How is pig manure used for earthworm farming after composting?

In recent years, with the rapid development of China's livestock and poultry farming industry, livestock and poultry manure waste has also increased. The manure waste from some livestock and poultry farms is discharged without treatment, which not only seriously pollutes the surrounding environment but also can lead to livestock and poultry infectious diseases. Using earthworms to treat livestock and poultry manure waste not only protects the environment but also yields certain economic benefits. At present, cow manure, sheep manure, etc. are mostly used for earthworm farming at home and abroad, and reports of using pig manure to farm earthworms are rare. To study how pig manure can serve as feed for earthworm farming, the experiment carried out research in the following three aspects: 1) studying the effects of different doses of Bacillus subtilis on the composting and maturation of pig manure and straw; 2) studying the effects of adding Bacillus subtilis on the composting and maturation of pig manure and straw with different carbon-to-nitrogen ratios; 3) studying the effects of compost substrate with different carbon-to-nitrogen ratios on earthworm farming.

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Materials

Pig manure was collected from pig pens of local free-range pig farmers in Xinxiang City, Henan Province; wheat straw was collected from the Qiaoxie Experimental Field of Henan Institute of Science and Technology; Bacillus subtilis (ACCC19743) was purchased from the China Agricultural Microbial Culture Collection and Management Center; earthworms were purchased from Henan Shijia Trading Company.

The pig manure was naturally air-dried to a moisture content of about 20%, ground, and passed through a 2 mm aperture sieve for later use. The wheat straw was naturally air-dried to a moisture content of about 6.3% and crushed to a length of 2.0-2.5 cm.

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Methods2.1 Effects of different doses of Bacillus subtilis on compost maturation

According to different inoculation amounts of Bacillus subtilis, the experiment was divided into 5 groups. The amounts of bacterial inoculum added to Group 1, Group 2, Group 3, and Group 4 were 1.0%, 1.5%, 2.0%, and 2.5% (weight-to-volume ratio), respectively; the control group was naturally composted without adding bacteria. The carbon-to-nitrogen ratio of each group was 25, the composting amount was 200 kg, the initial moisture content was 60%, the ambient temperature was controlled at (28±1) °C, and the humidity was about 65%. Each group had 3 replicates; when the temperature exceeded 50 °C, the pile was manually turned once every 3 d. Using the five-point sampling method, 3 mixed samples were collected from each group each time for the determination of pH value, moisture content, total nitrogen, and total organic carbon.

2.2 Effects of Bacillus subtilis on the composting and maturation of pig manure and straw with different carbon-to-nitrogen ratios

According to different carbon-to-nitrogen ratios of pig manure and straw, the experiment was divided into 5 groups: Group A, Group B, Group C, Group D, and Group E. The initial carbon-to-nitrogen ratios of the compost substrate were 20, 25, 30, 35, and 40, respectively. Each group was inoculated according to the optimal inoculation amount of Bacillus subtilis.

2.3 Effects of compost substrate with different carbon-to-nitrogen ratios on earthworm farming

The comparison of the effects of compost substrate on earthworm farming included a comparison of the effects of compost substrate on earthworm growth and adaptability. The control group in 2.1 (natural composting) and the compost substrate of each group in 2.2 were air-dried for about 1 week, and after the ammonia odor dissipated, they were used as earthworm feed and placed in indoor flower pots (upper opening diameter 17.5 cm, lower opening diameter 13.0 cm, height 9.5 cm). Earthworm soil was added at 1/4 of the flower pot volume (to reduce earthworm stress), and the height of the compost substrate was no higher than 3/4 of the flower pot. The number of earthworms added was 100 per pot, the temperature was (20±1) °C, and the humidity was 65%-70%. Each group was repeated 3 times, and compost substrate was added about once every 10 d at a thickness of 3 cm. The culture ended after 7 weeks.

2.4 Measurement items2.4.1 Physicochemical indicators during composting

Temperature: during composting, the temperature of each group was measured around 09:00 and 16:00 every day; pH value: a mixed sample of 5 g dry weight was taken, double-distilled water was added, the solid-to-liquid ratio was 1:10, and the mixture was shaken on a shaker at 180 r/min for 2 h, left to stand for 30 min, and the supernatant was taken, and the pH value was measured with a pH meter; moisture content: 100 g of mixed sample was placed in a ceramic iron tray and placed in a 95 °C oven until the change in total amount was less than 0.01 g, and the moisture content was calculated according to the weight loss of the mixed sample before and after drying; carbon-to-nitrogen ratio: the potassium dichromate-concentrated sulfuric acid method was used to determine total organic carbon content, the Kjeldahl nitrogen determination method was used to determine total nitrogen content, and then the carbon-to-nitrogen ratio was calculated.

2.4.2 Earthworm farming effects

Average individual earthworm weight = total weight / survival number; earthworm survival number = total number - dead number and escaped number; earthworm adaptability was judged according to the optimal activity range of earthworms in the substrate, the looseness of the substrate, and the size of the surface area of earthworm castings.

