It's almost time to start breeding the ewes for next spring's lamb crop. The rams have spent the last 321 days keeping each other company, finding things to do and eat. They're fat and spunky and aware of the short days and cool nights. They're ready for their annual month of procreation.
Most breeds of sheep are seasonal breeders. That means they will breed in the late summer, through the fall, and into the early winter. As days start to lengthen, the ewes stop ovulating and the rams lose some of their libido. The farther you are from the equator, the more pronounced this effect is and the shorter the breeding season is. What triggers the hormonal changes in seasonal sheep is a little bit confusing. It's not exactly day length, although day length plays a part. It's the rate of change in day length that does it. So as you go farther north in the Northern Hemisphere, the difference between the length of daylight on June 21 and Dec. 21 gets greater and greater, the rate of change gets faster and faster and hence the breeding season gets more and more compressed.
"So what? Who cares?" you might be tempted to ask. Shepherds do. This is one of the coolest things about sheep production -- it could almost make you believe in intelligent design if it weren't so perfectly explained by evolution. There comes a point in the breeding season where everything just lines up perfectly. Here in the mid-latitudes, seasonal ewes start to ovulate in late August. It'll just be a few to start with, say 15 percent of the flock. Seventeen days later, most of these ewes will ovulate again, and a few more will join them. As we move into October, nearly all the flock will be ovulating, but their cycles will be scattered. By the end of October, however, "the dormitory effect" will start to kick in.
The dormitory effect gets its name from the fact that, generally speaking, groups of female mammals living together will tend to synchronize their ovulation cycles. By late November, all the ewes are cycling, and they are all fertile within a day or two of one another on each cycle. If they are not bred, this will continue for several weeks, probably until early January when days start getting longer again. Then some of the ewes will stop cycling, and gradually the synchronization will start to end and by sometime in late Feburary or early March, the ewes would no longer be ovulating at all.
But in those weeks at the end of November and early December, it's possible to have so many ewes fertile at the same time that rams can breed dozens of ewes every day. I had one North Country Cheviot ram that had bred 30 ewes within his first 45 minutes of his introduction to them. This leads, 145 days later, to a lot of ewes lambing at the same time. And lo and behold, 145 days after the beginning of this peak of fertility is the onset of grass growth and the beginning of warm weather.
Shepherding a group of lambing ewes, and later, their lambs, is much much easier if all the ewes are lambing at the same time. We introduce the rams at the time of year when the most ewes are fertile at the same time, and they can breed the whole flock in two cycles, or 34 days. There's enough variation in gestation length that all the ewes don't lamb exactly 145 days later, but there's a definite peak about 10 days after lambing starts, and a valley a few days later, followed by another, smaller peak about 27 days after lambing starts, representing ewes that were not bred on the first cycle.
When lambing is brief, the nutritional needs of the flock through the winter are consistent and manageable. The lamb crop is ready to be weaned at the same time in late July or early August, and all the lambs are close enough to the same age that they can be managed in a single group from then until market.
When lambing is long and drawn out, some ewes are very close to lambing and have much higher nutritional needs than their fellow ewes bred later, who may still be months from delivery. The shepherd faces a choice of segregating sheep into different groups based on estimated lambing dates, or splitting the difference in their nutritional needs and hoping for the best. And hoping for the best is never very successful in shepherding, let me tell you.
The only way to control lambing time is to keep rams separated from the ewes for all but a carefully selected period of time. Put the rams in with the ewes in very good condition, let them breed for 34 days, and take the rams out. If you have ewes that haven't gotten bred in that time, don't keep them around. Within a few years, you'll have a flock of ewes that lambs in short order in the spring. My flock took just 18 days from the first lamb to the last in 2007.
I have four rams right now. Technically speaking, that's two and half more than I really need to breed my flock of 75 ewes. But having more rams allows me to produce distinct lines of ewes and market lambs, and ensures a little bit of genetic diversity in my flock. For 331 days of the year, these guys rest and relax, fight and feed, but for the other 34, they ensure the continuation of my flock and my sanity at lambing time.
Shepherds use a five-point scale to describe the condition of sheep. A 1 is emaciated, 5 is obese. I aim to have my rams at about 4 or even 4.5 at the beginning of breeding season. It's not uncommon for rams to lose two condition score points during breeding season, as they are too distracted to eat very much, and they're burning lots of calories herding their ewes. So eat up guys. November 30 is fast approaching, and I want you well rested and ready to party.