Feeding oscillating dietary crude protein concentrations increases nitrogen utilization in growing lambs and this response is partly attributable to increased urea transfer to the rumen

Effect of source and frequency of rumen-protected protein supplementation on mammary gland amino acid metabolism and nitrogen balance of dairy cattle

The AA profile of MP affects mammary gland metabolism and milk N efficiency of dairy cattle. Further, the frequency of dietary protein supplementation may influence N partitioning leading to reduced N excretion. This study investigated the effect of source and frequency of rumen-protected (RP) protein supplementation on apparent total-tract digestibility, milk production, mammary gland AA metabolism, and N balance of dairy cattle. Twenty-eight Holstein-Friesian cows (2.3 ± 0.9 lactations; 93 ± 27 DIM; mean ± SD) were used in a randomized complete block design and fed a basal TMR consisting of 41% corn silage, 32% grass silage, and 27% concentrate (DM basis) and formulated to meet 100% and 95% of net energy and MP requirements, respectively. Cows were adapted to the basal TMR in a freestall barn for 7 d, moved to individual tiestalls for 13 d of adaptation to dietary treatments, and then moved into climate respiration chambers for a 4-d measurement period. Treatments consisted of the basal TMR (CON; 159 g CP/kg DM) or the basal TMR including 1 of 3 iso-MP supplements: (1) 315-g mixture of RP soybean meal and RP rapeseed meal fed daily (ST-RPSR), (2) 384-g mixture of RP His, RP Lys, and RP Met fed daily (ST-RPAA), and (3) 768-g mixture of RP His, RP Lys, and RP Met fed every other day (OS-RPAA). The basal TMR with the addition of treatment supplements was designed to deliver 100% of required MP over a 48-h period. The mixture of His, Lys, and Met was formulated to deliver digestible AA in amounts relative to their concentration in casein. Compared with ST-RPSR, ST-RPAA increased milk protein and fat concentration, increased the arterial concentration of total His, Lys, and Met (HLM), decreased mammary clearance of HLM, and increased clearance of Phe, Leu, and Tyr (tendency for Leu and Tyr). Rumen-protected protein source did not affect N balance, but the marginal use efficiency (efficiency of transfer of RP protein supplement into milk protein) of ST-RPAA (67%) was higher than that of ST-RPSR (17%). Milk protein concentration decreased with OS-RPAA compared with ST-RPAA. Arterial concentration of HLM increased on the nonsupplemented day compared with the supplemented day with OS-RPAA, and there was no difference in arterial HLM concentration across days with ST-RPAA. Mammary uptake of HLM tended to increase on the nonsupplemented day compared with the supplemented day with OS-RPAA. Supplementation frequency of RP AA did not affect N balance or overall milk N efficiency, but the marginal use efficiency of OS-RPAA (49%) was lower compared with ST-RPAA. Overall, mammary glands responded to an increased supply of His, Lys, and Met by reducing efflux of other EAA when RP His, RP Lys, and RP Met were supplemented compared with RP plant proteins. Mammary glands increased sequestration of EAA (primarily HLM) on the nonsupplemented day with OS-RPAA, but supplementing RP AA according to a 24-h oscillating pattern did not increase N efficiency over static supplementation.

Crude protein oscillation in diets adequate and deficient in metabolizable protein: Effects on nutrient digestibility, nitrogen balance, plasma amino acids, and greenhouse gas emissions

Reducing dietary CP is a well-established means to improve N use efficiency. Yet, few studies have considered if transient restrictions in dietary CP could reduce the environmental footprint of late-lactation cows. We hypothesized that the effects of CP feeding pattern on digestibility and environmental outputs would be amplified at lower dietary CP. We tested CP levels below and near predicted requirements (low protein [LP], 13.8%; high protein [HP], 15.5%) offered in 2 feeding patterns: where diets alternated ±1.8 percentage units CP every 2 d (oscillating [OF]) or remained static. Our study used a 2 × 2 factorial design with 16 mid- to late-lactation Holsteins (mean = 128, SD = 12 DIM), divided into rumen-cannulated (n = 8) and noncannulated subsets (n = 8). For each 28-d experimental period, we recorded feed intake and milk production and took samples of orts (1×/d) and milk (2×/d) for 4 d. For the cannulated subset, we measured and sampled from the total mass of feces and urine production and collected plasma 2×/d across 4 d. For the noncannulated subset, we sampled carbon dioxide and methane emissions 3×/d for 4 d. For each subset, we fit linear mixed models with fixed effects for CP level, CP feeding pattern, the interaction of CP level and CP feeding pattern, period, and a random effect for cow. For plasma and urinary urea-N, we conducted time series analysis. Contrary to our hypothesis, we found no evidence that dietary CP level and CP feeding pattern interacted to influence N balance, nutrient digestibility, or gas emissions. Results showed HP resulted in similar milk N but increased manure N, reducing N use efficiency (milk true protein N/intake N) relative to LP. For OF, urea-N in urine and plasma peaked 46 to 52 h after the first higher-CP phase feeding. Nutrient digestibility and gas emissions were similar across treatments, except CO2 production was greater for OF-HP. In summary, measured variables were minimally affected by dietary CP alternating ±1.8 percentage units every 48 h, even when average dietary CP was fed below predicted requirements (LP). Although our findings suggest that mid- to late-lactation cows are resilient to oscillation in dietary CP, oscillating CP neither reduced the environmental footprint by improving nutrient use efficiencies nor reduced the potential for direct and indirect greenhouse gas emissions.

