Feed utilization, digestibility and carcass parameters of Tigray highland sheep fed urea treated wheat straw supplemented with mixtures of wheat bran and noug seed cake, in Southern Tigray, Ethiopia

Gebremeskel Gebretsadik and Kefelegn Kebede

Haramaya University, School of Animal and Range Sciences, Dire Dawa-Ethiopia
gere3582@yahoo.com

Abstract

The research was conducted in Maichew Agricultural Technical Vocational Education and Training College, located at 120 km distance south from Mekele, Ethiopia. The objectives of the study were to assess the impact of concentrate mixture on feed intake, digestibility and to evaluate the different treatment levels on carcass parameters of growing lambs fed on urea treated wheat (Triticuln astivum) straw as basal diet. Twenty male yearling growing lambs with an average live weight of 17 ± 1.6 kg were used. The treatment diets, namely, S0, S150, S200, and S250 g DM/head/day concentrate mixture were assigned randomly with a basal diet, urea treated wheat straw (UTWS). The experimental design was a randomized complete design with four treatments and five replications. For the analysis of carcass parameters slaughter weight was added as a covariate in the model.

The results showed that intake of UTWS was significantly depressed (p<0.05), whereas total DM intake, CP intake and digestibility of CP, DM and OM was significantly higher (p<0.05) for growing lambs supplemented with the highest level of concentrate mixture than the control treatment. The average values of hot carcass weight and dressing percentage on slaughter weight base were significantly (p<0.05) higher for S250 and S200 as compared to S0 and S150. In this experiment supplementation had seen to improve feed intake, digestibility, as well as the composition of carcass parameters. 

Key words: Carcass parameters, digestibility, feed intake, feeding trial, urea treated wheat straw

Introduction

The estimated livestock population of Ethiopia is 47.57 million cattle, 26.12 million sheep and 21.7 million goats (CSA 2008), believed to be the largest livestock population inventory in Africa. Despite the huge livestock resources of the country, the overall contribution of the livestock subsector achieved so far has been meager. Generally, the agricultural sector in Ethiopia, engaging 85% of the population, contributes 52% to the gross domestic product (GDP) and 90% to the foreign exchange earnings (CSA 2008). According to MoARD (2007) the livestock sector accounts for 16% of the national GDP and 27–30% of the agricultural GDP and 13% of the country’s export earning, which is comparatively low. Among the limiting factors poor feed supply and feeding systems are the most important. McDonald et al (2002) confirmed that all the straws and related by-products are extremely fibrous, most have a high content of lignin and all are of low nutritive value. Their high fiber content restricts their use as feed for ruminants. In addition to this, most dry forages and roughages found in Ethiopia have a crude protein content of less than 7% which indicates microbial requirement can hardly be satisfied unless supplemented with protein rich feeds (Vansoest 1994). When such feeds are fed alone, they are unable to provide even the net maintenance requirement of livestock (FAO 1999). 

Moreover, the rapid increase in human population puts a pressure on the land for crop production; resulting in less and less land available for grazing and leading to an increase feed shortage (Gemeda et al 2003).  Under such conditions, livestock meat demand by consumers cannot be satisfied and the current high price for meat cannot be maintained with the ever-increasing human population (Woldu et al 2004). Besides, the relatively huge number of livestock resources, proximity to the export markets in neighboring countries as well as the Middle East and the liberalization of the economy, diverse agro-ecologies, increasing number of export abattoirs, the expansion of agro-industries and the increase of by-product feeds give the country comparative advantage in the trading of livestock and their products (Belachew and Jemal 2003). Hence, the livestock sector has to look for mechanisms to balance the demand & supply of meat. In this regard small ruminants can provide an opportunity being they are small in size, have high reproduction rate, and their ability to thrive on locally available and low quality feeds. Likewise, small ruminants play a significant role in the rural families as they provide both meat and milk as a source of energy and protein for human consumption (ILRI 2000). Similarly feeding urea treated straw to ruminants had improved intake and digestibility of straws; however, such improvements can only support a little more than maintenance requirements of the animal Smith et al (1989) and Manyuchi et al (1994). Hence, further improvement in animal performance can be achieved when urea treated straw is supplemented with dietary proteins, energy and vitamin sources. Therefore, the objective of this experiment was to assess the impact of the mixture on feed intake, digestibility and to evaluate the different treatment levels on carcass parameters of Tigray highland sheep fed on UTWS as basal diet. 

