Phenotypic parameters for production characteristics of Angora goats
IT is of fundamental importance that scientific breeding knowledge should be available for the drafting of efficient breeding plans. Such breeding plans in turn, enable breeders to select more purposefully and thereby make faster breeding progress within their flocks. The procedure which is followed to develop a system of selection for the improvement of production characteristics through improved breeding, is generally described by the following stadia:
- Identification of the economically important characteristics and the determination of the relative value of each.
- Ascertaining the degree of variation in each of these characteristics.
- Determination of the degree to which characteristics are transferred to the offspring and whether any relationship exists between the various characteristics. Heredity and genetic correlations are of fundamental importance in the long-term improvement of breeding. Repeatability and phenotypic correlations are more inclined to give an indication of the improvement, which can be expected in the present flock, through selection. Repeatability is often referred to as the top margin of heredity and the concept of repeatability are generally defined as the degree to which individual animals maintain their superiority throughout their productive life with regard to specific characteristics.
The following results were found in a study in which a full fleece analysis was carried out every four months on a group of 75 unselected Angora goats in the flock of the: Grootfontein Agricultural College:
Variation in characteristics
Table 1 shows average production values, standard deviations and coefficient of variation. The pooled average can serve as an indication of the production level under veld conditions on the Jansenville experimental farm.
The figures in Table 1 correspond well with those of earlier findings. Although lustre may be of economic importance, Table 1 shows a low coefficient of variation for this characteristic. The potential for selection for this characteristic is thus low. The percentage clean yield and percentage pure mohair show similar low coefficients of variation.
Some of the characteristics shown in Table 1 can only be measured at great cost and should therefore not be considered as routine measurements for selection. These characteristics include for example straight length, staple size, felting ability, suint percentage and pH, S/P ratio, follicle density and thickness of skin. Repeatability estimates and phenotypic correlations will therefore be restricted to characteristics which can be considered for inclusion in a performance testing scheme which will eventually serve as information upon which selection can be based.
Repeatability can be defined as the ratio between the components of variation, which cause permanent differences between animals and those, which cause differences between measurements on the same animal.
When breeding plans are designed it is essential to know at which stage of an animal's lifespan selection can be done in order to increase its lifetime production and, even more important, to bring about genetic progress. Repeatability estimates can be used not only to determine the correct age for selection, but also to evaluate the relative value of one or more records per animal for purposes of selection. Since the performance-testing scheme for Angora goats is still in the development stage, the information discussed is of great importance in order to carry out the scheme correctly.
Table 2 shows repeatability figures and correlations with lifetime production for measurements which were carried out at an age of 12 months. This age seems to be the youngest age at which accurate selection can be done for increasing lifetime production.
Repeatability figures where one record per animal is concerned can be regarded as high if "t" has a value of 0,60, medium if "t" has a value between 0,30 and 0,60 and low if "t" is smaller than 0,30.
All estimates in Table 2 except that of lustre are higher than 0,30. The estimates for fibre diameter, frequency of curl and body mass are higher than 0,60 and can therefore be considered as being highly repeatable. With the exception of staple length, the correlation with lifetime production is medium to high and reasonable increases in lifetime production can be expected with selection at 12 months of age. In the case of staple length the accuracy of measurement is suspect and more appropriate techniques should be considered.
Due to the relatively limited number of animals in this study, the correlations in Tables 3 and 4 should only be used as a guide until more dependable results eventually become available.
It is interesting to note the high positive correlation between grease hair production and clean hair production. The implication of this correlation is obviously that it is not essential to determine clean hair mass as a measure of selection in order to increase hair production. A second important correlation is the high positive correlation between fibre diameter and both hair production and body mass. It can be expected that selection progress in both body mass and hair production will be seriously retarded if fibre diameter is required in order to improve the quality of the hair at the same time. A heartening factor is the relatively high positive correlation between body mass and hair production. Correlations between the other hair traits are generally low and/or variable, making it impossible to come to definite conclusions at this stage.
While certain correlations could possibly retard selection progress, the possibility of improving production traits on a scientific basis, is quite good.
Up to now selection procedures were based on the hand and eye method and the conclusion is that on a theoretical basis, the rate of selection progress can be accelerated by using more scientific procedures.
Angora goat and mohair journal 29 (1)