Net flux of nutrients across splanchnic tissues in sheep fed tropical vs. temperate grass hay of moderate or low qualities

ELSEVIER

Livestock Production Science 43 ( 1995) 49-61

Net flux of nutrients across splanchnic tissues in sheep fed tropical vs. temperate grass hay of moderate or low qualities * A.R. Patil, A.L. Goetsch”, K.K. Park, B. Kouakou, D.L. Galloway, Sr., C.P. West, Z.B. Johnson Department of Animal Science, University of Arkansas, Fayetteville, AR 72701, USA

Accepted 8 February1995

Abstract Crossbred wethers ( 18 months old; 44 f 0.7 kg body weight), with catheters in a hepatic vein, the portal vein and a mesenteric vein and artery, consumed ad libitum tropical or temperate grass hay each of three different qualities or stages of maturity. Splanchnic tissue energy consumption was similar among tropical grass diets but increased as quality of temperate grass declined. Portal-drained viscera oxygen consumption increased with increasing digestible energy intake and fecal neutral detergent fiber excretion. Energy available to extra-splanchnic tissues with highest quality grass was greater for temperate than for tropical grass because of lower splanchnic tissue energy consumption relative to digestible energy intake. Grass source, quality and nitrogen concentration did not significantly affect portal-drained viscera release of o-amino nitrogen. Hepatic uptake of cu-amino nitrogen was greater for tropical than for temperate grass, presumably because of higher nitrogen concentration with greater hepatic ammonia nitrogen uptake. Glucose uptake by the portal-drained viscera was greater for tropical than for temperate grass, and the potential contribution of propionate to hepatic glucose release tended to be greatest for grass highest in quality. Grass quality appears more important to achieve maximal energy availability to extra-splanchnic tissues with temperate than tropical grass. Keywords: Sheep; Forage; Forage quality; Metabolism

1. Introduction Both tropical and temperate grasses are consumed by ruminants in many parts of the world. Tropical and temperate grasses can differ in feed intake, digestibility, digestion endproducts and metabolic conditions. At the same relative stage of maturity, temperate grass is more digestible than tropical grass, and feed intake typically is similar or lower for tropical than for temperate grass (Minson, 1990). Primarily because of the lower non* Corresponding author. * Published with the approval of the Director of the Arkansas Agricultural Experiment Station, Manuscript No. 94108. 0301-6226/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SsD10301-6226(95)00006-2

structural carbohydrate level in tropical than temperate grass, acetate/propionate in ruminal fluid is less for tropical grass (Minson, 1990). Extent of ruminal degradation of protein in tropical and temperate grass does not appear to differ greatly (Jones et al., 1987, 1988; Brake et al., 1989; Sun et al., 1993), although intestinal amino acid absorption and ruminal volatile fatty acid production may be lower for tropical grass because of less fermentable organic matter and microbial protein synthesis (Doyle, 1987). Because of high metabolic activity of splanchnic tissues, effects of differences between tropical and temperate grass in net flux of nutrients across splanchnic tissues may not necessarily coincide with expectations

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based on feed intake, digestibility and concentrations of digestion endproducts in the gastrointestinal tract. For example, Sun et al. (1994) observed higher gut mass relative to digestible organic matter intake in growing wethers consuming tropical than temperate grass, implicating greater oxygen consumption and nutrient metabolism by the gastrointestinal tract. Differences between tropical and temperate grass in net flux of nutrients across splanchnic tissues have not been studied previously. Grasses are harvested directly by ruminants or first by man at varying stages of growth or quality. In practice, quality and digestibility are more variable for temperate than for tropical grass because of differences in growth characteristics and tissue composition and proportions (Wilson, 1993). Forage quality impacts feed intake and digestibility and, thus, influences intestinal amino acid flow and ruminal volatile fatty acid levels (Galyean and Goetsch, 1993). In addition, acetate/ propionate in ruminal fluid generally decreases with increasing nonstructural carbohydrate concentration in forage. Forage quality may also impact the quantity or characteristics of digesta in the gut to alter gastrointestinal tract mass and metabolic activity (Rompala et al., 1988, 1990). Influences of forage quality on net flux of nutrients across splanchnic tissues have not been extensively studied. Therefore, objectives of this experiment were to determine effects of grass source (tropical vs. temperate) and different growth stages or qualities on net flux of oxygen and nutrients across the portal-drained viscera and liver in sheep.

2. Materials and methods Twelve crossbred (Suffolk X Rambouillet-Dorset) castrate male sheep (approx. 18 mo of age; 44 f 0.7 kg body weight) were surgically fitted with chronic indwelling catheters in a hepatic vein, the portal vein and a mesenteric vein and artery (Ferrell et al., 1992). Catheters were fitted with a heparinized (100 U/ml) saline solution at surgery and between sampling periods. The sheep began the experiment at approx. 3 wk after surgery, being individually maintained in 1.2 X 1.2 m slatted floor pens with free access to water. Sheep were allotted to four groups with three per group for use in replicated simultaneous 3 X 3 Latin squares, with a split-plot design. Two groups consumed