Materials and Methods

Study site

The study was conducted in Maichew Agricultural Technical Vocational Education and Training College, Tigray, Ethiopia; situated at 12⁰ 47’N latitude and 39⁰ 32’E longitude, with an altitude between 2396 and 2472 m.a.sl. It receives average annual rainfall of 759.8 mm and the mean annual temperature was 17.6^oC. 

Experimental Feed Preparation

According to Ibrahim and Schiere (1989) a solution of 4 kg of urea in 80 liters of water was prepared to treat 100 kg air dried wheat straw.  A trench was prepared and covered with polyethylene sheet on the floor and all the four sides. The solution was sprayed thoroughly to layers of wheat straw, rubbed with hand to ensure proper penetration and trampled with foot to ensure proper packing. After filling the trench with urea treated straw, it was covered with plastic sheet, and covered with soil to create a hermetic sealing and left to incubate for 21 days. 

Experimental Sheep

Twenty yearling highland growing lambs weighing 17 + 1.6 kg were purchased from local markets based on their dentition and information from the owners. The animals were drenched with a broad spectrum (Albendazol) drug against internal parasites and sprayed (Accaricide or Ectoparasite) against external parasites. They were vaccinated against common diseases (anthrax and pasteurelosis) during the quarantine period. Then the experimental animals were housed in individual pens and offered UTWS and the supplements for 21days to get them adapted to the feeds prior to the beginning of the experiment.

Experimental Design and Treatments

The experiment was conducted using a complete randomized design with 4 treatments and 5 replications. Treatment diets were randomly assigned to each animal in the treatment in such a way that each animal had equal chance of receiving one of the treatment diets. The experimental animals were supplied UTWS, water, and salt blocks comprising sodium chloride ad libitum daily.  The concentrate mixture was offered at the graded level of 0 g (S0 = no supplement), 150 g (S150), 200 g (S200) and 250 g (S250) /head/day on DM bases at 0800 h and 1600 h. Daily offer and refusal of UTWS and concentrate mixture were recorded for each animal. Representative samples were then taken for the feed offer every morning before feeding, placed in deep freezer to minimize loss of ammonia until a sub-sample was taken for analysis. Treatment refusals were pooled over the experimental period and sub-sampled for analysis. Supplement and basal feed offers and refusals were weighed for each animal daily and their differences were recorded as a daily feed intake per animal. Feed conversion ratio was calculated by dividing the daily total DM intake by daily live weight gain.  

Measurements

Chemical Analysis

Representative samples of daily feed offers, refusals and fecal output were ground to pass through a 1mm sieve screen size using a laboratory mill and analyzed based on index no. 934.01 for DM, index no. 984.13 for nitrogen,  index no. 942.05 for organic matter (OM) and ash (AOAC 1990). The neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) were analyzed following the procedure of (Van Soest et al 1991). 

Statistical Analysis

The data obtained from the experiment were analyzed using PROC GLM (SAS 2005) with treatments as the main effect in the model. Treatment means of the parameters were separated using Tukey honestly significant difference test. The statistical model used for the analysis of feed intake and digestibility parameters was:

g_i  = µ + a_i + e_i;  where: g_i  is response variable, µ is overall mean, a_i is the  i^th treatment effect and  e_i  is the i^th random error

For the analysis of carcass parameters the slaughter weight (kg) was added as a covariate in the above statistical model and was kept if it reached significance. 

