Effect of dietary NDF levels on nutrient digestion in hindgut of sheep *

Four 4-month-old wethers fi tted with ileal canulas were fed iso-caloric and iso-nitrogenous diets containing different NDF levels (30.0 to 45.0%) in a 4×4 Latin square design to evaluate the effects of dietary NDF levels on nutrient digestion in hindgut of sheep. The percent of hindgut digestion of DM, OM, CP, NDF, ADF, hemicellulose and cellulose in total tract digestion was 28.0 to 31.8%, 19.3 to 21.9%, 14.1 to 16.2%, 22.3 to 25.1%, 22.0 to 25.2%, 22.7 to 25.1% and 18.1 to 21.5%, respectively. Lower (30%) or higher (45%) NDF content in diet of sheep depressed fi brous fraction digestion anterior to hindgut, and hindgut complementally increased digestion of fi brous fractions.


INTRODUCTION
The hindgut of ruminant includes caecum, colon and rectum.The caecum and fi rst region of the proximal colon act as a single compartment, which is the main site of fermentative digestion in hindgut.It was indicated that the hindgut made a signifi cant contribution to fermentative digestion in ruminants, particularly when they were fed diets of low nutritive value.Forage to concentrate ratio is one of the main factors on nutrient digestion in ruminants, however, limited studies determined its effect on nutrient digestion in hindgut (Lewis et al., 1985;Siciliano-Jones et al., 1989), and the results were variable.In these trials, the animals were fed ad libitum, thus, had different feed intake, and feed intake could affect digestibility of dietary nutrients.Neutral detergent fi bre (NDF) is a main index to refl ect forage to concentrate ratio of diet.This study was conducted to determine the effects of NDF levels of diets with iso-caloric and iso-nitrogen on nutrient digestion in hindgut of sheep with same feed intake.

Animals and diets
Four 4-mo-old cross-bred (Small-Tailed Han sheep×Poll Dorset) wether lambs (BW 23.5±0.6 kg) were used in a 4×4 Latin square design.Each was fi tted with an ileal cannula of 5 mm internal diameter.The ileal cannula was placed approximately 15 cm cranial to the ileo-caecal junction.The surgery was completed 3 weeks before the experiment.Animals were housed individually in pens.To ensure full consumption, diets were fed restrictedly and 900 g (DM basis) feed were fed at 06.00 and 18.00 in two equal allotments.Water was freely available at all times.The ingredients and chemical composition of experimental diets were presented in Table 1.Diets were formulated to be 30, 35, 40 and 45% NDF, respectively, be iso-caloric and iso-nitrogenous and meet the CP requirements for 50 g of gain/d (NRC, 1985).

Experimental procedure
Each experimental period lasted 15 days, with the fi rst 11 days for dietary adaptation and 4 days for sample collection.On days 12-13, faecal samples were collected.Ileal samples were collected on days 14-15.From 06.00 on d 14 to 04.00 on d 15, ileal digesta samples were taken at 2 h intervals.From 07.00 on d 15 to 05.00 on d 16, ileal digesta samples were taken at 2 h intervals.Twenty-four ileal samples from each lamb were composited for each period by the mixing of equal portions of each.Feed, faecal and ileal samples were dried at 50°C in an air-forced oven.

Calculations and statistical analyses
AIA was used as an endogenous marker for calculating the apparent digestibilities of major nutrients.According to the percentage values of AIA and nutrients in feed, faecal, and ileal samples, total tract and ileal apparent digestibilities were caculated.Hindgut apparent digestibility was obtained by subtracting ileal apparent digestibility from total tract apparent digestibility.
Data were analysed using ANOVA procedures of SAS (1999), and the means were compared by Duncan-test.

