Nutrient quality of forages in Ethiopia with particular reference to mineral elements

E. Kabaija and D.A. Little

International Livestock Centre for Africa
P.O. Box 5689, Addis Ababa, Ethiopia

Introduction
Methodology
Results and discussion
Conclusion
References

Abstract

Hays, range grasses, crop residues and browse trees from different parts of Ethiopia were analysed for nitrogen, fibre components and mineral concentrations. Pasture grasses and crop residues studied had very low crude protein contents, while the browse on the average had more than 15% crude protein. Almost all the forages had sufficient levels of K, Ca, Mg, Mn and Zn to meet requirements of ruminant animals, but the occurrence of marginal to deficient supplies of Na, Cu and P appear very likely. There is an urgent requirement for experimentation on supplementation regimes involving these minerals, so that appropriate recommendations can be formulated.

Introduction

In Ethiopia, as elsewhere in Africa, malnutrition impairs livestock production. Ethiopia's overall livestock productivity is below average. Although 12.7% of Africa's 524.61 million cattle, sheep and goats are found in Ethiopia, the country produces only 7.3% and 5.1% of Africa's total meat and milk production respectively (FAO, 1985). Grazing animals in Ethiopia subsist mainly on poor quality feedstuffs in the form of poor quality pastures in arid and semi-arid areas and hays and/or crop residues in the arable areas. In a few of the animal production centres where improved management is undertaken, the main supplements are energy and protein in the form of agro-industrial by-products such as cereal brans, molasses and oilseed cakes. Scant attention is given to the mineral content and nutritional balance of such diets. It has been widely established that available energy and protein of a feed are of primary importance to any animal but optimal performance is only possible if there is an adequate supply of minerals and vitamins (McDowell, 1985).

In Ethiopia, cattle rarely receive mineral supplements except occasionally common salt. Pastures are thus the main source of minerals, and only rarely can forages completely satisfy all mineral requirements of livestock (Miles and McDowell, 1983). Mineral status of grazing animals in Ethiopia and other African countries has received very little attention. This may be due partly to the fact that the methodology of mineral nutrition studies, especially of trace elements, is rather complicated and signs of marginal mineral deficiencies are not easily detected but frequently it is mistakenly assumed that the grazing animal will obtain its mineral needs from the pasture. Mineral deficiencies, even if marginal, can result in depression of animal performance. Subclinical mineral deficiencies are often widespread and are responsible for as yet unestimated, but probably great, economic losses in livestock production.

An earlier mineral study in Ethiopia (Faye et al., 1983) indicated probable widespread copper and zinc deficiencies, and although much more information on the supplies of essential minerals for livestock is required, it is very likely that mineral deficiencies contribute to the poor performance of livestock in Ethiopia. Before measures can be undertaken to correct these deficiencies, it is necessary to assess the mineral status of forages and of the grazing animals as well as their production responses to mineral supplementation. This study was undertaken to investigate the nutrient status, with special regard to minerals, of the common crop residues, pasture grasses, grass hays and browse trees used as livestock feeds in Ethiopia.

Methodology

Samples

Samples of meadow hay were collected from an ILCA site in Addis Ababa and from two other sites at Debre Berhan (an ILCA-station 120 km east of Addis Ababa and a Government sheep breeding station 132 km from Addis Ababa). Samples were also taken from a commercial source in Addis Ababa, whose supplies came from the Addis Ababa region. All the samples were from the highlands (2500-3000 m) and were obtained by pooling core samples from ten positions within the stack. Samples of oats hay (whole plant with grain), ryegrass hay and improved pasture hay (mostly ryegrass and native Trifolium species) were taken as described above from stacks of hay at ILCA sites in Debre Berhan harvested between September and November, 1986.

