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EFFECT OF DIFFERENT LEVELS OF UREA SUPPLEMENTATION ON REPRODUCTION, PRODUCTION AND PHYSIOLOGICAL PARAMETERS OF WOOL EWES

 

J.H. Hoon# & J.A. van Rooyen

Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900

#E-mail: Jan Hoon

 

 

INTRODUCTION

Supplemental feeding is a means to promote productivity by supplying limiting nutrients during deficient periods (Huston et al., 1999). Energy can be successfully supplemented with a rapidly fermentable carbohydrate, such as maize meal or molasses, to improve the utilisation of the low quality roughages (Gomes et al., 1994), while non-protein nitrogen (NPN) sources such as urea may improve the digestibility and intake of low quality roughages (Perdok et al., 1988). Undegradable dietary protein is normally necessary to improve milk production of ewes, as well as birth weight and early growth rate of lambs (Brand et al., 1997).

In contrast to the ability of the ewe to draw on body fat reserves when energy intake fails to meet her needs, there is little scope for sustaining milk production by drawing on body protein (Robertson, 1987). For autumn-mated ewes, late pregnancy and lactation normally coincides with a period when the quality of the natural grazing is very low (late winter and early spring in the summer rainfall areas), especially in the grassland areas. Protein is the first limiting nutrient for ruminants grazing natural grasslands during the dry or winter season (Allen et al., 1969) and therefore attention needs to be given to supplementing sufficient protein to uphold the functions of the late pregnancy and lactation phases.

Urea is commonly used at varying inclusion levels as a protein source in winter grass veld supplements for sheep. From an economic point of view, urea is regarded as an important protein source when compared to other natural protein sources, e.g. oil cake meals, fishmeal, etc. Urea is supplied at different levels as source of supplementary protein, depending on the quality and quantity of the grazing and the physiological stage of the animal (dry, pregnant, lactating or growing) and mainly positive results are obtained in terms of animal performance.  Observations in practice, however, indicate possible negative responses in late pregnant and lactating ewes receiving high levels of urea. These responses include, amongst others, strange ewe behaviour, lack of bonding of the ewe with her lamb and possible lower milk production with its subsequent negative effect on lamb growth rate. The aim of this study was therefore to determine the effect of different levels of urea in winter supplements for late pregnant and lactating ewes on reproduction, milk production, growth of lambs, as well as milk, blood, urine and rumen characteristics.

 

MATERIALS AND METHODS

The study was conducted on the farm Rietkuil of Mr. David King in the Steynsburg district and the farmer’s own animals were used. The veld type can be described as a transition between False Upper Karoo and Karroid Merxmuellera Mountain Veld (Acocks, 1988). A flock of 300 pregnant ewes due to lamb in spring 2010 were selected and divided into five groups of 60 animals each (Group 0, 1, 2, 3, 4) four to six weeks before lambing and placed into five different camps. Only ewes scanned pregnant with single fetuses were used. The age distribution of the ewes was the same for the different groups. The animals were kept under natural grazing conditions and received 300 g of feed supplement per animal per day for a period of 14 weeks (last 4 to 6 weeks of pregnancy and first 8 to 10 weeks of lactation). The supplements contained 0%, 1%, 2%, 3% and 4% urea respectively with an average crude protein content (CP) of 17-18% and energy content of 9.3-9.6 MJ ME per kg. The CP concentration of the different supplements was kept constant by adjusting the proportion of cottonseed oil cake meal. All existing management practices were maintained and kept the same for all groups. Conception rate, milk production and growth rate of lambs were recorded and milk, blood, urine and rumen characteristics were measured. The composition of the five supplements is presented in Table 1.

 

Table 1. Composition of the supplementary feeding mixtures

Ingredients

Diets

0

1

2

3

4

Maize meal (%)

40

49

58

67

71

Cottonseed oil cake meal (%)

35

25

15

5

0

Molasses meal (%)

8

8

8

8

8

Salt (%)

15

15

15

15

15

Feed grade urea (%)

0

1

2

3

4

Feed lime (%)

1

1

1

1

1

Dicalcium phosphate (%)

1

1

1

1

1

 

The ewes were weighed at the start of the supplementation period, at 42-day lamb age and when the lambs were weaned. The body weights of the lambs were recorded at 42-day age and at weaning. Ewes were scanned six weeks after the subsequent mating period in order to determine the effect of the different supplementary treatments during the 2010 spring-lambing season on the conception percentage during the 2011 mating season. The milk production of 25 ewes per group was determined 3 to 4 weeks and 7 to 8 weeks after lambing by using the oxytocin method (Bencini, 1995). Milk samples were collected from all 25 ewes and analysed for fat, protein, lactose and total solids.