tropical grass (TRO), and two groups consumed temperate grass (TEM) , with forages differing in quality or stage of maturity in each period. Tropical grass was bermudagrass (Cynodon dactylon) hay with three growing periods: 24-d regrowth (highest quality; H) , 42-d regrowth (moderate quality; M) and full season growth (lowest quality; L). Initially, TEM was orchardgrass (Dactylis glomerutu) hay harvested at three stages of maturity; however, hay of the highest quality was lost in a barn fire before the experiment began. Thus, highest quality TEM was endophyte-free fescue (Festuca arundinucea; early head emergence). Moderate and lowest quality TEM were orchardgrass hay harvested at post-anthesis and with seed in the dough stage, respectively. Bermudagrass hay was harvested from the same plot of a local farm; fescue and orchardgrass hays were harvested at a University farm. Sheep consumed hay ad libitum (offered at 1050 to 1100 g/kg of consumption on the preceding few days), with meals at 08:OOand 16:00 h on d 1 to 12 and at 08:OOand 20:00 h on d 13 to 21. In addition, sheep received at 08:OOh 3.5 g of a mineral mixture of 200 g/kg trace minerals (> 120 g/kg Zn, 100 g/kg Mn, 50 g/kg K, 25 g/kg Mg, 15 g/kg Cu, 3 g/kg I, 1 g/kg Co and 0.2 g/kg Se) and 800 g/kg salt. Composite feed samples were formed by sampling on d 14 to 21. Feces were collected in canvas bags on d 15 to 20 (4 d per sheep). Composite samples were formed by subsampling ( 100 g/kg) each day and then frozen between collections. Ruminal fluid was obtained by stomach tube on d 15 at 12:00 h (4 h post-feeding), strained through cheesecloth, acidified with H$O,, frozen and later analyzed for volatile fatty acids (VFA; Goetsch and Galyean, 1983). On the morning subsequent to removal of fecal bags, sheep (two to four per day) were placed in metabolism crates for sampling of blood, with feed and water availability maintained. Sheep had been previously accustomed to location in crates and sampling of blood and, thus, exhibited normal behavior. A priming dose ( 15 ml) of paru-aminohippuric acid (PAH; 30 g/l, w/v) was given at 05.30 h into the mesenteric vein catheter, followed by a continuous infusion (0.6 mllmin) for 8.5 h. Body weight was determined immediately after the last sample was obtained. Blood was withdrawn hourly from portal and hepatic vein and mesenteric arterial catheters from - 2 to 6 h post-feeding. A l-ml sample was obtained anaerobic-

A.R. Patil et al. /Livestock Production Science 43 (1995) 49-61

ally into a heparinized syringe and placed in ice. Hematocrit, oxygen saturation and hemoglobin concentration (OSM 2; Radiometer Corporation, Copenhagen, Denmark) were measured immediately. Oxygen concentration was calculated as described by Eisemann and Nienaber ( 1990). A lo-ml sample was taken in a tube with potassium oxalate and sodium fluoride and placed in ice. On the day of sampling, a 1.5ml aliquot was diluted with deionized water (4.5 ml) and subjected to automated procedures for PAH, ammonia nitrogen ( AMN) , urea nitrogen (UN) and cY-amino nitrogen (AAN), as described by Eisemann and Nienaber ( 1990). Concentrations of PAH and metabolites were averaged across times, and blood flow was calculated (Eisemann and Nienaber, 1990). Hematocrit concentration was used to estimate plasma flows. Remaining blood of the lo-ml sample was used to form composites for harvesting of plasma by centrifugation (3.7 Xg for 20 min; stored at - 80°C) and deproteinization with barium hydroxide and zinc sulfate (Yen et al., 1991) and perchloric acid (Harmon et al., 1991). Glucose concentration was determined in plasma (Sigma Diagnostics, Proc. No. 315; Sigma Chemical Company, St. Louis, MO, USA). Blood deproteinized with barium hydroxide and zinc sulfate was analyzed for VFA as described by Yen et al. ( 1991) , except that iso-caproic acid was added as an internal standard before gas chromatography. Blood deproteinized with perchloric acid was used to determine concentrations of r-lactate (Gutmann and Wahlefeld, 1974) and P-hydroxybutyrate (Williamson and Mellanby, 1974). Net metabolite fluxes were calculated as described by Burrin et al. ( 1991)) with plasma flows being used for glucose. Fecal composites were dried at 55°C and allowed to air-equilibrate. Hay and fecal samples were ground to pass a 1-mm screen and analyzed for dry matter (DM), ash, Kjeldahl nitrogen (N), energy (Association of Official Analytical Chemists, 1984) and neutral detergent fiber (Goering and van Soest, 1970; without sodium sulfite, decalin or ethoxyethanol). Hay samples also were analyzed for acid detergent fiber (nonsequential) and lignin (Goering and van Soest, 1970). Cellulose was estimated as loss in weight upon sulfuric acid treatment and hemicellulose as the difference between neutral and acid detergent fiber. The average of feed intake on the 6 d preceding and on the day of

51

blood sampling was used to calculate digestibilities. Data were analyzed as a split-plot by the General Linear Models procedure of Statistical Analysis System ( 1990) with forage source (or square), animal within forage source (error for forage source), period, forage quality (or stage of maturity) and the forage source X quality interaction. Differences among means were determined by least significance difference procedures when either the effect of quality or the forage source X quality interaction was significant (P < 0.05). The significance level of the forage source effect is presented in tables when less than 0.10. Simple correlation coefficients were determined for all data or within forage source. The Regression procedure of Statistical Analysis System ( 1990) was used to determine partial regression coefficients. Data from two animals were omitted from analyses because arterial catheters were patent only in the first period. In addition, the portal catheter of one sheep and the hepatic catheter of another were nonpatent in every period. The number of observations was 30 for feed intake, digestion and ruminal fluid concentrations of volatile fatty acids and ammonia, 27 for portal-drained viscera (PDV) and splanchnic tissue net fluxes and 24 for hepatic net flux.