Whole-body urea kinetics and functional roles of urea transporters and aquaporins in urea secretion into the rumen in sheep fed diets varying in crude protein content and corn grain processing method

The objectives were to determine the effects of dietary crude protein (CP) content and corn grain processing on whole-body urea kinetics and the functional roles of urea transporter-B (UT-B) and aquaporins (AQP) in serosal-to-mucosal urea flux (Jsm-urea) in ovine ruminal epithelia. Thirty-two Rideau-Arcott ram lambs were blocked by bodyweight into groups of 4 and then randomly allocated within blocks to 1 of 4 diets (n = 8) in a 2 × 2 factorial design. Dietary factors were CP content (11% [LP] vs. 16% [HP]) and corn grain processing (whole-shelled [WSC] vs. steam-flaked [SFC] corn). Whole-body urea kinetics and N balance were determined using 4-d continuous intrajugular infusions of [15N15N]-urea with concurrent collections of urine and feces with four blocks of lambs (n = 4). After 23 d on diets, lambs were killed to collect ruminal epithelia for mounting in Ussing chambers to determine Jsm-urea and the measurement of mRNA abundance of UT-B and AQP. Serosal and mucosal additions of phloretin and NiCl2 were used to inhibit UT-B- and AQP-mediated urea transport, respectively. Lambs fed HP had a greater (P < 0.01) N intake (29.4 vs. 19.1 g/d) than those fed LP; however, retained N (g/d or % of N intake) was not different. As a % of N intake, lambs fed SFC tended (P = 0.09) to have a lower N excretion (72.2 vs. 83.5%) and a greater N retention (27.8 vs. 16.6%) compared to those fed WSC. Endogenous urea-N production (UER) was greater in lambs fed HP compared to those fed LP (29.9 vs. 20.6 g/d; P = 0.02), whereas urea-N secreted into the gut (GER; g/d) and urea-N used for anabolic purposes (UUA; g/d) were similar. Lambs fed LP tended (P = 0.05) to have greater GER:UER (0.78 vs. 0.66) and UUA:GER (0.23 vs. 0.13) ratios, and a greater Jsm-urea (144.7 vs. 116.1 nmol/[cm2 × h]; P = 0.07) compared to those fed HP. Lambs fed SFC tended to have a lower NiCl2-insensitive Jsm-urea (117.4 vs. 178.4 nmol/[cm2 × h]; P = 0.09) and had a lower phloretin-insensitive Jsm-urea (87.1 vs. 143.1 nmol/[cm2 × h]; P = 0.02) compared to those fed WSC. The mRNA abundance of UT-B (0.89 vs. 1.07; P = 0.08) and AQP-3 (0.90 vs. 1.05; P = 0.07) tended to be lower in lambs fed SFC compared to those fed WSC. Overall, reducing CP content tended to increase the GER:UER ratio with no changes in the expression or function of UT-B and AQP. Although corn grain processing had no effects on GER, feeding SFC increased the portion of urea secretion into the rumen that was mediated via UT-B and AQP.

Dynamic lactation responses to dietary crude protein oscillation in diets adequate and deficient in metabolizable protein in Holstein cows

Limited research has examined the interaction between dietary crude protein (CP) level and CP feeding pattern. We tested CP level (low protein [LP], 13.8%; high protein [HP], 15.5% CP, dry matter [DM] basis) and CP feeding pattern (OF = oscillating, SF = static) using a 2 × 2 factorial in 16 mid- to late-lactation Holsteins (initially 128 ± 12 d in milk; mean ± SD). Cows ate total mixed rations formulated by exchanging soy hulls and ground corn with solvent soybean meal to keep constant ratios of neutral detergent fiber to starch (1.18:1), rumen-degradable protein to CP (0.61:1), and forage-to-concentrate (1.5:1) in DM. The OF treatments alternated diets every 48 h to vary CP above and below the mean CP level (OF-LP = 13.8% ± 1.8%; OF-HP = 15.5% ± 1.8% CP [DM basis]) whereas diets were constant in SF (SF-LP = 13.8%; SF-HP = 15.5% CP [DM basis]). In four 28-d periods, 8 rumen-cannulated and 8 noncannulated cows formed 2 Latin rectangles. On d 25 to 28 of each period, each cow's feed intake and milk production were recorded, and samples were taken of orts (1×/d) and milk (2×/d). We fit linear mixed models with fixed CP level, CP feeding pattern, and period effects, and a random intercept for cow, computing least squares means and standard errors. Neither CP level, CP feeding pattern, nor the interaction affected DM intake, feed efficiency, or production of milk, fat- and protein-corrected milk (FPCM), fat, true protein, or lactose. Milk urea-N (MUN) yield was lesser for LP. The LP and OF conditions decreased MUN concentration. The CP level tended to interact with CP feeding pattern so that milk protein concentration was greatest for OF-HP. The OF and LP conditions increased the ratio of true protein to MUN yield. Within OF, cosinor mixed models of selected variables showed that cows maintained production of FPCM across dietary changes, but MUN followed a wave-pattern at a 2-d delay relative to dietary changes. A tendency for lesser MUN with OF contradicted prior research and suggested potential differences in urea-N metabolism between OF and SF. Results showed that cows maintained production of economically-relevant components regardless of CP feeding pattern and CP level. Contrary to our hypothesis, the effects of 48-h oscillating CP were mostly consistent across CP levels, suggesting that productivity is resilient to patterned variation in dietary CP over time even when average CP supply is low (13.8% of DM) and despite 48 h restrictions at 12.2% CP.

