Introduction

Biodiesel has been produced for over 100 years, since Rudolf Diesel tested the use of vegetable oil as a fuel. Biodiesel use has increased by towns, school districts, businesses, and government agencies. Its production and the number of biodiesel plants have also increased rapidly. In 2003, about 450 million gallons of biodiesel were produced in Europe, and the United States produced 450 million gallons in 2007 (National Biodiesel Board). The cost to produce biodiesel depends on the feedstock.

Biodiesel is produced primarily through transesterification, a process where a fat or oil is mixed with an alcohol to form esters and glycerol. For every gallon of biodiesel produced, about 0.66 pounds of glycerol are generated. Glycerol is used in over 1,500 products, including candy, cake mixes, medicines, lotions, shampoo, soaps, detergents and make-up. It is also used in emollients, lubricants, solvents, and chemical dispersing products.

In October of 2008, crude glycerin was trading at 5 to 6 cents a pound. The high in 2008 was 25 cents a pound in the summer when the commodity prices increased. The low was in September of 2008 when glycerol traded at 1 cent per pound. FC-Stone, a commodity risk management company, projects that 600 million gallons of biodiesel will be produced in 2008. This means an extra 60 million gallons of glycerol on the market, which could keep the price low for some time.

Glycerol is a potential valuable and economical energy source for swine diets. The author conducted research to examine the potential for using glycerol in nursery pig diets and the impact that glycerol may have on feed manufacturing. Michael Shields conducted this work as part of his M.S. degree programme.

Feed Manufacturing Studies

To evaluate the effect of glycerol inclusion on feed flowability, we constructed three finisher diets to contain three levels of glycerol (0, 2.5 and 5%). The crude glycerol contained 86.95% total glycerol, 9.22% moisture, 1.26% sodium, and 1.86% chloride. Diets were manufactured in batches of 1,000 lb and replicated three times. Feed was manufactured at the N.C. State University Feed Mill Educational Unit, and samples were taken to determine flowability. We observed that flowabilty of finishing feed improved when glycerol was added. We subsequently manufactured the same diets (0, 2.5, and 5% supplemental glycerol) to determine the impact of glycerol on pelleting. In that study, we used two conditioning temperatures. All diets were pelleted at a conditioning temperature of 165 ºF, because glycerol-containing diets, especially at 5% inclusion, could not be pelleted at higher temperatures. In addition, a second batch of control feed was pelleted at 185ºF, a more common pelleting temperature for swine diets, resulting in four treatment comparisons.

Glycerol supplementation resulted in a linear improvement in production efficiency (P=0.005) and pellet durability index (P=0.001) and a linear reduction in hot pellet temperature (P=0.05) (Table 1). Increasing the conditioning temperature from 165°F to 185°F resulted in an improvement in pellet durability index (P<0.001) and tended to increase production efficiency (P=0.06). With an increase in production efficiency, feed is passed through the pellet mill at a higher rate because of less friction on the pellet die. This lowers electrical costs. Pellet durability improved when glycerol was added. Higher pellet durability decreases feed wastage, decreases selective feeding, and improves feed palatability. Pellet durability was nearly identical for the diet with 5% glycerol pelleted using a conditioning temperature of 165ºF compared to the diet with 0% glycerol and a conditioning temperature of 185°F. Adding glycerol could allow a feed mill to lower the conditioning temperature without sacrificing pellet quality, which would lower costs.

Pig Growth Performance Studies

Two experiments were conducted to evaluate the effect of dietary glycerol inclusion on nursery pig performance. In Experiment 1, a total of 126 pigs (body weight was 6.91±0.18 kg) were weaned at approximately 21 days of age at the Swine Educational Unit, Raleigh, N.C. Pigs were housed 3 pigs per pen using 42 pens and were given one of 6 dietary treatments. Dietary treatments were arranged in a 2×3 factorial randomised complete-block design. Factors consisted of: (1) glycerol inclusion in Phase 1 diets (0 or 5%), and (2) glycerol inclusion level in Phase 2 diets (0, 5, or 10%). Glycerol supplementation in Phase 1 diets replaced lactose on a weight-for-weight basis. Glycerol inclusion in Phase 2 diets replaced corn on a nutrient basis (thus accounting for the corn’s nutrient composition). Pigs were fed a two-phase dietary programme. The first phase diet was fed immediately following weaning for 14 days and the second phase diet was fed for three weeks.

