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The two most limiting factors affecting the rate of ruminal fiber digestion are pH and physical chemical interactions among cell wall constituents.

Ruminal pH affects fiber digestion through its influence on the specific growth rates of cellulolytic bacteria. Growth of cellulolytic bacteria is optimal at ruminal pH of greater than 6.5. Between pH of 6.5 to 6.0, the specific growth rate decreases 14%/hr for every 0.1 unit decrease in ruminal pH.

Cellulolytic bacteria do not grow at ruminal pH below 6.0. This toxicity is due apparently to the inability of cellulolytic bacteria to regulate intracellular anion concentrations at lower ruminal pH (Russell and Wilson, 1996).

In addition to specific growth rates of cellulolytic bacteria, the availability (or accessibility) of substrate to the cellulolytic process is also an important limitation to the rate of fiber digestion. Cellulose fibrils are cemented together in a matrix of hemicellulose, lignin, pectins, and extensins. In particular, the physical/chemical interactions of hemicellulose and lignin with cellulose present a formidable barrier to the cellulolytic process (Hatfield, 1993).

Ruminal deficiencies in fibrolytic enzymes may be partially overcome by dietary enzyme supplementation.

Combinations of cellulase and xylanase enzymes have enhanced in vitro (Feng et al., 1996; Howes et al., 1998) and in vivo digestion (Lewis et al., 1996), growth performance of steers fed forage-based diets (Beauchemin et al., 1995) and milk production (Howes et al., 1998).

Our objective in the present study was to evaluate the influence of a commercial enzyme supplement (Fibrozyme, Alltech, Inc.) containing xylanase and cellulase activity, on characteristics of digestion and performance of feedlot steers fed a high-energy growing diet. Hemicellulose is composed of dense pentose polymers called xylans. In this study we test the hypothesis that dietary enzyme supplementation will promote disruption of the hemicellulose-cellulose matrix thereby enhancing ruminal fiber digestion, and in turn, feed intake.


Experimental procedures

TRIAL 1. EFFECTS ON RUMINAL DIGESTION PARAMETERS

Eight Holstein steers (159 kg) with cannulas in the rumen and proximal duodenum (Zinn and Plascencia, 1993) were used in a crossover design to evaluate the influence of Fibrozyme supplementation (0 versus 15 g/steer/day) on digestive function.

Fibrozyme is a rumen-stable (not rapidly degraded by ruminal proteolytic enzymes) fibrolytic enzyme supplement prepared from fermentation extracts of Aspergillus niger and Trichoderma longibrachiatum. Steers were individually maintained in slotted-floor pens (7.6 m2) with ad libitum access to water. Composition of the basal diet is shown in Table 1.

Chromic oxide (0.40%) was included in the diet as a digesta marker.

Fibrozyme (7.5 g/feeding) was added to the basal diet at the time of feeding.

Intake of the basal diet was restricted to 3.5 kg dry matter per day. Diets were fed in equal portions at 0800 and 2000 daily. Experimental periods consisted of a 10-day diet adjustment period followed by a 4-day collection period. During the collection period duodenal and fecal samples were taken from all steers twice daily at the following times: day 1, 0750 and 1350; day 2, 0900 and 1500; day 3, 1050 and 1650; and day 4, 1200 and 1800.

Individual samples consisted of approximately 500 ml duodenal chyme and 200 g (wet basis) fecal material. Samples from each steer and within each collection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer at 4 hr after feeding via the ruminal cannula and ruminal fluid pH was determined. Ruminal fluidwas composited for isolation of ruminal bacteria via differential centrifugation (Bergen et al., 1968). The microbial isolates were prepared for analysis by oven-drying at 70wCand then grinding with mortar and pestle.

Feed, duodenal and fecal samples were first oven-dried at 70wC and then ground in a laboratory mill (Micro-Mill, Bell-Arts Products, Pequannock, NJ). Samples were then oven-dried at 105wC until no further weight loss and stored in sealed glass jars.

Samples were subjected to all or part of the following analysis: ash, Kjeldahl nitrogen (N), ammoniaN(AOAC, 1975), neutral detergent fiber (NDF) (Goering and Van Soest, 1970; corrected for neutral detergent insoluble ash), purines (Zinn and Owens, 1986), chromic oxide (Hill and Anderson, 1958) and starch (Zinn, 1989).

