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R A Kohn

Publications and source records attributed to R A Kohn.

27 records · Page 2Linked to original sources
PubMed1999

Milk urea nitrogen target concentrations for lactating dairy cows fed according to National Research Council recommendations.

The objectives of this study were to develop and evaluate a mathematical model to predict milk urea N and to use this model to establish target concentrations. A mechanistic model to predict milk urea N was developed using raw data from 3 studies (10 diets, 40 cows, and 70 observations) and was evaluated with 18 independent studies (89 treatment means). For the independent literature data set, the model prediction error was approximately 35%; the majority of the error was due to variation among experiments. A mean of at least 25 cows was determined to be necessary for reliable model predictions. This model, which uses such data as protein intake and milk production, was used to predict milk urea N concentrations when cattle are fed according to National Research Council recommendations. Target values calculated in this manner for a typical lactation were 10 to 16 mg/dl, depending on days in milk. Target concentrations were sensitive to changes in milk production and amount of N intake and were relatively insensitive to body weight, parity, and grouping strategy. Analysis of data from the Lancaster Dairy Herd Improvement Association (n = 133,057) indicated that cows in the region were being fed diets containing approximately 17% crude protein, regardless of parity. A comparison to target milk urea N concentrations for this data indicated that cows were being fed 8 to 16% more protein than recommended by the National Research Council. Target milk urea N concentrations have been established, and dairy farmers now have a definitive way to interpret milk urea N concentrations.

Animal Nutritional Physiological Phenomena↗
PubMed1998

Calculation of the buffering capacity of bicarbonate in the rumen and in vitro.

We describe a model to calculate the buffering capacity of bicarbonate in the rumen. The addition of NaHCO3 results in the release of CO2 from solution and eventually from the rumen via eructation. This process directly neutralizes ruminal acidity. The degree to which the process continues depends on the partial pressure of CO2 in the gas phase, the pH, and a constant (7.74), according to the Henderson-Hasselbalch equation: pH = 7.74 + log([HCO3-]/pressure of CO2 in atmospheres). The addition of NaHCO3 to buffer solutions and ruminal fluid under high pressure of CO2 increased pH as predicted. The buffering capacity of ruminal fluid under CO2 was greater at low pH than was previously determined by titration in air. In contrast, in vitro systems in which CO2 is not permitted to escape may result in reduced buffering capacity. In vitro systems in which excess CO2 may escape (under N2 gas pressure) may result in uncontrolled pH elevation. Dilution of ruminal fluid under constant pressure of CO2 decreased ruminal pH as predicted by the model. The pH under different pressures at equilibrium and the buffering capacity are easily calculated for in vitro and in vivo systems.

Animals↗
PubMed1998

Using milk urea nitrogen to predict nitrogen excretion and utilization efficiency in lactating dairy cows.

Because animal agriculture has been identified as a major source of nonpoint N pollution, ways to reduce the excretion of N by production animals must be examined. The objective of this research was to develop and evaluate a mathematical model that integrates milk urea N to predict excretion, intake, and utilization efficiency of N in lactating dairy cows. Three separate digestibility and N balance studies (10 diets, 40 cows, and 70 observations) were used to develop the model, and 19 independent studies (93 diets) were used for evaluation. The driving variables for the model were milk urea N (milligrams per deciliter), milk production (kilograms per day), milk protein (percentage), and dietary crude protein (percentage). For the developmental data set, the model accurately predicted N excretion and efficiency with no significant mean or linear bias for most predictions. Residual analysis revealed that a majority of the unexplained model error was associated with variation among cows. For the independent data set, model prediction error was approximately 15% of mean predictions. A mean of at least 10 cows was determined to be appropriate for model predictions. Target milk urea N concentrations were determined from expected urinary N excretion for cows that were fed according to National Research Council recommendations. Target values calculated in this manner were 10 to 16 mg/dl, depending on milk production. Milk urea N is a simple and noninvasive measurement that can be used to monitor N excretion from lactating dairy cows.

Animals↗
PubMed1998

Evaluation of models for balancing the protein requirements of dairy cows.