Feed intake

Daily feed intake of individual sheep was recorded as a difference between the feed offered and the refusal. The substitution rate was calculated by the equation adapted from (Ponnampalam et al 2004) as given below:

Digestibility trials

The digestibility trial was conducted after 21 days of adaptation period to the treatment feeds. Total collection of feces was undertaken for 7 consecutive days after 3 days of adapting the growing lambs to the carrying of fecal bags. Feces were collected and weighed every morning for each animal before feed offer. About 10% of the daily fecal excretion of each growing lamb was sampled and kept in air tight plastic containers and stored at -20 ^OC.  At the end of the collection period, the fecal sample for each animal was thoroughly mixed and sub-samples were taken to determine the chemical composition of the feces. The apparent digestibility coefficient of feed nutrients was determined using the following equation: 

Carcass parameters

At the end of the feeding and digestibility trial, all experimental animals from each feeding treatment were fasted overnight, weighed and slaughtered for carcass evaluation. The weights of the edible and inedible components of the carcass were measured and recorded for each growing lamb. The empty body weight was calculated by subtracting the weight of the alimentary tract contents from the live weight at slaughter. The cross-section of the rib-eye muscle was traced on transparency paper between the 11^th and 12^th ribs (Galal et al 1979) of the left half side of the carcass after freezing overnight and the area was measured. The rib-eye area was taken as the mean of the two sides of the ribs. Back fat thickness was measured over the rib-eye muscle on the 12^th rib using calibrated ruler.  Total edible offal component (TEOC) was taken as the sum of lung with trachea and esophagus, liver with gall bladder, empty gut, visceral fat (kidney fat + omental fat), kidneys, tongue and tail.  Total non-edible offal component (TNEOC) was computed as the sum of blood, spleen and pancreas, head, skin, testis and penis, total gut fill and feet.  Total usable product (TUP) was taken as the sum of hot carcass weight, TEOC and skin. 

Correlation

In the experiment different parameters were correlated using Pearson or product-moment correlation. 

Results and Discussion

Chemical composition of experimental feeds 

The CP content of UTWS due to treatment was nearly doubled in percentage units from 3.0% when untreated to 5.98% and tended to decrease NDF from 81.3% to 75.6% (Table1). The cell wall components of the straw were also affected by urea treatment. In this study the ADF and ADL contents of straw were slightly decreased from 50.2 to 48.4 and 5.6 to 4.58 respectively due to urea treatment. The UTWS refusals contained lower CP and higher ADL than in the UTWS offer in all treatments. The dry matter content of UTWS offered was almost comparable to the values of 69.9% reported by (Getahun 2006). The increment of CP due to urea treatment was observed to improve the nutrient content of wheat straw. Similar changes in CP and NDF of teff straw following urea treatment have been reported by (Awet 2007). However, the CP content of UTWS observed in this study was lower than that of previously reported by Rehrahie and Ledin (2001). The lower CP content of UTWS in the current study could be due to volatile nitrogen loss while ventilating the silo and during drying for analysis. According to SundstØl and Coxworth (1984) and Chenost (1995) up to two-third of the ammonia generated is usually evaporated to the environment in the process of urea straw treatment and until feeding to the animals; moreover, large increase in CP contents doesn’t necessarily indicate good urea treatment.  

The cell wall components of the straw were also affected by urea treatment. In this study the ADF and ADL contents of straw were slightly decreased and the NDF part of the straw was most affected by urea treatment probably hemicellulose is most sensitive to delignification. This brings about the advantage of the increased fermentation of cell wall components in the rumen which in turn probably enhance the amount of rumen fermentable sugars (Givens et al 1988). Similarly (Awet 2007) reported decreased values of ADF and ADL for urea treated teff straw. Contrary to the current finding Rehrahie and Ledin (2001) reported increased values of ADF and ADL for urea treated barley and teff straws. The CP content of wheat bran in this study was comparable to the values reported by Awet (2007) and Tesfay (2007) but lower than the values reported by Getnet et al (1999) and Alemu (1981). The variation might be due to the effect of processing in milling industries and the quality of the original grain used in the milling industries. The CP content of the noug seed cake was also in agreement with the findings of Seyoum and Zinash (1989) and Tesfay (2007). The NDF and ADF of NSC in this study were also comparable to the values reported by (Tesfay 2007) but lower than the NDF and ADF contents reported by (Alemu 1981). Generally, the need to use urea treated straw and concentrate supplement can be justified as, wheat straw had high cell wall contents that is usually taken as a negative index of feed quality (Van Soest 1994) and NDF content above 550 g/kg DM can limit DM intake (Van Soest 1967). Digestibility also decreases with increased NDF content and increased lignifications (McDonald et al 2002). Similarly, the treated UTWS had crude protein content of less than 7% which indicates microbial requirement can hardly be satisfied unless supplemented with protein rich feeds (Van Soest 1994). 

