How birth weight affects lamb survival
Updated: Apr 29
It is well established that the birth weight of a lamb affects its chance of survival.
Interestingly, between lambs born to mature ewes and lambs born to hoggets there are different ranges in lamb survival, based on birth weight.
There are also differences in survival based on lamb birth rank, within those born to hoggets and within those born to mature ewes.
Lambs that were 2.5 kg or lighter at birth had a high risk of dying, regardless of their birth rank or age of their dam.
Heavy lambs have different survival rates, dependent on their birth rank and the age of their dam.
Overall, lambs born to hoggets were lighter at birth and had lower rates of survival than lambs born to mature ewes.
If hoggets are to be bred, they need to be heavy enough at breeding to maintain a pregnancy, and successfully wean a lamb.
Emma Pettigrew has written this summary as a part of a series on the performance of hoggets and mature ewes. Emma is hoping to submit her PhD shortly on “The effect of birth rank and dam age on the reproductive performance of ewe replacements managed under New Zealand pastoral conditions”. We will link all of Emma's summaries at the end of this article if you want to read more about Emma's research.
Some of the main causes of lamb death during the first week of life include dystocia and starvation and/or exposure.
Dystocia can be due to:
feto-maternal disproportion: the lamb is too large for the birth canal
entanglement: multiple lambs trying to exit the birth canal at the same time
malpresentation: lambs are presented in the breach position for birth
Starvation and/or exposure can be due to:
lambs are born too light, and die due to a lack of body reserves, resulting in hypothermia.
dystocia: lambs have a difficult birth, and subsequently die due to starvation and/or exposure.
misadventure: lambs lose contact with the mother and die due to starvation and/or exposure.
Lambs that are heavier at birth are likely to die from dystocia related to the lamb being too large for the birth canal. Lambs that are lighter at birth are likely to die from starvation and/or exposure. Birth weight can play a large part in the chances of survival of the lamb.
It has been reported that lambs born to hoggets are smaller at birth and have lower rates of survival. However, due to the differences in lambing time and sires used, direct comparisons can’t be made. Thus, there is little comparable information about the lamb survival for lambs born to either mature ewe or hoggets when using the same rams.
This study was a preliminary step in breeding lambs to be selected as replacements born to mature ewes and hoggets.
Details of the study design and the performance of the ewes (live weights, condition scores, pregnancy rates and lambing percentages) can be found in Emma’s previous summary --> “Differences in lambing performance between ewes and hoggets”
Lambs were identified and tagged within 16 hours of birth and had their birth weight recorded. Lambs that were dead at the time of tagging were noted, and lambs that died after the tagging were also recorded.
Lambs born as singles to mature ewes were the heaviest at birth, averaging 5.65 kg as seen in Figure 1. Twins born to mature ewes were the next heaviest at birth averaging 5.16 kg. Triplets born to mature ewes were similar to singles born to hoggets, being the third heaviest at 4.25 and 4.38 kg on average. The lightest at birth were twins born to hoggets that were on average 3.45 kg.
Lambs born as singles to mature ewes had the greatest average survival at tagging (within 16 hours of birth), as seen in Figure 2. Lambs born as triplets to mature ewes and born as singles or twins to hoggets had the lowest survival at birth.
Based on lamb birth rank and the age of the dam there were differences in lamb survival, based on birth weight.
Figure 3 shows the predicted survival rate for lambs that were born as singles from mature ewes for different birth weights from approximately 3 kg to 8 kg. Singles born to mature ewes had a high rate of survival, up to a birth weight of around 7.5 kg, where survival rate declines.
Comparing this to lambs that were born as twins or triplets to mature ewes (Figure 4 below), lambs born as twins to mature ewes had a quadratic shape for survival. This means that with the extreme birth weights there was lower survival rates and the middle of the pack weights had the best survival (4-6 kg birthweight).
Survival of triplets born to mature ewes increased as birth weights increased, but had a lower overall survival.
In Figure 5 below is the survival of single and twin lambs born to hoggets. The singles born to hoggets have the largest range in birth weights for all the birth ranks ranging from below 2kg to almost 8kg, similar to that of a twin born to a mature ewe. This quadratic shape for survival shows again that the extreme birth weights have lower survival rates and the middle of the pack weights had the best survival. The birthweight range for optimal survival for lambs born as singles to hoggets was lower than twins born to mature ewes.
The twins born to hoggets also show a similar shaped survival graph, however, there is a smaller optimum range for survival (3.5 - 4.5 kg) and the overall survival is lower.
The heaviest twins born to hoggets were 5.6 kg, this is the average birth weight for singles born to mature ewes.
As seen in Figure 6 below (when all 5 birth rank-ewe age combinations are put together), all lambs (except singles born to mature ewes, which didn’t have low enough birth weights) had similar levels of survival when they were under 3 kg at birth, regardless of birth rank or ewe age.
This suggests that all lambs die equally at low birth weights, probably due to the effects of starvation and/or exposure.
Singles born to mature ewes had much greater survival than singles born to hoggets at birth weights of 7 kg. This is possibly because the mature ewe is larger than the hogget, and therefore has a larger reproductive tract capacity.
Smaller sheep will have higher rates of dystocia when giving birth to heavier lambs, compared with larger sheep. Therefore, lambs born to hoggets have lower rates of survival, at the same birth weight, than lambs born to mature ewes, if birth weights are high for that birth rank.
The conclusions from this study are that lambs born to hoggets were lighter at birth and had lower rates of survival than lambs born to mature ewes.
This highlights the need to ensure that if hoggets are to be bred, they are heavy enough at breeding to maintain a pregnancy, and successfully wean a lamb. Hoggets and ewes bearing triplets need to be offered the best paddocks to ensure lambs have the greatest chance at survival (more on triplet lambs here and here).
Lambs that were 2.5 kg or lighter at birth had a high risk of dying from starvation and/or exposure.
Heavy lambs have differing survival rates, dependent on their birth rank and the age of their dam.
Farmers can intervene in these cases, assisting the birth in cases of dystocia, or bringing a lamb home to give an extra feed for lighter lambs prone to starvation and/or exposure, to increase lamb survival rates. However, it is beneficial to know the relative differences in survival, depending on dam age and lamb birth rank.
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Find the full text of the article here:
Pettigrew, EJ, Hickson, RE, Blair, HT, Griffiths, KJ, Ridler, AL, Morris, ST and Kenyon, PR. 2020. Differences in lamb production between ewe lambs and mature ewes. New Zealand Journal of Agricultural Research, In Press.