Potential nutritional assessment of dwarf elephant grass (Pennisetum purpureum Schum. cv. Mott) by chemical composition, digestion and net portal flux of oxygen in cattle

Animal Feed Science and Technology 104 (2003) 29–40

Potential nutritional assessment of dwarf elephant grass (Pennisetum purpureum Schum. cv. Mott) by chemical composition, digestion and net portal flux of oxygen in cattle G.V. Kozloski a,∗ , J. Perottoni b , M.L.S. Ciocca c , J.B.T. Rocha d , A.G. Raiser e , L.M.B. Sanchez f a

e

Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, RS, Brazil b Universidade Federal de Santa Maria (UFSM), Santa Maria, RS, Brazil c Animal Science Department, UFRGS, Porto Alegre, RS, Brazil d Chemistry Department, UFSM, Santa Maria, RS, Brazil Small Animal Surgery and Clinics Department, UFSM, Santa Maria, RS, Brazil f Animal Science Department, UFSM, Santa Maria, RS, Brazil

Received 13 June 2001; received in revised form 30 October 2002; accepted 30 November 2002

Abstract This study was conducted to evaluate changes in chemical composition of dwarf elephant grass hay cut at 30, 40, 50 and 60 days of growth, and its effect on apparent digestibility and particle-phase passage through the gastrointestinal tract (Experiment 1) and on oxygen utilization by the portal-drained viscera of cattle (Experiment 2). The experiments were carried out using four Holstein steers in each experiment (mean live weight of 129 ± 17 kg and 139 ± 3 kg in the Experiments 1 and 2, respectively) in a 4×4 Latin square experimental design. The animals of Experiment 2 were implanted surgically with permanent indwelling catheters in portal and mesenteric veins. The experimental diets were chopped hay (10–15 cm length particles) fed two times a day in amounts restricted at 2% of the animal live weight. The content of neutral detergent fiber, acid detergent fiber, acid detergent lignin, titratable cross-ester links and acid detergent insoluble nitrogen increased, but

Abbreviations: DM, dry matter; OM, organic matter; NDF, neutral detergent fiber; ADF, acid detergent fiber; ADL, acid detergent lignin; TEL, titratable ester links; N, total nitrogen; NPN, non-protein nitrogen; ADIN, acid detergent insoluble nitrogen; TT, transit time; RTrr, retention time in the reticulum-rumen; RTcc, retention time in caecum-proximal colon; PRrr, passage rate of particle-phase through reticulum-rumen; PRrr, passage rate of particle-phase through caecum-proximal colon; DE, digestible energy; PDV, portal-drained viscera; PBF, portal blood flow; PFOx, portal flux of oxygen; PHP, heat production by the portal-drained viscera. ∗ Corresponding author. Present address: FEPAGRO, Centro de Pesquisa Noroeste e Missões, BR 285, km 455, Caixa Postal 191, Iju´ı 98700-000, RS, Brazil. E-mail address: [email protected] (G.V. Kozloski). 0377-8401/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-8401(02)00328-0

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the content of total, non-protein and soluble nitrogen decreased with plant growth. The apparent digestibility of different compounds of hay was not affected (P > 0.05), but retention times in the reticulum-rumen (P < 0.01) and in the caecum-proximal colon (P < 0.05) decreased linearly and, the passage rate of particles through reticulo-rumen (P < 0.01) and through caecum-proximal colon (P < 0.05) increased linearly with plant growth. In addition, oxygen utilization and estimated heat production by the portal-drained viscera, as a proportion on digestible energy intake, increased linearly with dwarf elephant grass growth (P < 0.01 and P < 0.05, respectively). In conclusion, if the nutritional value of forage decrease with plant maturity, the results of present study suggest that, at restricted level of intake and within of vegetative stage of dwarf elephant grass this is so due, at least partially, an increase of metabolizable energy use by the gastrointestinal tract. Therefore, energy availability for productive purposes, such as live weight gain or milk synthesis decrease. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Tropical forage; Nutritive value; Digestion; Oxygen portal flux

