Macro-minerals bioavailability study in goats fed forages of nitrogen fertilized Guinea grass and Guinea grass-Verano stylo mixture

Bamikole M A*

Department of Animal Science, University of Ibadan, Ibadan, Nigeria
*Present address and address for correspondence: Department of Animal Science, University of Benin, Benin City, Nigeria  bankymao@uniben.edu;    bankymao@yahoo.co.uk

Abstract

An investigation of macro-minerals bioavailability from forages of nitrogen fertilized Guinea grass (Panicum maximum, NFG), Guinea grass-Verano stylo (Stylosanthes hamata) mixture (GVSM) and unfertilized Guinea grass (UFG) was carried out, using fifteen West African dwarf goats with mean body weight of 7.61 ± 0.44 kg. The forages were six -weeks old and contained from 6.19 to 11.8 g crude protein/100g DM).

Calcium was best utilized in GVSM with the respective intake, digestibility, balance and retention values of 2.50 g/d, 35.6%, 0.68 g/d and 27.30 %. The utilization of Mg, Na, and Cl were best in NFG, having an intake of 1.34 g/d, balance of 0.45 g/d and retention value of 33.76 % for Mg with corresponding values of 0.056 g/d, 0.016 g/d and 27.99 % for Na as well as 0.33 g/d, 0.10 g/d and 29.01 % for Cl. Potassium was well utilized in all forages with no significant variation observed. Sulphur utilization was poor and no improvement was evident due to the grass treatment.

Generally, the results indicated that the strategy of applying nitrogen fertilizer to grass could be used to achieve better utilization of more macro-minerals in forages than legume incorporation into grass.

Key words: Bioavailability, goats, , Guinea grass, macro-minerals, nitrogen fertilizer, Verano stylo

Introduction

The bulk of the feed available to ruminant animals in the tropics is grass forage, as this can be sourced cheaply. Tropical grasses are poor in nutritive value (Minson and Mc Leod 1970) and attempts at enhancing the nutritive quality of the forage (particularly nitrogen component) have been through the use of nitrogen fertilizer or incorporation with legumes. The potential of forages so produced to meet the mineral needs of livestock, in the available form of the minerals, is of importance, as minerals could be regarded as salt of life. Life itself in animals is hinged on mineral elements as the beating of heart is brought about by balance between some minerals in the fluid bathing the heart muscle (McDowell 1992).

The assessment of the potential of forages to meet the mineral requirement of animals has been mostly through chemical assays and are well reported in the literature (Poland and Schnabel 1980; Jumba et al 1996). Information on biological availability of minerals in forages is relatively scanty. The in-situ method of studying mineral release in the rumen (Emanuele and Staples 1990; Pouli et al 1991) has some limitations as the release of mineral in soluble form in the rumen does not necessarily mean that the mineral will remain soluble until it is absorbed. Soluble mineral may interact with microbial cells, other minerals or other forage components to form insoluble compounds (Spears 1994). A complete study of mineral utilization in tropical forages is therefore very essential. In this study, the biological availability of macro-minerals in Guinea grass treated with Calcium Ammonium Nitrate (CAN) fertilizer or planted in association with Verano stylo legume was investigated.

Materials and Methods

The study was carried out at the small ruminant unit of the Teaching and Research farm of the University of Ibadan, Nigeria (7° 20"N, 3° 50"E., 200m above sea level). The average temperature during the period was in the range of 28 to 34° C.

A total of fifteen does of West African dwarf breed were used for the study. They were purchased from different locations within the neighbourhood of the farm. The animals were about 6 months old at purchase while at the time of the study they were 10 months old with an average body weight of 7.61±0.44kg. The moment they were brought to the farm, they were taken through a series of prophylactic treatment which included the given of long acting antibiotics (oxytetracycline) along with vitamin B complex at the dosage level of 1ml per 10kg body weight. They were also treated against endoparasites through a subcutaneous injection of levamisol (an injectable dewormer) as well as against ectoparasites through dipping in a solution of sumithion.

