Influence of sward height, daily timing of concentrate supplementation, and restricted time for grazing on forage utilization by lactating beef cows.

To establish the effect of sward height, concentrate feeding time, and restricted time for grazing on forage utilization by grazing cattle, 32 crossbred beef (24 Angus and eight Hereford) cows (632 kg BW) and calves (104 kg BW) were grouped by weight and calving date. They were assigned randomly to two sward height treatments (4 to 8 or 8 to 12 cm), replicated four times. The herbage comprised mainly Kentucky bluegrass, orchardgrass, some forbs, and white clover. The cows were restricted to 12 h/d grazing (0700 to 1900) or unrestricted to 24 h/d grazing and fed a concentrate supplement (4.1 kg DM.cow(-1).d(-1), approximately 0.65% of BW or 33% of total DMI) either at 0700 or 1800. The experiment was repeated over three 15-d periods in May, June/July, and August 2000. The herbage on high sward height pasture was higher (P = 0.06) in NDF and ADF and lower (P < 0.01) in CP than low sward height herbage. For cows restricted to 12 h/d grazing, supplementing at 0700 as opposed 1800 resulted in greater (P = 0.04) forage DMI (8.6 vs. 8.1 kg/d), whereas cows that were unrestricted showed little change (8.2 kg/d at 0700 vs. 8.4 kg/d at 1800). Supplementing at 1800 as opposed to 0700 resulted in greater (P = 0.03) herbage DM digestibility (67.7 vs. 64.5%) for cows on high sward height, whereas cows on low sward height exhibited minimal differences (65.4% at 1800 vs. 66.3% at 0700). Cows restricted to 12 h/d grazing and supplemented at 0700 as opposed to 1800 resulted in greater (P = 0.06) digestible DMI (5.0 vs. 4.7 kg/d), whereas unrestricted cows exhibited the opposite response (4.6 kg/d digestible DMI at 0700 vs. 4.9 kg/d at 1800). Supplementing at 1800 as opposed to 0700 increased the time spent grazing to a greater (P = 0.09) extent for restricted than for unrestricted cows. When forage availability or grazing time was limiting (due to a low forage allowance and restricted access to forage, respectively) supplementing concentrates at 0700 resulted in greater forage utilization and intake rate because of increased forage DMI, DM digestibility, and digestible DMI. However, when forage or grazing time was not limiting, supplementing concentrates at 1800 resulted in greater forage utilization because of increased forage DM digestibility.

Influence of mass of ruminal contents on voluntary intake and digesta passage in steers fed a forage and a concentrate diet.

To evaluate the influence of mass of ruminal contents on voluntary intake and ruminal function, five ruminally cannulated steers (550 kg) were fed an orchard grass hay diet ad libitum in a 5 x 5 Latin square experiment. The mass of ruminal contents was altered by adding varying weights of modified tennis balls to the rumen before the initiation of each 15-d experimental period. Treatments consisted of 50 balls with a specific gravity of 1.0, 1.1, 1.2, 1.3, or 1.4; the total weight of the balls was 7.45, 8.50, 9.25, 10.55, and 11.55 kg, respectively. Increasing the specific gravity of the balls added to the rumen decreased DMI and particle passage rate (P < 0.05) in a linear manner. A second experiment was conducted to evaluate the influence of mass of ruminal contents on voluntary intake and ruminal function of both forage and concentrate diets. Five ruminally cannulated steers (580 kg) were fed a 70% concentrate (DM basis) or an orchardgrass hay diet ad libitum in a 5 x 5 Latin square experiment. The mass of ruminal contents was altered as in the first experiment. Treatments consisted of 0 balls added to the rumen of steers fed concentrate diet (control), 75 balls with a specific gravity of 1.1 given to steers fed a concentrate diet, 75 balls with a specific gravity of 1.4 given to steers fed a concentrate diet, 75 balls with a specific gravity of 1.1 given to steers fed a hay diet, and 75 balls with a specific gravity of 1.4 given to steers fed hay diet. The addition of balls to the rumen of steers fed the concentrate diet decreased DMI (P < 0.05) compared with the 0-ball treatment, and increasing specific gravity of balls also decreased DMI (P < 0.01) for both concentrate and hay diets. Adding balls to the rumen of steers fed the concentrate diet decreased particle passage rate (P < 0.05), whereas increasing specific gravity of balls decreased particle passage rate for both concentrate and hay diet. The results of this study suggest that the density of ruminal digesta can have an influence on voluntary intake of both forage and concentrate diets.