1. Introduction In tropical climate areas, as well as during the summer in subtropical regions, the productivity of cattle fed on forages is limited due to the low quality of the forage species that grow in this area, among other factors. However, some tropical grasses, such as dwarf elephant grass (Pennisetum purpureum Schum. cv. Mott), stand out among species of this group as it produces large amounts of digestible dry matter (DM) per hectare and also for promoting high weight gains per animal and per area (Almeida et al., 2000). However, the knowledge of the intrinsic values of this forage and exactly how they affect the digestion, the metabolism, and the efficiency of feed utilization for productive processes by the animal still need to be determined. The chemical composition, specially in terms of cell wall components and their interactions, may limit the extraction of substrates by the ruminal microorganisms, and therefore represents a factor of forage which affects the availability of nutrients to the animal (Hatfield, 1993; Jung and Deetz, 1993; Van Soest, 1993; Wilson, 1997; Coleman et al., 1999; Satter et al., 1999). Moreover, digestion and absorption are the early steps of a complex process through which animals obtain from the environment energetic and constitutive components. In subsequent steps, absorbed nutrients enter the bloodstream and are transported, by the portal venous system, to the liver, which controls the levels of nutrients that will be distributed to the peripheral tissues (Seal and Reynolds, 1993). However, the functions carried out by the gastrointestinal tissues have a considerable energetic cost and are influenced by a variety of factors, such as diet composition, among others. Reviewing studies that measured the liver and portal flux of nutrients, Kozloski et al. (2001) observed that much of the variation of the efficiency of metabolizable energy utilization among feeds is associated with the metabolism of the gut. The efficiency of ruminant production systems based on forages, as the main source of protein and energy, are strongly dependent on forage maturity, which is considered a primary factor for the decrease in its nutritional quality (Nelson and Moser, 1994). However, although this has been well established for forages at different stages of development (e.g. vegetative versus reproductive or mature), there are limited data regarding the influence of maturity

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within the vegetative stage, particularly when no significant changes occur in the leaf:stem ratio. Therefore, this study was designed to evaluate the nutritional potential of the dwarf elephant grass for ruminants through its composition and effects on digestion, net oxygen portal flux, and how these factors are influenced by forage maturity.

2. Materials and methods 2.1. Experimental feedstuffs and treatments This study was carried out to evaluate the change in the chemical composition of the dwarf elephant grass (Pennisetum purpureum Schum. cv. Mott) cut at a height of 20 cm from the soil at 30, 40, 50, and 60 days of growth. Additionally, two experiments were carried out with animals in order to evaluate the effect of growth on apparent digestibility and particle-phase passage through the gastrointestinal tract (Experiment 1) and the use of oxygen by the portal-drained viscera (PDV) of cattle (Experiment 2). The experiments with animals were carried out with four Holstein steers in each experiment (mean live weights of 129 ± 17 kg and 139 ± 3 kg in Experiments 1 and 2, respectively) in a Latin square 4 × 4 experimental design. The experimental diets consisted of chopped hay (10–15 cm particles) supplied twice a day in restricted amounts of approximately 2% of the animals live weight in order to avoid refusals (1.7 times maintenance DM intake, estimated from NRC, 1989). A commercial mineral premix containing the following components (per kg): Ca: 60 g, P: 45 g, S: 4.12 g, Na: 152 g, Co: 39 mg, Cu: 1050 mg, Fe: 1300 mg, I: 50 mg, Mn: 1000 mg, Se: 9 mg, Zn: 2520 mg and F: 450 mg., was also fed at the proportion of 1% of the dry matter. 2.2. Experiment 1: Digestibility and particle-phase passage Before the experiment and for approximately 30 days, the animals received a dwarf elephant grass hay mixture of 40–50 days of growth ad libitum. Experiment 1 was carried out in four periods of 15 days, with the first 10 days for adaptation to the hay and the last 5 days for sample collection. To measure digestibility, hay intake was recorded and all feces were collected daily during the last 5 days of each experimental period. The feces were weighed, homogenized and sampled every day. There were no refusals of hay during sample collection in the experimental periods. Hay and feces samples were dried at 55 ◦ C for 96 h in a forced-air oven, ground through a 1 mm screen and analyzed for DM, ash and total nitrogen (N). The DM was determined by drying at 105 ◦ C to a constant weight. Ash was determined by combusting at 550 ◦ C to a constant weight. Organic matter (OM) was calculated as DM-ash. Total N was assayed by a Kjedahl method (Method 984.13, AOAC, 1995), modified by using a solution of boric acid (4%, w/v) to receive the free ammonia during distillation; a solution of 0.2% (w/v) of bromocresol green and 0.1% (w/v) of methyl red in ethanol as indicator; and a standard acid solution (sulfuric acid) for titration. Acid detergent fiber (ADF) and acid detergent lignin (ADL) were determined according to Robertson and Van Soest (1981) and, neutral detergent fiber (NDF), according to Van Soest et al. (1991) (without sodium sulfite and alpha amylase, and ash content excluded). The content of soluble

