DRIED BIOGAS SLURRY AS . A NUTRIENT SOURCE IN GROWING PIG DIETS

Two feeding experiments were conducted t o evaluate the effects of including dried biogas slurry (DBS) on the performance of growing pigs. In each experiment, 36 growing pigs were limit-fed diets containing 0,5.10, and 20% DBS substituted (wlw) for coconut oil cake and rice bran. The results suggest that DBS is potentially useful as a nutrient source in growing pig diets. The inclusion of up to 10% DBS in diets had no adverse effect on gain and feed efficiency, whereas 20% DBS inclusion depressed performance which may beattributed to its high fibre and low available energy contents. The merits of biogas slurry over the raw manure and the practical limitations t o its use are discussed. The need to develop renewablea sources of energy has generated much interest during the recent decades in biogas production from animal manures. The slurry (solid by-product) from biogas digesters is generally utilized as a fertilizer for crops as in fish ponds. Because the biogas slurry contains moderate amounts of protein,7 it is possible that the dried slurry could be used as an animal feeding stuff. Extensive reviews are available on the refeeding of various forms of animal manure, 9495 but corresponding data on the feeding value of bi,ogas slurry is almost non-existent. The experiments reported herein were carried out to evaluate dried biogas slyrry (DBS) as a source of nutrient in growing pig diets 2. Materials and Methods i 2.1 Source of DBS The DBS samples used in this study originated from a continuosly-fed Indian-type field scale biogas digester maintained at the piggery of the University of ~ e r a d e n i ~ a . ~ r e s h manure was washed daily into the digester from pigs housed in semi-open concrete-floored pens. The pigs were fed 'on a low-quality swill (8-10% crude protein, 10-16% crude fibre) and limited quantities of a mixture of rice bran and coconut oil cake. The solid effluent from the digester was removed, initially sun-dried on a concrete floor to 86438% solids content and finally dried overnight in an Unitherm oven at 196 V_ Ravindr~n and A.S.B. Rajaguru 6 0 ' ~ . The resultant hard crumbs were ground into a semi-powdery form before incorporation into the diets. Prior to the commencement of each feeding trial, the DBS was bulked and representative samples were taken for the analysis of proximate components.' Representative samples of rice bran and coconut oil cake were also subjected to similar analyses.' 2.2 Feeding experiments . The study consisted of two separate feeding experiments. In each experiment, thirty-six crossbred barrows were divided into three groups on the basis of weight. The average weight of pigs in Experiments 1 and 2 were 15.5 and 18.3 kg, respectively. Pigs within each weight group were assigned t o one of twelve pens, so that there were three pigs per pen. The four treatments were then ,randomly allocated to three pens each. he composition of the experimental diets is shown in Table 1. They were designed to contain 0,5,10 and 20% DBS, the DBS replacing coconut oil cake and rice bran in the control diet. Each five parts of DBS replaced (w/w) three parts of coconut oil cake and two parts of rice bran. The diets were iso-nitrogenous but not iso-caloric since the digestible energy value of DBS was not available. Pigs were housed in concrete-floored pens. h he diets were limit fed twice daily in equal amounts. The amount of feed to be given was determined by the animal group consuming the least during a 30-minute feeding period at each w'eighing. Feeding levels' were held constant until next weighing. Body weights were recorded at weekly intervals. The experiments were of eight weeks duration. Diets were mixed with water immediately prior to feeding to improve consumption and t o reduce wastage. Water was freely . available at all times. Average daily gainand feed to gain ratio were computed on a pen basis. The data were, subjected to analysis of variance' and the treatment means were compared using the Tukey's procedure.8 Dried Biogas Slurry as a Nutrient Source in Growing Pig Diets 197 Table 1. Percentage composition of pig grower diets used in the feeding trials Level of DBS (96) Control 5 10 20 Maize 2 5 25 25 2 5 Coconut oil cake 40 3 7 34 28 Rice bran DBS Fish meal (50% CP) Skim milk powder Bone meal Vitamin-trace mineral premix3 Salt Analysis (dry matter basis) Crude protein (%) b Digestible energy ( M ~ a l I k g ) ~ asupplied (per kilogram of premix): 6,600,000 IU vitamin A, 528,000 IU vitamin D3, 880 IU Vitamin E, 1.98 g riboflavin, 4.4 g calcium panthothenate, 8.8 g niacinarnide, 22.0 g choline chloride , 8.8 g vitamin B12, 1.32 g vitamin K, 5.28 g manganese sulfate, 1.98g copper carbonate, 0.99 g calcium iodate, 0.55 g cobalt sulfate and 0.22 g zinc oxide. b~e te rmined value. C~alculated value. 