Sheep 201: Calculating adjusted weaning weights

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Males grow faster than females

Singles grow faster than twins

Triplets don't grow as fast as twins

Yearlings don't produce as
much milk as mature ewes

Production is less for ewes over 6

Weighing lambs

Calculating Adjusted Weaning Weights

To be used properly in selection, all records need to be adjusted to a common basis. Growth measures such as weaning weight need to be adjusted for sex of lamb, type of birth/rearing, lamb age, and age of dam.

Calculating Age-Corrected Weaning Weights

The first step is to adjust weaning weights to a common day of age. For lambs that are weaned at less than 75 days of age (on-average), 60-day weaning weights should be calculated. For lambs weaned over 75 days of age, 90-day weaning weights should be calculated. To do this, you need to determine the lamb's pre-weaning average daily gain (ADG).

Pre-weaning ADG
Pre-weaning ADG = (weaning weight - birth weight) / weaning age

Pre-weaning ADG is used to predict the weight of the lamb at the common age of weaning (e.g. 60 days)

Age-corrected weaning weight
Age-corrected weaning weight = (pre-weaning ADG x 60 d) + birth weight

If birth weights are not known, pre-weaning weight per day of age (WDA) can be used instead of pre-weaning ADG

Pre-weaning WDA = weaning weight / weaning age

Age-corrected weaning weight = WDA x 60

Adjusted weaning weights

Once you determine the age-corrected weaning weight, you need to adjust it for the sex of the lamb, age of its dam, and type of birth and rearing. The table below gives the appropriate adjustment factors.

Adjusted weaning weight = age-corrected weight x adjustment factor

ADJUSTMENT FACTORS

Multiplicative adjustment factors to correct for effects of sex, age of dam, and type of birth and rearing on lamb weaning weights.

Sex of
Lamb
Age of Dam

Type of birth and rearing

1, 1

1, 2

2, 1

2, 2

3+, 1

3+, 2

3+, 3

Ewe

1

1.14

1.30

1.27

1.37

1.36

1.46

1.56

2

1.06

1.21

1.18

1.27

1.26

1.36

1.45

3-6

1.00

1.14

1.11

1.20

1.19

1.28

1.37

> 6

1.04

1.19

1.15

1.25

1.24

1.33

1.42

Ram

1

1.04

1.18

1.15

1.24

1.23

1.33

1.42

2

0.96

1.10

1.07

1.16

1.15

1.23

1.32

3-6

0.91

1.04

1.01

1.09

1.08

1.16

1.25

> 6

0.95

1.08

1.05

1.14

1.13

1.21

1.30

Wether

1

1.11

1.26

1.23

1.33

1.32

1.42

1.51

2

1.03

1.21

1.14

1.23

1.22

1.32

1.41

3-6

0.97

1.11

1.08

1.16

1.15

1.24

1.33

> 6

1.01

1.15

1.12

1.21

1.20

1.29

1.38

Source: Sheep Production Handbook
Different adjustment factors are available for different breeds.

Here are some examples:

Example #1
Determine the 60-day adjusted weaning weight of a ewe lamb with a birth weight of 8 lbs. and a weaning weight of 68 lbs. at 70 days of age. She was born and raised as a twin. Her dam is 2 years old.

Pre-weaning ADG = (68 - 8) / 70 = 0.857 lbs./day
Age-corrected weaning weight = (0.857 x 60 d) + 8 = 59.4 lbs
Adjustment factor for a ewe lamb, born and reared as a twin, out of a 2 year old dam is 1.27 (see table)
Adjusted 60-day weaning weight = 59.4 x 1.27 = 75.5 lbs.

Example #2
Determine the 60-day adjusted weaning weight of a ewe lamb with a birth weight of 12 lbs. and a weaning weight of 85 lbs. at 75 days of age. She was born and raised as a single. Her dam is 5 years old.

Pre-weaning ADG = (85 - 12) / 75 = 1.04 lbs/day
Age Corrected weaning weight = (0.97 x 60 d) + 12 = 70.4 lbs
Adjustment factor for a ewe lamb, born and reared as a twin, out of a 2 year old dam is 1.00 (see table)
Adjusted 60-day weaning weight = 70.4 x 1.00 = 70.4 lbs.

If you had only looked at actual weaning weights, you would have favored the second ewe lamb in your selection program. However, the first ewe lamb has superior performance and is the genetically superior choice.

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