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

Metabolomics and amino acid profiling of plasma reveals the metabolite profiles associated with nitrogen utilisation efficiency in primiparous dairy cows

Nitrogen (N) utilisation efficiency (NUE, milk N yield [g/d]/N intake [g/d]) is an important performance indicator in dairy farming. Determining the NUE-associated blood metabolite profile will contribute to the optimisation of nutritional strategies to further improve NUE among dairy cows. Here, 20 primiparous lactating cows with days in milk ranging from 95 to 115 days were selected from a total of 1 221 cows. Each cow's N intake and milk N yield were measured for 7 days. Subsequently, blood samples were collected before morning feeding. Based on analysis and calculations, cows were retrospectively classified into two groups based on their NUE values, namely, a low NUE group (LNUE, NUE = 24.8 ± 1.6%, n = 10, mean ± SD) and a high NUE group (HNUE, NUE = 35.2 ± 1.7%, n = 10, mean ± SD). Plasma samples were selected from six cows in each group for metabolomics and amino acid profiling. Among the 41 differential metabolites (DMs) identified in the metabolomic analysis, sucrose, MG(0:0/22:1(13Z)/0:0), 2-amino-6-hydroxyhexanoic acid, and L-glutamine exhibited significant correlations with NUE, milk yield, and BW (P < 0.05). Moreover, the five differential amino acids and amino acid metabolites (DAAs) identified in the amino acid profiling and 5 of the 6 differential amino acids and amino acid conjugates identified by plasma metabolomics were found to be less abundant in the HNUE group (P < 0.05). Specifically, there was a 39.4% decrease in L-arginine content and a 29.2% decrease in L-glutamine content (P < 0.05). Pathway analysis indicated that the DMs and DAAs were mainly involved in arginine biosynthesis, glutathione metabolism, arginine and proline metabolism, and tryptophan metabolism (pathway impact > 0.1). These results provided new insights into the new blood metabolite profile associated with NUE in dairy cows. These new insights can provide foundational information for the formulation of new strategies to further enhance NUE in dairy cows.

Impact of growth implants and low-level tannin supplementation on enteric emissions and nitrogen excretion in grazing steers

The primary objective of this experiment was to evaluate the effects of a growth-hormone implant (Revalor-G, Merck Animal Health., Rahway, NJ, USA) and tannin supplementation (Silvafeed BX, Silva Team, San Michele Mondovi CN, Italy) on enteric methane (CH4) emissions and estimated nitrogen (N) excretion in grazing steers. Steers (n = 20; initial body weight [IBW] = 343 ± 14 kg) were acclimated to use a portable automated head-chamber system (AHCS) to measure CH4 and a SmartFeed Pro automated feeder for dietary supplementation (C-Lock Inc., Rapid City, SD, USA). After the training period, steers were randomly assigned to a 2 × 2 factorial arrangements of treatments, with 2 levels of growth-hormone implants, no-implant (NO-IMP) or implanted (IMP), and 2 levels of tannin supplementation, no tannin supplementation (NO-TAN) or tannin supplementation (TAN). This created 4 treatment groups: (1) NO-TAN and NO-IMP, (2) TAN and NO-IMP, (3) IMP and NO-TAN, and (4) TAN and IMP. Tannin was offered daily at 0.30% dry matter intake (DMI) through 0.5 kg/hd/d sweetfeed supplement (Sweetfeed Mix, AgFinity., Eaton, CO, USA) with a targeted tannin intake at 48 g/hd/d. No (P ≥ 0.05) implant × tannin interaction was detected for any dependent variable, so only the main effects of implant (NO-IMP vs. IMP) and tannin supplementation (NO-TAN vs. TAN) are discussed. Implant status did not affect (P ≥ 0.56) final body weight (FBW) or average daily gain (ADG) during the 90 d grazing period. There was no effect (P ≥ 0.15) of growth implant on CH4 production or emission intensity (EI; g CH4/kg gain). Additionally, IMP steers tended (P ≤ 0.08) to have less CH4 yield (MY; g CH4/g DMI) and higher blood urea nitrogen (BUN) than NO-IMP steers. Tannin supplementation did not impact (P ≥ 0.26) FBW or ADG. However, NO-TAN steers tended (P = 0.06) to have a greater total DMI than steers supplemented with tannin. No effect (P ≥ 0.22) of tannin supplementation was observed for CH4 production and EI. Nitrogen utilization as measured through BUN, urine N, fecal N, or fecal P was similar (P ≥ 0.12) between TAN and NO-TAN animals. The findings indicate that low-level dietary supplementation to reduce enteric emissions is difficult in grazing systems due to inconsistent animal intake and that growth implants could be used as a strategy to improve growth performance and reduce EI of steers grazing improved pasture.

