INTRODUCTION

The high costs of conventional vegetable protein and energy ingredients such as groundnut cake, soybean meal and maize, sorghum and millet have largely contributed to the existing high prices of animal feeds and products in Nigeria. Rabbits though often neglected, are one of the sources of animal protein in Nigeria. To improve its production and consumption in Nigeria where per capita animal protein intake is much below the recommended levels, there is need to source for readily available, high quality alternative vegetable protein and energy sources that are cheaper and can reduce the production cost of feeds. The problem can be solved by using the known but neglected tropical grass and legume species such as Andropogon gayanus (Ag) and Centrosema pubescens (Cp). These two forages are under-utilized but they can be used as major source of protein and energy grass and legume species are not consumed by humans and do not have direct competition with industries in Nigeria and also they are all seasonal tropical forage plants¹. They are hardy, drought and disease resistant and bloom in the rainy season. The grass is called “Egbe Oku“ while the legume is called “Onori”. Currently, Nigeria is plagued with food crisis which is partly due to the unprecedented rise in population and the drastic drop in per caput food production particularly in the last decades. Food such as garri, beans and rice are not affordable, the animal protein intake in Nigeria is very low because of very high cost of animal feed leading to very high cost of finished animal products. The shortage of protein, particularly those of animal origin is prevalent in most parts of Africa including Nigeria where it is estimated that an average of 10 g of animal protein is consumed per day compared to a recommended daily intake of 35 g. The resulting high cost of feed, especially feed for monogastric animal species has brought about a low supply of animal products such as meat, milk, eggs and their by-products. The short supply of livestock products had been attributed to many factors including the problem of competition between man, industries and other livestock for the available feedstuffs. This competition has increased the cost of essential feed ingredients in our market thereby putting animal farmers at great disadvantage as they can hardly make ends meet in their business². This situation has led to the closing down of so many of the livestock farms as well as the production of poor quality feed by the feed mill operators in their attempt to make profit. Based on this precarious economic condition of low animal protein intake, animal nutritionists/researchers are calling for an urgent need for ways of ameliorating the food insecurity through the use of non-conventional sources such as Ag and Cp in place of maize and the rabbit can use these products so as to increase the animal protein intake of Nigerians³. Energy feedstuffs constitute 45-60% of finished feeds for monogastric animals and as at present, maize constitutes the bulk of energy component of various rations. Between 1982 and 2008, the price of maize rose from N270 t^–1 to over N60, 000 t^–1 - an increase of over 200%. Feed cost for livestock can only be reduced by incorporating cheaper and alternative sources of nutrients in the feed and which are not directly consumed by man. Grass and legume species should fit logically as grain replacers in livestock feeds by virtue of their high caloric values with several agronomic advantages. The nutritional value of some of these plants had already been reported⁴. Rabbits are known to perform better when fed with a mixture of forages and concentrate diets. Ag and Cp are found in great quantity in our environment, their effective use in rabbit production will contribute greatly to improve the animal protein intake of Nigerians⁵. This study aimed to evaluate Ag and Cp leaf meals as feed ingredients and their effects on growth performance, carcass characteristics and cost benefit in rabbit production.

 MATERIALS AND METHODS

Study site: This study was conducted at the Rabbitry Unit of the Department of Animal Science Teaching and Research Farm, Faculty of Agriculture and Natural Resources Management, Ebonyi State University, Abakaliki, Nigeria from 12th March to 16th July, 2018. Ebonyi State is partly located in the rainforest and derived savanna region of Nigeria. Abakaliki lies approximately on latitude 6°45’Nand longitude 8°30’E with an average temperature of 27.7°C and average rainfall of between 1500-1800 mm⁶.

Source of rabbits and forage plants: The rabbits used in this study were purchased from Akwa Ibom, Nigeria while Gamba grass and Centro (Ag and Cp) were harvested from the Ebonyi State University Teaching and Research Farm, Faculty of Agriculture and Natural Resource Management, Abakaliki, Nigeria.

Forage processing: Ag and Cp were harvested and washed separately in a trough using clean water to remove dirt. The leaves were washed once again with 1% salt water and rinsed with clean water. The forages were kept on top of a platform to drain off water and thereafter were cut into shorter lengths (2-5 cm) and spread on the cellophane for drying. They were air-dried for a period of 4 days, milled and bagged.