Dietary protein oscillation: effects on digestibility, nutrient balance and estimated microbial protein synthesis in lactating dairy cows

Various studies with growing ruminants report increases in nitrogen use efficiency (NUE) when feeding oscillating (OS) dietary CP, whereas limited research with lactating dairy cows demonstrates a lack of improvement in NUE when feeding OS diets. We hypothesised that a total mixed ration (TMR) delivering OS CP (48-h phases of 134 and 171 g CP/kg DM, respectively) compared to a static CP TMR (ST; 152 g CP/kg DM) would result in similar or increased urinary purine derivative excretion (as a marker of microbial protein synthesis (MPS)) and greater urinary nitrogen excretion in lactating dairy cows. Responses in intake, production, apparent total tract digestibility (ATTD), nutrient balance, and estimated MPS were evaluated using faecal and urine collection in 12 multiparous cows (172 ± 39 d in milk) in a randomised complete block design, where total urinary output was estimated indirectly. All measurements were taken during d 8 (at 1700) to d 16 (at 1700) of the 16-d study that followed a 28-d period in which cows already received their respective treatments. Dry matter intake, yields of milk, protein, fat, lactose, and fat- and protein-corrected milk were similar for ST and OS. Milk composition, BW, and body condition score also did not differ between treatments, except for a tendency for increased milk urea concentration with OS (13.7 vs 12.4 mg/dL). Feed efficiency, NUE and ATTD of organic matter, NDF, CP and gross energy did not differ, but ATTD of crude fat (658 vs 627 g/kg) and starch (980 vs 975 g/kg) increased, and ATTD of DM (702 vs 691 g/kg) tended to increase with OS. Milk energy as a proportion of digested energy tended to decrease with OS (34.6 vs 37.1%), but other energy metabolism variables were not affected by treatment. Estimated urinary nitrogen excretion increased (165 vs 144 g/d), estimated urinary nitrogen as a proportion of nitrogen intake tended to increase (25.3 vs 22.7%), and milk nitrogen as a proportion of digested nitrogen decreased (47.3 vs 51.8%) in response to OS. Estimated urinary excretion of creatinine (184 vs 165 mmol/d), uric acid (29 vs 20 mmol/d) and urea (3.1 vs 2.5 mol/d) increased, but other nitrogen metabolism parameters were not affected by OS. Overall, oscillating dietary CP content did not affect lactational performance, milk NUE, or estimated MPS. However, ATTD of some nutrients increased, postabsorptive energy use for milk synthesis tended to decrease, and estimated urinary nitrogen losses increased with OS.

Lipid microspheres containing urea for slow release of non-protein N in ruminant diets

ContextUrea is widely used in ruminant diets as a source of non-protein nitrogen (NPN), partially substituting for true protein in feed, but high levels of urea in the diet may cause toxicity.AimsThis study investigated the microencapsulation of urea in carnauba wax for slow release in the rumen to improve the N efficiency and growth of sheep.MethodsTwo microencapsulated systems were developed with urea:carnauba wax ratios (w/w) of 1:2 (U12) and 1:4 (U14). Based on the initial characterisation, only U12 was examined in an in vivo experiment with Santa Ines crossbred male sheep (n=40) initially weighing 28±0.6kg at 270 days of age. The experimental arrangement was a completely randomised design, and the animals were distributed into four treatments; i.e. four levels of inclusion (0, control; 15; 30; and 45g/kg DM) of microencapsulated urea in the diet.Key resultsThe dietary inclusion of microencapsulated urea was associated with linear increases (P≤0.05) in the intake of DM and metabolisable energy, the digestibility of crude protein, non-fibrous carbohydrates and fibre fractions, and N balance. Additionally, liveweight gain and feed efficiency increased quadratically (P&lt;0.001). There were also linear reductions (P&lt;0.001) in blood urea and urinary urea concentrations.ConclusionsThe study demonstrated that the inclusion of up to 30g/kg U12 microencapsulated urea can be recommended as a source of slow-release urea in sheep diets since it improved the performance and feed efficiency and promoted lower concentrations of blood urea and urinary urea.ImplicationsThe use of urea microencapsulated in carnauba wax can reduce the risk of urea toxicity and provide a safer way to supply NPN to ruminants and improve N utilisation.

The effect of sheep genetic merit and feed allowance on nitrogen partitioning and isotopic discrimination

Animal nitrogen (N) partitioning is a key parameter for profitability and sustainability of ruminant production systems, which may be predicted from N isotopic discrimination or fractionation (Δ¹⁵N). Both animal genetics and feeding level may interact and impact on N partitioning. Therefore, this study aimed to assess the interactive effects of genetic merit (G) and feed allowance (F) on N partitioning and Δ¹⁵N in sheep. The sheep were drawn from two levels of G (high G vs. low G; based on New Zealand Sheep Improvement Limited (http://www.sil.co.nz/) dual (wool and meat) growth index) and allocated to two levels of F (1.7 (high F) vs. 1.1 (low F) times Metabolisable Energy requirement for maintenance) treatments. Twenty-four Coopworth rams were divided into four equal groups for a N balance study: high G × high F, high G × low F, low G × high F, and low G × low F. The main factors (G and F) and the interaction term were used for 2-way ANOVA and regression analysis. Higher F led to higher N excretions (urinary N (UN); faecal N (FN); manure N), retained N, N use efficiency (NUE), and urinary purine derivatives excretion (P < 0.05). On the other hand, higher UN/N intake, and plasma Δ¹⁵N were observed with the lower F (P < 0.05). Higher G led to increased UN, FN, manure N, apparent N digestibility, and urinary purine derivatives excretion (P < 0.05). Higher F only increased UN in high G sheep, with no effect on low G sheep (P < 0.05). Regression analysis results demonstrated potential to use plasma Δ¹⁵N to reflect the effects of G and F on NUE and UN/N intake. Further research is urged to study interactive effects of genetic and feeding level on sheep N partitioning.

