendous diversity of processes regulating the N-metabolism impede the consistent determination of causal regions in the bovine genome. In order to map the genetic determinants affecting N-excretion, MU and eight other N-excretory metabolites in milk and urine were assessed in a genome-wide association study. Therefore phenotypes of 371 Holstein- Friesians were obtained in a trial on a dairy farm under near commercial conditions. Genotype data comprised SNP information of the Bovine 50K MD Genome chip (45,613 SNPs). Significantly associated genomic regions for MU concentration revealed GJA1 (BTA 9), RXFP1, and FRY1 (both BTA 12) as putative candidates. For milk urea yield (MUY) a promising QTL on BTA 17 including SH3D19 emerged, whereas RCAN2, CLIC5, ENPP4, and ENPP5 (BTA 23) are suggested to influence urinary urea concentration. Minor N-fractions in milk (MN) may be regulated by ELF2 and SLC7A11 (BTA 17), whilst ITPR2 and MYBPC1 (BTA 5), STIM2 (BTA 6), SGCD (BTA 7), SLC6A2 (BTA 18), TMCC2 and MFSD4A (BTA 16) are suggested to have an impact on various non-urea-N (NUN) fractions excreted via urine. Our results highlight genomic regions and candidate genes for N-excretory metabolites and provide a deeper insight into the predisposed component to regulate the N-metabolism in dairy 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.

Novel Crabtree negative yeast from rumen fluids can improve rumen fermentation and milk quality

Upgrading the nutritive value of rice straw (RS) is necessary to increase its contribution to enhancing meat and milk production. Present work verified whether novel Crabtree negative yeast inoculant could promote RS utilization, rumen fermentation, and milk quality in tropical crossbred lactating Holstein cows. The new stain of Crabtree negative yeasts (Pichia kudriavzevii KKU20 and Candida tropicalis KKU20) was isolated from the rumen of dairy cattle. This study used 6 multiparous crossbreds between Holstein Frisian × Zebu dairy cows in their mid-lactation period. Dairy cows were randomly allocated to three ensiled RS with various yeast stains including Saccharomyces cerevisiae, P. kudriavzevii KKU20, and C. tropicalis KKU20 according to a 3 × 3 replicated Latin square design. Crabtree-negative yeast (P. kudriavzevii and C. tropicalis) increased the apparent digestibility of dry matter by about 6.9% when compared with Crabtree-positive yeast (S. cerevisiae). Bacterial populations were highest with ensiled RS by C. tropicalis KKU20. Ensiled RS with Crabtree-negative yeasts were significantly increased with total volatile fatty acids, but they did not affect volatile fatty acid profiles. Milk protein precentage was highest at 35.6 g/kg when C. tropicalis was fed, and lowest when applied with S. cerevisiae and P. kudriavzevii KKU20 in ensiled RS at 34.5 and 34.1 g/kg, respectively. Thus, feeding ensiled RS with novel Crabtree negative yeast could improve RS digestion, rumen fermentation, and milk protein content in dairy cows.

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.

Proxy Measures and Novel Strategies for Estimating Nitrogen Utilisation Efficiency in Dairy Cattle

Simple Summary

Dairy cow diets contain nitrogen, mostly in the form of protein. However, dietary nitrogen is used with a low efficiency for milk production, and much of the unused nitrogen is converted to urea and excreted in urine and faeces (manure). Nitrogen within manure can then be lost to the environment, and this is a particular issue when dairy cows are offered diets containing excess dietary protein. As a result, there is increasing pressure on the dairy sector to improve the efficiency with which dairy cows utilise dietary nitrogen. While nitrogen utilisation efficiency can be measured accurately on research farms, this is more difficult on commercial farms. For that reason, there is much interest in developing low-cost and easy-to-use proximate measures that can provide accurate estimates of nitrogen utilisation. This review examines a number of proximate analyses that are already used as indicators of nitrogen use efficiency in dairy cows (e.g., blood urea and milk urea), and a number of more novel measures that may have potential for use in the future (including analysis of milk, blood, urine, breath, and predictions of intake). These ‘proxy’ measurements can be used to improve feeding management and might be used to monitor adherence to legislation.