Sesbania sesban grown at ILCA, Addis Ababa and in Yabello in the southern Ethiopian rangelands of Sidamo were harvested in February, 1987. Samples were taken by pooling leaves from mature trees (50 in Addis Ababa and 15 in Sidamo). Leaves from the trees in Addis Ababa were further separated into young (first ten leaves on twig) and old. Pods with seeds on trees from Addis Ababa were first air-dried and then separated into pods and seeds. Samples of Leucaena leucocephala cv Peru were taken in February, 1987 from a lowland area at a Government research station (Institute of Agricultural Research, Melka Worer) in eastern Ethiopia using the same procedures as for Sesbania.

Grass and browse species were collected from the Sidamo southern rangelands in February, 1987 by hand plucking during grazing observations over three days. The species collected included: Cenchrus ciliaris, Themeda triandra, Chrysopogon aucheri, Pennisetum mezianum, Acacia brevispica and Acacia nilotica. The Acacia samples were of the species mainly browsed by camels and goats.

Samples of crop residues were taken using the same procedures as for hays. The crop residues were of highland origin, and most of them were being used in feeding experiments at ILCA, Debre Zeit and ILCA, Addis Ababa.

Chemical Analysis

Dry matter was determined after drying in a microwave oven at 60°C for 24 hours. The dried samples were ground using a one-mm sieve and stored in plastic bottles ready for analysis. Neutral-detergent fibre (NDF), acid-detergent fibre (ADF), lignin and ADF ash were determined with a microfibre apparatus using the procedures of Goering and van Soest (1970).

Samples were prepared for mineral analysis by the wet digestion method using concentrated sulphuric acid in presence of hydrogen peroxide. Nitrogen (N) and phosphorus (P) concentrations in the extracts were determined with the autoanalyser which uses ascorbic acid as a reducing agent for P determination. The concentrations of potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu) were determined by atomic absorption spectroscopy. Sulphur (S) in the samples was quantified using the turbidimetric method of Cottenie et al. (1982).

Results and discussion

The analytical results for hay and range grasses are shown in Tables 1 and 2, for crop residues in Tables 3 and 4 and for leguminous trees in Tables 5 and 6.

Table 1. Dry matter, crude protein, NDF, lignin, ash and ADF ash of some hays from Ethiopian highlands and range grasses from southern Ethiopian rangelands.

Sample description		DM	CP	NDF	ADF	Lignin	Ash	ADF ash
		%
Highland hays
	Meadow hay, DBSBS	93.4	5.3	69.7	37.9	4.0	7.5	2.4
	Meadow hay, ILCA-DB	93.1	4.4	69.2	37.9	4.9	7.9	2.8
	Oats hay, ILCA-DB	91.2	5.1	71.0	38.1	4.5	4.6	1.5
	Rye grass hay, ILCA-DB	92.5	6.2	57.5	35.7	5.0	7.4	2.6
	Improved pasture hay, ILCA-DB	92.0	5.3	58.7	32.7	4.0	6.5	3.1
	Meadow hay, ILCA-Addis	94.1	4.6	72.2	43.7	5.7	8.7	3.4
	Meadow hay from commercial centre	91.9	3.7	77.0	41.1	5.1	8.4	3.4
Range grasses
	Cenchrus ciliaris	58.8	7.5	66.2	37.2	5.1	11.3	4.06
	Themeda triandra	84.7	5.0	77.2	50.2	8.0	10.9	7.61
	Chrysopogon aucheri	59.1	6.0	71.2	36.0	5.2	11.5	5.80
	Pennisetum mezianum	78.0	6.3	74.1	46.0	8.5	10.7	4.60

Table 2. Mineral content of some hays from Ethiopian highlands and range grasses from Ethiopian Sidamo southern rangelands.

