PSII-9 Preliminary Investigation of Nutrient Challenge During Pregnancy on Wether Lamb Development, Growth, and Performance from Birth to Harvest

Inadequate nutrition during gestation has been shown to impair growth and metabolism of affected offspring and can predispose them to lifelong inefficiency and poor performance. Development throughout preweaning has been studied and compared to carcass performance; however, the impact of nutrient challenge during gestation on post weaning growth, efficiency, and nutrition utilization is not well known. Thus, our objective was to investigate the impacts of maternal nutrient restriction in late gestation on wether lambs' lifelong performance, from birth through harvest. We hypothesized that lambs from nutrient restricted dams would have reduced growth efficiency, taking longer to reach market weight and yielding altered body and carcass composition. Twenty-two bred yearling ewes were fed a diet meeting 100% of National Research Council (NRC) requirements until the 110th day of gestion (dGA). From 110 dGA until lambing occurred, ewes were either maintained on a diet meeting 100% of NRC requirements (n = 10) or reduced to a diet of 65% of NRC requirements (n = 12). Beginning at birth, body weight and morphometrics were collected weekly for control (CONT; n = 4) and treatment (NR; n = 5) wether lambs. At approximately 71 days of age, wethers were weaned and placed into individual pens to simulate a controlled feedlot setting. Wether lambs were fed a commercial lamb finishing diet ad libitum and orts were collected to evaluate feed efficiency and growth. At 115 days of age, wethers were placed in metabolism crates for 5 days to allow for total collection or urine and feces in order to evaluate nutrient balance. Birthweight, weaning weight, and morphometrics did not differ between NR and CONT lambs from birth until weaning. However, there was a tendency (P < 0.10) for a slower rate of gain from birth until weaning in NR lambs compared to their CONT counterparts. Current research in feedlot performance and digestibility is ongoing; however, thus far there are no differences in feed intake or average daily gain. These data suggest that a moderate nutrient challenge in the last month of gestation is not substantial enough to induce adaptive changes that result in differences in postnatal growth and performance.

PSIII-9 Hempseed Meal as an Effective Protein Supplement for Finishing Wethers

As our global agriculture landscape continues to change it has become more important than ever to find sustainable alternatives to feed livestock. Hempseed and its derivatives may provide alternative sources of nutrients for inclusion in livestock diets, however, due to the paucity of research on hemp and its byproducts, there is no authorization of the inclusion of these products in food animal diets. We hypothesized that the digestibility and use of hempseed meal would be similar to other livestock protein sources. Forty Western White-Faced wethers were used in a completely randomized block design with 5 treatments. These treatments included diets formulated to be near isonitrogenous with 0, 5, 10, 15, or 20% of diet DM as hemp seed meal, primarily as a substitute for dried distillers grains with solubles. Wethers were fed the diets individually for 90 d, which was followed by a 5 d balance trial with a total collection of urine and feces. There were no differences in DM intake (P = 0.44) or average daily gain (P= .16) between treatments. There were no differences in DM digestibility (P = 0.86) or N digestibility (P = 0.29) between treatments, although there was a slight increase in P digestibility as hemp meal inclusion increased until it represented 15% of the dietary DM (P = 0.02). There were no differences in the digestibility of Ca (P=0.44), Mg (P = 0.10), K (P = 0.85), or Na (P = 0.54). There were no differences in urinary N excretion (P=0.33) or urinary urea excretion (P=0.34) between treatments. Additionally, blood chemistry constituents were also not affected by treatment (P ≥ 0.10). Based on these data, it is concluded that hemp seed meal is a comparable protein supplement for sheep with no identified deleterious effects.

Shrink, weighing accuracy, and weighing precision of mineral supplement in five commercial dairies in the Western United States

Shrink, accuracy, and precision of ingredient weighing are critical factors of efficiency in TMR-fed dairy systems. Those factors have been evaluated for major feeds; however, we are not aware of any reports for mineral supplement. Farms commonly mix mineral supplement with other low-inclusion ingredients into a premix which is used later as a single ingredient for TMR formulation. Our objectives were to evaluate shrink, weighing accuracy, and weighing precision of mineral supplement during premix formulation, and variation in concentration of minerals in mineral supplement and TMR, in five large dairies in the Western United States. We used the automated weight-tracking system at each farm to account for all the mineral supplement loaded into the mixing-wagon and collected samples of mineral supplement and TMR from time of mineral supplement delivery at the farm until 100% of it was consumed. Mean, standard deviation and coefficient of variation (CV) for each variable were calculated with SAS 9.4. Average shrink was estimated at 2.0% for mineral supplement during storage and loading, ranging from 0.37% to 3.25%. Mineral supplement weighing deviation from the targeted amount was 1.54% on average for the five dairies with a 95% CV. Mineral composition of mineral supplements averaged 11.3%, 0.27%, and 3.16% for Ca, P, and Mg, and 215, 881, and 1533 ppm for Cu, Mn, and Zn, respectively. Mineral compositions in TMR averaged 0.84%, 0.41%, and 0.37% for Ca, P, and Mg, respectively; and 15.1, 71, and 94.5 ppm for Cu, Mn, and Zn, respectively. The CV of all minerals except Ca, were larger for mineral supplement than TMR, and with the exception of P in mineral supplement, CV of trace minerals were larger than CV values for macro minerals. Our shrink estimates for mineral supplement represent an initial approximation to this issue. Results of our weighing deviation analysis suggest some room for improvement on the precision of weighing mineral supplement at the time when premix is prepared at the farm, which could improve consistency in chemical composition of the premix and consequently reduce the variation (CV values) of mineral concentrations in TMR.

391 Linking Nutrition, Production, and Environmental Aspects of Ruminant Livestock Production

The relationship between nutrition, production and environmental aspects of ruminant livestock production systems is a complex and highly nuanced subject that has long suffered from oversimplification and assumptions associated within these practices. However, with the advent of new and evolving models and a growing understanding of these complexities and their interactions with each other, there has been a large and welcome growth in recent literature regarding new and emerging technologies, and insights that will allow for appropriate and impactful changes in livestock management that will affect overall change for the benefit of society as a whole. The primary means through which environmental impacts may be modified are separated into four distinct, but interconnected mechanisms, which include 1) improvements in use of dietary nutrients, 2) use of dietary additives that impact certain functions in the digestive tracts of the animal, 3) improvements in genetics, and 4) improvements in productive efficiency. While it is obvious that there are significant overlaps between these practices, it is imperative to consider all these aspects to prevent “leakage” of impacts to other industries and processes. In this presentation, we will review recent developments in all of these areas with a specific emphasis on the use of energy in ruminant production systems.

