Effect of live yeast culture supplementation on apparent digestibility and rate of passage in horses fed a high-fiber or high-starch diet

Effect of live yeast culture supplementation on apparent digestibility
and rate of passage in horses fed a high-fiber or high-starch diet1
J.-P. Jouany,* J. Gobert,† B. Medina,†‡ G. Bertin,† and V. Julliand‡2
*INRA, UR1213 Herbivores, Site de Theix, 63122 Saint Genes Champanelle, France; †Alltech-France, 14 Place Marie-Jeanne Bassot, 92593 Levallois-Perret, France; and ‡Etablissement National d’Enseignement Supe´rieur Agronomique de Dijon (ENESAD), 26 Boulevard Dr Petitjean, BP 87999, 21079 Dijon, France ABSTRACT:
tract (MRTYb), Eu bound to the pelleted feeds, and Dy 5 yr and with BW of 305 ± 18 kg were used in pairs in bound to the fecal particles for MRT in the hindgut a 4 × 4 Latin square design with 4 ground and pelleted (MRTEu and MRTDy). Apparent digestibilities of DM, diets. Each pair included a cecum and right ventral OM, and CP were greater (P < 0.001) in the HS than colon-fistulated animal and a cecal-fistulated animal.
The 4 horse diets were a high-fiber diet (HF+0) based whereas ADF digestibility was greatest in the HF diet on dehydrated alfalfa, a high-starch diet based on bar- (P = 0.035). Cellulolytic activity estimated through the ley and wheat bran (HS+0), and the HF or HS diets in vitro disappearance rate of the dietary ADF fraction supplemented with Saccharomyces cerevisiae (SC) CBS 493.94 (HF+SC and HS+SC). The probiotic preparation ADF) was less (P < 0.001) in the HS than the HF diet. There was no dietary effect on NDF digestibility contained 4.5 × 109 cfu/g of live yeast mixed with the culture medium, and was top-dressed onto the feed pel- gut (P = 0.036), which compensated for the lower fibro- lets at a rate of 10 g/d, equally distributed between the lytic activity expressed per unit of time in the HS com- 2 daily meals. All 4 diets were offered in the samequantities (18.0 g of pelleted feed DM + 3.5 g of long pared with the HF diet. Supplementation with SC im- wheat straw/kg of BW per d). Each of the 4 experimental proved ADF digestibility (P = 0.038) and stimulated treatments was divided into a 21-d period of diet adap- DM (P = 0.030) and NDF (P = 0.038) intakes, but had tation followed by a 10-d period of total fecal collection no effect on the MRT of solid digesta. The absence of any for digesta flow rate and apparent digestibility mea- significant diet × SC interaction supports the strategy of surements. Three markers were used to measure mean using SC to stimulate cellulose digestion and improve retention time (MRT) of the feed particles: Yb bound the nutritional status of horses under both HF and to the pelleted feeds for MRT in the whole digestive Key words: dietary effect, digestibility, fiber digestion, horse, live yeast, transit time
2008 American Society of Animal Science. All rights reserved. INTRODUCTION
number of meals throughout the day, resulting in inges-tion of large quantities of starch by animals over a short In horses the energy supplied by forage diets does time. Potter et al. (1992) reported that only limited not meet the energy requirements for intense physical amounts of starch are digested in the small intestine exercise due to the relatively low digestibility of cell and that decreasing the dietary hay:grain ratio will wall carbohydrates. Consequently, athletic horse diets increase the amount of undigested starch entering the are supplemented with concentrate feeds, mainly com- hindgut, thus disturbing its microbial balance. The ac- prising cereal grains and often distributed in a small cumulation of lactate, and the subsequent drop in pH,impairs the activity of lower-gut cellulolytic bacterialpopulations and increases the horse’s susceptibility tocolic pain or laminitis (Kronfeld and Harris, 1997).
1The authors would like to thank D. Juniper for his support on Several strategies have been tested to reduce these reviewing the draft of this manuscript, and F. Glasser and A. Breuvart dysfunctions of the equine intestinal ecosystem. One for their contributions on statistical analysis.
strategy consists of adding probiotics such as Saccharo- Corresponding author: [email protected] myces cerevisiae, as recommended for ruminants fed high-starch diets (Wallace and Newbold, 1992; Nagar- aja et al., 1997). Previous studies indicated that live Table 1. Composition of the high-fiber (HF) and high-
yeasts can improve the microbial balance in the hindgut of horses, stimulating the population of cellulolytic bac-teria and their activity (Medina et al., 2002), and in- creasing the digestibility of dietary nutrients (Glade, 1991a,b). However, the diet composition-related ability of live yeasts to modify microbial digestion and fiber Ground and pelleted fraction of the dietDehydrated alfalfa degradation in horses has not been extensively studied.
