- Mating practices and their influence on reproduction
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Mating practices and their influence on reproduction
EM van Tonder
During the biennial congress of the South African Veterinary Medical association held at East London in September 1971, Dr. E. M. van Tonder, of the Veterinary Investigation Centre, Middelburg, Cape, delivered a comprehensive address dealing with the influence of mating methods on reproduction. This article is an abstract of his paper.
THE domestication of our farm animals and their incorporation into profitable farming enterprises inevitably lead to the employment of practices aimed at increased production and reproduction. Since these animals are also kept and bred under controlled circumstances, reproduction has become an economical consideration far beyond the level required for the survival of a particular breed.
Maximal reproduction can only be obtained when mating is practised during the breeding season when sexual activity is at its peak and all other factors preventing successful mating are limited to a minimum.
It is well known that in most animals sexual activity is restricted to more or less definite breeding seasons. This is apparently aimed at producing the young at a time most favourable for development and growth. In contrast with most animals where increased daylight irradiation acts as the most important stimulus of sexual activity, Angora goats and sheep respond more actively to a decrease in light.
The incidence of sexual activity, as presented in Fig. 1, follows a definite seasonal pattern, which commences during March and reaches its highest intensity during the following four months. Termination of oestrus occurs in individual ewes during July, August and finally, at the beginning of September. It must be borne in mind, however, that the breeding season of a particular flock is not representative of the duration of the breeding season of its individual members but merely the collective result of relatively short individual seasons distributed over a longer period.
It was found that the mean duration of the breeding season is 117 days, varying from 84 to 147 days. The breeding season is therefore short, but is characterised by fairly regular occurrence of oestrus. During this season the normal doe exhibits 6 to 8 oestrus cycles at regular intervals of 19 to 20 days. The duration of oestrus varies from 22 to 33 hours
Apart from a seasonal influence on ovarian activity as such, there is also conclusive evidence of a seasonal tendency in ovulation rate. The occurrence of twill and multiple, as opposed to single, ovulations closely follows the pattern of the breeding season.
As regards a seasonal incidence of sexual activity in Angora rams, Van Rensburg (1971) states that the decreasing length of daylight in autumn stimulates increased libido in conjunction with the characteristic buck odour. Personal observations strongly support this view, and it would also appear that Angora rams have a more clearly defined seasonal incidence of sexual activity, compared with rams of the different breeds of sheep.
Concerning seasonal changes in semen production, Moule (1963) suggested that these probably occur in all species showing seasonal rhythm in breeding. Seasonal changes are manifested as decreases in motility, concentration and total numbers of sperm, and increases in the proportion of dead and/or morphologically abnormal sperm were reported by various workers. Although the possibility of climatic influences, especially ambient temperature, cannot from personal experience be overlooked, it is nevertheless felt that these factors have a confusing superimposed effect on seasonally depressed seminal activity. It can be stated again that in Angora rams seasonal changes in the semen picture are more distinct:
FACTORS INFLUENCING REPRODUCTION
It has been proved that increased nutritional levels cause an increased duration of the breeding season and that low levels of feeding during the breeding season cause reduced oestrus activity. Hugo (1955) concluded that a decreased level of feeding cause lowered sexual activity, reflected both in the number of ewes exhibiting oestrus as well as the number of oestrus cycles exhibited by these ewes. As the breeding season therefore becomes more concentrated around the peak period of April - May, the practical implications are obvious, namely, that more ewes will be detected and served in a shorter space of time.
As the duration of the oestrus period was significantly reduced when the dietary level decreased from high to medium, Hugo concluded that duration of the oestrus period is indeed very sensitive to changes in nutritional levels
The influence of nutrition, by increasing sexual activity and stimulating the onset of the breeding season, is manifested in the practice of flushing or mating ewes whose condition is improving.
