Last update: December 2, 2010 10:14:35 AM E-mail Print

 

SELECTION FOR INCREASED REPRODUCTION IN DORPER SHEEP


Gretha Snyman & Buks Olivier

Grootfontein ADI, P/bag X529, Middelburg, 5900

 


In almost all South African sheep enterprises, reproductive performance is of the utmost importance in determining the economic efficiency of sheep production. This is even more so for mutton and dual purpose sheep breeds. As almost all the income from commercial Dorper sheep enterprises is generated through mutton production, the aim of the stud breeder should be to increase the efficiency of slaughter lamb production under natural grazing conditions. Selection should therefore be based on growth and reproductive traits. There are various selection strategies that can be followed in order to achieve this.

Body weight as selection criterion

Results obtained from the analysis of data from various experimental flocks indicated that there is a high genetic correlation between body weight and total weight of lamb weaned by a ewe over her lifetime. This implies that total weight of lamb weaned can be improved genetically through selection based on body weight.

It has been reported for beef cattle that continuous selection for increased growth rate or body weight in a specific herd, especially under intensive conditions, could have several adverse effects. For instance, selection for body weight per se could lead to a correlated increase in feed intake and fat deposition in the body, as well as a decrease in fertility and survival rate. However, there is no evidence of any adverse response in sheep where selection for body weight is done under extensive conditions.

Although there is a positive relationship between body weight and reproduction in other sheep breeds, it could be that there is no relationship, or even a negative relationship between these traits in Dorper sheep. It is therefore not possible to make recommendations based on scientific findings for suitable breeding plans for Dorper sheep. The possibility of direct selection for increased reproduction should therefore be investigated.

Total weight of lamb weaned as selection criterion to increase reproductive performance of the ewe flock

The aim with slaughter lamb production under extensive conditions is to produce slaughter lambs of good quality as early as possible from natural grazing. In view of the limited natural resources, an increase in number of lambs per ewe joined is not always 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.

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. The composite trait total weight of lamb weaned incorporates the component traits fertility, prolificacy, lamb survival and weaning weight of the lamb. The heritability of these component traits are of the same order, or even lower than that estimated for lifetime total weight of lamb weaned. Therefore, if the breeding objective is to increase lifetime reproductive efficiency of the flock, the composite trait total weight of lamb weaned should be the selection criterion.

Results of studies with Afrino and Merino sheep indicated that there is a relatively large variation in lifetime total weight of lamb weaned, regardless of the reproductive rate of the flock. This variation may have a genetic basis and could therefore be exploited to genetically increase lifetime reproductive efficiency in any flock. However, it would be impractical to select ewes after three or four parities. The high genetic and 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. The genetic variance exploited in this way, should also increase the genetic merit of these ewes' daughters in terms of lifetime reproductive efficiency.

The implications and response of selecting rams on the basis of their dam’s performance in terms of total weight of lamb weaned are, however, not known.

New research project at Koopmansfontein to evaluate total weight of lamb weaned as selection criterion

From these discussions it seems that selection on total weight of lamb weaned could be done in order to increase lifetime reproductive performance in the current flock and possibly also in future generations. A project was initiated at the Koopmansfontein Experimental Station (KES) during May 1997 to investigate and evaluate the possibility and implications of total weight of lamb weaned as a selection criterion for Dorper sheep under extensive conditions. Three flocks will be maintained. The Dorper control flock (200 ewes) at KES will serve as the control, while two experimental flocks of 200 ewes each will be kept.

An age structure of five ewe age groups will be maintained in all flocks, which implies that 20 - 25% of the ewes will be replaced annually. Young ewes will be mated at seven months of age for the first time. All ewes will be mated once a year for a five week period in June/July. Five percent rams will be used. An individual mating system (hand mating) will be followed and full pedigree records kept.

All ram and ewe lambs will be retained until the age of 9 months. Ram selection in experimental flock 1 will be done on a combination of BLUP of breeding values for weaning weight and 270 day-body weight. Selection of rams in experimental flock 2 will be done on a combination of BLUP of breeding values for weaning weight and 270 day-body weight, as well as breeding value of the ram's dam for total weight of lamb weaned. Ewe selection in experimental flock 1 will be done on BLUP of breeding values for weaning weight. Selection of ewes in experimental flock 2 will be done on their dam's breeding value for total weight of lamb weaned. After their first parity, final ewe selection will be done on the young ewe's own breeding value for total weight of lamb weaned in both flocks.

The following data will be recorded on both flocks :

Reproductive performance of ewes

Full pedigree

Birth weight of lamb

42-day body weight

100-day weaning weight and condition score

Monthly body weight and condition score from 5 to 9 months of age

Body weight and condition score of ewes before and after mating

Various subjective conformation and type traits will be assessed on a linear scale ranging from 1 - 50 at 6 months of age.

This project has the following long term objectives :

"DORPER PLAN"

The fact that reproduction can only be measured in ewes has the implication that the accumulation of a suitable number of records can take several years. A further limitation of data accumulated from experimental flocks is that only a small sample of the population is measured. In order to increase the number of records and the accuracy of the results, it is of the utmost importance to make use of similar records from the industry. The DORPER PLAN option of INTERGIS can play an important role in this regard. With this option a method of recording of a minimum number of records to gain maximum information on growth and reproduction on individual animals is proposed. It is expected from the breeder to capture data on parentage and weaning weight. These data will be analysed and the following three reports send to each breeder:

The methods and procedures involved are available from the Breed director or from the ARC office at Grootfontein. This will not only enable individual breeders to base their selection on measured performance, but will also assist in building a data base on production and reproduction of the Dorper breed as a whole.