Physical and mechanical characteristics of tibias from transgenic mice expressing mutant bovine growth hormone genes.

Physical and mechanical characteristics of tibia from mice expressing either the M4, M11, or G119K mutant bovine growth hormone (bGH) gene and displaying large, near-normal, or small-size phenotypes, respectively, were compared to those of non-transgenic, control mice (NTC). Three animals of each strain were euthanized at 28, 38, 48, 58, and 68 days of age. Variables were regressed against age to establish the pattern of change throughout the experiment, and the regression results are presented. Tibias from G119K were shorter (13.1 mm) and lighter (37.3 mg) than those from other strains, and M4 tibias were heavier (87.9 mg) and longer (16.6 mm) at 70 days of age. The ratio of tibia length to body weight suggests longitudinal bone growth was not reduced as much as overall growth in G119K mice. The external and internal dimensions of the G119K tibias were smaller than the other strains whereas the M4 tibias were somewhat larger. Differences in physical dimensions between the NTC and M11 mice did not greatly affect bone mechanical characteristics. Tibias from M4 mice resisted more load at both flexure and breaking compared to the other strains. At 50 days of age, stress at flexure was greater at all ages for G119K mice (12.4 kg/mm2) and was decreased in M4 mice (8.5 kg/mm2). The bGH mutations produce different effects on bone growth and its mechanical characteristics. There also may be differential tissue responsiveness to the mutant bGH analogs, as longitudinal growth was not as affected as empty body growth in the G119K mice. These transgenic mouse strains provide valuable models to study bone growth, formation, and reformation in response to GH regulation, and more importantly, the M4 and G119K mice may serve as a model in which the priorities for GH action on bone vs muscle may be determined.

Determining effects of monensin on energy value of corn silage diets for beef cattle by linear or semi-log methods.

An energy balance experiment was conducted to examine the effects of monensin on energy partitioning of cattle fed corn silage diets. One hundred and thirty Hereford steer feeder calves (average initial weight 247 kg) were used. Cattle were fed ad libitum for 211 days or limit-fed for 253 days. Carcass specific gravity was measured on all cattle for calculation of energy and tissue retention conclusions on energy utilization were based on linear regressions of energy retention on metabolizable energy and dry matter intake above maintenance, with comparisons to semilog calculations evaluated. Rates of live weight gain for ad libitum-fed cattle were similar with and without monensin (920 vs 923 g/day), while dry matter consumption was significantly reduced (5.91%) with monensin. Feed conversion was improved (5.84%) with monensin. Terminal body composition an total energy retained were not significantly altered by monensin. Limiting energy intake reduced daily rates of fat deposition more (-47.3%) than it reduced protein (-25.1%). Limit-fed steers had 10% less fat than ad libitum fed steers at equal empty body weights. Monensin tended to increase rates of protein and decrease rates of fat deposition in limit-fed cattle, and it induced a change (P < .08) in composition of growth. Monensin increased the apparent efficiency of energy use for maintenance by 5.7% but did not alter the efficiency of energy use for growth. Dry matter needed for maintenance was reduced by 5.4% with monensin. These data indicate that monensin may decrease maintenance requirements and (or) increase efficiency of diet energy use for maintenance.

Relationship of anabolic status and phase and rate of growth to priorities for protein and fat deposition in steers.

The effects of anabolic implants, growth phase (growing vs finishing) and rate of growth on the priorities for protein and fat deposition were determined in yearling cattle. Santa Gertrudis crossbred yearling steers weighing 290 kg were individually fed diets varying in forage and grain content and either not implanted (n = 16) or implanted (90-d intervals) with Ralgro (n = 13) or Synovex-S (n = 12) implants. The cattle were fed toward a similar expected final empty BW (455 kg). Initial and interim empty body composition was measured via deuterium oxide dilution; final composition was determined by carcass specific gravity. During a 100-d growing phase, rates of protein gain were increased (P less than .12) to 118 and 131 g/d from 98 g/d for Ralgro and Synovex vs nonimplanted cattle, respectively. Concurrently, the fraction of protein in empty body growth was increased (P less than .09) from 17.5% for controls to 23.8 and 19.7% for Ralgro- and Synovex-implanted steers, respectively. This change in protein growth occurred concomitant with mobilization of fat and a reduction (P less than .04) in fat gain with Ralgro and Synovex implants. During the 136-d finishing phase, protein accretion was 115 and 132 vs 93 g/d for Ralgro- and Synovex-implanted cattle vs nonimplanted cattle; this represented a 24 and 42% increase (P less than .03) with Ralgro and Synovex, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Anabolic effects on rate, composition and energetic efficiency of growth in cattle fed forage and grain diets.

