Influence of rainfall pattern on grass/legume composition and nutritive value of natural pasture in Bale Highlands of Ethiopia

Solomon Bogale, Solomon Melaku* and Alemu Yami**

Sinana Agricultural Research Center, PO Box 208, Bale Robe, Ethiopia
^*Department of Animal Sciences, Haramaya University, PO Box 138, Dire Dawa, Ethiopia
^**Debre Zeit Agricultural Research Center,PO Box 32, Debre Zeit, Ethiopia
solmelay@yahoo.com

Abstract

The study was conducted in Sinana Dinsho district of Bale highlands, Ethiopia. The objectives of the study were to assess the effect of moisture availability or pattern of rainfall on grass/legume composition and nutritive value of natural pasture in the district.  

Grass species dominated the natural pasture of the study area. Peak proportion of legume species was observed in September and October in both Sinana and Dinsho sub- districts. The proportion of grasses increased (P<0.05) with maturity of pasture on the grazing lands and a decline in rainfall from the wet to the dry season, whereas the proportion of legumes were higher (P<0.05) in the natural pasture early in the rainy season. The CP and ash contents of natural pasture were higher (P<0.05) in Dinsho sub- district than in Sinana, while IVDMD and fiber contents were similar (P>0.05) for the natural pasture in the two sub-districts. Annual metabolizable energy (ME) (P<0.001) and digestible crude protein (DCP) (P<0.001) supplied by forage from grazing lands was higher in Dinsho sub- district compared to Sinana sub- district. In vitro DM digestibility, CP, and ash contents decreased with advance in the rainy season, while the contrary was true for the contents of cell wall fibre and lignin.  

Therefore, it was concluded that delayed harvesting of natural pasture for hay making results in reduced nutritive value.

Key words:  chemical composition, grass, legume, natural pasture

Introduction

The highlands of Ethiopia are characterized by crop-livestock mixed farming systems. Nearly 90% of the total human population and 70% of the livestock population of the country inhabit in the highlands (Saleem and Tedla 1995). Despite enormous contribution of livestock to the livelihood of farmers in the highlands, they are faced with multifaceted problems in the production system, among which the major one is the quantitative and qualitative inadequacy of feed supply. Indeed, insufficient and poor quality feed, particularly during the dry periods was reported to be one of the most important constraints to Ethiopian livestock feeding systems (Anderson 1987). Similar studies (Bediye and Sileshi 1995; Sileshi et al 1995) reported feed resources to be the major bottleneck to livestock production in the highlands of Ethiopia, where natural pastures and crop residues are the major sources of feed. Saleem and Tedla (1995) also reiterated that herbage from natural pasture are usually inadequate quantitatively and qualitatively to support reasonable livestock production.  

The importance of natural pasture as a source of feed is mainly restricted to the wet season (Kusekwa et al 1990; Sileshi et al 1995). The availability of feed resources in the Ethiopian highlands is also a function of the quantity and distribution of rainfall, and these restrict the reliability of natural pasture as a feed source only to the wet season, although limited supplies of pasture are obtainable during the dry season on unusual patches of land and along riverbanks. The quality of herbage obtained from natural pasture depends on the composition of the plant species, with high proportion of legumes usually accompanied with better nutritive value (Gebremeskel 1993). However, the proportion of legumes or grasses depends on the period of harvesting in the growing season, and thus, late harvesting is usually associated with lower proportion of legumes in the natural pasture and a concomitant reduction in nutritive value. Moreover, continuous exposure of natural pastures to grazing by animals has an impact on the photosynthetic area of plants, species composition and soil compaction (Stoddart et al 1975), which usually lead to the disappearance of the legume components. In the highlands of Bale, the expansion of crop production, particularly the shift to cereal mono-cropping was reported (ICRA 2001). Such developments can have a direct impact on livestock production through shrinking the size of grazing land and/or by marginalizing the quality of feed for livestock. The objective of this study was therefore, to assess the grass/legume composition and nutritive value of natural pastures and recommend appropriate time for harvesting of forage for hay making in Sinana Dinsho district of Bale highlands, Ethiopia.  

Materials and methods 

Description of the study area 

The study was conducted in Sinana Dinsho district of Bale highlands, Southeastern Ethiopia in 2003/2004. The altitude of the district ranges from 2000 to 3600 masl with higher elevations in Dinsho sub- district. There are two major soil types in the district. The Sinana sub- district is characterized by chromic and pellic vertisols. These soil types usually occur on plains and are favorable for production of various crops. On the other hand, dystric and humic cambisols are dominant at high altitudes and on steep slopes in high rainfall areas, particularly in Dinsho sub- district (SARC 2001).  