Blood samples were collected from 10 animals per group 1 to 2 weeks and 3 to 4 weeks after lambing and analysed for progesterone. Urine samples from 10 animals per group were collected 1 to 2 weeks before lambing, 3 to 4 weeks after lambing and 7 to 8 weeks after lambing and the pH of the samples was determined immediately after collection. Rumen fluid samples from 10 animals per group were collected 1 to 2 weeks before lambing, 3 to 4 weeks after lambing and 7 to 8 weeks after lambing and the pH of the samples was determined immediately after collection. The rumen fluid samples collected 3 to 4 weeks after lambing for Groups 0, 2 and 4 were prepared and analysed for volatile fatty acids (VFA). Blood, urine and rumen fluid samples were collected from the same 10 animals per group at the different intervals. Statistical analysis was done using the Proc GLM-procedure of SAS (SAS, 2009).

 

RESULTS AND DISCUSSION

The body weights of the ewes and lambs are presented in Table 2.

 

Table 2. Body weight (± s.e.) of the ewes and lambs of the different groups

Body weight

Group

Ewes

0

1

2

3

4

4 weeks before

lambing (kg)

42.24 ± 0.76

41.94 ± 0.76

40.61 ± 0.76

42.64 ± 0.76

42.58 ± 0.76

42-day lamb

age (kg)

41.91a ± 0.37

38.67b± 0.34

41.65a ± 0.34

41.44a ± 0.37

42.52a ± 0.35

Weaning of

lambs (kg)

43.37ab ± 0.45

43.59ab ± 0.40

44.23a ± 0.41

43.65ab ± 0.44

42.94b ± 0.42

Lambs

 

 

 

 

 

42-day age (kg)

15.66ab ± 1.00

15.65ab ± 0.98

16.01a ± 0.94

16.40a ± 1.00

14.78b ± 0.99

Weaning (kg)

25.80ac ± 1.44

27.33bc ± 1.40

28.69b ± 1.33

26.34ac ± 1.42

25.63a ± 1.41

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

 

No differences (P>0.05) in body weight of the ewes were observed among the groups 4 weeks prior to lambing when the supplementary feeding started. At 42-day lamb age, the body weight of the ewes of the 1% Urea group was lower (P<0.05) than the other groups. At weaning of the lambs, the only difference (P<0.05) in body weight of the ewes was observed between the 2% and the 4% Urea group. The 42-day body weight of the lambs of the 4% Urea group was lower (P<0.05) than the 2% and 3% groups. At weaning, the body weight of the 2% Urea group was higher (P<0.05) than the 0%, 3% and 4% groups. The milk production of the ewes at the first and second milking is presented in Table 3.

 

Table 3. Milk production (± s.e.) of the ewes (3-hour production) of the different groups at first and second milking

 

Milk production

Group

0

1

2

3

4

3 to 4 weeks after lambing -

1st milking (ml)

95.71a

± 6.94

89.55a

± 6.78

87.71ab

± 6.94

77.39ab

± 6.63

69.74b

± 7.30

7 to 8 weeks after lambing -

2nd milking (ml)

99.76

± 8.03

97.61

± 7.68

97.38

± 8.03

97.14

± 8.03

99.17

± 8.68

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

 

The milk production of the ewes of the 4% Urea group, 3 to 4 weeks after lambing, was lower (P<0.05) than the 0% and 1% groups. This is in accordance with the lower body weight of the lambs of the 4% Urea group recorded at 42-day age. At 7 to 8 weeks after lambing, no differences (P>0.05) in milk production were observed among the groups. Milk production of the ewes during early lactation (3 to 4 weeks after lambing) was inversely related to urea concentration in the supplements. These results are in agreement with a study by Gonzalez et al. (1982) on the effect of different protein supplements on milk production of twin-suckling Finnish Landrace X Dorset Horn ewes. That study concluded that all protein supplements (blood meal, meat and bone meal, fish meal, linseed, soybean meal, groundnut) except urea led to higher levels of milk production. According to McDonald et al. (2002), sufficient evidence exists for reduced performance in milk production by high-producing animals given diets containing urea. The milk composition of the ewes at the first milking and second milkings is presented in Table 4.