Hydroponic barley supplementation fed with high protein diets improves the production performance of lactating dairy cows

The study investigated the effects of dietary protein level and the inclusion of hydroponic barley sprouts (HB) on lactation performance, blood biochemistry and N use efficiency in mid-lactation dairy cows. Treatments were arranged in a 2 × 2 factorial design with 2 crude protein (CP) levels [16.8% and 15.5% of dry matter (DM)], with HB (4.8% of DM, replacing 4.3% of alfalfa hay and 0.5% of distillers dried grains with solubles (DDGS)) or without HB. Forty-eight multiparous Holstein dairy cows (146 ± 15 d in milk, 40 ± 5 kg/d of milk) were randomly allocated to 1 of 4 diets: high protein diet (16.8% CP, HP), HP with HB (HP+HB), low protein diet (15.5% CP, LP), or LP with HB (LP+HB). An interaction between CP × HB on dry matter intake (DMI) was detected, with DMI being unaffected by HB inclusion in cows fed the high CP diets, but was lower in cows fed HB when the low CP diet was fed. A CP × HB interaction was also observed on milk and milk protein yield, which was higher in cows fed HB with HP, but not LP. Inclusion of HB also tended to reduce milk fat content, and feeding HP resulted in a higher milk protein and milk urea N content, but lower milk lactose content. Feed efficiency was increased by feeding HP or HB diets, whereas N efficiency was higher for cows fed LP or HB diets. There was an interaction on the apparent total-tract digestibility of DM and CP, which was higher when HB was fed along with HP, but reduced when fed with LP, whereas the digestibility of ADF was increased by feeding low protein diets. In conclusion, feeding a low protein diet had no adverse effect on cow performance, while feeding HB improved milk and milk component yield, and N efficiency when fed with a high CP diet, but compromised cow performance with a low CP diet.

Increasing the prepartum dose of rumen-protected choline: Effects on milk production and metabolism in high-producing Holstein dairy cows

