Influence of amount and degradability of dietary protein on nitrogen utilization by dairy cows

Effects of replacing hybrid giant napier with sugarcane bagasse and fermented sugarcane bagasse on growth performance, nutrient digestibility, rumen fermentation characteristics, and rumen microorganisms of Simmental crossbred cattle

This study investigated the effects of replacing hybrid giant napiers with sugarcane bagasse and fermented sugarcane bagasse on the growth performance, apparent nutrient digestibility, rumen fermentation characteristics, and rumen microorganisms of Simmental crossbred cattle. Twenty-one Simmental crossbred cattle with similar initial body weight (363.42 ± 8.67 kg) were randomly divided into three groups: Group CON (20% hybrid giant napier +45% distillers grains +35% concentrate mixture), Group SB (20% sugarcane bagasse +45% distillers grains +35% concentrate mixture), and Group FSB (20% fermented sugarcane bagasse +45% distillers grains +35% concentrate mixture). The average daily weight gain in the SB group was lower than in the CON group, no significant difference was found between the CON and FSB groups. The feed conversion ratio of the CON and FSB groups was lower compared to the SB group. The apparent digestibility of neutral detergent fiber and acid detergent fiber in the SB group was lower than in the CON group, no significant difference was found between the CON and FSB groups. The levels of NH3-N, microbial protein, acetate, propionate, butyrate, isobutyrate, and total volatile fatty acids were higher in the CON and FSB groups than in the SB group, no significant difference was found between the CON and FSB groups. The relative abundances of Christensenellaceae_R-7_group, Rikenellaceae_RC9_gut_group, Prevotellaceae_UCG-003, Saccharofermentans, and Eubacteriumcoprostanoligenes_group were lower in the CON and FSB groups compared to the SB group. The relative abundance of Succiniclasticum was highest in the FSB group, followed by the CON group and then the SB group. Correlation analysis showed that the relative abundance of Succiniclasticum was positively correlated with propionate and NH3-N content, while the relative abundance of Rikenellaceae_RC9_gut_group was inversely correlated with NH3-N content. Gene function prediction indicated that fermented sugarcane bagasse promoted rumen microbial amino acid metabolism. In conclusion, replacing hybrid giant napiers with 20% sugarcane bagasse negatively affected the growth performance of Simmental crossbred cattle, while the addition of 20% fermented sugarcane bagasse had no adverse effects on growth performance and rumen fermentation characteristics, and did not alter the abundance of the rumen core flora in Simmental crossbred cattle.

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.

Predictions of ruminal outflow of essential amino acids in dairy cattle

The objective of this work was to update and evaluate predictions of essential AA (EAA) outflows from the rumen. The model was constructed based on previously derived equations for rumen-undegradable (RUP), microbial (MiCP), and endogenous (EndCP) protein outflows from the rumen, and revised estimates of ingredient composition and EAA composition of the protein fractions. Corrections were adopted to account for incomplete recovery of EAA during 24-h acid hydrolysis. The predicted ruminal protein and EAA outflows were evaluated against a data set of observed values from the literature. Initial evaluations indicated a minor mean bias for non-ammonia, non-microbial nitrogen flow ([RUP + EndCP]/6.25) of 16 g of N per day. Root mean squared errors (RMSE) of EAA predictions ranged from 26.8 to 40.6% of observed mean values. Concordance correlation coefficients (CCC) of EAA predictions ranged from 0.34 to 0.55. Except for Leu, all ruminal EAA outflows were overpredicted by 3.0 to 32 g/d. In addition, small but significant slope biases were present for Arg [2.2% mean squared error (MSE)] and Lys (3.2% MSE). The overpredictions may suggest that the mean recovery of AA from acid hydrolysis across laboratories was less than estimates encompassed in the recovery factors. To test this hypothesis, several regression approaches were undertaken to identify potential causes of the bias. These included regressions of (1) residual errors for predicted EAA flows on each of the 3 protein-driven EA flows, (2) observed EAA flows on each protein-driven EAA flow, including an intercept, (3) observed EAA flows on the protein-driven EAA flows, excluding an intercept term, and (4) observed EAA flows on RUP and MiCP. However, these equations were deemed unsatisfactory for bias adjustment, as they generated biologically unfeasible predictions for some entities. Future work should focus on identifying the cause of the observed prediction bias.