Influence of pasture sward height and concentrate supplementation on intake, digestibility, and grazing time of lactating beef cows.

To establish the effect of sward height (SH) and concentrate supplementation on performance of grazing cattle, 24 crossbred Angus beef cows (535 kg BW) and calves (114 kg BW) were grouped by weight and calving date. They were randomly assigned to two SH treatments, either 4 to 8 cm or 8 to 11 cm, and fed three levels of supplement, high, low, or none, consisting of 6.24, 3.12, and 0 kg x animal(-1) x d(-1), respectively. The experiment was repeated over three 15-d periods in 1996: May (P1), June/July (P2), and August (P3). No SH x supplement level x period or SH x supplement level interactions (P > 0.10) were evident for responses tested. Cows on lower SH had greater (P < 0.08) DMI but spent an additional 1.3 h/d (P < 0.01) grazing compared with cows on higher SH. Sward height had no influence (P > 0.10) on forage DM digestibility (DMD). Forage DMI, DMD, and grazing time (GT) decreased (P < 0.05) as supplementation increased. Nonetheless, supplemented cows consumed more total DMI (P < 0.08) than unsupplemented cows. Cows consumed 2.4 kg/d more forage DM (P < 0.01) in P1 and P2 than in P3. Cows grazed 1.3 h/d (P < 0.01) less in P1 than in P2 and P3. Grazing efficiency (DMI/h GT) declined as supplementation increased and grazing season advanced to P3 (P < 0.01). Decreased forage DMI and grazing efficiency with increasing supplementation suggests that supplemented cattle should be able to maintain productivity while grazing at SH lower than unsupplemented cattle.

Influence of mass and volume of ruminal contents on voluntary intake and digesta passage of a forage diet in steers.

To assess the influence of volume and mass of ruminal contents on voluntary intake and related variables, five ruminally cannulated steers (550 kg) were fed a low-quality forage diet (43.1% ADF, 8.1% CP) in a 5 x 5 Latin square experiment. Mass and volume of ruminal contents were altered by adding varying numbers and weights of filled tennis balls (6.7-cm diameter) to the rumen immediately before the initiation of each experimental period. Treatments consisted of 0 balls (control), 50 balls with a 1.1 specific gravity (SG), 100 balls with a 1.1 SG, 50 balls with a 1.3 SG, and 100 balls with a SG of 1.3. The total volume of balls was 7.25 and 14.5 L for 50 and 100 balls, respectively. The total weight of balls was 8.5 and 17 kg for 50 and 100 balls with a 1.1 SG and 10.75 and 21.5 kg for 50 and 100 balls with a 1.3 SG, respectively. Daily DMI was 8.3, 7.3, 7.0, 6.5, and 6.0 kg for control; 50, 1.1 SG; 50, 1.3 SG; 100, 1.1 SG; and 100, 1.3 SG, respectively. Addition of balls to the rumen reduced (P < .01) DMI. Increasing the number (P < .01) and SG (P <. 01) of the balls decreased DMI further. However, digestibilities of DM, NDF, ADF, and CP were not influenced by treatment. Increasing the number of balls in the rumen increased (P < .05) rate of passage of digesta from the rumen, but increasing SG of the balls did not alter rate of passage. There was a treatment x hour interaction (P < .05) in the proportion of ruminal digesta with a functional specific gravity (FSG) less than 1.1, which decreased with time after feeding for the control but increased with time after feeding for other treatments. Ruminal passage rate of inert particles added in the rumen of different SG (1.1 and 1.3) and length (1 and 3 mm) decreased (P < .05) as SG of the balls increased. Mean fecal particle size was greater for those treatments with the heavier balls. Both the number and SG of balls (P < .10) influenced total VFA, and total concentrations were greater for the control and for the 1.1 SG than for the 1.3 SG treatments.

Influence of moisture content of forage on ruminal functional specific gravity and passage of digesta.

