Effect of dietary crude protein concentration on milk production and nitrogen utilization in lactating dairy cows

Forty lactating Holstein cows, including 10 with ruminal cannulas, were blocked by days in milk into 8 groups and then randomly assigned to 1 of 8 incomplete 5 × 5 Latin squares to assess the effects of 5 levels of dietary crude protein (CP) on milk production and N use. Diets contained 25% alfalfa silage, 25% corn silage, and 50% concentrate, on a dry matter (DM) basis. Rolled high-moisture shelled corn was replaced with solvent-extracted soybean meal to increase CP from 13.5 to 15.0, 16.5, 17.9, and 19.4% of DM. Each of the 4 experimental periods lasted 28 d, with 14 d for adaptation and 14 d for data collection. Spot sampling of ruminal digesta, blood, urine, and feces was conducted on d 21 of each period. Intake of DM was not affected by diet but milk fat content as well as ruminal acetate, NH₃, and branched-chain volatile fatty acids, urinary allantoin, and blood and milk urea all increased linearly with increasing CP. Milk and protein yield showed trends for quadratic responses to dietary CP and were, respectively, 38.3 and 1.18 kg/d at 16.5% CP. As a proportion of N intake, urinary N excretion increased from 23.8 to 36.2%, whereas N secreted in milk decreased from 36.5 to 25.4%, as dietary protein increased from 13.5 to 19.4%. Under the conditions of this study, yield of milk and protein were not increased by feeding more than 16.5% CP. The linear increase in urinary N excretion resulted from a sharp decline in N efficiency as dietary CP content increased.     

Effect of increasing dietary protein with constant lysine:methionine ratio on production and omasal flow of nonammonia nitrogen in lactating dairy cows

Eight lactating cows were fed 4 diets in which dietary crude protein (CP) was increased in steps of approximately 2 percentage units from 11 to 17% of DM by replacing high-moisture corn with soybean meal supplemented with rumen-protected Met to maintain a Lys:Met ratio of 3:1 in metabolizable protein. Trial design was a replicated 4 × 4 Latin square; experimental periods lasted 28 d, with data and sample collection being performed during wk 3 and 4 of each period. Digesta samples were collected from the rumen as well as the omasum to measure metabolite concentrations and ruminal outflow of N fractions using infusion of ¹⁵N-enriched ammonia to quantify microbial nonammonia N (NAN) and nonmicrobial NAN. Data were analyzed using the MIXED procedure of SAS (SAS Institute Inc.). There were linear increases in the yields of milk and true protein and concentration of milk urea N, and a linear decrease in N efficiency, with increasing dietary CP. Apparent ruminal and total-tract N digestibility increased linearly with increasing dietary CP, but estimated true total-tract N digestibility was not affected. Apparent digestibility of the other macronutrients was not influenced by diet. Ruminal ammonia, total AA and peptides, and branched-chain VFA also increased linearly with dietary CP. The ¹⁵N enrichment of liquid- and particle-associated microbes linearly declined with increasing dietary CP due to decreasing ¹⁵N enrichment of the ammonia pool. Although no effect of dietary CP on nonmicrobial NAN flow was detected, total NAN flow increased linearly from 525 g/d at 11% CP to 637 g/d at 17% CP due to the linear increase in microbial NAN flow from 406 g/d at 11% CP to 482 g/d at 17% CP. Under the conditions of this study, when dietary CP was increased by adding soybean meal supplemented with rumen-protected Met, improved milk and protein yields were driven not by RUP supply but by increased ruminal outflow of microbial protein.     

Effect of dietary cation-anion difference and dietary crude protein on performance of lactating dairy cows during hot weather

Thirty-two lactating Holstein cows (225 ± 63 d in milk) were used in a 6-wk trial to determine the effect of dietary cation-anion difference (DCAD) and dietary crude protein (CP) concentration on milk and component yield, acid-base status, and serum AA concentrations during hot weather. Treatments were arranged as a 2 × 2 factorial within a randomized complete block design to provide 15 or 17% CP and a DCAD of 25 or 50 mEq (Na + K - Cl)/100 g of dry matter (DM). A DCAD × CP interaction was detected for milk yield; milk yield was less for high DCAD than for low DCAD for the high-CP diets. No differences were noted at low dietary CP. Milk fat percentage was greater for high DCAD than for low DCAD, and high-CP diets supported greater milk fat percentage than low-CP diets. No differences were observed among treatments for dry matter intake or milk protein percentage. Serum total AA and essential AA concentrations and ratio of essential AA:total AA were greater for high DCAD. These results suggest that increasing DCAD improves AA availability for protein synthesis by taking the place of AA that would otherwise be used for maintenance of acid-base balance. A better understanding of the mechanisms behind this AA-sparing effect will improve management of protein nutrition in the lactating dairy cow.     

