NRC Model Research Articles

Metabolic Models of Ruminant Metabolism: Recent Improvements and Current Status

The NC-1009 regional research project has two broad goals of quantifying the properties of feeds and the metabolic interactions among nutrients that influence nutrient availability for milk production and that alter synthesis of milk, and using those quantitative relationships to challenge and refine computer-based nutrition systems for dairy cattle. The objective of this paper was to review progress in modeling. Significant progress has been made in model refinements over the past 10 yr as exemplified by the most recent NRC model (2001) and work on the Molly model of Baldwin and colleagues (1987). These models have different objectives but share many properties. The level of aggregation of the NRC model (2001) does not allow detailed analyses of specific metabolic reactions that affect nutritional efficiency. The Baldwin model is aggregated at the pathway level and is therefore amenable to assessment with a broad range of biological measurements. Recent improvements to that model include the addition of an ingredient based input scheme, use of in situ data to set ruminal protein degradation rates, and refinement of the representation of mammary cell numbers and activity. Although the Baldwin model appears to be appropriate structurally, several parameters are known to be inadequate. Predictions of ruminal N metabolism and total-tract starch digestions have similar accuracy as the NRC model. However, the NRC more accurately predicts total-tract fiber digestion and both models significantly overpredict total-tract lipid digestion. These errors contribute to overpredictions of weight retention when simulating full lactations with the Baldwin model and may result in performance prediction errors with the NRC model. Limitations remain in the descriptions of metabolism and metabolic regulation of the splanchnic, viscera, adipose tissue, body muscle, and mammary tissue. Integration of genetic control mechanisms can expand these efforts to assist genetic selection as well as feeding management decisions.

Integration of Ruminal Metabolism in Dairy Cattle1,2

An important objective is to identify nutrients or dietary factors that are most critical for advancing our knowledge of, and improving our ability to predict, milk protein production. The Dairy NRC (2001) model is sensitive to prediction of microbial protein synthesis, which is among the most important component of models integrating requirement and corresponding supply of metabolizable protein or amino acids. There are a variety of important considerations when assessing appropriate use of microbial marker methodology. Statistical formulas and examples are included to document and explain limitations in using a calibration equation from a source publication to predict duodenal flow of purine bases from measured urinary purine derivatives in a future study, and an improved approach was derived. Sources of specific carbohydrate rumen-degraded protein components probably explain microbial interactions and differences among studies. Changes in microbial populations might explain the variation in ruminal outflow of biohydrogenation intermediates that modify milk fat secretion. Finally, microbial protein synthesis can be better integrated with the production of volatile fatty acids, which do not necessarily reflect volatile fatty acid molar proportions in the rumen. The gut and splanchnic tissues metabolize varying amounts of volatile fatty acids, and propionate has important hormonal responses influencing milk protein percentage. Integration of ruminal metabolism with that in the mammary and peripheral tissues can be improved to increase the efficiency of conversion of dietary nutrients into milk components for more efficient milk production with decreased environmental impact.

Development and Evaluation of Models to Predict the Feed Intake of Dairy Cows in Early Lactation

Inaccurate prediction of dry matter intake (DMI) limits the ability of current models to anticipate the technical and economic consequences of adopting different strategies for production management on individual dairy farms. The objective of the present study was to develop an accurate, robust, and broadly applicable prediction model and to compare it with the current NRC model for dairy cows in early lactation. Among various functions, an exponential model was selected for its best fit to DMI data of dairy cows in early lactation. Daily DMI data (n=8,547) for 3 groups of Holstein cows (at Illinois, New Hampshire, and Pennsylvania) were used in this study. Cows at Illinois and New Hampshire were fed totally mixed diets for the first 70 d of lactation. At Pennsylvania, data were for the first 63 d postpartum. Data from Illinois cows were used as the developmental dataset, and the other 2 datasets were used for model evaluation and validation. Data for BW, milk yield, and milk composition were only available for Illinois and New Hampshire cows; therefore, only these 2 datasets were used for model comparisons. The exponential model, fitted to the individual cow daily DMI data, explained an average of 74% of the total variation in daily DMI for Illinois data, 49% of the variation for New Hampshire data, 67% of the variation for Pennsylvania data, and 64% of the variation overall. Based on all model selection criteria used in this study, the exponential model for prediction of weekly DMI of individual cows was superior to the current NRC equation. The exponential model explained 85% of the variation in weekly mean DMI compared with 42% for the NRC equation. Compared with the relative prediction error of 6% for the exponential model, that associated with prediction using the NRC equation was 14%. The overall mean square prediction error value for individual cows was 5-fold higher for the NRC equation than for the exponential model (10.4 vs. 2.0kg2/d2). The consistently accurate and robust prediction of DMI by the exponential model for all data-sets suggested that it could safely be used for predicting DMI in many circumstances.