3. Results 3. I. Forage composition Crude protein (CP) concentration declined within grass source as stage of maturity increased or quality decreased, and CP concentration was greater for TRO than for TEM at each quality (Table 1) . Neutral detergent fiber concentration was similar among different qualities of TRO and less for ‘EM-H than for EM-M and TEM-L. Acid detergent lignin concentration increased with decreasing quality, although change was much greater for TEIMthan for TRO. 3.2. Forage intake and digestion Dry matter and organic matter (OM) intakes were lowest (P < 0.05) for TEM-L and similar among other treatments. Organic matter, energy, NDF and N digestibilities were similar among TRO diets; however, digestibilities generally declined for TEM as quality decreased (Table 2). Digestible OM and energy

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A.R. Patil et al. /Livestock Production Science 43 (1995) 49-61

Table 1 Grass composition (dry matter basis) Item

Ash (g/k) Crude protein (g/kg) Energy (Meal/kg) Neutral detergent fiber (g/kg) Acid detergent fiber (g/kg) Acid detergent lignin (g/kg) Cellulose (g/kg) Hemicellulose (g/kg)

Tropical grass

Temperate grass

H

M

L

H

M

L

9.6 17.1 4.45 74.5 31.0 4.1 24.1 43.5

9.4 15.6 4.35 72.0 33.5 5.2 27.6 38.5

9.8 12.9 4.25 73.3 37.4 5.5 30.6 35.9

7.7 12.7 4.33 71.6 42.1 3.8 36.5 29.5

7.8 10.8 4.31 80.8 50.3 6.2 40.6 30.6

6.0 5.4 4.33 81.6 53.1 8.8 42.5 28.5

H = highest quality; M = moderate quality; L = lowest quality.

intakes were lowest (P < 0.05) for TEM-L and similar among other treatments. Digestible N intake declined with decreasing quality for both grass sources, and change was greater for TEM than for TRO. 3.3. Ruminalfluid AMN and VFA concentrations Ruminal AMN concentration was less (P < 0.05) for TEM-M and TEM-L than for TEM-H and for TROL than for TRO-M (Table 3). Total VFA concentration was lower (P < 0.05) for TRO-H than for TRO-L and for TEM-L than for TEM-H and TEM-M (Table 3). Molar proportions of acetate and propionate and the acetate/propionate ratio were similar among treatments. Molar proportions of isobutyrate, isovalerate and valerate were greater (P < 0.05) for TRO than for TEM. 3.4. Bloodflow and oxygen consumption Portal venous and hepatic venous and arterial blood flows were similar among treatments (Table 4). Oxygen concentrations were similar among TRO treatments and lower (P < 0.05) for TEM-M than for TEM-H and THM-L. Oxygen consumption by the PDV was lowest (P < 0.05) for TEM-L, hepatic oxygen consumption was similar among treatments, and splanchnit oxygen consumption was greater (P CO.05) for TRO than for TEM. 3.5.AAN, UNandAMN Concentrations of AAN did not differ among treatments (Table 4). Hepatic AAN uptake was greater for TRO than for TEM, although both PDV and splanchnic

AAN release were not altered by treatment. Urea N concentrations were greater (P < 0.05) for TRO than for TEM, similar among TRO treatments and greater for TEM-H than for TEM-M and TEM-L. Uptake of UN by the PDV was greater (P< 0.05) for TRO-L than for TRO-H and TRO-M and less (P < 0.05) for TEM-L than for TRM-H and TEM-M. Hepatic UN release was greater (P < 0.05) for TRO-L than for TRO-H and TRO-M and similar among TEM treatments. Splanchnic UN release was greater for TRO than for THM. Ammonia N concentration in portal blood was greater (P
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Table 2 Feed intake and digestion in sheep consuming different qualities of tropical and temperate grasses Item

Tropical grass

Dry matter intake (g/d) Organic matter Intake (g/d) Digestion g/kg g/d Energy Intake (Meal/d) Digestion kcal/Mcal Meal/d Neutral detergent fiber Intake (g/d) Feces (g/d) Digestion g/kg g/d Nitrogen Intake (g/d) Digestion

SE

Gm.SS source

H

M

L

H

M

L

1053b

997b

1109b

1004b

964b

595”

53.3

0.04

952b

902b

1002b

921b

889b

560a

49.3

0.07

625ab 593b

6258bc 562’

603” 603b

704’ 652b

66P 591b

589” 338”

14.8 32.1

785b 259

28.6’

P value

4.71b

4.35b

4.15b

2.51”

598* 2.59b

573” 2.69b

68W 2.96b

62gb 2.61b

515” 1.51”

15.1 0.157

717b 250

814b 299

7196 192

779b 204

485” 180

40.0 16.8

6340b 514be

734c 527’

739” 575’

617’ 306&

15.1 27.2

24.9’

702’ 19.Y

g/kg g/d

Temperate grass

22.5* 64oE

656’ 16.4’

14.4cd

20.4’ 689’ 14.0C

16.6b 55gb 9.3b

0.238

0.04

0.01

5.0”

1.09

0.01

392” 2.08

19.4 0.76

0.01 0.01

H = highest quality; M = moderate quality; L = lowest quality. “.b,C.““Means in a row without a common superscript differ ( P < 0.05).

