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This paper examines the educational impact of the implementation of the tutorial activity "Changes in Energy and Momentum" from "The Tutorials in Introductory Physics" in five different instructional settings. These settings include (1) a completely computer-based learning environment and (2) use of cooperative learning groups with varying levels of instructor support. Pre- and post-tests provide evidence that a computer-based implementation falls significantly short of classroom implementations which involve both collaborative learning groups and interactions with a teaching assistance. Other findings provide insight into the importance of certain elements of instructor training and the appropriate use of the tutorial as an initial introduction to a new concept. (Contains 3 figures and 4 tables.)

A finishing feedlot study was conducted with beef steers to determine effects of encapsulated nitrate (EN) on growth performance, carcass characteristics, methane production, and nitrate (NO3 -) residues in tissues. The 132 crossbred steers were backgrounded in a feedlot for 91 d and transitioned for 28 days to the high-concentrate diets evaluated in the present study, maintaining the treatment and pen assignments designated at the start of the backgrounding period. The steers were initially assigned to 22 pens (6 animals per pen) in a randomized complete block design with BW (18 pens) and animals designated for methane measurement (4 pens) as blocking factors. Five animals in each pen designated for methane measurement (total of 20 animals) were monitored for methane emissions in respiratory chambers twice during the experiment. Pens received 3 dietary treatments (7 pens each): Control, a finishing diet supplemented with urea; 1.25% EN, control diet supplemented with 1.25% encapsulated NO3 - in dietary DM that partially replaced urea; and 2.5% EN, control diet supplemented with 2.5% EN (DM basis) fully replacing urea. The final pen designated only for methane measurement received a fourth dietary treatment, 2.3% UEN, the control diet supplemented with unencapsulated NO3 - (UEN) fully replacing urea. The cattle weighed 449 ± SD 32 kg at the start of the 150-d finishing period. The 2.5% EN diet decreased (P < 0.01) DMI compared with Control and 1.25% EN diets. Feeding EN tended to increase (P = 0.092) ADG compared with Control, and G:F was improved (P < 0.01) for EN compared with Control. No differences in methane production (g/d) and yield (g/kg DMI) were observed among treatments. Inclusion of EN in the diets increased (P ≤ 0.03) sorting in favor of large and medium particles and against small and fine particles. Plasma NO3 - and NO2 - concentrations were elevated (P < 0.01) with EN in a dose-response manner, but total blood methemoglobin levels for all treatments were low, below the detection limit. Feeding EN increased (P < 0.01) NO3 - concentrations of samples from muscle, fat, liver, and kidney; NO3 - concentrations of these tissues were similar between 1.25% EN and 2.3% UEN. In conclusion, inclusion of 2.5% EN in a finishing diet (DM basis; about 2% NO3 -) did not cause NO3 - toxicity or any health problems in the long term. In comparison with supplemental urea, feeding EN improved feed efficiency despite increases in sorting against dietary EN. [ABSTRACT FROM AUTHOR]

A long-term experiment was conducted to examine the effects of feeding encapsulated nitrate (EN) on growth, enteric methane production, and nitrate (NO3 -) toxicity in beef cattle fed a backgrounding diet. A total of 108 crossbred steers (292 ± 18 kg) were blocked by BW and randomly assigned to 18 pens. The pens (experimental unit; 6 animals per pen) received 3 dietary treatments: Control, a backgrounding diet supplemented with urea; 1.25% EN, control diet supplemented with 1.25% encapsulated calcium ammonium NO3 - (i.e., EN) in dietary DM, which partially replaced urea; or 2.5% EN, control diet supplemented with 2.5% EN (DM basis) fully replacing urea. Additionally, 24 steers were located in 4 pens and randomly assigned to 1 of the above 3 dietary treatments plus a fourth treatment: 2.3% UEN, control diet supplemented with 2.3% unencapsulated calcium ammonium NO3 - (UEN) fully replacing urea. Animals in the additional 4 pens were used for methane measurement in respiratory chambers, and the pens (except UEN) were also part of the performance study (i.e., n = 7 pens/treatment). The experiment was conducted for 91 d in a randomized complete block design. During the experiment, DMI was not affected by inclusion of EN in the diet. Feeding EN had no effect on BW, ADG, and G:F (P ≥ 0.57). Methane production (g/d) tended to decrease (P = 0.099) with EN and UEN, but yield (g/kg DMI) did not differ (P = 0.56) among treatments. Inclusion of EN in the diet increased (P ≤ 0.02) sorting of the diets in favor of large and medium particles and against small and fine particles, resulting in considerable increases in NO3 -concentrations of orts without affecting DMI. Plasma NO3 -N and NO2 -N concentrations increased (P ≤ 0.05) for EN compared with Control in a dose response manner, but blood methemoglobin levels were below the detection limit. Nitrate concentration in fecal samples slightly increased (from 0.01% to 0.14% DM; P < 0.01) with increasing levels of EN in the diet. In conclusion, EN can be used as a feed additive replacing urea in beef cattle during a backgrounding phase in the long term without NO3 - intoxication or any negative effects on growth performance. In addition, the study confirmed that feeding EN tended to decrease enteric methane production in the long term. [ABSTRACT FROM AUTHOR]

Fecal nutrients and apparent total tract digestibility (ATTD) were predicted using near-infrared spectroscopy (NIRS) of feces collected from the pen floor or the rectum of feedlot cattle in 2 studies, and pen floor samples were assessed for their ability to predict NEg, ADG, and G:F. In study 1, 160 crossbred beef steers in 16 pens (4 pens per treatment) were fed dry-rolled barley or wheat (89% of diet DM) processed at 2 levels. Study 2 utilized 160 crossbred beef steers in 20 pens (5 pens per treatment) that were fed dryrolled barley with 4 levels of barley silage (0%, 4%, 8%, and 12% of diet DM). Both studies fed steers to a target weight of 650 kg. Differences in composition of feces collected from the rectum and the pen floor of a subset of steers (3 to 7) were examined. Fecal pats from the pen floor of each pen were collected throughout the feeding period and composited by pen. Except for DM, which was higher (P < 0.01) in pen floor than rectal fecal samples, there were minimal differences in fecal constituents between collection methods. In study 1, interactions between grain type and processing index (P ≤ 0.05) were observed, with a reduction in DM, OM, and starch and an increase in NDF and ADL concentrations being associated with more extensively processed wheat than barley. As grain was more extensively processed, ATTD of all nutrients increased (P < 0.01). In study 2, fecal ADF and ADL linearly increased (P < 0.01) with increasing silage in the diet, whereas fecal DM and N linearly decreased (P < 0.01). Digestibility of all nutrients except starch linearly decreased (P < 0.01) with increasing silage. Apparent total tract digestibility of GE predicted using NIRS was related to NEg of the diets as estimated by performance for the wheat-fed steers in study 1 (R² = 0.58, P = 0.03) and those fed increasing silage in study 2 (R² = 0.43, P < 0.01). Similarly, observed ADG could be predicted using NIRS for steers fed wheat in study 1 (R² = 0.48, P = 0.05) and silage in study 2 (R² = 0.40, P < 0.01), but G:F could not. Using NIRS of feces collected from multiple cattle off the feedlot pen floor demonstrated potential for predicting growth performance of finishing cattle. However, grain type and stage of maturity of the cattle impacted the predictability of equations. Increasing the sample size and sampling frequency may be necessary to improve predictions. [ABSTRACT FROM AUTHOR]