The effects of anabolic implants on rate, composition and energetic efficiency of growth were determined in steers fed diets varying in forage and grain content. Santa Gertrudis-cross steers averaging 337 kg were group-fed (n = 72) or individually fed (n = 45) ad libitum one of three diets and either not implanted or implanted (90-d intervals) with Ralgro or Synovex-S implants. Steers were fed to a similar empty body weight (463 kg). Initial empty body composition of individually fed steers was determined via D2O dilution, and final composition of all steers was determined by carcass specific gravity. Rate of empty body gain increased (P less than .05) from 695 g/d for nonimplanted steers to 798 and 844 g/d for Ralgro- and Synovex-implanted steers. Anabolic implants increased (P less than .01) daily empty body protein gain from 91 to 119 and 133 g for Ralgro and Synovex, an increase of 31 and 46%, respectively. The fraction of protein in empty body gain increased (P less than .01) from 13.8% to 15.6 and 15.9%, and the percentage of fat in empty body gain decreased (P less than .01) from 41.7% to 32.9 and 31.3% with Ralgro and Synovex, respectively. Daily rates of protein deposition increased at a decreasing rate, and rates of fat deposition increased at an increasing rate with increasing rate of empty body gain. Implanted steers deposited more protein and less fat at any rate of growth; the magnitude of this shift in nutrient partitioning from fat to protein growth increased with rate of growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dietary monensin and potassium on apparent absorption of magnesium and other macroelements in sheep.

Twelve ruminally cannulated lambs (average weight, 31.1 kg) were used to determine the effects of monensin and K on apparent absorption of Mg in lambs. Lambs were assigned randomly to two groups and fed a basal high concentrate diet containing .44% K with or without 20 mg/kg monensin for 19 d of dietary adjustment. During adjustment, lambs were housed by treatment group and given ad libitum access to their diets. Following dietary adjustment, lambs were placed in individual metabolism stalls and fed 450 g of their respective diet twice daily. Within each monensin group, lambs were infused ruminally with 0, 7.6 or 31.6 g K/d (equal amounts at each feeding in 158 ml of distilled-deionized water) during three periods in a 3 X 3 Latin-square arrangement of K treatments. Each period consisted of a 10-d preliminary phase and a 7-d collection phase followed by a 3-d D2O infusion phase to determine water turnover. Addition of monensin to the diet decreased (P less than .05) fecal Mg excretion 15.9%. Apparent absorption and retention of Mg were increased (P less than .05) with the addition of monensin. Increasing K level increased (P less than .05) fecal Mg excretion. Apparent absorption of Mg decreased (P less than .05) from .93 to .80 g/d when either level of K was infused into the rumen. Monensin decreased the acetate:propionate (A:P) ratio. There was a significant interaction between monensin and K level for acetate and propionate (molar %), and A:P ratio. In the presence of higher K concentrations, monensin appears to be more effective in decreasing the A:P ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of deuterium oxide to estimate body composition of growing swine.

An experiment involving 73 pigs was conducted to evaluate the efficacy of the deuterium oxide (D2O) dilution procedure for in vivo estimation of body composition in swine. Pigs were infused with known quantities of D2O at 6.4 and 18 kg and then at approximately 18 kg increments to 145 kg body weight. Postinfusion blood D2O concentrations were partitioned into early- and total-equilibrating pools. Results from D2O analyses were compared with empty (ingesta-free) and total body water values obtained from chemical analysis of the ground animals. Both early- and total-equilibrating D2O pools seemed to be good estimators of empty body weight. Total body D2O space was similar to total body water at 6.4 kg, but consistently overestimated body water (averaged 20%) as the pigs attained heavier weights. Early- or total-body D2O pool measurements accurately predicted chemically determined components with residual coefficients of variation below 5% and R2 greater than or equal to .90. However, total body D2O space provided as accurate an estimate of empty body water as the early-pool technique and may be easier to use because catheterization of blood vessels is not required. The incorporation of live weight and sex in multiple regression equations did not improve the accuracy of empty body water prediction over that obtained from D2O measurements alone. A method to estimate total body composition (water, protein, fat, ash) from the total body D2O space measurements is presented.