 

Table 4. Milk composition (± s.e.) of the different groups at first and second milkings

Milk composition

Group

Fat (%) 

0

1

2

3

4

1st milking

5.33 ± 0.24

5.87 ± 0.24

5.39 ± 0.24

5.52 ± 0.23

5.59 ± 0.27

2nd milking

7.19 ± 0.24

7.12 ± 0.23

6.72 ± 0.24

6.75 ± 0.24

6.61 ± 0.26

Protein (%)

 

 

 

 

 

1st milking

4.34a ± 0.10

4.89b ± 0.10

4.59ac ± 0.10

4.80b ± 0.10

4.57ac ± 0.11

2nd milking

4.92a ± 0.10

4.98a ± 0.10

5.10ab ± 0.10

5.43c ± 0.10

5.27bc ± 0.11

Lactose (%)

 

 

 

 

 

1st milking

5.01ac ± 0.05

5.08ab ± 0.05

5.20d ± 0.05

4.93c ± 0.05

5.20d ± 0.06

2nd milking

4.81ac ± 0.05

4.64b ± 0.05

4.94a ± 0.05

4.75bc ± 0.05

4.98a ± 0.06

Total solids (%)

 

 

 

 

 

1st milking

15.40a ± 0.28

16.56b ± 0.28

15.91ab ± 0.28

15.96ab ± 0.27

16.09ab ± 0.32

2nd milking

17.67 ± 0.28

17.46 ± 0.27

17.47 ± 0.29

17.65 ± 0.28

17.58 ± 0.30

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

 

No specific pattern in milk composition was observed with different inclusion levels of urea. No differences (P>0.05) in the fat content of the milk were observed among groups at the first milking or second milking. This is contrary to the results obtained by Hoon et al. (2002) with Merino ewes on Eastern Mixed Karoo veld where the milk of animals receiving high urea supplements had a lower fat and lactose content than animals receiving low urea supplements. The rumen pH of the ewes at 1 to 2 weeks before lambing, 3 to 4 weeks after lambing and 7 to 8 weeks after lambing is presented in Table 5.

 

Table 5. Rumen pH (± s.e.) of the ewes of the different groups

 

Rumen pH

Group

0

1

2

3

4

1 to 2 weeks before lambing

7.06a

± 0.08

6.92ab

± 0.08

6.75b

± 0.08

6.81b

± 0.08

6.79b

± 0.08

3 to 4 weeks after lambing

6.75ab

± 0.05

6.65a

± 0.05

6.83b

± 0.05

6.80b

± 0.05

6.83b

± 0.05

7 to 8 weeks after lambing

6.66a

± 0.06

6.87c

± 0.06

7.01bc

± 0.06

6.72ac

± 0.06

6.67a

± 0.06

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

 

No specific pattern in the rumen pH of the ewes was observed with different inclusion levels of urea. This is contrary to reports by NRC (1985) that large quantities of urea lead to the accumulation of high concentrations of ammonia near the rumen walls with a subsequent increase in rumen pH. Cellulolytic organisms grow optimally at a rumen pH of approximately 6.7, and deviations of pH substantially higher or lower than this point become inhibitory to fibre digestion (Van Soest, 1983). All the rumen pH values in this study correspond well with the optimal rumen pH of 6.7. The urinary pH of the ewes at 1 to 2 weeks before lambing, 3 to 4 weeks after lambing and 7 to 8 weeks after lambing is presented in Table 6.

 

Table 6. Urinary pH (± s.e.) of the ewes of the different groups

 

Urinary pH

Group

0

1

2

3

4

1 to 2 weeks before lambing

6.81a

± 0.23

7.87b

± 0.23

7.78b

± 0.23

6.89a

± 0.23

6.75a

± 0.26

3 to 4 weeks after lambing

6.15a

± 0.37

6.97ab

± 0.35

6.92ab

± 0.35

7.35b

± 0.35

6.12a

± 0.35

7 to 8 weeks after lambing

8.58a

± 0.09

8.41ab

± 0.08

8.32b

± 0.09

8.21c

± 0.09

8.48ab

± 0.09

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

 

No specific pattern in the urinary pH of ewes was observed with different inclusion levels of urea. This is contrary to reports by NRC (1985) indicating that excess urea is excreted as ammonia via the kidneys, leading to an increase in urinary pH.  The volatile fatty acid (VFA) composition of the rumen fluid of the ewes at 3 to 4 weeks after lambing is presented in Table 7.