The objective of the study was to investigate the effects of feed consumption rate on potential toxicity, rumen fermentation, and eating behavior when beef heifers were fed a diet supplemented with nitrate (NI). Twelve ruminally cannulated heifers (827 ± 65.5 kg BW) were used in a randomized complete block design. The experiment consisted of 10-d adaptation, 8-d urea-feeding, and 3-d nitrate-feeding periods. All heifers were fed a diet supplemented with urea (UR) during the adaptation and urea-feeding periods, whereas the NI diet (1.09% NO3- in dietary DM) was fed during the nitrate-feeding period. After adaptation, heifers were randomly assigned to ad libitum or restrictive feeding (about 80% of ad libitum intake) for the urea- and nitrate-feeding periods. Ad libitum DMI decreased (14.1 vs. 15.1 kg/d; P < 0.01) when heifers were fed the NI diet compared with the UR diet. The amount of feed consumed increased (P < 0.01) at 0 to 3 h and decreased (P ≤ 0.03) at 3 to 24 h for restrictive vs. ad libitum feeding of both the UR and NI diets. Compared to the UR diet, the NI diet decreased (P < 0.01) feed consumption at 0 to 3 h and increased (P < 0.02) feed consumption at 3 to 24 h (except feed consumption at 9 to 12 h; P = 0.90), indicating nitrate feeding changed the consumption pattern (a more even distribution of feed intake over the day). The increased feed consumption from 0 to 3 h after feeding the NI diet restrictively vs. ad libitum numerically decreased (P = 0.11) rumen pH and numerically or significantly increased (P = 0.01 to 0.28) rumen ammonia, NO3-, and NO2-; blood methemoglobin; and plasma NO3- and NO2- at 3 h. Regression analysis indicated that increased feed consumption (0 to 3 h) exponentially elevated (P < 0.01; R² = 0.75) blood methemoglobin, and plasma NO3- + NO2- among other rumen and blood variables had the greatest correlation (sigmoid response; P < 0.01, R² = 0.47) with feed consumption (0 to 3 h). Particle size distribution of orts was partially altered (P = 0.02 to 0.40) when the NI diet was fed compared with the UR diet. During the nitrate-feeding period, the nitrate content of orts on d 2 and 3 was greater (P = 0.02) than that on d 1. In conclusion, the increased consumption rate of a diet supplemented with nitrate was an important factor influencing risk of nitrate toxicity based on blood methemoglobin and plasma NO3-. In addition, the pattern of daily feed consumption was altered by nitrate (creating a "nibbling" pattern of eating) in beef heifers. [ABSTRACT FROM AUTHOR]

The objective of this study was to determine whether supplementing organic acids can prevent incidences of subacute ruminal acidosis (SARA) in beef heifers fed a diet consisting of 8% barley silage and 92% barley grain-based concentrate (DM basis). Ten ruminally cannulated Hereford crossbred heifers (484 ± 25 kg BW) were used in a replicated 5 × 5 Latin square design with 14-d periods including 10 d for dietary adaptation and 4 d for measurements. Dietary treatments included no supplementation (Control), low fumaric acid (61 g/d), high fumaric acid (125 g/d), low malic acid (59 g/d), and high malic acid (134 g/d). Organic acid supplementation had no effect on DMI (P = 0.77). Similarly, no effects were observed on mean (P = 0.74), minimum (P = 0.64), and maximum (P = 0.27) ruminal pH measured continuously for 48 h. Moreover, area under the curve for pH thresholds 6.2 (P = 0.97), 5.8 (P = 0.66), 5.5 (P = 0.55), and 5.2 (P = 0.93) was similar for all treatments. However, malic acid supplementation lowered the amount of time that ruminal pH was <6.2 compared with the Control (P = 0.02) and fumaric acid treatments (P < 0.01). No effects were observed on total VFA concentrations with organic acid supplementation (P = 0.98) compared with the Control, but greater total VFA concentrations were observed with fumaric acid compared with the malic acid treatments (P = 0.02). The population of total culturable bacteria 3 h after feeding was reduced with supplemental malic acid compared with the Control (P = 0.03) and fumaric acid treatments (P = 0.03). However, no effects were observed with organic acid supplementation on lactic acid-utilizing bacteria (P = 0.59). In conclusion, under the conditions of the present study, organic acid supplementation did not have any significant effects on ruminal fermentation parameters compared with the Control and were not effective in preventing SARA in beef cattle fed high-grain diets. [ABSTRACT FROM AUTHOR]

The objective of this study was to investigate effects of encapsulated nitrate (EN) on enteric methane emission and N and energy utilization in beef heifers. Eight ruminally-cannulated beef heifers (451 ± 21 kg BW) were used in a replicated 4 × 4 Latin square design. Four experimental diets were prepared and fed once daily for ad libitum intake: control, 1%, 2%, and 3% EN (0.15, 0.9, 1.5, and 2.5% NO3- in dietary DM, respectively). The control diet (55% forage and 45% concentrate) included encapsulated urea, which was gradually replaced with EN for the EN diets (iso-nitrogenous; 12.5% CP). In each period, EN was increased stepwise by 1% every 4 d during adaptation. A 7-d washout period (control diet offered to all heifers) was provided between experimental periods. Dry matter intake tended to decrease (10.4 to 10.1 kg/d; linear, P = 0.06) with EN levels. Enteric methane yield was linearly decreased (21.3 to 17.4 g/kg DMI; P < 0.01) by EN, and methane production (g/d) recovered to the level from heifers fed the control diet on the first day when EN was withdrawn from the diet. Apparent total-tract digestibility of DM and OM increased (P = 0.03) or tended to increase (P = 0.06), respectively, with EN levels. Starch digestibility tended to be greater (P = 0.07) for EN vs. control. The concentrations of rumen ammonia-N and plasma urea-N decreased linearly (P < 0.01) with EN. Total urinary N and urea-N excretion as proportions of N intake were linearly decreased (46.3 to 41.4%, P = 0.09 and 37.1 to 29.9%, P = 0.01, respectively) with EN addition. However, NO3--N excretion in urine increased linearly (P < 0.01) with EN levels. Fecal N excretion was not affected (P = 0.47) by EN, although fecal NO3--N excretion increased linearly (P < 0.01) with inclusion of EN (0.09 to 0.88% of total N, P < 0.01). Retained N tended to be increased (percentage of N intake; 16.6 to 21.4%, P = 0.08) by the EN. Supplementary EN lowered (6.64 to 5.46% of GE intake [GEI], P < 0.01) energy losses by enteric methane mitigation, which increased ME supply (calculated; 56.5 to 58.8% of GEI, P = 0.01) without changes in calculated heat production (P = 0.24). As a result, retained energy tended to increase (P = 0.07) with EN levels. In conclusion, feeding EN to beef heifers lowered enteric methane production in a doseresponse manner, which slightly increased energy supply. Total urinary N excretion was lowered for EN due to lower urinary urea-N excretion. [ABSTRACT FROM AUTHOR]

