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EFFECT OF ENERGY LEVEL IN LUCERNE (MEDICAGO SATIVA) HAY-BASED FINISHING DIETS ON CARCASS CHARACTERISTICS OF DORPER LAMBS

 

V.N. Shivambu#, J.H. Hoon, W.J. Olivier & B.R. King

Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (E.C), 5900

#E-mail: Vongani Shivambu

 

 

INTRODUCTION

Mutton is one of the major sources of red meat in South Africa. Sheep production mainly depends on natural grazing, which alone is not always sufficient for optimising live weight gain. Feedlotting is therefore a common practice on most sheep farms. However, different opinions exist with regard to the ratio of roughage to concentrate in sheep feedlot diets. The cost and availability of raw materials, such as lucerne hay and maize, often dictate the decisions of producers with regard to the inclusion rate of roughages and concentrates. Optimum growth can be obtained with appropriate combinations of concentrates and roughages in the diet. Feeding high concentrate diets to young animals has been shown to increase growth rate, carcass yield, carcass quality, etc. (Mahajan et al., 1976; Van Soest, 1982; Kochapakdee et al., 1994; Ferreira et al., 2002; Haddad & Husein, 2004; Abdullah & Hussein, 2007; Jabbar & Anjum, 2008; Shivambu et al., 2011). However, some degree of roughage inclusion is required in feedlot diets to maintain normal ruminal function. Inadequate roughage levels can lead to increased incidents of acidosis, liver abscesses and erratic feed intake, resulting in decreased animal performance and profitability (Cranston et al., 2005). Lucerne hay plays an integral role as a roughage component in finishing diets for ruminants in South Africa. However, it can be expensive and difficult to obtain in some years (Brand et al., 1991).

This article deals with the second in a series of three trials planned to determine the appropriate roughage to concentrate ratio in feedlot diets of different breeds of lambs for economical mutton production. Therefore, the aim of this study was to determine the most economical energy level in feedlot diets, leading to the increased and economical production of high quality meat from Dorper lambs.  

 

MATERIALS AND METHODS

Ninety weaned Dorper lambs were used in this study. At the start of the trial before the animals entered the facility, a standard animal health program was followed. The animals were fasted overnight, tagged with individually numbered ear tags and the initial body weights were recorded. The lambs were then divided on a stratified body weight basis into six groups of 15 animals each and placed in different pens where they were group fed.

Six experimental diets with different energy levels were used. Maize and lucerne hay were used as the base ingredients in all the mixtures. The respective diets were equivalent with regard to their crude protein content; urea and cottonseed oil cake meal were used to balance the protein. The ingredients and chemical composition of the experimental diets are summarised in Table 1. The animals were allowed a one-week adaptation period on the diets. During this period, the lambs were supplied with lucerne hay on an ad libitum basis, whilst the experimental diets were introduced gradually. Body weights of lambs were recorded at the commencement of the trial and on a weekly basis during the trial period. The lambs were given feed daily and the left over feed was weighed back and recorded weekly to determine the feed intake and feed conversion ratio on a group basis.

 

Table 1. Ingredients and chemical composition of the six experimental diets

Ingredients

Diet

20%

30%

40%

50%

60%

70%

Lucerne hay (%)

20.0

30.0

40.0

50.0

60.0

70.0

Maize meal (%)

62.1

54.6

46.1

38.1

30.0

20.5

Feed grade urea (%)

0.9

0.8

0.6

0.5

0.4

0.2

Cottonseed oil cake meal (%)

7.0

5.0

4.0

2.5

1.0

1.0

Molasses meal (%)

8.0

8.0

8.0

8.0

8.0

8.0

Feed lime (%)

1.1

0.8

0.6

0.6

0.3

0.0

Salt (%)

0.3

0.3

0.3

0.3

0.3

0.3

Acid buffer (%)

0.6

0.5

0.4

0.0

0.0

0.0

Premix added to the balanced diet (%)

0.15

0.15

0.15

0.15

0.15

0.15

Chemical composition (calculated) 

Crude protein (g/kg)

140.4

139.9

139.4

140.3

141.2

143.5

Energy ME (MJ/kg)