Four crossbred beef steers (392 kg BW) fitted with ruminal cannulas were used in a 4 x 4 Latin square experiment with treatments arranged in a 2 x 2 factorial to evaluate effects of forage species (F) and reconstitution (R) on apparent digestibility, ruminal kinetics, and digesta functional specific gravity (FSG). Dietary treatments were alfalfa (Medicago sativa L.) and timothy (Phleum pratense L.) hays (90% DM) and the same hays reconstituted with water to 38% DM. Apparent DM digestibility was unaffected by R or F. Proportion of ruminal digesta with a FSG of .9 to 1.1 or 1.1 to 1.2 decreased linearly (P < .05) after feeding, whereas the proportion with FSG > 1.2 increased linearly (P < .01). Reconstitution decreased (P < .10) the proportion of ruminal digesta with FSG of 1.1 to 1.2 and increased (P < .10) the proportion with a FSG > 1.2 compared with the hay diets. Reconstitution decreased digesta passage rate for timothy forage; no influence on passage rate for alfalfa was detected (F x R, P < .10). Ruminal DM pool was increased by reconstitution (P < .10). Inert particles of 1-mm length passed from the rumen at a faster rate (P < .05) than 3-mm particles. Inert particles with SG of 1.32 passed from the rumen at a faster rate (P < .05) in the reconstituted alfalfa than in the dry alfalfa diet but at a slower rate in the reconstituted timothy than in the dry timothy diet. These results suggest that factors other than SG of the digesta can have a profound effect on passage of forage particles from the rumen.

Influence of moisture content of forage diets on intake and digestion by sheep.

To determine the effect of forage moisture content on intake and digestion kinetics in sheep, a metabolism trial was conducted using 16 mature wethers (44 kg BW) in a completely randomized design. Forage was harvested at two maturities in early spring from a naturalized pasture composed of temperate grass and legume species. Herbage was harvested at either 8 (early) or 16 cm (late) in height and fed after freezing (high-moisture) or as a dried hay. Intakes of DM (grams/[kilogram BW.75.day]), NDF, ADF, and CP (grams/day) (P < .05) and coefficients for DM, NDF, and ADF digestibilities were greater (P < .01) for hay than for high-moisture forage. Mean particulate retention times (MRT) were shorter (P < .05) for high-moisture forage (23.3 h) than for hay (30.7 h) diets. Early-harvested forages had shorter (P < .05) MRT values (23.9 h) than late-cut forages (30.1 h). Fractional passage rates of 1-mm nylon particles of specific gravity (SG) .90, 1.14, and 1.32 through the alimentary tract were influenced by moisture content of the forage (P < .10) and were faster for frozen forages and increased (P < .01) with an increase in SG. Nitrogen retention was greater (P < .01) for the hay than for the high-moisture forage. In situ DM digestion rates, determined using four ruminally fistulated wethers, showed no differences (P = .67) among forages. The results of this study indicate that differences in digestibilities between hays and high-moisture forages are most likely due to differences in digesta passage rates.

Influence of forage species and diet particle size on the passage of digesta and nylon particles from the reticulorumen of steers.

To assess factors that influence the passage of digesta from the reticulorumen, ruminally fistulated steers (387 kg) were fed to appetite in a 4 x 4 Latin square design either orchardgrass (OG) (Dactylis glomerata L.) or switchgrass (SG) (Panicum virgatum L.) hays. The two hays were fed in either the long form or ground to pass a 2-cm screen. Steers fed OG diets had greater DMI (P < .05) than steers fed SG diets (11.7 vs 10.4 kg/d respectively). Particle size of ruminal digesta and feces determined by wet sieving was greater (P < .05) for the OG and long hay diets than for the SG and ground diets. Particle size of digesta collected from the anterior dorsal sac and anterior ventral sac of the rumen and from the reticulum did not vary (P > .25) with site. Conversely, ruminal concentrations of 1-mm nylon particles tended (P = .11) to be greater in the lower strata of the reticulorumen at 12 and 24 h after dosing. Neither a site effect nor a site x time-after-dosing interaction (P > .75) was observed for the 3- or 5-mm nylon particles, suggesting that a sorting process independent of specific gravity occurs in the rumen for smaller particles (before the reticulomasal orifice). Passage rate of the 1-mm nylon particles from the reticulorumen was greater (P < .01) for animals fed the SG than for those fed the OG diets, whereas for the 5-mm particles, passage rate was greater for the OG diets.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of increased postmilking prolactin concentrations on lactation, plasma metabolites, and pancreatic hormones in lactating goats.