Revised digestive parameter estimates for the Molly cow model

The Molly cow model represents nutrient digestion and metabolism based on a mechanistic representation of the key biological elements. Digestive parameters were derived ad hoc from literature observations or were assumed. Preliminary work determined that several of these parameters did not represent the true relationships. The current work was undertaken to derive ruminal and postruminal digestive parameters and to use a meta-approach to assess the effects of interactions among nutrients and identify areas of model weakness. Model predictions were compared with a database of literature observations containing 233 treatment means. Mean square prediction errors were assessed to characterize model performance. Ruminal pH prediction equations had substantial mean bias, which caused problems in fiber digestion and microbial growth predictions. The pH prediction equation was reparameterized simultaneously with the several ruminal and postruminal digestion parameters, resulting in more realistic parameter estimates for ruminal fiber digestion, and moderate reductions in prediction errors for pH, neutral detergent fiber, acid detergent fiber, and microbial N outflow from the rumen; and postruminal digestion of neutral detergent fiber, acid detergent fiber, and protein. Prediction errors are still large for ruminal ammonia and outflow of starch from the rumen. The gain in microbial efficiency associated with fat feeding was found to be more than twice the original estimate, but in contrast to prior assumptions, fat feeding did not exert negative effects on fiber and protein degradation in the rumen. Microbial responses to ruminal ammonia concentrations were half saturated at 0.2mM versus the original estimate of 1.2mM. Residuals analyses indicated that additional progress could be made in predicting microbial N outflow, volatile fatty acid production and concentrations, and cycling of N between blood and the rumen. These additional corrections should lead to an even more robust representation of the effects of dietary nutrients on ruminal metabolism and nutrient absorption, of animal performance, and the environmental impact of dairy production.

Effects of monensin and increasing crude protein in early lactation on performance of dairy cows

Twenty-four Holstein dairy cows were used to evaluate the singular and combined effects of different level of crude protein and monensin treatments during the early lactation on digestion and milk yield of dairy cows. The experiment was designed as completely randomized with a 3x2 factorial arrangement of treatments. The factors were three Concentrations of CP supplement (19.5, 21.4 and 23.4% of dry matter) and two levels of monensin (0 and 350 mg per cow per day). This experiment consist of three periods and each period was 3 week in length. Monensin did not affect DMI, milk yield, lactose and SNF but it reduced milk fat and protein percentage. Monensin premix significantly decreased rumen ammonia but rumen pH and microbial protein synthesis was not affected by monensin treatment. Although, Monensin treatment increased apparent digestibility of DM, NDF, ADF, CP, but they were not significantly. Increasing dietary CP, improved milk and protein production, but did not alter the other components of milk. Digestibility of NDF, ADF, CP were improved by increasing dietary CP. Increasing diet CP from 19.5 to 21.4% did not significantly increase ruminal ammonia, but increasing to 23.4% have significant effect on it. There was a linear relationship between level of crud protein in the diet and urine volume excretion. Microbial protein synthesis was affected by increasing CP level; on this way maximum protein synthesis was achieved in 21.4% CP.

Effect of Dietary Crude Protein Concentration on Ruminal Nitrogen Metabolism in Lactating Dairy Cows

Ten lactating Holstein cows fitted with ruminal cannulas that were part of a larger feeding trial were blocked by days in milk into 2 groups and then randomly assigned to 1 of 2 incomplete 5 x 5 Latin squares. Diets contained [dry matter (DM) basis] 25% alfalfa silage, 25% corn silage, and 50% concentrate. Rolled high-moisture shelled corn was replaced with solvent-extracted soybean meal to increase crude protein (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 data and sample collection performed during the last 8 d. Digesta samples were collected from the omasum to quantify the ruminal outflow of different N fractions. Intake of DM was not affected but showed a quadratic trend with maxima of 23.9 kg/d at 16.5% CP. Ruminal outflow of total bacterial nonammonia N (NAN) was not different among diets but a significant linear effect of dietary CP was detected for this variable. Bacterial efficiency (g of total bacterial NAN flow/kg of organic matter truly digested in the rumen) and omasal flows of dietary NAN and total NAN also showed positive linear responses to dietary CP. Total NAN flow increased from 574 g/d at 13.5% CP to 688 g/d at 16.5% CP but did not increase further with the feeding of more CP. Under the conditions of this study, 16.5% of dietary CP appeared to be sufficient for maximal ruminal outflow of total bacterial NAN and total NAN.