Modeling nutrient supply to dairy cattle from a feedstuff using NRC-2001 (a TDN-based model) with inputs based on in situ and mobile bag technique measurements

The objectives of this study were to use the NRC-2001 model with inputs based on in situ and mobile bag techniques to (1) predict the potential nutrient supply to dairy cows using an exampled feed- whole lupin seeds that were systematically toasted and (2) quantitatively determine the effects of toasting (which shifted degradation of protein from the rumen to the abomasum and small intestine without changing intestinal digestion) and optimal toasting conditions by the NRC-2001 model. The quantitative predictions were made in terms of: (1) rumen undegraded and degraded feed protein, (2) truly absorbed undegraded protein, (3) potential microbial protein synthesized in the rumen from rumen available protein or (4) from total digestible nutrients (TDN), (5) truly absorbed rumen synthesized microbial protein, (6) truly absorbed rumen endogenous protein, (7) total metabolizable protein (MP), as well as (8) the protein degradation balance (PDB). The results show that using the NRC-2001 model with inputs based on in situ and mobile bag techniques, the predicted PDB and MP supply to dairy cattle was significantly improved. However, no optimal condition could be obtained from this study due to high PDB values (>48 g kg-1 DM) in all the treatments, predicted by the model. With toasting, temperature and/or duration could go still higher than 136°C and/or longer than 15 min to further prevent potential N loss in the rumen if total tract digestion is not depressed. More study is needed. However, the results differed from that published with the DVE/OEB system (a non-TDN-based model) although the two models had significant correlations with high R (>0.99) values. Using the NRC model, the overall mean for MP was higher (+10 g kg-1 DM), but the PDB values were lower (-12 g kg-1 DM) in comparison with that predicted by the non-TDN based model for the whole lupin seeds. Key words: Modeling nutrient supply, dairy cattle; National Research Council, in situ, mobile bag technique

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.

Barley- versus protein-supplemented corn-based diets for feedlot cattle evaluated using the NRC and CNCPS beef models

A study was conducted to evaluate the effects of supplementing a corn-based finishing diet [10.0% crude protein (CP), 6.2% degradable intake protein (DIP), dry matter basis, no protein supplementation] with DIP in the form of urea (13.0% CP, 9.1% DIP) or canola meal and urea (12.8% CP, 8.6% DIP) on dry matter intake (DMI) and average daily gain (ADG). Animal performances were compared with those obtained by feeding a barley-based diet (13.9% CP, 9.6% DIP). Crossbred beef steers (398 ± 28 kg) were allocated to 24 pens and six pens were assigned to each diet. There were no differences (P > 0.05) in DMI and ADG between cattle fed the barley diet and those fed the corn diets supplemented to provide a concentration of DIP similar to that supplied by the barley diet. However, ADG was 10% lower and DMI was 8% lower for cattle fed the corn diet with no protein supplementation (P < 0.05). The NRC and CNCPS models predicted the substantially lower gain of cattle fed the corn diet with no protein supplementation, but underestimated the improvement in performance when the corn diet was supplemented with urea or canola meal and urea to supply the requirement for DIP. Key words: Feedlot cattle, protein, corn, barley, growth, models