In vivo body composition estimation in nongravid and reproducing first-litter sows with deuterium oxide.

An experiment was conducted with 64 first-litter sows to evaluate the efficacy of a D2O dilution procedure for measuring in vivo body composition during the reproduction cycle. Eight gilts were each infused at breeding, 57 and 105 d postcoitum and at 5 and 25 d postpartum, with equivalent numbers of nongravid controls infused at corresponding periods except at 5 d postpartum. Results from D2O dilution were compared with body water estimates obtained from chemical analysis. An early-equilibrating D2O pool (before 15 min) was similar quantitatively to empty body (ingesta free) water in nongravid and lactating animals, but not in pregnant sows. Because of inconsistent D2O equilibration patterns in gravid sows, the early pool was considered to have equilibrated with part but not all of the water in the conceptus products. Total body D2O space measurement obtained from data following equilibration of D2O in the entire body (1 to 2 h) overestimated total body water (including gastrointestinal water) by approximately 19%. Coefficients of determination for equations relating total body D2O space to empty body and maternal body water were .96 and .88, respectively, in gestating sows and .67 and .74, respectively, for lactating sows, while coefficients of variation were below 6% in all cases. Prediction equations were developed to estimate empty and maternal body components (protein, fat and ash) from body weight and D2O space. Accuracy of protein and ash weight prediction is lowest with this procedure because it involves the composite error of estimation of the other body components.

Site and extent of apparent magnesium absorption by lambs fed different sources of magnesium.

Ten abomasally cannulated crossbred wether lambs (avg wt, 33 kg) were used in a replicated 5 x 5 latin square design to determine the site and extent of apparent absorption of Mg when fed different sources of Mg. Lambs were fed twice daily 220 g of chopped mixed grass hay and 180 g of a corn-based supplement (control; .13% mg, DM basis), or the control diet supplemented with Mg (.26% Mg, DM basis) from MgO, magnesium citrate (MgC), smectite-vermiculite (Mg-Mica) or MgOH. Lambs were maintained in metabolism stalls during each of the five experimental periods. Each period consisted of a 7-d dietary adjustment followed by a 3-d collection of abomasal samples, feces and urine. Abomasal contents were sampled four times daily during the 3-d collection period. The diet contained .5% chromium oxide as a digestion marker. Apparent absorption of Mg was .17, .55, .85, .78 and .82 g/d for lambs fed the control, MgO, MgC, Mg-Mica and MgOH diets, respectively. Apparent absorption of Mg (g/d) was similar (P greater than .05) in the lambs fed the supplemented diets and greater (P less than .05) than in those fed the control diet. Preintestinal absorption of Mg was .21, .57, 1.08, .14 and .92 g/d when the control, MgO, MgC, Mg-Mica and MgOH diets were fed. Lambs fed the control and Mg-Mica diets absorbed similar (P greater than .05) quantities of Mg in the preintestinal region and less (P less than .05) than lambs fed the MgO, MgC and MgOH diets.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of season on the maintenance requirements of mature beef cows.

Sixty mature, nonpregnant, nonlactating beef cows of two breed types, Angus and Simmental, were used in a four-period, split-plot study to determine the effects of season on maintenance requirements. Periods were chosen to represent each of the four seasons and ran consecutively from June 14, 1986 through June 20, 1987. All cows were fed at one of four feeding levels (75, 87.5, 112.5, and 125% of estimated ME required for maintenance) a cottonseed hull-based diet in each of the four seasons and were rotated such that all cows received all feeding levels during the study. Retained energy for all cows within periods was determined through changes in body composition determined initially and at the end of each period. Mean DM digestibility (DMD) was similar for both breeds (54.4%), and in both breeds DMD was similar in the summer, fall, and spring and lower (P less than .05) in the winter. Mean DE was 61.4% and also was similar between breeds. Season affected DE, which was greatest (P less than .05) for both breeds in the fall (62.5%). Simmental cows had a 16.1% greater (P less than .05) overall daily ME requirement for weight maintenance than Angus cows (123.5 vs 103.6 kcal/kg.75), and both breeds had greater (P less than .05) requirements during the summer and lower (P less than .05) requirements during the winter (122.6 vs 91.4 and 145.9 vs 109.3 kcal/kg.75 for Angus and Simmental cows, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Visual and chemical tissue markers for bovine carcass components.