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 x 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.

Estimation of the stoichiometry of volatile fatty acid production in the rumen of lactating cows

The purpose of this study was to improve the prediction of the quantity and type of Volatile Fatty Acids (VFA) produced from fermented substrate in the rumen of lactating cows. A model was formulated that describes the conversion of substrate (soluble carbohydrates, starch, hemi-cellulose, cellulose, and protein) into VFA (acetate, propionate, butyrate, and other VFA). Inputs to the model were observed rates of true rumen digestion of substrates, whereas outputs were observed molar proportions of VFA in rumen fluid. A literature survey generated data of 182 diets (96 roughage and 86 concentrate diets). Coefficient values that define the conversion of a specific substrate into VFA were estimated meta-analytically by regression of the model against observed VFA molar proportions using non-linear regression techniques. Coefficient estimates significantly differed for acetate and propionate production in particular, between different types of substrate and between roughage and concentrate diets. Deviations of fitted from observed VFA molar proportions could be attributed to random error for 100%. In addition to regression against observed data, simulation studies were performed to investigate the potential of the estimation method. Fitted coefficient estimates from simulated data sets appeared accurate, as well as fitted rates of VFA production, although the model accounted for only a small fraction (maximally 45%) of the variation in VFA molar proportions. The simulation results showed that the latter result was merely a consequence of the statistical analysis chosen and should not be interpreted as an indication of inaccuracy of coefficient estimates. Deviations between fitted and observed values corresponded to those obtained in simulations.

Varying Protein and Starch in the Diet of Dairy Cows. I. Effects on Ruminal Fermentation and Intestinal Supply of Nutrients

The main objective of this experiment was to examine the effects of the percentage and source of crude protein (CP) and the amount of starch in the diet of dairy cows on ruminal fermentation, nutrient passage to the small intestine, and nutrient digestibility. For this purpose, 6 multiparous Holstein cows fistulated in the rumen and duodenum that averaged 73 d in milk were used in a 6 x 6 Latin square design with a 2 x 3 factorial arrangement of treatments. Two sources of CP [solvent-extracted soybean meal (SBM) and a mixture of SBM and a blend of animal-marine protein supplements plus ruminally protected Met (AMB)] and 3 levels of dietary protein (about 14, 16, and 18%) were combined into 6 treatments. On a dry matter (DM) basis, diets contained 25% corn silage, 20% alfalfa silage, 10% cottonseed, 26.7 to 37% corn grain, and 4 to 13.5% protein supplement. Intakes and digestibilities in the rumen and total tract of DM, organic matter, acid and neutral detergent fiber were unaffected by treatments. Increasing dietary CP from 14 to 18% decreased the intake and apparent ruminal and total tract digestion of starch, but increased the proportion of starch consumed by the cows that was apparently digested in the small intestine. At 14% CP, starch intake and total tract digestion were higher for the AMB diet than for the SBM diet, but the opposite occurred at 16% CP. Across CP sources, increasing CP in the diet from 14 to 18% increased the intakes of N and amino acids (AA), and ruminal outflows of nonammonia N, nonammonia nonmicrobial N, each individual AA except Met, total essential AA, and total AA. Across CP percentages, replacing a portion of SBM with AMB increased the intake of Met and Val and decreased the concentration of ammonia N in the rumen, but did not affect the intake of other essential AA or the intestinal supply of any essential AA and starch. The ruminal outflow of microbial N, the proportional contribution of Lys and Met to total AA delivered to the duodenum, and milk yield were unaffected by treatments. Data suggest that the intake of N by high-producing dairy cows that consume sufficient energy and other nutrients to meet their requirements can be decreased to about 600 to 650 g daily without compromising the supply of metabolizable protein if the source and amount of dietary CP and carbohydrate are properly matched.