Oscillating dietary crude protein concentrations increase N retention of calves by affecting urea-N recycling and nitrogen metabolism of rumen bacteria and epithelium

The purpose of this study was to investigate the effects of oscillating crude protein (CP) concentration diet on the nitrogen utilization efficiency (NUE) of calves and determine its mechanism. Twelve Holstein calves were assigned randomly into static protein diet (SP, 149 g/kg CP) and oscillating protein diet (OP, 125 and 173 g/kg CP diets oscillated at 2-d intervals) groups. After 60 days of feeding, the weights of total stomach, rumen and omasum tended to increase in calves fed OP. The apparent crude fat digestibility, NUE and energy metabolism also increased. In terms of urea-N kinetics evaluated by urea-¹⁵N¹⁵N isotope labeling method, the urea-N production and that entry to gastrointestinal tended to increase, and urea-N reused for anabolism increased significantly in calves fed OP during the low protein phase. These data indicate that urea-N recycling contributed to improving NUE when dietary protein concentration was low. In addition, the differentially expressed genes in rumen epithelium and the rumen bacteria involved in protein and energy metabolism promoted the utilization of dietary protein in calves fed OP.

Dietary protein oscillation: Effects on feed intake, lactation performance, and milk nitrogen efficiency in lactating dairy cows

Limited research with growing ruminants indicates that oscillating (OS) dietary crude protein (CP) concentration may improve nitrogen use efficiency (NUE). Our aim was to determine if a total mixed ration (TMR) based on OS CP (48-h phases of 13.4% and 16.5% CP, respectively) would increase NUE of lactating dairy cows compared with a static CP TMR (ST; 14.9% CP). The experiment was a randomized complete block design with 50 cows [150 ± 61 (mean ± SD) d in milk]. Cows were blocked by parity, days in milk, and milk protein yield. On average, diets were equal in composition over the total experiment. Cows were milked twice daily, and 8 milk samples were collected in each 4-d period. Each 48 h of low-CP (LP) and high-CP (HP) TMR offered to OS cows corresponded to milk collected at milkings 1 to 4 and 5 to 8, respectively. Dry matter intake (mean = 25.5 kg/d for both treatment groups); yields of milk (mean = 31.5 kg/d for both treatment groups), protein, fat, lactose, and fat- and protein-corrected milk (mean = 33.6 kg/d for both treatment groups); and milk concentration of protein, fat, and lactose did not differ between treatments. However, milk urea concentration was higher for OS compared with ST (12.2 vs. 11.3 mg/dL). Body weight, body condition score, NUE, and feed efficiency were unaffected by OS. Apparent total-tract digestibility of dry matter (695 vs. 677 g/kg), organic matter (714 vs. 697 g/kg), CP (624 vs. 594 g/kg), neutral detergent fiber (530 vs. 499 g/kg), and starch (976 vs. 973 g/kg) were higher for OS than for ST cows. Cows in OS responded transiently, and regression analysis of differences within block over time revealed changes in yield of milk (-531 g/d), milk protein (-25.6 g/d), and milk lactose (-16.7 g/d) in LP. Opposite effects were observed for yield of milk (+612 g/d), milk protein (+28.8 g/d), and milk lactose (+28.0 g/d) during HP. Changes in concentrations of milk protein (-0.050%/d), lactose (+0.030%/d), and urea (-3.0 mg/dL per day) during LP, and in milk lactose (-0.024%/d) and urea (+4.3 mg/dL per day) during HP, were observed. Milk yield, lactose yield, and protein yield were lower for OS than ST cows at the last milking of LP and at the first milking of HP. Milk urea concentration did not show such a lag and was lower in the last 2 milkings of LP, and higher in the last 3 milkings of HP, in OS compared with ST cows. Overall, performance and NUE were unaffected by OS treatment, but apparent total-tract digestibility and milk urea concentration increased, and transient effects on milk yield and composition occurred in OS cows.

Effects of oscillating dietary crude protein concentrations on production, nutrient digestion, plasma metabolites, and body composition in lactating dairy cows