3. Results and Discussion The compositional data of DBS confirm its moderate nutritional potential for, non-ruminants (Table 2). The crude protein content of DBS samples used in Experiments 1 and 2 were 16.6 and 14.7%, respectively. The crude fibre contents of the samples were however, high averaging 18.5%. The high ash content was related to sand contamination, as indicated by the high levels of acid-insoluble ash. The blocking of biogas digesters by sand is a common practical problem and contamination with sand may be expected to be more severe under field situations. But preliminary observations have shown that considerable reduction in contamination could be achieved by 'scoop shovelling' instead of directly washing the manure into the biogas digester. I? Ravindrizn and A.S.B. Rajaguru . , Table 2. Average chemical composition of dried biogas slurry, coconut oil cake and rice bran (dry matter basis) Constituent Dried bingas slurry Coconut Rice oil cake bran Trial 1 Trial 2 Crude protein (Nx6.25) 16.6 14.7 21.0 10.5 Ether extract 3.1 3.3 8.6 11.2 Crude fibre 18.5 18.4 12.1 16.8 Ash 11.4 13.5 6.9 12.5 Nitrogen-free extract 50.4 50.1 51.4 49.0 Acid-insoluble ash 6.1 8.2 0.6 5.1 The potential of biogas slurry as a non-ruminant feedstuff has been questioned by Summers et al. on the grounds that a well digested slurry will have minimal 'potentially' digestible carbohydrates left. The DBS samples used in the present study contained an average of 50.2% nitrogen-free , extracts, which is generally assumed to be indicative of the digestible carbohydrate fraction. The variable nature of biogas slurry needs special mention at this point. As could be expected, the composition of slurry would vary depending on the source of manure (animal species) which, in turn, would differ widely according t o the type of diet fed t o the donor animals. For example, preliminary evaluation of biogas slurry from cattle manure have shown it to be a poor quality feed for non-ruminants containing crude protein contents of less than 9% and crude fibre contents as high as 3 1 % ~ In Experiment 1, the performance of animals fed diets containing 5 and 10% DBS were similar to those on the control diet (Table 3). But the performance tended to be non-significantly(P > 0.05) higher at 5% DBS. The significance of this slight growth response is unclear. Although pigs fed diets with 20% DBS gained 11% slower and required 12% more feed per unit of gain than did controls, the differences were not statistically significant. In Experiment . 2, the performance of pigs were unaffected by the dietary inclusion of up to 10% DBS (Table 3). Gain and feed efficiency of pigs. fed diets with 20% DBS, however, were significantly poorer (P.<'0.05) when compared to other treatments. Dried Biogas slurry as a Nutrient Source in Growing Pig Diets 199 Table 3. Performance of growing pigs fed diets containing varying levels of DBS Avg. initial Avg. final Avg. daily Peed weight(kg) weight(kg) gain(kg) P a gain Experiment 1 C ~ n t r o I 5% DBS 10% DBS 20% DBS SEM* Experiment 2 Control , 5% DBS 10% DBS 20% DBS SEM* * Standard error of mean. a $ b ~ i t h i n each trial, means in the same column with ,different superscripts are significantly (P<0.05) different. The weight gains of animals in Experiment 1 were markedly lower than those inExperiment 2. Reasons for this difference are unclear, but may have been due to the extremely hot weather experienced. during the experimental period. The overall results suggest that DBS could be used as a nutrient source in growing pig diets up to 10% level without any adverse effect on growth performance. The pigs appear to have the ability to extract the benefits of microbial protein and vitamin B-complex up t o this level of inclusion. A very high content of vitamin.B (3000 mcglkg) in DBS has been reported.2 The growth depression observeh20n 20% DBS diets is probably related to the low available energy content, resulting from its high crude fibre content. It is known that animal manures have a low digestible energy c ~ n t e n t . ~ ~ ~ ~ ~ The available energy could be expected. to further lowered during the production of b i ~ g a s . ~ Thus the low energy content of DBS would be a major factar limiting its high level usage in non-ruminant diets. From the energy point of view, ruminants are the most desirable species for feeding .biogas slurry because of the ability of the rumen microbes to utilize fibre components. The digested slurry has several advantages over the raw manure. The slurry has only a faint odour and is greatly improved in this respect over the V. Ravindran and A.S.B. Rajaguru manure. The slurry. is also stable on long-term storage. A potential health hazard due to the survival of pathogens in DBS does exist, but studies have demonstrated that anaerobic digestion considerably reduces the nuniber of pathogens present in animal rnan~res .~ No health problems were observed in pigs in the present study. , While DBS is potentially useful as a feedstuff for growing pigs, utilization of this material requires some artificial dehydration. Even under sunny and dry weather-conditions, sun-drying was fo