A two-component, nontoxic, quantifiable animal/carcass tracing system was developed using riboflavin as an on-premises, initial carcass identifier visible under longwave ultraviolet (UV) light and deuterium oxide (D2O) as a tracer analytically quantified via fixed wavelength infrared spectrophotometry. Twenty-four cull cows and heifers were allocated into eight antemortem treatment groups (1, 12, 24, 48, 72, 96, 120, 144 h) for evaluation of the efficacy of riboflavin and D2O as tissue tracers in postmortem meat tissues. All cattle were slaughtered using conventional procedures and inspection. To study postmortem riboflavin marker changes due to constant light exposure over time, fluorescence and emission intensity scores were obtained by a trained panel 24, 48, and 168 h postslaughter. The riboflavin marker intensity rating means for UV fluorescence were classified as identifiable on all carcasses when evaluated under UV light, but were classified as not identifiable when evaluated under ambient light. Deuterium oxide levels in all tissue water samples, regardless of antemortem infusion group, contained D2O concentrations at least 2.5 times greater than those found in background water. Deuterium oxide was shown to disperse rapidly throughout living tissues. Correlations within animals for D2O levels from blood and muscle were all highly significant (r = .99).

Site and extent of apparent magnesium and calcium absorption in steers fed monensin.

Eight crossbred steers (avg wt, 350 kg) with duodenal and ileal cannulas were used in two experimental periods in a single-reversal design to determine the effects of monensin on site and extent of apparent absorption of Mg and Ca. Steers were fed a cottonseed hull:concentrate diet (30:70) with or without 25 mg monensin/kg. The diet contained .20% Mg and .38% Ca. Each diet contained .25% Cr2O3 as a digesta marker. During each period, steers were fed the control diet for 10 d before allotment to treatments. Each period consisted of 17 d for diet adjustment and 6 d for collection. During the collection phase, fecal samples were obtained beginning on d 1 and continuing through d 6 at 12-h intervals progressing 2 h each day. Apparent Mg availability (grams apparently absorbed expressed as a percentage of grams of intake) increased (P less than .05) from 18.4 to 32.5% when monensin was fed. Feeding monensin increased (P less than .05) the apparent Mg availability and amount of Mg absorbed in the preintestinal region (47.7 vs 29.8% and 8.5 vs 5.1 g/d, respectively). Larger quantities (P less than .05) of Mg were secreted into the small intestine when monensin was fed. Monensin did not alter (P less than .05) apparent Ca availability or absorption in any segment of the digestive tract. These data indicate that monensin increased Mg and did not change Ca availability in the preintestinal region.

Maintenance requirements and energetic efficiency of cows of different breed types.