Feed intake

From the result a significant difference (p<0.05) was observed on the DM and OM intake of UTWS among treatments (Table 2). As the level of supplementation increased, there was a depression of UTWS intake, DM intake and OM intake. The CP intake among the different treatment groups was significantly different (p< 0.05). There was an increasing trend of CP intake as the level of concentrate mixture increases, the highest being in S250 and lowest in S0.  

Similar results on DM intake and OM intake were reported by (Bonsi et al 1996). The study indicated an increment of 30.6% for the intake of total OM when sheep were fed on teff straw basal diet supplemented with cotton seed cake. Sheep in S0 and S150 had relatively consumed more DM and OM of UTWS as compared to S250, might be due to the relatively low CP and high NDF content (which limit intake due to fill effect) of the UTWS used in the experiment. This also indicated that, there was a substitution effect of supplement at the expense of intake of UTWS. The apparent substitution rate was 0.11, 0.22, and 0.22 for S150, S200 and S250, respectively. Type and amount of supplement can affect substitution rate and it has been generally found that substitution rate increases as the level of supplement increases (Ponnampalam et al 2004). The total DM intake per unit metabolic body weight of growing lambs in the study was within the range of values reported by (Bonsi et al 1996) for Ethiopian sheep. The CP intake of the experiment was comparable with Bonsi et al (1996) and Tesfay (2007). The increased CP intake can be explained by the increased total DM intake and higher CP content of the concentrate mix than the basal diet. The higher content of NDF and ADF in the treated straw could be the major cause for high levels of NDF and ADF intake in all growing lambs. Moreover, the non significant (p>0.05) content of  NDF and ADF intake in both, supplemented and unsupplemented treatments might be explained as, the potentially digestible substances in the supplemented treatments with concentrate mix could be escaped from microbial action (rumen) which would be digested by animal enzyme systems in the lower tract (Zhang et al 1994).

Feed utilization

The control treatment had significantly higher (p<0.05) feed conversion ratio than the supplemented treatments. From the figure below as the CP intake increased, the DM intake and LWG also increased linearly, this might explain CP intake is the main factor determining DM intake and live weight gain (LWG). Moreover, improved feed conversion efficiency could be due to high nutrient concentration of the supplement which leads to increased live weight gain Kefelegn and Gebremeskel (2010). From the experiment growing lambs in S250 were efficient in the utilization of nutrients for their live weight gain. Similarly, (Awet 2007) reported that there was a linear increment of feed utilization efficiency with the level of supplementation.

Figure 2. The trend in live weight gain of growing lambs fed on UTWS based diet and supplemented with graded levels of concentrate mixture.	Figure 3. The trend in DM feed conversion of growing lambs fed on UTWS based diet and supplemented with graded levels of concentrate mixture.

The proportion of CP in diet DM was highly correlated with DMI, LWG and FCR (R² = 0.99, 0.70 and 0.99, respectively)

Digestibility trial

Apparent digestibility of DM and OM of the basal diet, UTWS, was significantly increased (p<0.05) due to supplementation (Table 4).In agreement to the results of this study, Khanal et al (1999) reported that there were 18.1% and 13.3% increment in apparent DM digestibility for urea treated rice and wheat straw respectively. This might be due to UTWS has improved the apparent digestibility of DM. The apparent digestibility of CP was increased due to the high total CP intake of the supplemented growing lambs. According to (McDonald et al 2002) any increase in protein intake may result in an increase in apparent digestibility of CP, especially if the intake is marginally sufficient in protein; however, the apparent digestibility of NDF and ADF was not affected. This is also in agreement with (Tesfay 2007) who reported supplementation had little or no effect on the digestibility of NDF and ADF. 