M and similar among TRO treatments, and splanchnic

Lactate concentrations and net fluxes were not altered

release

by treatment.

of propionate

did not differ

among

treatments.

Table 3 Ruminal fluid ammonia nitrogen and volatile fatty acid concentrations in sheep consuming different qualities of tropical and temperate grasses Item

Tropical grass H

Ammonia nitrogen ( mg/ 100 ml) Volatile fatty acids Total ( mM/ 1) M/lOOM Acetate Propionate lsobutyrate Butyrate IsovaIerate Valerate Acetate/propionate

8.3bc

Temperate grass

M

L

9.8’

7.6b

H

M

SE

Grass source P value

L

7.1b

2.9”

3.3”

0.50

0.01 0.05

75.5b

86.9bc

91.1’

81.0be

77.5b

57.3”

4.47

73.3 16.8 1.21’ 6.44 1.32 0.92b 4.38

73.2 17.9 0.92b 6.31 0.94 0.17b 4.10

12.4 18.6 0.728b 6.79 0.72 0.81b 3.89

72.0 19.0 0.74* 6.63 0.79 0.7gb 3.82

75.4 17.1 0.62” 5.77 0.68 0.50” 4.45

72.5 18.3 0.84’b 6.98 0.88 0.49” 3.99

1.01 0.77 0.083 0.345 0.121 0.069 0.223

H = highest quality; M = moderate quality; L= lowest quality. ‘*b*cMeans in a row without a common superscript differ (P < 0.05).

0.01 0.05 0.01

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Production Science 43 (1995) 49-61

Table 4 Whole blood flow and oxygen and alpha-amino, urea and ammonia nitrogen measures in sheep consuming different qualities of tropical and temperate grasses Item

Tropical grass

Temperate grass

SE

GMSS

source H Blood flow (I/h) Portal vein 119 Hepatic vein 143 Hepatic artery 24 Oxygen Whole blood concentration (mM/l) Portal vein 3.11b Hepatic vein 2.11ab 4.90& Artery Consumption (mM/h) Portal-drained viscera 171c Hepatic 135 Splanchnic 302 a-Amino nitrogen Whole blood concentration (mM/l) Portal vein 5.67 Hepatic vein 5.32 5.32 Artery Net flux (mM/h) Portal-drained viscera 31.9 Hepatic -32.1 Splanchnic 3.2 Urea nitrogen Whole blood concentration (n&I/l) Portal vein 14.7c Hepatic vein 15.1C 15.0c Artery Net flux (mM/h) Portal-drained viscera - 24.8” Hepatic 47.0b 27.3 sp1anchnic Ammonia nitrogen Whole blood concentration (mM/l) 0.476 Portal vein Hepatic vein 0.286 Artery 0.308 Net flux (mM/h) Portal-drained viscera 20.6 -23.8 Hepatic Splanchnic -3.3

M

L

H

M

L

114 139 21

123 148 33

112 126 12

113 151 35

90 114 22

3.12b 2.83abc 4.74k 134b” 106 262

3.35” 2.6ga 4.49* 14gk 124 265

4.73 4.50 4.44 32.1 - 17.1 8.2

6.03 5.32 5.23 40.9 - 28.4 12.3

13.0’ 14.2” 13.9”

12.0c 13.1’ 12.9’

- 17.3* 46.gb 34.6

-43.0’ 68.5’ 23.6

0.388 0.176 0.180 25.6 - 24.5 -0.2

0.460 0.298 0.300 22.9 - 22.6 -0.3

3.75b 3.17bc 4.93’ 133b 97 235

4.74 4.65 4.47 32.8 -21.5 10.7

8.gb 9.2b 9.lb

3.04’ 2.51’ 4.264 137b 123 268

5.31 5.31 5.06 32.4 - 17.9 17.4

6.1’ 6.5” 6.9”

- 28.7& 42.gab 10.8

- 18.7* 35.9”s 16.8

0.440 0.248 0.286

0.312 0.232 0.230

17.8 - 18.4 0.0

8.8 - 10.6 -0.9

P value

3.69b 3.24’ 4.68& 86” 86 179

6.34 6.44

6.10 21.1 - 12.6 11.3

6.8 10.4 6.1

0.132 0.120 0.150 10.2 14.3 23.4

0.07 0.04

0.447 0.458 0.423 5.51 4.54 6.57

0.03

6.3” 6.5a 6.4”

0.94 0.79 0.75

0.01 0.01 0.01

- 9.8’ 28.9= 18.5

4.82 5.61 5.75

0.02 0.01 0.01

0.0583 0.0429 0.0420

0.04

3.27 2.67 1.31

0.02 0.06

0.378 0.246 0.258 11.1 -11.3 0.2

H = highest quality; M = moderate quality; L = lowest quality. a.b*cMeansin a row without a common superscript differ (P < 0.05).