Effect of dietary protein concentration and degradability on response to rumen-protected methionine in lactating dairy cows

An incomplete 8 × 8 Latin square trial (4-wk periods; 12 wk total) using 32 multiparous and 16 primiparous Holstein cows was conducted to assess the production response to crude protein (CP), digestible rumen-undegraded protein (RUP), and rumen-protected Met (RPM; fed as Mepron; Degussa Corp., Kennesaw, GA). Diets contained [dry matter (DM) basis] 21% alfalfa silage, 34% corn silage, 22 to 26% high-moisture corn, 10 to 14% soybean meal, 4% soyhulls, 2% added fat, 1.3% minerals and vitamins, and 27 to 28% neutral detergent fiber. Treatments were a 2 × 2 × 2 factorial arrangement of the following main effects: 15.8 or 17.1% dietary CP, with or without supplemental rumen-undegraded protein (RUP) from expeller soybean meal, and 0 or 9 g of RPM/d. None of the 2- or 3-way interactions was significant. Higher dietary CP increased DM intake 1.1 kg/d and yield of milk 1.7 kg/d, 3.5% fat-corrected milk (FCM) 2.2 kg/d, fat 0.10 kg/d, and true protein 0.05 kg/d, and improved apparent N balance and DM and fiber digestibility. However, milk urea N and estimated urinary excretion of urea-N and total-N also increased, and apparent N efficiency (milk-N/N-intake) fell from 33 to 30% when cows consumed higher dietary CP. Positive effects of feeding more RUP were increased feed efficiency and milk fat content plus 1.8 kg/d greater FCM and 0.08 kg/d greater fat, but milk protein content was lower and milk urea N and urinary urea excretion were elevated. Supplementation with RPM increased DM intake 0.7 kg/d and FCM and fat yield by 1.4 and 0.06 kg/d, and tended to increase milk fat content and yield of milk and protein.     

Effect of amount and ruminal degradability of soybean meal protein on performance of lactating dairy cows

Twenty-eight Holstein cows (4 with ruminal cannulas) were blocked by days in milk into 7 groups and then randomly assigned to 1 of 7 balanced 4 × 4 Latin square diet sequences. The diets contained [dry matter (DM) basis] 20% alfalfa silage, 35% corn silage, and 45% concentrate mainly from high-moisture corn and soybean meal. Diets differed in crude protein (CP) content and source of protein supplement: diet A) 15.6% CP, 3.7% solvent-extracted soybean meal (SSBM), 4.5% expeller soybean meal (ESBM); diet B) 16.6% CP, 9.6% SSBM, 0% ESBM; diet C) 16.6% CP, 4.6% SSBM, 5.9% ESBM; and diet D) 17.6% CP, 11.7% SSBM, 0% ESBM. Each experimental period consisted of 14 d for adaptation plus 14 d for collection of production data. Sampling of ruminal digesta and spot sampling of blood, feces, and urine was done on d 26 and 27 of each period. Planned contrasts compared included diet A vs. diet B, diet B vs. diet C, and diet B vs. diet D. There were no effects of diet on most of the production traits measured. However, milk yield tended to be higher for diet B vs. A. Trends were also detected for higher DM intake and weight gain and lower milk yield/DM intake in cows fed diet D vs. diet B. Milk lactose content was higher on diets A and C than on B. Ruminal NH₃ was higher on diet D vs. B, but other ruminal metabolites, apparent nutrient digestibility, and estimated bacterial CP synthesis did not differ across diets. Blood and milk urea-N were higher on diets C and D than on B; milk urea-N was higher on diet B than on A. Increasing dietary CP from 16.6% (diet B) to 17.6% (diet D) increased urinary N excretion by 54 g/d and reduced apparent N efficiency (milk N/N intake) by 2.5 percentage units, without altering yield. Under the conditions of this trial, milk production was not improved by feeding rumen-undegraded protein from ESBM or greater amounts of rumen-degraded protein from SSBM. Feeding more than 16.6% CP depressed N efficiency.     