PSII-22 Effects of Clinoptilolite inclusion on Ruminant Invitro Digestibility

The aim of this study was to assess the effects of Zeolite (ZE, clinoptilolite) inclusion on in vitro rumen fermentation. A modified procedure from Tilley and Terry (1963) was used to determine alfalfa in vitro dry matter digestibility (IVDMD) in the presence or absence of ZE. Test tubes (n = 96) were placed in a 39 °C bath and were blocked based on hours 0, 1, 2, 4, 6, 8, 12, 24, and 48. Substrate consisted of dried alfalfa grinded through a 1mm screen using a Wily Mill. Treatments consisted of, Control (no ZE inclusion), and 0.05g ZE, for each 1 gram of alfalfa. After incubation, IVDMD was calculated 24 hours after drying period. Data was analyzed using a randomized block design. The IVDMD was found to be similar across all treatments (P > 0.8464). The IVDMD was significantly different when incubated for the varying lengths of time (P < 0.001). There was an IVDMD of 20.18 ± 2.89 % for the control and 18.07 ± 5.49 % for the ZE at the zero hour. The IVDMD for Control and ZE for 48 hours were 54.30 ± 1.58 % and 53. 48 ± 1.04 % respectively. To predict the IVDMD of the effects of ZE treatment on digestibility over time, the corresponding regression f(x) = 20.60 + 0.421X1 + 0.730X2 with R2 = 0.8464. In conclusion these data demonstrate that inclusion of ZE, does not influence the in vitro digestibility of alfalfa. These findings are important as previous data would indicate that inclusion of ZE may reduce cost of gain and/or influence liver abscess rates, it was uncertain if that would come at the cost of reduced digestibility of feed. These data indicate that there is likely very little to no impact on feed digestibility when ZE is included in the ration.

PSI-9 Effect of Pen-Surface Temperature and Trace Mineral Source on Infrared Hide Temperature in Cattle Raised in Confinement Under a Natural-Feeding Protocol in Summer

Thirty-two pens housing between 249–282 beef cattle each were blocked by arrival date and sex and used to investigate effects of pen-surface temperature, trace mineral source and sex on hide temperature. A randomized complete block design was used with 2 treatments: 1) Control, with inorganic sources for all trace minerals of interest; and 2) Test, with ProPath (Zinpro Corporation, Eden Prairie, MN) used to provide additional AA complexes of Zn and Mn, complexed Co, and ruminally-protected folic acid to basal control diet. Cattle were fed to meet requirements of JBS “Aspen Ridge” and observed twice monthly from June–September. Three observations per observation day were made at these times: 1) 0700–1000; 2) 1015–1315; 3) 1430–1700 (all times ± 30 min). Hide temperatures were observed caudal to glenohumeral joints of 10 black-hided and, when available, 10 non-black-hided animals per pen. Within each pen, surface temperatures were observed at 3 locations on the cement bunk apron and 7 locations on the dirt surface. Temperatures were observed using a Fluke VT04 visual infrared thermometer. Temperatures were greater on black-hided cattle than non-black-hided cattle (P < 0.0001) and on steers than heifers (P < 0.0001). Temperatures on Test cattle were greater (P = 0.0008) than temperatures on Control cattle, but this effect was small (0.251o C) and inconsistent across observation days (treatment within date interaction: P < 0.0001). Pen-surface temperatures were greater in Time 2 than Time 1 (P < 0.0001), but not different between Time 2 and Time 3 (P = 0.37). Hide temperatures on all cattle were correlated with pen-surface temperature (R2 = 0.43). Trace mineral source had a small but statistically significant effect on hide temperature. Correlations between hide temperature and visible signs of heat stress are unclear and warrant further observation.

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Evaluation of performance and carcass traits for a five-cohort All Heifer, No Cow beef production system demonstration herd

The All Heifer, No Cow (AHNC) beef production system is an alternative to conventional cow/calf production that involves insemination of nulliparous heifers with sexed semen to produce female calves that are early weaned at 3 mo of age. Dams are finished on a high-concentrate diet and harvested before reaching 30 mo of age. Objectives of this research were to document reproductive, feedyard, calf, and carcass performance of an AHNC herd; evaluate effects of carcass maturity on carcass quality; and determine if performance of initial cohorts (i.e., cohorts 1 and 2) differed from sustaining cohorts (i.e., cohorts 3-5). A total of 272 heifers were enrolled in the AHNC system via five annual cohorts. The system was initiated with 51 yearling, Angus-based heifers, and a replicate set (n = 56) was started 12 mo after. Heifers in cohorts 3 (n = 53), 4 (n = 56), and 5 (n = 56) were primarily offspring of prior cohorts (i.e., cohort 3 heifers born to cohort 1 females), but some were purchased to maintain inventory. Angus replacement heifers were purchased in cohorts 3 (n = 26), 4 (n = 26), and 5 (n = 28). Mean (±standard deviation) pregnancy rate at 30 d after fixed-time artificial insemination (AI) with sexed semen was 50.8% ± 9.4%, and 140-d pregnancy rate was 93.0% ± 1.5%. With AHNC, 61.0% ± 6.5% of females replaced themselves with a heifer. During finishing, average daily gain (ADG) was 1.9 ± 0.4 kg • d^-1 and dry matter intake (DMI) was 14.9 ± 1.9 kg • d^-1. Hot carcass weight (HCW) was 367 ± 35 kg. The USDA grading system classified 20.5% of all carcasses (n = 220) as C maturity (A⁰⁰ = 100, B⁰⁰ = 200, etc.), 62.4% ± 29.1% of carcasses as USDA Choice. USDA yield grade (YG) was 2.6 ± 0.7. Based on cohorts 1 and 2, there were no differences (P = 0.96) in Warner-Bratzler shear force values between A and B maturity vs. C maturity carcasses. Across all cohorts, there were no differences in USDA YG, marbling score (MA), and lean maturity between A and B maturity vs. C maturity carcasses; there were differences in age (P < 0.001), bone maturity (P < 0.001), and overall maturity (P <0.001). A comparison of initial vs. sustaining cohorts showed that initial cohorts had lower (P < 0.001) DMI, heavier (P < 0.001) HCW, and more advanced (P < 0.05) bone maturity. However, there were no differences for 30- and 140-d pregnancy rates, ADG, USDA YG, and MA between initial and sustaining cohorts. The AHNC beef production system can effectively produce female calves and quality carcasses for harvest.