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N, non-protein N (NPN) and acid detergent insoluble N of hay samples were analyzed according to Licitra et al. (1996) and, the number of titratable ester links (TEL), according to Lau and Van Soest (1981). The estimation of particle-phase passage through the whole gastrointestinal tract was performed by the use of chromium mordanted fiber (CMF) prepared for each hay according to Udén et al. (1980). After giving each animal a dose of CMF, feces were collected directly from the rectum at 0, 3, 6, 9, 12, 18, 24, 30, 36, 48, 60, 72, 96 and 120 h. Feces samples obtained for particle-phase passage measurement were analyzed for DM (AOAC, 1995) and chromium using an adapted colorimetric method (Czarnocki et al., 1961): 500 mg of feces; OM digestion conducted at 200 ◦ C with 5 ml nitric acid, followed by the addition of 3 ml of hydrogen peroxide at 130 volumes; chromium solubilized at 200 ◦ C; wavelength reading at 360 nm; absorbance of chromium samples corrected by mean absorbance found in the feces collected before CMF supply. Passage parameters were estimated by the analysis of individual curves of chromium fecal excretion, according to the mathematical model of Grovum and Willians (1973). Natural logarithms of chromium concentration values of descending and ascending portions of the curve were analyzed by regression and the slopes were called K1 and K2 , respectively. The point at which started the regression analysis of descending phase was selected by eye and, for each data set, several curves were estimated by selecting different points of this portion of the curve. The values of chromium concentration obtained from estimated curves were compared with observed data using the χ 2 -test. The curve that resulted in the lowest χ 2 -value was selected. The retention time in the reticulum-rumen (RTrr) and in caecum-proximal colon (RTcc) were calculated as the reciprocal of the natural logarithmic slope of descending and ascending phase of curve (1/K1 and 1/K2 , respectively). Passage rate of particle-phase through reticulum-rumen (PRrr) and through caecum-proximal colon (PRcc) were the natural logarithmic slopes expressed as percentage per hour ((K1 and K2 ) × 100, respectively). 2.3. Experiment 2: Net portal flux of oxygen Four Holstein steers were implanted surgically with permanent indwelling catheters in the portal and mesenteric veins according with adapted method from Huntington et al. (1989). The portal catheters and one of the mesenteric catheters were made of Teflon tubes of 1.2 m length (1.5 mm o.d. × 2.3 mm i.d.). A second mesenteric vein was catheterized as a safety measure. In this case, a silicon gastric probe of 1.2 m length and 1 mm internal diameter (Levine gastric probe, no. 4, Mark Med Ind. e Com. Ltda., SP, Brazil) was used. For arterial blood collection, one of the carotids was surgically elevated in order to be placed closer to the skin. All animals recovered quickly, and feed intake was already normal in the day following surgery. On sampling days, a temporary catheter (Insyte 20 gauges, 3 cm length × 1.1 mm i.d.) attached to an extension with a three-way valve was introduced in the carotid artery. After a period of approximately 3 weeks before and 1 week after surgeries, when the animals were already housed and receiving ad libitum a dwarf elephant grass hay mixture of 40–50 days of growth, Experiment 2 was carried out in four periods of 8 days, being the first 7 days for adaptation to the hay, and the last for sample collection. In the morning of eighth day of each experimental period, hay was offered and kept available for the animals for only 1 h. After this period, hay residues were removed and weighed. Immediately after,