Effects of Dry Matter Intake Restriction on Growth Performance and Carcass Merit of Finishing Steers

Two studies were conducted to determine the effects of restricting DMI on growth performance and carcass merit of finishing steers. In Trial 1, 84 Angus cross steers (BW = 313 ± 7 kg) were randomly assigned to one of three treatment diets that were designed to achieve a 1.6 kg/d ADG at three intake levels: ad libitum, 90% of ad libitum DMI (LF90), or 80% of ad libitum DMI (LF80). In Trial 2, 84 Angus cross steers (BW = 327 ± 20 kg) were randomly assigned to one of two treatment diets that were designed to achieve an ADG of 1.6 kg/d at two intake levels: ad libitum or LF80. All diets delivered similar total NE, metabolizable protein, and P per day. Daily feed delivery (0800 h) for each treatment was determined based on a rolling 5-d average of DMI by steers fed ad libitum. No steers were treated for digestive disorders in either trial, and liver abscess scores were similar between treatments (Trial 1, P=0.65; Trial 2, P=0.73). Actual average DMI during Trial 1 were 2.45% BW for ad libitum steers, 2.26% BW for LF90 (92% of ad libitum intake), and 1.99% BW for LF80 (81% of ad libitum intake). During Trial 2, actual average DMI was 2.43% BW for ad libitum steers and 1.94% BW for LF80. Steer ADG was greater (P≤0.04) for LF80 than for ad libitum feeding during both trials. Similarly, growth efficiency [gain to feed ratio; G:F] by LF80 steers was greater (P<0.01) than that of ad libitum steers during both trials. During Trial 1, hot carcass weights of LF80 steers were greater (P=0.01) than those for ad libitum steers or for LF90 steers. Marbling scores of ad libitum steers and for LF80 steers were greater (P<0.05) than those for LF90 steers. Yield grade tended to be more favorable (P=0.11) in ad libitum steers and for LF90 steers compared with LF80 steers. Hot carcass weights, marbling scores, and yield grades were similar between treatments in Trial 2. Growth performance was greater than that predicted by NRC models when finishing steers were fed a diet formulated to promote 1.6 kg/d ADG but limited to approximately 80% of ad libitum DMI.

Recent Surveillance Data and a Revised Embrittlement Correlation

Abstract The mechanistically-guided embrittlement correlation model adopted in ASTM E 900-02 was based on U.S. surveillance results current through calendar year 1998. There now exists an extensive amount of new surveillance data that includes a large amount of boiling water reactor (BWR) results from a supplemental surveillance program designed to augment the plant-specific BWR surveillance programs. These recent data allow a statistical check of the ASTM E 900-02 embrittlement correlation, as well as the NRC correlation model currently being used in the pressurized thermal shock (PTS) re-evaluation effort and the older Regulatory Guide 1.99, Revision 2 correlation. Even though the ASTM E 900-02 embrittlement correlation is a simplified version of the NRC model, a comparison of the two embrittlement correlation models utilizing the new database has proven to be revealing. Based on the new BWR data, both models are inadequate in their ability to predict BWR results. Other aspects of the two models are presented, as well as plans to develop a revised embrittlement correlation.

Estimation of the total efficiency of metabolizable energy utilization for maintenance and growth by cattle in tropical conditions

Data of 320 animals were obtained from eight comparative slaughter studies performed under tropical conditions and used to estimate the total efficiency of utilization of the metabolizable energy intake (MEI), which varied from 77 to 419 kcal kg-0.75d-1. The provided data also contained direct measures of the recovered energy (RE), which allowed calculating the heat production (HE) by difference. The RE was regressed on MEI and deviations from linearity were evaluated by using the F-test. The respective estimates of the fasting heat production and the intercept and the slope that composes the relationship between RE and MEI were 73 kcal kg-0.75d-1, 42 kcal kg-0.75d-1 and 0.37. Hence, the total efficiency was estimated by dividing the net energy for maintenance and growth by the metabolizable energy intake. The estimated total efficiency of the ME utilization and analogous estimates based on the beef cattle NRC model were employed in an additional study to evaluate their predictive powers in terms of the mean square deviations for both temperate and tropical conditions. The two approaches presented similar predictive powers but the proposed one had a 22% lower mean squared deviation even with its more simplified structure.

