Repro: Breeding Seasons and OR Flashcards
(26 cards)
What is a “breeding season”?
The period of the year in which females and males show normal breeding activity and fertility
How is the breeding season typically timed?
So that offspring are born when the feed supply is at its greatest and the weather is the most favourable
How has domestication removed seasonality to some degree?
- selection against extremely seasonal animals
- selection for those with longer or earlier onset of a breeding season
- improved nutrition
What are the three main breeding groups?
- non-seasonal poly-oestrous breeders
- seasonal poly-oestrus breeders
- mixed breeders
What are non-seasonal poly-oestrous breeders (and some examples)?
- animals that breed all year round
- pigs
- cows
What are seasonal poly-oestrous breeders? (and some examples)
- animals with multiple oestrous cycles but only during a defined period of time (breeding season)
- sheep
- goats
What are mixed breeders? (and some examples)
- some breeds exhibit marked seasonality while others have little seasonality
- horses
- dogs
What are the types of season breeders? Define them and give examples
- short day breeders: breed when daylight to darkness ratio is declining
- goat, ewe, and doe
- long day breeders: breed when daylight to darkness ratio is increasing
- mares
What two principle factors thought to be responsible for breeding seasons?
- photoperiod changes detected by the optic nerve and signal sent to the hypothalamus
- signals from optic nerve stimulate pineal gland to produce melatonin
How does melatonin affect different seasonal breeders?
- induces reproductive activity in short-day breeders
- inhibits reproductive activity in long-day breeders
How does seasonality affect males?
-outside of the breeding season, males can have less sperm production, scrotal circumference, and testosterone concentration
What factors affect the breeding season?
- latitude/altitude (affect on day length)
- temperature
- breed and genetics (breed originating from the equator are less seasonal)
- other (social interaction, nutrition, stress, age)
Why do some systems aim for high ovulation rates?
- to increase the number of potential offspring (more to be sold, more to select replacements from)
- to increase efficiency
- in some species, multiple offspring can better utilise additional herbage supply grown in spring
Why is high ovulation undesirable in some species?
- metabolic stress of the dam
- greater embryo and fetal loss
- greater dam mortality (mare)
- increased birth difficulties (dystocia)
- lighter live weights at birth and weaning
- lower survival rates
- negative effects on the dam’s live weight and BCS
- limited number of teats to feed the litter (sow)
- mixed sex twins in cattle are usually associated with infertility (freemartins)
What is the dynamic effect in maximising ovulation rate (OR) by nutritional management?
- nutritional flushing
- high levels of nutrition days/weeks prior to breeding can increase ovulation rate
- can increase liveweight depending on the length of the flushing period
- a loss in live weight during this period will reduce OR
Does quality or composition of the diet affect ovulation rate (OR)?
- both energy and protein affect OR
- branched chained amino acids have a positive effect
- tannins can protect proteins from breakdown in rumen so more are absorbed so has a positive effect
- minimise/eliminate exposure to natural inhibitors in the pasture (phyto-oestrogens and mycotoxins)
Explain the max OR without intervention
- max OR in May, resulting in October born lambs
- late in the lambing season if the aim is to have lambs slaughtered by December for peak prices
Explain the max OR when advancing the breeding season
- peak OR in April, resulting in September born lambs
- lambs are, on average, a month older by early December and therefore more likely to reach the target slaughter weight
How can OR be maximised using hormonal manipulation?
- exogenous reproductive hormones to induce ovarian activity and ovulation
- melatonin supplementation to advance the breeding season
- immunisation against endogenous steroids that inhibit high OR (Androstenedione)
How can max OR be altered through genetic selection?
- animals can be selected for increased OR
- OR is of relatively low heritability (very slow progress)
- selecting based on OR is difficult as it can be a difficult trait to measure
What effect does the species breed have on OR?
- within species, breeds differ in OR
- owners can potentially change breed to increase OR
- crossbreeding can be used to increase OR
What are the specific genes that affect OR in sheep?
- the Booroola gene
- the Inverdale gene
What is the Booroola gene?
- first identified in a Merino sheep
- single gene mutation that increases ovulation rate
- one copy: increases OR by 1.4-1.6
- two copies: increases OR by approx. 3.0
What is the Inverdale gene?
- first identified in a Romney sheep
- found on the X chromosome only (up to 1 copy for males, up to 2 copies for female)
- one copy increases OR by 1
- two copies causes small, non-functional, ‘streak’ ovaries (infertile)