Effect of dietary fiber on endogenous nitrogen flows in lactating dairy cows

The effect of dietary fiber on endogenous N secretion was studied using a 15N isotope dilution technique in four fistulated Holstein cows. Two isonitrogenous diets differing only in fiber (NDF and ADF) content were used in a crossover design. One diet (HF) contained 37.4% NDF, while the other (LF) contained 23.3%. A new model was developed to estimate endogenous N secretions and losses for the preintestinal, intestinal, and the total sections of the gastrointestinal tract. Three precursor pools: TCA-soluble fraction of plasma, intestinal mucosa, and milk were compared. Although endogenous losses estimated with the model were numerically different for each precursor pool selected (TCA-soluble fraction > mucosa > milk), treatment effects were similar. As intestinal mucosa is probably closest to the precursor pool, these data are discussed. Non-urea N endogenous secretions contributed 13% of the duodenal N flow but were not affected by the fiber content of the diet. The nonurea N endogenous flow at the duodenum was comprised of approximately equal inputs from endogenous N direct, and that incorporated into the microbial biomass. Total endogenous N flows at the duodenum exceeded, by nearly twofold, estimated inputs of urea-N to microbial biomass. Metabolic fecal output averaged 17% of fecal N and was not affected by level of dietary fiber, but net losses from secretions occurring in the small intestine were higher with the low fiber diet. Overall, endogenous N secretions represented 30% of total digestive tract protein synthesis.     

Effect of dietary level of rumen-degraded protein on production and nitrogen metabolism in lactating dairy cows

Twenty-eight (8 with ruminal cannulas) lactating Holstein cows were assigned to 4 × 4 Latin squares and fed diets with different levels of rumen-degraded protein (RDP) to study the effect of RDP on production and N metabolism. Diets contained [dry matter (DM) basis] 37% corn silage, 13% alfalfa silage, and 50% concentrate. The concentrate contained solvent and lignosulfonate-treated soybean meal and urea, and was adjusted to provide RDP at: 13.2, 12.3, 11.7, and 10.6% of DM in diets A to D, respectively. Intake of DM and yield of milk, fat-corrected milk, and fat were not affected by treatments. Dietary RDP had positive linear effects on milk true protein content and microbial non-ammonia N (NAN) flow at the omasal canal, and a quadratic effect on true protein yield, with maximal protein production at 12.3% RDP. However, dietary RDP had a positive linear effect on total N excretion, with urinary N accounting for most of the increase, and a negative linear effect on environmental N efficiency (kg of milk produced per kg of N excreted). Therefore, a compromise between profitability and environmental quality was achieved at a dietary RDP level of 11.7% of DM. Observed microbial NAN flow and RDP supply were higher and RUP flow was lower than those predicted by the NRC (2001) model. The NRC (2001) model overpredicted production responses to RUP compared with the results in this study. Replacing default NRC degradation rates for protein supplements with rates measured in vivo resulted in similar observed and predicted values, suggesting that in situ degradation rates used by the NRC are slower than apparent rates in this study.     

Effect of dietary nitrogen content and intravenous urea infusion on ruminal and portal-drained visceral extraction of arterial urea in lactating Holstein cows

Urea extraction across ruminal and portal-drained visceral (PDV) tissues were investigated using 9 rumen-cannulated and multi-catheterized lactating dairy cows adapted to low-N (12.9% crude protein) and high-N (17.1% crude protein) diets in a crossover design. The interaction between adaptation to dietary treatments and blood plasma concentrations of urea was studied by dividing samplings into a 2.5-h period without urea infusion followed by a 2.5-h period with primed continuous intravenous infusion of urea (0.493 ± 0.012 mmol/kg of BW per h). Cows were sampled at 66 ± 14 and 68 ± 12 d in milk and produced 42 ± 1 and 36 ± 1 kg of milk/d with the high-N and low-N diets, respectively. The arterial blood urea concentration before urea infusion was 1.37 and 4.09 ± 0.18 mmol/L with low-N and high-N, respectively. Dietary treatment did not affect the urea infusion-induced increase in arterial urea concentration (1.91 ± 0.13 mmol/L). Arterial urea extraction across the PDV and rumen increased from 2.7 to 5.4 ± 0.5% and from 7.1 to 23.8 ± 2.1% when cows were changed from high-N to low-N, respectively. Urea infusion did not decrease urea extractions, implying that urea transport rates were proportional to arterial urea concentrations. Urea extraction increased more across the rumen wall than across the total PDV for low-N compared with high-N, which implies that a larger proportion of total PDV uptake of arterial urea is directed toward the rumen with decreasing N intake. The ruminal vein - arterial (RA) concentration difference for ammonia increased instantly (first sampling 15 min after initiation of infusion) to the primed intravenous infusion when cows were adapted to the low-N diet. The RA difference for ammonia correlated poorly to the ventral ruminal concentration of ammonia (r = 0.55). Relating the RA difference for ammonia to a function of both ruminal ammonia concentration and the RA difference for urea markedly improved the fit (r = 0.85), indicating that a large fraction of ammonia released to the ruminal vein is absorbed from an epithelial ammonia pool not in equilibrium with the ventral ruminal ammonia pool. Changing cows from high-N to low-N affected the relative blood urea clearance by kidneys and PDV. The clearance by the kidneys decreased from 41 to 27 ± 2 L/h and the clearance by the PDV increased from 52 to 105 ± 12 L/h when the diet was changed from high-N to low-N. In conclusion, urea transport across gut epithelia in cattle is adapting to N status and driven by mass action. Data are commensurable with a model for urea transport across gut epithelia based on regulated expression or activity of facilitative urea transporters.     