Milling and bagging: The forages were milled separately using a hammer mill with screen size 2.0 mm. This screen size was used to allow for proper milling. Andropogon gayanus and C. pubescens leaf meals strongly attract moisture and the product can reabsorb moisture during and after milling. For this reason, the leaf meals were stored in separate air-tight plastic containers and kept ready for use.

Chemical analysis: A fraction of each of the two milled samples was collected and used for chemical analysis to determine the proximate composition, Acid detergent fibre (ADF), Neutral detergent fibre (NDF), energy values, anti-nutritional factors tannin, trypsin inhibitor, saponin and oxalates and mineral composition.

Proximate and energy composition: Dried and ground samples of each of the diets were analyzed for their proximate composition: crude protein, fat, ash, fiber and total carbohydrate contents. These were carried out in triplicates according to standard methods of Horwitz and Albert⁷. The energy-values of the diets were calculated using the Atwater factors 4, 9 and 4 for protein, fat and carbohydrate, respectively. The Metabolizable Energy value (ME) of Ag and Cp leaf meal diets was calculated using the following formula as described by Folorunso et al.⁸:

ME = 35×CP%+81.8x E.E%+35.5×NFE

Anti-nutritional factors/phytochemical composition: Ground samples of feed and forages were analyzed for proximate and phytochemical components. The samples were screened for tannin, cyanide and saponin. Quantitative determination of phenols and trypsin inhibitors were carried out in triplicates using the methods of Horwitz and Albert⁷ and Spectrophotometry method as described by Avula et al.⁹, respectively.

Neutral detergent fiber (NDF): The method of Gidenne¹⁰ was used. The weight of the filter paper was subtracted from the total weight to obtain the weight of the residue:

• W₀ = Weight of the sample used
• W₁ = Weight of filter paper
• W₂ = Weight of filter paper+residue after drying

Acid detergent fiber (ADF): The acid detergent fraction was taken as the difference in weigh t.

Acid detergent lignin: The acid detergent lignin was taken as the difference in weight⁷.

Cellulose: The cellulose was estimated from the difference between ADF and lignin.

Hemicellulose: The difference between the NDF and ADF was estimated as hemicelluloses.

Dietary fiber (%) = NDF + ADF + Lignin + cellulose + Hemicellulose⁷.

Tannin: The tannin content was calculated as follows:

Tannin (%) - An/AsxCx100/w×five

Where:

Oxalate: The Calcium oxalate content was calculated as:

Where:

Trypsin inhibitor activity: The trypsin inhibitor activity was tested using the spectrophotometric method as described by Emmanuel and Deborah¹¹. The trypsin inhibitor activity was expressed as the number of trypsin units inhibited (TU) per unit weight (g) of the sample analyzed:

Where:

Where:

Minerals: Minerals such as calcium, phosphorous, sodium, chlorine, magnesium and some micro mineral elements (Zinc, Iron, manganese, iodine, selenium and chromium) of the leaf meals and experimental diets were determined using Atomic Absorption spectrophotometer¹².

Experimental rabbits/design: A total of 24 crossbred growing rabbits of mixed sex, aged between 8 and 10 weeks were used for the experiment. They were randomly assigned to four dietary treatments in a Completely Randomized Design (CRD). Each treatment group has 6 animals replicated three times with 2 animals per replicate. Only100 g of feed was offered per rabbit while fresh cool water was served ad libitum to the animals throughout the period of the experiment. The study lasted for 63 days.

Experimental diets: Four experimental diets were formulated such that Andropogon gayanus and Centrosema pubescens leaf meals were included at 0, 5, 10 and 15% dietary levels representing T1, T2, T3 and T4, respectively. The T1 (0% leaf meal) served as the control of the experiment. As the leaf meals increased in the diets, the maize components of the diets decreased proportionately. The energy and protein levels of the diets were kept within the National Research Council (NRC) nutrient requirement recommendation for grower rabbits. Other feed ingredients included in the diets were soybean meal, wheat offal, brewers spent grain, bone meal and salt and vitamin/mineral premix (Table 1).

Housing and management: In this experiment, animals were housed in 3 single tier wood/wire meshed cages (61×45×40 cm) consisting of 8 cells (pens) each. Each cage had a feed trough and water trough. The cages were kept in a well-ventilated asbestos roofed building with dwarf wall and wire mesh traversing the whole length of the wall. This was to facilitate proper ventilation and proper dissipation of heat as fast as possible. The floor of the house was prepared with concrete, this is to ensure easy and proper cleaning.