Abstract

The efficiency with which dairy cows convert dietary nitrogen (N) to milk N is generally low (typically 25%). As a result, much of the N consumed is excreted in manure, from which N can be lost to the environment. Therefore there is increasing pressure to reduce N excretion and improve N use efficiency (NUE) on dairy farms. However, assessing N excretion and NUE on farms is difficult, thus the need to develop proximate measures that can provide accurate estimates of nitrogen utilisation. This review examines a number of these proximate measures. While a strong relationship exists between blood urea N and urinary N excretion, blood sampling is an invasive technique unsuitable for regular herd monitoring. Milk urea N (MUN) can be measured non-invasively, and while strong relationships exist between dietary crude protein and MUN, and MUN and urinary N excretion, the technique has limitations. Direct prediction of NUE using mid-infrared analysis of milk has real potential, while techniques such as near-infrared spectroscopy analysis of faeces and manure have received little attention. Similarly, techniques such as nitrogen isotope analysis, nuclear magnetic resonance spectroscopy of urine, and breath ammonia analysis may all offer potential in the future, but much research is still required.

Bulk milk quality as affected by cattle breed composition of the herd in mountain area

The aim of this study was to investigate the variation of gross composition, somatic cell count, urea content, and fatty acids (FA) composition of bulk milk from single-breed and multi-breed farms in alpine area, keeping either Brown Swiss (BS), Holstein Friesian (HF), Simmental (SI), or their combinations. Gross milk composition, urea content, and FA composition were predicted using mid-infrared spectroscopy. Observations were grouped in 7 combinations consisting of 3 singlebreed and 4 multi-breed types of herd. A mixed linear model was used for data analysis, accounting for the fixed effects of herd composition (7 combinations), month of sampling, year of sampling, and the interactions between herd composition and month of sampling, and between herd composition and year of sampling. Farm was included as random effect. Results highlighted that about two thirds of South Tyrolean farms were single-breed and herds with more than 20 lactating cows changed herd structure over time, switching from multi- to single-breed. Single-breed BS farms produced milk with greater fat, protein, casein, lactose, and FA content than single-breed HF and SI farms. Further, multi-breed herds including BS cows produced milk with greater fat, protein, casein, and polyunsaturated FA content than multi-breed HF+SI herds. Overall, single-breed SI farms produced milk with lower somatic cell count than other herd combinations. Despite the number of BS cows in South Tyrol has decreased in favor of SI in the last years, this breed is still the most interesting for alpine dairy farming to achieve optimal milk quality in both single- and multi-breed scenarios. The tendency to move to SI is mainly related to good milk performance of SI cows coupled with their robustness, high carcass value, high market value of calves, and adaptability to mountain farming system.

Low dietary protein resilience is an indicator of the relative protein efficiency of individual dairy cows

Our objectives were to determine (1) the sources of variation in cow responses to dietary protein reduction, and (2) the association of low dietary protein resilience (LPR) with protein efficiency. Lactating Holstein cows in peak lactation (n = 166; 92 primiparous, 77 multiparous) with initial milk yield 41 ± 9.8 kg/d were fed high-protein (HP) or low-protein (LP) diets in 4-wk periods in a crossover design with half the cows fed LP first and half fed HP first. The study was repeated for 69 of these cows (42 primiparous, 27 multiparous) in late lactation. Low-protein diets were 14% crude protein (CP) in peak lactation and 13% CP in late lactation and were formulated to contain adequate rumen-degradable protein to maintain rumen function but inadequate rumen-undegradable protein for cows with average production in this study to maintain their production. High-protein diets were 18% CP in peak lactation and 16% CP in late lactation and contained extra expeller soybean meal to meet metabolizable protein requirements. Protein efficiency was defined as the protein captured in milk or in both milk and body tissues per unit of consumed protein. Low dietary protein resilience was calculated for each cow in peak and late lactation based on actual intake, production, and body weight measures. The ability of a cow to maintain total protein captured in milk and body gain when fed less protein varied considerably and the variation was mostly explained by a cow's captured protein per kilogram of metabolic body weight when fed HP, her parity, treatment sequence, and experiment. Protein efficiency was moderately repeatable across diets within lactation stage. Milk urea nitrogen was not associated with protein efficiency in individual cows within a diet and lactation stage. Cows with greater dietary protein resiliency (higher LPR) had similar protein efficiency on the HP diet as cows with lower LPR, but higher protein efficiency on the LP diet. In conclusion, cows generally maintained their protein efficiency rankings when switching diets between sufficient or insufficient protein; however, some high-producing cows are better able to maintain high production when fed less protein. We define this ability as LPR and suggest it might be useful for identifying cows that use protein more efficiently to enhance dairy sustainability.