Peripartum rumen-protected choline (RPC) supplementation is beneficial for cow health and production, yet the optimal dose is unknown. In vivo and in vitro supplementation of choline modulates hepatic lipid, glucose, and methyl donor metabolism. The objective of this experiment was to determine the effects of increasing the dose of prepartum RPC supplementation on milk production and blood biomarkers. Pregnant multiparous Holstein cows (n = 116) were randomly assigned to one of 4 prepartum choline treatments that were fed from -21 d relative to calving (DRTC) until calving. From calving until +21 DRTC, cows were fed diets targeting 0 g/d choline ion (control, CTL) or the recommended dose (15 g/d choline ion; RD) of the same RPC product that they were fed prepartum. The resulting treatments targeted: (1) 0 g/d pre- and postpartum [0.0 ± 0.000 choline ion, percent of dry matter (%DM); CTL]; (2) 15 g/d pre- and postpartum of choline ion from an established product (prepartum: 0.10 ± 0.004 choline ion, %DM; postpartum: 0.05 ± 0.004 choline ion, %DM; ReaShure, Balchem Corp.; RPC1RD▸RD); (3) 15 g/d pre- and postpartum of choline ion from a concentrated RPC prototype (prepartum: 0.09 ± 0.004 choline ion, %DM; postpartum: 0.05 ± 0.003 choline ion, %DM; RPC2, Balchem Corp.; RPC2RD▸RD); or (4) 22 g/d prepartum and 15 g/d postpartum from RPC2 [prepartum: 0.13 ± 0.005 choline ion, %DM; postpartum: 0.05 ± 0.003 choline ion, %DM; high prepartum dose (HD), RPC2HD▸RD]. Treatments were mixed into a total mixed ration, and cows had ad libitum access via a roughage intake control system (Hokofarm Group). From calving to +21 DRTC, all cows were fed a common base diet and treatments were mixed into the total mixed ration (supplementation period, SP). Thereafter, all cows were fed a common diet (0 g/d choline ion) until +100 DRTC (postsupplementation period, postSP). Milk yield was recorded daily and composition analyzed weekly. Blood samples were obtained via tail vessel upon enrollment, approximately every other day from -7 to +21 DRTC, and at +56 and +100 DRTC. Feeding any RPC treatment reduced prepartum dry matter intake compared with CTL. During the SP, no evidence for a treatment effect on energy-corrected milk (ECM) yield was found, but during the postSP, RPC1RD▸RD and RPC2RD▸RD treatments tended to increase ECM, protein, and fat yields. During the postSP, the RPC1RD▸RD and RPC2RD▸RD treatments tended to increase, and RPC2HD▸RD increased, the de novo proportion of total milk fatty acids. During the early lactation SP, RPC2HD▸RD tended to increase plasma fatty acids and β-hydroxybutyrate concentrations, and RPC1RD▸RD and RPC2RD▸RD reduced blood urea nitrogen concentrations compared with CTL. The RPC2HD▸RD treatment reduced early lactation serum lipopolysaccharide binding protein compared with CTL. Overall, peripartum RPC supplementation at the recommended dose tended to increase ECM yield postSP, but no evidence was seen of an additional benefit on milk production with an increased prepartum dose of choline ion. The effects of RPC on metabolic and inflammatory biomarkers support the potential for RPC supplementation to affect transition cow metabolism and health and may support the production gains observed.

Increasing the prepartum dose of rumen-protected choline: Effects of maternal choline supplementation on growth, feed efficiency, and metabolism in Holstein and Holstein × Angus calves