Trace mineral source influences ruminal distribution of copper and zinc and their binding strength to ruminal digesta1,2,3

Eight crossbred steers (BW 719.0 ± 65.0 kg) with ruminal and duodenal cannulae were used to examine the effect of trace mineral (TM) source on digestibility; ruminal and duodenal solubility of Cu, Zn, and Mn; and in vitro release of Cu, Zn, and Mn from the solid fraction of ruminal digesta. Experiment 1 determined the effect of TM source on DM and NDF digestibility in steers fed a corn silage and steam-flaked corn-based diet. Treatments consisted of 10 mg Cu, 20 mg Mn, and 30 mg Zn/kg DM from either sulfate TM (STM) or hydroxy TM (HTM) sources. Following a 14-d adaptation period, total fecal output was collected for 5 d. Dry matter digestibility was not affected by treatment, but NDF digestibility tended (P < 0.09) to be greater in HTM vs. STM supplemented steers. In Exp. 2, steers were fed a diet without supplemental Cu, Zn, or Mn for 19 d. Steers were then administrated a pulse dose of STM or HTM (2× the National Research Council requirements for Cu, Mn, and Zn) via the rumen fistula. Ruminal and duodenal samples were obtained at 2-h intervals starting at -4 and ending at 24 h relative to dosing. Ruminal soluble Cu and Zn concentrations were affected by treatment, time, and treatment × time. Soluble concentrations and percent soluble Cu and Zn in ruminal digesta increased (P < 0.05) above 0-h values for 10 h following dosing with STM, but not HTM. Concentrations of Cu and Zn in ruminal solid digesta were also affected by treatment, time, and treatment × time. Steers dosed with STM had greater (P < 0.05) solid digesta Cu concentrations at 2 and 4 h but lesser (P < 0.05) concentrations from 6 to 20 h post-dosing than those receiving HTM. Ruminal solid digesta Zn concentrations were greater (P < 0.05) in HTM vs. STM-dosed steers from 6 through 24 h post-dosing. Distribution of Mn in ruminal digesta was affected by TM source, but to a lesser extent than Zn and Cu. Duodenal soluble TM concentrations were variable and not affected by treatment. Binding strength of TM to ruminal solid digesta was estimated at 0, 6, and 12 h post-dosing using dialysis against chelating agents. The percentage of Cu and Zn released from ruminal solid digesta by dialysis against Tris-EDTA was greater (P < 0.05) at 12 h post-dosing from steers receiving HTM vs. STM. Results indicate that Cu and Zn from HTM have low solubility in the rumen and appear to be less tightly bound to ruminal solid digesta than Cu and Zn from STM.

Sensor analysis and initial assessment of detectable first hoof contacts and last break-overs as unique signal fluctuations for equine gait analysis

The objective of the control study was to assess 2 prominent fluctuations in a single optical signal as being either a true first hoof contact or a last break-over based on descriptive measures. The study builds on initial findings from a preliminary investigation of the embedded-optical-base system’s (EOBS) capabilities in signal capturing and feasibility as potential alternative to existing gait technologies, such as piezoelectric (e.g., load cell) systems. Hoof contacts and break-overs were measured (0 to 1 au; arbitrary units) using a 2.4-m (length) × 0.9-m (width) platform containing 1 EOBS. Three mixed-breed horses (n = 3) were injected with saline or either 100 IU or 200 IU Botox (i.e., onabotulinumtoxinA) with a 2.5-mL final volume. Injections were made into the deep digital flexor muscle at the motor end plates, with electromyography and ultrasound guidance. Horses were observed for 3 time points (pre-, post-, and recovery test days) over the span of a 4-mo period. Signal fluctuations [i.e., amplitude of hoof impacts based on true first hoof contacts (ΔS_TS) and true last break-overs (ΔS_TL)] and kinematics [i.e., complete gait pass (CGP) time duration (T)] were recorded from each horse. Visual observations and video analysis were used for determining gait pattern categories. Individual horse measurements were analyzed for each trial, compared with video data and classified. Comparison of primary signal fluctuations (i.e., ΔS_TS vs. ΔS_TL; forelimb vs. hindlimb) exhibited significant differences between hoof contacts and break-overs (P < 0.05). Right and left forelimb hoof contacts and hindlimb break-overs were not significantly different (P = 0.966; 0.063 ± 0.135; Estimate ± SE; P = 0.606; 0.176 ± 0.142; Estimate ± SE, respectively). Additionally, treatment vs. saline forelimbs did not exhibit significant difference (P = 0.7407; −0.098 ± 0.279; Estimate ± SE). Overall, data showed that the EOBS can collect repeatable and unique primary signal fluctuations as prominent and different gait measurements providing evidence to further development and research of the sensing system.

Nitrous Oxide and Ammonia Emissions from Cattle Excreta on Shortgrass Steppe

Grazing cattle redistribute nitrogen (N) consumed in forage through urine and feces patches. The high concentration of N in these patches often exceeds the uptake demands of the local plant community, thereby providing ideal conditions for losses of reactive N. However, knowledge on nitrous oxide (NO) and ammonia (NH) emissions from excretal patches on shortgrass steppe grassland is limited. We studied the effect of cattle urine (1002 kg N ha) and feces (1021 kg N ha) patches on NO and NH emissions in two sites with contrasting vegetation: (i) cool-season (C3) 'Bozoisky-Select' Russian wildrye [ (Fisch.) Nevski], pasture (C3Past) and (ii) C4-dominated native shortgrass steppe rangeland (C4SS). Nitrous oxide and NH were measured using semi-static and semi-open chambers, respectively. Cumulative NO emissions were 217 and 173% greater and cumulative volatile NH emissions were 339 and 157% greater on C3Past compared with C4SS from the urine and feces treatments, respectively. Nitrous oxide emission factors were 0.20 and 0.05% for urine and 0.07 and 0.03% for feces on C3Past and C4SS, respectively. Our findings suggest that using the IPCC Tier 1 default emission factor (2%, 95% CI = 0.7-6%) to estimate NO emissions from cattle excretal patches on shortgrass steppe grassland would result in a significant overestimation for these dryland systems. Ammonia emission factors were 35 and 10% for urine and 7 and 5% for feces on C3Past and C4SS, respectively. With the exception of the urine treatment on C3Past, observed NH emissions were consistent with the IPCC Tier 1 default assumption that 20% (95% CI = 5-50%) of excretal N is volatilized as NH+NO.