The need to develop renewablea sources of energy has generated much interest during the recent decades in biogas production from animal manures.The slurry (solid by-product) from biogas digesters is generally utilized as a fertilizer for crops as in fish ponds.Because the biogas slurry contains moderate amounts of protein,7 it is possible that the dried slurry could be used as an animal feeding stuff.
Extensive reviews are available on the refeeding of various forms of animal manure, 9 4 9 5 but corresponding data on the feeding value of bi,ogas slurry is almost non-existent.The experiments reported herein were carried out to evaluate dried biogas slyrry (DBS) as a source of nutrient in growing pig diets 2. Materials and Methods i

Source of DBS
The DBS samples used in this study originated from a continuosly-fed Indian-type field scale biogas digester maintained at the piggery of the University of ~e r a d e n i ~a .
~r e s h manure was washed daily into the digester from pigs housed in semi-open concrete-floored pens.The pigs were fed 'on a low-quality swill (8-10% crude protein, 10-16% crude fibre) and limited quantities of a mixture of rice bran and coconut o i l cake.The solid effluent from the digester was removed, initially sun-dried on a concrete floor to 86438% solids content and finally dried overnight in an Unitherm oven at 6 0 ' ~.The resultant hard crumbs were ground into a semi-powdery form before incorporation into the diets.
Prior to the commencement of each feeding trial, the DBS was bulked and representative samples were taken for the analysis of proximate components.' Representative samples of rice bran and coconut oil cake were also subjected to similar analyses.'

Feeding experiments .
The study consisted of two separate feeding experiments.In each experiment, thirty-six crossbred barrows were divided into three groups on the basis of weight.The average weight of pigs in Experiments 1 and 2 were 15.5 and 18.3 kg, respectively.Pigs within each weight group were assigned t o one of twelve pens, so that there were three pigs per pen.The four treatments were then ,randomly allocated to three pens each. he composition of the experimental diets is shown in Table 1.They were designed t o contain 0,5,10 and 20% DBS, the DBS replacing coconut oil cake and rice bran in the control diet.Each five parts of DBS replaced (w/w) three parts of coconut oil cake and two parts of rice bran.The diets were iso-nitrogenous but not iso-caloric since the digestible energy value of DBS was not available.
Pigs were housed in concrete-floored pens.h he diets were limit fed twice daily in equal amounts.The amount of feed t o be given was determined by the animal group consuming the least during a 30-minute feeding period at each w'eighing.Feeding levels' were held constant until next weighing.Body weights were recorded at weekly intervals.The experiments were of eight weeks duration.Diets were mixed with water immediately prior to feeding t o improve consumption and t o reduce wastage.Water was freely .available at all times.
Average daily gainand feed to gain ratio were computed on a pen basis.The data were, subjected to analysis of variance' and the treatment means were compared using the Tukey's procedure.8b ~e t e r m i n e d value.