Impacts of the source and amount of crude protein on the intestinal supply of nitrogen fractions and performance of dairy cows

The objective of this article was to review and summarize the significance of the amount and source of dietary crude protein supplements on the supply of nitrogen fractions passing to the small intestine and the performance of lactating dairy cows. A meta-analysis was used to evaluate 2 data sets, one for nitrogen flow to the small intestine and one for performance of cows. The response of dairy cows to rumen-undegradable protein supplements is variable. A portion of this variable response from research trials is explained by the source of crude protein in the control diet, the proportion and source of rumen-undegradable protein in the experimental diet, the effect of rumen-undegradable protein on microbial protein outflow from the rumen, the degradability and amino acid content of the rumen-undegradable protein, and the crude protein percentage of the diet. Compared with soybean meal, the mean milk production responses to feeding rumen-undegradable protein supplements ranged from -2.5 to +2.75%. Because of the large variation and small magnitude of response when rumen-undegradable protein supplements are fed compared with soybean meal, efficiency of nitrogen utilization and the cost to benefit ratio for these crude protein supplements may determine the source and amount of crude protein to feed to dairy cows in the future.

Development of a System to Predict Feed Protein Flow to the Small Intestine of Cattle

A data set constructed from research trials published between 1979 and 1998 was used to derive equations to adjust published tabular values for the rumen-undegradable protein (RUP) content of feeds to better predict the passage of nonammonia nonmicrobial N (NANMN) to the small intestine of lactating dairy cows. Both linear and nonlinear forms of equations were considered for making adjustments. Iterative processes were used to estimate equation parameters. A logistic equation was developed and considered to be the most optimal for adjustment of published tabular RUP contents of feeds. The equation is a function of dietary dry matter intake (DMI) and includes terms for tabular RUP and nonprotein N contents of individual feeds. The equation has a standard error of prediction of 69.29 g of NANMN/ d per cow and a root mean square prediction error of 104.63 g of NANMN/d per cow. Independent evaluation of the equation indicated that the concept of variable RUP content for feeds based on DMI is correct. Further refinements may be needed as other data become available to quantify the effects of additional factors on the RUP value of feeds.

Effects of inclusion of a blended protein product in 35 dairy herds in five regions of the country

We assessed the change in milk production in 35 dairy herds located in eight states when a blended animal-marine protein product was included in diets at 2 (22 herds) to 4% (13 herds) of dry matter. Average 305-d production across herds was 8844.1 kg (SEM = 153.7 kg) of milk, with a range of 6876.9 to 11,293.2 kg. The mean days in milk for all herds at initiation of the trial was 118.8 d (SEM = 3.7, herd mean range of 68.8 to 160.0 d), and the average daily milk production was 32.6 kg (SEM = 0.6 kg, range 24.2 to 39.6 kg). The animal-marine protein blend was included in herd rations for 30 d, beginning immediately after a DHI herd test month and ending immediately after the next DHI sample test. Cow milk records were collected for 1 to 2 mo before the protein blend was included and for 2 to 3 mo after the protein was removed. Sample days were assigned a dummy variable to indicate months off or on the animal-marine protein blend. A total of 33,190 milk records from 7135 cows were analyzed. The numbers within herd ranged from 35 to 2012 cows. Of the 35 herds, 19 were classed as having increased milk yield, 12 herds as having no change, and 4 herds as having decreased milk yield when the animal-marine protein blend was included in the diet. The population mean for change in milk yield with the inclusion of the animal-marine protein blend was 1.24 kg/d of milk (SEM = 0.05 kg). There was no significant effect of parity on mean response. Milk protein content was not influenced by animal-marine protein blend inclusion. Fat content was lower for the month on which the animal-marine protein blend was fed (3.51%) compared with the month prior (3.63%) and the month after (3.70%), respectively (SEM = 0.032). Stage of lactation influenced the method for calculating the production response and the actual response to the animal-marine protein blend.

Estimation of microbial nitrogen flow to the duodenum of cattle based on dry matter intake and diet composition

The objectives of this study were: 1) to evaluate the National Research Council equation used to predict microbial N flow to the duodenum in lactating cows, and 2) to determine whether improved equations could be developed by using dietary parameters used in the field. Treatment means from 55 trials with lactating and nonlactating cattle with duodenal cannulas were subjected to the backward elimination procedure of multiple regression. Variation within and among trials was accounted for by weighting the observations and including trial effects in all models. The equations to predict microbial N flow based on net energy for lactation (NEL) intake were different from the equation based on NEL intake used by the dairy National Research Council. Dry matter intake (DMI) estimated microbial N flow as well as did NEL intake, indicating that DMI drives predictions based on NEL intake. When multiple dietary factors [i.e., DMI; dietary percentages of crude protein, forage, and neutral detergent fiber; and all two-way interactions] were included, the resulting equation [microbial N (grams per day) = 16.1 + 22.9 x DMI (kilograms per day) - 0.365 x DMI2 - 1.74 x dietary neutral detergent fiber (percentage of dry matter)] tended to fit the data better than the equations based on NEL intake but not better than the equation based on DMI alone. The multiple-factor equation appeared to be the best overall equation for prediction; in contrast to the equation based on DMI, this equation is sensitive to diet composition. An asymptotic multiple-factor equation was developed, which may be more appropriate when extrapolating beyond the data range.