Dry, nonpregnant, mature cows (greater than 10 yr) of five breeds (Angus, A; Brahman, B; Hereford, He; Holstein, Ho; and Jersey, J) and their crosses (n = 60) were used in a 428-d experiment to determine maintenance energy requirements and efficiency of energy exchange. Cows were fed individually (via Calan electronic gates) a 70% cottonseed hull diet for four consecutive periods (127, 105, 97 and 99 d) at each of four levels (50, 83, 117 and 150% of each animal's estimated maintenance requirement). Each of four cows/breed group was assigned to one of the four feeding levels each period, with one cow fed each level each period. Body composition was measured initially and following each period in all cows via D2O dilution with a two-pool kinetics model procedure. Average ending live weight and empty body weight, protein and fat were similar to beginning values, indicating that cows began and ended in similar body composition. Dry matter digestibility (DMD) following the last period averaged 54.7%. Average DMD was 53.5, 57.8, 52.0, 55.0 and 51.7% (standard error of mean [SE] = 2.1) for A, B, He, Ho and J; values for He and J were lower (P less than .05) than for B. Diet digestible energy (DE) was similar for all breed types and averaged 62.4% of gross energy (GE). A small decrease in DE with increasing GE intake was noted for all breed types. The daily metabolizable energy requirement for weight equilibrium for A, B, He, Ho and J differed (P less than .01) and was 100, 98, 108, 119 and 152 kcal/kg.75 (SE 4.8), respectively, with an overall mean of 107. The ME for maintenance (MEm) was 91.6, 93.8, 95.3, 115.7 and 140.4 kcal/kg.75 for A, B, He, Ho and J (SE 6.0), respectively, with an overall mean of 101.9. Efficiency of weight change for A, B, He, Ho and J differed (P less than .01) and was 116, 135, 80, 116 and 58 g/Mcal ME intake, respectively, with an overall mean of 96. The respective efficiency of ME use for tissue energy gain or loss was 80.6, 66.8, 66.0, 36.5 and 36.2% for A, B, He, Ho and J, with an average energetic efficiency of 60.1%. In general, maintenance requirements for weight and energy equilibrium were lower in beef breeds and their crosses than in dairy breeds and their crosses. Efficiency of ME use also favored the beef breeds over the dairy breeds.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of monensin on monovalent ion metabolism and tissue concentrations in lambs.

A study consisting of two trials was conducted to determine the effects of monensin on the apparent absorption and retention of sodium (Na) and potassium (K) and to examine changes in tissue and ruminal fluid concentrations of these minerals in lambs. Eight lambs (39 kg) were used in trial 1 and 10 lambs (37 kg) were used in trial 2. Animals were used in randomized block designs, blocked by weight, and fed a high concentrate diet with or without 20 mg/kg monensin. Trials began with a dietary adjustment period lasting 18 d in trial 1 and 21 d in trial 2. Animals were then placed in metabolism stalls for a 10-d stall adjustment period followed by a 12-d collection period. Collections for mineral balance were made during the first 10 d of the collection period. Blood and ruminal fluid samples were obtained on d 11 of the collection period. Lambs were slaughtered on d 12 of the collection period and tissue samples were collected. Sodium retention decreased (P less than .05) 86.2% when monensin was fed. Apparent K absorption increased (P less than .05) 16.7%, while K retention increased (P less than .10) 52.6% when monensin was fed. In lambs fed monensin, ileal Na decreased (P less than .10) 13.8%. These results indicate that dietary monensin alters the metabolism of Na and K in lambs.

Effects of monensin on Mg, Ca, P and Zn metabolism and tissue concentrations in lambs.

A study consisting of two trials was conducted to determine the effects of monensin on the apparent absorption and retention of magnesium (Mg), phosphorus (P), calcium (Ca) and zinc (Zn) and to determine mineral changes in tissue and ruminal fluid. Eight lambs (39 kg) were used in trial 1, and 10 lambs (37 kg) were used in trial 2. Animals were blocked by weight and fed a high concentrate diet with or without 20 mg/kg monensin. Trials began with a dietary adjustment period lasting 18 d in trial 1 and 21 d in trial 2. Animals were then placed in metabolism stalls for a 10-d stall adjustment period followed by a 12-d collection period. Collections to determine mineral balance were made during the first 10 d of the collection period. Blood and ruminal fluid samples were taken on d 11 of the collection period. Lambs were slaughtered on d 12 of the collection period and tissue samples were collected. Monensin supplementation increased (P less than .05) Mg retention 42.0%. Urinary Ca excretion decreased (P less than .05) 60.0% when monensin was fed. Monensin supplementation decreased (P less than .05) liver Ca and bone Ca, 45.5 and 2.9%, respectively. Apparent P digestibility increased (P less than .05) 40.0% and P retention increased (P less than .10) 26.8% due to monensin supplementation. Both apparent absorption and retention of Zn increased (P less than .01) 50.0 and 45.0%, respectively, with monensin supplementation. Ruminal fluid Zn concentrations decreased (P less than .05) 33.0% with the addition of monensin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dietary potassium and sodium on magnesium utilization in sheep.

