This year we had a few lambs born as singles from mature ewes. This makes for very impressive specimens but it also makes it difficult to evaluate the potential of the lambs. The typical concerns are How do I compare growth and Will a ram who is single produce less lambs. This is how we evaluate these two things.
How to Compare Growth
Most breeders instinctively know that they should expect singles to grow faster than twins and twins should be expected to grow faster than triplets up to the point of weaning. Breeders also know that ram lambs generally grow faster than ewe lambs and that the milk produced by the ewe will vary by her age. If you have enough sample data and smart people to analyze the data you can derive conversion factors that can be applied to weaning weights so that you can compare apples with apples. This analysis has been done for Katahdins and there exists a conversion table to calculate corrected weights.
Calculating the corrected weight allows you to correct the weaning weight of the lamb for it's sex, ewe age, birth type and rear type. This corrected weight can then be used to create an even playing field to compare the weights of lambs.
When you weigh your lambs they are typically not the same age, the weights need to be adjusted for age. The age adjusted weight is a weight where all lambs are the same age. You need to calculate an age adjusted weight before you can calculate corrected weights. For example, we weighed our lambs on June 8th, so the oldest lamb was 73 days old and the youngest lamb 40 days old, so we need to adjust their weights to 60 days of age.
E.g. if you have an ewe lamb that weighed 7.5 lbs at birth, was 66 days old when weighed and weight 45 lbs and the weight needs to be age adjusted to 60 days you would do the following:
Now that all the lamb weights are age adjusted to 60 days you can calculate their corrected weights. The following table allows you to lookup all the conversion factors.
PDF Version of conversion factors.
Let's do a few corrected weight calculations so you can become comfortable with the process.
You have an ram lamb that weighed 13.6 lbs at birth, was 57 days old when weighed and weight 56 lbs, he is a twin, being raised as a twin and his mother is 5 years old.
To adjust his age to to 60 days you would do the following:
56 (current weight) - 13.6 (birth weight) = 42.4 (pounds he has gained since birth)
He had 57 days to gain 42.4 pounds so 42.4 /57 = 0.74 (pounds he gained per day)
We need to work out how much he gained by 60 days so 0.74 x 60 = 44.4 (pounds gained from birth to 60 days)
He weighed 13.6 pounds at birth so we need to add that back to get his weight at 60 days. 44.4 + 13.6 = 58 pounds
To correct his weight for his birth type, rear type, sex and mothers age we need to find the correct conversion factor from the table above:
He is a ram lamb so under "sex of lamb" we go to the "ram" row
His mother is 5 yrs old, so we go to the "3-6" in the ram row under "ewe age"
He is a twin, being raised as a twin so we go to the 2-2 column while staying in the "ram" and "3-6" row
This gives us 1.06 for a conversion factor
His corrected weight is 58 x 1.06 = 61 pounds
Example - Ewe lamb, age of ewe at birth was 1 year, lamb born as a single and raised as single:
Go to the Ewe row of Sex of Lamb, the 1 row of Ewe Age and the 1-1 column of Birth Type-Rearing column. This gives you a conversion factor of 1.17.
Example - Ram lamb, age of ewe at birth was 2 years, lamb born as a twin and raised as single:
Go to the Ram row of Sex of Lamb, the 2 row of Ewe Age and the 2-1 column of Birth Type-Rearing column. This gives you the value 1.03.
To calculate the corrected weight you multiple the age adjusted weight with the conversion factor you lookup in the table above. Now you can compare apples with apples.
Let's see if the answers make sense.
Ram lamb A: Born a single from a 1 year old ewe and raised as a single. Weighed 50 lbs at 60 days
Ram lamb B: Born a single from a 4 year old ewe and raised as a single. Weighed 57 lbs at 60 days
Ram lamb A corrected weight = 50 x 1.06 = 53.00
Ram lamb B corrected weight = 57 x 0.91 = 51.87
So ram lamb A has better growth than ram lamb B. It makes sense if you think about it, ram lamb B's dam is an adult at peak milk production while ram lamb A's dam is not at peak milk production.
To compare apples with apples you need increase the weight of ram lamb A (multiply by 1.06) because his dam does not produce a lot milk at her age, you also need to decrease the weight of ram lamb B (multiply by 0.91) because his dam is at her peak milk production.
Corrected weights allows you to compare the growth of the lambs with each other by removing the impact that their sex, the age of the dam, birth type and rear type had on their growth weaning. You are comparing the lambs not their dam's or the conditions they born into.
This is not something we just made up, most of the information here come from an article that Jim Morgan, PhD published in the The Katahdin Hairald, Fall 2009. You can also have a look at Sheep 201 where this topic is covered.
Will a Ram Who is a Single Produce Less Lambs?
The simple answer is NO, the more complex answer is MAYBE. A ram has no influence (as long as the ram is fertile) on how many lambs the ewes that he is bred to have, that is all determined by how many eggs your ewes produce. The number of eggs that an ewe produces is mostly determined by management and environment, so do you flush your ewes, do you utilize the ram effect and are you breeding out of season?
He does however influence how many eggs his daughters produce.
You definitely should look at his mothers whole reproductive history. A single ram lamb out of an ewe lamb or an adult ewe who has had twins and triplets before and just happened to have a single this one time will probably perform the same as a twin or a triplet lamb. A single ram lamb out of a mature ewe that has always had singles would be a more risky proposition.
Prolificacy has low heritability, it is estimated that for Katahdins only about 8% of the variation is because of genetics with the rest being determined by environment / management, compare that with post weaning weight with moderate heritability where about 29% of the variation is because of genetics.
When evaluating a trait such as post weaning weight you can be relatively accurate by looking at just the lamb you are thinking about buying. On the other hand when a trait has low heritability you need look more at their predecessors.
So when considering a lam that was born a single you need to look at the lambing records of the dam, daughters of the dam and siblings of the dam the get a better idea of their reproductive performance.
We hope that this will help you with your selection process. If you want more you might want to read the presentation from Susan Schoenian on Selection.