Feeding pregnant cows rumen-protected choline (RPC) may have the potential to affect the growth and health of offspring, but little is known about the optimal dose, or the potential mechanisms of action. The objectives of this experiment were to 1) determine if increasing RPC supplementation during late gestation in multiparous Holstein cows would improve calf growth and 2) determine if maternal choline supplementation alters global DNA methylation patterns. Pregnant multiparous Holstein cows (n = 116) were randomly assigned to diets targeting 0g choline ion (0.0 ± 0.000 choline ion, %DM, control; CTL), 15g of choline ion (recommended dose; RD) from an established RPC product (0.10 ± 0.004 choline ion, %DM, RPC1RD; ReaShure, Balchem Corp.; positive control), or 15g (0.09 ± 0.004 choline ion, %DM, RPC2RD) or 22g (0.13 ± 0.005 choline ion, %DM, high dose; RPC2HD) of choline ion from a concentrated RPC prototype (RPC2; Balchem Corp.). Treatments were mixed into a total mixed ration and cows had ad libitum access via a roughage intake control system (Hokofarm Group, Marknesse, Netherlands). All female Holstein (n = 49) and Holstein × Angus calves (male, n = 18; female, n = 30) were enrolled and fed colostrum from a cow within the same treatment. Holstein calves and Holstein × Angus calves were fed an accelerated and traditional milk replacer program, respectively, and offered ad libitum access to calf starter. Jugular vein blood samples were collected, and body weight was measured at 7, 14, 28, 42, and 56 d of age. Categorical treatment and continuous effects of actual prepartum maternal choline ion intake were analyzed using mixed effect models. An interaction of treatment with sex, nested within breed, resulted in any choline treatment increasing the proportion of methylated whole blood DNA in male, but not female calves. Although 37% of Holstein calves across all treatments experienced abomasal bloat, no evidence for differences in health measurements (signs of respiratory disease and fecal consistency) were observed across treatments. During the first 2 wk of life in Holstein calves, RPC2HD tended to increase average daily gain (ADG) and feed efficiency (FE) compared with CTL and increasing maternal choline ion intake linearly increased ADG and FE. Maternal choline supplementation increased plasma glucose compared with CTL, while increasing serum insulin-like growth factor-1 and decreasing serum lipopolysaccharide binding protein at 7 d of age in Holstein calves. In Holstein × Angus calves, the effect of treatment on ADG tended to interact with sex: in males, RPC2HD increased ADG after 2 wk of life compared with CTL, without evidence of a treatment effect in female calves. Increasing maternal choline ion intake linearly increased ADG after 2 wk of age in male Holstein × Angus calves, while quadratically increasing FE in both sexes. Altered global DNA methylation patterns in male Holstein × Angus calves, and changes in blood metabolites in Holstein calves, provide 2 potential mechanisms for observed improvements in calf growth. Continuous treatment models demonstrated that the effects of maternal choline supplementation are sensitive to the amount of maternal choline ion intake, with greater benefit to calves observed at higher maternal intakes.

Microbial signature inferred from genomic breeding selection on milk urea concentration and its relation to proxies of nitrogen-utilization efficiency in Holsteins

Increasing the nitrogen-utilization efficiency (NUE) of dairy cows by breeding selection would offer advantages from nutritional, environmental, and economic perspectives. Because data collection of NUE phenotypes is not feasible in large cow cohorts, the cow individual milk urea concentration (MU) has been suggested as an indicator trait. Considering the symbiotic interplay between dairy cows and their rumen microbiome, individual MU was thought to be influenced by host genetics and by the rumen microbiome, the latter in turn being partly attributed to host genetics. To enhance our knowledge of MU as an indicator trait for NUE, we aimed to identify differential abundant rumen microbial genera between Holstein cows with divergent genomic breeding values for MU (GBVMU; GBV_HMU vs. GBV_LMU, where H and L indicate high and low MU phenotypes, respectively). The microbial genera identified were further investigated for their correlations with MU and 7 additional NUE-associated traits in urine, milk, and feces in 358 lactating Holsteins. Statistical analysis of microbial 16S rRNA amplicon sequencing data revealed significantly higher abundances of the ureolytic genus Succinivibrionaceae UCG-002 in GBV_LMU cows, whereas GBV_HMU animals hosted higher abundances of Clostridia unclassified and Desulfovibrio. The entire discriminating ruminal signature of 24 microbial taxa included a further 3 genera of the Lachnospiraceae family that revealed significant correlations to MU values and were therefore proposed as considerable players in the GBVMU-microbiome-MU axis. The significant correlations of Prevotellaceae UCG-003, Anaerovibrio, Blautia, and Butyrivibrio abundances with MU measurements, milk nitrogen, and N content in feces suggested their contribution to genetically determined N-utilization in Holstein cows. The microbial genera identified might be considered for future breeding programs to enhance NUE in dairy herds.

The significance of N-carbamoylglutamate in ruminant production

Improving the efficiency and production of grazing ruminants to support food and fiber production, while reducing the environmental footprint and meeting the welfare needs of the animals, is important for sustainable livestock production systems. Development of new technologies that can improve the efficiency of nitrogen (N) utilization in ruminants, and that are effective and safe, has important implications for ruminant livestock production. N-carbomoylglutamate (NCG) is a functional micronutrient that stimulates endogenous synthesis of arginine, which can improve survival, growth, lactation, reproductive performance, and feed efficiency in mammals. There is a growing body of evidence to support the potential of dietary NCG supplementation to improve the productive capacity and N utilization efficiency of ruminants. This review summarizes the current literature on the effects of dietary supplementation with NCG in ruminants and impacts on production and potential to reduce the environmental footprint of farmed ruminant livestock. The current literature highlights the potential for commercial application in ruminant livestock to improve productivity and N utilization efficiency.

