Fly larvae, earthworms and duckweed as feeds for frogs in an integrated farming system

Phounvisouk Latsamy and T R Preston^*

Living Aquatic Resources Research Center (LARReC), NongThang Village, Vientiane, Laos
Meetouna@yahoo.com
^* TOSOLY, UTA-Colombia, AA#48, Socorro, Santander, Colombia

Abstract

Frogs, purchased as 20 day-old fingerlings, were raised in ponds and, after a period of adaptation, were submitted to 4 treatments arranged as a 2*2 factorial with 3 replications using a completely randomized design. The factors were: addition or not of duckweed to the feed; and earthworms or fly larvae as the main diet. At the start of the trial the frogs weighed on average 19 g and were fed the experimental diets for 90 days.

Growth rates and feed conversion (for DM and crude protein) were better, and mortality was lower, when frogs were fed earthworms rather than larvae and when they had access to fresh duckweed mixed with the larvae/earthworms. The net increase in live weight (252 g in 90 days) on the best diet (earthworms plus duckweed) was better than in one report concerning frogs fed an artificial diet (200 g in 120 days).

Comparisons with another aquatic species (catfish) indicated broadly similar results for growth rate and feed conversion.

Key words: Aquaculture, feed conversion, growth

Introduction

Frog culture is an important economic activity in Thailand with high demand for the product in foreign markets such as Malaysia, Singapore, Hong Kong, Japan, Germany and France (Akasay, 1994). There are also recent developments in Brazil, where frog meat is marketed in the form of entire carcasses or of frozen thighs (frog legs). (Moura and Ramos No Date)

In Laos there is an increasing demand for frogs for consumption in family households and local markets. There are restaurants in Vientiane that buy frogs on a regular basis for their customers (Bounsong 2001).

From hatching of the eggs to market size, it takes 3 to 4 months which is similar to other commercial aquatic species such as catfish and tilapia. Frogs can be raised in most locations, as they require a small area and lower quantity of water than other aquatic species. However, in Laos the cultivation of frogs is still not economically attractive because of high operational costs, chiefly of purchased feed. This is usually imported from Thailand and contains expensive ingredients (Table 1) (LARReC 2001). The proximate analysis of the feed from one feed manufacturer is shown (Table 2).

The common lowland frog (Rana rugulosa) is the most popular cultured frog in Laos and Thailand. Because the frog can take in oxygen by the lungs and through the skin, it can be housed at extremely high density (30 frogs/m²). The frogs are mainly carnivorous and prefer to eat creatures that are alive and moving; however, there are reports (Anon¹ No date) that they can be trained to consume "inert" food such as pelleted feeds.

There appears to be potential for the incorporation of frog culture in systems of recycling with duckweed being part of the feed for the frogs (Figure 1).

According to Akasay (1994), potential sources of "natural" feeds that could be cultivated as feed for frogs are fly larvae, earthworms, termites, duckweeds and snails. Earthworms are appropriate elements in systems of recycling live stock manure (Bay, 2002), as they are more suitable than biodigesters when the manure is derived from rabbits and goats (Preston and Rodríguez, No date). Earthworms are high in protein and would thus appear to be suitable components in a feeding system for frogs.

Fly larvae have been recommended as feed for frogs by Sheppard (No date), as they are rich in both fat and protein (Khan et al 1999).

The objectives of the study were to compare the growth of frogs fed with fly larvae or earth worms as part of an integrated farming system. The hypothesis was that frogs can grow at acceptable rates when their diet is composed primarily of earthworms or fly larvae.

Materials and methods

Location

The experiment was carried out in the Living Aquatic Resource Research Centre in Naong Thang village, Vientiane province from April to October 2006.

Experimental treatments

Frogs were raised in ponds and submitted to 4 treatments arranged as a 2*2 factorial with 3 replications using a completely randomized design. The factors were:

Addition of duckweed to the feed

• D: Duckweed

• ND: No duckweed

Source of protein supplement

• EW: Earthworms

• FL: Fly larvae

Design of the ponds

The ponds (n = 12) for the frogs were 1*1m in area and 0.5m deep. They were lined with polyethylene sheet which extended some 0.5m from the edges (Photo 1). The plastic sheet was secured by sticks at the four corners and each edge of the sheet was wrapped around a piece of bamboo in order to tighten the sheet. Four bamboo poles each 1.2 m high were erected at the four corners of each pond and a plastic net was placed around the poles to make a fence for the pond. The lower edge of the net was then secured to the surface with gravel. The total area including the pond was 2*2m. A sheet of dark plastic mesh was attached to the tops of the bamboo poles (Photo 2). All the ponds were fertilized with biodigester effluent at the rate of 250 ml/m²/week. When duckweed was seen to grow naturally in the ponds it was removed.