Parameters measured: The parameters measured were initial weight, feed intake, weight gain. Total weight gain, daily weight gain, Total feed intake, Feed conversion ratio, Protein intake, Protein efficiency ratio were calculated. Carcass, organs evaluation and cost-benefits of production were determined. With the data collected on feed intake and weight gain other performance indices such as Feed conversion ratio and Protein efficiency ratio were calculated.

Carcass and organ evaluation: At the end of the 63 days feeding trial, 3 rabbits per treatment (total 12 rabbits) were sacrificed to study the carcass traits. The animals were deprived of feed overnight but with free access to water. They were weighed prior to slaughter and the weights were recorded as pre-slaughter live weights. The slaughtered rabbits were hanged for proper bleeding. The remaining carcass from each slaughtered animals was weighed with a digital weighing balance of 5 kg capacity (model: 5 kg/11 Ib, d = 1 g/oz.) and expressed as a percentage of the live weight to obtain the dressing percentage. The carcass was cut into primal cuts according to the harmonized standard of world rabbit Science Association (WRSA).

Cost-benefit analysis: Per kg cost of diet was calculated by multiplying the percentage composition of the feedstuffs with the price per kg and summing all. Total feed intake x cost per kg feed gave the total feed cost. Feed cost per kg weight gain was calculated as FCR×Cost per kg diet. Total feed intake (TFI) is the total feed consumed in each treatment for the duration of the experiment. Total weight gain (TWG) is the total weight gained in each treatment for the duration of the experiment. Total variable cost (TVC) is the cost of inputs during the experiment. Total revenue (TR) is the amount realized from the sales of the carcass after slaughter. Gross margin (GM) is the difference between total revenue (TR) and the total variable cost (TVC).

Digestibility trial: After a week, faecal samples were collected from each of the treatments, oven dried at 105°C to a constant weight to determine the dry matter. At the 7th and 8th week another batch of faecal samples were collected from each of the four treatments for laboratory analysis. Total collection procedure was employed for the faecal collection. Mosquito nets were used underneath the cages (7.5 cm above the floor). The collected samples were dried in a forced air circulation oven at 65°C. The samples of each of the replicates were allowed to cool in a glass desiccator to prevent further absorption of moisture from the atmosphere. The 3 days samples were pooled ground and then assessed for proximate analysis, fibre and ether extract according to the method of Horwitz and Albert⁷.

Digestibility co-efficient: Digestibility co-efficient was calculated as:

Nutrient digestibility/utilization: Nutrient digestibility/ utilization is a fraction of a total nutrient digested and utilized in the body. Nitrogen free extract was calculated by subtracting the sum of percentages of all the nutrients already determined from 100.

Statistical analysis: The statistical model for the experiment is as follows:

Y_ij = μ+T_i+e_ij

Where:

The random error is independent, identical and normally distributed with zero mean and constant variance. Data collected were analyzed using one-way analysis of variance (ANOVA). Significant differences between treatment means where they occur were separated using new Duncan’s Multiple range Test¹³.

 RESULTS AND DISCUSSION

Experiment 1: The results of the proximate composition and mineral contents of the two forages (A. gayanus and C . pubescens leaf meals) are presented in Table 1 and 2 while the phytochemical analysis of both the forages and the experimental diets are presented in Table 3 and 4, respectively.

Proximate composition: The proximate composition of A. gayanus and C. pubescens gave the following results: 13.83% crude protein, 32.75 crude fibre, 0.88% ether extract 19.20% ash, 30.45% nitrogen free extract while for C. pubescens the results were -25.03% crude protein, 13.10% crude fibre, 1.0% ether extract, 14.75% ash and 49.48% nitrogen free extract, respectively (Table 5). Rabbits require a high amount of crude fibre content from different forages to manage normal digestion transit. Forages generally contain appreciable amount of protein, fibre, fat and minerals and that can support growth and production. The mineral composition analysis of the two forages and the experimental diets are presented in Table 1. Table 2 shows that Centrosema pubescens contains calcium (0.36%), magnesium (2.55%), sodium (2.05%), iron (0.22%), zinc (0.0005%) and manganese (0.00115%) and Andropogon gayanus contains calcium (0.24%), magnesium (1.61%), sodium (1.34%), iron (0.22%), manganese (0.01%). However, these results disagree with those of Peiretti¹⁴ and Waziri et al.⁴. The values reported in this study were lower than those of Waziri et al.⁴. These variations could be due to a number of factors such as soil type, the plant parts, the period of the year and the processing method. Results of the current study clearly showed that legume has higher mineral content than that of the grass species. It is noteworthy that minerals are very important in the diets of animals, they play important roles in their body functions. The result of the analysis showed that the forages are used in the diet to meet the energy requirements of the growing rabbits.