Feeding lower-protein diets based on red clover and grass or alfalfa and corn silage does not affect milk production but improves nitrogen use efficiency in dairy cows

Reducing the dietary crude protein (CP) concentration can decrease the financial cost and lower the environmental impact of milk production. Two studies were conducted to examine the effects of reducing the dietary CP concentration on animal performance, nutrient digestibility, milk fatty acid (FA) profile, and nitrogen use efficiency (NUE; milk N/N intake) in dairy cows fed legume silage-based diets. Thirty-six multiparous Holstein-Friesian dairy cows that were 76 ± 14 (mean ± SD) days in milk and 698 ± 54 kg body weight were used in a 3 × 3 Latin square design in each of 2 studies, with 3 periods of 28 d. In study 1, cows were fed diets based on a 50:50 ratio of red clover to grass silage [dry matter (DM) basis] containing 1 of 3 dietary CP concentrations: high (H) = 175 g of CP/kg of DM; medium (M) = 165 g of CP/kg of DM; or low (L) = 150 g of CP/kg of DM. In study 2, cows were fed 175 g of CP/kg of DM with a 50:50 ratio of alfalfa to corn silage (H50) or 1 of 2 diets containing 150 g of CP/kg of DM with either a 50:50 (L50) or a 60:40 (L60) ratio of alfalfa to corn silage. Cows in both studies were fed a total mixed ration with a forage-to-concentrate ratio of 52:48 (DM basis). All diets were formulated to meet the MP requirements, except L (95% of MP requirements). In study 1, cows fed L ate 1.6 kg of DM/d less than those fed H or M, but milk yield was similar across treatments. Mean milk protein, fat, and lactose concentrations were not affected by diet. However, the apparent total-tract nutrient digestibility was decreased in cows fed L. The NUE was 5.7 percentage units higher in cows fed L than H. Feeding L also decreased milk and plasma urea concentrations by 4.4 mg/dL and 0.78 mmol/L, respectively. We found no effect of dietary treatment on the milk saturated or monounsaturated FA proportion, but the proportion of polyunsaturated FA was increased, and milk odd- and branched-chain FA decreased in cows fed L compared with H. In study 2, DM intake was 2 kg/d lower in cows receiving L50 than H50. Increasing the alfalfa content and feeding a low-CP diet (L60) did not alter DMI but decreased milk yield and milk protein concentration by 2 kg/d and 0.6 g/kg, respectively, compared with H50. Likewise, milk protein and lactose yield were decreased by 0.08 kg/d in cows receiving L60 versus H50. Diet had no effect on apparent nutrient digestibility. Feeding the low-CP diets compared with H50 increased the apparent NUE by approximately 5 percentage units and decreased milk and plasma urea concentrations by 7.2 mg/dL and 1.43 mmol/L, respectively. Dietary treatment did not alter milk FA profile except cis-9,trans-11 conjugated linoleic acid, which was higher in milk from cows receiving L60 compared with H50. We concluded that reducing CP concentration to around 150 g/kg of DM in red clover and grass or alfalfa and corn silage-based diets increases the apparent NUE and has little effect on nutrient digestibility or milk performance in dairy cows.