Programas de alimentação para matrizes pesadas após o pico de postura, com base em modelos para predizer a exigência energética

Este trabalho foi conduzido com o objetivo de avaliar o desempenho de matrizes pesadas, submetidas a diferentes programas de alimentação estabelecidos pela aplicação de modelos para predizer as exigências energéticas, após o pico de postura. O experimento foi conduzido no setor de avicultura da UNESP Campus Jaboticabal, com duração de 84 dias (três períodos de 28 dias). Foram utilizadas 740 matrizes de corte Hubbard Hy-Yield e 80 machos Petterson, com 55 semanas de idade. O delineamento foi inteiramente casualizado, com quatro tratamentos e cinco repetições de 37 aves por repetição (box) e um modelo fatorial 4´3 (quatro tratamentos ´ três períodos). Os programas de alimentação avaliados foram: T1 - Fornecimento de ração de acordo com o padrão da linhagem (428 kcal/ave/dia de 55 a 66 semanas de idade); T2 - Redução semanal de energia (2 kcal de EM/ave em cada semana); T3 - Fornecimento de ração de acordo com o modelo de exigência de EM, UNESP (2000); e T4 - Fornecimento de ração de acordo com o modelo, NRC (1994). O programa de alimentação com redução semanal de energia foi adequado para manter os desempenhos produtivo e reprodutivo das aves, indicando a possibilidade de redução de 2 kcal/ave/dia, em cada semana, na alimentação de matrizes pesadas após 55 semanas de idade. Os modelos UNESP e NRC proporcionaram estimativas mais elevadas das exigências energéticas que o modelo padrão, provavelmente em decorrência do ganho de peso das matrizes, que esteve acima do recomendado para a linhagem, promovendo maiores exigências de energia para mantença.

Nutrient supply to dairy cows from processed white lupines

The objective of this study was to compare the DVE/OEB system (DVE = truly absorbed protein in the small intestine; OEB = degraded protein balance (DPB) in Dutch) and the NRC-2001 model in the prediction of supply of protein to dairy cows from processed white lupines (Lupinus albus L.). Comparisons were made in terms of (1) ruminally synthesized microbial CP, (2) truly absorbed protein in the small intestine, and (3) degraded protein balance. In addition, the systematic investigation of roasting of the white lupines at various temperatures (110, 130, or 150°C) and times (15, 30 or 45 min) on manipulation of digestive behaviour and the potential nutrient supply to dairy cows were also carried out, to obtain information on best processing conditions as intestinal protein sources (to achieve target values for potential high net absorbable protein in the small intestine while holding any N loss in the rumen to a low level). The results showed that the predicted values from the DVE/OEB system and the NRC-2001 model had significant correlations with high R ( > 0.83) values. However, using the DVE/OEB system, the overall average microbial protein supply based on available energy was 11% higher and the truly absorbed protein in the small intestine was 7% higher than that predicted by the NRC-2001 model. The difference was also found in the prediction of the degraded protein balances (DPB), which was 8% higher based on data from the NRC-2001 model. These differences are due to considerably different factors used in calculations in the two models, although both are based on similar principles. This indicates that a further refinement is needed for a modern protein evaluation and prediction system. In addition, this study showed that the roasting at higher temperature and time was effective in shifting protein degradation from rumen to intestines and it increased the DVE or MP values without reaching the negative degraded protein balance. The processing at 150°C for 30 or 45 min might be the best treatments for white lupine due to its higher DVE and MP values and the very low DPB values.

Site and extent of digestion and amino acid flow to the small intestine in beef cattle consuming limited amounts of forage.