An evaluation of postabsorptive protein and amino acid metabolism in the lactating dairy cow

The current protein system utilized in the US was formulated in 1985 with minor modifications in 1989 and has gained widespread acceptance. However, some of the assumptions that were adopted by the National Research Council (NRC) appear to be inconsistent with observational data. The marginal efficiency of conversion of absorbed protein to milk protein was assumed by NRC to be 70% until the requirement for absorbed protein was met and was 0% thereafter. The mean marginal efficiency observed for abomasal casein infusions reported in the literature and collected at the Purina Mills Research Center was 21%. Sorting the data into protein-sufficient and protein-deficient classes did not support the assumptions of 70% marginal efficiency in a deficient state and 0% marginal efficiency in the sufficient state. Analyses of nitrogen balance data and abomasal flow data and the work of Van Straalen et al. (77) indicated that energy status of the animal plays a role in determining the response to absorbed protein. Such a consideration was not included in the NRC model. The adoption of equations that describe metabolism at the organ level as opposed to the animal level would allow direct use of organ level data for parameterization and may provide better predictions. Simple representations of digestion and absorption, splanchnic metabolism, and mammary metabolism of amino acids or protein in aggregate are described. These representations could be used to improve the current system and could serve as a bridge to adoption of more complex models.

Modeling ruminal digestibility of carbohydrates and microbial protein flow to the duodenum

Carbohydrates are the major source of energy for dairy cows and for microbial protein synthesis in the rumen. The prediction of ruminal carbohydrate digestibility and of the flow of microbial protein to the small intestine is difficult because of the variability among various feeds in the kinetics of digestion and passage of neutral detergent fiber and starch. Disappearance of fiber and starch in vitro or in situ and gas production in vitro have been extensively evaluated, improved, and reviewed. Similarly, markers and models to measure ruminal passage rate have been extensively researched and improved. Sources of variation and decreased accuracy for these techniques are discussed. Variation and potential errors also remain for the prediction of microbial protein flow to the duodenum using in vivo procedures. However, when in vivo results were accumulated into a database, microbial N flow to the duodenum over a wide range of conditions could be predicted accurately by intake of net energy for lactation or by dry matter intake and percentage of neutral detergent fiber in the diet. Although evaluation of feeding interactions and specific dietary limitations for microbial protein production in the rumen are possible with some models but not with this regression approach, mechanistic models need further validation and more accurate rate constants for improved accuracy over a wide range of conditions.

Effects of rumen-undegradable protein on dairy cow performance: a 12-year literature review

In order to integrate and analyze knowledge on the use of protein supplements and protein nutrition of lactating dairy cows, we compiled a review of 108 studies published throughout the world, but principally in the Journal of Dairy Science between 1985 and 1997. In 29 comparisons from 15 metabolism trials, soybean meal was replaced by high amounts of rumen undegradable protein (RUP) as a supplement; the benefits were not consistently observed for flow to the duodenum, essential amino acids, or lysine and methionine. High RUP diets resulted in decreased microbial protein synthesis in 76% of the comparisons. However, fish meal provided a good balance of lysine and methionine when calculated as a percentage of total essential amino acids. In 127 comparisons from 88 lactation trials that were published from 1985 to 1997, researchers studied the effects of replacing soybean meal with high RUP sources, such as heated and chemically treated soybean meal, corn gluten meal, distillers grains, brewers grains, blood meal, meat and bone meal, feather meal, or blends of these sources; milk yield was significantly higher in only 17% of the comparisons. Fish meal and treated soybean meal accounted for most of the positive effects on milk yield from RUP; corn gluten meal resulted in mostly negative results. The percentage of fat in milk was depressed more by fish meal than by other RUP sources. Protein percentage was decreased in 28 comparisons and increased in only 6 comparisons, probably reflecting the decrease in microbial protein synthesis, as was observed for diets high in RUP. The data strongly suggest that increased RUP per se in dairy cow diets, which often results in a decrease in RDP and a change in absorbed AA profiles, does not consistently improve lactational performance.