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Results and analysis3.1 Effects of different doses of Bacillus subtilis on compost maturation3.1.1 Temperature changes

After composting began, the temperature of Groups 1, 2, 3, and 4 rose rapidly, entering the high-temperature fermentation stage (≥50 °C) from 0-4 d, and the composting temperature ≥50 °C lasted for 14-16 d; while the temperature of the control group rose slowly, reaching 50 °C on the 8th day, with the composting temperature ≥50 °C lasting for 5 d, and the highest temperature reaching 52 °C. At the end of the experiment, the temperatures of Groups 1, 2, 3, and 4 were close to the ambient temperature, while the temperature of the control group was slightly higher than the ambient temperature.

3.1.2 Moisture content changes

During composting, aerobic composting is both a process of oxidizing organic matter and a process of generating water, while also involving water evaporation and formation. The moisture content of each group showed an overall downward trend. At the end of composting, the moisture content of Groups 1, 2, 3, and 4 decreased to 33%-35%, and the decreases were all greater than that of the control group (moisture content 41%). This indicates that adding Bacillus subtilis can accelerate the loss of compost moisture.

3.1.3 pH value changes

During composting, the pH value of each group changed with composting time and temperature. At the initial stage of composting, the pH value of each group was around 7.9. During composting, the pH value generally first decreased, then increased, and then slowly decreased again. The final pH value of each group was 8.0-8.5.

3.1.4 Carbon-to-nitrogen ratio changes

The compost substrate of each group was prepared at an initial carbon-to-nitrogen ratio of 25. After 28 d of composting, the carbon-to-nitrogen ratios of Groups 1, 2, 3, and 4 all dropped below 20 and remained at a relatively stable stage; the carbon-to-nitrogen ratio of the control group was 21.

3.2 Effects of Bacillus subtilis on the composting and maturation of pig manure with different carbon-to-nitrogen ratios3.2.1 Temperature changes

The temperature of each group reached above 50 °C from 0-4 d, among which Group A ≥50 °C lasted for 18 d, Groups B, C, and D ≥50 °C lasted for 15 d, and Group E ≥50 °C lasted for 11 d. The cooling rate was Group C > Group B > Group D > Group A > Group E. After 32 d of composting, except that Group E dropped to 33.1 °C, all other groups dropped to ambient temperature (about 30 °C).

3.2.2 Moisture content changes

The moisture content of Groups B and C decreased relatively quickly because the carbon-to-nitrogen ratios of these two groups were suitable for microbial metabolism and growth, producing more heat; the moisture content of Groups A and E decreased slowly because Group A had a small amount of straw, producing anaerobic conditions and low temperature; while Group E had a high carbon-to-nitrogen ratio, which was unfavorable for microbial metabolism and growth. At the end of composting, Group E had the lowest moisture content (30.5%).

3.2.3 pH value changes

The pH value changes of each group generally showed a trend of first decreasing, then increasing, and then slowly decreasing. After 28 d of composting, the pH values were basically stable. The pH values of Group A, Group B, Group C, Group D, and Group E were 8.30, 8.10, 8.05, 8.10, and 7.80, respectively.

3.2.4 Carbon-to-nitrogen ratio changes

During composting, the carbon-to-nitrogen ratio generally tended to decrease, and all showed a rapid decrease in the initial stage. After 32 d of composting, the carbon-to-nitrogen ratios of Group A, Group B, Group C, Group D, and Group E decreased to 12.8, 15.6, 19.4, 21.5, and 29.6, respectively. The decrease in the carbon-to-nitrogen ratio of each group was because the total organic carbon content showed a downward trend, while the total nitrogen content relatively increased, causing the carbon-to-nitrogen ratio of each group's compost pile to gradually decrease.

3.3 Effects of compost substrate on earthworm farming3.3.1 Comparison of earthworm growth

Compared with Group A (436.23 mg), the final weights of earthworms farmed in Groups B, C, and D (481.70, 510.67, 474.10 mg) were higher; Group E had more initial escapes and deaths, and growth indicators could not be effectively measured, so no statistics were made.

3.3.2 Comparison of earthworm adaptability

Adaptability mainly refers to the optimal activity range of earthworms in the substrate, the looseness of the substrate, and the size of the surface area of earthworm castings. The number of escapes and deaths can indirectly reflect the earthworm survival rate. A wide activity range, large feed intake, large excretion amount, and high survival rate indicate strong adaptability of earthworms; conversely, adaptability is poor. The adaptability of earthworms in each group from strong to weak was Group C > Group B > Group D > Group A, among which the substrate of Group E was not suitable for farming earthworms.

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Conclusion

1) During composting, adding Bacillus subtilis can shorten the time for pig manure and straw to enter the high-temperature stage and accelerate the maturation progress, among which inoculation with 1.0% is the most cost-saving.

2) Pig manure and straw with different carbon-to-nitrogen ratios affect compost maturity. When the carbon-to-nitrogen ratio of pig manure and straw is 25-30, the composting effect is better.

3) When the carbon-to-nitrogen ratio of pig manure and straw is 30, the effect of farming earthworms after compost maturation is the best.

Authors | Song Gaojie, Li Han, Liu Xingyou

School of Animal Science and Technology, Henan Institute of Science and Technology

School of Resources and Environment, Henan Institute of Science and Technology

Xinxiang University

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