Eight Angus x Gelbvieh heifers (445 +/- 74.5 kg) fitted with ruminal and duodenal cannulas were used in a 4 x 4 Latin square double double-crossover designed experiment to assess the effect of restricted forage intake on site and extent of digestion and flow of essential AA amino acids to the small intestine. Heifers were fed chopped (2.54 cm) bromegrass hay (9.2% CP, 64% NDF on an OM basis) at one of four percentages of maintenance (30, 60, 90, and 120%). Experimental periods were 21 d in length, with 17 d of adaptation followed by 4 d of intensive sample collection, after which maintenance requirements and subsequent level of intake were adjusted for BW change. True ruminal OM, NDF, and N digestion (g/d) decreased linearly (P < 0.001) with decreasing forage intake. When expressed as a percentage of OM intake, true ruminal OM and N digestibility were not affected (P = 0.23 to 0.87), whereas ruminal NDF digestibility tended to increase (P = 0.09) as forage intake decreased. Total and microbial essential amino acid flow to the duodenum decreased linearly (P = 0.001) from 496.1 to 132.1 g/d and 329.1 to 96.0 g/d, as intake decreased from 120 to 30% of maintenance intake, respectively. Although the profile of individual essential amino acids in duodenal digesta (P = 0.001 to 0.07) and isolated ruminal microbes differed (P = 0.001 to 0.09) across treatment, the greatest difference noted for total and microbial essential amino acid profile was only 0.3 percentage units. Because total and microbial flow of essential amino acids to the small intestine decreased as OM intake decreased, but true ruminal degradability of individual essential amino acids (P = 0.17 to 0.99) and digesta essential amino acid profile were comparable across treatments, total essential amino acid supply to the small intestine was predicted using OM intake as the independent variable. The resulting simple linear regression equation was: total essential amino acid flow = (0.055 x OM intake) + 1.546 (r2 = 0.91). The model developed in this experiment accounted for more of the variation in the data set than the current beef cattle NRC model, which under-predicted total flow of essential amino acids to the duodenum. The prediction equation developed herein can be used to estimate the supply of essential amino acids reaching the small intestine when formulating supplements to compensate for potential amino acid deficiencies resulting from restricted forage intake.

Model Prediction of Nutrient Supply to Ruminants from Processed Field Tick Beans

The objective of this study was to compare the Dutch DVE/OEB system and the NRC-2001 model in the prediction of supply of protein to dairy cows from processed field tick beans. Comparisons were made in terms of 1) ruminally synthesized microbial CP, 2) truly absorbed protein in the small intestine, and 3) degraded protein balance. The results showed that the predicted values from the DVE/OEB system and the NRC-2001 model had significant correlations with high R (>0.90) values. However, using the DVE/OEB system, the overall average microbial protein supply based on available energy was 16% higher and the truly absorbed protein in the small intestine was 9% higher than that predicted by the NRC-2001 model. The difference was also found in the prediction of the degraded protein balances (DPB), which was 5% lower than that predicted based on data from the NRC-2001 model. These differences are due to considerably different factors used in calculations in the two models, although both are based on similar principles. It need to mention that this comparison was based on the limited data, the full comparison involving various types of concentrate feeds will be investigated in the future.

Comparison of the National Research Council-2001 Model with the Dutch System (DVE/OEB) in the Prediction of Nutrient Supply to Dairy Cows from Forages