Duration of maternal undernutrition differentially alters fetal growth and hormone concentrations

To investigate the impact of duration of maternal undernutrition in twin sheep pregnancies, ewes were either fed 100% (C) or 50% of their nutrient requirements from 28 to 78 d gestational age (dGA) and readjusted to 100% beginning at 79 dGA (LC) or continuously restricted from 28 to 135 dGA (LL). Weights of the fetus, empty carcass, brain, and liver were greater in the LC than LL fetuses at 135 dGA (P ≤ 0.05). Although umbilical vein (UmV) glucose concentrations did not differ, the UmV:umbilical artery (UmA) glucose gradient was smaller (0.26 ± 0.03 vs 0.38 ± 0.03 and 0.39 ± 0.04 mmol L(-1); P ≤ 0.05) in LL than C and LC fetuses, respectively. Umbilical vein concentrations of IGF-1 were less (46.7 ± 5.62 vs 74.3 ± 6.71 ng/mL; P ≤ 0.05) in LL than LC fetuses. Additionally, LL fetuses tended (P ≤ 0.10) to have lower UmA concentrations of insulin (0.24 ± 0.13 vs 0.70 ± 0.15 ng/mL) and IGF-1 (66.6 ± 7.51 vs 91.4 ± 8.97 ng/mL) than LC fetuses. Although most of the observed differences occurred between LC and LL pregnancies, LC fetuses tended (P ≤ 0.10) to have greater UmV and UmA pCO2 than C fetuses. Furthermore, the UmV:UmA O2 content gradient tended to be greater (5.02 ± 0.43 vs 3.41 ± 0.47; P ≤ 0.10) in C than LL fetuses. UmA placental lactogen also tended to be greater (46.6 ± 4.40 vs 31.1 ± 4.69 ng/mL; P ≤ 0.10) in LL than C fetuses. These data suggest that in twin pregnancies, maternal undernutrition followed by realimentation induces a different fetal outcome compared with continuous nutrient restriction, and both may differ physiologically from control fed pregnancies.

Evaluation of Saccharomyces cerevisiae fermentation product as an alternative to monensin on growth performance, cost of gain, and carcass characteristics of heavy-weight yearling beef steers

Two hundred fifty-two cross-bred yearling steers (406 ± 24 kg BW) were used in a completely randomized block design with a 2 × 2 factorial arrangement of treatments (7 pens/treatment) to evaluate the effects of dietary Saccharomyces cerevisiae fermentation product (SFP) and monensin (MON) on growth performance and carcass characteristics. Dietary treatments arranged as a 2 × 2 factorial were 1) with or without SFP and 2) with or without MON. Finishing diets contained 19.7% of DM as dried distiller's grains with solubles. Both SFP and MON were added in the total mixed ration in place of an equal amount of cornmeal (DM basis; target intake = 2.8 g of SFP and 33 mg of MON/kg of dietary DM). Each treatment group was offered ad libitum access to a transition ration from d 1 to 8 and then to the finishing ration from d 9 to 125. Body weights were collected on d 0, 28, 56, 84, 110, and 125. Initial and final BW was an average of 2-d weights (d -1 and 0 and d 124 and 125, respectively). Steers were shipped for harvest on d 125. Overall ADG was decreased (P = 0.03) in steers supplemented with SFP, but final BW was similar among treatments. Feeding SFP was associated with lighter (P < 0.01) HCW and a greater (P = 0.01) number of carcasses grading USDA Choice. Twelfth rib fat thickness was not affected by SFP (P = 0.82) or MON (P = 0.35), but numerical decreases in 12th rib fat thickness among cattle receiving SFP or MON alone contributed to a tendency (P = 0.07) for greater 12 rib fat thickness when SFP and MON were provided. There was no effect of treatment on cost of gain (P ≥ 0.21). The effects of SFP in the current study may have been limited in heavy yearling steers due to consumption of a finishing diet containing 19.7% dried distiller's grains with solubles.

The Modern Feedlot for Finishing Cattle

The modern beef feedlot has evolved into a complex system that is very dependent upon technology. Modern feedlots are organized into departments, often including the office, cattle, yard, feed milling, and feed departments, that allow for improvements in production efficiency through the specialization of management and labor. Regardless of size, feedlots must succeed at the following tasks: cattle procurement, cattle receiving, cattle processing, daily cattle observations, health treatments, cattle marketing, feed procurement, feed commodity receiving, feed commodity storage, diet formulation, diet delivery, bunk management, and environmental management. Apart from cattle ownership, feedlots create most of their gross income from feed sales, yardage, inventory gain on flaked grain, and combinations of these sources. The future of the industry is filled with economic and political challenges, including high grain prices owing to competition from the ethanol industry, environmental regulations, excess feedlot capacity, and a diminishing labor pool owing to declining rural populations.

Naturally colonized beef cattle populations fed combinations of yeast culture and an ionophore in finishing diets containing dried distiller's grains with solubles had similar fecal shedding of Escherichia coli O157:H7

Beef steers (n = 252) were used to evaluate the effects of dietary supplement on fecal shedding of Escherichia coli O157:H7. Seven pens of 9 steers (63 steers per treatment) were fed diets supplemented with or without yeast culture (YC) or monensin (MON) and their combination (YC × MON). YC and MON were offered at 2.8 g/kg and 33 mg/kg of dry matter intake, respectively. Environmental sponge samples (from each pen floor, feed bunk, and water trough) were collected on day 0. Rectal fecal grab samples were collected on days 0, 28, 56, 84, 110, and 125. Samples were collected and pooled by pen and analyzed for presumptive E. coli O157:H7 colonies, which were confirmed by a multiplex PCR assay and characterized by pulsed-field gel electrophoresis (PFGE) typing. On day 0, E. coli O157:H7 was detected in 7.0% of feed bunk samples and 14.3% of pen floor samples but in none of the water trough samples. The 71.4% prevalence of E. coli O157:H7 in fecal samples on day 0 decreased significantly (P < 0.05) over time. E. coli O157:H7 fecal shedding was not associated with dietary treatment (P > 0.05); however, in cattle fed YC and YC × MON fecal shedding was 0% by day 28. Eight Xba I PFGE subtypes were identified, and a predominant subtype and three closely related subtypes (differing by three or fewer bands) accounted for 78.7% of environmental and fecal isolates characterized. Results from this study indicate that feeding YC to cattle may numerically decrease but not eliminate fecal shedding of E. coli O157:H7 at the onset of treatment and that certain E. coli O157 subtypes found in the feedlot environment may persist in feedlot cattle.