Effect of supplementing rumen-protected methionine on production and nitrogen excretion in lactating dairy cows

Two 4 × 4 Latin square trials (4-wk periods; 16 wk total) were conducted to see whether supplementing rumen-protected Met (RPM; fed as Mepron) would allow feeding less crude protein (CP), thereby reducing urinary N excretion, but without losing production. In trial 1, 24 Holsteins were fed 4 diets as total mixed rations containing [dry matter (DM) basis]: 18.6% CP and 0 g of RPM/d; 17.3% CP and 5 g of RPM/d; 16.1% CP and 10 g of RPM/d; or 14.8% CP and 15 g of RPM/d. Dietary CP was reduced by replacing soybean meal with high-moisture shelled corn. All diets contained 21% alfalfa silage, 28% corn silage, 4.5% roasted soybeans, 5.8% soyhulls, 0.6% sodium bicarbonate, 0.5% vitamins and minerals, and 27% neutral detergent fiber. There was no effect of diet on intake, weight gain, or yields of protein, lactose, and solids-not-fat. However, production was greater at 17.3% CP plus RPM and 16.1% CP plus RPM than on the other 2 diets. Apparent N efficiency (milk N:N intake) was greatest on the lowest CP diet containing the most RPM. Linear reductions in milk urea N and urinary N excretion were observed with lower dietary CP. In trial 2, 32 Holsteins were fed 4 diets as total mixed rations, formulated from ingredients used in trial 1 and containing 16.1 or 17.3% CP with 0 or 10 g of RPM/d. On average, cows were calculated to be in negative N balance on all diets because of lower than expected DM intake. There was no effect of RPM supplementation on any production trait. However, higher CP gave small increases in yields of milk, protein, and solids-not-fat and tended to increase DM intake and lactose yield. Apparent N efficiency was greater, and milk urea nitrogen was lower, on 16.1% CP. In trial 1, feeding lower CP diets supplemented with RPM resulted in improved N efficiency and reduced urinary N excretion. However, in trial 2, reducing dietary CP from 17.3 to 16.1% reduced milk secretion, an effect that was not reversed by RPM supplementation at low DM intakes when cows were apparently mobilizing body protein.     

Effects of different protein supplements on omasal nutrient flow and microbial protein synthesis in lactating dairy cows

Eight ruminally cannulated Holstein cows that were part of a larger lactation trial were used in 2 replicated 4 × 4 Latin squares to quantify effects of supplementing protein as urea, solvent soybean meal (SSBM), cottonseed meal (CSM), or canola meal (CM) on omasal nutrient flows and microbial protein synthesis. All diets contained (% of dry matter) 21% alfalfa silage and 35% corn silage plus 1) 2% urea plus 41% high-moisture shelled corn (HMSC), 2) 12% SSBM plus 31% HMSC, 3) 14% CSM plus 29% HMSC, or 4) 16% CM plus 27% HMSC. Crude protein was equal across diets, averaging 16.6%. The CSM diet supplied the least rumen-degraded protein and the most rumen-undegraded protein. Microbial nonammonia N flow was similar among the true protein supplements but was 14% lower in cows fed urea. In vivo ruminal passage rate, degradation rate, and estimated escape for the 3 true proteins were, respectively, 0.044/h, 0.105/h, and 29% for SSBM; 0.051/h, 0.050/h, and 51% for CSM; and 0.039/h, 0.081/h, and 34% for CM. This indicated that CSM protein was less degraded because of both a faster passage rate and slower degradation rate. Omasal flow of individual AA, branched-chain AA, essential AA, nonessential AA, and total AA all were lower in cows fed urea compared with one of the true protein supplements. Among the 3 diets supplemented with true protein, omasal flow of Arg was greatest on CSM, and omasal flow of His was greatest on CSM, intermediate on CM, and lowest on SSBM. Lower flows of AA and microbial nonammonia N explained lower yields of milk yield and milk components observed on the urea diet in the companion lactation trial. These results clearly showed that supplementation with true protein was necessary to obtain sufficient microbial protein and rumen-undegraded protein to meet the metabolizable AA requirements of high-producing dairy cows.     