Therefore, a study was conducted to evaluate the effect of a live yeast culture of S. cerevisiae CBS 493.94 on in vivo digestibility of dietary components and on the rate of cell wall digestion in horses fed either a high-starch or high-fiber diet. Mean retention times of digesta in the whole digestive tract and the hindgut were measured to explain the observed changes in di- MATERIALS AND METHODS
This experiment was carried out in the animal re- search unit of ENESAD (Etablissement National d’Enseignement Supe´rieur Agronomique de Dijon) un- der a license delivered by the Health and Animal Wel- 1Supplied per kilogram of diet: vitamin A, 810 IU; vitamin D3, 135 fare department of the French Veterinary Authority.
IU; vitamin E, 7.3 IU; biotin, 10.8 ␮g; and Cu, 1.35 mg.
2Cellulose was calculated by the difference ADF − ADL.
3Hemicellulose was calculated by the difference NDF − ADF.
Animals
4DE was calculated from equations reported by Fonnesbeck (1981).
Eight crossbred male horses (12 ± 5 yr; mean BW 305 ± 18 kg) were allocated into pairs consisting of a CBS 493.94, including the growth medium (Yea-Sacc, cecum and right ventral colon-fistulated animal and Alltech Inc., Lexington, KY). The live yeast culture sup- a second animal that was only cecally fistulated. The plement containing 4.5 × 109 cfu/g was given at a rate cannula (polyvinyl chloride, i.d. 30 mm) were fitted sur- gically by a certified veterinary surgeon at least 6 mo The HF and HS diets were formulated to be isonitro- before the beginning of the trial (Drogoul et al., 2000).
genous, to provide significantly different NDF:starch Horses were wormed with a double dose of Pyrantel ratios (3.5 and 1.0 for the HF and HS diets, respec- (Strongid, Laboratoire Pfizer, Orsay, France) followed tively), and to meet the energy requirements of the 1 wk later by a single dose of Ivermectin (Eqvalan, horses. Minimum daily voluntary intake of wheat straw Laboratoire Merial, Lyon, France) given 15 d before the was measured for all horses during a pre-experimental experiment began. Indoor housing consisted of con- period performed with the same HF and HS concen- crete-floored, individual loose boxes bedded with flax trates. The lowest level of wheat straw intake recorded shavings (Ecolit, Croissanville, France). During the during this period (i.e., 3.5 g/kg of BW) was selected for diet-adaptation period (21 d), animals were given access to a sandy paddock for 10 h/wk. During the following All diets were given at the same rate (i.e., 18.0 g of 10 d they were individually tethered in stalls and fitted pelleted feeds + 3.5 g of long wheat straw = 21.5 g of DM/kg of BW per d) to avoid any effect of meal size onthe rate of passage of digesta (Drogoul et al., 2001; Diets
Pearson et al., 2001). At this intake level the HF dietprovided 100% of energy requirements, whereas the HS Feedstuffs as well as their physical form were chosen diet provided 130% of the energy requirements (Martin- to mimic normal French feeding practices conducted in Rosset et al., 1994). The daily rations (6.5 kg/horse per riding schools. Two diets were used (Table 1): a pelleted d, on average) were divided into 2 equal meals given high-fiber (HF) concentrate or a pelleted high-starch
in individual troughs at 0800 and 1700. The pelleted (HS) concentrate, both given in mixture with long
concentrates were offered first with the SC culture top- wheat straw to supply a minimum fiber content. Ingre- dressed (5 g/meal), and the wheat straw fraction was dients of the HF and HS concentrates were ground provided 30 min later. Mean daily ingestion of starch through a 1.5-mm sieve and pressed into 3-mm-diame- was equal to 3.1 and 1.2 g/kg of BW per meal for the ter pellets. The 2 diets were supplemented (HF+SC;
HS and HF diets, respectively (Table 2).