 

Table 7. Volatile fatty acid composition (± s.e.) of the rumen fluid of the ewes of the different groups

 

Volatile fatty acid

Group 

0

2

4

Acetic acid (mmol/L)

50.44 ± 2.64

57.55 ± 2.64

52.71 ± 2.64

Propionic acid (mmol/L)

10.49 ± 0.50

11.70a ± 0.50

 9.65b ± 0.50

Iso-Butyric acid (mmol/L)

0.53 ± 0.06

0.65a ± 0.06

0.47b ± 0.06

N-Butyric acid (mmol/L)

5.02 ± 0.47

6.09a ± 0.47

4.63b ± 0.47

Valeric acid (mmol/L)

0.41 ± 0.03

0.45a ± 0.03

0.32b ± 0.03

Total (mmol/L)

66.89 ± 3.43

76.38 ± 3.43

67.80 ± 3.43

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

 

A major factor determining the efficiency with which digestible feed energy is utilised by the ruminant is the relative rates of production of the three major VFA, namely acetate, propionate and butyrate. In addition, relative rates of VFA production in the rumen affect the partitioning of nutrients between synthesis of body fat and synthesis of milk fat (Armentano & Young, 1983). Acetate (acetic acid), the most prevalent of the VFA, is the precursor of butterfat in milk, which is the main source of energy to the young animal. A decrease in the supply of acetate could therefore lead to reduced butterfat content and a subsequent negative effect on the growth rate of the young animal (Kaneko, 1989). No differences (P>0.05) in acetic acid were, however, observed among the 0%, 2% and 4% urea groups. Higher values (P<0.05) were observed for propionic, iso-butyric, n-butyric and valeric acid for the 2% Urea group compared to the 4% group. However, the total volatile acids did not differ (P>0.05) among the groups. The blood progesterone levels of the ewes at 1 to 2 weeks before lambing and 3 to 4 weeks after lambing are presented in Table 8.

 

Table 8. Blood progesterone level (± s.e.) of the ewes of the different groups

 

Blood progesterone

Group

0

1

2

3

4

1 to 2 weeks before lambing (nmol/L)

0.52

± 0.07

0.51

± 0.07

0.72

± 0.07

0.57

± 0.07

0.62

± 0.07

3 to 4 weeks after lambing (nmol/L)

0.48

± 0.07

0.39

± 0.07

0.44

± 0.07

0.37

± 0.07

0.42

± 0.07

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

 

Poor nutrition is associated with higher plasma progesterone in late gestation (O’Doherty & Crosby, 1996) and a lower ratio of oestradiol to progesterone at birth (Dwyer et al., 2003).  High plasma progesterone is negatively related to colostrum and milk yield and therefore may threaten the survival of newborn lambs. In addition, progesterone and oestradiol are involved in the onset of maternal behaviour (Shipka & Ford, 1991). Elevated progesterone in underfed ewes might therefore contribute to poor maternal behaviour. No differences (P>0.05) in blood progesterone levels were, however, observed among the groups at either 1 to 2 weeks before lambing or 3 to 4 weeks after lambing. Blood progesterone levels of all the groups decreased from 1 to 2 weeks before lambing until 3 to 4 weeks after lambing, due to the change in physiological status of the ewes from pregnancy to lactation. The conception rate of the ewes of the different groups in the next breeding season is presented in Table 9.

 

Table 9. Conception rate of the ewes of the different groups

 

Group

0

1

2

3

4

Conception percentage (%)

95.2

95.8

97.8

86.7

82.9

Scanning percentage (%)

95.2

95.8

104.4

88.9

87.8

Conception %: number of ewes scanned pregnant/number of ewes mated

Scanning %: number of fetuses scanned/number of ewes mated

 

Lower conception and scanning percentages were recorded in the subsequent breeding season for the ewes receiving higher levels of urea (3%, 4%) during late pregnancy and lactation in the previous lambing season, indicating a possible carry-over effect. Hoon et al. (2002) reported that no carry-over effect was observed with regard to the reproduction rate of Merino ewes receiving high and low urea supplements respectively during late pregnancy and lactation of the previous lambing season.

 

CONCLUSIONS

From the results of this study, the hypothesis that excess urea can increase rumen and urinary pH and affect acetate production, butterfat of milk and blood progesterone levels negatively, could not be confirmed. No specific trend could be detected in milk composition, rumen pH, urinary pH, rumen VFAs (including acetate) and blood progesterone of animals receiving different levels of urea as part of a supplementary feeding program. Milk production of ewes at 3 to 4 weeks after lambing did, however, decreased with an increase in urea inclusion level, resulting in lower body weights for the lambs of the 4% Urea group at 42-day age. A lower conception rate was also recorded in the next breeding season for ewes receiving higher inclusion levels of urea, indicating a possible carry-over effect from the feeding treatments applied during late pregnancy and lactation.

The trends observed with regard to milk production, early growth rate and reproduction rate in the next breeding season warrant further investigation into this matter. This should, however, be done in a more controlled environment where the intake of urea by individual animals used for the collection of rumen, urine and blood samples, can be controlled and monitored more accurately.

 

ACKNOWLEDGEMENTS

The following people are thanked for their contribution to the project:

 

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Published

Grootfontein Agric 12 (1) : 5