Effect of dietary crude protein modification on ammonia and nitrous oxide concentration on a tie-stall dairy barn floor

Dietary crude protein (CP) reduction is considered a useful strategy to minimize cow N excretion and NH₃ and N₂O emissions. The aim of the current work was to relate dietary CP modification to whole-animal N balance and subsequent NH₃ and N₂O concentrations on a tie-stall barn floor. The effect of temperature on NH₃ and N₂O concentration was also studied. Three Holstein mid to late lactating cows were confined in separate tie-stalls and randomly assigned to 3 diets with varying CP content [low CP (LCP): 14.1%; moderate CP (MCP): 15.9%; high CP (HCP): 16.9%]. Increasing N intake (from 438.6 to 522.8 g of N/d) improved milk yield (from 22.1 to 24.2 kg/d). However, N use efficiency tended to decrease with increasing dietary CP, as shown by milk N use efficiency (from 23.9 to 22.6%), milk urea N (from 15.4 to 18.7 mg/dL), and excreted N per milk yield unit (from 14.7 to 16.4 g of N/kg of milk). Because of higher N excretion, NH₃ concentration on the dairy barn floor increased (LCP: 7.1 mg of NH₃/m³; MCP: 10.4 mg of NH₃/m³; HCP: 10.8 mg of NH₃/m³). In contrast, N₂O concentration did not respond to dietary manipulation (mean 1.1 mg of N₂O/m³). Temperature, which ranged between 12.6 and 18.0°C, did not affect NH₃ and N₂O concentrations at the stall level. However, when fecal and urinary samples were incubated at 4, 19, and 29°C in the laboratory, ammonia concentration increased for all diets, especially for the MCP and HCP diets, as the temperature increased. In contrast, N2O concentration was negatively related to increasing temperature. In conclusion, data from the current trial demonstrate that lowering dietary CP minimizes NH₃ concentration on dairy stall floors although temperature controls the rate of NH₃ volatilization. On the other hand, N₂O concentration is not affected by dietary treatments on tie-stall floors.     

Effect of source of rumen-degraded protein on production and ruminal metabolism in lactating dairy cows

Twenty-eight (8 with ruminal cannulas) lactating Holstein cows were assigned to seven 4 × 4 Latin squares in a 16-wk trial to study the effects on production and ruminal metabolism of feeding differing proportions of rumen-degraded protein (RDP) from soybean meal and urea. Diets contained [dry matter (DM) basis] 40% corn silage, 15% alfalfa silage, 28 to 30% high-moisture corn, plus varying levels of ground dry shelled corn, solvent- and lignosulfonate-treated soybean meal, and urea. Proportions of the soybean meals, urea, and dry corn were adjusted such that all diets contained 16.1% crude protein and 10.5% RDP, with urea providing 0, 1.2, 2.4, and 3.7% RDP (DM basis). As urea supplied greater proportions of RDP, there were linear decreases in DM intake, yield of milk, 3.5% fat-corrected milk, fat, protein, and solids-not-fat, and of weight gain. Milk contents of fat, protein, and solids-not-fat were not affected by source of RDP. Replacing soybean meal RDP with urea RDP resulted in several linear responses: increased excretion of urinary urea-N and concentration of milk urea-N, blood urea-N, and ruminal ammonia-N and decreased excretion of fecal N; there was also a trend for increased excretion of total urinary N. A linear increase in neutral detergent fiber (NDF) digestibility, probably due to digestion of NDF-N from lignosulfonate-treated soybean meal, was observed with greater urea intake. Omasal sampling revealed small but significant effects of N source on measured RDP supply, which averaged 11.0% (DM basis) across diets. Increasing the proportion of RDP from urea resulted in linear decrease in omasal flow of dietary nonammonia N (NAN) and microbial NAN and in microbial growth efficiency (microbial NAN/unit of organic matter truly digested in the rumen). These changes were paralleled by large linear reductions in omasal flows of essential, nonessential, and total amino acids. Overall, these results indicated that replacing soybean meal RDP with that from urea reduced yield of milk and milk components, largely because of depressed microbial protein formation in the rumen and that RDP from nonprotein-N sources was not as effective as RDP provided by true protein.     