HS+SC) or not (HF+0; HS+0) with a lyophilized floccu-
Horses were weighed on 2 consecutive days before lent culture of Saccharomyces cerevisiae (SC) strain
each diet-adaptation period to adjust feed supply to Effect of live yeasts on digestion of horses Table 2. Daily nutrient intake (g/kg of BW) in horses fed a high-fiber (HF) or high-starch
(HS) diet with (+SC)1 or without (+0) Saccharomyces cerevisiae (SC) supplementation
a–cMeans within a row with different superscript letters differ, P < 0.05.
1The probiotic preparation contained 4.5 × 109 cfu/g of live S. cerevisiae mixed with the culture medium and was top-dressed onto the feed pellets at a rate of 10 g/d, equally distributed between the 2 daily meals.
2Numbers of observations in HF+0, HF+SC, HS+0 and HS+SC were 7, 8, 6, and 8, respectively.
3Cellulose was calculated by the difference ADF − ADL.
4Hemicellulose was calculated by the difference NDF − ADF.
animal BW. All horses were given individual free access estimate the mean retention time (MRTEu) of small
to clean potable drinking water and a lick block of trace- particles (≤2 mm) in the hindgut (cecum + colon). To measure mean retention time (MRTDy) of large parti-
cles (>2 mm) in the hindgut, dysprosium (Dy) was bound
Experimental Design and Treatments
to the undigested fecal particles and introduced manu-ally through the cecal cannula. Fecal particles intended The 8 horses were randomly assigned by pairs in a to be labeled were collected during each period of animal double 4 × 4 Latin square design at the beginning of adaptation to harnesses and were then water-washed the experiment. Each pair included a cecum and right and filtered through a 2.5-mm sieve. The particles re- ventral colon-fistulated animal and a cecal-fistulated tained on the sieve were labeled using a Dy oxide solu- animal, and remained unchanged for the duration of tion (Dy2O3; 50 mg of Dy/g of DM; pH 2.5). The 2 pelleted the entire study. The 4 pairs of animals received 4 feeds (HF and HS) were labeled using either a Yb oxide dietary treatments over 4 periods. The 4 dietary treat- solution (Yb2O3; 35 mg of Yb/g of DM; pH 2.5) or a Eu ments described previously were applied as follows: 1) oxide solution (Eu2O3; 30 mg of Eu/g of DM; pH 2.5) the high-fiber diet (HF+0 treatment); 2) the high-starch according to the method described by Drogoul et al.
diet (HS+0 treatment) ; 3) the high-fiber diet supple- (2000). To prevent losses of small particles, pelleted mented with 10 g/d of a Saccharomyces cerevisiae CBS feeds were placed into nylon bags with a 100-␮m pore 493.94 preparation (HF+SC); and 4) the high-starch size (Blutex T120, Saati-France, Sailly Saillisel, diet supplemented with 10 g/d of the same SC prepara- France) during the labeling procedure. The labeled feeds were hand-pelleted with a syringe so that a pre- Each of the 4 experimental periods lasted 31 d, split cise dose of markers could be administered.
into a 21-d period that allowed for adaptation to experi- A single dose of Yb-labeled pellets (40 g) was fed to mental diets, followed by a 10-d period of total fecal each animal mixed into one-third of the morning meal.
collection, which comprised a 3-d period for animals to After either complete ingestion, or at least a 45-min adapt to the fecal bags followed by a 7-d period for delay, refusals, if any, were weighed and removed from measurements on apparent digestibility and rate of the trough. The rest of the pelleted concentrate meal was then given. During meal ingestion, all collection Indigestible Markers used for Measurement
harnesses were cleaned of feces. The Eu-labeled pellets of the Rate of Passage of Digesta
(30 g; particles ≤2 mm) and the Dy-labeled fecal parti-cles (30 g; particles >2 mm) were introduced through Three indigestible markers were used to calculate the cecal cannula 4 h after the oral dose of Yb-labeled rate of passage of feed particles through the hindgut pellets (Drogoul et al., 2000). After ingestion of the Yb- (cecum + colon) and whole digestive tract, as recom- labeled meal, excreted feces were collected from each mended by Pagan et al. (1998) and Drogoul et al. (2000).
horse at 4, 7, 10, 13, 16, 19, 22, 25, 28, 32, 36, 40, 46, Ytterbium was bound to the pelleted feeds and was fed 52, 58, 64, 72, 84, 96, 120, 144, and 168 h postingestion.
to horses to estimate mean retention time (MRTYb) of
Individual fecal collections were weighed and thor- small particles (≤2 mm) in the whole digestive tract.
oughly mixed, and a subsample (300 g of fresh matter) Europium (Eu) was bound to pelleted feeds and intro- was taken to measure DM and marker concentrations.
duced manually through the cecal cannula in order to Markers were solubilized according to Siddons et al.
