Last update: April 12, 2012 02:15:56 PM E-mail Print

 

BREEDING PLANS FOR AFRINO SHEEP

SELECTION FOR REPRODUCTION

Gretha Snyman

Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (E.C.), 5900

 

In almost any South African sheep enterprise, reproductive performance is of the utmost economic importance in determining the efficiency of sheep production. This is even more so for mutton and dual purpose sheep breeds.

Lambing percentages in excess of 130% under harsh, extensive conditions and in wool producing sheep breeds lead to the production of a high quantity of lambs, but the quality of these lambs are in many instances not acceptable. In view of the limited natural resources, an increase in number of lambs per ewe joined is not the answer to generate higher income from such a farming enterprise. Selection for increased reproductive performance in such flocks should be aimed at increasing the quality and monetary value of the product in terms of weight and carcass quality. The aim with slaughter lamb production under extensive conditions is to produce slaughter lambs that can be marketed as soon as possible after weaning, without the need for supplementary feeding. Selection for litter size, without taking the weaning weight of the individual lambs into consideration, would therefore be short sighted.

Furthermore, litter size is directly related to ovulation rate, which in turn is influenced by only a few hormones. Total weight of lamb weaned, however, is determined by litter size as well as several other factors, such as mothering ability, milk production of the ewe and growth potential of the lamb. The genes influencing these different traits would all have an effect on total weight of lamb weaned. Selection for reproduction should ideally be based on some measure closely resembling the true breeding objective, which is the total weight of lamb weaned per ewe joined. If selection is aimed solely at increasing litter size, the frequency of genes affecting total weight of lamb weaned would perhaps not be sufficiently influenced. Total weight of lamb weaned in the flock should therefore be the main selection objective.

As mentioned in the previous article, ewe selection has a dual purpose. In the first place it is aimed at increasing income from the current flock through own productivity and secondly, to contribute genetically to possible superior future generations. In practice, however, most selection emphasis is normally placed on rams and little or no attention is given to ewe selection. As discussed in the previous article, ram selection could concentrate on the economically important growth and wool traits, while ewe selection should be done mainly on reproductive performance.

Not all of the information required for the accurate identification of superior ewes is available at an early age. Adequate information on the productive performance (growth and wool traits) are available and accurate selection based on these traits is therefore possible. These traits, however, contribute much less to total lifetime productivity than the ewe's reproductive performance. At this stage little information on the young ewe's reproductive ability is known. The aim must therefore be to predict from the available information at selection age which ewes will be the highest producers, in order to increase gains in the current flock, as well as in future generations.

 

EWE SELECTION

Results obtained from analysing the Carnarvon Afrino data indicate that selection for weaning weight (WW) or 9-month body weight (W9) will lead to a correlated genetic increase in total weight of lamb weaned (TWW). However, the low phenotypic correlations estimated between TWW and WW (0.153) and between TWW and W9 (0.242) would not guarantee that the highest producers be selected for the current flock.

At selection age, total weight of lamb weaned by the young ewe’s dam is the best predictor of lifetime reproduction of the young ewe. After the first parity, total weight of lamb weaned by the young ewe herself, also gives the best indication of her lifetime reproduction in the current flock. The high genetic and especially phenotypic correlations estimated between total weight of lamb weaned after the first parity (TWW1) and future performance indicate that selection based on TWW1 will ensure that the highest producers will be selected and therefore that gains in the current flock would be increased. It is therefore recommended that ewe selection should take place in two phases.

In the first phase, ewes could be assessed subjectively for breed standards and conformation or wool faults. Preliminary selection on the basis of WW or W9 could then be done. If preferred, one of the selection indices recommended for Afrino ram selection could be used. Each breeder or commercial producer should decide which selection strategy is most suitable for his specific circumstances and requirements. As the production traits contribute much less to total productivity than the ewe's lifetime reproductive performance, more than the required number of young replacement ewes should be selected during the first phase. These ewes should then be mated and final selection (second phase) could be done after their first parity.

With the second phase of ewe selection, selection should be based solely on reproduction performance. Selection intensity at this phase is dependent upon several factors. The most important of these are the prevailing environmental conditions especially during mating, but also pregnancy and lactation, as well as the age at first mating. In extremely poor years it would be advisable to leave final selection of young ewes till after their second parity.

The commercial producer should concentrate his efforts on current flock gains, as genetic gains will entirely be due to the efforts of his ram supplier. Second phase selection could therefore be done by culling young ewes that fail to wean a lamb or produce below average TWW1.

For selection to be based on TWW1, records of ewes mated, dam-offspring identification and weaning weight of lambs should be kept. The recording of weaning weight is a prerequisite for Afrino breeders affiliated with the Afrino Breeders' Society. However, the keeping of records under extensive conditions is not an easy task and many commercial producers would not be able to do this, or be unwilling to make the extra inputs required for the keeping of the necessary records. Due to increasing input costs, breeders and commercial producers can not afford to keep unproductive ewes in the flock. It is therefore recommended that records of at least ewes that failed to lamb should be kept. By culling such ewes, genetic progress with regard to fertility, albeit slow, could be achieved in the flock.

 

SELECTION OF RAMS FOR REPRODUCTION

As discussed in the previous article, TWW would increase due to a positive genetic correlation with body weight, if selection is based on any of the recommended selection indices.

New and better methods are continuously sought to improve efficiency of selection. The latest tendency is to include estimated maternal breeding values for weaning weight of the sire in the selection programme for breeding sires. These estimated maternal breeding values are assumed to be an indication of the milk production potential of the daughters of the sire. Afrino ewes at the Carnarvon Experimental station were milked at four, eight and twelve weeks post partum in order to determine milk production.

The results of that study indicated that estimated maternal breeding value for weaning weight of the sire of the young ewe had a negative relationship with her milk production measured at four, eight and twelve weeks after parturition. The relationship between estimated maternal breeding value for weaning weight of the young ewe with her own milk production measured at four and eight weeks are also negative, but it became positive at twelve weeks after parturition. This could be ascribed to the fact that milk production becomes less important relative to direct growth as the lamb/s grew older.

Estimated breeding values for total weight of lamb weaned had a low positive relationship, which also increased from four to twelve weeks after parturition, with milk production. As expected, total weight of lamb weaned by the young ewe had a significant positive relationship with her own milk production.

From these results it is evident that a further in depth investigation regarding the relationships between milk production, maternal breeding values for weaning weight and total weight of lamb weaned should be done before maternal breeding value for weaning weight could be recommended as a possible criterion in the selection programme of breeding sires.

As far as selection for improved reproduction in rams are concerned, it would therefore suffice to accommodate it through indirect selection on body weight.

 

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Die Wolboer