Non-Invasive Indicators Associated with Subacute Ruminal Acidosis in Dairy Cows

The aim of the study was to characterize the interrelationship between decreased ruminal fluid pH during subacute ruminal acidosis (SARA) and concentrations of principal constituents of milk and biochemical indices associated with nitrogen utilizations such as rumen ammonia nitrogen (RAN), blood urea nitrogen (BUN) and milk urea nitrogen (MUN). Ruminal fluid samples were obtained by rumenocentesis from 305 cows representing 13 dairy herds. The cows were divided according to ruminal fluid pH into three groups: low, moderate, and high rumen pH cows. The herds were divided into three groups on the basis of the percentages of cows with an assigned value of ruminal fluid pH: SARA-positive, SARA-risk and SARA-negative. SARA-positive herds were characterized by higher concentrations of RAN (12.6 vs. 6.9 mg/dL), BUN (16.2 vs. 10.1 mg/dL) and MUN (12.4 vs. 9.1 mg/dL) compared to SARA-negative herds. Similarly, low-rumen pH cows had greater concentrations of RAN, BUN and MUN than high-rumen pH cows (11.9 vs. 5.8 mg/ dL, 19.9 vs. 14.1 mg/dL, and 12.3 vs. 9.5 mg/dL, respectively). Moreover, SARA-positive herds and low-rumen pH cows had the highest lactose and the lowest fat concentrations in milk. The study demonstrated that the concentration of milk urea nitrogen could be considered one of the indirect and non-invasive indicators of the occurrence of subacute ruminal acidosis in dairy herds.

Milk yield of cows submitted to different levels of olive pomace in the diet

The cultivation of olive trees is expanding in Brazil, mainly in Rio Grande do Sul in order to meet the demand for olive oil, the main product of the industrialization of olives. However, from the extraction, there is a significant generation of waste. This residue has high moisture and an appropriate destination is necessary given its environmental and economic importance. The chemical composition of the residue is similar to other products used in animal feed, such as corn silage or sorghum, in relation to DM, CP and NDF, the differential being high levels of EE and ADL. In order to evaluate the replacement of corn silage by the extraction residue of olive oil (olive pomace), eight Holstein dairy cow, between 90 and 100 days of lactation, were used, making a double Latin square (4x4). Analyzes were made regarding animals, feed and milk product in order to verify the feasibility of the substitution. The inclusion of olive pomace, conserved as silage, to replace corn silage, in the diet of lactating cows up to 15% (dry basis) does not alter milk production, as well as its composition and feed efficiency.

Quantitative and qualitative detailed milk protein profiles of 6 cattle breeds: Sources of variation and contribution of protein genetic variants