Characterization and identification of hepatic mRNA related to copper metabolism and homeostasis in cattle

Copper is an essential trace mineral required for growth and development. Copper homeostasis within the cell is mediated by the expression of the Cu transporter protein (CTR1), ATPase7A (ATP7A), ATPase7B (ATP7B), Cox17, and Cu chaperone for Cu-Zn superoxide dismutase (CCS) which helps to regulate Cu uptake, export, and intracellular compartmentalization in non-ruminants. Copper also serves as a cofactor of antioxidant, superoxide dismutase1 (SOD1). Liver tissue from eighteen Holstein bull calves (average BW 201 +/- 58.5 kg, 7.3 +/- 1.9 months) from a previous experiment were utilized to characterize and identify hepatic mRNA related to Cu metabolism and homeostasis in cattle. Hepatic Cu concentration was determined via flame atomic absorption, and total RNA was extracted using TRI reagent and purified using RNeasy. Hepatic Cu concentrations ranged from 86 to 801 mg of Cu/kg DM. Real-time polymerase chain reaction analysis revealed that CTR1, ATP7A, and ATP7B mRNA expressions were negatively correlated with hepatic Cu concentration, while CCS (P = 0.0887) and SOD1 had a tendency (P = 0.0733) to be negatively correlated to hepatic Cu concentration. These data indicate that higher than normal hepatic Cu concentration downregulates gene expression of CTR1, ATP7A, ATP7B, and Cox17, which are involved in bovine liver copper homeostasis.

Urea metabolism in beef steers grazing Bermudagrass, Caucasian bluestem, or gamagrass pastures varying in plant morphology, protein content, and protein composition1,2,3,4

Pastures of Bermudagrass (Cynodon dactylon, BG), Caucasian bluestem (Bothriochloa caucasica, CBS), and gamagrass (Tripsacum dactyloides, GG) were evaluated from the perspectives of forage composition, selection during grazing, and N metabolism in beef steers. All pastures were fertilized with 78 kg/ha of N approximately 60 and 30 d before sample collection. In 2000 and 2001, 12 steers (250 kg of BW) were blocked based on BW and then assigned randomly to a replicated, randomized complete block design, with 2 pastures of each forage and 2 steers per pasture. Three other steers with esophageal fistulas were used to collect masticate samples to represent intake preferences. Herbage mass was >1,900 kg/ha. After at least 14 d of adaptation, urine and blood samples were collected for determination of serum urea N and percentage of urinary N in the form of urea. One steer per pasture (6 steers per year) was infused i.v. with (15,15)N urea for 50 h before collecting urine for 6 h to measure urea N enrichment, urea entry rate, urinary urea excretion, gut urea recycling, and return of urea N to the ornithine cycle. The canopy leaf:stem DM ratio differed (P = 0.01) among BG (0.50), CBS (1.01), and GG (4.00). Caucasian bluestem had less CP (% of DM) than GG or BG in the canopy (9.6 vs. 12.0 or 12.3, P = 0.07) and in the masticate (9.8 vs. 14.7 or 13.9, P = 0.04). Bermudagrass had less true protein (% of CP) than CBS or GG in the canopy (72.9 vs. 83.3 or 83.0, P = 0.07) and in the masticate (73.7 vs. 85.8 or 88.0, P = 0.04). Compared with GG and BG, CBS had less serum urea N (10.1 or 12.2 vs. 2.5 mM, P = 0.01), urea entry rate (353 or 391 vs. 209 mmol of N/h, P = 0.07), and urinary urea excretion (105 or 95 vs. 18 mmol of N/h, P = 0.04), and a greater return of urea N to the ornithine cycle as a proportion of gut urea recycling (0.109 or 0.118 vs. 0.231, P = 0.02). Urea production and recycling in these steers responded more to the N concentration in the grasses than to differences in plant protein fractions. There was no evidence of improved N capture by the steers due to changes in the leaf:stem ratio among the grasses at the herbage mass evaluated.

Fatty acid indices of stearoyl-CoA desaturase do not reflect actual stearoyl-CoA desaturase enzyme activities in adipose tissues of beef steers finished with corn-, flaxseed-, or sorghum-based diets

We hypothesized that stearoyl-CoA desaturase (SCD) enzyme activity would not correlate with fatty acid indices of SCD activity in steers fed different grains. Forty-five Angus steers (358 +/- 26 kg BW) were individually fed for 107 d diets differing in whole cottonseed (WCS) supplementation (0, 5, or 15% of DM) and grain source (rolled corn, flaxseed plus rolled corn, or ground sorghum grain) in a 3 x 3 factorial arrangement. Flaxseed- and corn-fed steers had greater (P < 0.01) G:F (0.119 and 0.108, respectively) than sorghum-fed steers (0.093). Marbling score was decreased by WCS (P = 0.04), and LM area was decreased (P < 0.01) by sorghum. Plasma 14:0, 16:0, 16:1n-7, and 18:2n-6 were greatest in corn-fed steers, whereas plasma 18:3n-3 and 20:5n-3 were greatest in the flax-seed-fed steers (P < 0.01). Plasma 18:1trans-11 was least in sorghum-fed steers, and plasma cis-9,trans-11 CLA was barely detectable, in spite of high intestinal mucosal SCD enzyme activity (118 to 141 nmol*g tissue(-1).7 min(-1)). Interfascicular (i.f.) and s.c. cis-9,trans-11 CLA remained unchanged (P > or = 0.25) by treatment, although 18:1trans-11 was increased (P < or = 0.02) in steers fed corn or flaxseed. Steers fed flaxseed also had greater (P < 0.01) i.f. and s.c. concentrations of 18:3n-3 than steers fed the other grain sources. Oleic acid (18:1n-9) was least and total SFA were greatest (P < 0.01) in i.f. adipose tissue of steers fed 15% WCS. Lipogenesis from acetate in s.c. adipose tissue was greater (P < 0.01) in flaxseed-fed steers than in the corn- or sorghum-fed steers. Steers fed flaxseed or corn had larger i.f. mean adipocyte volumes (P < 0.01) than those fed sorghum and tended (P = 0.07) to have larger s.c. adipocyte volumes. Several fatty acid indices of SCD enzyme activity were decreased (P < or = 0.03) by WCS in i.f. adipose tissue, including the 18:2cis-9,trans-11/ 18:1trans-11 ratio. The 18:2cis-9,trans-11/18:1trans-11 ratio also tended to be decreased (P = 0.09) in s.c. adipose tissue by flaxseed; however, SCD enzyme activities in i.f. and s.c. adipose tissue were not affected by dietary WCS (P > or = 0.47) or grain source (P > or = 0.37). Differences in SFA seemed to be independent of SCD enzyme activity in both adipose tissues, suggesting that duodenal concentrations of fatty acids were more important in determining tissue fatty acid concentrations than endogenous desaturation by SCD.