RESULTS AND DISCUSSION
The dietary NDF levels effected (P<0.01) the ileal DM digestibility of sheep, with lowest value for sheep fed the diet containing 45% NDF, and had not effect (P>0.05)  NRC (1985) on the hindgut digestibility of DM or OM and its percent in total tract digestion, but, with highest value for sheep fed the diet containing 45% NDF (Table 2).
Dietary NDF levels had not effect (P>0.05) on CP digestibility in ileum and hindgut.But, the CP ileum digestibility of sheep fed the diet containing 45% NDF was lower than others, and its percent of hindgut digestion in total tract digestion was higher.Lewis et al. (1985) found that the percentage of CP caecum digestion of sheep fed diets of, %: 60 maize: 40 hay and 80 maize: 20 hay were 21.6 and 22.0, which accounted for 37.96 and 34.10% of total tract digestion of CP, respectively.In present experiment, the percent of CP digestion in hindgut accounted for 14.1 to 16.2% of the total digestible CP.However, Siciliano-Jones et al. (1989) indicated that CP digested in the large intestine of cow fed diets containing, %: 20 hay: 80 grain and 80 hay: 20 grain only accounted for 3.2 and 0.3% of total tract digestion of CP, respectively, which may be resulted from incomplete recovery of the indicator (chromic oxide).Hoover (1978) mentioned that ammonia absorption from the hindgut contributed 39% of the total nonprotein nitrogen absorbed into body fl uids, and was probably a major source of nitrogen recycled to the rumen.The ileal NDF and HC digestibility of sheep fed four diets were similar, but the ileal ADF and CEL digestibility of sheep fed 30 and 45% NDF diets were lower than those of sheep fed 35 and 40% NDF diets (Table 3).The result indicated that lower (30%) or higher (45%) NDF in diet decreased fi brous fractions digestion anterior to hindgut.Dietary NDF level did not affect the hindgut NDF, ADF, HC and CEL digestibility of and the percent of hindgut digestion of fi brous fractions in total tract digestion of sheep, but common trend was that corresponding values of 30 and 45% NDF diets were higher than those of 35 and 40% NDF diets, and it indicated that digestion of fi brous fractions escaped rumen fermentation then reaching hindgut complementally increased.Lewis et al. (1985) reported that hindgut digestion of NDF, ADF and CEL of sheep fed diets with, %: 60 maize: 40 hay and 40 maize: 60 hay accounted for 15.9 and 12.9%, 29.7 and 33.6%, and 23.4 and 36.8% of the total digestible NDF, ADF and CEL, respectively.In present experiment, the percent of hindgut digestion of NDF and ADF in total tract digestion was 22.3 to 25.1% and 22.0 to 25.2%, respectively.Callison et al. (2001) mentioned that large intestinal NDF digestion of dairy cow accounted for 9.5 to 18.9% of the total digestible NDF.Siciliano-Jones et al. (1989) reported that large intestinal NDF and ADF digestion of sheep accounted for 4.9 to 15.5% and 5.0 to 14.9% of the total digestible NDF and ADF, respectively.Possible reasons causing the difference of fi brous fractions digestion in hindgut may include feed sources, forage maturity, concentrate to forage ratio, feed intake, and so on.Hoover (1978) reviewed that, in ruminants, the hindgut digestion of HC accounted for 30 to 40% of the total HC digested per day, and the hindgut digestion of CEL accounted for 18 to 27% of the total CEL digested.In present experiment, the percent of hindgut digestion of HC and CEL in total tract digestion of sheep was 22.7 to 25.1% and 18.1 to 21.5%, respectively.

CONCLUSIONS
It is concluded that increasing dietary NDF levels from 30 to 45% a little increased the percent of hindgut digestion of DM, OM and CP in total tract digestion of sheep, and dietary NDF level had not signifi cant effect on the percent of hindgut digestion of NDF, ADF, HC and CEL in total tract digestion, but corresponding values fed 30 and 45% NDF diets were higher than those of 35 and 40% NDF diets.Hindgut digestion has a compensating or equalizing effect on total tract digestibility.

Table 1 .
Ingredients and chemical composition of experimental diets

Table 2 .
Effect of dietary NDF levels on DM, OM and CP digestion in sheep

Table 3 .
Effect of dietary NDF levels on fi brous fraction digestion in sheep