Two metabolism trials were conducted to determine the effect of altering dietary Na and K on Mg availability. Three dietary treatments (normal K-normal Na, high K-normal Na and high K-high Na) were imposed upon nine crossbred wether lambs in a randomized block design. Each trial consisted of a 5-d diet adjustment, a 10-d preliminary period and a 5-d collection of feed, feces and urine. Increasing K and Na intake increased the percentage of Mg excreted in feces. Both elevated K and K-Na depressed (P less than .10) urinary Mg excretion, primarily a reflection of depressed Mg absorption in animals fed these diets. Apparent K absorption and retention increased when animals were supplemented with K. Addition of Na to the diet did not alter absorption or retention of K. Apparent K and Na availability increased when K and Na supplements were included in the diet. This increased availability resulted from increased total absorbed K and Na compared with a fixed endogenous excretion. Calcium balance in lambs was not affected by addition of K or Na. Lambs were able to absorb enough Mg from their respective diet to maintain normal serum Mg levels. Addition of Na to a diet high in K did not enhance Mg absorption in lambs.

Growth patterns and body composition of transgenic mice expressing mutated bovine somatotropin genes.

The objectives of this study were to determine growth rates, feed intakes, feed efficiencies, and chemical composition of mice from three transgenic lines in 10-d periods from weaning to near maturity. Lines M4, M11, and G119K express bovine somatotropin (bST) mutations E117L, L121P-E126G, and G119K and display phenotypes of large, near normal, and small body size, respectively. M4 mice were 28% larger at 28 d and 84% larger at 68 d than non-transgenic control (NTC) mice. M11 mice were the same size at 28 d as NTC but were 25% larger at 68 d. G119K mice were 34% and 25% smaller than NTC at 28 and 68 d, respectively. Growth rates of G119K mice and NTC were similar, whereas growth rates of M11 and M4 mice were increased (P < .05). Feed intakes of M4 and M11 mice were greater than those of NTC mice (P < .05), whereas feed intakes of G119K mice were lower than those of NTC mice (P < .05). Feed efficiency (gain/feed) was improved in M4 and M11 mice (P < .05) and not altered in G119K mice compared to that of NTC mice (P > .05). Chemical composition was also altered by expression of bST analogs in transgenic mice. G119K and M4 mice had increased body fat percentages and decreased body protein percentages in comparison to M11 and NTC mice (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nutritional level and biological type on gonadotropin-releasing hormone-induced luteinizing hormone release and plasma progesterone, estrone and estradiol concentrations in pre- and post-partum beef heifers.

Forty-six beef heifers (16 to 23 mo) of two biological types (small = Red Poll-sired, large = Charolais-sired) were individually fed from d 90 of gestation through parturition to evaluate the effects of nutritional restriction on plasma LH and steroid hormone concentrations. Heifers were allotted to one of two nutritional treatments to achieve a BW reduction (loss, fed at 1% of BW/d) or to maintain initial BW (maintenance, fed 1.5% of BW/d) to parturition. Gonadotropin-releasing hormone (100 micrograms) was injected i.m. three times during gestation (d 130; d 200; d 270) and twice after parturition (d 1 to 14; d 23 to 36). Blood samples were collected at 20-min intervals after GnRH for 4 h. Maternal BW change from d 90 to parturition differed (P less than .01) between loss and maintenance heifers. Mean plasma progesterone concentrations were greater (P less than .05) at d 130 and 270 of gestation in small than in large heifers and were greater (P less than .01) at d 23 to 36 postpartum in maintenance than in loss heifers. Mean concentrations of estrone and estradiol were greater (P less than .05) in large than in small heifers at d 200 of gestation. Mean plasma LH concentrations following GnRH injection were greater (P less than .01) in loss than in maintenance heifers at 200 and 270 d of gestation. Metabolizable and retained energy were related inversely to LH release during mid and late gestation.(ABSTRACT TRUNCATED AT 250 WORDS)

Gonadotropin-releasing hormone-induced luteinizing hormone release in heifers: effect of nutrition during gestation.