Effect of dietary supplementation with heat-treated canola meal on ruminal nutrient metabolism in lactating dairy cows

An experiment was conducted to quantify the effects of incremental levels of heat-moisture-treated canola meal (TCM) fed to dairy cows on the relationship between ruminal nutrient digestion and milk production. Experimental diets were fed to 4 multiparous rumen-cannulated Nordic Red cows, averaging (mean ± standard deviation) 681 ± 54.8 kg of body weight, 111 ± 16 d in milk, and 29.1 ± 9.1 kg of milk/d at the start of the study, in a Latin square design with four 21-d periods. The 4 experimental dietary treatments consisted of a basal diet of grass silage and crimped barley, and 3 diets in which the crimped barley was replaced with TCM, giving 3 incremental levels of protein supplementation. Nutrient flow was quantified by the omasal sampling technique using 3 markers (Cr, Yb, and indigestible neutral detergent fiber). Continuous infusion of ¹⁵N was used to label bacterial crude protein. Additionally, ruminal sampling and evacuations and measurements of total-tract digestibility were conducted. The experimental diets provided 132, 148, 164, and 180 g of crude protein/kg of dry matter. The increased level of TCM linearly increased dry matter intake from 15.1 to 16.6 kg/d and energy-corrected milk yield from 21.0 to 25.6 kg/d. The increased proportion of TCM when substituting barley with TCM was associated with greater total-tract digestibility of neutral detergent fiber and potentially digestible neutral detergent fiber, which could be explained by increased digestion rate of potentially digestible neutral detergent fiber. Omasal flow of nonammonia N naturally increased with greater dietary TCM inclusion, but the increased intestinal supply of rumen-undegradable protein was partly offset by diminished microbial protein synthesis when feeding more TCM. This was also reflected in a decreased proportion of milk protein from ruminal bacterial protein when TCM supplementation increased.     

Effect of varying dietary ratios of alfalfa silage to corn silage on omasal flow and microbial protein synthesis in dairy cows

Eight ruminally cannulated multiparous Holstein cows that were part of a larger production trial were used to study the effects of varying dietary ratios of alfalfa silage (AS) to corn silage (CS) on omasal flow of nutrients and microbial protein. Cows were blocked by DIM and randomly assigned to 2 replicated 4 × 4 Latin squares (28-d periods). Diets fed contained (dry matter basis): A) 51% AS, 43% rolled high-moisture shelled corn (HMSC), and 3% solvent soybean meal (SSBM); B) 37% AS, 13% CS, 39% HMSC, and 7% SSBM; C) 24% AS, 27% CS, 35% HMSC, and 12% SSBM; or D) 10% AS, 40% CS, 31% HMSC, and 16% SSBM. Crude protein (CP) contents were 17.2, 16.9, 16.6, and 16.2% for diets A, B, C, and D. All 4 diets were high in energy, averaging 49% nonfiber carbohydrates and 24% neutral detergent fiber. Total microbial nonammonia nitrogen flow was lower on diet D (423 g/d) compared with diets A (465 g/d), B (479 g/d), and C (460 g/d). A significant quadratic effect indicated that microbial protein synthesis was maximal at 38% AS. Supply of rumen-degraded protein decreased linearly from 3,068 g/d (diet A) to 2,469 g/d (diet D). Omasal flow of rumen-undegraded protein did not differ among diets and averaged 1,528 g/d. However, when expressed as a percentage of dry matter intake, rumen-undegraded protein increased linearly from 5.59% (diet A) to 6.13% (diet D), probably because CP from SSBM was more resistant to degradation than CP from AS. Essential AA flow was lowest on diet D, and Lys flow tended to be lower on diet D, which may explain the lower milk and protein yields observed on that diet.     

Effect of supplement crude protein concentration on milk production over the main grazing season and on nitrogen excretion in late-lactation grazing dairy cows

The objectives of this study are to evaluate the effects of (1) a potential interaction between supplement crude protein (CP) concentration and differing cow genotypes on milk production, (2) differing cow genotypes on milk production, and (3) decreasing the supplement CP concentration on milk production and N excretion during the main grazing season within a spring-calving herd. A 2 × 2 factorial arrangement experiment, with 2 feeding strategies [14%; n = 30 (lower CP; LCP) and 18%; n = 28 (higher CP; HCP) CP concentrate supplements] offered at varying levels according to pasture availability and days in milk (DIM) was conducted over the main grazing season from April 3 to September 3, 2019, at University College Dublin Lyons Farm. Cows were also grouped into 2 genotype groups: lower milk genotype; n = 30 [LM; milk kg predicted transmitting ability (PTA): 45 ± 68.6 (mean ± SD); fat kg PTA: 10 ± 4.9; and protein kg PTA: 7 ± 2.3] and higher milk genotype; n = 28 [HM; milk kg PTA: 203 ± 55.0; fat kg PTA: 13 ± 3.8; and protein kg PTA: 10 ± 2.4]. A total of 46 multiparous and 12 primiparous (total; 58) Holstein Friesian dairy cows were blocked on parity and balanced on DIM, body condition score, and Economic Breeding Index. Cows were offered a basal diet of grazed perennial ryegrass pasture. The N partitioning study took place from August 25 to 30, 2019 (187 ± 15.2 DIM). No interactions were observed for any milk production or milk composition parameter. No effect of supplement CP concentration was observed for any total accumulated milk production, daily milk production, or milk composition parameter measured. The HM cows had increased daily milk yield (+1.9 kg), fat and protein (+0.15 kg), and energy-corrected milk (+1.7 kg), compared with the LM cows. Furthermore, HM cows had decreased milk protein concentration (?0.1%) compared with LM cows. For the N partitioning study, cows offered LCP had increased pasture dry matter intake (PDMI; +0.9 kg/d), dietary N intake (+0.022 kg/d), feces N excretion (+0.016 kg/d), and decreased N partitioning to milk (?2%), and N utilization efficiency (?2.3%). In conclusion, offering cows LCP had no negative influence on milk production or milk composition over the main grazing season where high pasture quality was maintained. However, any potential negative effects of offering LCP on milk production may have been offset by the increased PDMI. Furthermore, offering cows LCP decreased N utilization efficiency due to the higher PDMI and feed N intake associated with cows on this treatment in our study.     

