Last update: November 22, 2010 11:00:54 AM E-mail Print

 

Influence of ewe age structure on reproductive efficiency of Angora goats


Dr Gretha Snyman


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

Email: Gretha Snyman

 


Reproductive performance is the most important trait that determines income from all livestock enterprises. The relatively low reproductive rate and high kid mortality rate of Angora goats are well documented. There are numerous internal and external factors that contribute to the actual number of kids born per ewe that was put to the rams. There are even more factors which determine whether a kid born alive will survive to weaning age. Age of dam is one of the internal factors that have a marked influence on overall efficiency of the flock. There is a tendency among Angora goat stud breeders to retain stud ewes in the breeding flock until the age of up to 12 years. A study was done on 12 Angora goat studs to investigate the factors influencing reproductive rate and kid mortalities in Angora goats. Some aspects with regard to the effect of body weight and age of dam on reproductive efficiency of the flock will be discussed in this paper.

Data recorded from 2000 to 2003 on 12 studs (± 4100 ewes per year) were analysed. Reproduction data collected included number of ewes mated, number of ewes scanned pregnant, number of kids scanned, number of ewes kidding, number of kids born and number of kids weaned. Body weights of ewes before mating and at scan (42 days after mating), as well as birth weight, weaning weight, 8- and 16-month body weights of kids were also recorded.

There was a wide range with regard to reproductive performance among the various studs. Reproductive data of the participating studs for the 2002 kidding season, for example, were as follows: Kidding percentage (ewes kidded per 100 ewes mated) varied from 73% to 91% for the different studs (average = 82%). Percentage of ewes scanned dry varied from 4.9 % to 22.1 % among the different studs (average = 14.1 %), while scan percentage ranged from 79 % to 131 % kids per ewe scanned (average = 100%). Percentage multiple births varied between 3% and 34% (average = 18%), while kid survival rate ranged from 81% to 94% (average = 89%). Weaning percentage (kids weaned per 100 ewes mated) ranged from 59% to 102% (average = 85%). Apart from a high kid mortality rate, which is regarded by breeders as the most important factor contributing to low weaning percentages, nearly 20 % of the ewes did not conceive or lost their fetuses before birth. There was an average of 9 % of the ewes that were scanned pregnant, that did not kid, which implies that they have lost their kids between scanning and kidding.

Body weights of ewes before mating ranged from 33.2 kg to 46.0 kg (average = 34.1 kg) among studs, while body weight at scan ranged from 33.6 kg to 43.3 kg (average = 36.2 kg). Body weight before mating had a significant influence on the number of kids scanned and born, but no influence on number of kids weaned. Ewes carrying twin kids also had higher body weights at scan than dry ewes or ewes carrying single kids.

The age structure of the ewes mated in the various flocks from 2000 to 2003 is presented in Table 1. It is evident that 13% of the ewes in the flock was 8 years and older.

Table 1. Age structure of ewes mated from 2000 to 2003

Age

Number

(n=12032)

%

2

1820

15.1

3

2134

17.7

4

2000

16.6

5

1812

15.1

6

1477

12.3

7

1221

10.2

8

940

7.8

9

474

3.9

10

107

0.9

11

39

0.3

12

6

0.1

The effect of ewe age on reproductive performance is illustrated in Figure 1. All reproductive parameters recorded had a typical U-shaped relationship with age of dam; 2-and 3-year old ewes and ewes older than 9 years of age reproducing poorer than 4- to 9-year old ewes. Although 11- and 12-year old ewes had the highest scan percentages, their kidding percentages were considerably lower, indicating that they have lost their kids between scanning and kidding. The same trends are prevalent in all studs, regardless of the reproductive rates of the different flocks.

A curvi-linear relationship existed between age of dam and kid mortality rate from birth to weaning, where 17.4% of kids born to 2-year old dams died, compared to a 7% kid mortality rate for 10-year old dams (Figure 2).