We hypothesized that dairy cows fed oscillating metabolizable protein (MP) and crude protein (CP) concentrations on a 24-h frequency for a diet formulated to be below MP requirements would use N more efficiently (i.e., increased milk protein yields and less manure N) without increasing mobilization of body protein stores than would cows fed the same deficient MP diet continuously, although both treatments would on average have equal MP concentrations. In a randomized block design, 30 Holstein cows (119 ± 21 d in milk; 667 ± 69 kg of body weight) were blocked according to milk yield within a parity (3 primiparous and 7 multiparous blocks) and fed 1 of 3 treatments: (1) diet with 16.2% CP (109% of MP requirements) fed continuously (109MP), (2) diet with 14.1% CP (95% of MP requirements) fed continuously (95MP), or (3) diets oscillating on a 24-h cycle from the 109MP diet and a diet with 11.9% CP (∼78% of MP requirements) such that average CP and MP concentration would be the same as 95MP (OSC). Dry matter intake was similar between 109MP and 95MP (22.9 vs. 23.2 kg/d) but tended to be lower for OSC (22.2 kg/d) compared with 95MP. Milk yield was greater for 109MP compared with 95MP (36.6 vs. 35.1 kg/d) and similar between 95MP and OSC (35.3 kg/d). Milk protein and energy-corrected milk yields were similar among treatments. Milk urea N (MUN) concentration was higher for 109MP compared with 95MP (12.8 vs. 10.2 mg/dL), and tended to be higher for OSC (10.9 mg/dL) compared with 95MP. Higher MUN concentration for OSC occurred despite lower N intake (474 vs. 512 g of N/d) and similar milk N outputs compared with 95MP (164 vs. 179 g/d). On days when cows on OSC were fed high versus low MP diets, yields of milk (34.8 vs. 36.3 kg/d) and milk protein (1.0 vs. 1.1 kg/d) and MUN concentration (9.3 vs. 12.5 mg/dL) followed the oscillation pattern but lagged the change in diet CP by 1 d, whereas dry matter intake, yields of milk fat, plasma energy metabolites, AA, and 3-methyl-His were similar between days. Nutrient digestibility was similar for major nutrients across treatments except for CP, which was greater for 109MP (65.2%) and OSC (65.3%) compared with 95MP (61.7%). Compared with 95MP, OSC did not increase milk N relative to N intake (averaged 0.35 g of milk N/g of N intake) or N balance; however, urinary N output was increased for OSC versus 95MP (0.32 vs. 0.24 g of urine N/g of N intake). Body composition estimated using urea dilution was similar across treatments, and all cows accreted lipid and energy during the trial. Empty body CP did not change over the 50-d treatment period. Overall, greater CP digestion, urinary N excretion, and MUN concentrations with lesser N intake and similar milk N outputs for OSC compared with 95MP suggests that the lower energy intake by OSC cows may have limited potential responses to altered N metabolism.

Influence of forage level and corn grain processing on whole-body urea kinetics, and serosal-to-mucosal urea flux and expression of urea transporters and aquaporins in the ovine ruminal, duodenal, and cecal epithelia

The objectives were to determine the effects of forage level and grain processing on whole-body urea kinetics, N balance, serosal-to-mucosal urea flux (Jsm-urea), and messenger ribonucleic acid (mRNA) abundance of urea transporter-B (UT-B; SLC14a1) and aquaporins (AQP) in ovine ruminal, duodenal, and cecal epithelia. Thirty-two wether lambs were blocked by body weight into groups of four and assigned to one of four diets (n = 8) in a 2 × 2 factorial design. Dietary factors were forage level (30% [LF] vs. 70% [HF]) and corn grain processing (whole-shelled [WS] vs. steam-flaked [SF]). Four blocks of lambs (n = 4) were used to determine urea kinetics and N balance using 4-d [15N15N]-urea infusions with concurrent fecal and urine collections. Lambs were killed after 23 d of dietary adaptation. Ruminal, duodenal, and cecal epithelia were collected to determine Jsm-urea and mRNA abundance of UT-B and AQP. Lambs fed LF had greater intakes of dry matter (DMI; 1.20 vs. 0.86 kg/d) and N (NI; 20.1 vs. 15.0 g/d) than those fed HF (P < 0.01). Lambs fed SF had greater DMI (1.20 vs. 0.86 kg/d) and NI (20.6 vs. 14.5 g/d) than those fed WS (P < 0.01). As a percentage of NI, total N excretion was greater in lambs fed HF compared with those fed LF (103% vs. 63.0%; P < 0.01) and was also greater in lambs fed WS compared with those fed SF (93.6% vs. 72.1%; P = 0.02). Retained N (% of NI) was greater in lambs fed LF compared with those fed HF (37.0% vs. -2.55%; P < 0.01). Lambs fed SF had a greater (P = 0.02) retained N (% of NI; 28.0% vs. 6.50%) compared with those fed WS. Endogenous urea production (UER) tended (P = 0.09) to be greater in lambs fed HF compared with those fed LF. As a proportion of UER, lambs fed HF had a greater urinary urea-N loss (0.38 vs. 0.22) and lower urea-N transferred to the gastrointestinal tract (GIT; 0.62 vs. 0.78) or urea-N used for anabolism (as a proportion of urea-N transferred to the GIT; 0.12 vs. 0.26) compared with lambs fed LF (P < 0.01). Ruminal Jsm-urea was unaffected by diet. Duodenal Jsm-urea was greater (P < 0.01) in lambs fed HF compared with LF (77.5 vs. 57.2 nmol/[cm2 × h]). Lambs fed LF had greater (P = 0.03) mRNA expression of AQP3 in ruminal epithelia and tended (P = 0.06) to have greater mRNA expression of AQP3 in duodenal epithelia compared with lambs fed HF. Expression of UT-B mRNA was unaffected by diet. Our results showed that feeding more ruminally available energy improved N utilization, partly through a greater proportion of UER being transferred to the GIT and being used for anabolic purposes.