The objective of this study was to compare the DVE/OEB system (DVE = truly absorbed protein in the small intestine; OEB = degraded protein balance) and the NRC-2001 model in the prediction of supply of protein to dairy cows from selected forages: alfalfa (Medicago sativa L. cv. Pioneer and Beaver) and timothy (Phleum pratense L. cv. Climax and Joliette). Comparisons were made in terms of 1) ruminally synthesized microbial CP, 2) truly absorbed protein in the small intestine, and 3) degraded protein balance. In addition, the effects of variety and cutting stage of the selected forages on the potential nutrient supply to dairy cows were also studied. The results showed that the predicted values from the DVE/OEB system and the NRC-2001 model had significant correlations with high R (>0.96) values. However, using the DVE/OEB system, the overall average microbial protein supply based on available energy was 12% higher, and the truly absorbed protein in the small intestine was 15% lower than that predicted by the NRC-2001 model. The difference was also found in the prediction of the degraded protein balances, which was 11% higher based on data from the NRC-2001 model. These differences are due to considerably different factors used in calculations in the two models, although both are based on similar principles. This indicates that a further refinement is needed for a modern protein evaluation and prediction system. In addition, this study showed that the two alfalfa varieties studied (Pioneer vs. Beaver) had no effect, but cutting stage had a profound influence on ruminally synthesized microbial CP (93, 96, 86 g/kg DM at stage of early bud, late bud, and early bloom, respectively) and truly absorbed intestinal protein predicted by the DVE/OEB system (80, 79, 67 g/kg DM at stage of early bud, late bud, and early bloom, respectively). With timothy, both variety (Climax vs. Joliette) and cutting stage had significant impacts on the potential protein supply predicted by both models. The potential protein supply (DVE or MP) to the dairy cow from Climax timothy was higher than that from Joliette timothy (DVE: 46 vs. 32 g/kg DM; MP: 61 vs. 38 g/kg DM). With increasing stage of cutting, the potential protein supply (DVE or MP) was reduced (DVE: 53, 39, 25 g/kg DM; MP: 62, 51, 36 g/kg DM at stage of joint, prebloom head, and full head, respectively).

An evaluation of models predicting beef cattle performance using data from bulls on performance testing

Data from 372 bulls on performance testing were used to compare predictions of intake and gain with observed performance when the forage-to-concentrate ratio (F:C) in the ration ranged from 57:43 to 45:55. Predictions were derived from the National Research Council (NRC 1996) and the Agricultural and Food Research Council (AFRC 1993) models. Increasing the F:C resulted in a decline in dry matter intake as a percent of body weight, but did not influence observed average daily gain (ADG). The NRC (1996) model increasingly under-predicted ADG as forage inclusion in the ration increased, whereas AFRC (1993) was a better predictor of daily gain as F:C increased. Contrary to predictions, inclusion levels of silage can be increased in totally mixed rations without compromising animal performance. Key words: Forage-to-concentrate ratio, bulls, performance, model

Performance and meat quality of beef steers fed corn-based or bread by-product-based diets.

A feeding trial was conducted with beef breed steers (120) to determine the effects of substituting bread by-product (BBy) for whole shelled corn on performance and meat quality. Chemical analysis of each diet ingredient and in vitro rates of digestion from gas production of BBy and corn were determined to provide accurate information for diet evaluations using the 1996 Beef NRC Model Level 2. Bread by-product contained 16% CP (75.6% degradable) and 75.1% non-structural carbohydrates (70% as starch, which had a digestion rate of 16%/h). The steers were given one estrogenic implant (Synovex-S) and started on the experiment at 15 mo of age and an average weight of 364 kg. The cattle were commercially slaughtered in three groups (40 steers at 101, 60 steers at 126, and 20 steers at 160 d on feed) weighing an average of 553 kg when they reached a small degree of marbling. Carcasses were electrically stimulated to prevent cold shortening of muscles. Warner-Bratzler shear force values were measured in rib steaks at 5, 14, and 21 d after slaughter (n = 76). Rib steaks from 30 steers per treatment were evaluated for palatability traits. Use of BBy at 55% of the diet (substituted for 75% of the corn) significantly improved feed efficiency by 8.1%. There were no statistically significant differences between the two diets for effects on ADG, carcass characteristics, shear force values, or sensory panel ratings of tenderness, juiciness, flavor, or overall acceptability. After adjusting intestinal starch digestibility in Level 2 to 63% for the whole corn and 90% for the BBy, predicted ADG matched that observed. Apparent NE(g) values for BBy and corn were 1.57 and 1.41 Mcal/kg, respectively.

Predicting amino acid adequacy of diets fed to Holstein steers.