Effects of Macleaya Cordata Extract on Performance, Nutrient Apparent Digestibilities, Milk Composition, and Plasma Metabolites of Dairy Goats

In this study, we aimed to investigate the effects of Macleaya cordata extract (MCE) supplementation on performance, nutrient apparent digestibilities, plasma metabolites, and milk quality in dairy goats. Twenty-four lactating Guanzhong dairy goats (n = 24) were randomly divided into two groups (each containing 12 goats) in a 52-day trial: the CON group was fed a basal diet; the MCE group was fed a basal diet supplemented with 400 mg/kg MCE. The results indicated that the 4% fat corrected milk yield (4% FCM); uncorrected milk yield; milk-fat concentration; content of C4:0, C18:0, and C18:1n9c fatty acids in milk; and apparent digestibility of neutral detergent fiber (NDF) and acid detergent fiber (ADF) in the MCE group were significantly higher (p < 0.05). Furthermore, the lactoferrin (LTF), alpha-lactalbumin (α-La), and beta-lactoglobulin (β-Lg) of the milk and feed conversion rate (FCR) of the goats were significantly greater (p < 0.01) in the MCE group than in the CON group. In contrast, the somatic cell count (SCC) (p < 0.01), content of C14:0 fatty acids (p < 0.01) of milk, and blood urea nitrogen (BUN) concentrations (p < 0.05) were significantly lower in the in the MCE goats. These results show that the feeding of MCE can increase the performance and apparent nutrient digestibility of fiber in dairy goats, improving the quality of goat milk.

Untargeted metabolomics confirms the association between plasma branched chain amino acids and residual feed intake in beef heifers

This study explored plasma biomarkers and metabolic pathways underlying feed efficiency measured as residual feed intake (RFI) in Charolais heifers. A total of 48 RFI extreme individuals (High-RFI, n = 24; Low-RFI, n = 24) were selected from a population of 142 heifers for classical plasma metabolite and hormone quantification and plasma metabolomic profiling through untargeted LC-MS. Most efficient heifers (Low-RFI) had greater (P = 0.03) plasma concentrations of IGF-1 and tended to have (P = 0.06) a lower back fat depth compared to least efficient heifers. However, no changes were noted (P ≥ 0.10) for plasma concentrations of glucose, insulin, non-esterified fatty acids, β-hydroxybutyrate and urea. The plasma metabolomic dataset comprised 3,457 ions with none significantly differing between RFI classes after false discovery rate correction (FDR > 0.10). Among the 101 ions having a raw P < 0.05 for the RFI effect, 13 were putatively annotated by using internal databases and 6 compounds were further confirmed with standards. Metabolic pathway analysis from these 6 confirmed compounds revealed that the branched chain amino acid metabolism was significantly (FDR < 0.05) impacted by the RFI classes. Our results confirmed for the first time in beef heifers previous findings obtained in male beef cattle and pointing to changes in branched-chain amino acids metabolism along with that of body composition as biological mechanisms related to RFI. Further studies are warranted to ascertain whether there is a cause-and-effect relationship between these mechanisms and RFI.