Effect of dietary cation-anion difference and dietary crude protein on milk yield, acid-base chemistry, and rumen fermentation

Eight primiparous lactating Holstein cows (47 ± 10 d in milk) fitted with ruminal cannulae were used to determine the effect of dietary cation-anion difference (DCAD) and dietary crude protein (CP) concentration on milk yield and composition, acid-base chemistry, and measures of N metabolism in lactating dairy cows. Treatments were arranged as a 2 × 2 factorial in a randomized complete block design to provide 15 or 17% CP and DCAD of 25 or 50 mEq (Na + K - Cl)/100 g of feed dry matter [15 or 39 mEq (Na + K) - (Cl + S)/100 g of feed dry matter]. High DCAD improved dry matter intake, milk yield, and concentrations of milk fat and protein. An interaction of DCAD and CP was observed for uric acid excretion, an indicator of microbial protein yield. Uric acid excretion was higher for high DCAD than for low DCAD in low CP diets and was similar for low and high DCAD with high CP. Serum bicarbonate concentration, urinary bicarbonate excretion, blood pH, and serum Na were elevated for high DCAD compared with low DCAD. Fractional excretion of Na, K, Cl, and Ca increased for high DCAD. Blood urea N and urinary urea N were greater for high than for low CP diets. No differences due to DCAD were observed for these parameters. Results of this study suggest that, in early lactation cows, blood acid-base chemistry is altered by differences in DCAD that range between the high and low ends of the desired DCAD range. Modifications of acid-base chemistry and the corresponding changes in protein metabolism may allow for more efficient feeding of protein and better nutritional management of the lactating dairy cow.     

Effects of feeding formate-treated alfalfa silage or red clover silage on omasal nutrient flow and microbial protein synthesis in lactating dairy cows

Eight ruminally cannulated Holstein cows that were part of a larger lactation trial were blocked by days in milk and randomly assigned to replicated 4 × 4 Latin squares to quantify effects of nonprotein N (NPN) content of alfalfa silage (AS) and red clover silage (RCS) on omasal nutrient flows. Diets, fed as total mixed rations, contained 50% dry matter from control AS (CAS), ammonium tetraformate-treated AS (TAS), late maturity RCS (RCS1), or early maturity RCS (RCS2). Silages differed in NPN and acid detergent insoluble N (% of total N): 50 and 4% (CAS); 45 and 3% (TAS); 27 and 8% (RCS1); 29 and 4% (RCS2). The CAS, TAS, and RCS2 diets had 36% high-moisture shelled corn and 3% soybean meal, and the RCS1 diet had 31% high-moisture shelled corn and 9% soybean meal. All diets contained 10% corn silage, 27% neutral detergent fiber, and 17 to 18% crude protein. Compared with RCS, feeding AS increased the supply of rumen-degraded protein and omasal flows of nonammonia N and microbial protein, which may explain the improved milk yield observed in the companion lactation trial. However, omasal flow of rumen-undegraded protein was 34% greater on RCS. Except for Arg, omasal flows of individual AA, branched-chain AA, nonessential AA, essential AA, and total AA did not differ between cows fed AS vs. RCS. Within AS diets, no differences in omasal AA flows were observed. However, omasal flows of Asp, Ser, Glu, Cys, Val, Ile, Tyr, Lys, total nonessential AA, and total AA all were higher in cows fed RCS1 vs. cows fed RCS2. In this trial, there was no advantage to reducing NPN content of hay-crop silage.     

