In order to provide safe and specialized nutrition for domesticated animals like rabbits, researchers and farmers are currently attempting to assess alternate sources of feed ingredients for livestock feeding. However, with an emphasis on supplying the people's protein demands in Nigeria, the availability of animal protein has continued to be impacted by the growing demand for agricultural products brought on by population expansion. This is particularly true for animals with a relatively short generation interval and the ability to grow rapidly, such as rabbits [1].
Numerous products, including glue, gelatin, ossein, dinner's foot, dicalcium phosphate, fertilizer, ceramics, and pottery, are made from animal bones [2].

In various West African nations, particularly Togo, Senegal, Benin Republic, and Nigeria, there is an abundance of rock phosphate. Agedeson et al [1]. indicated that commercial quantities of this mineral are found in Imo, Ogun, and Sokoto staes of Nigeria. Calcium and phosphorus are the most valuable mineral elements required in large amount by all types of animals as revealed by Agedeson et al [1]. hence alternative and more readily available sources of calcium and phosphorus should be evaluated for use in animal diets. Calcium sources in the country have been identified, such oyster shell, periwinkle shell, limestone, di-calcium phosphate and bone ash/meal. However, there aren't many efforts focused on finding local substitutes for the important mineral nutrients, such as phosphorus and calcium (Tumova et al.) [3].

Rock phosphate, is a naturally occurring sedimentary rock with high calcium and phosphorus content. Although it is considerably less expensive in some areas of Nigeria, its use has been restricted due to its high fluoride content [4]. The technique to reduce the fluoride concentration of crude rock phosphate is heat treatment. Deflourinated or soft rock phosphate is another name for HTRP, which likewise has fluoride concentrations much lower than RRP (Agbu et al. [2].

Bone meal has grown unappealing for animal diets due to contaminants and worry of disease transfer from animal bone meal to animal through feeds as described by Tion et al [5].

Rabbits are commonly referred to as pseudo-ruminants, a medium-sized, hopping animals with big ears and short tails, rabbits are known to adapt quickly to backyard rearing systems. They are also known to efficiently convert feed to meat, according to Agedeson et al [1].

Digestibility is the outcome of both the passage and the rate of digestion of a feedstock, which may be impacted by meal intake level [6].

Compared to ruminants, rabbits seem to digest fat and protein rather well, but their digestibility of fiber is very low. Although the digestibility ratings of individual feed ingredients are helpful predictors of their potential, the digestibility of the feed mixture as a whole is not always the same as the average of its component parts. It is frequently noted that different constituent proportions with comparable proximate analysis perform differently. The best strains and breeds are meat breeds, albeit there are variations. White people in New Zealand are just marginally more productive than those in California, according to Olaleru and Abu (2021) [6].

After weaning, rabbit growth and digestibility gradually decline until stabilizing at approximately 9 weeks, according to (Olaleru and Abu 2021) [6]. Digestibility is comparable across the sexes in young rabbits, however in adults; the buck digest less than the doe.

There are few data on how sensitive rabbits are to fluoride exposure. According to experimental results, a 6-month accumulation of 150 mg F/L in drinking water did not cause any obvious clinical symptoms and did not change the activity of antioxidant enzymes (Reddy et al, 2003) [7]. The liver is one of the vital organs responsible for the metabolism of toxic compounds produced by systemic mechanisms and exogenous toxins that enter the body from the environment [8]. Studies revealed cellular disarray, congestion, cellular degeneration, cellular vacuoles, nuclear fragmentation coupled with nuclear degeneration, and substantial necrosis in hepatocytes.

Shashi and Thapar (2001) reported that they observed hepatocellular necrosis, hepatic hyperplasia, significant vacuolisation in hepatocytes, and hepatic sinusoids and central vein enlargement in albino rabbits administered sodium fluoride [9].

Rabbits exposed to fluoride for 15 days to 16 weeks showed notable alterations in their livers, including a degenerative change in the liver cells (Santosh et al., 2015) [8].

In order to achieve maximum productivity, low production costs, and a sufficient return on investment, it is therefore essential to look for substitutes with high nutritional value.

Experimental site

The study was carried out at the University of Ibadan Teaching and Research Farm's Rabbitry Unit, which is situated at latitude 7.27 °N and longitude 3.54 °S on the Oyo State GPS (2023).

Ethical Approval

University of Ibadan Animal Care and Use Research Ethics Committee approved the experimental procedures before the commencement of the study (Approval ID: 23/049).

Collection of Phosphate Rocks

Phosphate sediments were obtained from Ogun State, between Ososun and Ifo Junction, about 43 and 48 kilometers north of Lagos, Nigeria.