The influence of oscillating dietary protein concentrations on finishing cattle. I. Feedlot performance and odorous compound production1,2

We hypothesized that oscillation of the dietary CP concentration, which may improve N retention of finishing beef steers, would reduce production of manure odor compounds and total N inputs while yielding comparable performance. Charolais-sired steers (n = 144; 303 +/- 5 kg of initial BW) were used in a completely randomized block design (6 pens/treatment). The steers were fed to 567 kg of BW on the following finishing diets, which were based on dry-rolled corn: 1) low (9.1% CP), 2) medium (11.8% CP), 3) high (14.9% CP), or 4) low and high oscillated on a 48-h interval for each feed (oscillating). Steers fed low tended (P = 0.08) to have less DMI (7.80 kg/d) than steers fed medium (8.60 kg/d) or oscillating (8.67 kg/d), but not less than steers fed high (8.12 kg/d). Daily N intake was greatest (P < 0.01) for steers fed high (189 g), intermediate for medium (160 g) and oscillating (164 g), and least for low (113 g). The ADG was lower (P < 0.01) for steers fed low (1.03 kg) than for those fed medium (1.45 kg), high (1.45 kg), or oscillating (1.43 kg). Similarly, steers fed low had a lower adjusted fat thickness (P < 0.01) and yield grade (P = 0.05) and tended (P = 0.10) to have less marbling than steers fed the other 3 diets. In slurries with feces, urine, soil, and water, incubated for 35 d, nonsoluble CP was similar among slurries from steers fed medium, high, or oscillating, but was less (P < 0.01) in slurries from steers fed low. However, throughout the incubation period, slurries from steers fed high or oscillating had greater (P < 0.01) concentrations of total aromatics and ammonia than those from steers fed low or medium. Also, the slurries from steers fed oscillating had greater (P < 0.01) concentrations of branched-chain VFA than manure slurries from steers fed any of the other diets. These data indicate that although there is no apparent alteration in the performance of finishing steers fed diets with oscillation of the dietary protein, there may be undesirable increases in the production of compounds associated with malodor.

The influence of oscillating dietary protein concentrations on finishing cattle. II. Nutrient retention and ammonia emissions

We hypothesized that oscillation of the dietary CP concentrations would improve efficiency of N use and reduce N loss to the environment. Charolais-cross steers (n = 8; 315 +/- 21 kg of BW) were used in a replicated 4 x 4 Latin square design. The steers were allowed ad libitum access to the following diets: 1) 9.1% CP (low), 2) 11.8% CP (medium), 3) 13.9% CP (high), or 4) low and high oscillated on a 48-h interval on each diet (oscillating). Dry matter intake did not differ among treatments (P = 0.46), but N intake differed (P < 0.01) from 94 (low) to 131 (medium), 142 (high), and 133 g/d (oscillating), as designed. Dry matter digestibility increased (P < 0.01) from 71.8% (low) to 75.8% (medium), 77.7% (high), and 77.5% (oscillating). Nitrogen digestibility increased (P < 0.01) from 62.2% (low) to 67.2% (medium) to 70.1% (high) and 70.9% (oscillating). Nitrogen retention was greater (P < 0.01) in steers fed oscillating (55.0 g/d) than in the steers fed low (34.8 g/ d) or high (40.2 g/d), but N retention of steers fed medium (49.8 g/d) differed (P = 0.02) only from that of steers fed low. Urinary urea N did not differ between steers fed medium (19.5 g/d) or oscillating (21.3 g/d) but was lowest (P < 0.01) for those fed low (8.2 g/d) and greatest for those fed high (39.2 g/d). Daily heat production (kcal/BW(0.75)) tended (P = 0.09) to be less for the steers fed low (177) than those fed medium (189), high (188), or oscillating (182). Cumulative in vitro ammonia volatilization from the manure of steers fed oscillating was lower (P < 0.01) for the initial 5 d of incubation than from manure of those fed medium, but there was no difference after 11 d of incubation. Additionally, there was a decrease (P < 0.01) in in vitro ammonia volatilization as protein concentration in the diet decreased from high to medium to low. These data indicate that oscillation of the dietary protein improved N retention of finishing steers compared with those in high and low N diets and that these changes were great enough to correspondingly alter ammonia volatilization from manure.

Dietary supplementation of high levels of saturated and monounsaturated fatty acids to ewes during late gestation reduces thermogenesis in newborn lambs by depressing fatty acid oxidation in perirenal brown adipose tissue

We hypothesized that dietary supplementation of (n-6) plus (n-3) PUFA during late gestation would increase uncoupling protein-1 (UCP1) gene expression and thereby increase thermogenic capacity of newborn lambs. Thirty twin-bearing ewes were fed rumen-protected fat (2, 4, or 8%) high in saturated and monounsaturated fatty acids (SMFA) or high in (n-6) and (n-3) PUFA. Lambs (n = 7-10 per ewe treatment group) were placed in a cold chamber at 0 degrees C for 2 h. Rectal temperature was higher at birth and increased more with cold exposure in lambs from ewes fed 2 or 4% supplemental fat than in lambs from ewes fed 8% SMFA (fat type x fat level interaction, P = 0.001). Cytochrome c oxidase activity was greatest in brown adipose tissue (BAT) lambs from ewes fed 2% SMFA or 4% PUFA (fat type x fat level interaction, P = 0.01). BAT of lambs from ewes fed 2 or 4% PUFA had nearly 7-fold more (P = 0.05) UCP1 mRNA than BAT of lambs from ewes fed 8% PUFA. UCP1 expression decreased by over 80% by 24 h of age. Supplementation of 8% fat tended to depress palmitate esterification into lipids (P = 0.07) and decreased palmitate oxidation (P = 0.003) in lamb BAT in vitro, especially in those lambs from ewes fed 8% SMFA. Thus, supplementing the diets of ewes with 8% SMFA depressed cold tolerance in newborn lambs, which was reflected in their decreased ability to oxidize fatty acids in vitro.