With regard to overfeeding, research workers report the detrimental effect on sexual activity of fat deposition in and around the ovaries. Some workers regard fat as the chief cause of infertility in two-teeth ewes. These findings, however, were contradicted by Wallace (1961) who found higher conception rates and lambing percentages in ewes kept under excellent feeding conditions from birth. However in this country it is doubtful whether overfeeding would ever present a problem.
It is generally accepted that fertility in the ewe increases with age up to an optimum age. Investigation on this aspect by Hugo (1955) indicated that the number of ewes exhibiting oestrus reached a peak at the early age of 19 months while the average number of oestrus cycles of the flock showed a gradual increase. The optimum age at which maximal sexual activity (number of cycles per ewe as against the possible maximum) was reached was calculated at 5 years while the maximum length of the breeding season (mean number of oestrus cycles) was attained at 4 to 5 years.
(iii) Mohair Production
The influence of increased mohair production on fertility and hence reproduction has been shown by experiments indicating a physiological antagonism between mohair production and reproduction. It was clearly indicated that higher mohair production decreases the duration of the breeding season and oestrus period and retards the onset of puberty. It is common knowledge that an over-packed goat is usually small and of low fertility.
(iv) Mating behaviour
The stimulating effect of the presence of rams on ewes prior to and at the onset of the breeding season has been widely studied. Research workers reported earlier onset of the breeding and kidding season when ewes were prejoined with vasectomised rams. In later experiments it was proved that, when joined with intact rams, ewes which had some previous association with rams exhibited oestrus within 17 days as compared to other ewes which did so between the 18th and 26th day. It was also found that direct contact was not essential, as smell and sound had the necessary stimulating affect.
As the mating season in relation to the natural breeding season is of vital importance in determining the reproduction rate, the selection of mating practices most suitable to a specific set of circumstances remains one of the basic considerations.
1. April versus February mating
The specific seasonal incidence of sexual activity in goats has already been pointed out as well as the fact that the sexual season ranges from late February/March to July. It has also been determined that the peak of the breading season occurs round about April to May, Let us compare a February/March (early season) mating with an April/May (mid or late season) mating to point out the advantages and disadvantages of each.
As the breeding season of a particular flock is comprised of the seasons of its individual members, it obviously follows that the peak represents the time when the maximum number of ewes in the flock exhibit sexual activity. It is thus clear that mating during this period (April/May) is to be recommended if maximal conception and therefore reproduction is to be attained. It has also been proved that the conception rate is higher with progressive increase prior to the peak of the breeding season than at any other time
With regard to the length of the mating period it should be pointed out that too long a mating and hence kidding period is not desirable from a management and economical point of view. In practice a period of 6 to 8 weeks is usually preferable. Thus the rams should be put with the ewes for eight weeks when the sexual activity is at it’s highest.
Owing to a tendency for the oestrus period to become longer, as well as an increased incidence of twin or multiple ovulations towards the peak of the breeding season, the chances of ewes being served and conceiving will also be greatly increased in mid-season mating as against early-season mating.
The cooler weather conditions prevailing during April would be more conducive to successful mating as it benefits both ewes and rams. In rams, high ambient temperatures and overheating during active seeking and mating, adversely affect the semen picture, while in ewes increased temperatures have been reported to be detrimental to fertilization and early embryonic growth.
In favour of April mating it can finally be mentioned that, in the main Angora goat areas, it usually occurs under good natural grazing conditions, with the result that flushing is obtained by natural means and need not be effected by supplementary feeding.
2. September versus July Kidding
Apart from the substantial difference in kidding percentages that can be expected in favour of an April mating (September kidding), there are distinct advantages and disadvantages in both kidding practices.
Since April mating is carried out at the peak of the breeding season, and will therefore be concentrated in a short period, the September kidding season will also be of short duration in comparison with the more protracted July kidding. This will be the case even if the February mating period is limited to the same extent as the April mating period. From a management as well as an economic point of view September kidding is therefore preferred.