(1985). Ytterbium, Eu, and Dy concentrations were as- tion intervals and calculated as ⎜ti + ti − 1 ⎟ with ti sayed by atomic absorption spectrophotometry (Spec- trAA 300, Zeeman–Varian, Les Ulis, France) with being the time to the end of the ith interval; and Mi is wavelengths set at 398.8, 421.2, and 459.4 nm for Yb, the amount of marker excreted in the ith interval with all markers being excreted by the nth interval. TheMRT of small particles from mouth to cecum was calcu- Collection of Samples for In Vivo Apparent
lated by the difference MRTYb − MRTEu.
Digestibility and Rate of Passage
All data were processed by ANOVA using the GLM of Digesta Measurements
procedure (SAS Inst. Inc., Cary, NC). The model in-cluded the effects of horse (which was considered ran- In vivo apparent digestibility and rate of passage of dom), period, diet, yeast supplementation, and the in- digesta were determined by the total fecal collection teractive effects of diet type and yeast supplementation.
method using harnesses and collection bags for 7 d.
No effect of period was detected; therefore, period was Amount of feed offered, refused, and feces were weighed removed from the final statistical model. Least squares every day at 0800. Aliquots of individual feces (10% of means were calculated for each variable and separated total excreted fresh feces), offered feeds (500 g of fresh using the pairwise t-tests (PDIFF option of SAS).
matter) and all refusals, if any, were taken daily and A repeated-measurements ANOVA was performed to dried in a forced-air oven until constant weight at 65°C compare differences in marker excretion (cumulative for DM determination. At the end of each period, ali- Yb excretion and Eu excretion) at differing time points quots were ground through a 0.8-mm mesh, and repre- using the repeated time option of SAS. The model used sentative samples from each collection (offered feeds, included the effects of horse, diet, and yeast supplemen- refusals, and feces) were pooled for each animal over tation. Statistical significance was set at P < 0.05.
the 7-d period for subsequent analysis. Acid detergentfiber, NDF, and ADL were determined according to theprocedure described by Van Soest and Wine (1967).
Organic matter was determined after ashing in a mufflefurnace (550°C for 6 h). Crude protein determination Only the values obtained from horses that had no was based on nitrogen content measured using a semi- significant feed refusals (less than 5% of offered DM) automated micro-Kjeldahl method using a VDK 126A of pelleted feeds and for which the pelleted feed ratio distillation unit (Velp Scientifica, Milan, Italy). Starch in the total feed intake did not differ from 0.84 ± 0.02 content was determined according to the method of Thi- are reported here. Thus, 29 of the total 32 observations vend et al. (1965). All analyses were performed in du- were retained for statistical analyses of digestibility and MRTYb values. Due to missing data for 1 horse,only 25 observations were retained for the MRTEu, Calculations and Statistical Analyses
MRTDy, and (MRTYb − MRTEu) calculations.
All animals gained approximately 10% of their initial Apparent digestibility of DM was calculated as the BW and there were no obvious signs of digestive or amount of digested DM (ingested DM − excreted DM metabolic disorders during the duration of the study.
in feces) per unit of ingested DM. Total DM intakes andfecal DM outputs were measured over the 7-d collection Nutrient Intake
period. In vitro apparent disappearance rate (IVAD) of
NDF was calculated as the amount of digested NDF
There were no diet effects on DM, CP, and OM intakes (ingested NDF − excreted NDF in feces) per unit of (P > 0.068; Table 2). The greater proportion of barley retention time (h) of solid particles in the hindgut. The in the HS diets led to an increase in starch intake (P same relationship was applied to ADF, and to cellulose < 0.001) and a decrease in NDF, ADF, cellulose, and (ADF − ADL) and hemicellulose (NDF − ADF) cell hemicellulose intakes (P < 0.001). Yeast addition had a positive effect on DM (P = 0.03), NDF (P = 0.038), The MRT of small particles in the whole digestive and hemicellulose (P = 0.023) intakes. A significant diet tract and small and large particles in the hindgut com- × SC interaction (P = 0.042) was observed on OM partment were calculated using the model established by Faichney (1975) and applied to the kinetic excretionsof Yb, Eu, and Dy in feces as: Rate of Passage of Digesta
There was no effect of SC supplementation on the MRT of solid particles in either the whole digestive tract or the hindgut (Table 3). Conversely, the MRT of small particles in the hindgut was 3.5 h longer with the HS diet than the HF diet (P = 0.036). A similar where ⌬ti is the time elapsed between introduction of pattern was observed for the MRT of large particles (+3 the markers (time zero) and the middle of the ith collec- h), but the difference was not significant (P = 0.08). The Effect of live yeasts on digestion of horses Table 3. Total and partial mean retention time (MRT, h) in the digestive tract of horses
fed a high-fiber (HF) or high-starch (HS) diet with (+SC)1 or without (+0) Saccharomyces
cerevisiae (SC) supplementation
1The probiotic preparation contained 4.5 × 109 cfu/g of live S. cerevisiae mixed with the culture medium, and was top-dressed on to feed pellets at a rate of 10 g/d, equally distributed between the 2 daily meals.
