Warm lambing

Although today’s high is predicted to reach 40F and last night’s low sat at about 25F, these temperatures are particularly warm for this time of year in Iowa. I frequently hear from people who wonder why we don’t push our lambing into the spring, which would provide more reliable warmth. You’d think I’d be thrilled to be doing my barn checks in balmy spring temps, but warmth during lambing isn’t as great as it might at first seem.

We have discovered, after many years of lambing between Valentine’s Day and April Fool’s Day, that lambs generally do better when born in cold weather. Iowa weather being what it is, late February is typically still quite cold, with an average high near freezing and an average low of about 19F. Yet before lambing is over, the highs average about 50F or higher and the lows typically average at or above freezing. This has allowed years of comparison between our early-born lambs (who come in colder temps) and those born during the warmer spring weather — and I was surprised at what we uncovered!

If we divide our lambs into two groups — those born before about March 4 and those born after — there is an obvious difference. Keep in mind that this date moves back and forth a bit, from about March 2 to about March 10, depending on the weather. Yet the results are the same from year to year. The early-born lambs gain weight better and mature earlier, while their later-born half-siblings (since the sires are common between many of our lambs) are smaller and slower-growing. You can almost see a dividing line on the calendar each year — those born before that date do very well, while those born afterwards can’t be fully assessed until their next year when they’ve had more time to grow and mature.

I know it sounds as though the difference I’m seeing is potentially insignificant, but that’s not the case at all. In some years, the average daily gain of the early group can be 0.25 pounds or more higher than that of the latter group. Think about what that means for two nearly identical lambs born at, say, ten pounds each — one on Feb. 15th and the other a month later on March 15th. Assuming an average daily gain of 1.0 lb per day for our early-born lamb (which is not unusual in our flock), that lamb would weigh 17 lbs after a week, 38 lbs at 4 weeks, and 70 lbs at weaning — a very nice size! Our late-born lamb, in contrast, will weigh 15.3 lbs at one week, 31 pounds at 4 weeks, and 55 lbs at weaning. Quite a difference!

This difference between the lambs continues throughout the summer while they are on pasture. Their weight gains slow a bit as they get bigger, but the weight-gain difference continues. By the end of the summer, if we put these lambs into a breeding group, the early-born lamb might weigh around 130 pounds while the slower-growing late-born lamb will only come in at 80 pounds or so. This difference in weight reflects a true difference in maturity, and it is likely that the early-born lamb will breed in that first breeding season and safely deliver a lamb in spring. Meanwhile, the late-born lamb would likely be too small and immature to breed, forcing us to wait another year to see what they can produce.

I’m sure you’re wondering why nicer weather would result in lambs who don’t gain as well. Intuitively, we think it would be the other way around: since the early-born lambs must use their nutritional intake for warmth, they would be smaller overall; the lambs who come in warmer weather can use all of their nutrition for growth. Yet year after year, this has proven to be wrong. So why do the early-born lambs, arriving in our sometimes sub-zero February temperatures, out-gain the late-born lambs time and time again?

I’ve developed a hypothesis. A lamb that is born during the very cold temps of an Iowa February must nurse aggressively and often in order to survive. If they don’t, they will perish in the cold, despite our heat lamps and everything else we do to keep them alive. Their survival depends on nutrition. Once the cold weather begins to warm (only weeks after they are born), they continue the habit they’ve developed: continually filling their bellies to capacity.

The late-born lambs come in warmer weather, and because they don’t have to worry about freezing, they nurse less often and take in only what they feel they need. As the weather continues to warm — and actually through their whole first year — they continue this habit of a less-filling nutrition. During the next winter, while the big, early-born lambs often continue to show weight gains even when eating hay during our snow and cold, the warm-weather lambs typically don’t gain much at all.

So although I do like the warmth for my own benefit, as far as the lambs are concerned, I much prefer the cold for lambing. I can think of nothing better than a flock of fast-gaining, fast-maturing lambs, ready to take their place in the world come fall! And in fact, we’ve already had lambs from two of last year’s early-born: Ossidy and Olive have given us lambs of their own (Peabody and Pickles, respectively), and O’Chloe and Olympia, both born before the cold weather cut-off last year, are yet to deliver. Warm weather is delightful for vacations — but for lambing, nothing can beat a good freezing-cold week in the 20s!

Skirting progress: The fleeces of both breeds are all skirted, and I hope to post them to our notification list sometime around March 10th. Stay tuned for details as to date and time.


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  • Jane says:

    Thank you for explaining this. It is very unintuitive but when the difference is still apparent in the following winter it has to be taken seriously. Do the smaller lambs ever catch up? And speaking of smaller lambs did I miss what happened to the really teeny tiny lambs?

    (I love the name Pickles.)

    • Dee says:

      The lambs do eventually catch up to their genetic potential in size, but often not until their yearling summer. When they come in from the pastures in fall, I sometimes fail to recognize them due to their surprisingly larger size!

  • Erika says:

    I wonder if the milk of the cold weather lambing ewes has a higher fat content for more calories and if this continues through to weaning?

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