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The Seasonality of Breeding

THE EFFECT OF DAY LENGTH ON THE BREEDING SEASON

The sheep is a seasonal breeder in the UK, and the main factor influencing when breeding will occur is the proportion of hours of daylight to dark photoperiod.
At the base of the brain is the pineal gland, which starts producing the hormone called melatonin as soon as it gets dark, and ceases production when dawn breaks. When the days get shorter in the autumn, sufficient melatonin is produced to initiate the start of breeding cycles in the ewe. As the days begin to lengthen once more in the spring, there is insufficient production of melatonin and the breeding season peters out until the following autumn.
It is not simply the day length which is important, but also whether the day length is steadily declining or increasing. Increasing day length is more inhibitory to breeding and this accounts for the fact that it is more difficult to bring ewes into heat in spring and early summer, rather than after the longest day (21 June) has passed.
In the tropics where day and night are of roughly equal length, the native sheep breeds have no breeding and non-breeding seasons as in northern or southern regions of the world, so that ewes may breed all year round. However, there are still some periods of the year when breeding is more successful than at others. Sheep moved from the northern to the southern hemisphere, or vice versa, will eventually adjust and breed at the same period as the native sheep, although it may take them a year or two to settle down.
Other factors have an effect on the onset of the breeding season, but in the fine tuning rather than in its overall control. Shepherds will be aware that a cold spell of weather in autumn may 'bring ewes on' sooner than when the weather is warmer. Sheep at high altitudes may be slower to start breeding, but it is difficult to separate this from the poorer level of nutrition associated with hill sheep which also has an inhibitory effect. Ewes in very poor body condition are likely to be slower in coming to the ram, to have a poorer conception rate and a higher barren rate and to produce fewer lambs, than ewes in average to good body condition. Disease may also affect the body condition of ewes and rams and thus exert an indirect effect upon fertility.

LENGTH OF THE BREEDING SEASON

Breeds vary considerably in the length of their breeding season. The shortest day (21 December) is the mid point of sexual activity for most breeds, but the peak of activity is generally in October or November, and that is when they are usually most fertile. The mountain breeds such as the Scottish Blackface, Swaledale, Cheviot and Welsh Mountain have a short breeding season of about three to four months—time to fit in five to seven cycles of around 17 days each. Crossbreds such as the Mule, Masham, Greyface and Scottish Halfbred have an intermediate breeding season of around five months in length, whilst Suffolks and Suffolk crosses may fit in 10 or 11 cycles in a period of around six months.
Unique among British breeds is the Dorset Horn, which has a very long sexual cycle of around eight months. The peak of its breeding activity tends to be several weeks earlier in the year than for other breeds. In flocks where rams are run with the ewes all year round (an ill-advised thing to do) a few Dorset Horn lambs may be dropped during every month of the year. This feature of the breed is often made use of in early, or out-of-season, breeding flocks. Crossing the Dorset Horn with a breed of high fecundity, such as the Finnish Landrace, produces female progeny with intermediate fertility and intermediate length of breeding season which can be used to advantage in these systems.
Crossing any two breeds with different lengths of breeding season will produce female progeny with an intermediate breeding season. In general, breeds with a short breeding season start their sexual activity later in the autumn and finish earlier in the spring.
Apart from the major differences between breeds, there is also con¬siderable variation between individuals of the same breed and this fact has practical significance. In flocks with a mixture of breeds, the variation may be even greater. In the absence of the ram, ewes tend to trickle into sexual activity and there may be several weeks between the time when the first ewe and the last ewe have their first heats. These events will be detected only by the ram of course.
Generally speaking, it is best to mate a particular breed at the time of year when the fertility will be optimal, rather than to attempt ambitious early breeding schemes with an unsuitable breed. Also, changing breed is not always acceptable, and so the ewes may be manipulated in one way or another to persuade them to breed when they would not normally do so, hopefully without compromising fertility too greatly. However, the further away from the peak of breeding activity that mating takes place, the lower will be the conception rate, the higher the barren rate, the smaller the lamb crop and the more protracted the lambing period.

The Swaledale - The Veterinary Book for Sheep Farmers

Plate 2.1 The Swaledale and other mountain breeds have a very restricted breeding season.

Scottish Greyface - The Veterinary Book for Sheep Farmers

Plate 2.2 Scottish Greyface (above) and Scottish Halfbreds (below) have a breeding season of intermediate length.

Scottish Halfbreds - The Veterinary Book for Sheep Farmers

THE BREEDING SEASON IN RAMS
It is not often appreciated that rams also have a breeding season. It is true that rams of most breeds are capable of working all year round, but it is perhaps not so widely appreciated that their performance—in terms of both sexual drive and the fertilising capacity of their semen—may decline during the period of the year when ewes are not sexually active. This is due to the same factors as in the ewe, since day length affects the hormone levels of both sexes. This has important implications for early and out-of-season breeding programmes. The factors affecting ram performance are discussed in detail in the following pages.
A successful lambing is dependent upon planning which begins at least seven or eight months earlier, with the selection and preparation of both ewes and rams for the forthcoming mating season. It will be assumed here that the flock is a spring lambing one. Flocks attempting early lamb pro¬duction or frequent breeding have some special preparatory requirements which will be described in the chapter on the manipulation of breeding.

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