Six lactating dairy goats were utilized in a replicated 3 x 3 Latin square design to examine the effect of increased postmilking prolactin concentrations on lactational performance. Thirty minutes after each milking, either placebo or pituitary-derived ovine prolactin at 2 mg/h or 4 mg/h was infused via jugular cannula for 2 h. Goats were housed in a controlled environment at 19 degrees C and a cycle of 16 h light and 8 h dark. Mean plasma prolactin concentrations after milking were increased 100 and 180% for 2 and 4 mg/h prolactin infusion, respectively. Milk yield was slightly but significantly increased by 2 mg/h prolactin infusion compared with placebo and 4 mg/h prolactin infusion. However, milk composition and yields of milk components were not influenced by treatments. No differences in dry matter intake, digestibility of the diet, or N retention were observed. Circulating concentrations of glucose, nonesterified fatty acids, insulin, and glucagon were not altered.

Influence of forage diets on ruminal particle size, passage of digesta, feed intake and digestibility by steers.

To assess the influence of forage diets on particle size of digesta in the rumen, three ruminally fistulated steers were fed alfalfa, orchardgrass or switchgrass hays in a 3 x 3 Latin square design. Mean sieve size of ruminal digesta for alfalfa, orchardgrass and switchgrass diets was 671, 652 and 466 microns, respectively. A second experiment examined the influence of ruminal particle size on digesta passage, digestibility and intake. Four ruminally fistulated Angus-Hereford steers were given ad libitum access to different proportions of alfalfa and switchgrass in a 4 x 4 Latin square design. Either a 100% alfalfa, 50% alfalfa:50% switchgrass, 25% alfalfa:75% switchgrass or 100% switchgrass diet was offered once daily. Mean sieve sizes of the ruminal digesta of these diets responded cubically (P less than .01; 1,066, 946, 1,003 and 925 microns, respectively). Mean ruminal turnover times were 24.3, 24.8, 24.7 and 29.8 h, respectively. Dry matter intake increased linearly (P less than .10) as the proportion of legume in the diet increased; no influence of diet on DM digestibility was observed. Passage rate of dosed nylon particles 1, 3 and 5 mm in length was influenced (P less than .01) by size; however, no difference in the passage rates of the nylon particles due to diet was evident. Particle size of ruminal digesta did not respond in a linear manner to the proportion of legume in the diet. A smaller ruminal particle size was not associated with a faster turnover of digesta.

Effect of prolactin infusion on lactation, glucose kinetics, and pancreatic hormones in lactating goats.

To determine lactational and glucogenic responses to increased prolactin concentrations, five lactating dairy goats were used in a crossover design. Prolactin at .85 mg/h or placebo was infused on each of 8 consecutive d via jugular catheter for 14 h initiated 30 min after the a.m. milking. To examine the effect of treatments on glucose metabolism, goats were given a single isotope injection of [6-3H]glucose 2 h after initiation of treatment infusion on the final day of the treatment period. Milk yield and milk protein percentage were not affected by prolactin infusion; however, milk fat and SNF percentage were significantly decreased. No differences in dry matter intake and digestibility of the diet were observed. Prolactin increased plasma prolactin and decreased plasma glucagon concentrations, but did not affect plasma insulin or glucose concentrations. Although glucose pool size per kilogram of BW was significantly lower in the prolactin-treated group than in the control group, no difference in glucose turnover rate was observed.

Effect of indomethacin administration on glucose homeostasis and pancreatic hormones in lactating goats.

To determine glucogenic and hormonal responses in ruminants to indomethacin, a prostaglandin synthetase inhibitor, four lactating Toggenburg goats were used in a crossover design. Fifteen milligrams of indomethacin or 3 ml of saline were injected subcutaneously twice daily with propionate infused intrajugularly at a rate of 2 mM/min for 75 min as a challenge. Plasma glucose concentrations were increased by indomethacin injections. Plasma concentrations of insulin, glucagon, and 6-keto-prostaglandin F1 alpha were not affected by indomethacin injections. Propionate infusion elevated plasma glucose, insulin, and glucagon concentrations. A decreased insulin release in response to propionate challenge was observed with indomethacin injections.

