Ruminal degradability of corn forages depending on the processing method employed

The in sacco degradabilities of starch and fibre in corn were compared between kernel grains and the whole corn plant before and after ensiling using the nylon bag technique. The same plants were used, in order to exclude the effects of genotype or maturity at harvest. The incubation time course was carried out over 48 h on four cannulated dairy cows. The effective degradability of starch was lower in kernel grains (70.2%) than in the whole plant before (83.9%) and after (92.3%) ensiling. Starch degradation in whole plants was accelerated compared to kernel grains by a shift from the slowly degradable (from 61.3% to 31.9%) to the rapidly degradable fraction (from 35.9% to 65.6%) without significantly affecting the degradation constant rate (7.7 and 8.0% per h respectively for kernel grains and whole corn plants). The ensiling process improved starch degradation even further compared to whole fresh plants by significantly increasing the rapidly degradable fraction (80.7% versus 65.6%) and by a higher degradation constant rate (12.4% per h versus 8.0% per h). The fibre degradation was similar between grains and whole corn plants despite differences in their content and composition of NDF. However, ensiling significantly increased the rapidly degradable NDF fraction (15.2 versus 9.9%) and doubled its degradation constant rate (from 1.6% per h to 3.2% per h). This effect was probably due to improved hemicellulose degradation, because cellulose and lignin were not degraded differently between corn plants before and after ensiling. Mechanical cracking such as chopping at harvest improves ruminal starch degradation without altering fibre degradation but the ensiling process simultaneously increases the degradability of starch and fibres. Dégradation ruminale du maïs fourrage en fonction de sa forme de présentation. Les dégradations in sacco de l'amidon et des fibres ont été comparées entre le maïs grain et la plante entière avant et après fermentation en utilisant la technique des sachets nylon. L'utilisation des mêmes plantes a permis de contrôler les effets du génotype ou du stade de maturité à la récolte. L'étude a porté sur 48 h d'incubation avec 4 vaches fistulées. La dégradabilité théorique de l'amidon du maïs grain (70,2 %) est plus faible que celle de l'amidon de la plante entière avant (83,9 %) et après (92,3 %) fermentation. La dégradation d'amidon de la plante entière a été accélérée par rapport à celle du maïs grain par un déplacement d'une partie de