Lamb size comparisonsI’ve been noticing a worrisome advertising trend in Katahdin sheep lately, and that is the bigger is always better motto. Some people are touting the adult weight of their sheep, and seeming to assume that goliath proportions are somehow desirable in a breeding animal of a meat breed variety. They tend to give the animals registered names which suggest mammoth sizes, names like Gastronormous (just kidding, I made that up, but I dibs it.) I fear that soon we could be seeing Katahdin rams advertised that tip the scale at a quarter ton by age five. But is this really what we should be shooting for?

Of course, with a meat breed, we are aiming for fast growth in lambs. Time is money, and most of us have a short window of available grass on which to grow lambs. Even people who feed lambs in a “hothouse” are anxious to get those babies on the truck to the butcher. The longer they hang around, the more food they eat, and the more labor they consume. The goal is to get them to butcher weight by six months (some intensive operations pull it off in four, and some grass-fed operations need more like nine). Of course, any wise breeder is always trying to select for genetics which can pull in that butcher date by a few weeks; and certainly avoid sliding it out.

When we look at the growth curves of four lambs from birth to 6 months, we’ll probably see a spread like this:


We’ll assume this farmer has a goal of a butcher weight of 90 lbs for grass-fed lambs. Now let’s assume these are ewes, and the farmer is going to choose two to keep as replacement ewes, and two will be sold. We’ll assume these are all twins from 3-year-old ewes, so they are on an even playing field (if they’re not, some extra math is in order to fairly compare them). So which ones have the best genetics?

A lot of people will jump right to those top two: the ones with the biggest weights at 180 days, right?? If this were 4-H or FFA judging, we’d place them like this, no?


Ah… But, what if we look into the future, to examine their adult weights?


We know that if our primary selection is on big lambs, we’ll get two things from this genetic pressure. We’ll get some lambs with accelerated maturity, where they approach adult weight sooner. And, we’ll also get some lambs that are just destined to be very large adult animals.

Lambs A, C and D all have a knee-bend in the curve, they accelerate quickly to 180 day weights, then they slow down heading towards adulthood. But, Lamb B- she just keeps going at the same rate, doesn’t she? So she actually has a slower early growth rate than the other three, she’s only at 52% of her adult weight at six months; whereas the others cluster around 60%.

If we look ahead at how these animals turn out as adults, we can see that Ewe B is going to eat the most later. Ewe B turns out to be 70 pounds heavier than Ewe C as an adult. Ewe B will eat 2 more pounds of hay per day as an adult than Ewe C! That’s 360 lbs of hay over a six-month winter period, approaching a quarter ton more food annually. And because Ewe B has genes for late maturation, when she does have lambs which are destined to be average-sized adults, they won’t hit butcher weight on time. So, she actually has two genetic detractors. And it’s possible she even has a third strike: she may not be sexually mature enough to lamb as a yearling.

It’s possible that in a feed-intensive operation, if Ewe B is able to produce big twins and triplets which hit butcher weight a month early most of the time, the tradeoff in feed might be equitable or even advantageous. But, for most operations, it’s not. Especially if Ewe B ever slips up and has just one, big single, or misses lambing her first year. Her ROI will go down the drain because she’s so much more expensive to maintain. And there are other factors to consider: animals that approach gigantism compared to their peers are known to have shorter lives, they break down earlier, and have more physical problems.

So what of the other three ewe lambs? We know that ewes mature a little slower than rams and wethers. So if I were picking replacement ewes, I might not worry so  much about Lamb C being a little underweight at six months. She will likely still be able to produce male butcher lambs which hit butcher weight on time, especially if paired with a high-growth sire, and if her milk is good. As a 140 pound adult, she’ll be easy to feed and handle.

Lamb A is awesome: she’s right on target and will probably produce ram lambs with more aggressive growth than herself, pulling in that butcher date by a few weeks. Lamb D is also in the ballpark, but she might be edging towards bigger-than-necessary. So, I’d place them like this, if this were the only criteria I were using to choose replacements:


That’s a reversal of which lambs I’d keep compared to what many believe. Of course, when we’re choosing them at 6 months of age, we don’t know their future adult weight. But we can project based on their genetics; and often even by their appearance. Sheep which will eventually be huge tend to be really big-boned and long-limbed as lambs; whereas more moderate sheep are more lithe and square when they are young.

Also, hopefully nobody is selecting breeding stock solely on this lone trait. If one lamb in that group of three had some terrible conformation trait that would compromise her physical soundness, a really bad temperament, a coat that didn’t shed, an irresponsible mother, or she lacked twinning genetics, I’d pass her over for one of the others.

When choosing ram lambs, the considerations are the same if his daughters may be kept. But if we’re going to be using a ram as a terminal sire, meaning we’re not keeping any breeding stock out of him and just want big butcher lambs, then a big ram might be great (but still, up to a point: we don’t want him so big he has trouble pairing with small ewes, and we don’t want him to skew birth weight into the territory of birthing problems.). And in that case, we may want to go further and use a different breed, to take advantage of heterosis.

In general, however, do we really want to be breeding a sheep that turns out as big as a pony, so we can dub him Gastronormous? Or should we be shooting for really beefy, highly matured teenagers that turn into modest-sized adults? I think the latter!