Seasonality of Reproduction - Anatomy & Physiology

Introduction

Unlike polyoestrus animals such as the cow or sow which can reproduce all year round some species such as the mare, ewe, goat, rabbit, hamster and doe have an anoestrus phase to their reproductive cycling. This means that for part of the year they are not experiencing oestrous cyclicity and so are not reproductively active. Such animals can be long or short day polyoestrus breeders. Seasonality of reproduction is due to the effect of light levels on the brain. Reproductive seasonality maximises the chance of offspring survival by timing parturition to coincide with spring when nutritional and climate conditions are optimum for lactation and for growth of the offspring. It is important to be able to distinguish between seasonal anoestrus and pathological infertility. Both males and females exhibit reproductive seasonality. However, the female is more strongly influenced by season.

Mechanism of seasonality

During the day, light levels are detected by the retina which sends impulses to the suprachiasmatic nucleus of the hypothalamus and then to the superior cervical ganglion. Postganglionic nerve fibres synapse with inhibitory nerve fibres within the pineal gland. These fibres cause inhibition of the pinealocyte cells of the pineal gland preventing synthesis of the hormone melatonin. At night there is reduced retinal firing and so reduced inhibition of pinealocytes by postganglionic superior cervical gangion fibres and so melatonin is synthesised and released. Melatonin stimulates the synthesis and release of GnRH from the hypothalamus leading to pulses of luteinising hormone release from the pituitary gland. High frequency, low amplitude LH pulses lead to breeding activity. Where as low frequency, high amplitude LH pulses lead to anoestrus and inactive gonads.

Long day breeders

Hamsters
The mare
The mares breeding period is from April-June, these months having increasingly long hours of daylight.
Reproductive cycling occurs with increased photoperiod (increasing light levels).

Short day breeders

Ewe, Doe, Goat
Their breeding season is at the end of summer and beginning of autumn.
Respond to decreased photoperiod.

Manipulation of Seasonal breeders

Apart from pharmalogical manipulation of the oestrous cycle, photoperiod, nutrition and temperature may also play a part in the controlling entry into the breeding season in some species.

Reasons for controlling breeding seasons

The Mare

To shorten the Anoestrus period after foaling.
In order to synchronize a group of mares so that they may all be mated at the same time or to make Embryo Transfer possible.
Control of entry into the breeding season is particually important in producing foals for the racing industry. Racehorses run from the age of two years old, with the age of the horse being measured from the 1st of January. Producing horses born as close to the 1st of January as possible gives them a performance advantage when racing.

The Ewe

In order to produce lambs for the easter markets ewes have to be mated earlier in the year.

Photoperiod control

In the mare exposure to 16 hours light/day from mid november can bring the first ovulation of the breeding season forward from early april to early february, advancing pregnancy by up to two months.

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