A slow-release form of nitrate (encapsulated nitrate [EN]) was investigated for effects on risk of toxicity, feed consumption rates, and feed sorting behavior in 2 experiments. In Exp. 1, 5 beef heifers (806 ± 72 kg BW) were fed once daily at 75% of ad libitum intake. Supplementary EN (85.6% DM; 71.4% NO3- on a DM basis) was increased by 1% every 4 d from 0 to 1.0, 2.0, 2.9, 3.9, 4.8, and 5.8% of dietary DM (from 10.3 to 15.6% CP and 0.11 to 4.8% total NO3-). During the study, a heifer was removed due to nitrate poisoning with 59% blood methemoglobin (MetHb; % of total hemoglobin) at 2.9% EN and another due to refusal to eat the 2% EN diet. When dietary EN increased from 0 to 5.8%, DMI (8.8 to 7.6 kg/d; P < 0.001) and feed consumption from 0 to 3 h were decreased (70.3 to 48.6% of total feed offered on an as-is basis; P = 0.001) and feed consumption from 12 to 24 h was increased (0.6 to 22.6%; P < 0.001). Blood MetHb at 1% EN was negligible (<1.5% of total hemoglobin). However, MetHb levels were greater (average 9.8 vs. 3.1% and maximum 23.6 vs. 13.6% at 3 h) at 2.0 and 2.9% EN than at 3.9, 4.8, and 5.8% EN. In Exp. 2, 8 beef heifers (451 ± 21 kg BW) were used in a replicated 4 × 4 Latin square design and fed for ad libitum intake, once daily, diets containing 0, 1, 2, and 3% EN (isonitrogenous, 12.7% CP, and 0.15 to 2.5% total NO3- in dietary DM). Each period consisted of 14 d for adaption and 14 d for sampling followed by a 7-d washout. During adaptation, EN was provided to heifers in a stepwise manner (an increase by 1% every 4 d). In Exp. 2 with ad libitum feeding, feed consumption rates were not different (41.8% of total feed consumed from 0 to 3 h; P = 0.56) among EN levels, but DMI tended to decrease linearly (0 to 3% EN; 10.4 to 10.1 kg/d; P = 0.06) and feed was sorted (linear, P < 0.05) against concentrates (containing EN) with EN inclusion. Potential toxicity of nitrate based on blood MetHb was not observed (all samples <2.0% of total hemoglobin). In both studies, ruminal pH increased numerically (Exp. 1) and significantly (Exp. 2; linear, P = 0.04) with EN. In conclusion, studies indicated that the organoleptic properties of diets containing >3% EN may have caused lower feed intake, reduced feed consumption rates, and feed sorting. In addition, feed consumption rate, which was altered by restrictive or ad libitum feeding, was an important factor affecting the potential toxicity of nitrate (e.g., MetHb). Ad libitum feeding minimized risks of nitrate toxicity. [ABSTRACT FROM AUTHOR]

The objectives were to characterize the effects of supplemental CP concentration and rumi-nal degradability in barley-based finishing diets on microbial protein synthesis, ruminal fermentation and nutrient digestion, and route and chemical form of N excretion in beef cattle. Four Angus heifers (564 ± 18 kg BW) with ruminal and duodenal cannulas were used in an experiment designed as a 4 x 4 Latin square with four 28-d periods (9 d for diet adaptation and 19 d for measurements). The basal diet consisted of 9% barley silage and 91% barley-based concentrate (DM basis). Dietary treatments included the basal diet with no add-ed protein (13% CP) or diets containing 14.5% CP by supplementation with urea (UREA), urea and canola meal (UREA+CM), or urea, corn gluten meal, and xylose-treated soybean meal (UREA+CGM+xSBM). Nutrient digestion was determined using Yb as a diges-ta flow marker and purine N as a microbial marker with the collection of ruminal, duodenal, and fecal samples over 5 d. The next week, total collections of feces and urine were performed for 5 d to quantify route and chemical form of N excretion. Feed offered was restricted (95% of ad libitum) and there was no effect of the dietary treatments on DMI (P = 0.55); therefore, N intake was less (P < 0.05) in heifers fed the 13% CP diets than the 14.5% CP diets. Supplemental RDP and RUP had no effect on ruminal NH3-N (P = 0.17), peptide N {P = 0.46), and VFA (P = 0.62) concentra-tions, flow of microbial (P = 0.69) and feed (P = 0.22) N, and ruminal and total tract nutrient digestibility (P > 0.18). Nutrient digestion in the rumen and total tract averaged 75.4 ± 3.8% and 84.6 ± 0.9% for OM, 80.8 ± 3.6% and 95.8 ± 0.8% for starch, and 41.2 ± 7.9% and 60.4 + 3.3% of intake for NDF, respectively. Daily output of N in feces {P = 0.91) and urine (P = 0.14) were not affected by the dietary treatments. Fecal N output averaged 19.9 ± 1.9% (P = 0.30) and urine N output averaged 44.1 ± 2.8% (P = 0.63) of N intake. Urea N output, however, was greater (P < 0.05) in heif-ers fed the 14.5% CP than the 13% CP diets and was the major form of N in urine (68.3% in heifers fed the 13% CP diet and 78.7 ± 2.9% in heifers fed the 14.5% CP diets; P < 0.10). Beef cattle fed barley-based finish-ing diets containing 13% CP do not require additional RDP or RUP to meet microbial or host N requirements. Barley-based finishing diets with no supplemental CP minimized urea N excretion and the potential loss of N from the system. [ABSTRACT FROM AUTHOR]