Methods of Rock Phosphate Processing

Rock Phosphate was extracted, the slurry sent to a flotation plant, its particles were washed and dried, then milled into the powder form as indicated by Kaankuka, (1990) [10].

Heat Treatment of RRP

Phosphate ore was ground into powder and sieved using 200 µm sieves. Samples (1000 g) were heated for five hours at varying temperatures of 10 °C/min between 550 °C and 600 °C until the color changed from brownish grey to reddish brown (Mgaidi et al. (2004); Fayiga and Obigbesan (2017) [11,12].

Housing and Management of Rabbits

Fifty-four (54) cross-bred seven-weeks-old mongrel weanling rabbits were utilised. The rabbits, with average initial weights of 599.65±2.7 g were purchased from a commercial rabbit farm in Ibadan. Weighed amount of feeds were given to the animals every morning, leftover feeds were weighed the following morning and the difference was calculated. In a fully randomised planned arrangement, the rabbits were assigned to each of the nine feed regimens based on their initial live body weights. The rabbits were put on dietary regimens that included 0, 25, 50, 75, and 100% replacement of bone meal for a period of 161 days.

Experimental Diet

Table 1: Compositions of Diets Containing Raw Rock Phosphate and Heat Treated Rock Phosphate for Weanling Rabbits.

Table 2: Apparent Nutrient Digestibility of Rabbits Fed inclusion levels of Raw Rock Phosphate and Heat Treated Rock Phosphate Supplementary Diets.

Digestibility is the outcome of both the passage and the rate of digestion of a feedstock which may be impacted by meal intake level (Olaleru and Abu 2021) [6]. Although the digestibility ratings of individual feed ingredients are helpful measures of their potential, the digestibility of the whole feed mixture does not always equal the meal of its parts. In fact, it is frequently noticed that different constituent proportions with comparable proximate analyses behave differently.

It was observed that the digestibility of mineral content in this study was very low. No significant difference (P>0.05) was observed in dry matter, ash and ether extract across treatments. Rabbits fed 100% HTRP and 75% RRP had lower crude protein digestibility of (32.74% and 30.99%) compared to those fed control (52.96%). Compared to ruminants, rabbits seem to digest protein quite well. The significant (P<0.05) impact could also be linked to the fact that the animals did not utilised the sufficient protein content in diets provided and inclusion replacement levels of rock phosphate adversely affected protein intake due to the presence of aluminum and magnesium oxide at varying inclusion levels that might interfered with crude protein metabolic activities. The apparent digestibility coefficient of crude protein found in this study differed from the findings of (Agedeson and Kaakuka, 2015) [15] who reported increase in crude protein digestibility where weanling rabbits were fed varying inclusion levels of rock phosphate. This could also likely link to different geographical sources. It has been found that the rabbit’s digestibility of fiber in fodder is very low compared to ruminants. The statistically analyses on crude fibre revealed that rabbits fed control diet had high fibre digestibility of (17.67%) when compared to those with low 100% HTRP (6.64%) and 50% RRP (9.20%). Olukayode (2005) revealed the range values of 83.00 - 91.00% and Zita et al. (2007) reported 72.73 - 74.20%. The significant difference (P<0.05) could also be attributed to insufficient lignin contents in the diets. The result was in line with Djago et al. (2010) who reported that the amount of crude fiber in the diets of growing rabbits ranges from 14 to 16%, whereas the levels in the diets of reproductive rabbits ranged from 12 to 13%. According to Houndonougbo et al. (2012), fibers are one of the main ingredients in rabbit diets because they are important for feeding since they boost caeca activity, and promotes effective digestion. The orthogonal contrast of ash in rabbits had significant (P<0.05) impact across treatments, except control diet against (HTRP and RRP) [16-19].

According to Santosh et al. (2015), the liver is one of the essential organs in charge of the metabolism of environmental exogenous toxins as well as hazardous chemicals generated by systemic processes. Furthermore, sodium fluoride was proposed to alter the liver's appearance and metabolism. According to Santosh et al. (2015), recent investigations of hepatocytes showed significant necrosis, cellular disarray, congestion, cellular degeneration, cellular vacuoles, and nuclear fragmentation combined with nuclear degeneration. After 15 to 16 weeks, rabbit liver treated with sodium fluoride exhibited bleeding in the pycnotic nucleus and central vein Santosh et al. (2015) [8].

