A more accurate efficiency benchmark is the kilograms of lamb produced (sold or retained) per kilogram of ewe mated.  The target for lowland farms is a ratio of 1:1 or 1 kg lamb sold per 1 kg of ewe mated. Expressed as a percentage this is 100%.  For example:

• 72 kg ewes (average weight at mating) at lambing percentage of 173%
• Selling lambs at 42 kg liveweight (LW)
• 1.73 lambs/ewe x 42 kg/lamb =  72.66 kg of lambs produced per ewe
• 72 kg (average ewe LW at tupping)  =  1.01 this is above the 1:1 ratio) or 101%

The “adjusted 90 day weaning weight” benchmark is similar to but offers a more thorough evaluation of ewe efficiency than the 1 to 1 calculation, before post-weaning management affects performance.

This benchmark uses the average lamb weaning weight standardised at 90 days (calculated from the mid point of lambing - point at which half of the ewes have lambed).  If weaning earlier, add the average lamb daily liveweight gain, multiplied by the difference of days, to the weaning weight.  Conversely, if weaning later, subtract the weight gain after 90 days.  This benchmark offers producers an accurate figure for the percentage of ewe liveweight weaned.  For Lowland target 0.95 (range 0.65-0.85); for Upland aim for 0.85 (range 0.55-0.80).

To calculate, multiply the average lamb weaning weight at 90 days by the weaning percentage (number of lambs weaned per ewes mated) and divide the result by the average ewe weight at tupping. For example, take a flock of 72 kg ewes at tupping, lambing from 1 April onwards, weaning at 176% with an average lamb weight of 31 kg on 16 July (90 days):

31 kg x 1.76  =  0.76  =  76%
72

The advantage of this ewe efficiency benchmark is that they are improved by different factors, such as increasing lamb growth rate to weaning, increasing lambs reared and lighter ewes.  As these factors are influenced by maternal genetics, the ewe efficiency calculation can be used to select replacements within different breeds once farm-specific parameters are set.

If some of your lambs are sold deadweight, you can either adjust their deadweight to liveweight or use a killing out percentage of 46% to calculate the carcase weight of any lambs sold live, store or retained as breeding replacements.  To calculate this benchmark, multiply the lambing percentage by the average carcase weight.  For example:

• 172% lambing, selling lambs at average carcase weight of 19.2 kg
• 1.72 x 19.2 =  33 kg lamb deadweight/ewe mated
• The target for lowland flocks is more than 30 kg. For hill flocks, the target range is 20-25 kg

Lambing percentage is dependent on:

• How many ewes get in lamb
• Number of lambs scanned, scanning percentage
• Lamb survival

The scanning percentage reflects the number of eggs shed from the ovaries that survive to 80 days (approx.) and is influenced by season, breed, ewe age, and body condition score at mating.