Effects of feeding nonforage fiber sources on site of fiber digestion

Although many nonforage fiber sources have high extents of neutral detergent fiber (NDF) digestion, most have rates of digestion similar to or slower than the rates of forage NDF digestion. Rates of NDF digestion vary considerably among and within sources of by-products. Digestion kinetics also vary because of the technique used (in vitro versus in situ) and because of high amounts of dietary concentrate. Based on available data for passage rate and specific gravity measurements, rates of passage of nonforage fiber sources from the rumen of high producing cows appear to be faster than those of forages. Therefore, the potential to shift NDF digestion to the hindgut has been discussed. To account for variability in ruminal and total tract digestibility of NDF, multiple regression analysis was used to indicate that nonforage NDF percentage in the diet had about two-thirds the positive response on total tract NDF digestion that forage NDF percentage did. Although the loss of potentially digestible NDF may occur, DMI does not appear to decrease much until forage NDF is below 14 to 16% of dietary DM. Conversely, replacement of starch with nonforage NDF appears to increase digestibility of fiber, mostly in diets with high concentrations of nonfiber carbohydrates, apparently because of reduced negative associative effects. Increasing the concentration of total NDF above 35% also can decrease DMI with little improvement in NDF digestibility. Increased knowledge of the kinetics of digestion and the passage of various nonforage fiber sources used to replace forage or concentrate should increase the accuracy and precision of dynamic models, thereby increasing the flexibility and utility of nonforage fiber sources in dairy rations.

Relationship between fermentation acid production in the rumen and the requirement for physically effective fiber

The content of ruminally fermented OM in the diet affects the fiber requirement of dairy cattle. Physically effective fiber is the fraction of feed that stimulates chewing activity. Chewing, in turn, stimulates saliva secretion. Bicarbonate and phosphate buffers in saliva neutralize acids produced by fermentation of OM in the rumen. The balance between the production of fermentation acid and buffer secretion is a major determinant of ruminal pH. Low ruminal pH may decrease DMI, fiber digestibility, and microbial yield and thus decrease milk production and increase feed costs. Diets should be formulated to maintain adequate mean ruminal pH, and variation in ruminal pH should be minimized by feeding management. The fraction of OM that is fermented in the rumen varies greatly among diets. This variation affects the amount of fermentation acids produced and directly affects the amount of physically effective fiber that is required to maintain adequate ruminal pH. Acid production in the rumen is due primarily to fermentation of carbohydrates, which represent over 65% of the DM in diets of dairy cows and have the most variable ruminal degradation across diets. The non-fiber carbohydrate content of the diet is often used as a proxy for ruminal fermentability, but this measure is inadequate. Ruminal fermentation of both nonfiber carbohydrate and fiber is extremely variable, and this variability is not related to the nonfiber carbohydrate content of the diet. The interaction of ruminally fermented carbohydrate and physically effective fiber must be considered when diets for dairy cattle are evaluated and formulated.

Effects of amount and ruminal degradability of protein on nutrient digestibility and production by cows fed tallow

Five cows with ruminal cannulas were used in a 5 x 5 Latin square design to determine the effects of fat and amount and ruminal degradability of dietary crude protein (CP) on nutrient digestibility and production of milk and milk components. Treatments were 1) control; 2) 15% CP, soybean meal; 3) 15% CP, by-product proteins; 4) 18% CP, soybean meal; and 5) 18% CP, soybean meal and by-product proteins. Diets 2 through 5 contained 3.5% tallow. Diets consisted of 28% alfalfa haylage, 22% corn silage, and 50% concentrate on a dry matter (DM) basis. Fat did not affect dry matter intake or percentages and yields of fat and CP in milk but increased milk yield 2.5 kg/d. Fat did not affect N fractions in milk but decreased the percentages of short- and medium-chain fatty acids (C6:0 to C16:0) and increased the percentages of long-chain fatty acids (C18:0 and C18:1) in milk fat. Fat did not affect ruminal fermentation characteristics or the percentages of dietary DM, organic matter, CP, acid detergent fiber, neutral detergent fiber, starch, ether extract, and energy that were digested. An increase in dietary CP from 15 to 18% increased dry matter intake 1.7 kg/d; increased intake of gross energy 8 Mcal/d; increased the percentages and quantities of DM, organic matter, CP, and energy digested; increased the quantities of acid detergent fiber and neutral detergent fiber digested; decreased ruminal pH; increased concentrations of total volatile fatty acids; and increased NH3 N in ruminal fluid. However, the difference in dietary CP did not affect milk yield or composition. Replacement of soybean meal in the diet with a mixture of by-product proteins decreased NH3 N in ruminal fluid, tended to decrease concentrations of total volatile fatty acids and increase pH of ruminal fluid, but did not affect milk yield or percentages and yields of milk components.

