Digestion of starch and crude fi bre in segments of the digestive tract of sheep fed different types and amounts of starch

The experiment was carried out on 6 Polish Merino sheep of average 60 kg BW with reentrant cannulas into the proximal duodenum and terminal ileum. The animals were fed a basic diet consisting of meadow hay and rapeseed oilmeal with the addition of 46 or 23% of potato, maize, or wheat starch. The amount of digesta entering the duodenum and large intestine was measured, sampled and the amount of starch and crude fi bre determined. All types of starch were digested predominantly in the rumen. Of every 100 g of starch ingested with the high(or low-, in parenthesis) starch diet, its output from the rumen was: 10.4, 3.3 and 9.6 (15.2, 8.2 and 11.3) g; from the small intestine, 5.3, 0.7, and 2.7 (5.0, 1.9, and 3.2) g; excreted with faeces 1.1, 0.3 and 0.3 (0.4, 0.2 and 0.3) g, of potato, maize or wheat starch, respectively. Maize and wheat starch were more susceptible to digestion in the rumen and small intestine than potato starch. Only a small amount of starch reached the large intestine where it was nearly totally digested and only its traces were excreted with faeces. A higher ratio of starch to crude fi bre in the diet depressed crude fi bre digestion in the rumen and its total digestibility in the digetsive tract.


INTRODUCTION
Starch in the diet for ruminants affects nutrient digestion processes, bacterial protein synthesis, and dietary nitrogen utilization.High levels of starch in the diet usually decrease structural fi bre digestibility due to the rapid fermentation of starch to volatile fatty acids that decrease the pH and cellulolytic enzyme activity in the rumen.The disappearance rate of starch in the rumen depends on the amount of starch ingested, rate of ruminal digesta output, and the origin of the starch.According to literature data, cereal starch is fermented more easily in the rumen than starch of maize, sorgo, legumes or potatoes (Żebrowska et al., 1997;Mills et al., 1999;Monteils et al., 2002).Data concerning the amount of starch leaving the rumen and entering the further part of the digestive tract and its digestion are scanty.
The aim of the study was to determine the effect of the amount and origin of starch in the diet on nutrient digestion in the rumen, small and large intestines of sheep.

MATERIAL AND METHODS
The experiment was carried out on 6 Polish Merino sheep of average 60 kg BW with re-entrant cannulas into the proximal duodenum and terminal ileum, fed a basic diet consisting of meadow hay, rapeseed oilmeal and mineral mixture, and supplemented, instead of hay, with 46 or 23% of potato, maize or wheat starch for respective groups of animals.The diets were isonitrogenous (11.5% crude protein) and contained 12.3 or 11.1 MJ ME, respectively, in the diets with 46 or 23% of starch.The animals were fed 1.2 kg/day in two portions at 8.00 and 16.00 at free access to water.
After 14 days of adaptation, faeces was collected for 3 days; duodenal and ileal digesta were collected and sampled from 8.00 to 16.00 and pooled for 3 days.The samples were frozen until analysis for nutrient content (AOAC, 1990).
The obtained results were subjected to statistical analysis of variance according to the Statgraphics ® ver.7.0 (1993) procedure.

RESULTS
The main site of all types of starch digestion was the rumen and in the total digestive tract, starch was digested nearly completely.Of each 100 g of starch ingested with a high-starch diet, its output (respective data for low level of starch are given in paranthesis) from the rumen was: 10.4, 3.3 and 9.6 (15.2, 8.2 and 11.3) g; from the small to large intestine 5.3, 0.7, and 2.7 (5.0, 1.9, and 3.2) g; excreted with faeces 1.1, 0.3 and 0.3 (0.4, 0.2 and 0.3) g, respectively, of potato, maize or wheat starch.Ruminal digestibility of starch at a high level of maize in the diet differed (P<0.01)from that of potato or wheat but there were no differences between starch type fed at low levels (P>0.05).In the small intestine, both levels of maize starch were digested better than potato starch.In the large intestine, potato and wheat starch were digested better than maize starch.Total digestive tract digestibility did not differ between types and levels of starch except for slightly but signifi cantly higher (P<0.05)than for potato starch.
Ruminal digestibility of crude fi bre when feeding a high potato starch diet was signifi cantly (P<0.01)higher than at a similar level of wheat starch, but at feeding STARCH AND FIBRE DIGESTION IN THE RUMEN AND INTESTINES low starch diets, a higher digestibility was obtained for maize starch (P<0.01).The crude fi bre of all diets with a high level of starch was digested worse than in diets containing less starch (P<0.01).Crude fi bre digestibility coeffi cients in the small intestine did not differ statistically between levels or type of starch (P>0.05).In the large intestine, crude fi bre was digested better when high-than low-starch diets were fed.Total digestive tract crude fi bre digestion was the highest when the diet with a low level of maize starch was fed.

DISCUSSION
The results demonstrate that the main site of starch digestion, independently of its type and content in the diet, is the rumen where about 90% of dietary starch is fermented.However, potato starch appeared more resistant to decomposition in the rumen than maize or wheat starch, of which only a small proportion reached the duodenum.It confi rms that bacterial α-amylase excreted by Clostridium butiricum or Streptococcus bovis hydrolyses potato starch slowly, but extracellular α-amylase is not able to hydrolyse this type of starch.The reason for the lower digestibility in the low-starch diets could be explained by the changed diet composition, as in this case the fi bre content was signifi cantly higher and cellulytic enzyme activity prevailed over amylolytic activity.The digestibility coeffi cient of all types of starch in the small intestine was lower than in the rumen as the digesta in the small intestine does not contain developed amylotylic microorganisms.However, maize and wheat starch was, similarly as in the rumen, KOWALIK B. ET AL. more susceptible to digestion in the small intestine than potato starch, particularly when fed in larger amounts.Only a small amount of starch reached the large intestine where conditions for its digestion were similar to those in the rumen.Only about 1% of ingested starch, including potato starch, avoided fermentation in the large intestine and was excreted in faeces.
Better digestion of crude fi bre in the rumen when a diet with a high level of potato starch was fed can be explained by the lower fermentation of potato starch leading to fewer volatile fatty acids and a higher pH in the rumen, creating better conditions for development of fi brolytic bacteria.Feeding the diets with high levels of all types of starch signifi cantly depressed fi bre digestibility compared with results obtained on low-starch diets.Digestion of fi bre reaching the small intestine was low, demonstrating low fi brolytic activity of small intestine digesta.Differences in fi bre digestion in the large intestine of animals fed different types of starch were not signifi cant and should be attributed to differentiated levels of fi bre in the diet and low digestibility of fi bre in the rumen of animals fed high levels of starch.As a result, total digestibility of crude fi bre was not affected by the type of starch fed but a lower ratio of starch to crude fi bre in the diet stimulated fi bre digestion.

CONCLUSIONS
Potato, maize and wheat starch in diets fed to sheep are digested predominantly in the rumen and only a small percentage of ingested starch reaches the small intestine where the rate of its digestion is lower than in the rumen.Maize and wheat starch are more susceptible to digestion in the rumen and small intestine than potato starch.Only a small amount of starch reaches the large intestine where it is nearly totally digested and only traces are excreted with faeces.A higher ratio of starch to crude fi bre in the diet depresses crude fi bre digestion in the rumen and its digestibility in the total digetsive tract.

Table 1 .
Starch and crude fi bre digestibility (%) in total, or partial digestive tract in sheep fed different types of starch at high (46%) or low (23%) levels in the diet