Adjusting Weaning Weights

Sheep producers can make better-informed lamb selection and ewe culling decisions at weaning by adjusting lamb weaning weights. Both genetics and the environment influence animal performance, so adjusting weaning weights is a useful tool for comparing individual lambs on the same farm, since these animals have, in theory, all been exposed to the same environmental conditions. Producers can also use adjusted weaning weights to evaluate ewe performance over time and ram offspring performance.

Weaning weights can be adjusted for:

• birth type (single, twin, or triplet)
• rearing type (single, twin, or triplet)
• sex of the lamb
• age of the dam or mother
• age at weaning

To adjust for the age at weaning:

1. Determine an age to adjust to. Weaning weights are typically adjusted to 60 days of age, but can also be adjusted to other ages, such as 90 or 120 days.
2. Subtract the birth weight from the weaning weight. This is the weight gained.
3. Divide by the age of the lamb (in days) when it was weighed. This is the average daily gain.
4. Multiply the average daily gain by the weaning age you are adjusting to (usually 60 days).
5. Add the birth weight.

Estimated 60-day weight = (((weaning weight – birth weight)/age at weaning) X 60) + birth weight.

The estimated 60-day weight can then be adjusted for more factors. The following table lists adjustment factors for ewe age, lamb sex, birth type, and rearing method for three sheep breeds (Targhee, Suffolk, and Polypay), as well as generic breed adjustments.

The tables below are adapted from the Breeding and Selection Chapter of the Sheep Production Handbook produced by the American Sheep Industry Association, Inc.

Adapted from Sheep Production Handbook

Class explanation

• Ewe Age: in years
• Sex: Ram, Ewe, or Wether (castrated male)
• Type of birth and rearing:
  • 1/1 = single raised as a single
  • 1/2 = single raised as a twin
  • 2/1 = twin raised as a single
  • 2/2 = twin raised as a twin
  • 3/1 = triplet raised as a single
  • 3/2= triplet raised as a twin
  • 3/3 = triplet raised as a triplet

Example of Adjusting a Weaning Weight

A twin-born ewe lamb was born on January 2^nd, weaned on March 4^th, and was raised as a twin. The dam is 2 years old.

• Birth weight = 10 lb.
• Weaning weight = 62 lb.
• Actual age at weaning = 61 days
• Weaning age to adjust to: 60 days

1. Adjust the weaning weight for sixty days. Use the following formula:

   Estimated 60 day weight = (((weaning weight – birth weight)/age at weaning) X 60) + birth weight

    (((62-10)/61) X 60) + 10 = 61.15 lbs.

2. Now adjust the weaning weight for ewe age, sex of the lamb, and the type of birth and rearing, by multiplying by the generic adjustment factors found in the table:

• 1.08 = adjustment factor for a 2-year-old ewe
• 1.00 = adjustment factor for a ewe lamb
• 1.21= adjustment factor for a twin raised as a twin

       61.15 lb. X 1.08 X 1.00 X 1.21 = 79.91 lb.

Therefore, the adjusted 60-day weight of this ewe lamb is 79.91 lbs.

Producers can then compare this adjusted weaning weight to other adjusted weaning weights for lambs within a flock. This allows the producer to better compare lambs from different age dams, different lamb sexes, and different birth types and rearing methods.

The Sheep Production Handbook also contains tables for adjustments to adult fleece weights for ewe age and for the expected number born adjustments for ewe age. This information will not be covered in this course. Go to the Resource page to find a link for ordering the Sheep Production Handbook.