1697
Effect of particle size and time of rumen fluid collection on in-vitro starch digestibility of corn and sorghum
Starch digestibility affects rumen health and milk production in dairy cows. Corn and sorghum are among the most common starch sources, but the least digestible when unprocessed. Grinding is the preferred way to increase starch digestibility. However, time of rumen fluid collection is often not considered as source of variation when analyzing in-vitro starch digestibility. In addition, interaction between particle size and time of feeding may further bias the digestibility results. Objectives of the study were to assess the effects of: 1 - different particle sizes in corn and sorghum, 2 - time of rumen fluid collection relative to feeding (ad libitum feeding, i.e. 8 h after first morning feeding, AL vs. collection after 8 h of fasting, FA) and 3 - their interaction on in-vitro starch digestibility (IVSD). Several cultivars of corn and sorghum were analyzed for starch and CP content and ground at 1, 2, 4 and 6 mm, using an ultra-centrifugal mill. Rumen fluid was collected from two cows fed a lactation diet containing 25% starch. Residual starch of the fermented samples were obtained at 0, 1, 2, 4, 6, 8, 12 and 24 h. Density of grains were also measured for all particle sizes and correlated with IVSD. Rates of starch digestion were computed assuming a first order decay and the parameters were estimated using PROC NLIN in SAS. Rates of digestion were on average higher for the FA rumen fluid (P<0.05) compared to the AL rumen fluid with 0.21 vs. 0.16 and 0.14 vs. 0.10 h-1 for corn and sorghum, respectively. Furthermore, interaction between particle size and rumen fluid was highly significant (P<0.001). Finer corn and sorghum had in fact consistently higher rates of starch digestion, increasing with decreasing particle size (P<0.001), only within the AL rumen fluid. When using the FA rumen fluid, particle size did not have any influence on starch digestibility (P=0.67). Both density and CP were also negatively correlated (-0.65 and -0.78, P<0.05) with IVSD. This proves the importance of time of collection of rumen fluid but it could also imply important consequences for rumen health when feeding starch separately, after hours of fasting.
Keywords:
in-vitro starch digestibility, rumen fluid, rumen health