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Gang of lambs

Which lamb is the best?


You can't tell by looking!

Thor, 5 days old

Males grow faster than females

twin lambs


Single ram lamb

Singles grow faster than twins

 Triplets
Triplets don't grow as fast as twins

 Yearling mother
Yearlings don't produce as
much milk as mature ewes

 11 year old ewe
Production is less for ewes over 6

Weighing lambs

Weighing lambs

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 age (at weighing), sex of lamb, type of birth/rearing, and age of dam. If such adjustments are not made, selection will favor single-born lambs from mature ewes (3-6 years), which aren't necessarily genetically superior to their flockmates. The sex adjustment is not necessary if comparison is only among same sex groups.

Once adjusted weaning weights have been calculated, it is important to only compare lambs in the same contemporary group. A contemporary group is one in which animals are of similar age and have been fed and managed the same. For example, lambs born in January would not be in the same contemporary group as lambs born in April. If a lamb's dam was separated for preferential feeding or grazing, it would not be in the same contemporary group as lambs whose dams were not separated. Lambs that were dewormed would not be in the same contemporary group as those which were not, even if all other feeding and management was the same.

Calculating Age-Corrected Weaning Weights

The first step is to correct weaning weights to a common day of age. Weaning age varies by production system. For lambs that are weaned at less than 75 days of age, 60-day weaning weights should be calculated. For lambs weaned between 75 and 100 days of age, 90-day weaning weights are usually calculated. 120 day weaning weights are common for lambs raised in more extensive production systems, such as those common to the range. Weaning weights do not have to coincide with physical weaning, though they mostly do.

To calculate an age-corrected weaning weight, first 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 weaning age of 60, 90, or 120 days.

Age-corrected weaning weight
Age-corrected weaning weight = (pre-weaning ADG x weaning age) + 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 weaning age

An alternative would be to used breed average birth weights.

An age-corrected weaning weight is what the lamb would have been expected to weigh if it had been weighed at 60, 90, or 120 days.


Adjusted weaning weights

Once you've determined the age-corrected weaning weight, you need to adjust it for the fixed environmental factors which affect weaning weights. These include the sex of the lamb, the age of its dam, and type of birth and rearing (singe, twin, or triplet).

Adjusted weaning weight = age-corrected weight x age of dam adjustment factor x sex adjustment factor x type of birth and rearing adjustment factor.


ADJUSTMENT FACTORS

The following table gives adjustment factors for age of dam, sex of lamb, and type of birth. The adjustment factors are generic and can be used across breeds or crossbreeds.

Item
Class
Adjustment factor
Ewe age
1
1.14
2
1.08
3-6
1.00
>6
1.05
Sex
Ram
0.91
Wether
0.97
Ewe
1.00
Type of birth and rearing
1-1
1.00
1-2
1.17
2-1
1.11
2-2
1.21
3-1
1.19
3-2
1.29
3-3
1.36
Source: Sheep Production Handbook, 2015

Here are some examples of adjusted weaning weight calculations.

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.

      1. Pre-weaning ADG = (68 - 8) ÷ 70 = 0.857 lbs./day
      2. Age-corrected weaning weight = (0.857 x 60 d) + 8 = 59.4 lbs
      3. Adjustment factor for age of dam (2) is 1.08
      4. Adjustment factor for sex (ewe) is 1.00
      5. Adjustment factor for a lamb born and raised as a twin (2-2) is 1.21
      6. Adjusted 60-day weaning weight = 59.4 x 1.08 x 1.00 x 1.21 = 77.6 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.

      1. Pre-weaning ADG = (85 - 12) ÷ 75 = 0.973 lbs/day
      2. Age Corrected weaning weight = (0.973 x 60 d) + 12 = 70.4 lbs
      3. Adjustment factor for age of dam (5) is 1.00
      4. Adjustment factor for sex (ewe) is 1.00
      5. Adjustment factor for born and raised as single (1-1) is 1.00
      6. 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.


If data is available, adjustment factors can be determined for individual breeds. The following table gives the multiplicative adjustment factors for Katahdin sheep.

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
1.17
1.43
1.31
1.49
1.35
1.60
1.81
2
1.03
1.23
1.13
1.27
1.17
1.30
1.45
3-6
1.00
1.14
1.05
1.17
1.08
1.23
1.38
7 +
1.01
1.20
1.10
1.23
1.13
1.25
1.44
Ram   
1
1.06
1.30
1.29
1.36
1.23
1.46
1.65
2
0.94
1.12
1.03
1.16
1.06
1.18
1.32
3-6
0.91
1.04
0.96
1.06
0.98
1.12
1.26
7 +
0.92
1.09
1.00
1.12
1.03
1.14
1.31
Wether   
1
1.12
1.37
1.26
1.43
1.30
1.54
1.74
2
0.99
1.18
1.08
1.22
1.12
1.25
1.39
3-6
0.96
1.09
1.01
1.12
1.04
1.18
1.32
7 +
0.97
1.15
1.06
1.18
1.08
1.20
1.38
Source: A Guide to Katahdin Hair Sheep, 2016 (page 40)

Example 3

Calculate the 90-day adjusted weaning weight of a Katahdin ram lamb with a birth weight of 10 lbs. and a weaning weight of 90 lbs. at 100 days of age. The lamb was born and raised as a triplet out of a 10 year old ewe.

  1. Pre-weaning ADG = (90 - 10) ÷ 100 = 0.80 lbs/day
  2. Age Corrected weaning weight = (0.80 x 90 d) + 10 = 82.0 lbs
  3. Adjustment factor for sex, age of dam, and type of birth/rearing is 1.31
  4. Adjusted 90-day weaning weight = 82.0 x 1.31 = 107.4


<== SHEEP 201 INDEX

 

Late updated 04-Feb-2018 by Susan Schoenian.
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