The objectives of the study were to characterize the effects of CP concentration and ruminal degradability of barley-based backgrounding diets on route and chemical form of N excretion, ruminal fermentation, microbial protein synthesis, and nutrient digestion in beef cattle. Four Angus heifers (479 ± 14.6 kg average BW) with ruminal and duodenal cannulas were used in an experiment designed as a 4 x 4 Latin square. The basal diet consisted of 54% barley silage and 46% barley grain-based concentrate (DM basis). Dietary treatments included the basal diet with no added protein (12% CP) or diets formulated to contain 14% CP by supplementation with urea (UREA), urea and canola meal (UREA+CM), or urea, corn gluten meal, and xylose-treated soybean meal (UREA+CGM+xSBM). The amount of feed offered was restricted to 95% of ad libitum intake. There was no effect of the diets on DM1 (P = 0.38), and therefore, N intake was less (P < 0.05) in heifers fed the 12% CP diets than the 14% CP diets. Fecal N output was not affected by the diet (P = 0.15), but urine N (P < 0.10) and urea N output were greater (P < 0.05) in heifers fed the 14% CP than the 12% CP diets. There was no effect of CP degradability (P >0.10) on the amount of urine N output. Urine N output was 38.9 and 45.1 ± 5.50% of N intake in heifers fed the 12% CP and 14% CP diets (P < 0.05), respectively. Urea N, the form of N most susceptible to NH3-N volatilization and loss, was the major form of N in urine (75.5% in heifers fed the 12% CP diet and 81.4 ± 1.7% in heifers fed the 14% CP diets; P < 0.05). Supplemental RDP (UREA+CM) and RUP combined with urea (UREA+CGM+xSBM) to provide 14% CP increased (P < 0.05) ruminal NH3-N but had no effect on ruminal peptide N (P = 0.62) and free AA N (P = 0.18) concentration, the flow of microbial (P = 0.34) and feed (P = 0.55) N, and ruminal (starch, P = 0.11; NDF, P = 0.78) and total tract nutrient digestibility (OM, P = 0.21; starch, P = 0.16). Supplementation of barley-based backgrounding diets containing 12% CP with NPN alone or combined with ruminally degradable and undegradable true protein to attain 14% CP had no effect on fecal N output, but urine N and urea N increased irrespective of protein source. In addition, the ruminal degradability of the protein sources did not influence the composition of protein flowing to the intestine and site and extent of nutrient digestibility. [ABSTRACT FROM AUTHOR]

Crossbred beef steers (« = 312) were used in an experiment with a completely randomized design during the growing (235 ± 1.6 kg initial BW) and fin-ishing (363 ± 2.7 kg) phase to determine the effects of dietary CP concentration and rumen degradability on NH3-N emissions, growth performance, and carcass traits. Diets were barley based and consisted of 55% silage and 45% concentrate in the backgrounding phase and 9% silage and 91% concentrate in the finishing phase. For each phase, there were 4 dietary treatments (6 pens of 13 cattle per diet): the basal diet with no protein supplementation (12% CP backgrounding and 12.6% CP finishing) or supplemented (14% CP) with urea (UREA), urea and canola meal (UREA+CM), or urea, corn gluten meal, and xylose-treated soybean meal (UREA+CGM+xSBM). Feed intake and BW of cattle were measured at 3-wk intervals. One pen of steers fed the 12 or 12.6% CP and 1 pen fed 1 of the 14% CP diets were housed in 2 isolated pens to quantify NH3-N emis-sions using the integrated horizontal flux technique with passive NH3 samplers. In the backgrounding phase final BW, ADG, and G:F were less (P < 0.05) in cattle fed the 12% CP and UREA compared with the UREA+CM and UREA+CGM+xSBM diets. Nitrogen-use efficiency of cattle fed UREA+CM and UREA+CGM+xSBM was equal to that of cattle fed 12% CP and averaged 19.8%. In the finishing phase, there was no effect (P > 0.10) of CP supplementation on BW, DMI, ADG, G:F, N-use efficiency, and carcass traits. The NH3-N emissions from December to February during the backgrounding phase ranged from 4.3 to 25.6 g N/(steerd) and 3.8 to 16.3% ofN intake and from April to July during the fin-ishing phase ranged from 9.7 to 76.4 g N/(steer-d) and 4.4 to 26.7% of N intake. Differences in NH3-N emis-sions between the pens of cattle fed the backgrounding diets with 12 and 14% CP were not detected. For cattle fed the 12.6 and 14% CP finishing diets, NH3-N emis-sions tended (P < 0.16) to be less for 2 of the 5 periods and averaged 14.4 and 28.1 g N/(steer-d) and 7.7 and 12.7% of N intake, respectively. The NH3-N emitted as a % ofN intake averaged 42% less for cattle fed 12.6% compared with 14% CP. Feeding the barley-based con-centrate diet to finishing cattle with 12.6% compared with 14% CP diets reduced NH3-N emissions with no effect on performance. Feeding tbe barley-based forage diet to backgrounding cattle with 12% CP, however, reduced performance compared with growing cattle fed supple-mentary degradable and undegradable true protein. [ABSTRACT FROM AUTHOR]

The objective of this investigation was to study the effects of feeding backgrounding and finishing diets on selected acute phase proteins in the plasma of feedlot steers. Two groups of 12 steers each, at the backgrounding and finishing stages, were offered either a backgrounding (45% barley grain-based concentrate and 55% barley silage on a DM basis) or a finishing (91% barley grain-based concentrate and 9% barley silage) diet for 12 and 15 wk, respectively. Steers at the backgrounding and finishing stages had initial BW of approximately 250 and 380 kg, respectively, at the beginning of the experiment. Blood samples were obtained from a jugular vein at 3-wk intervals during the experimental period beginning at wk 3 or 0 for the backgrounding and finishing periods, respectively. Plasma samples were analyzed for serum amyloid A (SAA), lipopolysaccharide-binding protein (LBP), haptoglobin, and α1-acid glycoprotein. Steers fed the finishing diet showed peak plasma SAA, LBP, and haptoglobin with- in 3 wk from the initiation of the diet (20, 23, and 1,940 μg/mL for SAA, LBP, and haptoglobin, respectively). Although plasma α1-acid glycoprotein reached a peak concentration (449 μg/mL) at the beginning of the finishing phase, no diet effect was obtained for this variable. Steers fed the backgrounding diet showed less variation in the concentrations of plasma acute phase proteins measured; plasma haptoglobin reached a peak concentration (1,720 μg/mL) 9 wk after the beginning of this diet. In conclusion, feeding feedlot steers the backgrounding or finishing diet was associated with increased peak concentrations of acute phase proteins in the plasma. More research is warranted to elucidate the mechanisms behind the inflammatory responses observed in feedlot steers and their implications for health issues and the production efficiency of feedlot operations. [ABSTRACT FROM AUTHOR]