2Numbers of observations for Yb measurements in HF+0, HF+SC, HS+0, and HS+SC were 7, 8, 6, and 8, respectively. Numbers of observations for Eu, Dy, and for the calculated MRT in the foregut in HF+0,HF+SC, HS+0 and HS+SC were 7, 6, 5, and 7, respectively.
MRT of feed particles in the foregut section (stomach + between-diet difference in peak shift of Eu excretion small intestine), as estimated by the difference between noted (6.7 h longer with HS) indicated differential rates the MRTYb in the whole digestive tract and the MRTEu of passage of dietary solid particles through the hindgut in the hindgut compartments, was not different (P = 0.972) between the 2 diets. No diet × SC interactionwas detected on rates of passage of digesta through the In Vivo Apparent Digestibility of the Diets
whole digestive tract, the foregut, or the hindgut.
Because of the absence of any SC effect on MRT in the Apparent digestibility of DM, OM, and CP were whole digestive tract, cumulative Yb excretion curves greater (P < 0.001; Table 4) in HS diets than in HF were constructed from the HF+0 and HF+SC values diets. In contrast, there were no diet effects on the and the HS+0 and HS+SC values for the HF and HS digestibility of total dietary cell wall fractions (NDF; diets, respectively. The curves showed that 46 and 39.5 P = 0.995), whereas the ADF fraction was better di- h were required to excrete 75% of Yb-labeled particleswith the HS and HF diets, respectively (Figure 1). The Figure 2. Europium (Eu) excretion per 12-h sampling
Figure 1. Cumulative Yb excretion (expressed as a per-
period (expressed as a percentage of the total Eu excreted) centage of total Yb excreted) through the whole digestive through the large intestinal compartment (cecum + colon) tract of horses. Because of the absence of any effect of of horses. Because of the absence of any effect of Saccharo- Saccharomyces cerevisiae supplementation on Yb excretion, myces cerevisiae (SC) supplementation on Yb excretion, data with and without SC were pooled for each high- data with and without SC supplementation were pooled fiber (HF); n = 15) and high-starch (HS; n = 14) diet.
for each high-fiber (HF; n = 13) and high-starch (HS; n = *Means differed at time marked by an asterisk (P < 0.05); 12) diet. *Means differed at time marked by an asterisk (P < 0.05); bars represent the SEM.
Table 4. Apparent digestibility (g/kg of DM) of dietary components in horses fed a high-
fiber (HF) or high-starch (HS) diet with (+SC)1 or without (+0) Saccharomyces cerevisiae
(SC) supplementation
a,bMeans within a row with different superscript letters differ, P < 0.05.
1The probiotic preparation contained 4.5 × 109 cfu/g of live S. cerevisiae mixed with the culture medium, and was top-dressed on to feed pellets at a rate of 10 g/d, equally distributed between the 2 daily meals.
2Numbers of observations in HF+0, HF+SC, HS+0, and HS+SC were 7, 8, 6, and 8, respectively.
3Cellulose was calculated by the difference ADF − ADL.