Glucogenic and hormonal responses to abomasal casein and ruminal volatile fatty acid infusions in lactating goats.

To determine responses to abomasal protein infusion and ruminal acetate: propionate ratios, four lactating Toggenburg goats fed hourly a 70% roughage and 30% concentrate diet were used in a Latin-square design with a factorial arrangement of treatments. Either acetate or propionate was infused ruminally and casein or saline infused abomasally. Estimated net energy and volume of the infusates were similar for all treatments. To examine the effects of treatments on glucose metabolism, 2-carbon-14 propionate was infused ruminally and 6-hydrogen-3 glucose was infused intravenously for 9 and 5 h, respectively. Although glucose concentration in plasma was higher and propionate turnover greater with propionate treatment, percentage of glucose derived from propionate, amount of propionate coverted to glucose, and glucose turnover remained unchanged. No differences in glucose metabolism due to the abomasal casein infusion were evident. To determine the effects of treatment on insulin, glucagon, growth hormone, and prolactin in plasma, samples were collected at 10-min intervals for 3 h at 0400 and 1600 h. No diurnal variation or consistent peaks were observed for any of the hormones nor were treatment effects on plasma concentrations of insulin, growth hormone, or prolactin evident. Glucagon concentration was higher with casein treatment; however, no relationship existed between glucagon in plasma and glucogenic parameters measured.

Comparative digestion, rumen fermentation and kinetics of forage diets by steers and wethers.

Four rumen fistulated wether and beef steers were used to evaluate differences in dry matter digestibility (DMD) between cattle and sheep. They were fed either perennial ryegrass or switchgrass hay at an ad libitum or restricted level for four experimental periods. Significant ruminant species X forage and ruminant species X level of intake (P less than .05) interactions were observed for digestible dry matter. The steers digested the switchgrass 7 percentage units greater than the wethers while ryegrass was digested equally. Digestibility differences between the steers and wethers were 6 percentage units at the ad libitum level of intake and 1 unit at the restricted level of intake. Crude protein digestibility tended to be greater (P less than .10) for sheep with a 7 unit difference for switchgrass and a 3 unit difference for ryegrass. The mean ruminal solids retention time of the digesta was approximately (P less than .01) 50% greater (26.0 vs 17.4 h) in cattle, with no difference in ruminal liquid dilution rate (LD) between animal species. Total ruminal volatile fatty acid concentration differed (P less than .01) with level of intake; however, no influence due to intake on the molar proportion of acetate (P greater than .10) or propionate (P greater than .10) was evident in spite of a difference (P less than .01) in LD. Rumen pH (P less than .05) and osmolality (P less than .01) were affected by both level of intake and forage, with the ryegrass and high level of intake decreasing pH and increasing osmolality.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of ytterbium chloride and chromium sesquioxide as fecal indicators.

To assess the potential of ytterbium chloride (Yb) as an indicator of fecal output, we dosed 13 mature beef cows with Yb and chromic oxide (Cr2O3) either once (0800) or twice (0800 and 1600 hr) daily. Fecal output and digestible dry matter were estimated from fecal marker concentration determined in grab samples collected at 0800, 1600 or 0800 and 1600 hr, and compared with values obtained by total fecal collection. With single daily dosings, fecal outputs estimated from Yb at all collection times did not differ from values obtained by total collection, whereas Cr2O3 underestimated (P greater than .05) fecal production at 0800 hr and in the composited (0800 and 1600 hr) collections. With twice-daily dosing, fecal outputs estimated from Yb differed to a greater extent from the total collection values for the 1600 hr collection; however, the Cr2O3 values more closely approximated those estimated by total fecal collection at 0800 hours. Relative marker concentration in the feces collected from grab samples at 4.hr intervals indicated that twice-daily dosing appeared to reduce diurnal variation. Diurnal fecal excretion patterns of Yb and Cr2O3 differed (P greater than .05); however, the variation observed for both markers within dosing schedule appeared to be as great for Yb as it was for Cr2O3. Although fecal output was accurately estimated from Yb with one simultaneous collection and dosing period, the response appeared to be due to time of collection rather than to an actual reduction in diurnal variation.