Most diets for dairy cattle in the US are formulated using the mathematical model developed by the National Research Council (NRC). This model is simpler than more mechanistic models and is largely empirical. Based on the research reviewed in this paper, the simpler empirical approach is recommended for routine diet formulation at the present time. Under typical conditions using feed tables for most feed descriptions, the NRC model was more accurate than the Cornell Net Carbohydrate and Protein System, and, in its present form, the model developed by Baldwin et al. (4) is too difficult to run routinely in the field. However, the more mechanistic approaches are recommended to investigate diet and animal interactions under nonstandard environmental conditions, animals, or feeds. Because the NRC model does not address many of the potential limitations of some diets or management conditions, more complex models are needed to identify why some herds appear to underperform. Mechanistic models can be used to study or explain nutritional or physiological concepts and to develop and test research hypotheses. Ultimately, it is the fundamental research in nutrition modeling that enables advances in routine diet formulation procedures. Nonetheless, models used routinely to balance diets need to be as simple and as accurate as possible.

Animal Nutritional Physiological Phenomena↗
PubMed1995

Effect of plant maturity and preservation method on in vitro protein degradation of forages.

The influence of maturity and method of conservation on protein degradation was determined for four different forage species. Alfalfa, smooth bromegrass, and reed canarygrass were harvested at three maturities, and whole plant corn was harvested at two maturities. Samples of each forage were freeze-dried or wilted and then ensiled in mini silos at two DM contents. Additional samples of all forages except corn were field-dried to hay. Ground sample was incubated for 0, 2, and 24 h with crude enzyme extract from ruminal contents. Degraded protein as a percentage of total CP was determined as the amount of protein that was soluble in TCA (80 g/L) after degradation. Increased maturity resulted in lower protein degradation for alfalfa, bromegrass, and canarygrass. For example, the most mature alfalfa or bromegrass, respectively, had 77 or 63% as much N that was soluble in TCA after 2 h of incubation with ruminal enzyme than the least mature forage of the same species. Although protein degradation was higher for ensiled than for dried forage, silage DM content had no consistent effect. Freeze-dried material generally had less degraded protein than hay, but protein degradation of bromegrass at 24 h was lower for hay than for freeze-dried samples. Protein degradation of forages was highly variable and depended on plant maturity and conservation method.

Animal Feed↗
PubMed1995

Prediction of protein degradation of forages from solubility fractions.

Two experiments were conducted to determine the relationship between enzymatic digestion of forage protein and fractionation based on solubility. The first experiment used 42 forages, each replicated three times, including different species, stages of maturity, and methods of conservation. Crude protein was fractionated into six parts for each forage by sequential extraction in TCA, bicarbonate-phosphate buffer, acetone, detergent at pH 7, and detergent with acid. Multiple regression analysis, with all the solubility fractions as independent variables, resulted in prediction of CP degradation by ruminal enzyme extract at 2 and 24 h; R2 were .88 and .81, respectively. Greater solubility in the buffer and the detergent at pH 7 was associated with higher protein degradation; solubility in acetone, detergent with acid, and insolubility were associated with lower degradation. In the second experiment, eight forages each replicated twice were digested with ruminal enzyme for 0, 2, 6, and 24 h and then were extracted as described. Solubility in TCA and acetone increased during degradation, but solubility in buffer, detergent with acid, and insolubility decreased. For both experiments, buffer-soluble CP was the only uniform fraction across forages; other fractions contained proteins that degraded at diverse rates. Solubility of CP is related to degradation properties, but further research is needed to improve the accuracy of predictions based on solubility.

Animal Feed↗
PubMed1993

Effect of intestinal blood flow on absorptive site blood flow and lysine absorption as examined in situ in Holstein calves.

Total blood flow from an intestinal segment (TBF) was altered to determine effects on blood flow at the absorptive site (ASBF) and lysine absorption. Venous blood flow was restricted using a peristaltic pump to 20, 35, 50, 65, and 80% of the initial unrestricted rate. Lysine absorption and ASBF were determined from recovery of 14C and 3H in blood from intestinally perfused [14C]L-lysine and 3H2O, respectively. Fluid flux in the intestinal lumen was estimated from the difference in polyethylene glycol concentrations in luminal infusate and effluent. Restriction of TBF proportionally reduced ASBF, which composed 3 to 6% of TBF. Lysine absorption was reduced linearly during reduction of TBF. Fluid absorption varied among calves but was independent of TBF. Differences between loss of radioactive marker from perfusate and recovery in blood suggested a loss of 3H2O from the intestinal segment that was independent of TBF. Changes in blood flow to the small intestine may affect nutrient absorption in ruminants.

Animals↗