Effect of dietary fiber sources on feed intake, nutrient digestibility, growth rate, and economic returns of growing crossbred rabbits fed sweet potato vine (Ipomoea batatas) and para grass (Brachiaria mutica) 

Nguyen Thi Kim Dong and Nguyen Van Thu

Cantho University,  Vietnam

ntkdong@ctu.edu.vn

Abstract

A study was conducted in the experimental farm of Cantho University to evaluate the effects of dietary fiber sources from sweet potato vine and para grass (PG) as potential feed for rabbits. Two experiments were 2*3 factorial designs of two factors and three replications and four male rabbits in one experimental unit. The first factor was diet (sweet potato vine and sweet potato vine plus para grass with rate of 1:1), the second one was the levels of DM feed offered (8, 9 and 10% of LW). The first trial was done on growing rabbits at 8 weeks of age to evaluate feed intake, growth performance and economic returns, while the second experiment of feed digestibility, nitrogen retention and caecum fermentation of two rabbits was determined at twelve weeks of age.

In Expt. 1, the dry matter (DM) and organic matter (OM) intakes were significantly higher (P<0.001) on the diet included sweet potato vine (SP) plus para grass (PG), while those were quite similar (P>0.05) among the three levels of feed offered. Crude protein (CP) intake was not significantly different between the two feeds and three levels of feed offered. NDF and ADF intakes were not different among levels of feed offered, but these values were significantly lower (P<0.001) in the only SP diet. The daily gain was not significantly different for both the two factors, however, the lower FCR was found  (P<0.001) in the diet without PG. Lower feed cost resulting in higher benefit was found for the diet including SP plus PG and the level of DM feed offered at 8% of live weight.

In Expt. 2 the apparent digestibility of DM, OM, CP, NDF and ADF were significantly higher (P<0.001) in the diet fed only SP. The significantly lower nitrogen retention (P<0.001) was found in the diet included SP plus PG. The total volatile fatty acids (VFAs) was significantly higher (P<0.01) for rabbits fed only SP.

It was concluded that there are no nutritional benefits from supplementing sweet potato vine with para grass for growing rabbits.

Key words: Digestibility, growing rabbits, para grass, sweet potato, volatile fatty acid, weight gain  

Introduction

Rabbit production has a considerable potential in the developing countries for the supply of needed animal protein for human beings due to the low capital investment and space requirements, short generation interval, rapid growth rate, high reproductive potential and ability to utilize the abundant forages and fibrous agricultural by- products (Cheeke 1986).

Crossbred rabbits (local and improved pure breeds) are popularly raised in the Mekong delta because of a good adaptation to the local climate and feeds. Organic rabbit farming based on locally available feeds resources, such as natural grasses and vegetables have a very important role for production. Particularly, sweet potato (Ipomoea batatas L.) is easy to plant and gives high yield of biomass with CP content ranged from 19-22% (An et al  2003) and CF content is 13.0 -19.3% (Linh  2005), while para grass (Brachiaria mutica) from natural pastures is a good fiber feed source of 25.3% CF and protein source of 10.4-15.0% CP (Linh  2005 and Trang  2006). They can be fed rabbits by farmers. Gidenne et al (2000) found that there were effects of dietary fiber levels (12, 16 and 20% acid detergent fiber) on rabbit digestion. At two weeks after weaning the volatile fatty acid (VFA) level decreased linearly and significantly with reduction of ADF level. Samkol, et al (2006a) reported that increasing the offer levels of water spinach from 8-18% of live weight increased the proportion of leaves consumed, the intake of crude protein and the digestibility of the DM and CP. Therefore, the finding of the optimum levels of the dietary crude fiber and of feeds offered on live weight of rabbits would be important for raising them with local plants and the results of the study will be disseminated to producers for practice.

Materials and methods

Experiment 1.  Feeding trial

Animals and experimental design

The experiment was conducted in the Experimental farm of Cantho University. Seventy two young male crossbred rabbits (local x improved breeds) at 8 weeks of age with similar live weight were arranged in a factorial design with 2 factors and four male rabbits in an experimental unit. The first factor was the diet (sweet potato vine [SP] and sweet potato vine plus para grass [PG] with ratio 1:1), the second one was the levels of feed offered of 8 (SP8), 9 (SP9) and 10% (SP10) of live weight (DM basis). Three replications were applied for all treatments in the study. Sweet potato vine (SP) was fed in a trough in the treatments of factor one, while SP and Para grass (SP-PG) was fed separately with a ratio of 1:1 (DM basis). Paddy rice was supplied at the same level of 10g per day per rabbit for all treatments to provide energy. The experimental period lasted 9 weeks.

Feeds, feeding and management

The animals were fed three times a day at 8:00h, 15:00h and 19:00h. Sweet potato vine was offered separated into leaves and stems, and refusals recorded separately and daily. The diets were adjusted weekly following the live weights. Fresh water was available for all rabbits almost all day and night time. The refusals and spillage were collected and weighed daily in the morning to calculate the feed intake. The animals were vaccinated to prevent hemorrhagic and parasite diseases.