The observation made on rabbit’s haematoxylin and Eosin stain (100x) had no visible lesion. Anatomy land mark were intact without sign of injury or cellular infiltration in lungs, kidney and liver of rabbits exposed to HTRP fluoride for trial duration in 0, 25, 50, 75 and 100 % respectively. However, no cellular infiltration of lungs across treatments except 100 % of lungs, kidney and livers with mark vascular congestion and moderate portal inflammation as well as moderate congestion of pulmonary oedema (eosinophilic fluid deposition in alveoli) increasing alveoli wall cellularity of rabbits exposed to RRP (6.30% fluoride) for trial duration in 25, 50, 75 and 100 % respectively. The findings of this study were consistent with those of other authors, such as Thomas et al. (2007) and Khandarel et al. (2007), who found that feeding flouride to mice and chicken, respectively, likewise decreased feed consumption and body weight growth (BWG). The outcome was also comparable with the findings of Khandarel et al. (2007), who reported that in a study of rats given 50 ppm flourine for six weeks, flouride was found to have an inhibitory effect on the duodenum and gene transcription, which reduced the absorption of calcium (Tiwari et al., 2004) [20,21]. According to Susheela and Bhatnagar (2002) [22], rats' reactions to fluoride (F) therapy were examined in relation to the impact of feed calcium for five weeks, the rats were given NaF in their drinking water and were fed both high (2.0%) and adequate (0.5 %) dietary Ca. The minimum amounts of NaF that hindered survival and diminished body growth were 300 mg/L with 0.5% diet Ca and 550 mg/L with 2.0% diet Ca. Adults appeared to be lethal at dosages of 16–64 mg/kg of F, while even lower levels of 3–16 mg/kg were dangerous to neonates (Vakdevi et al., 2022). Chronic fluoride (F) poisoning is caused by high fluoride intake (> 1.5 mg/L), which is prevalent in many countries across the world, according to the World Health Organization (2003) [23,24].

Results for the histopathology of rabbits were observed to be impacted with some issues in treatment fed with 100% RRP. The liver kidney and lungs had severe vascular congestion with moderate hepatocellular vacuolar degeneration in rabbits. (Vakdevi et al., 2022) found that flouride in rat diets inhibits glycolysis pathways that lead to inactive ATP synthesis, which causes rats to not survive. The histopathological photos of some of the analyzed rabbit organs were consistent with their findings [23]. Experimental findings showed that a 6-month buildup of 150 mg F/L in drinking water did not alter the activity of antioxidant enzymes or result in any overt clinical symptoms (Reddy et a.l, 2003) [7]. However, when 10 mg NaF/kg body weight is administered daily for 18 months, sperm cell abnormalities are severe; suggesting that fluoride may contribute to the male rabbit's decreased fertility (Kumar et al., 1994) [25-27].

The finding from the digestibility of rabbits revealed that the heat treated rock phosphate and raw rock phosphate in the nutrition of rabbits were bioavailable as a viable alternative to bone meal. Integrated farming system of production systems such as mineral enhancement for rabbitry would require limited investment in the production of quality rabbit feeds through the heat treated rock phosphate and raw rock phosphate utilisation. During absorption of the rock phosphate, anti-nutritional complexes such as fluoride inhibitors may play also a beneficial impact in the decrease bioavailability. The threat of nutrient loss to complexes was reduced through the adopted processing method of heat treatment which tends to deflourinate the fluoride contents.

Authors’ contributions: Olatunbosun Odu: Conceptualisation, methodology, manuscript reviewer editing; Joshua Terseer Agedeson: Data collection, writing original draft , and Timothy Tartenger KUKA: Data analysis, Manuscipt review, and submission.

Data Availability Statement

Data presented in this study are available on request from the corresponding author.

Conflict of interest: The authors declare that there are no conflicts of interest that have influenced the content of this manuscript.