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portal blood flow was measured by the downstream dilution of a primed (8 ml) followed by continuous infusion (1 ml min−1 ) of p-aminohippurate solution (7%, w/v, pH 7.4). Beginning 30 min after p-aminohippurate infusion, two samples of each arterial and portal blood were simultaneously collected in heparinized syringes hourly during 8 h. Between sampling intervals, catheters were kept heparinized with a physiological solution containing about 20 IU ml−1 of heparine. One of the samplings (±20 ml) was used for the analysis of packed cell volume (micro-centrifuge), hemoglobin (iron cyanide method, LABTEST kit) and p-aminohippurate concentration (Huntington, 1982). The other sampling (±2 ml) was obtained anaerobically for immediate measurement of the oxygen concentration (automatic system of blood gasometry, AVL 990 busy, Austria). Portal blood flow (PBF) was estimated by the portal dilution of p-aminohippurate according to Huntington et al. (1989). The oxygen blood concentration resulted from the sum of the oxygen associated to hemoglobin plus the oxygen dissolved in the blood aqueous phase, calculated according to Huntington and Tyrrel (1985). Net PDV oxygen flux was calculated multiplying PBF by the difference between portal and arterial concentration of oxygen. The amount of aerobic heat produced by the PDV was estimated from the resulting use of oxygen taking into account a heat equivalent of 4.89 kcal l−1 of oxygen uptake (Huntington and Tyrrel, 1985). 2.4. Statistical analysis The change in the chemical composition of hay as plant ages was analyzed by regression. Different trend lines were tested for each hay chemical component, and the best fit line, expressed by the highest r2 -value, was chosen. Digestibility and passage results (Experiment 1) were submitted to analysis of regression, which included the effects of animals, periods, and treatments. The degrees of freedom of the treatments were divided into linear and quadratic components, and the latter was removed from the model if not significant (P > 0.05). The results of the PDV flux of oxygen (Experiment 2) were submitted to analysis of regression, which included the effects of animals, periods, treatments, time and interaction time versus treatment. The degrees of freedom the treatments were divided into linear and quadratic components, and the latter was removed from the model if not significant (P > 0.05). If the effects of time and of the time versus treatments interaction were not significant, they were also removed from the model and the analysis of regression was performed using the mean of the values obtained during the 8 h of sampling. In addition, the relationship between the change in the composition of hay and the different variables studied in both animal experiments was also analyzed. All analysis was performed using the general linear models procedures of SAS (1990).

3. Results and discussion 3.1. Chemical composition of hay The results of the composition of dwarf elephant grass hay at different ages are presented in Table 1. DM and OM levels were relatively similar and did not fit in any of the tested

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Table 1 Chemical composition of hay as a function of dwarf elephant grass age Componenta

Cut age (days)

DM (g kg−1 ) (g kg−1

Non-nitrogenous compounds OM NDF ADF ADL ADL (g kg−1 in NDF) TEL (meq. HCl g of DM−1 ) TEL (meq. HCl g of NDF−1 )

30

40

50

60

846

843

851

845

881 619 331 40.8 65.9 2.20 3.55

882 628 348 43.8 69.7 2.23 3.56

888 648 353 45.4 70.0 2.53 3.90

22.5 8.2 7.8 3.1

22.4 6.3 6.3 3.2

22.4 5.7 5.3 3.4

in DM) 878 608 330 40.6 66.8 1.97 3.25

Nitrogenous compounds (g kg−1 in DM) Total N Soluble N NPN ADIN

27.6 10.6 9.1 3.4

Nitrogenous compounds (g kg−1 of total N) Soluble N 384 NPN 330 ADIN 123

365 345 138

281 280 141

254 236 152

a DM, dry matter; OM, organic matter; NDF, neutral detergent fiber; ADF, acid detergent fiber; ADL, acid detergent lignin; TEL, titratable ester links; N, total nitrogen; NPN, non-protein nitrogen; ADIN, acid detergent insoluble nitrogen.