The effects of nutrition during the last two trimesters of gestation on GnRH-induced LH release were assessed in crossbred heifers. Heifers (n = 58) were allotted at 90 d gestation to one of three levels of an experimental diet fed at 1, 1.5 or 2% of BW to attain maternal BW loss, BW maintenance or BW gain, respectively, at parturition. Twenty-two heifers were injected (i.m.) once with 100 micrograms GnRH between d 14 and 1 before parturition, and 32 heifers were injected (i.m.) once with 100 micrograms GnRH between d 8 and 21 after parturition. Jugular blood samples were collected before and at 30-min intervals after GnRH for 4 h. Least squares means for BW change differed (P less than .01) among BW loss (-17.6%), BW maintenance (-6.0%) and BW gain (7.0%) heifers. Basal plasma LH concentration was not influenced by nutritional treatment and was similar before and after parturition for all groups. However, in response to GnRH, peak plasma LH concentration was greater (P less than .10) for prepartum than for postpartum heifers. Mean LH peak amplitude in prepartum heifers was approximately twofold greater (P less than .10) in the BW loss and maintenance groups compared with the BW gain group. Prepartum LH release was related inversely (r = -.64) to change in heifer BW and increased (P less than .01) as BW loss increased during gestation. After parturition, mean LH peak amplitude and area under the response curve averaged 50% less (P less than .10) in the BW loss and maintenance groups than in the BW gain group.(ABSTRACT TRUNCATED AT 250 WORDS)

Trona and sodium bicarbonate in beef cattle diets: effects on pH and volatile fatty acid concentrations.

A study was conducted to evaluate the effectiveness of NaHCO3 and trona in beef cattle diets. Trace element (n = 28) analysis revealed no toxicological or safety concerns with the use of trona. Trona was more (P less than .05) soluble in ruminal fluid than Na2CO3, and NaHCO3 and had greater (P less than .05) buffering capacity (9.6 meq/g) than NaHCO3 (6.1 meq/g) but less (P less than .05) than Na2CO3 (11.1 meq/g). Calcium carbonate was insoluble and did not buffer ruminal fluid. Six yearling (avg 272 kg) Hereford X Angus steers, each with ruminal, duodenal and ileal cannulas, were fed 50:50 (cracked corn-based concentrate:cottonseed hulls) or 90:10 concentrate diets with no buffer, 1% NaHCO3 or with 1% trona. Intake, across all treatments, averaged 2.4% of body weight. Propionate (mmol/liter) increased (17.6 vs 13.5; P less than .05) and butyrate decreased (3.5 vs 5.2; P less than .05) with trona in the 90:10 diet as compared with no buffer. Propionate (16.8) increased (P less than .05) with NaHCO3 in the 90:10 diet. Average ruminal pH was greater (P less than .05) in 90:10 diets with trona or NaHCO3 than with no buffer (5.61, 5.61 vs 5.55); duodenal pH was greater (P less than .01) with trona than with no buffer (2.66 vs 2.55). Trona reduced ruminal pH-hours (P less than .05) and pH-area (P less than .12; time and area below mean pH of control) below control for both concentrate levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Trona and sodium bicarbonate in beef cattle diets: effects on site and extent of digestion.

Six yearling Hereford X Angus steers (avg 272 kg), each with ruminal, duodenal and ileal cannulas were used in a 6 X 6 Latin-square metabolism trial to evaluate the impact of NaHCO3 and trona (a ground, nonrefined ore with chemical composition NaHCO3-Na2CO3-2H2O) on site and extent of digestion of nutrients in the digestive tract. The diets were 50:50 or 90:10 (cracked corn-based concentrate:cottonseed hulls) with no buffer, 1% NaHCO3, or 1% trona. Intake, across all treatments, averaged 2.4% of body weight. Dry matter (DM) and starch digestibility (via indigestible acid detergent fiber) before the duodenum was decreased (P less than .10) with trona in the 50:50 diet. Digestibility of DM, crude protein and starch before the ileum were greater (P less than .05) in the 90:10 diet vs 50:50 diet. Total tract digestibility was similar across buffer treatments in the 90:10 diet. Addition of NaHCO3 increased (P less than .05) digestibility of dry matter and cell solubles in the 50:50 diet. Organic matter and crude protein digestibility were also increased (P less than .10) with NaHCO3. Apparent crude protein and cell solubles digestibility were greater (P less than .10) with trona than NaHCO3 in the 50:50 diet. This trial indicates that buffers provide overall enhancement of diet digestibility in mixed grain/roughage diets.