Why 60 Day Weights?
We weighed the lambs to take their 60 day weights the end of May. We did it when the average age of the lambs were about 60 days. The growth of lambs up to 60 days are greatly influenced by:
Environment
Growth Genetics (inherited from dam and sire)
Dam Milk Production
Environment
When grazing and being 100% dependent on pasture you quickly learn that no one year is the same as the another. We don't creep feed nor do we supplement the ewes so there's no making up for bad pasture quality or difficult grazing conditions. The largest impact we see on the lambs produced from year to year comes from the environment.
Growth Genetics
The growth genetics come from the dam and the sire, typically estimated as 50% from the dam and 50% from the sire. In NSIP the WWT (weaning weight) EBV (estimated breeding value) quantifies the growth potential of an animal at 60 days old. The heritability of WWT is around 24%, this means that 24% of the variance you see among animals are because of genetics the rest of the variance is because of non-growth genetic factors.
Dam Milk Production
If a lamb does not get milk it cannot grow, no matter how much growth genetics it has. Even though we do not wean at 60 days, this is a good time to measure as the ewes are still producing a fair amount of milk. In NSIP the MWWT (Maternal Weaning Weight) EBV captures the milk producing ability of the ewe. This EBV ultimately comes down to "What is the impact the ewe has on lamb growth other than the growth genetics that she contributes". MWWT is about 15% heritable and it takes having and testing a good number of female relatives and offspring that lamb to get a good estimate for this EBV.
You might think that you don't need to concern yourself with MWWT when purchasing a ram, but you do need to consider it. The ram does not produce milk himself but he does provide milk genetics (50%) to ewe lambs that he produces that you might keep as replacements.
2021 Data
Looking at raw data can be tricky because not all farms are the same and most of the growth of lambs is due to the environment / management. It is hard to compare one year to another or even one grass fed operation to another. There are some things to take into consideration when comparing lambs. How old is the dam, what was the birth and rear type and where did the animal rank relative to it's contemporaries.
Below is a table that shows the weight of the top 10th percentile for ram lambs for birth and rear types 1-1, 2-2 and 3-3, the table also shows the dam age at lambing, 1, 2 and Adult.
In an ideal world the ram lambs we sell as breeding stock this year will all be in the top 10th percentile for growth. In reality they wont be, we try and balance growth with parasite resistance and milk production (as it relates to performance metrics).
The image below show a graph of the same data. This year the data is almost text book, you can see how big of an impact the age of the dam has as well as the difference in birth and rear type.
Summary
There's a lot to consider when selecting a ram. You have to like the ram and the ram has to help you improve your flock and product. When we provide data for selection we will include WWT and MWWT EBV data, 60 weights (including rankings), birth / rear types as well as the age of the dam (as well as other data).