1. Agedeson TJ, Kuka TT, Kaankuka FG. Rock phosphate as replacement for bone meal in rabbit diets. Nigerian J Anim Sci. 2021; 23: 150-156.
2. Agbu CS, Kaankuka FG, Tion MA, Gambo 1D. Evaluation of different sources of Phosphorus on Performance and blood characteristics of broiler chickens. FULafia J Sci Technol. 2017; 3: 21-27.
3. Tumova E, Skrivanova V, Zita L, Skrivan M, Fucikova A. The Effect of Restrictionon Digestibility of Nutrient, Organ Growth and Blood Picture in Broiler and Rabbit. Pp 1008-1014 in proc. 8th World Rabbit. Cong. Puebla. Mexico, WRSA. 2004.
4. Thomas A, Kawatra C, Bagel RPS, Kayak S. Use of Heat-Treated Rock Phosphate instead of Dicalcium Phosphate on Broiler Performance. J Anim India Nutr. 2007; 22: 193-197.
5. Tion MA, Offiong SA, Njoku PC. The Effect of Limestone Deposits as Calcium Source on the Performance of Broiler Chickens. Nigerian Journal of Animal Production. 2012; 39: 122-125.
6. Olaleru IF, Abu OA. Nutritive value of composite meal from two varieties of sweet potato (Ipomoea batatas) (Lam.) and its effect on performance and carcass characteristics of growing rabbits. Slovak J Anim Sci. 2021; 54: 75-85.
7. Reddy GS, Rao BSN. Effect of dietary calcium, vitamin C and protein in development of experimental skeletal fluorosis II calcium turnover with ⁴⁵Ca; calcium and phosphorus studies. Metabolism. 2003; 20, 650-656.
8. Santosh KS, Mishra DN, Pradhan S, Prusti JS. Panda SK, Agrawal D, et al. Int J Pharm Sci Rev Res. 2015; 30: 184-188.
9. Shashi A, Thapar SP. Histopathology of fluoride-induced hepatotoxicity in rabbits. Fluoride. 2001; 34: 34-42.
10. Kaankuka FG. Bone Meal and Rock Phosphate as Source of Calcium and Phosphorus for weaner and Grower Pigs. M.Sc. Thesis: Department of Animal Science A.B.U. Zaria, Nigeria. 1990.
11. Mgaidi A, Brahim FB, Oulahna D, Nzihou A, El Maaoui, M. and De Gruyter. Chemical and structural changes of raw phosphate during heat treatment. High-temperature materials and processes. 2004; 23: 185-194.
12. Fayiga AO, Obigbesan GO. Physico-Chemical Characterization of Ogun and Sokoto Phosphate Rocks Doi: Global Journal of Pure and Applied Sciences. 2017; 23: 27-34
13. Perez JM, Lebas F, Gidenne T, Maertens L, Xiccato G, Parigibini R, et al. European Reference Method for world Rabbit Sci. 1995; 3: 41-43.
14. Official Method of Analysis. 2005. Association of Official Analytical Chemist, 18^th edition, Arlington, Virginia, USA.
15. Agedeson TJ, Kaankuka FG. Rock phosphate as replacement for bone meal in rabbit diets. Department of Animal Nutrition, Federal University of Agriculture, Makurdi, Benue State, Nigeria.
16. Olukayode BS. Performance and Carcass Characteristics of Growing Rabbits Fed Low Technology Diets: Dept. of Anim. Prod. (M.Sc. Thesis Uni. Agric. Makurdi, Nig.) 2005.
17. Zita L, Tumowa E, Skrivanova V, Ledvinka Z. The Effect of Weaning Age on Performance and Nutrients Digestibility of Broiler Rabbits. Chech Anim Sci. 2007; 52: 341-347.
18. Djago Y, Kpodekon M, Lebas F. 2010. Practical Guide of Rabbits Farmer in Tropics.2nd edition. Benin: CECURI. 
19. Houndonougbo FM, Chrysostome CA, Amoussa AM, Tchoukoutou ZLAO. Residue and yogurt as feed additives in broilers feed. Research Opinions in Animal and Veterinary Sciences. 2011; 1: 597-600.
20. Khandare AL, Kumar PU, Shankar HN. Effect of calcium deficiency induced by fluoride intoxication on lipid metabolism in rabbits. Fluoride. 2007; 40: 184-189.
21. Tiwari S, Gupta SK, Kumar K, Trivedi R, Godbole MM. Simultaneous exposure of excess fluoride and calcium deficiency alters VDR, CAR, and calbindin D 9 k mRNA levels in rat duodenal mucosa. Calcif Tissue Int. 2004; 75: 313-320.
22. Susheela AK, Bhatnagar M. Reversal of fluoride induced cell injury through elimination of fluoride and consumption of diet rich in essential nutrients and antioxidants. Mol Cell Biochem. 2002; 234: 335-340.
23. Vakdevi V, Khandare A, Dheeravath S, Venkata M, Kurella S, Sinha SN. A study on dose and time-dependent effect of fluoride in rats. 2022. ICMR-National Institute of Nutrition.
24. World Health Organization. 2003. Background Document for Preparation of WHO Guidelines for Drinking-water Quality. Fluoride in Drinking-water. Geneva:WHO.
25. Kumar AL, Susheela AK. Ultrastructural Studies on Spermiogenesis in rabbits exposed to chronic fluoride toxicity. Int J fertility. 1994; 39: 164-171.
26. GPS, 2023. (Global Positioning System). A soft Ware in Hand Set Device Produced by Infinix.
27. SAS Institute Inc. 2023. (Statistical Analysis System). Base SAS 9.4 Procedure Guide. Cary. NC. USA.