Chemical and physical factors affecting starch digestibility in vitro and interactions with fibre

[摘要] ENGLISH ABSTRACT: Maize is a valuable and expensive resource in the dairy industry. It is routinely used in ruminants' diets as an energy concentrate to ensure that the high energy demands of top performing animals are met. The purpose of this study was to investigate chemical and physical factors affecting starch digestibility in vitro as well as possible interactions with fibre digestibility. Milling of grains is considered to have a great impact on the rate and extent of starch digestion, however differences in milling conditions lead to variation in particle and often an inconsistent product. In our first experiment hard and soft maize produced a NGMPS of 274.58 ± 0.87 and 470.91 ± 0.87 respectively when milled at 3mm; and a NGMPS of 396.64 ± 0.87 and 576.66 ± 0.87 respectively. There was significant interaction between the type of maize and screen size used. In the second experiment ground maize was divided into five different fractions and combined with a forage (lucerne or oat hay) to create combinations of either high or low starch-to-neutral detergent fibre (NDF) ratios. The chemical constituents were determined for the different maize fractions as well as the forages. Subsequently the individual ingredients as well as the combinations were analysed for 24 hour in vitro starch digestibility, rate of starch digestion, 48 hour in vitro NDF digestion (NDFd), and rate of NDF digestion (KNDF). Starch digestibility for the maize fractions Very fine, Fine, Medium, Coarse, and Cracked maize were 64.33, 62.28, 59.84, 47.58, and 42.15%, respectively, and rate of starch digestion was 18.24, 13.48, 10.02, 7.16, and 3.77 %/h, respectively, when pooled for forages and starch-to-NDF ratio. Fibre digestion was influenced by particle size, starch level and forage, resulting in NDF digestibility being the highest when combined with coarse or cracked maize, 43.15% and 44.15% respectively, and lowest with fine maize, 32.99%. The rate of NDFd for oat hay and lucerne was 3.11 and 5.11 %/h, respectively and it was influenced by particle size, with very fine maize reducing the rate. In our second experiment, we investigated how different proportions of starch type (amylose/amylopectin) impact the rumen digestion of grains. Hylon VII (74% amylose starch) and Amioca (98% amylopectin starch) were combined with forages (lucerne or oat hay) in order to create combinations of either high or low starch-to-NDF ratios. The chemical constituents of Amioca, Hylon, oat hay and lucerne were determined. Consequently, the individual ingredients as well as the combinations were analysed for 24 hour in vitro starch digestibility, rate of starch digestion, 48 hour in vitro NDF digestion, and rate of NDF digestion. Amioca had the greatest starch digestibility and the addition of forages increased starch digestion. Rate of starch digestion was 12.55 %/h and 6.13 %/h for Amioca and Hylon respectively and the rate was influenced forage type, but not by starch level. The KNDF was 7.35%/h for lucerne and 3.87%/h for oat hay (when pooled for starch type and starch level). The rate of NDFd for oat hay was, 3.15 %/h when combined with Amioca and 3.30 %/h with Hylon, but the difference between the control and the starch types was not significant. For lucerne the rate of NDF digestion was reduced by the addition of starch, 7.07 %/h when combined with Amioca and 5.88 %/h with Hylon. Enhanced characterization of grains', with regards to particle sizes and starch type, has the potential to better describe a specific feed's starch digestibility, the possible interactions with cell wall digestion and to more effectively satisfy the nutritional requirements of animals in different physiological stages.

[发布机构] Stellenbosch University