Biofuel co-products as livestock feed - Opportunities and challenges

158Biofuel co-products as livestock feed – Opportunities and challengesFIGURE 5Acetate:propionate ratio following in vitro fermentationof starch+glycerin mixtures by a mixed ruminalinoculum from grain-fed steersA:P Ratio0.900.850.800.750.700 2 4 8 10 12 16Glycerol, % of substrateinhibited cellulolytic ruminal fungi far more than cellulolyticbacteria. Paggi, Fay and Faverin (2004) also reported deleteriouseffects of glycerin on cellulolysis, and suggested thatthe concentrations necessary for inhibition were consistentwith levels capable of suppressing Neocallimastix frontalis,a ruminal fungus integrally involved in cellulolysis. Fungalcolonization is an important step in the digestion of cellulose,especially for low quality forages. These observationscould have important implications for diets that containsubstantial amounts of cellulosic materials, including dietscontaining fibrous byproduct feeds derived from processedcereal grains, oilseeds, sugar cane and other agriculturalcommodities.The impact of glycerin on fibre digestion has been thesubject of studies conducted by several research groups,measuring fermentative end-products and concentrationsof specific microbial populations often associated withfibre digestion. Abo El-Nor et al. (2010) investigated theimpact of increasing proportions of glycerol (0, 3.6, 7.2or 10.8 percent of substrate DM) on ruminal fermentationusing continuous culture systems fed a substrate consistingof 60 percent alfalfa hay in combination with groundmaize, soybean meal and soybean hulls. Total volatilefatty acid (VFA) production was greatest with the highestconcentration of glycerol, A:P ratio declined linearly withincreasing levels of glycerol addition, and digestibility ofneutral-detergent fibre (NDF) decreased with the additionof 7.2 or 10.8 percent glycerol, perhaps suggestingthat digestion of non-fibrous substrate was improved.Concentration of DNA from Butyrivibrio fibrisolvens, akey fibre-digesting organism in the rumen, decreasedlinearly in response to increasing levels of glycerin.Additionally, Selenomonas ruminantium and Clostridiumproteoclasticum decreased with higher levels of glycerol,and total bacterial DNA decreased by nearly 32 percentwith the highest level of glycerol addition. Based on theseobservations, it is conceivable that high levels of glycerinaffect not only fungi, but also may have deleteriousconsequences for ruminal bacteria. Krueger et al. (2010)reported decreases in A:P ratio with glycerol addition,though no negative effects on NDF digestibility werenoted. Van Cleef et al. (2011a) found that the impact ofglycerin on in vitro digestion was substantially influencedby prior exposure of donor animals to glycerin. In vitrodigestibility of diets decreased in response to glycerinaddition when ruminal digesta contents were obtainedfrom cattle fed diets without glycerin, whereas dietdigestion increased in response to glycerin addition whenthe ruminal inoculum was recovered from animals thathad been adapted to a diet containing 15 percent glycerin(interaction, P