Measurements

The feeds and refusals were taken for analysis of DM, OM, CP, EE, NDF, ADF, and ash following procedures of AOAC (1990) and Van Soest et al (1991). During the experiment four rabbits per experimental unit were weighed individually every week. Daily feed intakes, growth rate, and feed conversion ratios were measured and calculated. After finishing the experiment the rabbits were slaughtered for evaluating carcass and meat quality. The economic analysis was also done among the treatments.

Experiment 2: Digestibility trial

Animals and experimental design

The second experimental design was similar to that of the feeding trial, however,  two 12-week old rabbits in one experimental unit were used. The animals had two weeks for adaptation and another week for getting samples for 7 days. Feeds and refusals were daily measured. Urine was also collected for nitrogen analysis to calculate the nitrogen retention.  DM, CP, EE, NDF and ADF digestibility were employed according to Mc Donald et al (2002). At the end of the third week of this experiment the rabbits in the experimental unit were slaughtered at 9.00 am (after feeding 3 hours) to get the caecum content and it was treated immediately for measuring volatile fatty acid concentration by distillation (Barnett and Reid  1957).

Statistical analysis

The data from both experiments were analyzed by analysis of variance using the ANOVA option of the General Linear Model of Minitab Reference Manual Release 13.21 (Minitab 2000). Economic analyses were done using current prices in Vietnamese Dong (VND) to compare differences of income and the feed cost in different treatments.

Results and discussion

Expriment 1: Feeding trial

Feed characteristics

The chemical composition of the feed ingredients of rabbit is presented in Table 1.

Sweet potato had twice as much protein and less than half the ndf component compared to the para grass. dm and cp contents were higher, while ndf and adf were lower in sweet potato leaves (spl) compared to these of sweet potato stems (sps). the dm content of sp used in our experiment is slightly higher, but cp content is lower than the values reported by dao hung (2006). however, these values are lower than those reported by Gang et al (2006). This might have been caused by  different varieties and harvest seasons of sweet potato.

Feed and nutrient intakes

Daily intakes of para grass, sweet potato leaves and sweet potato stems are shown in Table 2 and Figures 1 and 2.

Daily intake of PG was lower for feed level of 10 %, but  not significantly different among three feed levels (p>0.05).  The daily intake of SPL and SPS was quite close among three feed levels offered (p>0.05). This indicated that increasing feed level offered (10% of live weight) to the  rabbits had little effect on intake of leaves and stems. The results were different to those reported by Samkol et al(2006a) that increasing the offer level of water spinach from 8 to 18% of live weight (DM basis) increased the proportion of leaf consumed, the intake of crude protein and the digestibility of the DM and the crude protein.

Daily intakes of feed of growing rabbits

The total DM intake in rabbits fed SP only was significantly lower (p< 0.001) than in those fed the diets that included SP plus PG, possibly due to the higher DM content in PG and the lower DM in SP (Table 3). However, the value was unaffected by the inclusion of graded feed levels offered of  SP plus PG . The results are considerably higher than the values of 51.4 - 64.1 g/day and 65-75.5g /day of the previous studies in which the rabbits were fed para grass supplemented with sweet potato vines and water spinach basal diets (Dong and Thu 2006 and Samkol et al  2006a, respectively). There was no significant difference in intake of CP (p>0.05). The findings of CP intakes in the current study are similar to those stated by Dao Hung (2006), however, the results are considerably higher than those from 8.79 to 11.3g/day reported by Phimmasan et al (2004). NDF and ADF intakes were significantly lower (p<0.001) in the SP diet than the SP plus PG diet.    

Growth rate, feed conversion ratio and economic analysis

Final live weight and daily weight gain did not differ among sources of feed or levels offered (Table 4).   The results of the present study are promising for production in crossbred rabbits. Ha et al (1996) stated that the weight gain of pure New Zealand rabbits fed grasses and concentrate and legume leaves were from 15 to per 20 g/day. The reported growth rates were also within the ranges (14.4 to 20.3g/day and 15.9 to 19.4g/day) reported by Samkol et al (2006a) and Nakkitset (2007), respectively. Feed conversion ratio was lower for animals fed SP alone (p<0.01), as a result of better daily gain. FCR values were similar among feed levels offered. The obtained values for feed conversion ratio were consistent with the results from 4.5 to 5.5 and from 4.6 to 5.2 reported by Phiny and Kaensombath (2006) and Nakkitset (2007), respectively. However, they were better than those (6.2 to 10.7) stated by Gang et al (2006).

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 The economic analysis showed that the cost for feed was lower in the SP plus PG diet and feed level at 8% of live weight, with nearly similar income among dietary treatments, resulting in more benefits in these diets. The results showed that the promising diets for the rabbits could be SP+PG and feed level at 8%.

Carcass traits and meat quality

The carcass weight, thigh meat, lean meat and meat composition were not significantly affected by diets and feed levels offered (Table 5). Nguyen Van Thu and Nguyen Thi Kim Dong (2005) in a previous study of sweet potato vine replacing para grass in the diets reported that the percentage of carcass (without head) and lean meat of growing crossbred rabbits were from 41.6 to 47.1% and from 67.8 to 79.2%, respectively. The contents of crude protein and ether extract of rabbit meat in the present experiment were from 18.7 to 19.1% and from 6.65 to 7.02 %, respectively. These values can be compared with 21% and 8%, respectively, reported by Lebas et al (1986). The difference could be caused by different breeds and nutrition.