Ruminal background of predisposed milk urea (MU) concentration in Holsteins

Efforts to reduce nitrogen (N) emissions are currently based on the optimization of dietary- N supply at average herd N requirements. The implementation of the considerable individual differences and predispositions in N- use efficiency and N- excretion in breeding programs is hampered by the difficulty of data collection. Cow individual milk urea (MU) concentration has been proposed as an easy-to-measure surrogate trait, but recent studies questioned its predictive power. Therefore, a deeper understanding of the biological mechanisms underlying predisposed higher (HMUg) or lower (LMUg) MU concentration in dairy cows is needed. Considering the complex N- metabolism in ruminants, the distinction between HMUg and LMUg could be based on differences in (i) the rumen microbial community, (ii) the host-specific transcription processes in the rumen villi, and (iii) the host-microbe interaction in the rumen. Therefore, rumen fluid and rumen epithelial samples from 10 HMUg and 10 LMUg cows were analyzed by 16S sequencing and HiSeq sequencing. In addition, the effect of dietary-N reduction on ruminal shifts was investigated in a second step. In total, 10 differentially abundant genera (DAG) were identified between HMUg and LMUg cows, elucidating greater abundances of ureolytic Succinivibrionaceae_UCG-002 and Ruminococcaceae_unclassified in LMUg animals and enhanced occurrences of Butyvibrio in HMUg cows. Differential expression analysis revealed genes of the bovine Major Histocompatibility Complex (BOLA genes) as well as MX1, ISG15, and PRSS2 displaying candidates of MU predisposition that further attributed to enhanced immune system activities in LMUg cows. A number of significant correlations between microbial genera and host transcript abundances were uncovered, including strikingly positive correlations of BOLA-DRA transcripts with Roseburia and Lachnospiraceae family abundances that might constitute particularly prominent microbial-host interplays of MU predisposition. The reduction of feed-N was followed by 18 DAG in HMUg and 19 DAG in LMUg, depicting pronounced interest on Shuttleworthia, which displayed controversial adaption in HMUg and LMUg cows. Lowering feed-N further elicited massive downregulation of immune response and energy metabolism pathways in LMUg. Considering breeding selection strategies, this study attributed information content to MU about predisposed ruminal N-utilization in Holstein-Friesians.

Can machine learning algorithms perform better than multiple linear regression in predicting nitrogen excretion from lactating dairy cows

This study aims to compare the performance of multiple linear regression and machine learning algorithms for predicting manure nitrogen excretion in lactating dairy cows, and to develop new machine learning prediction models for MN excretion. Dataset used were collated from 43 total diet digestibility studies with 951 lactating dairy cows. Prediction models for MN were developed and evaluated using MLR technique and three machine learning algorithms, artificial neural networks, random forest regression and support vector regression. The ANN model produced a lower RMSE and a higher CCC, compared to the MLR, RFR and SVR model, in the tenfold cross validation. Meanwhile, a hybrid knowledge-based and data-driven approach was developed and implemented to selecting features in this study. Results showed that the performance of ANN models were greatly improved by the turning process of selection of features and learning algorithms. The proposed new ANN models for prediction of MN were developed using nitrogen intake as the primary predictor. Alternative models were also developed based on live weight and milk yield for use in the condition where nitrogen intake data are not available (e.g., in some commercial farms). These new models provide benchmark information for prediction and mitigation of nitrogen excretion under typical dairy production conditions managed within grassland-based dairy systems.

Relationship Between Nitrogen Isotopic Discrimination and the Proportion of Dietary Nitrogen Excreted in Urine by Sheep Offered Different Levels of Dietary Non-Protein Nitrogen

Urinary nitrogen (N) excretion (UN) as a proportion of N intake (NI; UN/NI) is a major determinant of N excretion from ruminants and could be predicted from the N isotopic discrimination occurring between dietary and animal proteins (Δ15N). This study investigated the usefulness of Δ15N and other plasma biomarkers to reflect changes in UN/NI from sheep offered different levels of dietary urea. Eighteen Merino rams (age, 1–2 years; live weight, 41 ± 3 kg) were allocated to three dietary N treatments for a N balance study. Treatments were control (C), control + 0.5% urea (C+0.5%), and control + 1.2% urea (C+1.2%) and designed to provide maintenance, maintenance plus an additional 15%, and maintenance plus an additional 33% NI, respectively. The urea effect term was used for one-way ANOVA and regression analysis. As NI increased, the UN and retained N (RN) increased linearly (p &lt; 0.001), but UN/NI only increased in treatment C+1.2% compared with C (p &lt; 0.05). Plasma Δ15N was positively and significantly correlated with UN and UN/NI (r = 0.52, p = 0.028; and r = 0.68, p = 0.002, respectively) and increased linearly (p &lt; 0.001) with the highest values observed in C+1.2%. Urine δ15N changed linearly between C and C+1.2%, but plasma δ15N increased quadratically (p &lt; 0.05). Plasma urea N increased in a linear way across dietary urea levels (p &lt; 0.001). The N isotopic difference between plasma and urine (plasma δ15N–urine δ15N) of C did not vary from either of the other treatments; however, it differed between C+0.5% and C+1.2% (p &lt; 0.05). The study confirmed the potential usefulness of plasma Δ15N to estimate UN/NI from sheep. Moreover, plasma δ15N–urine δ15N can be proposed as a new biomarker of N excretion from small ruminants. These approaches, however, need to be tested in various study conditions.