Characterization of urea transport mechanisms in the intestinal tract of growing pigs

Pigs are capable of nitrogen salvage via urea recycling, which involves the movement of urea in the gastrointestinal tract. Aquaporins (AQP) and urea transporter B (UT-B) are involved in urea recycling in ruminants; however, their contribution to urea flux in the intestinal tract of the pig is not known. The objective of this study was to characterize the presence and relative contribution of known urea transporters to urea flux in the growing pig. Intestinal tissue samples (duodenum, jejunum, ileum, cecum, and colon) were obtained from nine barrows (50.8 ± 0.9 kg) and analyzed for mRNA abundance of UT-B and AQP-3, -7, and -10. Immediately after tissue collection, samples from the jejunum and cecum were placed in Ussing chambers for analysis of the serosal-to-mucosal urea flux (Jsm-urea) with no inhibition or when incubated in the presence of phloretin to inhibit UT-B-mediated transport, NiCl2 to inhibit AQP-mediated transport, or both inhibitors. UT-B expression was greatest (P < 0.05) in the cecum, whereas AQP-3, -7, and -10 expression was greatest (P < 0.05) in the jejunum. The Jsm-urea was greater in the cecum than the jejunum (67.8 . 42.7 ± 5.01 µmol·cm-2·h-1; P < 0.05), confirming the capacity for urea recycling in the gut in pigs; however, flux rate was not influenced (P > 0.05) by urea transporter inhibitors. The results of this study suggest that, although known urea transporters are expressed in the gastrointestinal tract of pigs, they may not play a significant functional role in transepithelial urea transport.NEW & NOTEWORTHY We characterized the location and contribution of known urea transporters to urea flux in the pig. Aquaporins are located throughout the intestinal tract, and urea transporter B is expressed only in the cecum. Urea flux occurred in both the jejunum and cecum. Transporter inhibitors had no affect on urea flux, suggesting that their contribution to urea transport in the intestinal tract is limited. Further work is required to determine which factors contribute to urea flux in swine.

Oscillating and static dietary crude protein supply. I. Impacts on intake, digestibility, performance, and nitrogen balance in young Nellore bulls

Effects of dietary crude protein (CP) supply on intake, digestibility, performance, and N balance were evaluated in young Nellore bulls consuming static or oscillating CP concentrations. Forty-two young bulls (initial BW of 260 ± 8.1 kg; age of 7 ± 1.0 mo) were fed ad libitum and were randomly assigned to receive one of six diets with different CP concentrations for 140 d: 105 (LO), 125 (MD), or 145 g CP /kg DM (HI), and LO to HI (LH), LO to MD (LM), or MD to HI (MH) oscillating CP at a 48-h interval for each feed. At the end of the experiment, bulls were slaughtered to evaluate carcass characteristics. Linear and quadratic effects were used to compare LO, MD, and HI, and specific contrasts were applied to compare oscillating dietary CP treatments vs. MD (125 g CP/kg DM) static treatment. Dry matter intake (DMI) was not affected (P > 0.26) by increasing or oscillating dietary CP. As dietary N concentration increased, there was a subsequent increase in apparent N compounds digestibility (P = 0.02), and no significant difference (P = 0.38) was observed between oscillating LH and MD. Daily total urinary and fecal N increased (P < 0.01) in response to increasing dietary CP. Significant differences were observed between oscillating LM and MH vs. MD, where bulls receiving the LM diet excreted less (P < 0.01; 71.21 g/d) and bulls fed MH excreted more (P < 0.01) urinary N (90.70 g/d) than those fed MD (85.52 g/d). A quadratic effect was observed (P < 0.01) for retained N as a percentage of N intake, where the bulls fed LO had greater N retention than those fed HI, 16.20% and 13.78%, respectively. Both LH and LM had greater (P < 0.01) daily retained N when compared with MD. Performance and carcass characteristics were not affected (P > 0.05) by increasing or oscillating dietary CP. Therefore, these data indicate that although there is no alteration in the performance of growing Nellore bulls fed with oscillating CP diets vs. a static level of 125 g CP/kg DM, nor static low (105 g CP/kg DM) and high (145 g CP/kg DM) levels; there may be undesirable increases in environmental N excretion when the average dietary CP content is increased. The results suggest that dietary CP concentrations of 105, 125 g/kg DM, or within this range can be indicated for finishing young Nellore bulls, since it reaches the requirements, reduces the environmental footprint related to N excretion, and may save on costs of high-priced protein feeds.

Recent advances to improve nitrogen efficiency of grain-finishing cattle in North American and Australian feedlots

Formulating diets conservatively for minimum crude-protein (CP) requirements and overfeeding nitrogen (N) is commonplace in grain finishing rations in USA, Canada and Australia. Overfeeding N is considered to be a low-cost and low-risk (to cattle production and health) strategy and is becoming more commonplace in the US with the use of high-N ethanol by-products in finishing diets. However, loss of N from feedlot manure in the form of volatilised ammonia and nitrous oxide, and nitrate contamination of water are of significant environmental concern. Thus, there is a need to improve N-use efficiency of beef cattle production and reduce losses of N to the environment. The most effective approach is to lower N intake of animals through precision feeding, and the application of the metabolisable protein system, including its recent updates to estimation of N supply and recycling. Precision feeding of protein needs to account for variations in the production system, e.g. grain type, liveweight, maturity, use of hormonal growth promotants and β agonists. Opportunities to reduce total N fed to finishing cattle include oscillating supply of dietary CP and reducing supply of CP to better meet cattle requirements (phase feeding).