The Cornell Net Carbohydrate and Protein System (CNCPS) and NRC (1985) models were evaluated for accuracy in predicting metabolizable protein (MP) and essential amino acid (EAA) allowable ADG, using chemical body and feed composition data from feeding trials with Holstein steers. Nine Holstein steers (113 to 200 kg) were slaughtered and determined to have the following whole-body essential amino acid composition of (grams/100 grams of protein): arginine, 5.94; histidine, 2.07; isoleucine, 2.28; leucine, 5.72; lysine, 5.81; methionine, 1.99; phenylalanine, 3.04; threonine, 3.52; tryptophan, .57; and valine, 3.32. The NRC and CNCPS were then tested against data from 25 feeding periods, each representing the 56-d growth of 10 Holstein steers (mean BW of 162 kg), to determine their ability to predict the gain allowed by the supply of MP and the first-limiting EAA. The NRC (1985) system accounted for 46% of the variation in MP allowable gain, with an average bias of -30%. The CNCPS accounted for 87 and 73% of the variation in MP and EAA allowable gain, with a bias of 8 and 5%, respectively. The bias was reduced to 3% (R2 of .82) when ADG was predicted by the factor (ME, MP, or EAA) first-limiting ADG.

Variation in Neutral Detergent Fiber Intake of Holstein Cows

Published data representing 1284 Holstein cows over 149 treatment periods were used to evaluate alternative methods of predicting DMI. Stepwise multiple regression analyses of ration and cow characteristics against DMI and NDF intake were used (regression set) to develop DMI prediction equations and to test against DMI models (test set). After 70 DIM, DMI was correlated with BW, DIM, FCM, and ration NDF (R2 = .99; standard deviation of the regression = 1.79). Intake of NDF was correlated with DIM, FCM, and ration NDF (R2 = .99; standard deviation of the regression = .096), and NDF intake increased with increasing ration NDF. When evaluated against the test set, the regression model had an average bias of 3% and an error of 1.6 kgld. A DMI model of Chase and Sniffen (11) that predicts DMI from BW and FCM underestimated DMI but had less bias and error than the NRC model (1.5 vs 1.7kg), which overestimated DMI. Using a constant NDF intake of 1.2% of BW, the model of Mertens, which predicts DMI from NDF, had the greatest error (2.5kg). When variable NDF intake, which was estimated by the NDF regression, was used in the model of Mertens, the DMI error decreased to 1.6 kgld of DM.

The accuracy of some simple models for predicting particulate interception and retention in agricultural systems.

The accuracy of three radionuclide transfer models for predicting the interception and retention of airborne particles by agricultural crops was tested using Pu-bearing aerosols released to the atmosphere from nuclear fuel facilities on the U.S. Department of Energy's Savannah River Plant, near Aiken, SC. The models evaluated were: 1) NRC, the model defined in U.S. Nuclear Regulatory Guide 1.109; 2) FOOD, a model similar to the NRC model that also predicts concentrations in grains; and 3) AGNS, a model developed from the NRC model for the southeastern United States. Plutonium concentrations in vegetation and grain were predicted from measured deposition rates and compared to concentrations observed in the field. Crops included wheat, soybeans, corn and cabbage. Although predictions of the three models differed by less than a factor of 4, they showed different abilities to predict concentrations observed in the field. The NRC and FOOD models consistently underpredicted the observed Pu concentrations for vegetation. The AGNS model was a more accurate predictor of Pu concentrations for vegetation. Both the FOOD and AGNS models accurately predicted the Pu concentrations for grains.

Prediction versus reality: An evaluation of the NRC traffic noise model

In order to evaluate the NRC traffic noise prediction model a program of taffic noise measurements has been undertaken. An array of ten microphones, sampled simultaneously, is used to provide information on the propagation of traffic noise, in terms of Leq and other statistical descriptors, for a variety of traffic speeds, car‐truck mixes, and site configurations. At present, eight sites with free‐flowing traffic have been studied. Five of these were open sites with most of the traffic stream visible from the measuring positions, and three were located in residential streets perpendicular to a freeway. The attenuation with distance of Leq is presented as a function of the effective receiver height, and compared to that predicted by the NRC model.