Microbial organic matter and microbial nitrogen leaving the abomasum were calculated using purines as a microbial marker (Zinn and Owens, 1986). Organic matter fermented in the rumen was considered equal to organic matter (OM) intake minus the difference between the amount of total OM reaching the duodenum and microbial OM reaching the duodenum. Feed N escape to the small intestine was considered equal to total N leaving the abomasum minus ammonia N and microbial N, and thus includes any endogenous contributions. The trial was analyzed as a crossover experiment (Cochran and Cox, 1950).


Table 1. Composition of diet fed to steers (Trials 1a and 2).


^a In Trial 1, chromic oxide (0.40%, dry basis) was substituted for steam-flaked corn in the basal diet as a digesta marker.
^b Trace mineral salt contained CoSO₄, 0.068%; CuSO₄, 1.04%; FeSO₄, 3.57%; ZnO, 1.24%; MnSO₄, 1.07%; KI, 0.052%; and NaCl, 92.96%. cBased on tabular NE values for individual feed ingredients (NRC, 1996).



TRIAL 2. EFFECT ON PERFORMANCE OF STEERS

Ninety-six crossbred steer calves were used in a 64-day receiving trial to evaluate the Fibrozyme supplementation on growth performance. The trial was initiated July 21, 1998. Calves were blocked by weight and assigned within weight groupings to 16 pens (6 steers/pen).

Upon initiation of the trial steers were implanted with Synovex-S (Fort Dodge Animal Health, Overland Park, KS). Treatments were the same as in Trial 1 (Table 1). Calves were allowed ad libitum access to experimental diets. Fresh feed was provided twice daily (roughly 0700 and 1500 hr). Fibrozyme (7.5 g/steer, twice daily) was topdressed on the feed at the time of feeding.

Assuming the primary determinant of energy gain was weight gain, energy gain was calculated by the equation: EG = (0.0557MBW^0.75)ADG^1.097, where EG is the daily energy deposited (Mcal/day), ADG is weight gain (kg/d) and MBW is the mean body weight (kg) (NRC, 1984).

Maintenance energy expended (Mcal/day, EM) was calculated by the equation: EM = 0.077MBW^0.75 (NRC, 1984). From the derived estimates for energy required for maintenance and gain, the NE for maintenance (NE_m) and gain (NE_g) of the diets were obtained by means of the quadratic formula below. The trial was analyzed as a randomized complete block experiment (Cochran and Cox, 1950).

Formula for deriving net energy estimates for maintenance (NE_m) and gain (NE_g):



where a = -0.41EM, b = 0.877EM + 0.41DMI + EG, and c = -0.877DMI, and NE_g = 0.877NE_m - 0.41.


Results and discussion

TRIAL 1

Treatment effects on characteristics of ruminal and total tract digestion (Trial 1) are shown in Table 2. There were no treatment effects (P>0.10) on ruminal pH, ruminal microbial efficiency or ruminal and total tract digestion of organic matter and starch. Fibrozyme supplementation increased (P <0.05) ruminal digestion of NDF (23%) and feedN(5%). The increase in ruminal degradation of feed N is consistent with the associative effects of fiber on accessibility of forage protein to the proteolytic process. Total tract digestion of NDF and N were similar (P>0.10) across treatments.


Table 2. Influence of Fibrozyme supplementation on characteristics of ruminal and total tract digestion of organic matter, NDF, starch, and nitrogen (Trial 11).



^a Treatments differ, P<0.05.
^b Microbial N, g/kg organic matter fermented.
^c Duodenal nonammonia N/N intake.



TRIAL 2

Treatment effects on growth performance of feedlot calves during a 64-day feeding period (Trial 2) are shown in Table 3. Fibrozyme supplementation increased final weight (3%, P<0.10), average daily gain (6%, P = 0.13), and dry matter (DM) intake (4.5%, P<0.05). Consistent with the metabolism trial (Trial 1), Fibrozyme did not influence the NE value of the diet.


Table 3. Influence of Fibrozyme supplementation on feedlot cattle growth performance (Trial 2).


^a Initial and final weights were reduced by 4% to account for digestive tract fill.
^b Treatments differ, P<0.10.
^c Treatments differ, P<0.05.