Four ruminally cannulated and four ruminally and duodenally cannulated beef heifers (476 ± 25 kg, initial BW) were used in a replicated 4 × 4 Latin square to determine the effects of feeding condensed and hydrolysable tannin extracts with a high protein diet containing wheat distillers grains on ruminal fermentation, digestibility, and route of nitrogen (N) excretion. Periods were 4 wk and included a 1-wk washout, 2 wk for adaptation to the tannin extracts, and 1 wk for measurements. Dietary treatments were control (CON, basal diet), 2.5% Acacia mearnsii (ACA), 2.5% quebracho (QUE), and 2.5% chestnut (CHE; DM basis). The basal diet was composed of 71% barley grain concentrate, 20% wheat distillers grains and solubles, and 9% barley silage and contained 16.8% CP (DM basis). Diets were fed as total mixed rations once per day for ad libitum intake. Feed offered and refused were measured daily. Total collection of urine and feces were made for 6 d. Plasma was collected at 3 h and rumen fluid was collected at 0, 3, 6, 9, 12, 15, and 18 h after feeding at the end of the collection period. Data were analyzed using a mixed linear model with square, period, and treatment as fixed effects and animal within square as a random effect. Dry matter intake tended (P = 0.10) to be lower when heifers were fed the CHE compared to the CON and QUE, and was intermediate for the ACA. Total tract digestibility of DM was similar among heifers fed the tannin extracts (78.1, 77.4, and 77.6% for ACA, QUE, and CHE, respectively), however, all tannin extracts reduced DM digestibility compared to the CON (80.8%, SEM 0.98%; P < 0.05). Plasma urea-N and ruminal NH3-N were also similar among heifers fed the tannin extracts and were reduced (P < 0.05) compared to the CON. There was no effect of the dietary treatments on total N output (P > 0.05), but fecal N output was increased (P < 0.05) and conversely urinary N output was decreased (P < 0.05) for heifers fed ACA and QUE compared to the CON. Fecal and urinary N outputs were intermediate for the CHE. Feeding tannins to beef cattle fed high protein diets containing wheat distillers grains reduced ruminal NH3-N and plasma urea-N and shifted the route of N excretion from labile forms in urine to feces although there was a 4% decrease in DM digestibility. [ABSTRACT FROM AUTHOR]

In situ and in vitro experiments were conducted to determine the nitrate (NO3 -) release rate from encapsulated NO3 - (EN; 71% NO3 -) in the rumen and to examine metabolism of ENNO 3 - in conjunction with methane (CH4), hydrogen (H2), and nitrous oxide (N2O) production. Three ruminally-cannulated beef heifers were used to incubate the following substrates in the rumen for 72 h: soybean meal, EN, and unencapsulated NO3 - (UEN). Because of immediate solubilization, the soluble nitrogen (N) fraction of UEN was assumed to be 100%. A non-linear regression model fitted to N disappearance of EN (r2 = 0.93, P < 0.01) indicated 35.4% and 54.7% of soluble and slowly degradable N fraction, respectively. Two in vitro experiments were conducted where UEN and EN were incubated for 24 h in buffer alone or with buffered-rumen fluid with substrates containing urea, UEN, or EN as sole N sources. When UEN was incubated with buffer, 99.9% of UEN-NO3 --N was recovered as NO3 --N in the buffer over 24 h. However, recovery of EN-NO3 - as NO3 --N gradually increased over 24 h up to 58%. When urea, EN or UEN were incubated with buffered-rumen fluid, total gas production decreased (P = 0.022) for UEN compared with urea and EN. Methane production decreased (10.7 and 13.3 vs 19.6 mL; P < 0.01) for UEN and EN, respectively, compared with urea. Hydrogen production increased (0.2 vs 0.07 and 0.08 mL; P < 0.01) for UEN compared with urea and EN, respectively. Nitrous oxide production was greater for UEN (20.8, 7.5 and 0.02 μL, respectively; P < 0.05) followed by EN and urea. During the incubation with buffered-rumen fluid, 100% of UEN-NO3 - was recovered as NO3 --N in the medium until 6 h and then the recovery gradually decreased to 18% at 24 h while recovery as nitrite (NO2 -)-N increased up to 22% at 24 h. However, recovery of EN-NO3 --N as NO3 --N in the medium increased by 20% at 12 h and then gradually decreased to 5% at 24 h without accumulation of NO2 --N in the medium. In conclusion, EN released NO3 - comparatively slowly in the rumen and decreased CH4 production without negatively affecting microbial fermentation. Increases in H2 and N2O production and accumulation of NO3 - and NO2 - in the medium were not observed for EN compared with UEN, indicating that EN could be a promising NO3 - source to lower CH4 production with less risk of NO3 -/NO2 - toxicity. [ABSTRACT FROM AUTHOR]

A finishing feedlot study was conducted with beef steers to determine effects of encapsulated nitrate (EN) on growth performance, carcass characteristics, methane production, and nitrate (NO3-) residues in tissues. The 132 crossbred steers were backgrounded in a feedlot and transitioned to the high concentrate diets evaluated in the present study. The steers were initially assigned to 22 pens (6 animals per pen) in a randomized complete block design with BW (18 pens) and animals designated for methane measurement (4 pens) as blocking factors. Five animals in each pen designated for methane measurement were monitored for methane emissions in respiratory chambers twice during the finishing period. Pens received 3 dietary treatments (7 pens each): Control, a finishing diet supplemented with urea (0.03% NO3- and 0.9% urea in dietary DM); 1.25% EN, the control diet supplemented with 1.25% EN (1.1% NO3-) which partially replaced urea; 2.5% EN, the control diet supplemented with 2.5% EN (2.0% NO3-) fully replacing urea. The final pen designated only for methane measurement received a fourth dietary treatment, 2.3% UEN; control diet supplemented with unencapsulated NO3- (UEN; 2.0% NO3- in dietary DM) fully replacing urea. The cattle weighed 449 ± 32 kg at the start of the finishing period. The 2.5% EN diet decreased DMI (9.9 vs 10.7 and 10.7 kg/d; P < 0.01) compared with Control and 1.25% EN. Feeding EN tended to increase ADG (1.41 vs 1.36 kg/d; P = 0.09) and increased gain:feed (0.137 vs 0.127 kg/kg; P < 0.01) for EN compared with Control. No differences in methane production and yield were observed among treatments. Inclusion of EN in the diets increased (P  0.03) sorting in favor of large and medium particles and against small and fine particles. Plasma NO3- and NO2 - concentrations were elevated (0.06 to 3.68 mg/L of NO3- and 1.47 to 9.23 g/L of NO2 -; P < 0.01) with EN in a dose response manner, but blood methemoglobin levels for all treatments were below 1%. Feeding EN increased (P < 0.01) NO3- concentrations in muscle, fat, liver, and kidney and NO3- concentrations of those were similar between 1.25% EN and 2.3% UEN. In conclusion, including up to 2.5% of EN on a DM basis in a finishing diet did not cause NO3- toxicity or health problems in a long term study. In comparison with supplemental urea, feeding EN improved feed efficiency despite increases in sorting against dietary EN. [ABSTRACT FROM AUTHOR]