10.82

10.48

10.12

9.77

9.41

9.03

Fat (g/kg)

32.9

30.8

28.4

26.2

23.9

21.4

Crude fibre (g/kg)

90.2

114.9

140.6

165.8

191.1

217.8

Calcium (g/kg)

8.8

8.6

8.7

8.6

8.7

8.8

Phosphorus (g/kg)

2.8

2.6

2.5

2.4

2.3

2.3

 

The rib eye muscle area and the fat depth were recorded and measured at the start and the end of the trial (before slaughter), using an Aloko Prosound2 ultrasound scanner. As a specific group reached an average body weight of 39 kg, the animals were fasted overnight for slaughtering the following day. The animals were slaughtered at the Grootfontein abattoir using standard slaughtering techniques. Animals were weighed before slaughtering and the warm carcass weight, kidney fat and abdominal fat were recorded just after slaughtering. The carcasses were hung in a cooler at 2 °C for 48 hours. The cold carcass weight was recorded after 48 hours and the following carcass and fat measurements were taken:

 

Carcasses were graded (A1-A6) according to the South African Standard Chart classification system (Agricultural Product Standards Act; Act No. 119 of 1990; Government Notice No. R. 1948, 26 June 1992). Carcass yield (dressing percentage) and growth rate (average daily gain) were calculated for each animal, while the feed intake and feed conversion ratio were calculated for each treatment group. Statistical analysis was done using the Proc GLM-procedure of SAS (SAS, 2009).

 

RESULTS AND DISCUSSION

Dietary energy level contributed significantly (P<0.05) to variation in weekly live body weights. During the first two weeks of feeding, no significant differences in body weights were observed among the groups. The body weight of the group receiving the diet with the 20% roughage inclusion level differed significantly from the other groups from week three onwards. The groups receiving diets with low roughage levels (20% and 30%) have shown an increase in body weight of approximately 4 kg per week during the last two weeks of the trial, whilst the animals on the high roughage diets showed an increase in body weight of approximately 2 kg per week during the same period.

The growth curves of the different treatment groups are illustrated in Figure 1. All the groups followed more or less the same trend with regard to their growth curves. This concurs with the trend observed in a study conducted by Shivambu et al. (2011) with Merino lambs. Lambs in the 50% and 70% roughage groups took two weeks extra to reach the predetermined slaughter weight of 39 kg when compared to lambs in the 20% roughage group.

 

 

Figure 1. Growth curves of the treatment groups at different roughage inclusion levels

 

Total feed intake and average daily gain, feed conversion ratio and days till slaughter of the six groups are summarised in Table 2.


Table 2. Total feed intake and average daily gain (± s.e.), feed conversion ratio and days till slaughter of the six groups

Roughage group

 

Feed intake DM (kg)

Average daily gain (g/day)

Feed conversion ratio

Days till slaughter

20%

909.5

336.54a ± 13.10

4.07

43

30%

993.9

271.90bc ± 13.54

4.86

51

40%

1016.2

282.30b ± 13.06

4.55

48

50%

1244.7

244.34cd ±13.07

5.39

56

60%

1008.5

260.75bcd ± 13.51

5.15

48

70%

1238.2

228.89d ± 13.05

5.69

56

a,b,c Values in columns with different superscripts differ significantly (P<0.05)

 

Dietary energy level influenced the feed conversion ratio (FCR) of the lambs. Animals in the groups that were fed the high energy diets (20%, 30% and 40%) had better FCR (4.07, 4.86 and 4.55 respectively) than those that were fed high roughage diets (50%, 60% and 70%). The FCR of the Dorper lambs in all the groups ranged between 4.07 and 5.69, while those in the first trial conducted on Merino lambs ranged between 5.8 and 8.9 (Shivambu et al., 2011). Dietary energy levels contributed significantly (P<0.05) to the variation in the average daily weight gain (ADG), with the 20% roughage group having a significantly higher ADG than the other groups and the 70% roughage group having a significantly lower ADG than the 20%, 30% and 40% groups.