On the other hand, it has been proved that the cool weather prevailing in late winter favours growth and development of kids in comparison with the high ambient temperatures of the September season.
If it is considered that summer rains seldom occur in the goat farming areas, with relatively poor grazing and milk production, both, but especially the mid-summer kidding season, appears to be unfavourable from a kid rearing point of view. Since supplementary feeding will then have to be resorted to in both cases, there is very little to choose between them. However, parasitic infestations and kid diseases are more restricted during the July season. .
The September season furthermore has the advantage that kids are born in warmer weather conditions instead of the cold weather of the July season, when big losses may occur due to adverse weather.
The choice of a particular breeding and therefore kidding season must be considered, bearing in mind the fewest disadvantages or the relative ease and minimal costs with which the kid can be reared. It will therefore have to be decided upon on a purely economical basis.
3. Mating systems
Although four natural mating methods are used, flock or mass mating is the one most commonly employed. It is also known as group mating, which term is often incorrectly used to denote individual mating Since ewes are always mated in groups it is felt that the terminology should be based on the number of rams associated with a group of ewes.
As general management remains the same for all methods, individual i.e. natural and hand mating only will be dealt with briefly.
(i) Group mating - In this instance a number of rams are placed in a camp with a number of ewes for varying periods.
(a) Mating season - As the peak of the breeding season is reached during May, and it has also been proved that better conception figures are obtained by mating during the period of increasing sexual activity, it would appear as if a mating season centred around mid April would be most advantageous.
(b) Management of ewes - Preparation of the ewe flock for mating actually starts a few weeks before mating, when the necessary feeding, dipping, shearing and vaccinations are carried out. Breeding ewes should be protected from severe nutritional stress at any time during their lives. As it is generally agreed that improved conditions before and during mating is most conducive to fertility, flush feeding for a period of 3 weeks before mating should be introduced.
(c) Management and use of rams - Rams should be examined for genital soundness as near the mating season as possible and those of doubtful fertility, libido and general mating dexterity replaced by rams exhibiting the desired qualities. All inoculations should be done at least six weeks prior to mating. As rams will have to search for and mate a large number of ewes repeatedly, optimal semen reserves and energy will be required, and as feeding time will be restricted, it is deemed absolutely necessary that supplementary concentrates or a complete ration should be given for at least three, but preferably 6 to 8 weeks before mating commences. A practical and rewarding procedure that is strongly recommended is to divide rams into two groups and to put alternative groups with the ewes for a period of 14 days, except for the last fortnight when all the rams are joined.
(d) Flock Composition - The available evidence indicates that it is sound practice to separate maiden does entirely, especially at their first mating. The fact remains that their presence in an existing flock will have a definite influence on the reproductive performance of that flock.
(e) Camps and Camp Sizes - Apart from choosing rested camps for mating, on account of their flushing value, it should always be endeavoured to select the more open type of camps with a more even surface and favourable topography. Since more watering points and large camps enhance group formation smaller camps with single watering points should, where available, always be selected for mating.
(f) Flock Size - No definite recommendation can be made on this aspect although flocks smaller than 200 to 300 ewes are preferred.
(ii) Individual Mating - This system is mainly used by ram breeders for progeny testing. (recording purposes). Rams to be tested are individually mated with a specific number of ewes in order to identify their progeny. The advantage of this system lies in the fact that more ewes can be allocated per ram, thus reducing the ram-cost per kid. Proper records of individual rams can be kept while their breeding potential and mating ability can also be assessed.
(iii) Hand mating - This is the most effective natural mating system, by which accelerated breeding progress can be achieved with a minimal ram-cost per kid.
As it is usually carried out under direct supervision of each mating, with retesting of ewes after 19 to 21 days, and, as a rule, using rams that have been tested beforehand, high kidding percentages are ensured. The fact remains that any possible cause of a lower kidding percentage will be identified within the course of one oestrus cycle and can be corrected timeously.
Angorabok en sybokhaarblad 14 (1)