3.7. Acetate, butyrate, BHBA and minor VFA Portal venous blood acetate concentration was similar among treatments, acetate concentration in hepatic venous blood was less (P < 0.05) for TEM-L than for

TEM-H and TEM-M and similar among TRO treatments, and arterial acetate level was not affected by treatment (Table 6). Acetate release by the PDV and splanchnic bed were similar among treatments but

A.R. Patil et al. /Livestock Production Science 43 (I 995) 49-61

55

Table 5 Glucose, propionate and lactate measures in sheep consuming different qualities of tropical and temperate grasses Item

Tropical grass H

Glucose Plasma concentration (mM/l) 2.92 Portal vein 3.02ab Hepatic vein 2.94 Artery Net flux (mM/h) -5.2 Portal-drained viscera 11.9 Hepatic 9.1 Splanchnic Propionate Whole blood concentration (mM/l) 0.149’b Portal vein 0.028b Hepatic vein o.017be Artery Net flux (mM/h) Portal-drained viscera 16.3”’ - 14.9ab Hepatic Splanchnic 1.5 Lactate Whole blood concentration (mM/l) 1.39 Portal vein 1.09 Hepatic vein 1.15 Artery Net flux (mM/h) Portal-drained viscera 25.5 Hepatic - 35.5 -7.3 Splanchnic

Temperate grass M

2.87 3.10b 2.94 - 7.9 24.1 17.4

0.144” 0.023’h 0.014”b 14.4ab - 13.6ab 1.1

1.35 1.24 1.31 5.5 - 12.1 -6.4

L

2.77 3.00ab 2.86 -8.3 30.5 18.0

0.164b 0.029” 0.021Cd

H

2.75 2.96ab 2.78 -3.1 20.0 16.3

SE M

2.97 3.11b 3.03 -5.1 21.9 17.3

0.213b 0.036’ 0.023d

L

2.75 2.86” 2.78 -2.5 10.8 9.0

0.095” 0.020” 0.012”

18.2’ - 17.2” 1.2

22.5b -21.9’ 1.9

18.1b - 17.8” 1.8

7.6” - 6.7b 0.8

1.54 1.32 1.37

1.41 1.37 1.33

1.58 1.23 1.42

1.25 0.98 1.11

33.7 -38.1 - 8.9

8.3 - 10.9 -4.5

19.0 - 27.5 - 2.2

13.5 - 20.3 -6.6

Grass source P value

0.074 0.054 0.064 3.13 6.27 4.20

0.07

0.0206 0.0020 0.0017 2.95 3.10 0.36

0.154 0.116 0.150 8.23 11.04 4.16

H = highest quality; M = moderate quality; L = lowest quality. “.b.cdMeansin a row without a common superscript differ (P < 0.05)

numerically lowest for TEM-L. Hepatic net flux of acetate was greater (P ~0.05) for TEM-M than for TRO treatments and TEM-H and greater (P
Differences among treatments were observed in portal venous blood concentrations of isobutyrate, isovalerate and valerate and in arterial concentration of valerate (Table 6). Portal-drained viscera release of isobutyrate and isovalerate tended to be lowest for L treatments. Splanchnic net fluxes of minor VFA did not differ among treatments.

4. Discussion 4. I. Forage, composition

The observed greater differences among TEM than TRO grasses in concentrations of NDF and acid detergent lignin and digestibilities were expected based on

56

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Table 6 Acetate, butyrate, beta-hydroxybutyrate and minor volatile fatty acid measures in sheep consuming different qualities of tropical and temperate grasses Item

Tropical grass

Temperate grass

SE

Grass SOUIIX

H Acetate Whole blood concentration Portal vein Hepatic vein Artery Net flux (mM/h) Portal-drained viscera Hepatic Splat&tic Butyrate Whole blood concentration Portal vein Hepatic vein Artery Net flux (mM/h) Portal-drained viscera Hepatic Splanchnic BHydroxybutyrate Whole blood concentration Portal vein Hepatic vein Artery Net flux (n&i/h) Portal-drained viscera Hepatic Splanchnic Isobutyrate Whole blood concentration Portal vein Hepatic vein Artery Net flux (mM/h) Portal-drained viscera Hepatic Splanchnic Isovalerate Whole blood concentration Portal vein Hepatic vein Artery Net flux (mM/h) Portal-drained viscera Hepatic Splanchnic Valerate Whole blood concentration Portal vein Hepatic vein Artery Net flux (mM/h) Portal-drained viscera Hepatic Splanchnic

(mM/ 1) 1.50 1.52’ 1.04 69.7 - 16.7” 69.4 (mM/l) 0.014* 0.008’b o.005ab l.31ab - 0.96 0.43 (mM/l) 0.548’ o.594b 0.487 5.4 7.5bc 15.1b (mM/l) O.OloE 0.002 0.005 l.lOb -l.Wb 0.16

