Bell curve of lamb birth weights over multiple years

I have written before about how I watch the trend of birth weights in my lambs. Here’s an updated analysis with this year’s data.

Birth weight is tricky. We know it’s statistically correlated to growth later on, so we want it as big as is tolerable. Small birth weight lambs not only grow slower, they are at risk in cold weather: their petite frame size can limit their ability to keep warm. On the other hand, big lambs can cause trouble during birth. They can get hung up, requiring assistance, and put themselves and the ewe at risk. Even if they deliver unassisted, lambs which spend a lot of time in the birth canal can be exhausted when they are finally born. Lambs aren’t born with a lot of energy reserves. So if a lot of energy is wasted just getting out of the womb, there isn’t much left to power the arduous task of learning to stand and nurse. If it’s cold, these lambs can waste the rest of their energy shivering; then just lay there and die if nobody is around to help them get re-fueled.

Ewe with nice-sized lambSo, there is a sweet-spot for birth weights. Different people have different goals. Some people are willing to tolerate big lambs because they are there to assist with births 24/7. Other people are willing to tolerate small lambs because they lamb  indoors and prioritize easy births over early growth. I prefer neither extreme. I want lambs born between seven and eleven pounds. Seven-pound lambs, I find, are plenty big enough to withstand cold, driving rain. Eleven pound lambs still deliver easily, even in smallish ewes. Lambs outside of this bracket are ok, but if they are way outside, that’s not so good. Five-pounders are awfully tiny, and thirteen pounders are often a long struggle for a ewe to deliver on her own.

This year, I don’t think I could have done any better at nailing the sweet-spot between the goal posts. The green line shows a perfectly centered, and fairly narrow bell curve, with a mean of 9.0 pounds and a standard deviation of 1.5  pounds. This is achieved by both managing ewe nutrition before and during pregnancy, as well as choosing the right genetics.

Tiny LambI got rid of a ram which was throwing small birth weights, that helped a lot. And, I was just lucky with feeding this year; I had good hay, managed parasites well, the winter was unchallenging, and things went swimmingly. I also didn’t have any ewelambs twin, which has its own pro’s and con’s; but that’s where some of the tiny tots come from. Of the lambs that were above twelve pounds, most were singles. This is why we have to be careful with the top end: usually twins and triplets don’t push the envelope of being too-big at birth. But you get one baby in there with a mature ewe, and watch out, that thing is going to be the size of a colt!

Now, the goal will be to fight to keep the bell curve here. As I move the bell curves around sixty and 120-day weights to the right for lamb growth, that genetic trend is going to put pressure on this bell curve to also move to the right. I don’t want that. So now it’s time for me to start scrutinizing the birth weight NSIP scores of my breeding stock, and holding the line of moderate-sized lambs at birth that can take off and grow once they’re on the ground.

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