At Athenry the policy has been not to cull for barrenness. The percentage of ewes that died was similar (4.6pc) for each ewe age group up to six years of age.
The incidence of ewes culled for mastitis varied from 1.6pc to 13.3pc among age groups, with an average of 5.7pc culled annually for mastitis.
As ewes got older the percentage of ewes being culled for condition of the teeth (mouth) increased.
This means 12.7pc of ewes that joined the flock to produce their first litter at two years of age were either culled, primarily for mastitis, or died within one year of joining the flock.
Of ewes that joined the flock to produce their seventh litter at eight years of age, 83pc were culled primarily for teeth (mouth) and poor body condition.
A recent research study carried out in Athenry found that barrenness is a chance event. For the purposes of the study, barren ewes were not culled from the flock.
The study showed that a total of 16pc of the ewes, which were up to six years of age, were barren at least once. On the basis that barreness is a chance event we would expect about 3pc of the ewes to be barrent twice - the observed incidence was 3.5pc.
Just two of the 10 cases where ewes were barren more than once involved ewes who required major assistance at the previous lambing.
This evidence implies that culling young ewes for barrenness merely serves to increase replacement costs unnecessarily as being barren in one year does not mean that the risk of being barren again is increased.
The cost of bringing a replacement into the ewe flock at 18 months of age is equivalent to approximately the value of 25pc of the lamb carcass weight that she will produce during her lifetime.
Body condition score
It has long been recognised that body weight per se is not a reliable indicator of the body reserve status of the ewe unless age and breed are known - and even then it can be misleading.
For example the mature body weight of Belclare, Texel and Suffolk breeds of sheep are 76kg, 80kg and 85kg respectively.
The most important time for ewes to be at target body condition is at joining.
While having the ewes in good body condition prior to the breeding period will have a positive impact on subsequent litter size it also provides a reserve of body tissue which can be mobilised, if required, during the following pregnancy and lactation.
Furthermore, ewes that are in good body condition are at a lower risk of metabolic disease (e.g., twin lamb disease) during late pregnancy, produce larger volumes of colostrum, and produce lambs that have greater viability and develop a stronger bond with their dam.
In addition, the ewes have condition (tissue reserves) to mobilise, if necessary, during early lactation (i.e., milk off their backs).
Body condition score (BCS), which is assessed on a scale of 1 to 5, is a 'hands on' method of assessing the fatness (condition) of animals.
Body condition scoring does not require any equipment, is easy to learn, and overcomes differences in ewe weight due to age, breed or physiological state (e.g., pregnancy).
The target condition score for ewes at joining is 3.5 to 4.0.
Body condition and weaning rate
Research undertaken at Athenry has shown that each one unit increase in condition score (within the range 2.5 to 4) at mating increases litter size by about 0.13.
This increase in litter size can be expected to increase the number of lambs reared per ewe joined (put to the ram) by about 0.1.
Mating ewes at condition score of less than 2.5 increases the risk of barrenness, consequently, further reducing weaning rate.
In flocks which produce mid-season prime lamb fit for slaughter, each 0.1 extra in the number of lambs reared per ewe joined is currently worth approximately €9.50 per ewe joined.
Weight and condition score
Results from analysis of data on flocks at Athenry shows that each 1 unit increase in body condition score is equivalent to an increase of around 12kg in body weight for lowland breed types within the condition score range of 2.5 to 4.0.
Consequently, for a flock to be at the target condition score (3.5 to 4.0) at ram joining (ram turnout) it is essential to condition score the flock well in advance to allow time for any required improvement in body condition to be achieved. In general, ewes on good grass swards have the ability to gain approximately 1kg/week.
Body condition scoring the flock well in advance of ram joining provides adequate time, if required, to increase ewes' BCS cost effectively through increased grass intake.
For example, ewes may have to graze good pasture for five to six weeks to increase their BCS by 0.5.
In the past, ewes were 'flushed', which is the practice of reducing condition post weaning by tight grazing and then raising the plane of nutrition for about three weeks prior to going to the ram so that the ewes would be gaining in body weight and body condition at mating.
Later studies failed to demonstrate that there was any benefit from having the ewes gaining in weight (or condition) at mating.
Based on available evidence, if ewes are in good condition after weaning they should not be deliberately slimmed down with the intention of improving condition immediately prior to mating.
This practice would also be an inefficient use of energy intake by the ewe and delivers no extra benefit.
Dr Tim Keady is principal research scientist at the Animal and Grassland Research and Innovation Centre, Teagasc, Athenry, Galway