The minimum and maximum temperature in Dinsho sub- district is 2^oC and 20^oC, respectively (Williams 2002). Temperature tends to be low with a high probability of frosts during the night time, particularly at higher altitudes of Dinsho sub- district, which is characterized with a mono modal rainfall pattern extending for eight months from late March to October. The greater bulk of the rain falls in April and then from August to October (Williams 2002), whereas precipitation from May to July is usually not very intense. The rainy season is followed by four months of dry season extending from November to February. According to Williams (2002), the lower altitudes of Dinsho sub- district receive between 600 and 1000 mm rainfall annually, whereas the higher altitude areas receive up to 1200 mm rainfall annually. The minimum and maximum temperatures are 9.4 ^oC and 21.2 ^oC, respectively in Sinana sub- district (SARC 2001). The rainfall pattern is bimodal and the main rainy season extends from August to December and the short rainy season stretches from March to July. The precipitation during the main rainy season ranges from 270 to 560 mm and during the short rainy season it ranges from 250 to 560 mm. The dry season in Sinana sub- district covers from December to March. 

Selection of the study area and sampling techniques 

Sinana Dinsho district was selected because it is among the areas known to have high potential for livestock production. The district was first stratified into two ‘recommendation domains’ based on the rainfall pattern and cropping system (ICRA 2001). Thereafter, a total of eight sites, five from Sinana and three from Dinsho sub-districts were purposively selected based on population density and accessibility. 

Determination of grass/legume composition 

Assessment of grass-legume composition was conducted on seasonally closed communal and private grazing lands as well as culturally reserved areas using a 1´1 m quadrat randomly placed in sampling paddocks at representative sites. Each sampling site was divided into three randomly selected sub-sites. Four herbage samples from each sub-site were randomly collected. Herbages within the quadrat were cut at ground level, weighed, and separated into grass and legume components. Dry matter (DM) of herbage was determined by drying the samples in an oven at 105^oC for 24 hour. Grass-legume composition was calculated following the method described by Tothil et al (1978); 

Where:

T_fw  = total fresh weight

S_fw  = sub- sample fresh weight

S_dw  =  sub- sample dry weight

T_dw  =  total dry weight

G_tdw = grand total dry weight

Chemical composition and in vitro digestibility of feed samples 

Representative feed samples were collected during subsequent visits and stratified based on type, location and month of harvest. Feed samples were oven-dried and ground to pass a one millimetre sieve size, and analysed for DM, CP and ash content (AOAC 1990). Neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) were determined according to Van Soest et al (1991). In vitro DM digestibility (IVDMD) was determined by the Tilley and Terry method as modified by Van Soest and Robertson (1985). Metabolizable energy (ME) and digestible crude protein (DCP) content were estimated using the formulae;

   (AAC 1990)

   (Church and Pond 1982)

Data analysis 

The data were analysed using Minitab, version 12.0 (Minitab 1998). Means were separated by Tukey pair-wise comparison procedure. The following statistical model was used for data analysis.

Y_ij = m + S_i + L_j + e_ij ,

where

Y_ij = quality and composition of natural pasture

m = overall mean

S_i = the effect of i^th season/month

L_j = the effect of j^th location

e_ij = random error
 

Results and discussion 

Grass-legume composition of natural pasture 

The native pasture from seasonally closed communal and private grazing lands as well as culturally reserved areas had higher (P<0.05) legume proportion in Dinsho than in Sinana sub- district (Table 1).

This indicated that the natural pasture in Dinsho sub- district to be of a better quality that may satisfy nutrient requirements of animals for production purposes, particularly from July to November when the proportion of legumes in the natural pasture is at its peak. In Dinsho sub- district, the overall proportion of legumes in the natural pasture was almost comparable from July to October and declined at the end of the rainy season, and in December, the proportion of legumes was lower (P<0.05) than in the previous months. Similarly, the proportion of legumes in the natural pasture was lower (P<0.05) in December than in October in Sinana sub- district. Such phenomenon could be attributed to the early maturing nature of legumes (Gebremeskel 1993; Denekew 2004) which results in their decline in pasture with reduction in precipitation. This has unfavourable effect on the chemical composition of the available herbage from natural pastures, with ultimate fall in its nutritive value. The overall results of this study is in agreement with FAO (2001) report that stated the proportion of grasses to be more dominant on  grazing lands in the central highlands of Ethiopia.

Chemical composition and in vitro dry matter digestibility 

Chemical composition and IVDMD in samples of natural pasture collected at different periods of the year in Sinana Dinsho district are given in Tables 2 and 3. Crude protein content of the herbage decreased (P<0.001) as harvesting period advanced in the rainy season with associated advance in maturity of plants, which is in agreement with other studies (Gebremeskel 1993; Denekew 2004).