Effect of dietary nitrogen level and source on mRNA expression of urea transporters in the rumen epithelium of fattening bulls

This paper aims to study the effect of the dietary treatments on mRNA expression of urea transporter B (UT-B) and some aquaporins (AQP) in rumen epithelium of Italian Simmental young bulls. Eighty animals allocated to 16 pens were fed from about 500 to 650 kg body weight with four experimental diets, which resulted from the combination of two crude protein levels (125 and 110 g/kg dry matter, diets M and L, respectively) and two nitrogen sources (soybean meal (SBM) or SBM partly replaced by an isonitrogenous mixture of corn and urea; diets -U and +U, respectively). At slaughtering samples of blood and rumen epithelium were collected from six bulls for each diet. Blood samples were analysed for haematological parameters and quantitative PCR was carried out on the mRNA extracted from the rumen epithelium samples. The bulls fed diets M had lower plasma concentrations of aspartate aminotransferase than those receiving diets L (78.9 vs. 88.3 U/l, p = 0.04). Plasma urea was higher (p = 0.03) for diets M and lower for diets +U (2.0 vs. 2.5 and 1.73 vs. 2.00 mmol/l, respectively, in M and L diets, p = 0.04). The effect of dietary treatments on rumen UT expression were limited to AQP3, which was down regulated (p = 0.01) in diets +U. Finally, a high positive correlation (R2 = 0.871) between the expressions of AQP7 and AQP10 was found. In conclusion, the AQP3 appears very responsive to dietary treatments and therefore it is a candidate to be further studied in rumen metabolism experiments. The close relationship between mRNA expression of AQP7 and AQP10 indicates a similar function of these two proteins.

Effects of Static or Oscillating Dietary Crude Protein Levels on Fermentation Dynamics of Beef Cattle Diets Using a Dual-Flow Continuous Culture System

The objective of this study was to evaluate the effects of increasing dietary crude protein (CP) levels and also comparing the effects of static versus oscillating dietary CP on ruminal nutrient digestibility, ruminal fermentation, nitrogen (N) metabolism, and microbial efficiency in beef cattle diets using a dual-flow continuous culture system. Eight fermenters (1,223 ± 21 mL) were used in a replicated 4 x 4 Latin square design with periods lasting 12 d each (8 d for adaptation and 4 d for sampling). Dietary treatments were: 1) 10% CP, 2) 12% CP, 3) 14% CP, and 4) 10 and 14% CP diets oscillating at 48-h intervals. Experimental diets consisted of 50% orchard hay and 50% concentrate. Fermenters were fed 72 g/d and solid and liquid dilution rates were adjusted to 5.5 and 11%/h, respectively. Data were analyzed using the MIXED procedure in SAS with α = 0.05. Apparent and true ruminal digestibilities of dry matter and organic matter were not affected (P > 0.05) by increasing dietary CP, nor by oscillating dietary CP. Total volatile fatty acids concentration and molar proportions of acetate, propionate, butyrate, valerate, iso-butyrate and iso-valerate were not affected (P > 0.05) by increasing or oscillating dietary CP. Ruminal NH₃-N concentration increased linearly (P < 0.01) in response to increasing dietary CP. Total N, non-ammonia N, and rumen undegraded protein flows did not differ among treatments or between oscillating dietary CP and static 12% CP. Microbial N and NH₃-N flows and microbial efficiency did not differ when comparing oscillating versus static CP (P > 0.05). However, there was a quadratic effect (P < 0.05) for these variables when dietary CP was increased. These results indicate that either ruminal microorganisms do not respond to oscillating CP levels or are capable of coping with 48-h periods of undernourishment.

The Role of Proanthocyanidins Complex in Structure and Nutrition Interaction in Alfalfa Forage

Alfalfa (Medicago sativa L.) is one of the main forages grown in the world. Alfalfa is a winter hardy, drought tolerant, N-fixing legume with a good longevity, high yield, high nutrient levels, high digestibility, unique structural to non-structural components ratio, high dry matter intake, and high animal productivity per hectare. However, its main limitation is its excessively rapid initial rate of protein degradation in the rumen, which results in pasture bloat and inefficient use of protein with consequent excessive excretions of nitrogen into the environment. Proanthocyanidins are secondary plant metabolites that can bind with protein and thereby reduce the rate and extent of ruminal protein degradation. However, these secondary metabolites do not accumulate in alfalfa. This review aims to firstly describe the events involved in the rapid release of protein from alfalfa and its effect on ruminant nutrition, environmental pollution, and pasture bloat; secondly, to describe occurrence, structure, functions and benefits of moderate amounts of proanthocyanidin; and finally, to describe the development of alfalfa which accumulates moderate amounts of proanthocyanidins. The emphasis of this review focuses on the role of proanthocyanidins compounds in structure and nutrition interaction in ruminant livestock systems.

Serosal-to-mucosal urea flux across the isolated ruminal epithelium is mediated via urea transporter-B and aquaporins when Holstein calves are abruptly changed to a moderately fermentable diet