Housing during this study was in individual metabolism cages, in a house constructed to achieve cross ventilation. The cages were equipped with facilities, which allowed separate collection of urine and faeces. An adaptation period of seven days was allowed before sample collection so that the animals could adjust to the system of feeding inside the cages, as they had been on the feed before in a previous study that involved weight gain monitoring.

The forages served were nitrogen-ertilized Guinea grass (NFG), Guinea grass-Verano Stylo mixture (GVSM) and unfertilized Guinea grass (UFG). They were obtained from experimental plots where they were established on treatment basis. Harvesting for animal feeding was at 6 weeks old, and this was made possible on a daily basis through a cutting regime already described (Bamikole et al 1998). Thus the forages were harvests from the third phase of cutting. The forages, chopped to about 3cm pieces were served twice a day (0800 and 1600 hours) such that each animal was served a forage quantity amounting to 50g DM/kg LW/day with 20% allowance above the previous day's consumption. In the grass-Verano stylo diet, Verano stylo was served separately and at the level of 40% of the total forages made available to animals on this diet. During the 7-day data collection period, records were taken of the total quantity of the forage served as well as the remnants. Samples were taken for dry matter determination. The total quantity of faeces excreted was weighed, volume of urine voided measured and a 10% aliquot taken on daily basis and bulked over the entire period. The urine was collected in plastic containers containing 20 ml of 10% sulphuric acid (H₂S0₄) to prevent loss of nitrogen through volatilization. The samples together with that of the faeces were later frozen pending laboratory analysis. The forage and faecal samples were dried at 60°C in a hot air oven to constant weight, and milled for chemical analysis. The contents of CP and OM were determined according to the official method of analysis (AOAC 1990), while NDF determination was by the method of Van Soest and Robertson (1985).

Mineral analysis in the samples was by the wet digestion method involving nitric-perchloric acid mixture. The concentrations of Ca, k, Mg, S, Cl and Na were determined by the atomic absorption method using Bulk Scientific model 200a spectrophotometer (East Norwalk, USA). Data were analysed as a completely randomized design using the general linear model procedure (SAS 1993).

Results

The level of crude protein (CP) was found to be highest in Verano stylo (Table 1).The mineral composition of the forages indicated adequate levels except for Na, Cl and S.

There were significant differences in the utilization of Ca in the forage diets (Table 2). Calcium intake was significantly highest in GVSM and least in UFG. Its digestibility, balance and retention were also found best in GVSM. Calcium digestibility was poor in NFG (13.13%) and negative in UFG (-59.66%). Negative Ca balance and retention were also recorded for both the NFG and UFG diets.

Magnesium utilization pattern of the animals (Table 2) indicated a significantly highest intake of this mineral in NFG. The intake of Mg from the GVSM was not significantly different from that of UFG. About 34% more of Mg was consumed in NFG than both in GVSM and UFG. There was a considerable high excretion of Mg consumed in the faeces for all the forage diets, which was significantly highest in NFG. Digestibility of Mg was not influenced by forage diets. Values obtained for all the three forages were not significantly different. However, Mg balance and retention were better in the treated forages than the untreated.

The intake of Na (Table 3) was generally low in all the forages, although nitrogen fertilization of grass significantly enhanced this as well as Na digestibility and retention better than in GVSM and UFG. Forage treatment effect was not significant in the intake of K, as well as its balance and retention (Table 3), although retention value was good in NFG and GVSM and below average in UFG. The digestibility of K was good in all the forage diets irrespective of the form of grass treatment used. The higher loss of K in the urine than the faeces is noteworthy. The K urinary output was not significantly different in NFG and GVSM diets and both were significantly lower than value obtained in UFG.

Chloride utilization pattern is presented in Table 4. The intake, digestibility, balance and retention of this mineral were enhanced by nitrogen fertilizer application to the grass. Significant differences were observed in all these parameters except retention values in NFG and GVSM and balance values in GVSM and UFG. The distribution of Cl output was more in the urine than the faeces and this is applicable to all the forage diets irrespective of the treatment given.