Results and Discussion
The compositional data of DBS confirm its moderate nutritional potential for, non-ruminants (Table 2).The crude protein content of DBS samples used in Experiments 1 and 2 were 16.6 and 14.7%, respectively.The crude fibre contents of the samples were however, high averaging 18.5%.The high ash content was related to sand contamination, as indicated by the high levels of acid-insoluble ash.The blocking of biogas digesters by sand is a common practical problem and contamination with sand may be expected to be more severe under field situations.But preliminary observations have shown that considerable reduction in contamination could be achieved by 'scoop shovelling' instead of directly washing the manure into the biogas digester.The potential of biogas slurry as a non-ruminant feedstuff has been questioned by Summers et al. on the grounds that a well digested slurry will have minimal 'potentially' digestible carbohydrates left.The DBS samples used in the present study contained an average of 50.2% nitrogen-free , extracts, which is generally assumed t o be indicative of the digestible carbohydrate fraction.
The variable nature of biogas slurry needs special mention at this point.As could be expected, the composition of slurry would vary depending on the source of manure (animal species) which, in turn, would differ widely according t o the type of diet fed t o the donor animals.For example, preliminary evaluation of biogas slurry from cattle manure have shown it to be a poor quality feed for non-ruminants containing crude protein contents of less than 9% and crude fibre contents as high as 3 1 % ~ In Experiment 1, the performance of animals fed diets containing 5 and 10% DBS were similar t o those on the control diet (Table 3).But the performance tended t o be non-significantly(P > 0.05) higher at 5% DBS.
The significance of this slight growth response is unclear.Although pigs fed diets with 20% DBS gained 11% slower and required 12% more feed per unit of gain than did controls, the differences were not statistically significant.
In Experiment . 2,the performance of pigs were unaffected by the dietary inclusion of up to 10% DBS (Table 3).Gain and feed efficiency of pigs.fed diets with 20% DBS, however, were significantly poorer (P.<'0.05) when compared t o other treatments.The weight gains of animals in Experiment 1 were markedly lower than those inExperiment 2. Reasons for this difference are unclear, but may have been due to the extremely hot weather experienced.during the experimental period.
The overall results suggest that DBS could be used as a nutrient source in growing pig diets up to 10% level without any adverse effect on growth performance.The pigs appear to have the ability t o extract the benefits of microbial protein and vitamin B-complex up t o this level of inclusion.A very high content of vitamin.B (3000 mcglkg) in DBS has been reported.2 The growth depression observeh20n 20% DBS diets is probably related t o the low available energy content, resulting from its high crude fibre content.It is known that animal manures have a low digestible energy c ~n t e n t .~~~~~ The available energy could be expected.to further lowered during the production of b i ~g a s .~ Thus the low energy content of DBS would be a major factar limiting its high level usage in non-ruminant diets.From the energy point of view, ruminants are the most desirable species for feeding .biogasslurry because of the ability of the rumen microbes t o utilize fibre components.
The digested slurry has several advantages over the raw manure.The slurry has only a faint odour and is greatly improved in this respect over the manure.The slurry. is also stable on long-term storage.A potential health hazard due to the survival of pathogens in DBS does exist, but studies have demonstrated that anaerobic digestion considerably reduces the nuniber of pathogens present in animal r n a n ~r e s .~ No health problems were observed in pigs in the present study.

,
While DBS is potentially useful as a feedstuff for growing pigs, utilization of this material requires some artificial dehydration.Even under sunny and dry weather-conditions, sun-drying was found to reduce the moisture content .
of the slurry'to only 12-14%.Cheaper ways of drying the slurry need be evolved before its practical use can be encouraged.An additional factor of concern is the ~ossible wide variability in the chemical composition of the slurry.Considerable variations have been reported with regard to the crude protein (14.9-18.2%),crude fibre (17.4-26.4%)and ash (11.2-&Q.O%I) contents of DBS samples?This would prevent any general recommehdation regarding the potential use of slurry in animal diets.
The study was partly funded by the International Foundation of Science, Sweden.Financial assistance was also provided through an F A 0 grant on 'non-traditional-feedstuff evaluation'. .

Table 2 .
Average chemical composition of dried biogas slurry, coconut oil cake and rice bran (dry matter basis)

Table 3 .
Performance of growing pigs fed diets containing varying levels of DBS * Standard error of mean.a $ b ~i t h i n each trial, means in the same column with ,different superscripts are significantly (P<0.05)different.