Different fractions of milk nitrogenous compounds (not only caseins) have different effects on the nutritional value of milk, its coagulation and curd firming properties, and its cheese-making efficiency. To assess different sources of variation, especially the cows' breed and genetic variants of the main protein fractions, milk samples were collected from 1,504 cows belonging to 3 dairy breeds (Holstein-Friesian, Brown Swiss, and Jersey) and 3 dual-purpose breeds (Simmental, Rendena, and Alpine Grey) reared in 41 multibreed herds. Beyond crude protein, casein (CN), and urea, 7 protein fractions were analyzed using HPLC, and 5 other N fraction traits were calculated. All 15 traits were measured qualitatively (% of milk N) and quantitatively (g/L of milk). The HPLC technique allowed us to discriminate between the main genetic variants of β-CN, κ-CN, and β-lactoglobulin and thus to genotype the cows for the CSN2, CSN3, and BLG genes, respectively. Data were analyzed using 2 mixed models, both including the effects of herd-date, breed, parity, and lactation stage, and only one also including the effects of the genotypes of the milk proteins. Breed of cow explained 2 to 36% of phenotypic variability for all the N fractions, with the exception of the urea and total casein contents of milk and the urea and β-CN proportions of total milk N. Lactation stage had a considerable influence on the amount (g/L) of almost all the protein fractions in milk, but neither the nonprotein N fractions nor the percentage of milk N protein profile were affected. The inclusion of the CSN2, CSN3, and BLG genotypes in the model explained a large part of the total variability in all the milk protein and nonprotein fractions except urea. It also reduced the variance explained by breed and residual factors. An exception was shown by the proportion of α_S1-CN variance explained by breed that moved from 13 to 28%. Similarly, for amount (g/L) of β-CN, the effect of breed became significant (12%), whereas it was almost null before inclusion of genotypes. In terms of percentage of milk N, the genotypes of CSN3 notably affected all the casein fractions, whereas the BLG genotypes had a much greater influence on most noncasein traits. The genotypes of the CSN2 gene exerted an appreciable effect on α_S2-CN and not β-CN, as expected. Comparing the 2 models, we were also able to discriminate the effect of the breed on a milk N fraction, both quantitatively and qualitatively, in 2 quotas: the first due to the milk protein polymorphisms (major genes) and the second due to other genetic factors (polygene), after correcting for the effect of herd-date of sampling, parity, and lactation stage. The knowledge about the detailed milk protein profile of different cattle breeds provided by this study could be of great benefit for the dairy industry, providing new tools for the enhancement of milk payment systems and breeding program designs.

Short communication: Genetic aspects of milk urea nitrogen and new indicators of nitrogen efficiency in dairy cows

Milk urea nitrogen (MUN), a trait routinely measured in the national milk recording system, is a useful indicator of nitrogen utilization efficiency of dairy cows, and selection for MUN and MUN-derived traits could be a valid strategy to produce better animals with regard to efficiency of nitrogen utilization. Therefore, the aim of the present study was to explore the genetic aspects of MUN and new potential indicators of nitrogen efficiency, namely ratios of protein to MUN, casein to MUN, and whey protein to MUN, in the Italian Brown Swiss population. A total of 153,175 test-day records of 10,827 cows in 500 herds were used for genetic analysis. Variance components and heritability of the investigated traits were estimated using single-trait repeatability animal models, whereas genetic and phenotypic correlations between the traits were estimated through bivariate repeatability animal models, including fixed effects of herd-test-date, stage of lactation, parity, calving year, and calving season, and the random effects of additive genetic animal, cow permanent environment, and the residual. Heritability estimates for MUN (0.20 ± 0.01) and the 3 new indicators of nitrogen utilization efficiency (0.15 ± 0.01 for protein-to-MUN and casein-to-MUN ratios, and 0.12 ± 0.01 for ratio of whey protein to MUN) suggested that additive genetic variation exists for these traits, and thus there is potential to select for greater organic nitrogen and lower inorganic nitrogen in milk. Genetic association between MUN and the 3 ratios was high (-0.87 ± 0.01) but not unity, suggesting that ratios could provide some further information beyond that provided by MUN with regard to efficiency of nitrogen utilization. Genetic trend of the investigated traits by year of birth of Brown Swiss sires showed how the selection applied in the last 30 yr has led to an increase of both quantity and quality of milk, and a decrease of somatic cell score and MUN. The inclusion of MUN in breeding programs could speed up the process of increasing organic nitrogen such as protein, which is useful for cheese-making, and reducing inorganic nitrogen (MUN) in milk.

Association of bulk tank milk urea nitrogen concentration with elevated individual cow values and investigation of sampling frequency for accurate assessment