The intake of S (Table 4) was significantly enhanced by Verano stylo incorporation to grass. Grass fertilization also has a slight effect on S intake from the grass diet. The digestibility of S was poor and below average in all the forage diets. Sulphur balance and retention could not be studied due to the sulphur contamination from the sulpuric acid introduced as preservative.

Discussion

The CP composition of all the forages was higher than 6-7 g/100g which is considered as the critical level at which intake could be depressed (Minson 1983). Some of the forage mineral compositions were also adequately in line with the requirement of ruminant animals, going by the recommendation levels of 0.21 % Mg for sheep (Suttle 1983), 0.5-1.0 % K for ruminant (Mc Dowell 1992) and 0.16 % Ca for lactating goats (NRC 1981).

The better intake of Ca in GVSM forage diet was due to the Verano stylo inclusion to the diet. Legumes are known to be richer in Ca than grasses (Minson 1983). This Verano stylo inclusion effect was also apparent in better digestibility, balance and retention of Ca in the GVSM forage diet. The negative Ca digestibility value (-59.66 %) in respect of UFG is in agreement with digestibility values reported for sheep grass (Festuca ovina, -40.6 %) as well as pangola grass (Digitaria decumbens, -40.0 %) by Perdomo et al (1977). The negative Ca balance on the other hand in NFG and UFG could be attributed to the occurrence of endogenous secretion of Ca in the lower part of the gastro-intestinal tract (GIT) and which could mask the absorption of the upper part of the GIT (Ben-Ghedalia et al 1975).

Fertilization of grass in this study enhanced better Mg intake, digestibility, balance and retention. However, Rosero et al (1980) gave a contrary report. This disparity in results could be attributed to factors ranging from nitrogen dosage level (Moore et al 1972) to age of the forage offered (Hannaway 1975). The observed higher excretion of Mg in the faeces than the urine was a reflection of the generally poor apparent Mg absorption from the GIT, due to the impermeability of the rumen epithelium to Mg²⁺ (Phillipson and Storry 1965), coupled with no net absorption of Mg in the abomasum and duodenum (Care and Van't Klooster 1965).

The generally low intake of Na was due to insufficient level of this mineral in the forage irrespective of the treatment imposed. Thus, an indication of the necessity to supplement this mineral in animals grazing such forages. The improvement in digestibility and retention of Na in NFG was an indication of the possibility of using nitrogen fertilization of grass to an advantage in enhancing Na utilization in goats.

The intake of K which was high in all the diets was a reflection of its high level in the forages and thus confirmed the generally high availability of K in actively growing grass and legumes (McDowell et al 1985). The good digestibility of K corroborated the observation of Hemkem (1983) for most feed stuffs. Moreso, the observation of no significant effect of forage treatment on K balance and retention was in line with the report of Rosero et al (1980) on different forage species. The higher K output in the urine than in the faeces is a commonly observed phenomenon. This is a pointer to high absorption of K in the GIT of animals, which could take place in lower small intestine and large intestine (Church and Pond 1974) as well as in the rumen and omasum (Kay and Pfeffer 1970).

The bioavailability of Cl was albeit good in all the forage diets, improvement on this could still be achieved through grass fertilization as it has been demonstrated in this study. The improved intake of S in GVSM diet was due to the legume component. The low digestibility value was an indication that improvement on utilization of S component of Guinea grass is not achievable through any of these grass treatment methods.

Conclusion

The study of macro-mineral bioavailability in grass treated with nitrogen fertilizer or planted with Verano-stylo legume has indicated that legume incorporation into grass can improve only the utilization of Ca, while utilization of Mg, Na, and Cl is better enhanced with nitrogen fertilization of grass. K utilization is good in both treated and untreated forages, while S utilization cannot be reasonably enhanced by grass treatment with nitrogen fertilizer or incorporation with legumes.

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