Effects of dietary fat with or without nicotinic acid on nutrient flow to the duodenum of dairy cows

Four Holstein cows, fitted with ruminal and duodenal cannulas, were utilized in a 4 x 4 Latin square design to investigate the effects of supplementing nicotinic acid to diets that contained 35% alfalfa haylage, 15% corn silage, and either 50% of a low fat concentrate or 10% whole raw soybeans and 40% of a high fat concentrate containing tallow. Treatments in a 2 x 2 arrangement were 1) low fat, no supplemental nicotinic acid; 2) low fat, 12 g/d of nicotinic acid; 3) high fat, no supplemental nicotinic acid; and 4) high fat, 12 g/d of nicotinic acid. The DMI and OM apparently and truly digested in the rumen and apparently digested postruminally were not different among treatments. Addition of fat to the diet decreased the concentration of total VFA in ruminal fluid but did not alter the molar proportions of any of the VFA; supplementation of nicotinic acid tended to decrease the molar proportion of acetate. Amounts of NAN, microbial N, nonammonia nonmicrobial N, and AA that flowed to the duodenum were similar among diets. The concentration of urea N in plasma decreased, and concentrations of cholesterol and triglycerides increased, when cows were fed supplemental fat. Milk composition and production of milk, 4% FCM, and milk components were not altered by addition of fat or nicotinic acid to the diet. Supplementation of fat or nicotinic acid to diets of dairy cows was not beneficial in this experiment.

Evaluation of chemical and physical properties of feeds that affect protein metabolism in the rumen

The goal of the NC-185 Cooperative Regional Research Project is to provide the information needed to improve the nutrition and feeding of dairy cattle, a major factor determining composition of milk and cost of milk yield. Emphasis is placed on understanding how energy and protein nutrition of lactating cows can be manipulated to increase the quantity and improve the profile of AA passing to the small intestine and to improve yield of milk and milk protein. To achieve this goal, one of the major objectives of this project has been to evaluate quantitatively the chemical and physical properties of protein and energy sources that determine AA availability to lactating cows. Reliable measurements of microbial protein synthesis and protein degradation in the rumen are critical in the evaluation process. Therefore, one of the ongoing areas of investigation of this research project has been to determine the most appropriate methods for estimating microbial protein synthesis and dietary protein degradation in the rumen. Other areas have been investigated, using continuous culture fermenters and ruminally and duodenally cannulated cows, including factors that alter microbial metabolism of N in the rumen and subsequently protein supply to the small intestine, such as sources of carbohydrate, protein, and fat and interrelationships of protein and carbohydrate. Findings of the NC-185 Cooperative Regional Research Project Committee and other investigators are summarized in this review.

Influence of amount and degradability of protein on production of milk and milk components by lactating Holstein cows

Forty-one multiparous cows were utilized in a completely randomized design with a 2 x 2 factorial arrangement of treatments to evaluate the amount and degradability of dietary CP on production of milk and milk components. The TMR contained 25% alfalfa haylage, 25% corn silage, and 50% concentrate to provide either 16.4 or 19.4% CP with a calculated ruminal degradability of 55 or 70%. Intakes of DM, ADF, and NDF and BW were not different among treatments. Production of milk, 4% FCM, fat, CP, and SNF was not affected by amount or degradability of dietary CP. Milk CP percentage was not affected significantly by amount or degradability of CP. Milk fat percentage was increased by the diet that was high in ruminally undegradable protein (2.90 and 3.12; low and high ruminally undegradable protein, respectively). These data suggest that synthesis of milk and milk components was not limited by a shortage of AA or that the different dietary sources of supplemental CP did not alter AA availability.