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Management and selection for optimal birth weight


Optimum birth weight is the weight at which lamb mortality is minimised and at which lambs are born with minimal assistance ( .


The objective is to produce lambs at the optimum birth weight. Why?

  1. Each 0.5 kg increase in lamb birth weight increases subsequent weaning weight by around 1.7 kg
  2. Lamb birth weight effects lamb mortality as shown in Figure 1. Regardless of litter size, as lamb weight increases mortality declines initially but levels out around the optimum birth weight, which varies by litter size. Subsequently, as birth weight increases above the optimum, lamb mortality increases again – probably reflecting difficulties immediately prior to and during delivery. The optimum birth weight for lambs born as twins is 0.93 that of singles; the corresponding proportion for triplets is 0.78.


Figure 1.  Relationship between lamb birth weight and total mortality for lambs born as singles, twins and triplets



(Hanrahan and Keady, 2013)

Factors influencing lamb birth weight

The following factors influence lamb birth weight

  1. Gender: Male lambs are heavier (approx. 0.2 kg) than female lambs
  2. Ewe genotype: Ewe genotypes with larger mature body weight produce heavier lambs
  3. Litter size: Each one lamb increase in litter size reduces lamb birth weight by approximately 20%
  4. Nutrition: The level of nutrition offered to ewes in late pregnancy effects lamb birth weight (

Benefits to achieving the optimum birth weight

Achieving the optimum birth weight

  1. Reduces labour associated with increased lambing difficulty with large lambs and additional care required for small lambs.
  2. Reduces lamb mortality.
  3. Improves flock productivity and thus profitability.

Additional information:



This post is also available in: Spanish