How does bodyweight and daylength affect your success at joining?
Static body weight is body weight measured at any one point in time. Dynamic body weight measures changes in body weight. If weaning has been early, ewes will have regained body reserves and achieved high static body weight before pasture deteriorates in quantity and quality over summer.
Body weight usually declines later in autumn but an early break in the season will see a dynamic increase in body weight. Recent research has shown that change in fat score (dynamic weight) before and during joining had little effect on conception rates. The fact that they reached the joining target was more important than wether they were gaining or losing fat score. Assessing pasture quantity and quality will provide a good guide to the likely performance of ewes feeding on the pasture.
An increase of 1 fat score unit will correspond to an increase in body weight of 7-8 kg, regardless of frame size. One extra fat score unit in Merino ewes at joining results in about 13 extra lambs born per 100 ewes joined.
There is considerable variation between flocks in this response so it is important to determine if this response is profitable for any given flock. Producers wishing to ensure high lamb-marking percentages should target a fat score 3 for their ewes at joining. Monitoring fat score when ewes are yarded is a useful exercise to measure the nutrition of your ewes and a good guide as to how they are likely to respond at joining.
Ewes joined at high static weights and at fat score 5, and who maintain this fat score throughout gestation, are likely to experience difficult births and pregnancy toxaemia.
Daylength and effect on ovulation:
Body weight is not the only factor affecting ovulation. The time of year is also important. Decreasing daylength triggers breeding activity, that is, the oestrus cycle. Also, studies with Merino sheep show that higher ovulation rates occur in autumn. Research has shown that 33 per cent more Merino lambs were weaned per ewe joined in autumn compared with lambs weaned per ewe joined in spring. The summer solstice is on December 21st and day length starts to decrease after that date.
High conception rates are therefore a combination of factors including the date of joining in relation to seasonal feed supply.
- Early weaning gives ewes the potential to regain body weight but also relies on this date in relation to the break in the season or drying-off of the season.
- Ewes joined in December in southern NSW rely on high body weight/fat score to give multiple ovulations coming out of spring, as daylength is not yet decreasing.
- Ewes joined in February in southern NSW may have a lower static body weight and lower fat score; they benefit from decreasing daylength.
- Provided an early autumn season break, ewes joined in April benefit from a dynamic increase in both body weight and fat score as well as from positive effects from decreasing daylength.
Acknowledgements: Edward Joshua (NSW DPI)
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