Inclusion of 5% glycerol in Starter 1 diets had no effect (P>0.29) on body weight, ADG, ADFI or feed efficiency (gain/feed) (Table 2). Final body weight after five weeks increased (P = 0.03) with increasing inclusion of glycerol. This response was primarily observed in pigs that had consumed diets with 5% glycerol in the Starter 1 phase. Average daily gain was greater in pigs fed glycerol in Starter 2 diets during the Starter 2 phase (P=0.002) and overall (P=0.03). ADFI increased when glycerol was added to Phase 2 diets during Starter 2 phase (P=0.003) and overall (P=0.02). Feed efficiency was improved with glycerol supplementation to Phase 2 diets during the Starter 2 phase (P=0.04).

In Experiment 2, a total of 144 pigs (body weight was 6.68±0.17 kg) were weaned at approximately 21 days of age at the Swine Educational Unit, Raleigh, N.C. Pigs were housed 3 pigs per pen using 48 pens and fed one of six dietary treatments. Treatments consisted of the following:
1) Control treatment containing 20% lactose and 0% glycerol
2) Diet with 17.5% lactose and 2.5% glycerol
3) Diet with 15% lactose and 5% glycerol
4) Diet with 12.5% lactose and 7.5% glycerol
5) Diet with 10% lactose and 10% glycerol
6) Diet with 10% lactose and 0% glycerol

The control diet contained 20% total lactose, therefore, glycerol replaced up to 10% of the lactose. A second control diet was included in the design that contained 10% lactose and no added glycerol. Replacement of lactose with glycerol was conducted on a weight-for-weight basis because we expected the DE of lactose and glycerol to be similar. Diets were manufactured by creating a basal diet first that contained all ingredients, except lactose or glycerol. The basal was divided into 6 portions to which lactose (with additional salt and water) and glycerol were added to create the final dietary treatments. This ensured that diets were identical in composition except for lactose and glycerol content. Pigs were fed a two-phase dietary program. The first phase diet was fed immediately following weaning for 14 days, and the second phase diet was fed for two weeks.

Supplementation of Starter 1 diets with glycerol linearly increased (P<0.05) pig body weight when measured on weeks 2 and 3 (Table 3). Average daily gain increased linearly during the Starter 1 period (P=0.01) with increasing levels of glycerol. Feed intake increased linearly during the Starter 1 phase (P=0.04) as level of glycerol in the diet increased. Feed efficiency was not impacted by glycerol supplementation. Supplementation of 20% lactose compared to 10% lactose in the Starter 1 diet had no effect on ADG, ADFI, and gain/feed. Glycerol supplementation (10%) to diets that had 10% lactose resulted in heavier pig body weights at week 2 (P=0.01), greater ADG during the prestarter period (P<0.01), and improved feed efficiency during the Starter 1 period (P<0.01) compared to the control diet with 10% lactose and no supplemental glycerol. No differences in performance were observed during the Starter 2 period during which no glycerol was fed.

Conclusions

Supplementation of glycerol to corn-soybean-meal-based finisher diets improved flowability, production efficiency, and pellet durability. Application of glycerol before pelleting may improve pelleting efficiency, resulting in a 50% reduction in energy use during the pelleting process. Supplementation of glycerol improved starter pig performance when added at 5% and 10%. In newly weaned pigs, pig performance was linearly improved with the supplementation of glycerol up to 10%, and this effect appeared to be independent from dietary lactose levels.