IN-SITU DEGRADATION KINETICS OF MAIZE HYBRID STALKS

An understanding in plant factors influencing digestion kinetics could help develop maize cultivars, thus improving digestibility and conseq uently the production of ruminants. The stalk fraction of maize hybrid s was used to study the cell wall digestion of hybrids varying from th e brown midrib-3 (bm3) and their isogeneic normal (N) to normal hybrid s. Cell wall constituent contents (NDF, ADF, ADL), soluble carbohydrat es (SC), in vitro dry matter digestibility (IVDMD) and ruminal degrada tion kinetics of the dry matter (DM) and the cell wall (measured as ND F) were determined on stalk samples harvested at silage stage (30-35% DM, whole plant). Six comparative studies of in situ degradability inv olved 3 comparisons of the bm3 and their N isogeneic hybrids (Ex223, E x234 and Adonis) and 3 comparisons involving 2 extreme genotypes (DK26 5 and Stefania): each were compared to the control genotype (Adonis). A comparison of the three sets of N and bm3 hybrids demonstrated that the mutation contained a lower concentration of the cell wall constitu ents. Hemicellulose rather than cellulose was generally present in low er amounts in the cell walls. Higher SC content and IVDMD were also se en in the bm3 maize. Mean effective degradabilities for both DM (64.4 versus 53.2%) and NDF (36.3 versus 29.5%) were higher (P<0.01) in the bm3 maize stalks than the N counterpart, respectively. In the maize no rmal material comparisons, the DK265 genotype presented a lower amount in all the cell wall constituents and was greater in SC content and I VDMD than in the Stefania genotype. The effective DM degradability for DK265 (59.9%) was not as high as the bm3 hybrids, but it showed a gre ater variation within the genotypes (36.1% for Stefania). A higher in situ DM degradability of bm3 maize compared to their counterpart N or between normal material, could be due to the lower cell wall constitue nt contents, to a decrease in the undegradable fraction as well as inc reases in the rapidly degradable or soluble fraction and to less indig estibility of the cell walls (NDF). The amount of lignin does not have a significant effect on the degradation rate constant associated with NDF digestion for most maize comparisons, but is closely related to t he extent of degradation. (C) 1997 Elsevier Science B.V.