A backgrounding feedlot study was conducted to examine the effects of feeding encapsulated calcium ammonium nitrate (EN) on growth, enteric methane production, and nitrate (NO3 -) toxicity in beef cattle. A total of 108 crossbred steers (292 ± 18 kg) were blocked by BW and randomly assigned to 18 pens. The pens (experimental unit; 6 animals per pen) received 3 dietary treatments: Control, a backgrounding diet supplemented with urea (0.2% NO3 - and 0.9% urea in dietary DM); 1.25% EN, control diet supplemented with 1.25% EN (1.2% NO3 -) in dietary DM, which partially replaced urea; or 2.5% EN, control diet supplemented with 2.5% EN (2.3% NO3 -) in dietary DM, fully replacing urea. Twenty four steers were allocated to additional 4 pens and randomly assigned to 1 of the above 3 dietary treatments plus a fourth treatment: 2.3% UEN, control diet supplemented with 2.3% unencapsulated NO3 - (UEN; 2.4% NO3 -) fully replacing urea. Animals in the additional 4 pens were used for methane measurement in respiratory chambers and the pens (except UEN) were also part of the performance study (i.e., n = 7 pens/treatment). The experiment was conducted for 91 d in a randomized complete block design. During the experiment, DMI was not affected by inclusion of EN in the diet. Changes in BW, ADG, and gain:feed were not observed (P ≥ 0.57) by feeding EN. Methane production (g/d) tended to decrease (P = 0.099) by NO3 -(188, 174, 156, and 152 g/d for Control, 1.25%, 2.5% EN, and UEN, respectively), but yield (g/kg DMI) did not differ (P=0.56) among treatments. Inclusion of EN in the diet increased (P ≤ 0.02) sorting of the diets in favor of large and medium particles and against small and fine particles resulting in considerable increases in NO3 - concentrations of orts without affecting DMI. Plasma NO3 - and NO2 - concentrations increased (P ≤ 0.05) for EN compared with Control in a dose response manner, but blood methemoglobin levels were below the detection limit. Nitrate concentration in fecal samples slightly increased (from 0.01% to 0.14%; P < 0.01) with increasing levels of EN in the diet. In conclusion, EN can be used as a feed additive replacing urea in beef cattle diets during the backgrounding phase without NO3 - intoxication or any negative effects on growth performance. In addition, the study confirmed that feeding EN tended to decrease enteric methane production in the long term. [ABSTRACT FROM AUTHOR]

Selenium is an essential trace mineral for dairy cattle and can be provided in the diet in various forms that may differ in bioavailability. The objective of this study was to determine how source of Se affects animal performance, Se status, retention, and apparent and true absorption. Multiparous Holstein cows (n = 24; 597 ± 49 kg of BW) were blocked by DIM (161 ± 18) and randomly assigned to receive 0.3 mg Se/kg of DM (100% of National Academies of Sciences, Engineering, and Medicine requirements) of either organic Se (ORG; selenized yeast) or inorganic Se (INO; sodium selenite). The Se premix was top dressed on a common TMR fed daily and mixed into the top 15 cm directly before feeding. Following an 11-wk adaptation period, cows received simultaneous infusions of an intraruminal isotope dose of 77 Se in the same chemical form as the premix, and an intravenous dose of 82 Se in an inorganic form. Infusions were followed by a 4-d period of blood and rumen fluid sampling, and total collection of feces, urine, and milk. Daily DMI (23 ± 0.6 kg), milk yield (35 ± 1.2 kg), and serum Se (0.11 ± 0.003 µg/g) were not different between treatments during the adaptation period, but milk Se concentrations were greater for ORG compared with INO. Serum 77 Se maximum concentration and area under the curve (AUC) were not different between treatments for 72 h following infusion, but rumen fluid 77 Se AUC was higher for ORG than INO. Apparent absorption (64% ± 1.4%), and retention (44% ± 1.5%) of the 77 Se dose did not differ between treatments. True absorption was calculated using 82 Se enrichment in serum and feces and was determined to be 69% ± 1.3% and did not differ between treatments. Fecal excretion of the 77 Se dose was not different between treatments (36% ± 1.4%), but ORG had lower urinary excretion and higher milk excretion compared with INO. These results indicate that organic Se resulted in greater Se concentration of milk and lower urinary Se excretion into the environment, but absorption, Se status, and performance of the cow were not affected by Se source at this supplementation level., (© 2024, The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Effects of the extent of grain processing and the percentage of silage in barley-based feedlot diets on microbial protein synthesis and nutrient digestibility were evaluated using four steers (initial BW of 442 +/- 15 kg) with ruminal and duodenal cannulas. The experiment was a 4 x 4 Latin square with four periods of 21 d each. Dietary treatments were arranged as a 2 x 2 factorial with two levels of barley silage (20 and 5% DM basis) and two degrees of barley grain processing (coarsely and flatly steamrolled to a processing index [PI] of 86 and 61%, respectively). The PI was quantified as the volume weight of the barley grain after processing, expressed as a percentage of the volume weight prior to processing. Digest a flow (Yb) and microbial (15N) markers were continuously infused into the rumen for a period of 13 d. Ruminal, duodenal, and fecal samples were collected at various times over the last 6 d of marker infusion. Diurnal ruminal pH was measured for 48 h. Intake of DM averaged 1.8% of BW, and was not different among the dietary treatments (P > 0.10). Ruminal starch digestibility was higher (P < 0.05) for the more extensively processed grain and tended (P < 0.10) to be highest when the more extensively processed grain was combined with 5% barley silage. In contrast, ruminal fiber digestibility for the 5% silage diets was reduced (P < 0.05) when the grain was more extensively processed. There was, however, no effect of grain processing on ruminal OM digestibility (P > 0.10), and hence, no inhibitory effect on microbial N flow to the intestine (P > 0.10). There was also no effect of the level of silage on microbial N flow (P > 0.10), but there was a tendency for improved efficiency of microbial protein synthesis for the 20% silage diets (P = 0.072). Ruminal escape of nonmicrobial N (P = 0.003) was greater, and thus, protein flow to the intestine was greater for the 5% silage diets. Diurnal ruminal pH was lower (P < 0.05) for 11 of the 24 hourly time points in steers fed the 5% silage diets than those fed the 20% silage diets. In conclusion, barley grain rolled to a PI of 86 to 61% and combined with 20 and 5% barley silage had little effect on microbial protein supply. Microbial protein supply was not inhibited when the barley grain was extensively processed (PI of 61%) and the silage was limited to only 5% of the diet DM, but feed intake of steers in this study was lower than would be expected in the feedlot.