Individual milk urea nitrogen (MUN) levels ≥ 19.63 mg/dL have been recently reported to significantly affect fertility. The objectives of the present study were to (a) predict the percentage of cows with elevated MUN within a herd using bulk tank (BTMUN) levels, in the absence of individual MUN records, and (b) establish a sampling frequency protocol for the assessment of actual BTMUN levels. A database of 17,687 monthly individual MUN and concurrent 229 monthly BTMUN records from 24 dairy herds was used. A ROC analysis was performed to determine the BTMUN threshold over which cows in the herd have elevated MUN concentrations that, based on literature, affect fertility. Moreover, a regression was run to predict the percentage of cows with elevated MUN within a herd from BTMUN values. A second database of 10,687 daily BTMUN records from 29 herds was used to identify an appropriate sampling frequency to assess the actual BTMUN levels. Eleven different sampling frequencies ranging from once to 8 times per month were assessed. A BTMUN value of 15.76 mg/dL was the optimum threshold over which cows with elevated MUN concentrations are included in a herd. The percentage of cows with elevated MUN values can be accurately predicted using BTMUN values (R² = 0.872; P < 0.001). A bulk tank sampling frequency of once per week seems appropriate for most herds in order to assess the actual BTMUN levels, in case daily BTMUN values are not available from milk processors.

Factors affecting the milk urea nitrogen concentration in Chinese Holstein cows

In order to investigate the factors affecting milk urea nitrogen in Chinese Holstein cows, a large commercial dairy farm participated in a 30-month study. In this study, the mean milk urea nitrogen concentration was 11.75 mg/dl. The milk urea nitrogen reached its maximum value on day 90 of lactation for the first parity and the third or higher parities, but it peaked at the end of lactation for the second parity. The milk urea nitrogen of the first parity was lower than that of other parities. The milk urea nitrogen showed its minimum level in January, and reached its maximum in July. The milk urea nitrogen at the first month of lactation in cows calving in summer was higher than other seasons, while at the fourth month of lactation, the milk urea nitrogen of cows calving in autumn was significantly lower than in cows calving in other seasons. Positive correlations were observed between daily milk yield, net energy for lactation, crude protein and milk urea nitrogen for the first and third parities, but negative correlations were observed in the second parity. The milk urea nitrogen showed significantly positive correlations with fat content, total solid content and daily matter intake for all parities. A negative correlation was observed between milk urea nitrogen and protein content, with the exception of the second parity. For all data, as milk urea nitrogen concentration increased, milk protein content decreased. It has been recommended that milk urea nitrogen concentration should be evaluated in combination with parity, days in milk, season (or month), daily matter intake and dietary nutritional components, in order to improve the management and economic benefits of dairy farm.

Monitoring Process Water Quality Using Near Infrared Spectroscopy and Partial Least Squares Regression with Prediction Uncertainty Estimation

Reuse of process water in dairy ingredient production-and food processing in general-opens the possibility for sustainable water regimes. Membrane filtration processes are an attractive source of process water recovery since the technology is already utilized in the dairy industry and its use is expected to grow considerably. At Arla Foods Ingredients (AFI), permeate from a reverse osmosis polisher filtration unit is sought to be reused as process water, replacing the intake of potable water. However, as for all dairy and food producers, the process water quality must be monitored continuously to ensure food safety. In the present investigation we found urea to be the main organic compound, which potentially could represent a microbiological risk. Near infrared spectroscopy (NIRS) in combination with multivariate modeling has a long-standing reputation as a real-time measurement technology in quality assurance. Urea was quantified Using NIRS and partial least squares regression (PLS) in the concentration range 50-200 ppm (RMSEP = 12 ppm, R²= 0.88) in laboratory settings with potential for on-line application. A drawback of using NIRS together with PLS is that uncertainty estimates are seldom reported but essential to establishing real-time risk assessment. In a multivariate regression setting, sample-specific prediction errors are needed, which complicates the uncertainty estimation. We give a straightforward strategy for implementing an already developed, but seldom used, method for estimating sample-specific prediction uncertainty. We also suggest an improvement. Comparing independent reference analyses with the sample-specific prediction error estimates showed that the method worked on industrial samples when the model was appropriate and unbiased, and was simple to implement.