Experimental feeds

Duckweed

The duckweed was cultivated in separate ponds and fertilized with biodigester effluent following the recommendation of Rodriguez and Preston (No date).

Earthworms

The earthworms were collected from soil in areas adjacent to the channels used to transport the pig manure from the pens. Only the large worms were collected, while the small ones were left to grow in the soil. Laboratory analysis at the National University of Laos identified the worms as belonging to two species: Microchaetidae and Monilgastridae. The average weights and lengths were 0.6 to 1.2 g and 6 to 10 cm for Microchaetidae, and 0.8 to 1.6 g and 15 to 30 cm for Moniligastridae.

Larvae

The larvae were produced using as substrate the combination of pig manure and fermented fish waste, which gave the highest yield of larvae in Experiment 1. Fifteen enclosures were used in rotation so that each day the larvae from three enclosures were harvested, following a 6 day cycle of production as practiced in Experiment 1.

Origin, management and feeding of the frogs

The frogs (Rana rugulosa) were obtained as 21 day-old fingerlings (n = 600) from a commercial farm in Champasak province. They were a crossbreed between local females and "improved" males the parent stock of which had been imported originally from Europe. In the commercial farm they had been fed on commercial concentrate feed. On arrival at the Aquatic Centre they were kept in a nursery pond for one month in order to let them grow to an adequate size. During this time they were fed a mixture of commercial feed and fresh larvae, in proportions in succeeding weeks of 70:30, 50:50, and 30:70 gradually changing to 100% larvae at the end of the month. For the experiment, 480 of the strongest frogs (360 females and 120 males) were selected and allocated to the 12 experimental ponds. At this time they had an average weight of 20 g and an average length of 8 cm. After one week, the smaller females were removed in order to maintain 30 frogs per pond; among which 20 were females and 10 were males.

The frogs were fed twice per day at 06.00 h and 17.00 h. For the "D" treatment, fresh duckweed was mixed with either fresh larvae or fresh earthworms in proportions of 30% of duckweed and 70% earthworms (or larvae) (fresh basis). The quantities were controlled according to the appetite of the frogs with the objective of avoiding feed residues. Prior to mixing the larvae with the duckweed the larvae were suspended in water for 30 minutes to reduce their motility, as it was observed that otherwise they migrated from the duckweed into the water, where they were consumed by the frogs, which then did not consume the duckweed. On the "ND" treatment, the larvae or earthworms were offered as separate feeds. The experimental feeds were given over a period of 90 days. The water in the ponds was replaced daily at between 05.00 and 06.00h.

Measurements

The amounts of feeds offered were recorded daily. At the start of the experiment, two frogs (one male and female) were selected in each treatment / replicate and identified with a colored string placed around the leg of the frog. These "tagged" frogs were then weighed every 2 weeks during the 90 days of the experiment. Samples of the feeds were analysed once at the beginning of the experiment for dry matter (DM) and crude protein in the case of the duckweed and for DM, crude protein and fat for the earthworms and the larvae. Samples of water from each pond were taken weekly immediately after changing the water, for measurements of temperature, pH and dissolved oxygen. All the chemical analyses were done according to the procedures in AOAC (1990).

Statistical analysis

The growth rates of the frogs were calculated from the linear regression of body weight (Y) on days (X) from the beginning of the experiment. Results for growth rate and feed conversion were analysed by the General Linear Model in the ANOVA option of the software in Minitab 2002 (version 3.1). Sources of variation were: protein source, duckweed, interaction protein source*duckweed and error.

Results

Water quality

The range in measurements of water quality during the experiment (dissolved oxygen 6.5 to 8.2 ppm, pH 6.5 to 8.2 and water temperature 20.0 to 26.0 ºC) indicated that these parameters were within the range recommended for raising frogs (Uodone 2004).

Chemical composition of dietary ingredients

The larvae were much richer in fat but with slightly less protein than for the earthworms (Table 3).

Similar data for fly larvae were reported by Khan et al (1999). (42% crude protein and 35% ether extract, DM basis). The crude protein content of the earthworms was similar to that reported by Nguyen Duy Quynh Tram et al (2005) (59% in DM) but the fat content was lower than the 7.9% in DM that was reported by these authors. Bay (2002) reported a crude protein content of 60% and fat of 7.5% in DM. The crude protein content of the duckweed was close to the maximum level of 40% in DM, reported in the review by Leng (1999). On a DM basis the contribution of the duckweed to the diet was less than 10% (Table 4).