curves (low r2 -value), but NDF and ADF levels increased linearly (r 2 = 0.96 and 0.89, respectively), ADL and TEL levels increased curvilinearly (r 2 = 0.97 and 0.93, respectively), whereas the levels of total N, soluble N and NPN decreased curvilinearly (r 2 = 0.71, 0.99 and 0.99, respectively) with plant growth (DM basis). On the other hand, although the range of the difference between the values was relatively small, ADIN level had a quadratic relation (r 2 = 0.93) and, as a proportion of NDF, the levels of ADL and TEL also increased curvilinearly (r 2 = 0.70 and 0.91, respectively) with forage growth. As a proportion of the total N, the level of soluble N decreased linearly (r 2 = 0.94), the level of NPN had a quadratic relation (r 2 = 0.93), and the level of ADIN increased curvilinearly (r 2 = 0.96) with the dwarf elephant grass growth. The effect of aging on forage composition was also reported, among others, by Merchen and Bourquin (1994), Wilson (1994), Jung and Allen (1995) and Deschamps (1999). 3.2. Experiment 1 The results of intake and digestibility of the hay components are presented in Table 2. Except for N intake, the intake and the digestibility of the several hay chemical components were not influenced (P > 0.05) by the plant growth. On the other hand, N intake linearly declined (P < 0.05) due to the decrease of the concentration of N in the hay as the dwarf elephant grass aged.

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Table 2 Feed intake and apparent digestibility of dwarf elephant grass as a function of age Componenta

Intake (g day−1 ) DM OM NDF ADF ADL N

Cut age (days) 30

40

50

60

2242 1969 1364 740 91 62

2271 2000 1405 751 92 51

2254 1988 1414 784 98 50

2240 1989 1452 792 102 50

Apparent digestibility DM OM NDF ADF ADL N

0.685 0.703 0.798 0.727 0.067 0.631

0.670 0.694 0.777 0.711 0.049 0.594

0.701 0.722 0.807 0.749 0.118 0.644

0.680 0.705 0.794 0.731 0.079 0.616

S.D.b

Pc

282 248 176 96 12 14

ns ns ns ns ns <0.05

0.030 0.028 0.024 0.030 0.135 0.042

ns ns ns ns ns ns

a

See Table 1 for explanations. Standard deviation of the means, where n = 4 per treatment. c P: probability of a linear effect (error Type I). b

The maturity, and consequent changes in the chemical composition, are considered a primary factor for the decrease in the nutritional quality of the forage (Nelson and Moser, 1994). In addition, the ADIN fraction, besides being considered indigestible, seems to be negatively associated to total N apparent digestibility of forages (Van Soest, 1994). However, the influence of the factors mentioned above on digestibility seems more pronounced in the stem, but are not as evident among leaves of different ages of a same grass plant (Wilson, 1994). Studying tall elephant grass varieties, Deschamps (1999) observed that aging, from 28 to 126 days, decreased the leaf:stem ratio, increased NDF and lignin levels, and decreased DM and NDF degradability (in situ) for both stem and leaf, but more pronounced for the stem fraction. In the present study, no significant presence of stem was observed in relation to leaf sheaths and blades. This may be the result of using a relatively shorter range of cutting days for dwarf elephant grass, reducing changes of the botanical, as well as chemical composition of the hay. The increase of plant age from 30 to 60 days increased 6.5, 11.8 and 28% the NDF, ADL and TEL levels (DM basis) and increased 23.5% the ADIN level (as proportion of total N). At restricted feed intake, these changes were not sufficient large to affect the digestibility of the dwarf elephant grass. Retention time in the reticulum-rumen and in the caecum-proximal colon decreased linearly (P < 0.01 and P < 0.05, respectively), whereas the passage rate through reticulumrumen and caecum-proximal colon, as well as the transit time of the particle-phase through the gastrointestinal tract, increased linearly (P < 0.05, P < 0.01 and P < 0.01, respectively) with dwarf elephant grass growth (Table 3). Consequently, the retention time of particles in the gastrointestinal tract (TT + RTrr + RTcc) decreased from 76 to 66 h with plant growth. However, the range of variation of retention time was approximately

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Table 3 Transit time (TT), retention time in the reticulum-rumen (RTrr) and in caecum-proximal colon (RTcc) and passage rate of particle-phase through reticulum-rumen (PRrr) and through caecum-proximal colon (PRcc) in cattle fed dwarf elephant grass cut at different ages Item