Lambs in the group with the highest energy inclusion level (20% roughage group) had the highest ADG and those in the group with the lowest energy inclusion level (70% roughage group) had the lowest ADG. The results of this study concur with the results reported by Haddad (2005) that increasing the concentrate portion of fattening  diets increased ADG and improved feed efficiency as well as carcass characteristics of growing Baladi kids. A linear increase in ADG with an increasing level of dietary concentrates was further observed. Other researchers observed similar results in weight gain and ADG for sheep (Fluharty & McClure, 1977). These results are also in line with the findings of Gabrovska & Ganovski (1986), who reported that when two groups of female lambs were fed roughages and concentrates with different combinations (25:75 and 75:25) from the age of two weeks to one year, maximum weight gain was observed in the group fed 25% roughage. In the present study with Dorper lambs, the groups with the 50% and 70% roughage inclusion levels had the highest cumulative feed intake, poorer feed conversion ratios and took longer to reach the predetermined slaughter weight. The best results were obtained with the 20% roughage diet, compared to the 30% roughage diet with Merino lambs (Shivambu et al., 2011). The effects of diet on rib eye muscle area and on fat depth are summarised in Table 3.

 

Table 3. Effect of diet on rib eye muscle area and fat depth (± s.e.)

Roughage group

Fat depth

at start of trial

(cm)

Fat depth

at slaughter (cm)

Change

in

fat depth (cm)

Rib eye muscle area at start of trial (cm2)

Rib eye muscle area      at slaughter

(cm2)

Change

in eye muscle area (cm2)

20%

0.29 ± 0.02

0.57a ± 0.02

0.29a ± 0.02

5.73a ± 0.12

8.50ac ± 0.14

2.77a ± 0.15

30%

0.30 ± 0.02

0.48b ± 0.02

0.19b± 0.02

6.71b ± 0.12

8.73ab ± 0.14

1.99bd ± 0.15

40%

0.31 ± 0.02

0.59a ± 0.02

0.28a ± 0.02

6.14c ± 0.12

8.65ac ± 0.14

2.51ac ± 0.15

50%

0.29 ± 0.02

0.59a ± 0.02

0.31a ± 0.02

6.19c ± 0.12

8.32c ± 0.14

2.12bc ± 0.15

60%

0.31 ± 0.02

0.60a ± 0.02

0.29a ± 0.02

6.71b ± 0.12

9.06b ± 0.14

2.36ab ± 0.15

70%

0.26 ± 0.02

0.55a ± 0.02

0.29a ± 0.02

7.01b ± 0.12

8.57ac ± 0.14

1.63d ± 0.15

a,b,c,d Values in columns with different superscripts differ significantly (P<0.05)

 

Dietary energy level contributed significantly (P<0.05) to the increase in rib eye muscle area from the start until the end of the trial (before slaughter). With the exception of the 30% roughage group, the general trend observed with regard to the change in eye muscle area is a decrease with an increase in the roughage inclusion level. Except for the 30% roughage group, dietary energy level did, however, not have an effect on the change in fat depth from the start until the end of the trial.

The slaughter traits of the six groups are summarised in Table 4. Dietary energy levels contributed significantly (P<0.05) to variation in carcass weight, carcass yield, hind leg length (inside), carcass length, hind leg circumference, fat depth (1, 3, 4 and 5) and kidney fat. This concurs with the results obtained in a similar trial with Merino lambs (Shivambu et al., 2011). No specific trend in carcass yield was observed in Dorper lambs, whereas Shivambu et al. (2011) reported that carcass yield of Merino lambs increased with a decrease in the roughage inclusion level.  The effect of dietary energy levels on carcass classification of the Dorper lambs is summarised in Table 5.

 

Table 4. Slaughter traits (± s.e.) of the six groups

Trait

20%

30%

40%

50%

60%

70%

Slaughter weight (kg)

39.20 ± 1.08

38.99 ± 1.12

39.68 ± 1.08

39.79 ± 1.08

38.29 ± 1.12

38.72 ± 1.08

Carcass weight (kg)

18.24± 0.24

17.55b ± 0.25

17.47b ± 0.24

18.06ab ± 0.24

18.37a ± 0.25

18.07ab ± 0.24

Carcass yield (%)