Net portal and hepatic flux of nutrients in growing wethers fed high-concentrate diets with oscillating protein concentrations

We hypothesized that oscillating dietary CP would improve N retention by increasing the uptake of endogenous urea N by portal drained viscera (PDV), compared with static dietary CP regimens. Chronic indwelling catheters were surgically implanted in the abdominal aorta, a mesenteric vein, a hepatic vein, and the portal vein of 18 growing Dorset x Suffolk wethers (44.6 +/- 3.6 kg of BW). Wethers had ad libitum access to the following diets in a completely randomized block design: 1) Low (9.9% CP), 2) Medium (12.5% CP), or 3) Low and High (14.2% CP) diets oscillated on a 48-h interval (Osc). Dry matter intake was greater (P = 0.04) for the Osc diet (1,313 g/d) than the Low diet (987 g/d) and was intermediate for the Medium diet (1,112 g/d). Nitrogen intake was not different between the wethers fed the Osc (25.4 g/d) and Medium diets (22.2 g/d), but was lower (P < 0.01) in wethers fed the Low diet (16.0 g/d). Wethers fed the Osc diet (6.7 g/d) retained more (P < 0.04) N than did those fed the Medium diet (4.0 g/d). Hepatic arterial blood flow was not different (P = 0.81) between wethers fed the Osc (31 L/h) or Medium diet (39 L/h) but was greater (P = 0.05) in wethers fed the Low diet (66 L/h). Net release of alpha-amino N by the PDV did not differ (P = 0.90) between the Low (37.8 mmol/h) and Medium diets (41.5 mmol/h) or between the Osc (53.0 mmol/h) and Medium diets (P = 0.29). Net PDV release of ammonia N was less (P = 0.05) for the Low diet than for the Medium diet, and this was accompanied by a similar decrease (P = 0.04) in hepatic ammonia N uptake. Urea N concentrations tended to be (P = 0.06) less in arterial, portal, and hepatic blood in wethers fed the Low diet compared with those fed the Medium diet. Wethers fed the Osc diet tended (P = 0.06) to have a greater PDV uptake of urea N than did those fed the Medium diet, but there was no difference between the Osc and Medium diets (P = 0.72) in hepatic urea N release. Net PDV uptake of glutamine tended to be greater (P < 0.07) in wethers fed the Low diet (6.7 mmol/h) than those fed the Medium diet (2.7 mmol/h). These data indicate that oscillating dietary protein may improve N retention by increasing endogenous urea N uptake by the gastrointestinal tract.

Influence of genotype and diet on steer performance, manure odor, and carriage of pathogenic and other fecal bacteria. II. Pathogenic and other fecal bacteria1,2

This study assessed the influence of cattle genotype and diet on the carriage and shedding of zoonotic bacterial pathogens and levels of generic Escherichia coli in feces and ruminal contents of beef cattle during the growing and finishing periods. Fifty-one steers of varying proportions of Brahman and MARC III [0 (15), 1/4 (20), 1/2 (7), and 3/4 Brahman (9)] genotypes were divided among 8 pens, such that each breed type was represented in each pen. Four pens each were assigned to 1 of 2 diets [100% chopped bromegrass hay or a diet composed primarily of corn silage (87%)] that were individually fed for a 119-d growing period, at which time the steers were switched to the same high-concentrate, corn-based finishing diet and fed to a target weight of 560 kg. Feces or ruminal fluid were collected and analyzed at alternating intervals of 14 d or less. Generic E. coli concentrations in feces or ruminal fluid did not differ (P > 0.10) by genotype or by growing diet in the growing or finishing periods. However, the concentrations in both feces and ruminal fluid increased in all cattle when switched to the same high-corn diet in the finishing period. There was no effect (P > 0.25) of diet or genotype during either period on E. coli O157 shedding in feces. Forty-one percent of the steers were positive for Campylobacter spp. at least once during the study, and repeated isolations of Campylobacter spp. from the same steer were common. These repeated isolations from the same animals may be responsible for the apparent diet (P = 0.05) and genotype effects (P = 0.02) on Campylobacter in feces in the finishing period. Cells bearing stx genes were detected frequently in both feces (22.5%) and ruminal fluid (19.6%). The number of stx-positive fecal samples was greater (P < 0.05) for 1/2 Brahman steers (42.9%) than for 1/4 Brahman (25.0%) or 3/4 Brahman steers (22.2%), but were not different compared with MARC III steers (38.3%). The greater feed consumption of 1/2 Brahman and MARC III steers may have resulted in greater starch passage into the colon, accompanied by an increase in fecal bacterial populations, which may have further improved the ability to detect stx genes in these cattle. There was no correlation between either ADG or daily DMI and the number of positive samples of E. coli O157, Campylobacter spp., or stx genes, which agrees with our current understanding that these microorganisms occur commonly in, and with no measurable detriment to, healthy cattle.

Influence of genotype and diet on steer performance, manure odor, and carriage of pathogenic and other fecal bacteria. III. Odorous compound production1,2

Three beef cattle diets were assessed for their potential to produce odorous compounds from cattle feces excreted during the growing and finishing periods. Eight pens containing 51 steers of varying proportions of Brahman and MARC III genotypes were fed either a chopped bromegrass hay diet or a corn silage diet for a 119-d growing period. After the growing period, all steers were switched to the same high-corn finishing diet (high corn) and fed to a target weight of 560 kg (finishing period). Fecal slurries were prepared from a composite of fresh fecal pats collected in each pen during both periods and incubated anaerobically. In additional flasks, starch, protein, or cellulose was added to the composite fecal subsamples to determine the preferred substrates for fermentation and odorous compound production. The content and composition of the fermentation products varied both initially and during the incubation, depending on the diet fed to the steers. The corn silage and high corn feces had the greater initial content of VFA (381.0 and 524.4 micromol/g of DM, respectively) compared with the bromegrass feces (139.3 micromol/g of DM) and accumulated more VFA than the bromegrass feces during the incubation. l-Lactic acid and VFA accumulation in the high corn and corn silage feces was at the expense of starch, based on starch loss and the production of straight-chain VFA. In the bromegrass feces, accumulation of branched-chain VFA and aromatic compounds and the low starch availability indicated that the protein in the feces was the primary source for odorous compound production. Substrate additions confirmed these conclusions. We conclude that starch availability was the primary factor determining accumulation and composition of malodorous fermentation products, and when starch was unavailable, fecal microorganisms utilized protein.