Cut age (days)

TT (h) RTrr (h) RTcc (h) PRrr (% per hour) PRcc (% per hour) a b

30

40

50

60

8.4 46.2 21.3 2.22 4.93

8.3 49.4 21.3 2.06 4.85

11.3 39.6 18.7 2.62 5.66

16.7 34.4 14.8 3.0 6.87

S.D.a

Pb

3.5 7.1 4.9 0.4 1.2

<0.01 <0.01 <0.05 <0.01 <0.05

Standard deviation of the means, where n = 4 per treatment. P: probability of a linear effect (error Type I).

two-fold in the reticulum-rumen (±12 h), suggesting that the changes in hay components interfere more on its passage through this segment of the digestive tract than through the caecum-colon, where the variation in retention time was reduced to about 6 h. The non-degraded material in the rumen generally consists of highly lignified fibers, requiring rumination for its fragmentation (Wilson, 1997). In addition, TEL level may limit the rumen degradation rate of the fiber (Jung and Allen, 1995). Therefore, taking into consideration that there was an increase in ADL and TEL levels in DM, as well as in NDF, an increase, but not a decrease of RTrr, as well as a decrease, but not an increase, of PRrr was expected with dwarf elephant grass growth. However, it is known that the fiber level of the material present inside the rumen is one of the main factors stimulating gastrointestinal motility (Van Soest, 1994). Therefore, the increase in NDF and ADL levels with dwarf elephant grass growth may have stimulated the motility and increased the particle passage rate through the rumen and the gastrointestinal tract as a whole. 3.3. Relationship between digestibility and passage The extension of OM degradation depends on its indigestible fraction, on the digestion rate of its digestible fraction, and on the retention time of the forage in the rumen (Mertens, 1993), and also inside the caecum-colon. Due to the obtained results, a reduction of DM and/or NDF digestibility of the hay with plant growth would be expected. Thus, it is possible that even the lower retention time of the fiber in the rumen, as found in this study for the hay of the plant cut at 60 days of growth (RTrr of approximately 34 h), was sufficient for maximum degradation of the potentially degradable components of the hay under ruminal conditions. The highest PRrr, obtained at the cut age of 60 days, was 3% per hour. Consequently, the ruminal digestion of the hay particles, especially of the cell wall components, could be limited if the degradation rate of these particles were lower than the PRrr. However, in their literature revision, Satter et al. (1999) did not report degradation rate values lower than 3.7% per hour for the NDF fraction of several forage species. Furthermore, Cabral et al. (1997a,b) reported relatively high NDF degradation rates, ranging from 3.36 to 7.03% and 3.1 to 6.63% per hour, in tall elephant grass samples cut at 42 and 63 days of growth, respectively.

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Table 4 Arterial blood composition, dry matter (DM) and digestible energy (DE) intake, portal blood flow (PBF), oxygen use (PFOx) and heat production by the portal-drained viscera (PHP) of cattle fed dwarf elephant grass cut at different ages Item

30 Arterial blood composition Cellular volume (%) Hemoglobin (g dl−1 ) Oxygen (ml l−1 ) DM intake (g)c DE intake (kJ)d PBF (l h−1 ) PFOx (ml h−1 ) PHP (kJ h−1 ) PHP/DEI (% per hour)e

S.D.a

Cut age (days) 40

50

Pb

60

25.0 7.8 105

25.3 9.0 113

26.0 8.4 115

26.5 8.1 108

1.5 0.7 10

1079 12259 254 6090 125 1.03

1208 13590 279 6561 134 1.00

1148 13464 294 7445 152 1.12

1078 12426 313 7986 164 1.34

65 753 48 1046 21 0.20

ns ns ns ns ns ns <0.01 <0.01 <0.05

a Standard deviation of the means, where n = 4 per treatment, except for 50-day cut age, where n = 3, due the loss of one plot. b See previous tables. c Intake observed in the meal immediately before blood samplings. d Estimated from OM digestibility results of the hays obtained in Experiment 1, where DE intake = digestible OM intake (g) × 18.41. e (PHP (kJ h−1 )/DE intake (kJ)) × 100.