48.44ab ± 0.56

48.14ab ± 0.58

47.02a ± 0.56

48.36ab ± 0.56

48.74b ± 0.58

48.46ab ± 0.56

Hind leg length inside (cm)

33.47± 0.51

31.19b ± 0.52

31.64b ± 0.51

31.41± 0.51

31.38b ± 0.52

31.12b ± 0.51

Hind leg length outside (cm)

41.12 ± 0.54

41.29 ± 0.56

41.13 ± 0.54

41.96 ± 0.54

40.46 ± 0.56

41.07 ± 0.54

Hind leg circumference (cm)

62.92ab ± 0.50

61.58a ± 0.51

61.95ab ± 0.50

61.66a ± 0.50

63.17b ± 0.51

61.84ab ± 0.50

Carcass length (cm)

97.04a ± 0.60

98.64ab ± 0.62

98.87b ± 0.60

98.95b ± 0.60

98.02ab ± 0.62

99.08b ± 0.60

Fat depth 1 (mm)

8.28ab ± 0.71

6.78a ± 0.74

8.35ab ± 0.71

8.82b ± 0.71

6.68a ± 0.74

8.19ab ± 0.71

Fat depth 2 (mm)

5.88 ± 0.67

4.96 ± 0.71

5.15 ± 0.69

5.92 ± 0.69

5.14 ± 0.71

5.48 ± 0.69

Fat depth 3 (m)

8.09a ± 0.71

5.16b ± 0.73

6.62ab ± 0.71

8.00a ± 0.71

6.12ab ± 0.74

7.49ab ± 0.71

Fat depth 4 (mm)

3.26a ± 0.41

2.91a ± 0.42

2.61a ± 0.41

4.54b ± 0.41

3.15a ± 0.42

3.54ab ± 0.41

Fat depth 5 (mm)

2.82ab ± 0.55

2.56ab ± 0.57

2.44ab ± 0.55

3.42a ± 0.55

1.68b ± 0.57

3.44a ± 0.55

Kidney fat (kg)

0.32ab ± 0.03

0.32ab ± 0.03

0.32ab ± 0.03

0.35a ± 0.03

0.25b ± 0.03

0.25b ± 0.03

Abdominal fat (kg)

0.62 ± 0.05

0.60 ± 0.05

0.55 ± 0.05

0.63 ± 0.05

0.49 ± 0.05

0.55 ± 0.05

a,b Values in rows with different superscripts differ significantly (P<0.05)

Table 5. Effect of diet on distribution of carcasses over the different carcass grades

Roughage group

Grade A1

(n)

Grade A2

(n)

Grade A3

(n)

Grade A4

(n)

Grade A5

(n)

Grade A6

(n)

20%

0

2

5

6

2

0

30%

1

6

5

2

0

0

40%

0

3

7

5

0

0

50%

0

0

6

8

0

1

60%

0

5

5

3

1

0

70%

0

2

8

4

1

0

 

From Table 5 it follows that 73% of the carcasses in all the groups were graded into A3 and A4 fat grades and 27% of the carcasses were graded into A1, A2, A5 and A6 fat grades. Only 2% of all the carcasses were graded into fat grade A1 and A6. However, in a similar study conducted by Shivambu et al. (2011) with Merino lambs, 71% of the carcasses were graded as A2 and A3, indicating leaner carcasses compared to the Dorper lambs.

 

CONCLUSIONS

It is evident from this study that differences in the energy level of feedlot diets had an effect on growth rates, slaughter traits and feed conversion ratio of Dorper lambs. High concentrate diets improved the average daily gain and feed conversion ratio compared to high roughage diets. It also decreased the cumulative feed intake and the number of days till slaughter. It is also evident that the average feed conversion ratio of Dorper lambs was better than that of Merino lambs. The results of this study indicate that a roughage inclusion level of 20% in feedlot diets for Dorper lambs yielded the best results. A further trial will be conducted using lambs from a different breed (Dohne Merino) to determine the effect of different dietary energy levels on the same traits that were measured in this trial. At the end of the project when all the data from the three trials have been obtained and analysed, the results will be used to develop a computer model to determine the most economical energy inclusion level in feedlot diets for lambs.

 

REFERENCES

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Published

Grootfontein Agric 12 (1) : 15