4Hemicellulose was calculated by the difference NDF − ADF.
gested (P = 0.035) and the hemicellulose fraction tended (P = 0.030). Data on ingestion indicated that non-yeast- to be less digested (P = 0.080) in HF than in HS diets.
supplemented animals tended to refuse a portion of No significant diet × SC interaction was detected on in their ration; whereas, yeast-supplemented animals in- gested the whole ration. The positive effect of SC supple- Yeast supplementation increased digestibility in the mentation on mean DM, OM, and NDF intakes were whole digestive tract of the ADF fraction of both diets greater for HF than HS diets, which explains both the (P = 0.038), but had no significant effect on digestibility overall positive effect of SC on NDF (P = 0.038) and hemicellulose (P = 0.023) intakes, and the significantdiet × SC interaction on OM intake (Table 2).
In Vivo Apparent Disappearance Rate of Dietary
Cell Wall Fractions
Effect of Starch Supplementation on Digestion
in Horses
Because the hindgut is the only site of cell wall diges- tion in the digestive tract of horses, the disappearance A negative effect of starch on microbial cellulolysis rate of dietary cell wall fractions (i.e., IVAD) will be in the hindgut of horses has been reported in previous considered as an indicator of the microbial cellulolytic experiments in which native grains were incorporated activity. A large diet effect was noted on IVADNDF, IVA- into the diet (>50% of DM intake) at the expense of DADF, and IVADcellulose, which were much greater (P < forage (Thompson et al., 1984; Martin-Rosset and Dul- 0.001) in HF than in HS diets. However, the diet had phy, 1987; Palmgren-Karlsson et al., 2000; Drogoul et no effect (P = 0.341) on IVADhemicellulose. There was no al., 2001). The amount of starch intake per se is one diet × SC interaction on estimated IVAD parameters.
factor that could explain such negative interactions Yeast supplementation had no effect on IVADNDF, with cellulolysis and the degree of cellulolysis inhibition (P = 0.141), IVADhemicellulose (P = 0.453), or IVADcellulose (Ott, 1981; Martin-Rosset and Dulphy, 1987). Further- (P = 0.124) in either diet but tended to improve IVADADF more, technological processes applied to grains (grind- in both HF and HS diets (P = 0.077).
ing, pelleting) can influence the enzymatic digestion ofstarch in the small intestine, and therefore, alter the DISCUSSION
amount of starch reaching the large intestine and inter-acting with fiber digestion (Julliand et al., 2006).
Data collected in the present experiment were used In the current experiment the amount of starch fed to estimate the impact of a live yeast culture prepara- in the HS diet (3.1 g/kg of BW per meal) was similar tion of Saccharomyces cerevisiae CBS 493.94 on in vivo to the 3.5 to 4.0 g/kg of BW per meal limit defined by apparent digestibility, rate of cell wall digestion, and Potter et al. (1992) and Kienzle (1994) as the maximum rate of passage of digesta in horses fed 2 different diets capacity of the small intestine for starch digestion.
(i.e., a high-fiber diet vs. a high-starch diet). It was also Therefore, it can be hypothesized that intestinal undi- possible to assess the apparent digestibility of these 2 gested starch can enter the hindgut of horses fed the HS diet. It is likely that the transit time of finely ground Feed Intake
and pelleted diets is shortened when compared withunground diets, thus allowing more starch to escape Although animals received similar amounts of feed, small intestine digestion, reach the hindgut, and inter- there was a general positive effect of SC on DM intake act with the local microbial environment (Julliand et Effect of live yeasts on digestion of horses Table 5. Apparent disappearance rate (IVAD)1 (g/h per 100 kg of BW) of cell wall
components in the hindgut of horses fed either a high-fiber (HF) or high-starch (HS) diet
with (+SC)2 or without (+0) Saccharomyces cerevisiae (SC) supplementation
a,bMeans within a row with different superscript letters differ, P < 0.05.
1IVADNDF was calculated as the amount of digested NDF (ingested NDF − excreted NDF in feces) per unit of retention time (h) of solid particles in the hindgut. The same relationship was applied to ADF,cellulose and hemicellulose cell wall fractions.
2The probiotic preparation contained 4.5 × 109 cfu/g of live S. cerevisiae mixed with the culture medium, and was top-dressed on to feed pellets at a rate of 10 g/d, equally distributed between the 2 daily meals.