M

1.10 l.llab 0.68 47.5 - 6.3” 57.6

0.018b 0.008”b 0.005” 1.39* - 0.97 0.44

0.427ab 0.484’ 0.392 2.9 7.1* 12.2*

0.008bc 0.002 0.001 0.78* - 0.74& 0.00

L

1.29 1.28& 0.90 66.1 - 17.6’ 54.1

0.022b O.Olob 0.C@8b l.85b - 1.42 0.38

0.403* 0.473a 0.408 4.6 5.1a 9.7’

H

1.44 1.22& 0.84 68.0 - 16.7’ 55.3

0.025b 0.012b 0.008b 2.14b - 1.67 0.59

0.461bc 0.492a 0.402 6.7 6.7”b 13.9”

0.00~ 0.002 0.007

0.008” 0.001 0.001

0.19O - l.loL - 0.73

0.81* - 0.83”= 0.00

M

1.30 1.30& 0.72 66.2 10.6’ 79.7

0.017b 0.w 0.004’ 1.4ob -0.90 0.62

0.377” 0.442a 0.342 3.7 8.6” 13.F

o.007b 0.001 0.001 0.68* -0.71c 0.03

L

0.90 0.86” 0.52 34.6 o.4k 36.1

0.007” 0.004” 0.003* 0.37’ -0.20 0.13

P value

0.139 0.115 0.109 12.20 4.62 10.52

0.0035 0.0018 0.0010

0.348 0.162

0.41Pb 0.480’ 0.369

0.0309 0.0263 0.0335 0.03

4.5 5.4’ 9.9”

1.13 0.59 1.48

o.005L 0.001 0.001

0.0006 0.04 0.0003 0.06 0.0014

0.37a - 0.406 - 0.08

0.225 0.090 0.253

(mM/l ) o.o12b 0.005 0.003 1.03b - 0.93” 0.22 (rnM/l) 0.007b 0.002 0.002* 0.61 - 0.56 0.01

0.012b 0.003 0.002 1.02b - 0.85’ 0.12

o.oo2’b 0.001 0.001’ 0.16 -0.04 0.11

0.006b 0.006 0.003 0.51* - 0.08b 0.39

0.001* 0.001 0.001* 0.13 0.06 0.03

o.o12b 0.005 0.006 0.67” - 0.72’ -0.13

0.005* 0.004 0.004b 0.09 -0.13 - 0.08

0.009ab 0.002 0.002 0.76b - 0.85’ - 0.07

0.001’ 0.001 0.003’ 0.08 -0.11 -0.02

0.06

0.007’ 0.004 0.003 0.31P -0.16b -0.10

0.0013 0.0014 0.0011 0.165 0.199 0.120

0.001” 0.003 0.001’

0.0019 0.0918 0.0010

0.04 0.27 0.31

0.173 0.345 0.180

H = highest quality; M = moderate quality; L = lowest quality. a*b%katts in a row without a common superscript differ (P< 0.05).

A.R. Patil et al. /Livestock Production Science 43 (1995) 49-41

different growth characteristics and tissue composition and proportions (Wilson, 1993). Higher concentrations of CP in TRO than TEM were not expected and may relate to different growing conditions (e.g., precipitation) and management practices. Overall, quality of these grasses should be regarded as low to moderate. Therefore, results of this experiment may not be directly applicable to grasses higher in quality or harvested at less mature stages of growth.

57

similar acetate/propionate. However, propionate concentration was 12.7, 15.5, 16.9, 15.5, 13.3 and 10.5 mM/l for TRO-H, TRO-M, TRO-L, TEM-H, TFM-M and TEM-L, respectively (SE 1.12), being greater (P < 0.05) for TRO-L than for TRO-H and for TEMH than for TEM-L. Greater concentrations of total VFA and propionate for TRO-L than for TRO-H correspond to a presumed increase in nonstructural carbohydrate concentration in TRO as quality declined, as indicated by similar NDF concentration and decreasing CP level.

4.2. Forage intake and digestion 4.4. Oxygen consumption Typically, DE intake is greater for TEM than for TRO (Minson, 1990). In this experiment, similar DE intake among treatments except TEM-L may involve the high age of these sheep and a limited capacity for protein accretion, which restricted feed intake potential (Webster, 1993). Lowest OM digestibility for TEM-L presumably was associated with the high concentration of acid detergent lignin relative to NDF and low digestibility. In addition, the low concentration of CP in TEM-L suggests that ruminal availability of nitrogenous compounds may have contributed to low intake via limited microbial growth. However, because the concentration of AMN in ruminal fluid was similar for TEM-M and TEM-L yet OM digestibility was greater for TEM-M, low availability of AMN to ruminal microbes for TEM-L did not seem to markedly limit ruminal OM degradation. 4.3. Ruminalfluid concentrations of VFA and AMN Ruminal fluid AMN concentration did not markedly vary among TRO treatments despite a considerable range in CP concentration, perhaps because of substantial ruminal recycling of N regardless of forage CP level. Similar ruminal AMN concentration for TEMM and TEM-L despite an appreciably higher CP concentration in TEM-M presumably was the sum effect of greater OM digestibility and a tendency for greater PDV uptake of UN for TEM-M. However, ruminal fluid was collected at 4 h post-feeding, probably slightly after peak AMN concentration. Minson ( 1990) summarized that acetate/propionate in ruminal fluid typically is lower for TEM than for TRO primarily because of the higher level of nonstructural carbohydrate in TEM. The absence of marked differences among treatments in NDF concentration in this experiment, thus, may have been responsible for