Sample description		K	Na	Ca	Mg	P	S	Fe	Mn	Zn	Cu
		g/kg						mg/kg
Highland hays
	Meadow hay, DBSBS	17.6	0.4	3.4	1.6	1.6	3.0	695	207	45	5.9
	Meadow hay, ILCA-DB	18.4	0.3	4.4	1.6	1.4	2.6	974	274	39	6.1
	Oats hay, ILCA-DB	12.6	0.9	1.6	1.0	1.8	1.8	192	96	30	7.0
	Ryegrass hay, ILCA-DB	16.3	1.1	3.8	1.7	2.2	2.1	397	214	32	7.9
	Improved pasture hay, ILCA-DB	20.8	1.2	3.7	1.7	1.6	2.6	358	206	28	7.1
	Meadow hay, ILCA-Addis	17.5	0.6	4.9	1.7	1.4	2.4	191	246	29	6.2
	Meadow hay, from commercial centre	13.1	0.5	4.6	2.1	1.6	3.0	589	322	37	7.9
Range grasses
	Cenchrus ciliaris	23.5	1.0	2.0	1.6	1.5	2.1	830	45	24	3.6
	Themeda triandra	12.0	0.1	4.1	1.5	1.2	1.0	625	72	31	3.5
	Chrysopogon aucheri	1.0	0.08	2.2	1.6	1.0	1.1	882	63	32	5.6
	Pennisetum mezianum	12.0	0.1	3.2	2.0	1.4	1.1	452	72	40	4.7

Table 3. Dry matter, crude protein, NDF, ADF, lignin, ash and ADF ash of some crop residues from Ethiopian highlands.

	DM	CP	NDF	ADF	Lignin	Ash	ADF ash
	(%)
Wheat straw	90.5	1.6	70.7	50.6	11.1	10.8	5.5
Barley straw	88.2	1.8	65.4	56.2	7.0	7.7	1.7
Teff straw	89.7	1.7	71.3	49.1	7.9	7.8	3.7
Oats straw	88.9	1.5	57.2	41.9	6.0	8.3	2.1
Linseed straw	93.0	3.0	68.7	-	-	8.4	3.1
Maize stover	87.2	1.5	68.1	47.1	7.9	9.2	3.1

Table 4. Mineral concentration of some crop residues from the Ethiopian highlands.

	K	Na	Ca	Mg	P	S	Fe	Mn	Zn	Cu
	g/kg						mg/kg
Wheat straw	14.8	0.3	4.1	1.5	1.3	1.4	325	78	11	3.0
Barley straw	10.7	0.5	4.6	1.4	1.9	1.2	1175	90	12	5.0
Teff straw	11.7	0.3	4.3	1.9	1.6	1.6	170	59	26	6.5
Oats straw	17.7	0.4	3.9	1.8	1.7	1.7	196	191	17	14.0
Linseed straw	10.7	0.6	7.7	2.2	1.3	1.6	103	71	24	14.0
Maize stover	17.8	0.5	3.3	2.5	1.7	0.9	408	61	24	5.9

Table 5. Dry matter, crude protein, NDF, ADF, lignin, ash and ADF ash of Leucaena leucocephala, Sesbania sesban and Acacia species.

Sample description		DM	CP	NDF	ADF	Lignin	Ash	ADF ash
		%
Leucaena:
	Young leaves	20.7	31.2	25.1	10.6	0.3	6.5	-
	old leaves	34.1	28.6	23.4	13.4	2.4	8.9	-
	Pods	79.1	7.9	67.8	61.1	19.6	5.7	-
	Seeds	97.3	34.8	33.9	21.2	2.1	4.2
Sesbania:
	Small twigs with leaves	29.0	27.5	14.0	12.4	2.8	12.4	0.3
	First ten leaves	31.3	28.5	13.3	9.2	2.4	6.3	0.2
	Old leaves	30.4	28.5	12.7	9.8	2.6	7.8	0.4
	Pods with seeds	27.5	21.0	50.4	40.4	6.6	2.7	0.5
	Pods alone	90.8*	10.6	60.9	52.6	13.9	6.4	0.3
	Seeds alone	89.8**	34.6	33.0	22.4	2.6	3.7	0.3
	Old and young leaves*	33.6	25.9	13.0	9.9	1.7	13.6	0.8
Acacia:
	Acacia brevispica	38.9	26.3	47.3	29.0	13.0	8.4	0.60
	Acacia nilotica	40.1	15.0	30.7	28.1	11.0	5.9	0.60

* Taken from the low-mid altitude southern Ethiopia rangelands.
** First air-dried.