Contribution of Ruminal Bacteriome to the Individual Variation of Nitrogen Utilization Efficiency of Dairy Cows

High nitrogen utilization efficiency (NUE) is important for increasing milk protein production and decreasing the feed nitrogen cost and nitrogen emission to the environment. Currently, there is a limited whole picture of the relationship between ruminal bacteriome and the NUE of dairy cows, even though some information has been revealed about the bacteriome and milk or milk protein production of dairy cows. The purpose of this study was to compare the rumen bacterial community in dairy cows with different nitrogen utilization efficiency under the same diet. The natural abundance of 15N between the animal proteins and diet (Δ15N) was used as a simple, non-invasive, and accurate biomarker for NUE in ruminants to mark the individual variation. Dairy cows with high NUE (HE_HP, n = 7), medium NUE (ME_MP, n = 7), and low NUE (LE_LP, n = 7) were selected from 284 Holstein dairy cows with the same diet. Measurement of the rumen fermentation indices showed that the proportion of propionate was higher in HE_HP cows and ME_MP cows than in LE_LP cows (P < 0.05). The diversity of rumen bacterial community was higher in LE_LP cows than in ME_MP cows and HE_HP cows by 16S rRNA sequencing analysis (P < 0.05). Moreover, at the genus level, the relative abundances of Succinivibrionaceae_UCG_001, uncultured_Selenomonadaceae, and Acidaminococcus were higher in HE_HP cows than in LE_LP cows (P < 0.05). Interestingly, we found that these bacteria were positively correlated with milk protein yield and negatively correlated with Δ15N (P < 0.05). However, Clostridia_UCG_014, Saccharofermentans, Bacilli_RF39, and Desulfovibrio were lower in HE_HP cows and ME_MP cows than in LE_LP cows (P < 0.05), which were negatively correlated with milk protein yield and positively correlated with Δ15N (P < 0.05). In conclusion, the study showed that the diversity and relative abundances of rumen bacteria differed among different NUE cows, indicating that rumen bacteriome contributes to nitrogen metabolism in dairy cows.

Plasma concentrations of branched-chain amino acids differ with Holstein genetic strain in pasture-based dairy systems

In pasture-based systems, there are nutritional and climatic challenges exacerbated across lactation; thus, dairy cows require an enhanced adaptive capacity compared with cows in confined systems. We aimed to evaluate the effect of lactation stage (21 vs. 180 days in milk, DIM) and Holstein genetic strain (North American Holstein, NAH, n = 8; New Zealand Holstein, NZH, n = 8) on metabolic adaptations of grazing dairy cows through plasma metabolomic profiling and its association with classical metabolites. Although 67 metabolites were affected (FDR < 0.05) by DIM, no metabolite was observed to differ between genetic strains while only alanine was affected (FDR = 0.02) by the interaction between genetic strain and DIM. However, complementary tools for time-series analysis (ASCA analysis, MEBA ranking) indicated that alanine and the branched-chain amino acids (BCAA) differed between genetic strains in a lactation-stage dependent manner. Indeed, NZH cows had lower (P-Tukey < 0.05) plasma concentrations of leucine, isoleucine and valine than NAH cows at 21 DIM, probably signaling for greater insulin sensitivity. Metabolic pathway analysis also revealed that, independently of genetic strains, AA metabolism might be structurally involved in homeorhetic changes as 40% (19/46) of metabolic pathways differentially expressed (FDR < 0.05) between 21 and 180 DIM belonged to AA metabolism.