 

Figure 1. Effect of age of dam on reproductive traits

 

Figure 2. Effect of age of dam on pre-weaning mortality rate in kids

 

The change in ewe body weight with age is illustrated in Figure 3. Body weight of ewes at mating increased from 27.4 kg in young ewes, to 38.2 kg in 5-year old ewes, after which it increased steadily to 42.5 kg in 11-year old ewes. Body weight of ewes at scan followed the same trend.

 

Figure 3. Effect of age of ewe on ewe body weight

The lower reproductive efficiency of young ewes could most probably largely be ascribed to their lower body weight. A body weight of 27 kg at 18 months of age is generally regarded as a minimum to ensure that young ewes conceive and are able to carry their fetuses to parturition. However, 30% of young ewes in this study had mating body weights of less than 25 kg; the range being from 15 to 45.8 kg, depending on the respective rearing environments.

 

Similar relationships to those of the reproduction traits were observed between age of dam and body weights of their kids (Figures 4 and 5). From these figures it is evident that kids born to young ewes (2-year old ewes) had lower body weights up to 16 months of age than kids born to 4- to 8-year old ewes. Birth, weaning and 8-month body weights of kids born to ewes older than 10 years of age, were also lower.

Figure 4. Relationship between age of dam and birth and weaning weight of kids

 

 

Figure 5. Relationship between age of dam and 8- and 16-month body weight of kids

 

Ewe selection has a dual purpose; the first is the immediate improvement of the productive performance of the flock and, therefore, the income from the flock. The second aim is the long-term genetic improvement of the stud. To optimize the rate of genetic improvement in the stud, a balance between selection intensity (number of replacement animals needed per year) and generation interval (the average age of the parents when the progeny is born) must be established.

When there are a large number of ewe age groups in the stud, the numbers of replacement young ewes are less than when fewer ewe age groups are kept. For example, with 5 ewe age groups, the replacement percentage will be between 20 and 25%. From Table 1, 15 to 20% replacements are needed with 11 ewe age groups. The fewer replacement animals that need to be selected, the better their productive performance will be, as only the best animals can be kept as replacements. This will enhance the rate of genetic improvement. However, when there is a large number of ewe age groups in the stud, the generation interval will be longer, which in turn will decrease the rate of genetic improvement. With fewer ewe age groups, the rate of genetic progress will be enhanced, but more replacement young ewes are needed. The most important factor that will determine the replacement rate, and hence the number of ewe age groups that can be kept, is kidding percentage and kid survival rate. The more young goats available at selection age, the more animals can be replaced. With a weaning percentage of 80%, replacement rates of 20 to 25% could be easily achieved.

It could be argued that a high proportion of young ewes in the flock (with fewer ewe age groups) will negatively affect the current performance of the flock due to the generally lower efficiency of the young ewes. This should be weighed against achieving optimum genetic gain as well as the lower efficiency of older ewes. As mentioned above, the lower reproductive efficiency of young ewes could largely be ascribed to their low body weight. An average growth rate of 24 g/day was recorded for ewe kids from weaning to 8 months of age, and a growth rate of 6.5 g/day from 8 to 12 months of age. This implies that the growth rate of ewe kids is severely stunted after weaning, for at least an 8-month period. This contributes directly to the young ewes not being able to reach an acceptable weight before mating at 18 months of age. It would be advantageous to take special care that ewe kids have every opportunity to grow out sufficiently to reach the required body weight before mating at 18 months of age.

Analysis of data on South African stud Angora goats revealed a generation interval of 5.11 years, which could be reduced considerably. Theoretically, genetic progress in selection for a single trait could be increased by approximately 12 % if the number of ewe age groups is decreased to five and that of ram groups to two.

From the above-mentioned results and discussion it is evident that the practice of keeping ewes older than 7-8 years of age in the breeding flock is detrimental for optimizing overall productivity. Not only will it slow down genetic progress, but income of the current herd is also adversely affected by retaining unproductive older ewes. To optimize productive and reproductive efficiency and rate of genetic progress, breeders should keep a maximum of 5 to 6 dam age groups, and ensure that young replacement ewes have every opportunity to grow out sufficiently to reach the required body weight before mating at 18 months of age.