In addition, although normally very little digestion of fiber occur after the rumen, if some partially digested material exited the rumen due to the increase of PRrr, these fractions were completely digested during the period they were retained in the caecum-colon, equalizing the apparent digestibility among treatments. 3.4. Experiment 2 The results of Experiment 2 are presented in Table 4. One of the plots of the 50-day treatment, corresponding to the last experimental period, was rejected as there were no differences of p-aminohippurate concentrations between portal and arterial blood. This was probably to a dislocation of the catheter tip from its liver location, returning to the mesenteric vein. In order to perform the analysis of variance of the results obtained in this experiment, the data of the lost plot were estimated, the degree of freedom of the error were decreased in one unit, and the sum of the squares of treatment effects were corrected according to Gomez and Gomez (1984). Measured in the meal before blood sampling, when feed was available for steers during only 1 h, DM and DE intake (P > 0.05) were not influenced, but along of 8 h after meal, oxygen use (PFOx) and the heat production by the portal system (PHP) increased linearly with dwarf elephant grass growth (P < 0.01). As a proportion of DE intake, portal heat production (PHP/DEI) increased linearly (P < 0.05) with dwarf elephant grass growth, accounting for about 8–11% of DE intake along of 8 h of sampling. Goetsch (1998) compiled data from experiments with sheep fed forages

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ad libitum and found that daily energy use by the PDV accounted for about 6.6% of DE intake. Although the mechanisms are not elucidated, the increase in oxygen utilization by the PDV with dwarf elephant grass growth is probably associated to the increase of NDF, ADF, and ADL (Table 1), and to differences in the work of digestion, retention and propulsion of the digesta through the gastrointestinal tract (Huntington, 1999). In the present experiment, the PRrr was linearly and inversely related to the PDV energy efficiency (PHP/DEI) (P < 0.01, r 2 = 0.24). In fact, although the use of oxygen by the PDV is relatively proportional to the total use of oxygen by the organism, it tends to be higher in animals fed forage-based diets than in those fed concentrates (Seal and Reynolds, 1993; Huntington, 1999). In addition, the PDV use of oxygen, as a proportion of DE intake, was higher in sheep fed bermudagrass hay (Cynodon dactylon—vegetative stage), with higher NDF and ADL levels, that in sheep fed a mixture of ryegrass (Lolium multiflorum—end of vegetative stage) and wheat (Triticum aestivum—initial stage of milk grain) hay, which NDF and ADL levels were lower (Goetsch and Ferrel, 1995; Patil et al., 1995; Goetsch et al., 1997). In addition, McLeod and Baldwin (2000) observed larger mass of the gastrointestinal tissue in sheep fed forage-based diets as compared to those fed concentrate-based diets. Consequently, the increase in fiber level of the dwarf elephant grass hay could have resulted in an increase in the mass of the gastrointestinal tissue, and thus, in the use of oxygen by the PDV of the animals of the present study. However, this is unlikely, taking into consideration that the experimental periods were relatively short (8 days) as compared to the ones used in the study of McLeod and Baldwin (2000), where the animals were fed the same diets for 52 days.

4. Conclusions The increase in the age of the dwarf elephant grass, cut from 30 to 60 days of growth, changed the chemical composition of the hay, but when offered at restricted amounts of approximately 1.7 times animal maintenance requirements, did not affect the apparent digestibility in cattle. However, an increase in the particle-phase passage through the gastrointestinal tract and in the use of oxygen by the PDV was observed. Also, the estimated heat production by the PDV as a proportion of DE intake, increased linearly with maturity. Thus, if the nutritional value of forage decrease with plant maturity, the results of present study suggest that within of vegetative stage of dwarf elephant grass this is so due, at least partially, an increase of metabolizable energy use by the gastrointestinal tract. Therefore, energy availability for productive purposes, such as live weight gain or milk synthesis decrease. However, it is recognized that intake level affects digestibility and, thus, others experiments using higher intake levels must be conducted to validate of results obtained in the present study.

Acknowledgements Financial support was provided by the Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnológico (CNPq fellowship no 300585/01-2).

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