Summary In this study preileal starch digestibility of starchy feeds (oats, corn, barley, potatoes, manioc) was determined in seven jejunofistulated horses. The grains were fed whole (oats, corn), rolled (oats, barley), crushed, ground and expanded (corn); the potatoes were fresh, the manioc rolled. Ground corn was also fed in combination with amylase. The feeds were fed partly isolated or in combination with alfalfa meal or hay (Table 1). At least four horses with a cannula in the terminal jejunum were used for each diet. Two meals per day were offered at 12 h intervals. The starch intake was mostly about 2 g/kg bw/meal, except one period with oats (3.9 g starch/kg bw) and with expanded corn (1.4 g/kg bw). Jejunal chyme was postprandially collected 11 times (from 1st to the 11th h after the morning meal for 15 min). Starch was determined polarimetrically. The preileal digestibility of starch was calculated by the marker method (chromic oxide 0.25% DM) and by estimating the total jejunoileal chyme flow during 12 h postprandially extrapolating the sample volume from the 15 min sampling periods. The results of both methods agreed quite well. Preileal digestibility of oat starch (80–90%) was (independent of doses or preparation or of the combination with hay, Table 4) significantly higher than that of whole or crushed corn (30%) or barley (26%). Grinding of corn significantly increased preileal digestibility to 51%, expanding to 90%. The addition of amylase improved digestion of ground corn by 10% (absolute). The preileal digestibility of potato or manioc was less than 10%. Individual factors in the horse (chewing intensity, amylase activity) had also considerable influence on preileal starch digestibility.

Summary Structure of starch in feed and chyme of horses (7 with a cannula at the caudal end of the jejunum and 2 with a cecal fistula) after feeding high starch diets (maize: whole, broken, ground, expanded and as silage, oats: whole, rolled or ground, rolled barley, raw potatoes, and tapioca) was investigated by light and electron microscopy. Structure of feed starch and morphology or starch degradation in the chyme corresponded to data on preileal starch digestibility which was investigated in a parallel study. Barriers for starch digestion in the gastrointestinal tract of the horse were structure of the plant storage organ, as, for example, a tight connection between starch granules in maize grains as well as the structure of the starch granules itself. The highly digestible oat starch was degraded by exocorrosion around the grains, whereas in other, less digestible, starch types degradation occurred by endocorrosion via pin holes. The number and size of the pin holes increased with increasing preileal starch digestibility. The effect of various ways of decomposition on preileal digestibility increased with advanced destruction of the original starch structure. Expanding was most effective. The granules were destroyed completely and the starch became soluble. Simple examination by light microscopy is a fast method to evaluate the degree of starch decomposition in the feed.

Zusammenfassung Die Aktivität der Amylase im Magen‐Darm‐Kanal des Pferdes An insgesamt 18 Pferden (146–410 kg KM) wurde die Aktivität der Amylase im Pancreasgewebe (postmortal, n = 8), im Chymus des terminalen Jejunums (Fistel am Ende des Jejunums, n = 8) bzw. des Caecums (Caecumfistel, n = 2) nach Fütterung von Getreide (Hafer, Mais, Gerste in verschiedenen Zubereitungen), Maissilage und Corn‐Cob‐Mix sowie Knollenstärken (Tapioka, rohe Kartoffeln) im Vergleich zu ausschließlicher Heufütterung überprüft. Der Effekt eines Amylasezusatzes (50 g Amylase HT1000 aus Bacillus subtilis) im Futter auf die Aktivität im Jejunum wurde ebenfalls untersucht. Außerdem erfolgte die Bestimmung des pH‐Optimums der Amylase. Der Einfluß einer 3stündigen in vitro‐Fermentation des Jejunumchymus auf Amylaseaktivität und pH‐Optimum wurde überprüft. Die Amylaseaktivität im Pancreasgewebe bewegte sich zwischen 85 und 909 U/g Gewebe, ein Einfluß der Fütterung war nicht offensichtlich. Im Mittel betrug die Amylaseaktivität im Dünndarm 22,3 U/g Chymus. Sie lag bei Rationen mit Getreide deutlich höher (30,8 ± 15,4 U/g Chymus; n = 161) als bei ausschließlicher Heufütterung (15,0 ± 3,4 U/g; n=12) bzw. bei Maissilage und Corn‐Cob‐Mix (7,0 ± 6,6 U/g; n = 32) oder bei Knollenstärke (12,5 ± 9,8 U/g; n = 50). Zwischen einzelnen Pferden bestanden erhebliche Differenzen in der Höhe der Amylaseaktivität im Jejunumchymus, die sich bei allen Rationen reproduzieren ließen. Der Zusatz von Amylase zum Futter führte zu einem erheblichen Anstieg der Aktivität im Jejunum (132,3 ± 50,1 U/g; n = 8). Die Aktivität der Amylase nahm vom Jejunum‐ zum Caecumchymus sowie nach in vitro‐Fermentation ab. Das pH‐Optimum der Pancreasamylase lag bei pH 7. Der Verlauf des pH‐Optimums war im Jejunumchymus ähnlich wie im Pancreasgewebe, während die Kurve im Caecumchymus und nach in vitro‐Fermentation flacher verlief.