This may be attributed to the reduction in the proportion of legume as well as the decrease in leaf to stem ratio leading to the reduction in CP content and IVDMD (McDonald et al 2002).  

Except for the months of December and January, feeds harvested from natural pasture in Sinana sub- district had CP content of more than 7% DM, whereas the CP content of the natural pasture was above 7% DM except for January in Dinsho sub- district. Therefore, the CP content of the natural pasture satisfies the minimum CP content of 7% DM (Van Soest 1982) for acceptable rumen microbial function as well as to satisfy the maintenance requirement of ruminants.

The study revealed the importance of harvesting natural pasture latest in November in Sinana sub- district, and in December in Dinsho sub- district when the CP content is still high to satisfy the requirements of rumen microbes for optimal activity (Van Soest 1982) as well as to replace endogenous losses of CP in animals. Delay in harvesting of natural pasture beyond this period resulted in CP content of less than 6% that may cause negative nitrogen balance due to protein malnutrition (Bondi 1987). Generally, natural pasture in Dinsho sub- district contained higher (P<0.05) CP than that in Sinana sub- district, and this could be attributed to the relatively high proportion of legumes in the natural pasture in Dinsho sub- district. It has been reported (Van Soest 1982) that high temperatures result in lower digestibility due to increased lignification of plant cell wall, and more rapid metabolic activity which decreases the pool size of metabolites in the cellular contents such as CP and soluble carbohydrate, and increases the structural cell wall components. Moreover, enzymatic activities associated with lignin biosynthesis are enhanced by increased temperature, whereas low temperature during plant growth allows plants to remain vegetative and less lignified (Van Soest 1982).  Therefore, the relatively higher environmental temperature in Sinana sub- district might have promoted rapid physiological development leading to the dilution of CP content and lignification of the cell wall fibers that reduced the digestibility of the natural pasture harvested in the sub- district as compared to that harvested in Dinsho sub- district.  

The NDF, ADF and cellulose contents increased with advance in harvesting period from May to January. However, variation in NDF content of natural pasture at different growing periods was not significant (P > 0.05), even if the fibre content at the end of the rainy season was higher. Generally, the NDF content of natural pasture in this study was high enough to limit DM intake and digestibility, since NDF content of more than 55% was reported to limit DM intake (Van Soest 1967). Moreover, feeds with more than 65% NDF content were classified as low quality roughages (Singh and Oosting 1992), and the natural pasture in the study area falls in this category. The increase in ADF content of natural pasture with advance in stage of harvesting agrees with the results of Gebremeskel (1993). Content of ADL exceeded 5% in the natural pasture at the end of rainy season, which is probably due to the positive correlation between lignification and plant maturity (McDonald et al 2002). This has a limiting effect on feed utilization since an increase in ADL content from 5-6% may reduce cellulose digestibility by 20% (McDonald et al 2002).  

In vitro DM digestibility of natural pasture was different (P < 0.05) across growing periods, whereby the digestibility at the early and mid of rainy season was higher than at the end of the rainy season (Table 2). The overall IVDMD reported in this study was higher than the IVDMD of hay from the central highlands of Ethiopia (Bediye and Sileshi 1989). The IVDMD value of natural pasture reached critical level at the beginning of the dry season in Dinsho, whereby it dropped to less than 50%, and this could have a limiting effect on feed intake (Van Soest 1982) that needs to be ameliorated by supplementation with a source of protein.  

Metabolizable energy content of natural pasture in this study was in agreement with the results of Keftasa (1988). Generally, the ME content of natural pasture decreased with advance in period of harvesting (Figures 1 and 2).

Metabolizable energy supplied by natural pasture from May to October was almost similar with a mean of 10.2 MJ/kg in Sinana sub- district and 10.4 MJ/kg in Dinsho sub- district. Standing hay had energy content of 7.9 MJ/kg DM in Sinana sub- district and 6 MJ/kg DM in Dinsho sub- district. It was reported that nitrogen, and not energy, is the first limiting nutrient for animal performance via its effect on intake and digestibility during the dry season (Preston and Leng 1984), and therefore, animals maintained on the natural pastures during this period in Sinana Dinsho district require supplementation for reasonable performance.

Content of DCP of natural pasture was higher at the beginning and mid of the rainy season, and declined with progress in the rainy season. The DCP content was higher for natural pasture in Dinsho sub- district as compared to Sinana sub- district, and this may be due to differences in climate and soil fertility. 
 

Conclusions 

• The quality of feed obtained from natural pasture declined with advance in the rainy season.
  
  
• High proportion of legumes in the natural pasture of the study area was observed during the early rainy season.
  
  
• Therefore, delayed harvesting of natural pasture for hay reduces its feeding value.
   

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Received 1 February 2007; Accepted 2 December 2007; Published 1 March 2008

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