Correlation between nutrient intake and digestibility

The result of the correlation analysis indicated that DM intake was positively correlated with DM digestibility (R² = 0.66), OM intake (R² = 0.99), OM digestibility (R² = 0.70), CP intake (R² = 0.96), CP digestibility (R² = 0.86) and NDF intake (R² = 0.53). 

Carcass parameters

Hot carcass weight, dressing percentage and empty body weight

The average values of hot carcass weight and dressing percentage on slaughter weight base were significantly (p<0.05) higher for S250 and S200 as compared to S0 and S150 (Table 5). Similarly, dressing percentage on empty body weight base was significantly (p<0.05) higher for S250 as compared to S0 and S150. However, there were no significant differences among treatments in their rib eye area and empty body weight contents. 

The results observed in this study were similar to Sandros (1993) who reported grazing lambs supplemented with concentrate had significantly higher hot carcass weight, and dressing percentage than the non-supplemented lambs. Dressing percentage values on the empty body weight basis were higher than on live weight at slaughter basis, implying the influence of digesta (gut fill) on dressing percentage. From this point of view it is more meaningful to express dressing percentage as the proportion of empty body weight than live weight at slaughter base. Similarly, Gibbs and Ivings (1993) and  El-khidir et at (1998) reported that gut content constitutes a large portion of the body weight and contribute 4 - 14% of fasted live weight in sheep and goats fasted for about 24 hours before slaughter. It has been reported that the dressing carcass coefficient for sheep generally falls between 40% -50%, (Gatenby 1991) and increase with age. The rib-eye muscle area observed in this study were comparable to results reported by (Asnakew 2005) which ranges from 5.2 to 8.8cm² for Hararghe goats. Generally, growing lambs in the highest level of supplementation (S250) exhibited higher rib-eye area than the other treatment feeds.  

Edible and Non-edible offals of the carcass

Due to differences in testing and eating habit and social taboos of the people, what are salable and edible portions of the carcass in one area of the country may not be acceptable in other parts. Therefore, characterization of offals as edible and non-edible was made based on the eating habit of the people in the study area. In this study, the size of heart, liver with gallbladder, empty gut, tail and kidney were significantly (p<0.05) affected due to supplementation (Table 6). Kirton et al (1972) reported that live weight and nutritional status of the animals can affect the production efficiency of carcass offals. Similarly, there was a significant difference (p<0.05) due to supplementation on blood, spleen and pancreas, TNEOC kg and TNEOC% (Table 7). The total usable products were also significantly lower (p<0.05) for S0 and S150 as compared to S250. The lowest TUP was recorded on S0 and the highest on S250; which implies, by increasing the nutritional densities of the diet, it was possible to obtain heavy and fleshy carcasses (Alexandre et al 2010).  

Correlation between live weight at slaughter, dressing percentage based on live weight at slaughter, rib-eye area and offal components

The correlation analysis between carcass parameters of growing lambs revealed that live weight at slaughter was positively and significantly (p<0.05) correlated with empty body weight (R² = 0.85), hot carcass weight (R² = 0.96), rib eye area (R² = 0.81); liver (R² = 0.59), heart (R² = 0.71),  kidney (R² = 0.62) and dressing percentage on live weight at slaughter base (R² = 0.50).  

Conclusions

Generally, the present study indicated that supplementation of growing lambs with graded levels of concentrate mixture on UTWS had improved feed intake, digestibility, and carcass parameters. Moreover, it was concluded that supplementation of 250 g DM concentrate mix resulted in better feed intake, and carcass traits in UTWS based feeding of growing lambs compared to other supplementations and could be recommended.

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Received 25 October 2010; Accepted 24 July 2011; Published 1 September 2011

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