Combustion Ash Can Serve As A Mineral Supplement In Swine Diets

by 5m Editor
15 May 2006, at 12:00am

By Eric van Heugten, Swine Nutrition Specialist, North Carolina State University - Environmental concerns surrounding the disposal of swine waste have prompted investigation of combustion or thermal decomposition methods for management of these wastes. Such processes occur at temperatures in excess of 1500°F and produce only energy and a sterile ash. This ash is potentially valuable as a feed ingredient since it contains minerals that typically are supplemented in animal feeds.

Dr Eric van Heugten
Swine Nutrition Specialist
North Carolina State University


Therefore, the objectives of this work were to characterize various ash materials, to determine growth performance and digestibility in pigs fed this ash, and to assess any health impacts on animals maintained on ash supplemented diets throughout the grow-finish period. This report presents data on ash characterization, growth performance, and nutrient digestibility.

Materials and Methods: Ash characterization

Ash samples were collected from several different sources in order to determine the degree of variability among samples. Prestage Farms generously provided two incinerator ashes from swine mortalities. One was from a Farrow to Finish (FF) operation, and the other from a Grow-Finish (GF) facility. Three gasifier ashes were also evaluated. These differed by gasifier design as well as by the feedstock gasified. The Brookes Gasification Process (BGP) gasifier, a batch-fed, staged combustion unit located at NCSU, was used to process swine waste solids. Similarly, the Bud Klepper Technologies (BKT) gasifier, an entrained flow, high-pressure design also located at NCSU, produced an ash from swine waste solids. Finally, an ash from gasification of turkey litter, processed in the Energy Products of Idaho (EPI) gasifier was evaluated.

Animal Experiments

Diets: Diets were corn-soybean meal based and were formulated to contain identical concentrations of minerals and equal acid base balance. For this purpose, HCL, KOH MgSO4 were used to balance the diets (Table 1). Each treatment diet included one of four different ashes under study. These were: grower-finisher incinerator ash (GF), belt harvested swine manure ash processed in the BKT gasifier (BKT), belt harvested swine manure ash processed in the Brookes Gasification Process gasifier (BGP), and turkey litter ash (EPI) processed by Energy Products of Idaho as a part of a Smithfield Foods project. In addition, a negative control diet without supplemental minerals and a positive control diet with a commercial micro-mineral premix and supplemental Ca and P (from limestone and dicalcium phosphate) were included in the experimental design. Feed was offered on a restricted basis at a rate of 45 g x BW0.75 per meal. Feed was offered two times per day (8:00 and 16:00 hr). Chromium oxide (0.3%; calculated to provide 2053 ppm of Cr to each of the test diets) was added as an indigestible marker for the digestibility trial.

Growth experiment:

A total of 54 barrows were selected by body weight from the Swine Educational Unit. Pigs were transported to the Grinnells Laboratory and housed individually in one of four identical rooms with 14 pens in each room. Each pen had plastic-coated slats as the flooring material, one feeder in the front, and a waterer in the back. Pigs were acclimated to the pens for a period of three days before placing them on the treatment diets. During this acclimation period, animals were fed a commercial diet on ad libitum basis. At the beginning of the experimental period, pigs were weighed and distributed into nine weight blocks. Each block had six pigs corresponding to the six dietary treatments. However, during the growth experiments, only five dietary treatments were used (the negative control diet was omitted from the experimental treatments for the growth experiment; two pigs per block were assigned to the positive control diet). Thus, eighteen pigs received the positive control diet and nine pigs, each, received one of the four test diets. Pigs were weighed at the end of each week for three weeks and daily gain and feed efficiency were calculated for each period.

Balance experiment:

A balance trial was conducted for a period of 7 d (4 d feeding and 3 d collection) after the end of the growth trial using six dietary treatments (the same four test treatments as the growth trial plus both positive and negative control diets (nine pigs fed the positive control diet were switched to the negative control diet for the balance trial). Sample collection and analytical procedures: During the collection period, representative fecal samples were collected immediately after defecation in the morning when the pigs were fed. Samples were placed in plastic containers. Subsequently, samples were freeze-dried and used for chemical analyses. Dry matter, energy and mineral content of the diets were evaluated in triplicate and fecal samples were analyzed in duplicate. Dry matter of diets was calculated by drying to a constant weight in a 60ºC oven. Gross energy was determined in an adiabatic bomb calorimeter (Model C5000, IKA, Wilmington, NC). Chromium, and mineral analyses of the feed and freeze-dried fecal samples was conducted at the Department of Soil Science, North Carolina State University, using modified AOAC procedures (AOAC, 1995).

Results and Discussion: Ash characterization

Mineral composition of the various ashes differed (Table 2), which can be attributed to the nature of the starting feedstock or to the processing method used to generate the ash. Due to the char remaining in the BKT ash, the mineral content was especially low relative to the BGP ash from the same feedstock. This was clearly a processing difference. When these values were corrected to a 100% ash value, there was little difference in the composition of the two swine waste solids ashes (data not shown). Mineral composition of the “true” mineral ash can also differ depending on the initial feedstock processed. Calcium was elevated in mortality ashes due to the bones present in this material, and turkey litter resulted in a much higher concentration of copper in the ash. The observed variation in ash composition points to the importance of knowing the ash profile before beginning diet formulation because incorporation levels will have to be adjusted accordingly.

Growth performance

No differences in weight gain or feed efficiency were detected between the positive control diet and the diets containing the various ash sources (Table 3). Therefore, short-term supplementation of minerals to the diet of growing pigs by supplementing ash can result in growth performance that is similar to that of pigs supplemented with minerals from a commercial premix. Long-term studies are currently underway to verify these results under practical conditions.

Digestibility of energy and minerals

Apparent digestibility of energy was reduced (P < 0.05) in the diet supplemented with BKT ash compared to all other diets. In addition, the diet with EPI ash had greater (P < 0.05) GE digestibility than the diet containing BGP ash, however, digestibility of GE was not different from the negative control, positive control, or the diet with GF ash. Apparent diet digestibility of P was improved (P < 0.05) with the supplementation of dicalcium phosphate (positive control), or ash from GF, BKT, BGP, or EPI compared to the negative control diet. All of the P in the negative control diet originated from plant materials, which have low P bioavailability in nonruminants (NRC, 1998), resulting in low digestibility of P in the diet.

The addition of dicalcium phosphate or ash improved P digestibility, indicating that the P provided by these sources was bioavailable. Phosphorus digestibility of the ash sources did not differ (P > 0.10) from the positive control, except BKT ash, which had lower P digestibility (P = 0.05) compared to the positive control. Digestibility of P from ash sources was calculated to be 81.0%, 56.8%, 76.5%, and 119% for GF, BKT, BGP, or EPI ash compared to 77.7% for dicalcium phosphate (SEM = 11.4%). Equivalent P digestibility values were observed for GF, BKT, and BGP ashes, whereas EPI had greater P digestibility compared to dicalcium phosphate. Interestingly, the BKT ash had the lowest concentration of ash (indicating the lowest completeness of combustion) and the lowest digestibility, whereas EPI ash had the greatest level of ash and the highest digestibility.


All ash sources supported equal levels of growth performance compared to diets with dicalcium phosphate. Completeness of combustion may be related to ash digestibility, with more pure ash having the best digestibility.

Literature Cited

AOAC. 1995. Official Methods of Analysis. 17th ed. Assoc. Off. Anal. Chem., Washington, DC.
NRC. 1998. Nutrient Requirements of Swine. 10th ed. Natl. Acad. Press, Washington, DC.

Reproduced Courtesy

Source: North Carolina State University Extension Swine Husbandry - April 2006