Ruminal escape of various amounts of methionine hydroxy analog [D,L-2-hydroxy-4-(methylthio)-butanoic acid (HMB)] was measured in an experiment designed as a 4 x 4 Latin square using four lactating dairy cows with cannula in the rumen and duodenum. The cows were fed a diet composed of corn silage, alfalfa haylage, rolled barley grain, canola meal, and blood meal, three times per day. The cows were fed the liquid analog each day for 1 wk before the experiment was started. On the day of the experiment, each cow received an intraruminal bolus dose of 0, 25, or 50 g of the liquid analog (Alimet feed supplement, 88% HMB) or 51.2 g of a dry calcium salt of the analog (86% HMB; MHA) mixed with 0.5 kg of ground barley grain. A liquid phase marker (Co-EDTA) was administered as a bolus dose into the rumen at the time of administration of the methionine hydroxy analogs. Rumen and duodenal contents, and blood serum were collected at 0, 1, 3, 6, 9, 12, and 24 h relative to the time of dosing. Rumen and duodenal samples were analyzed for Co and HMB, and serum was analyzed for free methionine. Fractional rate constants for the passage of the liquid marker (k(p)) and the decline of HMB concentration in the rumen (k(rHMB)) were determined by nonlinear regression. Liquid passage from the rumen was similar among the four analog treatments (0.136 +/- 0.012/h; mean +/- SEM). Ruminal escape of HMB as a percentage of the dose (100% x k(p)/k(rHMB)) did not differ between cows receiving 25, 50, and 51.2 g of the methionine analogs (42.5, 41.0, and 34.9 +/- 9.0%, respectively) and averaged 39.5%. Duodenal appearance of HMB as a percentage also did not differ between cows receiving 25, 50, and 51.2 g of the methionine analogs (16.2, 26.8, and 22.7%, respectively) and averaged 22%. Omasal absorption of HMB was variable ranging from 12.3 to 26.3% and averaged 17.6%. Serum methionine concentration peaked at 3 and 6 h after dosing and increased in proportion to the amount of the analog administered. It was concluded that 39.5% of the methionine hydroxy analog escaped rumen degradation, the percentage of the dose that escaped the rumen was not affected by the amount or form of the methionine analog fed, and the analog that escaped ruminal degradation was likely absorbed and metabolized to methionine.

Bioavailability of Met from a rumen-protected Met product was evaluated in two experiments using three ruminally and duodenally cannulated lactating (experiment 1) and nonlactating (experiment 2) dairy cows. In the first experiment, the ruminal in situ and mobile bag technique was used to assess ruminal degradability and intestinal disappearance of Met from the protected Met product. Effective ruminal degradability of Met at a ruminal outflow rate of 0.11/h was 21.7%. Combining effective ruminal degradability with intestinal digestibility yielded an estimate of Met availability of 25%. In the second experiment, designed as a 3 x 3 Latin square, Met availability was assessed by determining the response of plasma Met to supplementation of the protected Met product relative to that of duodenally administered Met. The periods were 1 wk with cows fed a meal containing 0, 20, or 63 g of protected Met on d 1 and infused intraduodenally with 10.7 g of Met on d 4. Blood was collected at various times relative to the time of oral dosing and the commencement of the duodenal infusion. Plasma Met response measured as area under the curve increased linearly with increasing protected Met. The response of plasma Met increased by 33 and 65.5% of the control values for 20 and 63 g of protected Met, respectively. Intestinal bioavailability of Met in the protected Met product ranged from 22 to 34%.

The effects of protozoa on ruminal NH3-N kinetics and bacterial N recycling were measured in five sheep (57.6+/-7.1 kg BW, x +/- SD) with ruminal and duodenal cannulas in naturally faunated, defaunated, and refaunated periods. The sheep were fed a diet of 239 g of alfalfa haylage and 814 g of barley concentrate per day (DM basis) divided into 12 equal portions and allocated at 2-h intervals. A pulse dose of 300 mg of 15N as [15N]NH4Cl was administered into the rumen (on d 1 and 15) and 300 mg of 15N as [15N]urea was administered intravenously to the blood (d 8). Enrichment of 15N was measured in ruminal NH3-N, bacterial N, and plasma urea N over a period of 35 h. Total collection of urine was made for 5 d and analyzed for purine derivatives to calculate the flow of microbial N. Ruminal parameters and nutrient digestibilities were also measured. Sheep were defaunated using a rumen washing procedure 50 d prior to measurements in the defaunated period. Sheep were refaunated with ruminal contents from a faunated sheep receiving the same diet. Measurements began 26 d following refaunation, at which time protozoal numbers had returned to those in the originally faunated sheep. Data reported in parentheses are for faunated, defaunated, and refaunated sheep, respectively. Total culturable and cellulolytic bacterial numbers were unaffected by defaunation, but there was an increase in flow of microbial N from the rumen (10.8, 17.3, and 11.1 g N/d; P < .05) in the defaunated period. Flux, irreversible loss, and intraruminal recycling of NH3-N and recycling of NH3-N from plasma urea N were not affected by defaunation. Defaunation had no effect on reducing the absolute amount (13.8, 10.0, and 11.3 g N/d; P > .20) of bacterial N recycling and the percentage of N flux through the bacterial N pool. Total-tract digestion was reduced in defaunated compared with faunated sheep by 8, 17, 15, and 32% for OM, N, NDF, and ADF, respectively. In conclusion, defaunation improved ruminal N metabolism through the enhancement of bacterial protein synthesis, and improvement in the flow of microbial protein to the host animal.