Lowest PDV oxygen consumption for TEM-L presumably resulted from low OM intake and digestion. Energy consumption by the PDV, liver and splanchnic bed (assuming 110 kcal/M oxygen; McLean, 1972) related to digestible energy (DE) intake (Fig. 1) , as indicated by Huntington (1990) and Johnson et al. ( 1990). Relationships in Fig. 1 are results of differences in intake and digestibility among grass sources and qualities. The rate of increase in PDV energy consumption with increasing DE intake was slightly greater than that of the liver (0.079 vs. 0.062 Meal/ Meal of DE intake). Goetsch and Ferrell ( 1995) observed slightly greater values in sheep consuming ad libitum different forage-based diets, with similar change noted for the PDV and liver (0.107 and 0.098, respectively). These differences in results probably relate to a wider range in quality of forage and 0,20 or 40% maize in diets consumed by younger wethers in the Goetsch and Ferrell ( 1995) study.

T

z

cr

‘.OO I- - PDV 0.80

I--HEP SPL

0

1

2

Dlgeetible energy

3

4

intake (Mcel/d)

Fig. 1. Relationships between portal-drained viscera (PDV), hepatic (HEP) and splanchnic hed (SPL) energy consumption (Meal/d) and digestible energy intake ( DEI, M&/d). PDV = 0.15 1 + 0.079 DEI; R’=0.32; HEP=0.138+0.062 DEI; R2=0.21; SPL = 0.299 + 0.143 DEI; R2= 0.40.

A.R. Patil et al. /Livestock Production Science 43 (1995) 49-61

c -

0.00

---

0.50

100

g/dfeed

NDF

240 g/d fecal NDF .’

900 g/d ,..%I NDF

_.__ 0

1

Digestible

2

energy

3

4

intake (Ycel/d)

2. Relationship between portal-drained viscera (PDV) energy consumption (Meal/d) and digestible energy intake (DEL Meal/ d) with 180,240 and 300 g/d of fecal neutral detergent fiber (NDP) excretion(g/d)(-8.9+(5ODEI)+(l.lOfecalNDF);R2=O.58).

Splanchnic tissue energy consumption, particularly that of the PDV, may have been altered by the quantity of digesta in the gut independently of DE intake. In support, the R2 of a regression of PDV energy consumption on DE intake and fecal NDF (Fig. 2) was considerably greater than that of the regression on DE intake alone (0.58 vs. 0.32). In Fig. 2, PDV energy consumption was predicted for fecal NDF excretion of 180,240 and 300 g/d. Standardized partial regression coefficients for this equation indicated that DE intake and fecal NDF accounted for 40 and 60%, respectively, of explained variation. Similarly, Sun et al. (1994) observed that gastrointestinal tract mass relative to digestible OM intake was greater for growing wethers consuming 75% forage diets with TRO than TEM, attributing the difference to greater mass of digesta in the gut with TRO. Also, Huntington et al. (1988) observed greater PDV oxygen consumption in relation to DE intake in steers consuming orchardgrass vs. alfalfa silage of higher digestibility. Splanchnic tissue energy consumption as a percentage of DE intake was not appreciably altered by grass quality with TRO but rose as TEM quality declined (32,26,29,21,28 and 35% for TRO-H, TRO-M, TROL, TEM-H, TEM-M and TEM-L, respectively; SE 2.50). In part, this may relate to greater differences in chemical composition (e.g., acid detergent lignin/ NDF) and digestibility among qualities of TEM than TRO. In accordance, Goetsch and Ferrell ( 1995) suggested that potential to decrease the proportion of DE intake comprised by energy consumed by splanchnic tissues is greater for low compared with moderate or

high quality forage. The difference between DE intake and splanchnic tissue energy consumption in our experiment, representing apparent energy available for use by extra-splanchnic tissues, was similar among TRO treatments but ranked H > M > L for IEM diets ( 1.79, 1.83, 1.78, 2.32, 1.91 and 1.03 Meal/d for TRO-H, TRO-M, TRO-L, TEM-H, TEM-M and TEM-L, respectively; SE 0.164). The low value for TEM-L was because of low DE intake and high splanchnic tissue energy consumption/DE intake; the difference between TEM-H and TEM-M was a consequence of similar DE intake but different splanchnic tissue energy consumption/DE intake. 4.5. AAN, UN and AMN netfluxes Apart from TEM-L, for which forage CP concentration and OM digestibility were lowest, forage CP concentration did not appear to impact PDV release of AAN, regardless of forage source. Similarly, Reynolds et al. (1992) observed that intake of metabolizable energy rather than of N dictated PDV AAN release in growing beef steers consuming 75% alfalfa or concentrate diets. Huntington et al. ( 1988) noted similar PDV release of AAN by beef steers consuming alfalfa or orchardgrass silage despite considerably greater N intake with alfalfa. Release of AMN by the PDV in our experiment did not increase with increasing forage CP concentration, which, coupled with similar release of AAN by the PDV among treatments, reflects variation among treatments in, and a large contribution of, recycled UN to N released by the PDV in amino acids and ammonia. Long-term recycling of urea to the reticula-rumen presumably is regulated by arterial UN concentration (van der Walt, 1993) and urease activity of rumenwall adherent bacteria, which is elevated by low ruminal AMN concentration (Cheng and Costerton, 1980). Short-term regulation of N recycling to the reticulorumen appears via differences in arterial UN concentration (van der Walt, 1993), osmotic pressure, epithelial area involved in transfer and rumen wall permeability to urea (Remond et al., 1993a). Factors influencing permeability include ruminal levels or absorption of carbon dioxide, VFA, particularly butyrate, and ammonia (Remond et al., 1993b). Uptake of UN by the PDV was moderately correlated with ruminal concentrations (mM/l) of total VFA

