Crude glycerin alters beef cow ruminal fluid degradability of nutrients in vitro

Crude glycerin (GLY) is used as an energy supplement for cattle and has been shown to increase propionate and decrease acetate concentrations in the rumen. Glycerin also decreases ruminal NH₃, and therefore, we hypothesized that GLY in rumen fluid would decrease protein degradability of feedstuffs. The objective was to evaluate the effect of GLY in bovine rumen fluid on extent of protein degradability of alfalfa hay (ALF), dried distiller's grains (DDG), and soybean meal (SBM). The study was conducted using an in vitro incubator with four incubation jars. Each incubation jar contained 18 filter bags with 0.5 g of ground feed sample (six bags contained ALF, six contained DDG, and six contained SBM), six blank bags, 1,600 mL of buffer solution, 400-mL rumen fluid (collected from two ruminally cannulated cows), and one of the two treatments. Treatments were 12.7 g of deionized water (-GLY) or 12.7 g of GLY (+GLY) added to the 2-liter contents of each jar. Incubation jars were purged with CO₂ and allowed to incubate at 39°C for 48 h. The 48-h incubation period was repeated for a total of three runs (the study was a randomized complete block design). Fluid samples from incubation jars were collected at 0 and 48 h into 25-mL vials for measurement of pH and analysis of VFA and NH₃. At the end of the 48-h incubation period, filter bags containing feed samples were collected for determination of DM, NDF, and CP disappearance. The pH and NH₃ concentrations of the contents of jars were lower (P ≤ 0.01) and total VFA concentrations were greater (P < 0.01) for +GLY than -GLY after 48 h of incubation. Molar percentages of acetate, isobutyrate, and isovalerate were lower (P < 0.01) and molar percentages of propionate and valerate were greater (P ≤ 0.02), resulting in a lower (P < 0.01) acetate-to-propionate ratio for +GLY compared with -GLY after the 48-h incubation period. A GLY × feed interaction (P < 0.01) occurred for DM and CP disappearance, where +GLY decreased DM and CP disappearance of SBM more than that of ALF and DDG. Disappearance of NDF from ALF, DDG, and SBM was lower for +GLY compared with -GLY (GLY; P < 0.01). These results suggest that supplementing glycerin to ruminants may cause an increase in RUP fraction escaping to the small intestine for potential digestion and absorption.