Evaluation of between-cow variation in milk urea and rumen ammonia nitrogen concentrations and the association with nitrogen utilization and diet digestibility in lactating cows

Concentrations of milk urea N (MUN) are influenced by dietary crude protein concentration and intake and could therefore be used as a biomarker of the efficiency of N utilization for milk production (milk N/N intake; MNE) in lactating cows. In the present investigation, data from milk-production trials (production data set; n=1,804 cow/period observations from 21 change-over studies) and metabolic studies involving measurements of nutrient flow at the omasum in lactating cows (flow data set; n=450 cow/period observations from 29 studies) were used to evaluate the influence of between-cow variation on the relationship of MUN with MNE, urinary N (UN) output, and diet digestibility. All measurements were made on cows fed diets based on grass silage supplemented with a range of protein supplements. Data were analyzed by mixed-model regression analysis with diet within experiment and period within experiment as random effects, allowing the effect of diet and period to be excluded. Between-cow coefficient of variation in MUN concentration and MNE was 0.13 and 0.07 in the production data set and 0.11 and 0.08 in the flow data set, respectively. Based on residual variance, the best model for predicting MNE developed from the production data set was MNE (g/kg)=238 + 7.0 × milk yield (MY; kg/d) - 0.064 × MY(2) - 2.7 × MUN (mg/dL) - 0.10 body weight (kg). For the flow data set, including both MUN and rumen ammonia N concentration with MY in the model accounted for more variation in MNE than when either term was used with MY alone. The best model for predicting UN excretion developed from the production data set (n=443) was UN (g/d)=-29 + 4.3 × dry matter intake (kg/d) + 4.3 × MUN + 0.14 × body weight. Between-cow variation had a smaller influence on the association of MUN with MNE and UN output than published estimates of these relationships based on treatment means, in which differences in MUN generally arise from variation in dietary crude protein concentration. For the flow data set, between-cow variation in MUN and rumen ammonia N concentrations was positively associated with total-tract organic matter digestibility. In conclusion, evaluation of phenotypic variation in MUN indicated that between-cow variation in MUN had a smaller effect on MNE compared with published responses of MUN to dietary crude protein concentration, suggesting that a closer control over diet composition relative to requirements has greater potential to improve MNE and lower UN on farm than genetic selection.

Genetic and environmental determinants of the urea level in cow's milk

Abstract. This study was conducted on a sample of 2237 Polish Holstein-Friesian cows. The aim was to estimate the effect of selected environmental factors on the level of urea in cow's milk and on its genetic parameters, i.e. the heritability coefficients, and genetic correlation with other selected traits of milk production. The present study has revealed the existence of a highly significant influence of herd, year of calving, parity, lactation phase, and milk performance level on the urea content in cow's milk. A high urea level in milk was detected in samples collected from older animals, both during the winter season and the middle phase of lactation (101–200 days). The heritability estimates were generally at a low level, particularly in terms of milk yield (0.183) and urea content (0.152–0.159), which may indicate the dominant role of the environment in shaping them. Relatively low values of genetic correlation (−0.097–0.140) between the urea content and other traits suggest that improvement of milk yield and its composition modify the urea level in milk only to a small degree.

Potential use of milk urea nitrogen to abate atmospheric nitrogen emissions from wisconsin dairy farms

Urinary urea N (UUN) is the principal nitrogen (N) source controlling emissions of ammonia (NH) and nitrous oxide (NO) from dairy manure. The objectives of this study were (i) to study the integrative nature of dietary crude protein (CP) management, secretion of milk urea N (MUN), excretion of UUN, and N emissions from dairy production systems; (ii) to evaluate how associative changes in dietary CP, MUN, and UUN affect atmospheric N emissions from dairy farms; and (iii) to discuss some of the challenges and opportunities to an expanded use of MUN to enhance dietary CP use and decrease UUN excretion and N emissions from dairy farms. Milk urea N records of 37,889 cows in 197 herds in Wisconsin revealed that approximately one half of tested cows were likely consuming dietary CP in excess of requirement. Farm simulations were used to quantify the effect of dietary CP on whole-farm N emissions. At a statewide average MUN of 12.5 mg dL, 48 to 87% of UUN was emitted as NH, with the lowest loss from pasture-based farms and the greatest loss from tie-stall farms. Each 1 mg dL decrease of MUN (range, 16-10 mg dL) provided an associated daily decrease in UUN of 16.6 g per cow, which decreased NH and NO emissions from manure by 7 to 12%. Although more site-specific information is required on herd MUN-UUN relationships and more a reliable interpretation of MUN assay results is needed, monitoring of MUN may be used to enhance dietary CP use and to reduce UUN excretion and N emissions from Wisconsin dairy farms.