A.R. Patil et al. /Livestock Production Science 43 (1995) 49-61

(r=0.44;P<0.02) andbutyrate(r=0.34;P<0.08). Arterial UN concentration did not correlate with PDV uptake of UN (all data or within grass source). In part, this might relate to varying proportions of UN entering the reticulorumen and postruminal digestive tract. Egan et al. ( 1986) summarized that the majority of N recycled is to the reticula-rumen, although van der Walt (1993) suggested that as plasma UN concentration rises above 8 mM/l, recycling to the postruminal tract increases. Grass N concentration, digestible N intake/DE intake and ruminal AMN concentration for TRO diets were negatively related to UN uptake by the PDV (I= -0.68, P
59

energy for ureagenesis and uptake of amino acids. However, with younger animals possessing greater potential for peripheral protein accretion and feed intake (Webster, 1993)) greater microbial capture of forage N in microbial cells and perhaps greater ruminal escape of forage protein should lessen these negative impacts of high forage N concentration, depending on adequacy of energy available to support peripheral protein accretion. 4.6. Glucose and propionate netfluxes Because negligible intestinal glucose absorption was expected and PDV glucose uptake was greater for TRO than for TEM, metabolism of glucose by the PDV may have been greater for TRO (2-fold), although factors responsible are unclear. However, perhaps the quantity of digesta in the gut altered glucose use by the PDV as postulated for oxygen consumption. Nonetheless, hepatic glucose release was similar for TRO and TBM and, thus, the effect of grass source on PDV glucose metabolism may not have altered hepatic energy consumed for gluconeogenesis or precursor uptake. This may have been a consequence of low glucose demand by peripheral tissues with minimal peripheral lipogenesis resulting from limited DE intake with the allforage diets. That propionate concentration in ruminal fluid increased with TRO and decreased with TEM as quality declined, and PDV propionate release did not vary among treatments with similar DE intake (TRO-H, TRO-M, TRO-L, TEM-H and TEM-M), imply that PDV propionate metabolism increased as propionate availability rose, as suggested by Veenhuizen et al. ( 1988). Assuming complete use of propionate taken up by the liver for gluconeogenesis, the potential proportion of released glucose arising from propionate tended to be greater for H than for M and L (76, 35, 31, 66, 45 and 36% for TRO-H, TRO-M, TRO-L, TEM-H, TEM-M and TBM-L, respectively; SE 13.1) . In conclusion, with low to moderate quality grass as used in our experiment, besides greater energy loss in feces, grasses of low digestibility may be metabolized less efficiently than ones more digestible because of differences in splanchnic tissue energy consumption. In addition, grass quality may impact energy available to extra-splanchnic tissues more with TEM than TRO, at least in part because of greater differences in com-

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A.R. Patil et al. /Livestock

Production Science 43 (1995) 49-61

position and digestibility among TEM. Lastly, with grass of at least 10% CP, CP concentration may not affect PDV release of amino acids.

Acknowledgement

Appreciation is expressed to the NRI Competitive Grants Program/USDA (Award No. 92-37208-8 185) and Arkansas Science & Technology Authority (Award No. 94-B-02) for partial financial support.

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by injecting phlorizin and feeding propionate. J. Nutr., 118: 1366-1375. Webster, A.J.F., 1993. Energy partitioning, tissue growth and appetite control. Proc. Nutr. Sot., 52: 69-76. Williamson, D.H. and Mellanby, J., 1974. d-( - )-3-Hydroxybutyrate. In: H. Bergmeyer (Ed.), Methods of Enzymatic Analysis (Vol. 4). Academic Press, New York, NY, pp. 1836-I 839. Wilson, J.R., 1993. Organization of forage plant tissues. In: H.G. Jung, D.R. Buxton, R.D. Hatfield and J. Ralph (Eds.),Forage Cell Wall Structure and Digestibility, Amer. Sot. Agron, Crop Sci. Sot. Am., Soil Sci. Sot. Am. Agron, WI, pp. l-32. Yen, J.T., Nienaber, J.A., Hill, D.A. and Pond, W.G., 1991. Potential contribution of absorbed volatile fatty acids to whoie-animal energy requirement in conscious swine. J. Anim. Sci., 69: 20012012.