We investigated the effects of two new hollow-fiber membrane oxygenators, the Capiox SX10 and the Lilliput 901, on pulsatile versus nonpulsatile perfusion in an in vitro model designed to simulate a 3 kg infant. The experiments were divided into eight groups (six pulsatile and two nonpulsatile), according to the equipment and settings used. Each group included six tests. In all experiments, the pseudo-patient's mean arterial pressure was 40 mmHg, and the pump flow rate was 550 ml/min. During pulsatile cardiopulmonary bypass, the pump's base flow was set at 30%, and the pump rate was set at 80, 100, 120, 140, or 150 beats/min. The PUMP START and PUMP STOP timing points were adjusted to produce different pulse-width settings. We were especially interested in evaluating the pre- and postoxygenator extracorporeal circuit pressure (ECP), the oxygenator pressure drop, and the precannula ECP. When used with a pulsatile roller pump, the Capiox produced a significantly lower preoxygenator ECP than the Lilliput (p < 0.001); moreover, the Capiox yielded a significantly lower oxygenator pressure drop (p < 0.001). During nonpulsatile perfusion, the Capiox again produced a lower preoxygenator ECP than the Lilliput (p < 0.001). These results suggest that the Capiox may be more suitable than the Lilliput when the pulsatile flow is employed, and pulsatile flow does not increase the ECP with either oxygenator.

Availability of liquid methionine hydroxy analog [D,L-2-hydroxy-4-(methylthio)-butanoic acid] was evaluated in two experiments using four cannulated lactating dairy cows. The first experiment was designed as a 4 x 4 Latin square. Each cow received a pulse dose of 0, 30, 60, or 90 g of the methionine analog in the rumen each day for 10 d. Duodenal samples were collected at 16, 20, and 24 h after dosing for the last 5 d and pooled. The methionine analog was not detected in duodenal contents because it passed rapidly from the rumen relative to the sampling protocol. In the second experiment, cows were offered 90 g of the methionine analog and 600 ml of Cr-EDTA (3.5 g of Cr) mixed with ground corn for a period of 20 min after which any remains of the treatment were placed in the rumen. The concentration of the analog peaked in ruminal and duodenal fluid at 1 and 3 h, respectively. Based on the fractional rate constants for ruminal and duodenal disappearance of the methionine analog and passage of the liquid, it was determined that 50.0 +/- 2.8% of the methionine analog escaped ruminal degradation and became available for intestinal absorption (44.6 +/- 5.7%) or was absorbed from the omasum (5.4 +/- 3.3%). Serum methionine concentration peaked 6 h after analog dosing at a level that was three times the predose level, indicating that the methionine analog that escaped ruminal degradation was absorbed and metabolized to methionine.

Six lactating, cannulated Holstein cows were used in a double 3 x 3 Latin square design to compare the effects of hull-less barley with barley and corn on ruminal fermentation, rate of passage, flow of nutrients to the duodenum, and milk production. Diets consisted of 60% concentrate, 30% barley silage, and 10% alfalfa hay (dry matter basis). Concentrates contained steam-rolled grains: hull-less barley, barley, or corn. Dry matter intake was unaffected by grain source, but starch intake tended to be greatest when hull-less barley or corn was fed. The barley diet was more degradable in the rumen than was the hull-less barley or corn diet, and, therefore, flow of microbial organic matter to the duodenum was greatest for cows fed the barley diet. Flow of microbial N to the duodenum was greater (50 g/d) for cows fed the barley diet than for cows fed the other diets, and the flow of ruminally undegradable N was greater (43 and 28 g/d) for cows fed the hull-less barley and corn diets, respectively, than for cows fed the barley diet. As a result, flow of nonammonia N to the duodenum was unaffected by grain source. Total tract apparent digestibility was highest for cows fed the barley and corn diets. Despite its low digestibility, cows fed the hull-less barley diet produced a similar amount of milk as did cows fed the barley and corn diets. Further studies are needed to evaluate the effects of processing hull-less barley on its utilization by dairy cows.

The effects of diet composition and chemical form of Se on intestinal flow, absorption, and retention of Se were determined in sheep by the balance technique and by disappearance of Se from sites along the gastrointestinal tract with reference to dual-phase digesta markers. Six sheep with ruminal and duodenal cannulas were used in a crossover design with a split-plot arrangement of the Se isotope treatments. Sheep were fed a forage (alfalfa hay)-based (.37 mg Se/kg) or concentrate (barley)-based (.27 mg Se/kg) diet at 90% of ad libitum intake. Selenium stable isotopes (enriched [77Se]yeast, enriched [82Se]selenite) and fluid (Co-EDTA) and particulate (Cr-mordanted fiber) markers were administered simultaneously into the rumen four times daily for 7 d, and total collections of feces and urine were made every 24 h for these and the following 7 d. A larger proportion (51 to 61%) of the Se tracers flowing to the duodenum was associated with the particulate fraction, mainly as bacteria-associated Se, than with the fluid fraction. The [82Se]selenite was more available (P < .05) for absorption and retention than [77Se]yeast, indicating that inorganic chemical forms of Se are as available to the ruminant as organic forms of Se commonly found in feedstuffs. Selenium absorption and retention were greater (P < .05) in sheep receiving the concentrate-based diet than in sheep receiving the forage-based diet. Thus, the availability of Se from inorganic and organic sources in sheep seems to be influenced by diet composition.

An inductively coupled plasma mass spectrometer with a newly designed continuous flow hydride generator was used for the determination of Se in biological materials. The design of the hydride generator was important in minimizing interference from HCl and in maximizing analytical sensitivity. Two sample preparation procedures incorporating either 3.8 or 7.2 M HCl in the final sample solutions were compared. Interference from Cu was eliminated by the addition of 0.2 M NaI to the sodium borohydride solution (3.8 M method) or by maintaining a high concentration of HCl in the sample solution (7.2 M method). The 3.8 M method had the advantage of minimizing exposure of expensive equipment to corrosive HCl fumes, whereas the 7.2 M method did not contaminate equipment with I and had no measurable sample-to-sample cross-contamination. In practice, cross-contamination from sample to sample in both methods was negligible during analysis. An important factor in minimizing cross-contamination from sample to sample was the elimination of the air bubble normally entrained between samples. Determination of isotopic tracer enrichment was linear from 0 to 320% enrichment, which provided a broad range for isotope dilution analysis. A detection limit of 6.4 pg of Se was observed under optimum conditions, whereas a detection limit of 1.3 ng of Se was found for routine analysis of 1-g samples of plant material. Selenium was accurately determined by isotope dilution analysis in a variety of biological reference materials.