Feeding high-moisture corn instead of dry-rolled corn reduces odorous compound production in manure of finishing beef cattle without decreasing performance1,2,3

We hypothesized that feeding steers ground high-moisture ensiled corn (HMC) in lieu of dry-rolled corn (DRC) would reduce the amount of starch being excreted in the manure and the associated odorous compound production. One hundred forty-eight crossbred steers (363 +/- 33 kg of BW) were fed a DRC-or HMC-based diet in a feeding trial, and 8 Charolais-sired steers (447 +/- 22 kg of BW) were used in a nutrient balance study. Steers fed HMC tended to have a slightly lower DMI (P = 0.09), ADG (P = 0.06), and yield grade, but G:F, final HCW, and quality grade did not differ (P > or = 0.23) between treatments. Compared with feeding DRC, feeding HMC decreased (P = 0.02) starch intake from 5,407 to 4,846 g/d, decreased (P < 0.01) fecal excretion of starch from 448 to 292 g/d, and increased (P = 0.03) starch digestibility from 91.7 to 94.1%. Nitrogen intake was greater (P < 0.01) for steers fed DRC than HMC in both studies, but N retention did not differ (P = 0.55). Heat production and energy retention did not differ between the 2 treatments (P > or = 0.55). In manure slurries incubated for 35 d with soil and water, total VFA concentration was lower (P < 0.01) in manure from steers fed HMC (1,625 micromol/g of DM) compared with steers fed DRC (3,041 micromol/g of DM). Lower initial (d 0) starch concentrations and greater initial pH was also observed in the slurries from the HMC manure. By d 3 of slurry incubation, there was an increase (P < 0.01) in free glucose and l-lactic acid in the DRC slurries but not in the HMC slurries. During manure incubation, alcohol and VFA content increased (P < 0.01) and pH declined, but to a lesser extent (P < 0.01) in the HMC slurries. However, branched-chain VFA increased more (P < 0.01) in the HMC slurries than in the DRC slurries. These data suggest that feeding HMC instead of DRC decreased fecal starch and production of some potentially odorous compounds in a finishing cattle system but had little impact on animal productivity.

Nitrogen metabolism of beef steers fed endophyte-free tall fescue hay: effects of ruminally protected methionine supplementation

Level of nitrogen (N) intake and ruminally protected methionine supplementation were evaluated in eight Angus growing steers (initial BW 253+/-21 kg, final BW 296+/-21 kg) in a replicated 4+/-4 Latin square design. The steers were fed two endophyte-free tall fescue (Festuca arundinacea) hays that contained 2.2 (LO) or 2.8% (HI) of DM as N and were either supplemented or not with ruminally protected methionine (10 g metabolizable methionine/d). Diets were fed to provide adequate energy for 0.5 kg ADG and sufficient protein for maintenance (LO), or protein to support 0.5 kg ADG (HI). Following at least 14 d of adjustment, N balance was measured for 6 d. Isotopic urea was infused (15N15N-urea, 0.164 mmol urea N/h) via a jugular catheter for 56 h and urine was collected from 48 to 56 h to measure urea kinetics. Jugular blood was collected during the balance trial, and serum was analyzed for serum urea N (SUN). By design, daily N intake was greater (P < 0.05) for HI (112 g) than for LO (89 g). Compared with LO, steers when fed HI had greater (P < 0.05) daily DMI (4,217 vs 4,151 g), fecal N (34.4 vs 31.2 g), N digested (77.1 vs 57.7 g), urine N (48.3 vs 37.5 g), urine urea N excretion (34.6 vs 24.8 g), and N retained (29.8 vs 21.1 g). When fed HI steers also had higher (P < 0.05) urine urea N concentration (276 vs 219 mM), SUN (8.7 vs 6.7 mM), N digestibility (69.1 vs 64.9%), percentage of urinary N present as urea (71.5 vs 66.7%, P < 0.053), and rate of urea N production (59.6 vs 49.2 g/d) but lower (P < 0.05) percentage of urea N produced that was returned to the ornithine cycle (15.03 vs 19.2 1%) than when fed LO. Methionine supplementation decreased daily urine N (41.2 vs 44.6 g, P = 0.10) and increased both the amount of N retained daily (27.9 vs 23.7 g, P < 0.089) and the percentage of N digested that was retained (40.4 vs 34.6%, P < 0.094). In summary, supplemental methionine met a specific dietary limitation by increasing the amount of digested N that was retained by the steers.

Urea flux in beef steers: effects of forage species and nitrogen fertilization

The effects of two forage species and N levels on urea kinetics and whole-body N metabolism were evaluated in eight Angus steers (initial BW 217+/-15 kg). In a replicated, 4 x 4 Latin square design, steers were fed gamagrass (Tripsacum dactyloides L.) or switchgrass (Panicum virgatum L.), each of which had 56.2 (LO) or 168.5 (HI) kg of N fertilization per hectare. Diets provided adequate energy for 0.5 kg ADG. Nitrogen balance and urea kinetics were measured from d 22 to 27 of each period. Urine samples collected during intravenous infusion of bis 15N urea were used to calculate production and recycling of urea N from relative abundance of urea isotopomers. Jugular blood serum was analyzed for serum urea N (SUN). Gamagrass differed from switchgrass (P < 0.05) in daily DMI (4,273 vs 4,185 g), N intake (72 vs 67 g), DM digestibility (61.0 vs 63.6%), fecal N (30.6 vs 28.3 g/d), urine urea N (10.5 vs 8.0 g/d), and percentage of urinary N present as urea N (53.5 vs 40.0%). After adjustment for differences in N intake, fecal N still tended to be greater (P < 0.09) for gamagrass than for switchgrass. The LO differed from the HI (P < 0.01) in daily N intake (63 vs 76 g), DM digestibility (61.3 vs 63.3%), urine N (13.6 vs 25.9 g/d), and N retained as a percentage of N digested (57.3 vs 43.5%). Compared to switchgrass, gamagrass had greater SUN, N digestibility, and N digested as N level increased (forage x N level interactions, P < 0.05). As N level increased, N retention increased from 19.5 to 23.5 g/d in gamagrass and decreased from 20.5 to 18.1 g/d in switchgrass (interaction, P < 0.07). The HI group was greater than the LO intake group (P < 0.03) in endogenous production of urea N (44.4 vs 34.0 g/d), gut entry rate of urea N (31.6 vs 28.2 g/d), and the amount of urea N that re-entered the ornithine cycle (9.4 vs 7.9 g/d). However, the percentage of urea N entering the gastrointestinal tract that was recycled was constant among treatments (29.1%), indicating that almost 70% of the urea N that entered the gastrointestinal tract was potentially available for anabolic purposes of the steers as a component of microbial products that were absorbed or excreted in the feces. In summary, N levels affected N metabolism of steers more when they were fed gamagrass than when they were fed switchgrass. Although the absolute amounts of N moving through the system changed with variations in intake, the proportions remained similar, with a greater efficiency of N use at low N intakes.