Urea transport (UT-B) proteins are known to facilitate urea movement across the ruminal epithelium; however, other mechanisms may be involved as well because inhibiting UT-B does not completely abolish urea transport. Of the aquaporins (AQP), which are a family of membrane-spanning proteins that are predominantly involved in the movement of water, AQP-3, AQP-7, and AQP-10 are also permeable to urea, but it is not clear if they contribute to urea transport across the ruminal epithelium. The objectives of this study were to determine (1) the functional roles of AQP and UT-B in the serosal-to-mucosal urea flux (Jsm-urea) across rumen epithelium; and (2) whether functional adaptation occurs in response to increased diet fermentability. Twenty-five Holstein steer calves (n=5) were assigned to a control diet (CON; 91.5% hay and 8.5% vitamin and mineral supplement) or a medium grain diet (MGD; 41.5% barley grain, 50% hay, and 8.5% vitamin and mineral) that was fed for 3, 7, 14, or 21 d. Calves were killed and ruminal epithelium was collected for mounting in Ussing chambers under short-circuit conditions and for analysis of mRNA abundance of UT-B and AQP-3, AQP-7, and AQP-10. To mimic physiologic conditions, the mucosal buffer (pH 6.2) contained no urea, whereas the serosal buffer (pH 7.4) contained 1 mM urea. The fluxes of (14)C-urea (Jsm-urea; 26 kBq/10 mL) and (3)H-mannitol (Jsm-mannitol; 37 kBq/10 mL) were measured, with Jsm-mannitol being used as an indicator of paracellular or hydrophilic movement. Serosal addition of phloretin (1 mM) was used to inhibit UT-B-mediated urea transport, whereas NiCl2 (1 mM) was used to inhibit AQP-mediated urea transport. Across treatments, the addition of phloretin or NiCl2 reduced the Jsm-urea from 116.5 to 54.0 and 89.5 nmol/(cm(2) × h), respectively. When both inhibitors were added simultaneously, Jsm-urea was further reduced to 36.8 nmol/(cm(2) × h). Phloretin-sensitive and NiCl2-sensitive Jsm-urea were not affected by diet. The Jsm-urea tended to increase linearly as the duration of adaptation to MGD increased, with the lowest Jsm-urea being observed in animals fed CON [107.7 nmol/(cm(2) × h)] and the highest for those fed the MGD for 21 d [144.2 nmol/(cm(2) × h)]. Phloretin-insensitive Jsm-urea tended to increase linearly as the duration of adaptation to MGD increased, whereas NiCl2-insensitive Jsm-urea tended to be affected by diet. Gene transcript abundance for AQP-3 and UT-B in ruminal epithelium increased linearly as the duration of MGD adaptation increased. For AQP-7 and AQP-10, gene transcript abundance in animals that were fed the MGD was greater compared with that of CON animals. These results demonstrate that both AQP and UT-B play significant functional roles in urea transport, and they may play a role in urea transport during dietary adaptation to fermentable carbohydrates.

Modulation of sheep ruminal urea transport by ammonia and pH

Ruminal fermentation products such as short-chain fatty acids (SCFA) and CO2 acutely stimulate urea transport across the ruminal epithelium in vivo, whereas ammonia has inhibitory effects. Uptake and signaling pathways remain obscure. The ruminal expression of SLC14a1 (UT-B) was studied using polymerase chain reaction (PCR). The functional short-term effects of ammonia on cytosolic pH (pHi) and ruminal urea transport across native epithelia were investigated using pH-sensitive microelectrodes and via flux measurements in Ussing chambers. Two variants (UT-B1 and UT-B2) could be fully sequenced from ovine ruminal cDNA. Functionally, transport was passive and modulated by luminal pH in the presence of SCFA and CO2, rising in response to luminal acidification to a peak value at pH 5.8 and dropping with further acidification, resulting in a bell-shaped curve. Presence of ammonia reduced the amplitude, but not the shape of the relationship between urea flux and pH, so that urea flux remained maximal at pH 5.8. Effects of ammonia were concentration dependent, with saturation at 5 mmol/l. Clamping the transepithelial potential altered the inhibitory potential of ammonia on urea flux. Ammonia depolarized the apical membrane and acidified pHi, suggesting that, at physiological pH (< 7), uptake of NH4 (+) into the cytosol may be a key signaling event regulating ruminal urea transport. We conclude that transport of urea across the ruminal epithelium involves proteins subject to rapid modulation by manipulations that alter pHi and the cytosolic concentration of NH4 (+). Implications for epithelial and ruminal homeostasis are discussed.

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

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

Supplemental butyrate does not enhance the absorptive or barrier functions of the isolated ovine ruminal epithelia

Our objective was to determine if increasing the ruminal butyrate concentration would improve the selective permeability of ruminal epithelia. Suffolk wether lambs (n = 18) with an initial BW of 47.4 ±1.4 kg were housed in individual pens (1.5 × 1.5 m) with rubber mats on the floor. Lambs were blocked by initial BW into 6 blocks and, within block, were randomly assigned to either the control (CON) or 1 of 2 butyrate supplementation amounts (i.e., 1.25% or 2.50% butyrate as a proportion of DMI). With the exception of butyrate supplementation, all lambs were fed a common diet (90% concentrate and 10% barley silage). After a 14-d feeding period, lambs were killed, and ruminal epithelia from the ventral sac were mounted in Ussing chambers. To facilitate the Ussing chamber measurements, only 1 lamb was killed on an individual day. Thus, the starting date was staggered so that all lambs were exposed to the same experimental protocol. In Ussing chambers, epithelia were incubated using separate mucosal (pH 6.2) and serosal (pH 7.4) bathing solutions. Then 1-14C-butyrate (74 kBq/10 mL) was added to the mucosal side and was used to measure the mucosal-to-serosal flux (J(ms-butyrate)) in 2 consecutive 60-min flux periods with simultaneous measurement of transepithelial conductance (G(t)). During the first (challenge) flux period, the mucosal buffer solution was either acidified to pH 5.2 (ACID) or used as a control (pH 6.2; SHAM). Buffer solutions bathing the epithelia were replaced before the second flux period (recovery). Total ruminal short-chain fatty acid and butyrate concentrations were greater (P = 0.001) in lambs fed 2.50% compared with those fed 0% or 1.25% butyrate. The J(ms-butyrate) was less for lambs fed 1.25% and 2.50% butyrate [3.00 and 3.12 μmol/(cm2·h), respectively] than for CON [3.91 μmol/(cm2· h)]. However, no difference (P = 0.13)was observed for G(t). An ex vivo treatment × flux period interaction was detected (P = 0.003) for J(ms-butyrate), where no differences were present between ACID and SHAM during the challenge period, but the Jms-butyrate was less for ACID than for SHAM during recovery. These results indicate that large increases in the ruminal butyrate concentration decrease the selective permeability of the isolated ruminal epithelia.