Phytochemical analysis: Table 3 and 4 show the results of the photochemical analysis in both the qualitative and the quantitative fractions. The tannin contents of A. gayanus and C. pubescens were 8.2 and 2.5%, respectively while the saponin contents were 0.64 and 0.36%, respectively. It is clear from this result that tannin and saponin contents of A. gayanus was higher than that of the C. pubescens. The values however reflected positively with those reported by Apory¹⁵, Ojiako and Igwe¹⁶. The results of this study confirmed earlier reports that forage/legume leaf meals contain anti-nutritional factors. The tannins and saponin contents in these two forages also increased the tannin and the saponin contents of the diets. (Table 3 and 4). This also meant that the diets contained anti-nutritional factors which are protease inhibitors. Protease inhibitors are widely distributed within the plant kingdom, including the seeds of most cultivated legumes. Protease inhibitors have the ability to inhibit the activities of photolytic enzymes within the gastrointestinal tract of animals. Trypsin inhibitors, chymotrypsin inhibitors and protease inhibitors are found in raw legume seeds. Protease inhibitors are the most commonly encountered class of anti-nutritional factors of plant origin. These inhibitors have been reported to be partially responsible for the growth-retarding property of raw legumes. The retardation has been attributed to inhibition of protein digestion but there is evidence that pancreatic type enzyme activity increased the production of trypsin and chymotrypsin with consequent loss of cysteine and methionine. Trypsin inhibitors had been implicated in reducing protein digestibility and in pancreatic hypertrophy. Trypsin inhibitors are polypeptides that form well characterized stable complexes with trypsin on a one-to-one molar ratio, obstructing the enzymatic action. Table 6 shows the result of the carcass (head, neck, rib, fore leg and shoulder, loin, hind leg and rump) and organs-(heart and lungs, intestine with content, empty intestine, spleen, caeca with content, empty caeca and colon) evaluations for grower rabbits fed diets containing A. gayanus and C. pubescens leaf meal at 0, 5, 10 and 15% levels of inclusion. There were no significant (p>0.05) differences in the live weights (LW), carcass weights (CW) (fore parts-FP, Mid parts-MP and hind parts –HP) among the dietary treatment groups. In the present study the live weight at slaughter was 1155.28-1284.12 g. In the present study the average carcass yield of the rabbit in various treatment groups (T₁ to T₄) ranged from 640-807 g which translate the average percentages of the fore part (44.71-52.42%), mid part (20.47-26.52%) and hind part (21.06-30.62%) respectively. In the present study the dressing percentage (carcass yield) was 48.04-62.84%. This result disagrees with that of Oteku and Igene¹⁷, who reported values ranging from 43. 24-53.83%, 48.70-49.45% and 52.05-53.36% but agrees with Abubakar et al.¹⁸ who reported the values ranging from 50.70-58.5%, 51.61-59.00% and 52.00-59.00%. This variation in the present results could be due to the disparity in the nutrition, live and slaughter weights, age and breeds of the experimental rabbits. Significant improvement was observed in T4 (15%), T3 (10%) and T2 (5%) when compared to T1 (0%-control), T4 (15%) and T3 (10%) showed more promising effect when compared with the control (0%) treatment. This result agrees with a previous study conducted by Olafadehan¹⁹ who observed values for dressing percentage ranging from 50.03-58.51% and live weight ranging from 1640.00 to 1860±34.89 g, slaughter weight from 1594.57-1794.57 g. In this study, the hind part values ranged from 21.06-31.72. However, little differences were observed in most carcass parameters among different breeds of rabbits. Difference in the meat quality due to different breeds or live weight seems to be small, implying certain consistency in rabbit meat. The carcass traits observed used for carcass quality evaluation differ significantly due to difference in fat and dressing percentages. Dressing percentage is an important economic variable in the meat market and the commercial criterion used in rabbits and the dressing percentage of rabbits ranged from 56-58%. This result is consistent with previous reports about dressing percentages ranging from 55.56-59.72% for growing rabbits²⁰. This study also revealed that feed type was more influential on carcass quality due to the low fat content in the carcass. The general effect of diet containing A. Gayanus and C. pubescens leaf meal on the carcass characteristics was a clear indication that the diet has no deleterious effect on the growth and development of the growing rabbits.