Table 6. Mineral content of Leucaena leucocephala, Sesbania sesban, and Acacia species.

Sample description		K	Na	Ca	Mg	P	S	Fe	Mn	Zn	Cu
		g/kg						mg/kg
Leucaena:
	Young leaves	20.4	0.3	3.7	1.9	3.1	3.4	181	31	39	10.0
	Old leaves	16.7	0.3	18.4	3.9	2.1	2.5	214	53	19	11.8
	Pods	13.6	0.4	15.4	0.6	0.4	-	198	43	11	3.4
	Seeds	15.6	0.3	3.4	1.9	3.8	4.9	131	29	40	11.0
Sesbania:
	Small twigs with leaves	13.0	1.2	21.3	2.8	2.1	2.3	274	165	41	9.5
	First ten leaves	10.9	1.2	23.4	2.7	2.1	2.3	285	200	48	8.1
	Pods with seeds	19.9	1.0	6.3	1.2	2.6	1.7	109	45	37	5.2
	Pods alone	12.8	1.1	12.0	0.9	1.1	1.2	581	63	31	5.3
	Seeds alone	9.3	0.8	3.7	1.4	4.6	1.9	181	52	52	7.6
	Old and young leaves	13.2	1.0	28.3	4.9	2.4	3.2	280	410	62	6.4
Acacia brevispica		16.9	1.04	7.7	2.9	2.0	3.4	151	137	46	6.9
Acacia nilotica		8.1	0.5	18.0	1.9	2.2	2.3	664	66	31	9.0

* Taken from the low-mid altitude southern Ethiopian rangelands.

All the pasture grasses and crop residues studied had very low crude protein contents which is characteristic of mature tropical forages whose protein content declines rapidly following the rapid growth of the rainy season. Those figures were all below the level of 7.5% considered to be required for optimum rumen function (van Soest, 1982).

For reasonable levels of production, animals subsisting on these forages will therefore require supplementary protein which may be in the form of oilseed cakes and/or NPN sources such as urea which could be easily obtained locally. While use of such sources is practicable on smallholder units, it is not easy to do so for cattle on range in pastoral areas. In such situations it is possible to achieve some success through the use of high protein browse and tree crops such as Sesbania, Leucaena, Gliricidia and Acacia species (Table 5). On smallholdings in the highlands, where feeding hay is practised, effort could be made to improve the quality of the hay by practising early cutting, although the problem here is to compromise quantity with quality. Since hay is now being marketed in Ethiopia, a degree of quality control could be achieved if analyses were carried out and a system of price premium for good quality was introduced. The high levels of ADF ash in most of the grasses indicates the presence of large amounts of silica which may seriously reduce digestibility (van Soest, 1982).

In relation to the essential mineral elements, the level of K in all the forages was above the level of 8 g/kg recommended for grazing animals (Underwood, 1981). It has, however, been suggested (McDowell, 1985) that weaned calves and high producing dairy cows under stress, such as heat stress, may require K level above 10 g/kg, but since only the straws of barley, teff and linseed approached this figure, it seems most unlikely that problems of K deficiency are likely to arise. The same conclusion appears valid for S, Fe and Mn, the contents of which in almost all the forages studied were above the levels of 1 g/kg, 50 mg/kg and 40 mg/kg respectively proposed as adequate for grazing animals (McDowell, 1985). The requirement for S is basically related to that for N and S-containing amino acids, such that the ARC (1980) suggested that an N:S ratio of 14:1 is indicative of adequacy of S; in this regard, all of the materials examined are satisfactory.

