- Effect of body weight and age of dam on reproduction in South African Angora goats
|Last update: November 22, 2010 03:27:35 PM|
Effect of body weight and age of dam on reproduction in South African Angora goats
M. A. Snyman
Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900, South Africa
email : Gretha Snyman
The poor reproductive performance and high kid mortality rate of Angora goats are well documented. There are numerous internal and external factors which contribute to the actual number of kids born per doe that was put to the bucks. 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 which has a marked influence on reproductive efficiency of the herd. There is a tendency among Angora goat stud breeders to retain stud does in the breeding herd until the age of up to 12 years. A study was done on 12 Angora goat studs to investigate the factors contributing to a low reproductive rate and high kid mortalities in Angora goats. Reproduction rate, with emphasis on the effect of body weight and age of dam thereon, will be discussed in this paper. Data recorded from 2000 to 2002 on 12 studs (± 4100 does per year) were analysed. Reproduction data collected included number of does mated, number of does scanned pregnant, number of kids scanned, number of does kidding, number of kids born and number of kids weaned. Body weights of does before mating and at scan (42 days after mating), as well as birth and weaning 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 (does kidded per 100 does mated) varied from 73% to 91% for the different studs (average = 82%). Percentage of does 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 doe 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 does 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, there was nearly 20 % of the does which did not conceive or lost their fetuses before birth. Body weights of does 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. Does carrying twin kids also had higher body weights at scan than dry does or does carrying single kids. Body weight of does at mating increased from 26.3 kg in young does, to 37.5 kg in 5-year old does, after which it increased steadily to 43 kg in 12-year old does. Body weight of does at scan followed the same trend. All reproductive parameters recorded had a typical U-shaped relationship with age of dam; 2-and 3-year old does and does older than 9 years of age reproducing poorer than 4- to 9-year old does. Similar relationships were observed between age of dam and birth as well as weaning weights of kids. 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. The lower reproductive efficiency of young does 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 does conceive and are able to carry their fetuses to parturition. However, 30% of young does 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. From the results it is evident that the practice of keeping does older than 8 to 9 years of age in the breeding herd is detrimental for optimizing reproductive efficiency of the herd. Not only will it slowed down genetic progress, but income of the current herd is also adversely affected by retaining unproductive older does. To optimize reproductive efficiency, breeders should keep a maximum of 5 to 6 dam age groups, and ensure that young replacement does have every opportunity to grow out sufficiently to reach the required body weight before mating at 18 months of age.