3Numbers of observations in HF+0, HF+SC, HS+0, and HS+SC were 7, 8, 6, and 8, respectively.
al., 2001). Also, we assumed that starch was more easily The apparent digestibility of CP was greater in the hydrolyzed by endogenous amylases in ground cereals, HS diet than in the HF diet and independent of SC and the resultant effect of grinding on starch digestibil- supplementation. This is explained by the greater avail- ity in the small intestine was probably minimal.
ability of soybean proteins to the endogenous enzymes Medina et al. (2002) analyzed the concentrations and in the small intestine of horses than the proteins from activities of hindgut microflora of horses fed diets simi- alfalfa. This could be due to a protective effect of cell lar to those used in the current study. They observed wall components on intracellular proteins, and to differ- that the microbial profiles were altered for a high-starch ences in the secondary or tertiary structure of proteins diet. The number of cellulolytic bacteria was decreased between alfalfa and soybean meal as indicated by Wal- benefiting lactobacilli and streptococci; thus, the lactic lace and Cotta (1988). Also, greater proteolytic activity acid concentration was increased and pH of the hindgut of hindgut digestive microbial ecosystem with HS diet content decreased, which impaired microbial fibrolytic could explain such a result, given that starch favors activity. These results are consistent with the decreased the growth of amylolytic bacteria (Medina et al., 2002) disappearance rate of cell wall components (IVAD and that most amylolytic bacteria in the digestive tract are also proteolytic (Wallace et al., 1997).
ADF) observed here in the HS diet (Table 5), which explains the subsequent reduction of ADF digest- Because the dietary ingredients of both diets were ibility in horses fed the HS diet compared with the HF finely ground and pelleted, the MRT of small particleswas considered a good indicator of the MRT of whole diet (Table 4). As observed in the rumen (Hoover et al., diets. The mean whole-tract digestive MRT in horses 2006), it is likely that negative interactions on microbial fed the HF diet (32.3 h) was identical to that reported fiber digestion originating from starch fermentation oc- by Pagan et al. (1998) for thoroughbred geldings fed a curred in the hindgut of horses fed the HS diet. In forage diet. Also, the mean MRT of feed particles in the addition, the lower IVADNDF and IVADADF are likely hindgut of animals fed the HF diet (25.0 h) was in the due to the decreased amount of cell wall material sup- same range as the MRT in the large intestines (23.3 h) plied to the hindgut and used as substrate by cellulo- of ponies fed timothy hay (Ude´n et al., 1982). Further- lytic bacteria when animals were fed the HS compared more, the mean MRT of small particles in the prececal tract of horses fed the HF diet (8.0 h) was similar to The significant increase in DM and OM digestibility that (8.7 h) reported for ponies (Cabrera, 1995).
observed in the HS+0 diet compared with HF+0 diet The greater MRT of small particles and the trend for were in the same range of those observed by Van der greater MRT of large particles in the hindgut of horses Noot and Gilbreath (1970), Yoder et al. (1997), and fed the HS diet, which are in agreement with the results Palmgren-Karlsson et al. (2000). The same pattern has obtained by Yoder et al. (1997) on ponies, could contrib- been observed when forage-based diets were supple- ute to the improvement in DM and OM digestibility mented with raw cereals (Hintz et al., 1971b; Ott, 1981; that was noted. Contrary to Nuss et al. (1982), who Thompson et al., 1984; Pagan et al., 1998; Palmgren- observed that supplying long-form forages accelerated Karlsson et al., 2000; Drogoul et al., 2001) or pelleted the rate of passage of digesta in the proximal part of concentrates (Hintz et al., 1971a; Martin-Rosset and digestive tract located before the large intestine, a simi- Dulphy, 1987; Vermorel et al., 1997). The greater DM lar transit time of solid digesta (8 h) was observed in and OM digestibility in the HS diet compared with the both the HS and HF diets. The observation that animals HF diet is due to diet composition, because starch is were fed a limited amount of finely ground and pelleted digested to a greater extent than the cell wall fractions feed may have reduced the strict fiber effect on transit that was observed with long forages.
Effect of Saccharomyces cerevisiae CBS 493.94
In conclusion, the current study indicated that SC Supplementation on Digestion in Horses
significantly improved the digestibility of the cellulosefraction in horses, regardless of diet. The results sug- Yeast supplementation had a positive overall effect gest that SC supplementation improved chiefly the ac- on ADF apparent digestibility independently of diet.
tivity of the complex microflora involved in digestion of Because microbial digestion of the cell wall fraction occurs exclusively in the hindgut, the extent of micro-bial digestion is the result of the product of [microbialcellulolytic activity expressed as the amount of de- LITERATURE CITED
graded cellulose per h = IVADADF] × [Time available fordigestion expressed as the MRT in the hindgut, in h].
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