With reference to Na, there is some debate in the literature concerning the dietary concentration required. While Underwood (1981) recommended 1 g/kg for most grazing animals, the findings of Morris (1980) and Little (1987) indicate that 0.7 g/kg is adequate for non-lactating cattle, and the ARC (1980) suggested that a level of 1.4 g/kg is required by sheep. The present data show that with the exception of Sesbania, most of the materials examined were very poor sources of Na, such that routine supplementation is likely to be necessary. This characteristic of Sesbania has been recorded elsewhere in the tropics (Little, 1986).

While dietary Ca concentrations of 2-6 g/kg, with higher requirements for lactation have been variously recommended for cattle and sheep (NRC, 1978, 1984, 1985: ARC, 1980), the findings of Sykes and Field (1972) suggest that levels of 2.53.0 g/kg are adequate in most circumstances. The results indicate that problems of Ca deficiency would not be expected. It is still widely believed that a large excess of Ca over P. resulting in Ca:P ratios approximating 10 or more, is deleterious to ruminants, although much conflicting evidence occurs in the literature (reviewed by Little, 1970). In this context it is noteworthy that the ARC (1980) concluded that "...it is not possible to state the optimal ratio of calcium to phosphorus for animal performance or whether such a ratio actually exists." Where wide ratios occur, the dietary concentration of P per se is almost certain to be inadequate.

Underwood (1981) considered a dietary P level of 1.7 g/kg to be marginal for grazing animals, in essential agreement with work of Little (1980, 1985) which indicated that a figure of 1.4 g/kg should be regarded as minimal for growing cattle. The data in Tables 2 and 4 show that most grasses and crop residues examined were marginal to deficient in P. and supplementation with P is likely to be beneficial. For animals grazing on range, this would best be supplied through incorporation into complete mineral licks or in mixtures with common salt and bone meal. Animals on smallholder units, which may receive some concentrate feeds, may receive adequate P if offered high protein oilseed cakes such as cotton seed cake and noug cake.

McDowell et al (1978) considered 30 mg/kg to be a critical level of dietary Zn, although the ARC (1980) has suggested that concentrations of 12-20 mg/kg are adequate for growing cattle. The crop residues may thus constitute a marginal supply of Zn (Table 4); the necessity for supplementary Zn needs to be kept under review particularly for sheep, which require some 35 mg Zn/kg diet (ARC, 1980).

It is commonly accepted that the dietary requirement of cattle for Cu lies in the range 8-14 mg/kg (ARC, 1980; NRC, 1978, 1984). Clearly most of the materials examined will provide a marginal to deficient supply of this mineral. This situation may be even further complicated by high levels of dietary Fe which can be elevated by soil ingestion during grazing. Humpries et al. (1981) showed that dietary concentrations exceeding 1 g Fe/kg can profoundly reduce the availability of ingested Cu; relatively slight and doubtless common levels of dietary soil contamination can produce Fe concentrations of this order (Healy, 1973), necessitating the serious consideration of - supplying supplementary Cu. either by injectable preparations, oral dosing with copper oxide needles or providing mineral licks containing Cu. In this context it should be remembered that the Cu requirement of sheep is only about 5 mg/kg however, so that the provision of supplementary Cu to this species is much less likely to be required.

It should be stressed that data of the type presented here can provide only an indication of the existence of potential mineral deficiency problems, since animal selectivity usually results in the consumption of material of somewhat higher quality than that of the total available, and conclusive diagnosis must be based on the occurrence of a positive response to supplementary supply of the mineral in question. However, such data are vital in the formulation of critical supplementation experiments, upon which recommendations for practical supplementation regimes should be based.

Conclusion

The pasture and crop residues examined had very low crude protein contents, and for increased levels of animal production the primary need is for supplementary protein. The most appropriate means of providing this will vary with local circumstances; protein-rich oilseed cakes or urea-containing blocks are valuable sources where they are available. But there is great potential for encouraging the use of leguminous trees, as well as the establishment of forage legumes as crops or oversown in the pasture.

The minerals most widely present in inadequate amounts are Na, Cu and P. and supplementation regimes involving these elements are very likely to produce beneficial results. There is an urgent need for appropriate experimentation so that soundly-based supplementation packages can be devised.

References

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