Growth performance

Intakes of DM and crude protein tended to be higher when the frogs received duckweed (Tables 5 and 6) (P=0.06 and 0.12), and when they were fed earthworms rather than larvae (P=0.097 and 0.048).

However, growth rates and feed conversion (for DM and crude protein) were better when the frogs were fed earthworms rather than larvae and when they had access to fresh duckweed mixed with the larvae/earthworms (Tables 5 and 6 and Figures 2 and 3).

The net increase in live weight (252 g in 90 days) on the best diet (earthworms plus duckweed) was greater than for frogs fed an artificial diet containing fish meal, dextrin, starch, rice bran, vegetable oil, tuna oil, vitamins, minerals, BHT and Choline chloride (200 g in 120 days) (Bounsong 2001). The growth rate of frogs raised with pelleted feed and house fly larvae in Mexico (Rodriquez-Serna et al 1996) were reported to be 0.63 g/day which is also considerably less than recorded for the house fly larvae diets in our experiment (0.89 g/day for larvae alone and 1.14 g/day for larvae plus duckweed).

Mortality

Mortality was lower when the frogs ate earthworms rather than larvae and there was a strong indication (P=0.059) that it was less when duckweed was included in the feed (Table 7; Figure 4). The fact that mortality was higher on the larval diet is in agreement with the poorer growth rates and could be interpreted as being due to the fly larvae being nutritionally inferior to the earthworms for raising frogs.

Discussion

In the present study, the growth rates of the frogs on the diets with earthworms and duckweed (2.84 g/day) were much higher than was reported for Catfish (0.69 g/day) given diets containing rice bran (60%), trash fish (10%) and earthworms (30%) (Nguyen Duy Quynh Tram et al., 2005). The feed conversion rates for the frogs (1.46 and 0.79 for DM and CP) were better than was reported for DM conversion for the catfish (1.58) and poorer for crude protein conversion (0.46 for catfish). Support for the high nutritive value of the earthworms for frogs is the finding in the study of Nguyen Duy Quynh Tram et al (2005) that the growth rates of the Catfish increased from 0.39 to 0.69 g/day when earthworms replaced 75% of the trash fish in the diet.

It is difficult to explain the much higher growth rates on the diets with earthworms, and the lower mortality, compared with the diets based on fly larvae. The earthworms contained slightly more protein but very much less fat than the larvae, and for both feeds, the protein was considerably above the recommended levels for frog growth. According to Somsueb and Boonyaratpalin (2001), the optimum protein content of diets for raising frogs intensively is 37% in DM. This is much lower than the protein level provided by the diets containing either the larvae or the earthworms in the present experiment (49 and 54%, respectively). It was observed that the frogs ate the earthworms more quickly than they ate the larvae; however, the overall intakes of DM and crude protein were only slightly higher for the earthworm diets. The other element affecting the nutritive value of the larvae may have been the high fat content (31%). Frogs contain very little fat in the carcass, most being stored as a "fat body" attached to the gonads (Zancanaro et al 1999). It is not known if frogs can digest a diet with 30% fat. The fact that commercial feeds for frogs contain only 4% fat suggests that fat may not be well utilized.

There appear to be major differences in the content of the first limiting essential amino acids between earthworms and fly larvae (Table 8).

Further research is required in order to corroborate the superior feeding value of earthworms compared with fly larvae for raising of frogs.

Conclusions and recommendations

• Growth rates and feed conversion (for DM and crude protein) were better, and mortality was lower, when frogs were fed earthworms rather than larvae and when they had access to fresh duckweed mixed with the larvae/earthworms.

• The net increase in live weight (252 g in 90 days) on the best diet (earthworms plus duckweed) was better than in one report concerning frogs fed an artificial diet (200 g in 120 days).

• Comparisons with another aquatic species (catfish) indicated broadly similar results for growth rate and feed conversion.

Acknowledgements

The authors wish to acknowledge the Swedish International Development Authority (Sida) for financial support of this study. I also would like to thank the Living Aquatic Recourse Research Center, pig station of the National Institute of Agriculture and Forestry, for allowing me to use their facilities, and in particular Miss. Nang, Mr. Touy and Mr. Kamphon for help in the experiment.

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