These management practices are inherent in the New Zealand farming calendar and they do have an important impact on lamb production.
Another pre-mating management strategy is arguably one of the most important for improving sheep productivity. That strategy is flushing and it is the first step in managing ewes to achieve a high lambing percentage.
Flushing can be defined as a rising plane of nutrition before or during mating that increases the ovulation rate of individual ewes. Ovulation rate is the number of eggs released per oestrus cycle which determines a ewe’s potential litter size. The higher the ovulation rate, the greater the potential for multiple lambs and the lower the occurrence of dries which leads to a higher overall lambing percentage.
A rising plane of nutrition effectively means an increase in feed supply that leads to an increase in ewe body weight. It is the ewe body weight that is associated with a response in ovulation rate and the result can be achieved in two ways.
A ewe fed before the mating date to add condition before the start of mating will have a higher ovulation rate and therefore a higher potential lambing rate than a similar ewe with a low body weight. This is known as the static effect. A ewe fed to increase body weight during the mating period will also show an increase in ovulation rate and therefore a higher potential lambing rate. This is known as the dynamic effect.
Flushing is not a newly discovered phenomenon. Some of the first recorded effects of flushing were published in the Journal of Agricultural Science more than 70 years ago. Since then many more studies using different breeds in different environments with different feeds have continued to provide supporting evidence.
On average, one extra kilogram of ewe body weight can increase flock lambing by 1.3% and it has been shown that this relationship continues for ewes over 70kg. Because of the variation in mature size across NZ ewe flocks it may be inaccurate to apply this rate across the board.
To identify opportunities to improve ovulation rate by flushing it may be more useful to assess ewe body condition scores than ewe body weight. This is because body condition score can be assessed on the same scale regardless of frame size. Weighing alone may identify smaller framed ewes as light and in need of flushing as well as larger framed ewes as heavy and in no need of flushing.
This may lead to the inefficient allocation of pasture to small ewes and a lost opportunity to improve lambing percentage by providing higher feed allowances to the large ewe.
The challenge of flushing is that it occurs at a time of year when feed planning and prioritising stock can be difficult. Surplus lambs late to reach finishing weights, ewe hoggets growing to reach mating weights, and recently weaned cattle are just some of the stock classes competing for pasture over late summer and autumn. A seasonal drought on top of this creates an even bigger challenge and given the recent reports on climate change this may be an increasingly common scenario.
Studies conducted under NZ pastoral farming conditions recommended that flushing could be achieved by increasing pasture allowance. Given that the average ewe needs 1-1.3kg of good quality drymatter to maintain body condition during autumn, allowances may be required to be as high as 1.5-2kg of quality drymatter to increase body condition and improve ovulation rate.
These allowances can be reduced by feeding clover dominant pastures because of the higher protein content. Although such pastures may not be typical to sheep farms at this time of year, preferential grazing can help increase diet quality by allowing ewes to selectively graze pastures before pastures are cleaned up by other stock classes. In general farmers should aim to achieve a condition score of 3 or greater for each and every ewe at mating.
Beef + Lamb NZ’s Making Every Mating Count publication states that increasing the condition score of an average-sized ewe from 2 to 3 is equivalent to 5kg of body weight. Therefore each ewe that requires a 5kg increase in her postweaning body condition significantly increases the feed demand over flushing.
The publication also provides information on the feed allowances needed to achieve this weight gain depending of ewe size and pasture quality.
The key to successful flushing is assessing ewe condition score as early as possible and planning feed allocation to ewes based on their need to increase, maintain, or even reduce condition. It is a crucial opportunity to push reset by eliminating variation in ewe condition carried over from the previous breeding season and setting up the flock to achieve high productivity in the coming season.
