BODY-WEIGHT AND BIGGER KID CROPS
J. F. K. Marais
The average price-differences between kids and adult goats' hair during the winter and summer seasons of 1968 were; R1.00 and R1.50 per lb respectively. This fact once more emphasises the importance of large kid crops. The higher the percentage of young animals in the flock the higher will be the average income per animal and consequently the total income for the flock.
In the course of a survey of the Angora goat industry, Van Rensburg (1963) concluded that the annual reproduction losses in Angoras amounted to 27.9 per cent.
Aborters contributed 11.6 per cent towards this figure, and non-breeders 16.3 percent. Thanks to brilliant work by Dr K. M. van Heerden, who recommended that all aborters be culled annually, the problem of abortion has virtually been overcome.
The non-breeding ewes must therefore be subjected to a close scrutiny. This group may consist of ewes which do not come into season, or which have been mated but do not become pregnant, also ewes which do become pregnant but the embryo is resorbed: Resorption is the process whereby the foetus, having died at an early stage, disintegrates and is absorbed into the bloodstream of the wall of the uterus. Unlike abortion, resorption takes place only in the early stages of pregnancy.
The possibility can therefore not be excluded that resorption takes place on a fairly large scale among Angoras. Professor C. H. van Niekerk of the Department of Animal Anatomy and Physiology at Stellenbosch University has already shown that under nourishment has led to the resorption of the embryo in mares and sheep. In this connection it must be remembered that the areas where Angoras are farmed are subject to periodical and seasonal droughts with concomitant deterioration of the grazing. Prof. Van Niekerk found that the period wherein the chances of survival of the embryo are most influenced by under-nourishment is that when the foetus becomes attached to the uterine mucous membrane. In sheep this significant period is on or about the twelfth day of pregnancy.
Since little was known of prenatal development of the foetus In Angora goats, it had first to be established at what stage of pregnancy this attachment takes place. This was accomplished by slaughtering Angora ewes at different stages of pregnancy to investigate the development of the embryo. When the attachment period became known, pregnant ewes were subjected to diet deficiencies to determine whether resorption could be induced experimentally. One hundred Angora ewes and 5 rams were used for this purpose, and tests were conducted on the Welgevallen experimental farm outside Stellenbosch.
It was found that early foetal development is significantly slower in Angoras than in sheep. The first signs of attachment, which can be observed in sheep after 10 days, could only be seen in Angoras after 13½ days. The time of attachment now being known, tests were planned so that Angora ewes were subjected to restricted diet over this period.
What it really amounted to was that one group of goats received high grade and the other low grade diet. Each of these two main groups was in turn subdivided into three. A control group, which received ad lib. feeding for 23 days after mating, and two starvation diet groups. One of these was starved from the 10th tot the 14th day and the other from the 14th to the 18th day. Twenty three days after mating all the goats were slaughtered and the contents of the uteri investigated.
There were no significant differences between members of each group, but great differences between the main groups. The ewes in the low level feeding group lost 18 per cent in weight during the test period, as against 4 per cent in the case of high level feeding.
Two cases of embryo resorption were observed in the starved animals, and judging by the size and state of the remnants of the foetus and membranes found in the uterus after 23 days, the embryos must have died during the period of starvation. The loss of weight of these two ewes during the test period was 22 percent and 27 percent respectively, considerably more than the average of 18 percent for the low level group.
The low incidence of resorption (4 per cent) even in the low level feeding groups leads to the question whether resorption plays a significant role in practice, where seldom, if ever, such high weight losses occur as a result of poor or insufficient grazing. Embryos in ewes which received poor feed showed delayed development. However, better feeding in the later stages of pregnancy could quite easily have masked an earlier setback.
Relation of mature breeding weight to performance the following season
The most important information derived from these experiments, is the extent to which body-weight at mating influenced the breeding potential of the Angora ewe. The average live weight of the eight ewes which were normally in season and mated but did not give birth was 56.4 lbs. after 23 days. The weight at the time of mating of the 37 ewes which produced single kids was 74.0 Ibs while 6 ewes which produced twins weighed 93.3 lbs.
These results emphasise once more the important role of body-weight in reproduction. They are also in agreement with work done at the McGregor Research Station in Texas on Angoras by Shelton (1965). where adult ewes (3 yrs and over) were divided into groups with 10 lbs difference in each. The results obtained are set out in the accompanying table.
In a previous article Shelton states that young ewes should weigh at least 55 lbs before being mated. Adult ewes should weigh at least 75 lbs at the start of the mating season. These figures are of course based on the results of American research.
It does not follow that the difference in weight between the more and the less fertile Angora will be applicable to all Angora farms in South Africa, but the broad tendency that the heavier goats will be more fertile should be apparent, while the chances that the smaller ones will kid are rather slim.
In these days of large price spreads between kids, young goats and adult hair, kidding percentage is playing an increasingly important role in the profitability of Angora farming. The question thus arises whether every farmer should not get rid of his lighter and possibly infertile ewes, irrespective of the desirability of their mohair characteristics.
Young ewes should of course not be treated the same as adult ewes. The ideal would be to cull the two-tooth ewes every year, so that selection for fertility can be initiated at an early stage. At the end of each kidding season those ewes which have not produced a viable kid for the second time in succession should be culled. The breeding potential, and thus the profitability of Angora farming can accordingly be enhanced permanently.
MARAIS, J. F. K. (1968). Aspekte van die vroeë embrioniese ontwikkeling by die Angorabok en die invloed van voeding daarop. M.Sc. (Agric.)-skripsie, Univ. Stellenbosch.
SHELTON, M., (1965). The relation of size to breeding performance of Angora does and Factors effecting kid production of Angora does. Sheep and Angora goat, wool and mohair report. Texas A and M.
SUlD-AFRIKAANSE SYBOKHAARRAAD, (1968). Statistiese ontleding van die Republiek se SybokhaarskeerBel. (Voorlopig). Somer- en Winterseisoen.
VAN HEERDEN, K. M. (1964). The effect of culling aborting ewes on the abortion rate in Angora ewes. The Angora Goat and Mohair Journal 2, 15.
VAN NIEKERK, C. H. (1965). Early embryonic resorption in Mares. (A preliminary report). J. S. Afr. Vet. Med. Ass. 36 (1).
VAN NIEKERK, C. H. (1966). Resorpsie van die embrio van die skaap in die vroeë stadium van dragtigheid.
In: Die Skaap en sy Vag (Red.) J. C. Swart. Kaapstad: Nasionale Boekhandel 1968.
VAN RENSBURG, N. A. J. (1963). 'n Opname van Produksie- en Reproduksie - probleme in die Angorabokbedryf van die Republiek van Suid-Afrika.
Angora goat and mohair journal 12 (1)