Dr. Maurice Eastridge, Dairy Specialist, The Ohio State University
Are you a label reader? Probably not, unless you get bored when you are on "business" in the bathroom and you don't have the Readers Digest or the local daily newspaper in arms reach. But take a look at the labels on the bottles of hand or body lotion, sun screen, and skin moisturizer. Very high on the list (and ingredients are listed in order from highest to lowest concentration), you will find glycerin (another name for "glycerol"). You also will find it in teat or udder ointment. Glycerol is an odorless, colorless, and sweet tasting viscous liquid, even at low environmental temperatures. It is the backbone for fatty acids to form fat (or triglycerides). Fat from soybeans, tallow, restaurant grease, etc. is being used presently for biodiesel production. The fatty acids are the integral part of the fuel, but the glycerol is a byproduct from the process. Every gallon of biodiesel produced will general about 0.75 lb of crude glycerol. So, with the increased availability of glycerol as a byproduct of biodiesel production, it is being investigated as a feed ingredient for dairy cattle. The bacteria in the rumen (first stomach compartment) can use glycerol to primarily produce propionate and the propionate will be converted to glucose by the liver of the cow.
In a recent Purdue University study, 60 dairy cows were fed diets containing 0, 5, 10, or 15% glycerol (replaced corn). The diets contained 31.9% corn silage, 10% alfalfa haylage, 12.2% alfalfa hay, and 45.9% concentrate (including the glycerol). The concentration of nonfiber carbohydrates (NFC) across the diets was approximately 39%. Feed intake, milk yield, milk fat, and milk protein were similar among the dietary treatments (53.5 lb/day, 81.2 lb/day, 3.60%, and 2.85%, respectively). Concentrations of milk urea nitrogen were lower with diets containing glycerol and cows consuming 10 or 15% glycerol gained more weight than cows not fed glycerol. We also recently completed a study at OSU whereby 48 cows (averaged 112 days in milk) were fed different concentrations of glycerol and NFC: 1) 0% glycerol and 37% NFC, 2) 5% glycerol and 37% NFC, 3) 10% glycerol and 37% NFC, or 4) 10% glycerol and 42% NFC. Diets contained 37.4% corn silage, 9.1% hay, and 53.5% concentrate (including the glycerol). Feed intake, milk yield, milk protein, and milk urea nitrogen were similar among the dietary treatments (52.6 lb/day, 87.1 lb/day, 3.06%, and 14.3 mg/dl). Milk fat percentage was decreased with glycerol addition (3.52, 3.18, 3.19, and 2.93%, respectively), especially when 10% glycerol was fed with 42% NFC.
In each of these studies, cows were fed the diets for eight weeks, and based on the results from these two studies, as well as some other published research, we can conclude that glycerol has value as a feed ingredient for dairy cattle. Feeding glycerol may have to be limited more when diets are rather high in NFC. The effect observed in milk fat percentage in the OSU study may not be a negative effect on the ruminal fermentation because feed intake was not altered; therefore, the effect may be on fat synthesis in the mammary gland. Feeding glycerol at 5 to 10% of the dietary dry matter should be valuable nutritionally and should pose limited risks to animals. Given the viscous nature of the product, it may help reduce sorting of total-mixed rations by dairy cows.
However, there is caution with the findings of these two studies. Both projects were conducted using food-grade glycerol. Crude glycerol from biodiesel production will contain unused catalyst (e.g. sodium hydroxide or potassium hydroxide), methanol, and salts. The actual amount of glycerol in crude glycerin may range from 75 to 90%. The Food and Drug Administration has issued a letter stating that if methanol is over 150 ppm (0.015%), then it should not be used for animal feed. So, additional research is going to be needed to determine the variation in composition of crude glycerol, including the concentration of contaminates that are of particular risk to animal health. Once these are better identified, then more defined feeding guidelines for crude glycerol can be established based on not only the nutritional value of the ingredient but also the limitations due to the impurities that may cause some risks.
(This article first appeared in Farm and Dairy, 6/21/2007, http://www.farmanddairy.com)