Treatment effects on DM intake (and consequently weight gain), can be explained by changes in ruminal NDF digestion (Trial 2). The rumen has an upper limit on its capacity. As energy density of the diet decreases, the amount of slowly digestible OM in the rumen increases.

In situations where energy density of the diet is limiting, maximal DM intake (DMImax) can be explained on the basis of effective NDF (eNDF) intake and ruminalNDFdigestion.Asimple basic-language programfor calculating DMImax is given in Table 4.

Inputs for initial weight of cattle when first placed in the feedlot, average weight during the feeding trial, dietary NDF, dietary eNDF, and ruminal NDF digestion were 180 kg, 265 kg, 19%, 80%, and 28.2%, respectively, for controls and 180 kg, 271 kg, 19%, 80%, and 34.7%, respectively, for Fibrozyme treatment. Accordingly, the 23% increase in ruminal NDF digestion due to Fibrozyme supplementation (Trial 1, Table 2) is expected to permit a 12% increase inDMintake.

Because the forage level of the basal diet was only marginally limiting energy intake, the observed increase in DM intake (4.5%, Table 3) with Fibrozyme supplementation was less then projected.


Table 4. Basic language program for calculation of maximal dry matter intake, and ruminal passage and digestion rates of NDF for feedlot cattle, as influenced by eNDF intake and ruminal NDF digestion.

References

AOAC. 1975. Official Methods of Analysis (12th Ed.). Association of Official Analytical Chemists, Washington, DC.

Beauchemin, K.A., L.M. Rode and V.J.H. Sewalt. 1995. Fibrolytic enzymes increase fiber digestibility and growth rate of steers fed dry forages. Can. J. Anim. Sci. 75:641-644.

Bergen,W. G., D. B. Purser and J. H. Cline. 1968. Effect of ration on the nutritive quality of rumen microbial protein. J. Anim. Sci. 27:1497.

Cochran,W. G. and G. M. Cox. 1950. Experimental Designs. JohnWiley & Sons, Inc., New York.

Feng, T., C. W. Hunt, G. T. Pritchard and W. E. Julien. 1996. Effects of enzyme preparations on in situ and in vitro digestive characteristics of mature cool-season grass forage in beef steers. J. Anim. Sci. 74:1349.

Goering, H. K. and P. J. Van Soest. 1970. Forage fiber analysis. Apparatus, reagents, procedures and some applications. ARS, USDA Agr. Handbook No. 379.

Hatfield, R. D. 1993. Cell wall polysaccharide interactions and degradability. In: Forage CellWall Structure and Digestibility. Am. Soc. Agron., Crop Sci. Soc. Am., and Soil Sci. Soc. Am., Madison, WI. Page 285.

Hill, F.N., andD. L. Anderson. 1958. Comparison of metabolizable energy and productive energy determinations with growing chicks. J. Nutr. 64:587.

Howes,D., J.M. Tricarico,K.Dawson, and K.Karnezo. 1998. Fibrozyme, the first protected enzyme for ruminants: Improving fiber digestion and animal performance. In: Biotechnology in the Feed Industry, Proceedings of the 14th Annual Symposium (T.P. Lyons and K.A. Jacques, eds). Nottingham University Press. UK.

Lewis, G. E., C. W. Hunt, W. K. Sanchez, R. Treacher, G. T. Pritchard and P. Feng. 1996. Effects of direct fed fibrolytic enzymes on the digestive characteristics of a forage-based diet fed to beef steers. J. Anim. Sci. 75:3020.

NRC. 1984. Nutrient Requirements of Beef Cattle (6th Rev. Ed.). National Academy of Press, Washington, DC.

NRC. 1996. Nutrient Requirements of Beef Cattle (7th Rev. Ed.). National Academy of Press, Washington, DC.

Russell, J. B., andD. B.Wilson. 1996.Why are ruminal cellulolytic bacteria unable to digest cellulose at low pH? J. Dairy Sci. 79:1503.

Zinn, R. A. 1989. Influence of steaming time on site of digestion of flaked corn in steers. J. Anim. Sci. 68:776.

Zinn, R. A. and F.N. Owens. 1986. A rapid procedure for purine measurements and its use for estimating net ruminal protein synthesis. Can. J. Anim. Sci. 66:157.

Zinn, R. A. and A. Plascencia. 1993. Interaction of whole cottonseed and supplemental fat on digestive function in cattle. J. Anim. Sci. 71:11.

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