Rapid Production Of Maggots As Feed Supplement And Organic Fertilizer By Two-Stae Composting Of Fresh Pig Manure

A burst of pig industry in China has brought about a huge amount of manure that is not only a reservoir of pathogens, parasites and weed seeds but also a favorable substrate for breeding the larvae (maggots) of the housefly Musca domestica. This study sought to develop a two-stage composting technology for the use of fresh pig manure in the production of maggots as feed supplement and organic fertilizer. The objective was to maximize maggot production in the first stage and accelerate the composting to maturity in the second stage by monitoring changes in temperature, moisture, pH, phytotoxicity, microbial enzyme activities, and low-molecular organic substances. Harvested maggots were also evaluated for the quality of their feed-purpose application in terms of the contents of nutrients and heavy metal ion. The results are summarized below.A triplicate experiment was conducted to determine a proper level of wheat bran-vectoring maggot inoculum (～2000neonate maggots per gram) for the composting of fresh pig manure (with72.3%moisture) at Deqing Pig Farm (Hangzhou, Zhejiang, China). Batches of200kg manure were flatly piled to～7cm thickness in cement trays (2.4x2.3x0.2m) under greenhouse conditions and inoculated with maggot inoculum at the weight ratios of0.25%,0.5%,0.75%,1.0%and1.5%respectively, forming five treatments plus control (not inoculated). After7-day composting under ambient conditions, the yields of aging maggots (near pupation) harvested by means of their photophobotaxis differed significantly among the treatments. A maximal yield of23±1.7kg aging maggots per tray (Fig. la) resulted from the inoculum treatment of0.5%(w/w) fresh manure weight, accompanied by moisture reduction to56.2%(±1.2%), which is well in agreement with the optima of50-60%for manure composting to maturity. The body weights of harvested maggots were averagely9-19.3mg per capita, decreasing with the inoculum level. Dried maggots contained54.62%proteins,1.23%methionine,4.16%lysine, and the trace contents of0.38,1.93,0.16,0.67and1.73mg/kg for the heavy metals of As, Pb, Hg, Cd and Cr, respectively. All these indices indicate that the maggot product meets commercial fishmeal requirements (GB13078-2001and GB/T19164-2003) in China. The maggot treated manure became granular in texture.In an enlarged two-stage composting experiment, the first7-day composting of1.8ton fresh pig manure in three large cement trays (7.3x2.3x0.2m) inoculated at the optimized level of0.5%maggot inoculum resulted in a harvest of193kg aging maggots. This maggot yield was equivalent to10.7%of the fresh manure weight and close to the percent yield of11.5%at the same inoculum level in the previous experiment. After maggot harvest on day7, all the manure was piled up into a peak-shaped compost (4.5m in length,2.2m in width, and0.8m in height), namely maggot-treated compost (MC), on the ground in a rainproof workshop and a natural compost (NC) was constructed as a control in the same way using1.8ton manure composted for7days in other three trays without maggot inoculum. Both MC and NC were covered with plastic film for24h to stifle possible residue maggots and then entered the second-stage composting to maturity for12weeks, during which both composts were turned upside down every3days. As a result, reaching the thermophilic phase and final maturity faster was characteristic of MC versus NC in the two-stage composting. Upon MC transit to the second stage, the composting temperature maintained around55℃for9days and the moisture decreased to-40%. Moreover, higher pH, faster detoxification and different activity patterns for some microbial enzymes were observed from the MC samples taken daily (first stage) or every3-6days (second stage). There was a strong material loss (35%water-soluble carbon and16%total nitrogen) caused by the maggot culture in the first stage.Moreover, a headspace solid-phase microextraction method (HS-SPME) was modified to assess the contents of low-molecular organic components in the samples from MC and NC by gas chromatography with mass spectrometric detection (GC/MS). Technical parameters were optimized as the extraction head of CAR/PDMS, the extraction temperature of60℃and the extraction time of30min. With the modified method,87low-molecule organic compounds were identified from the samples taken during the two-stage composting, including some substances that may resist plant diseases or promot plant growth. Based on the temperal changes of such substances, detected phytotoxicity and other physiochemical and biological parameters, the composting of fresh pig manure to maturity needed no more than25days (～22days) for MC but45days for NC.Overall, the results highlight that a higher economic value of pig manure can be achieved by rapid production of maggots as feed supplement and qualified organic fertilizer through the two-stage composting without bulking agents.