S for Seasonal Breeders

If you’ve been watching farming programmes on tv, or paying attention to what you see in the fields at different times of the year, you might have noticed you usually see newborn lambs bounding around in Spring, but see piglets and calves at all times of the year.

In the wild, animals evolved to breed at certain times of the year to ensure their newborns had the best chance of survival. Usually, their biological clocks timed it so they’d give birth when the temperatures began to lift and food became less scarce, so usually Springtime.

Through domestication, we’ve bred that seasonality out of cows and pigs and they now cycle all year round (on average, every 21 days), and can therefore give birth at any time of the year, but most sheep breeds have held onto that trait.

There’s not much that works more in-tune with the seasons than sheep farming!

Most sheep breeds are Seasonal Breeders, or Seasonally Polyoestrous to give it the proper term.

Polyoestrous Animals – Animals who have oestrous cycles throughout the whole year, not determined by day length (cattle & pigs).

Seasonally Polyoestrous Animals – Animals who only have oestrous cycles during certain periods of the year, determined by day length (horses, most sheep & goats).

Sheep are short-day breeders, meaning as the daylight hours reduce, they begin to cycle and become fertile. The ewes then come into season around every 17 days until the days lengthen again.

It’s quite amazing how their body can determine when to begin cycling by how many hours of light are entering their eyes each day!

Some breeds are less seasonal than others and can begin cycling as early as July, so lambs can be born in December and be the first to be fully-grown and finished for an Easter roast. And a few breeds have the ability to breed almost all year round!

You can also slightly fiddle with the dates ewes start cycling by using different types of hormonal interventions, or by using something called “the ram effect”.


Pheromones given off by an intact ram or a vasectomised one (known as a ‘teaser’ – who has all the testosterone but can no longer impregnate anything) can cause the ewes to begin ovulating when they’re introduced to them. Isn’t nature clever?

It’s most effective when the ewes are almost ready to start cycling, and if the ram is from a breed who would be naturally cycling at that time, because their fertility is also affected by the day length but not as much as a ewe’s is.

Using a Teaser can bring the breeding season forwards by 2 to 3 weeks, and synchronise the batch of ewes, so they all start ovulating around the same time and lamb closer together!

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