Effect of two levels of crude protein and methionine supplementation on performance of dairy cows

Sixteen Holstein cows in midlactation were randomly assigned to treatments in a replicated 4 x 4 Latin square. Two levels of CP (16.1 vs. 18.8%) with or without supplemental methionine (0.07 g/100 g of DM) were tested in a 2 x 2 factorial arrangement of treatments. Dry matter intake, milk production, milk composition, and N excretion were determined. No interactions between CP level and methionine supplementation were observed. Milk production and dry matter intake were not different among treatments. Milk protein concentration increased from 3.17 to 3.26% with the addition of methionine and decreased from 3.24 to 3.17% with increased CP. No differences were observed among treatments in milk protein yield. Milk fat concentration was low across all diets, but was increased from 2.33% with 16.1% CP diets to 2.68% with 18.8% CP diets. No significant treatment effects were observed for SNF, lactose concentration in milk, or casein N as a fraction of skim milk N. Increased dietary CP increased milk urea N by 3.9 mg/dl. Methionine supplementation did not affect N excretion in urine or feces. The higher protein diets increased estimated urine volume by 2.9 L/d and increased N concentration by 1.7 percentage units in both urine and feces. Feeding higher protein increased milk urea and urine N excretion as expressed as a percentage of total N excreted (44 vs. 38% for 18.8 and 16.1% CP, respectively). Overall, feeding 16.1% CP produced milk and milk protein yields similar to feeding 18.8% CP, but reduced the N losses in urine and milk urea.     

Effect of casein and propionate supply on mammary protein metabolism in lactating dairy cows

The effects of casein (CN) and propionate (C3) on mammary AA metabolism were determined in 3 multiparous Holstein cows fitted with both duodenal and ruminal cannulas and used in a replicated Youden square with six 14-d periods. Casein (743 g/d in the duodenum) and C3 (1,041 g/d in the rumen) infusions were tested in a factorial arrangement. For each period, l-[1-¹³C]Leu (d 11) and NaH[¹³C]O₃ (d 13) were infused into a jugular vein, and blood samples were taken from the carotid artery and the mammary vein to determine Leu kinetics and net uptake of AA. Both CN and C3 treatments separately increased milk protein concentration and yield. With CN there was a general response in mammary protein metabolism, involving increases in Leu net uptake (30%), the uptake:output ratio (8%), protein synthesis (11%), secretion in milk protein (21%), and oxidation (259%). In contrast, C3 treatments tended to increase only Leu in milk protein (7%) and, when in combination with CN, to reduce Leu used for protein synthesis (5%). Across all treatments, most Leu uptake by the mammary gland was accounted for as Leu in milk or oxidized, and the Leu balance was therefore achieved without involvement of either net peptide use or production. Mammary uptake of group 1 AA increased to match milk output with all infusions. In contrast, mammary uptake of group 2 AA exceeded output to a greater extent with CN than with C3 infusions, whereas the increment in uptake of group 3 AA increased with C3 treatments. Overall, these data suggest that different mechanisms operate to improve milk protein production when either protein or energy is supplied.     

Effect of dietary carbohydrate composition and availability on utilization of ruminal ammonia nitrogen for milk protein synthesis in dairy cows

A trial with four ruminally and duodenally cannulated, late-lactation dairy cows was conducted to investigate the effect of dietary carbohydrate (CHO) composition and availability on ruminal ammonia N utilization and transfer into milk protein. Two diets were fed at 8-h intervals in a crossover design. The diets differed in CHO composition: the ruminally fermentable non-structural carbohydrates (RFSS) diet (barley and molasses) contained a larger proportion of ruminally available CHO in the nonstructural carbohydrate fractions and the ruminally fermentable fiber (RFNDF) diet (corn, beet pulp, and brewer's grains) contained a larger proportion of CHO in ruminally available fiber. Nitrogen-15 was used to label ruminal ammonia N and consequently microbial and milk N. Fermentation acids, enzyme activities, and microbial protein production in the rumen were not affected by diet. Ruminal ammonia concentration was lowered by RFNDF. Ruminal and total tract digestibility of nutrients did not differ between diets except that apparent ruminal degradability of crude protein was lower for RFNDF compared with RFSS. Partitioning of N losses between urine and feces was also not affected by diet. Milk yield and fat and protein content were not affected by treatment. Average concentration of milk urea N was lower for RFNDF than for RFSS. Proportion of milk protein N originating from ruminal microbial N (based on the areas under the 15N-enrichment curves) was higher for RFNDF than for RFSS. Cumulative recovery of 15N in milk protein was 13% higher for RFNDF than for RFSS indicating enhanced transfer of 15N-ammonia into milk protein with the former diet. The results suggested that, compared to diets containing higher levels of ruminally fermentable starch, diets providing higher concentration of ruminally fermentable fiber may enhance transfer of ruminal ammonia and microbial N into milk protein.