Effects of corn particle size and ratio NDF:starch on in-vitro NDF degradability

Abstract Text: Reducing particle size is the most common technique to increase starch digestibility, especially for corn, and both amount and size of starch source may affect rumen environment. The objective of our study was to verify the effects of specific ranges of corn particle size and NDF:starch ratios on starch and NDF degradability (NDFd). The same batch of corn was milled using a Wiley mill with either a 1 or 2 mm screen. The corn was sieved and separated using the following sizes: <250, 250-500, 500-1180 and 1180-2000 μm. All sizes were analysed for starch, CP, EE and NDF. Oat hay and alfalfa were used separately to evaluate the effects of particle size on NDFd using the NDF:starch ratios of 1:1 and 1:1.25. Ratios were adjusted for each size class based on actual starch and NDF contents of both corn and forages. Higher ratios were excluded because the buffering capacity of the medium avoided changes in pH and NDF digestibility. Residual starch and NDF of the fermented samples were obtained at 3, 6, 9, 12, 24 and 48 hours. Rates of digestion (kdStarch and kdNDF) were calculated using a first order decay model and estimated indigestible starch and NDF using the 48 and 240 h fermentation residuals, respectively. Indigestible starch was numerically different across sizes. Data were analyzed according to a randomized complete block design with a factorial arrangement of treatments. The main tested effects were size, ratio, forage and their interactions. Fermentation run was considered random effect. With increasing particle size, starch decreased from 79 to 55%, NDF increased from 3.4 to 33.1%, EE increased from 2.17 to 3.78% and CP increased from 6.40 to 8.21%. Both forage type and NDF:starch ratio did not have any effect (P>0.45) on ivSD and kdStarch. As expected, kdStarch linearly increased (P<0.01) with smaller particle size with no interaction with either forage or NDF:starch ratio, from 0.14 to 0.47 h^-1. Interaction was present (P<0.01) between forage and particle size for ivSD and kdStarch, with more starch degraded for the two smallest size and alfalfa. Particle size, and not amount, affected NDF digestibility for both forages (P<0.01), with consistent higher NDFd and kdNDF for the largest size. Particle size affected NDF fermentation more than amount for corn, even if the medium might have decreased the effect of larger amount of starch. 

Keywords: in-vitro, NDF, particel size, starch