Repro physiology 11 – Breeding season in animals

The breeding season is...

• the period of the year in which females (male) show normal breeding

activity

The non-breeding season is termed....

• seasonal anestrous

The breeding season is timed so that....

• the young ones are born when feed supply is at its greatest and the

weather is most favorable

Domestication has removed seasonality to some degree

via,

• selection against extremely seasonal animals

• selection for those with longer or earlier onset of a breeding

season

• improved nutrition

From a breeding perspective there are three main groups of

animals:

1. Continuous breeders

2. Seasonal polyestrous animals

3. Mixed breeders

Continuous breeders – are what animals

• Sow, cow

Defintion of Continuous breeders

• Breed all year round but still exhibit some seasonal variation in

fertility

Seasonal polyestrous animals are

• Ewe, hind and doe

Definition of Seasonal polyestrous animals

Multiple estrous cycles but only during a defined breeding season

Breeding seasons occur during

short days (hours of daylight decreasing)

Breeding seasons tend to be poorly

defined near the equator (e.g., Kenya), where photoperiod varies little throughout the year

Mixed breeders are

Mare

Mixed breeders definition

• Some strains/breeds exhibit marked seasonality while others have

little seasonality

Short day breeders

• Breed when daylight to darkness ratio is declining

• e.g., ewe and doe

Long day breeders

• Breed during periods with increasing daylight to darkness ratio

• e.g., mare

Normal ovarian function

occurs in the

breeding season

This process is dependent

on the

hypothalamus and

pituitary responding to the

positive feedback effect of

estrogen produced by large

follicles

During the non-breeding (anestrous) season,

estrogen exerts a

negative feedback effect on GnRH

secretion and pituitary

sensitivity to GnRH

Two principal factors thought to be responsible

1. Photoperiod change detected by the optic nerve and signal sent

to the hypothalamus

2. Signals from optic nerve stimulate pineal gland to produce

melatonin

Melatonin secreted by

§ Secreted by the pineal gland

Melatonin produced during

§ Produced during hours of darkness

Melatonin Has different effects in

short day breeders versus long day breeders

oInduces reproductive activity in short day breeders

oInhibits reproductive activity in long day breeders

Melatonin Also affects fiber growth and color?

yes

oEspecially in wild animals

Melatonin (contd..) § Does it affect the male?

oYes

oSperm production, scrotal circumference – greater during short

days

oSeasonal changes in sensitivity of the hypothalamus to circulating

levels of testosterone

oBut, females are generally more sensitive to changes in day

length

Factors affecting the breeding season

• Season

• Temperature

• Breed origin

• Genetics

• Social

• Nutrition

• Latitude/Altitude

• Stress

• Age

Factors affecting the breeding season Most important factor is the

daylight to darkness ratio

• Season

• Most research is with sheep

• If moved from Northern hemisphere to Southern hemisphere, ewe

will adjust to the new photoperiod

Factors affecting the breeding season (contd..) Ambient temperature

• Naturally tends to reinforce day length changes

• i.e., short days are associated with declining ambient temperatures

• Is less important than photoperiod

Where breed/species originated

• Those that originated from closer to the equator are less seasonal

• e.g., Merino and merino-crosses show longer breeding seasons than

those breeds originating closer to the poles

• Photoperiod changes and changes in feed supply are less

marked

• Other less seasonal sheep breeds include Dorset, Rambouillet, Finn

sheep, Romanov, Karakul, and hairy sheep

• The most seasonal breeds are the British long-wool and meat breeds

• e.g., Lincoln, Leicester, Romney, Suffolk

Genetic factors

• Within species, there are breed differences

• Selection within breed, or crossbreeding can be used to manipulate

breeding season

Social factors - presence of other animals

• Introduction of a ram to ewes just before the onset of the breeding

season can advance the date of first estrus and synchronize estrus

• “Ram effect”

• Social facilitation

• Ewes that are in estrus early may help induce other ewes to

come into heat

Social factors: ram/buck effect

• Ewes will ovulate without estrus (silent heat) 3-6 days

following ram introduction – due to LH surge following

GnRH release in response to the presence of the rams

• Silent heat will be followed by normal fertile estrus 17 days

later (i.e., one estrous cycle length)

• Advancing the breeding season by about 7 days

Factors affecting the breeding season Nutrition

• Effect relatively minor

• Very poor nutrition can decrease body weights that inhibit estrus

activity and therefore delay onset of breeding season

• High levels of nutrition (flushing) may increase lambing % but will

not advance the onset of the breeding season

Factors affecting the breeding season (contd..)

• Altitude/latitude

• Where an animal is living affects the onset of breeding season

• This is due to the effect on day length

Factors affecting the breeding season stress

• Stress can delay onset of breeding season

Factors affecting the breeding season age

• Younger animals tend to be slightly later

Seasonal effects on lactation

• Lactation tends to occur at the same time of year so

observing effects of season on milk production is limited

• It is often difficult to separate effects of nutrition, temperature and

day length

Seasonal effects on lactation (contd..) Temperature and humidity affect milk production

• The level of this effect is dependent on breed

• i.e., Jersey cattle are considered to be more tolerant of hot conditions compared to Holstein-Friesian

• Bos indicus breeds even better at tolerating high temperatures

What happens to animal intake and pasture growth and quality?

• Pasture quality/quantity and intake is reduced

• Animals spend more time in the shade further reducing intake

Thyroid hormone (thyroxine) production

decreases, reducing metabolic activity

Increased energy is used to maintain

body temperature – increased sweating and respiration rate

Very cold weather (cold stress):

• Will also reduce milk yield

• May need to increase heat production

• Thyroxine levels are increased which increases metabolic rate

• If extra feed is available the animal will increase intake reducing

efficiency, if extra feed is not available more energy will be

partitioned towards heat production and less towards milk

production

• Large cows cope better than small cows

• Experienced at different temperatures for different animals

• Exacerbated by wet and/or windy conditions

• Increase heat loss

• Discourage feeding and/or reduce feed availability

• Milk production will fall if food intake falls or if the increase in

intake is insufficient to supply additional ME requirements for

thermoregulation