Puberty Onset Influencing Factors - Anatomy & Physiology
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Revision as of 14:52, 14 August 2009 by A.allison
Female Threshold Body Size
- Nutritional intake of the newborn is directed towards body maintenance.
- The priority of the neonate is to maintain vital physiological functions.
- Non-essential processes, such as reproduction are of low priority.
- As the neonate grows, energy consumption increases and the body mass increases with a relative decrease in body surface area.
- This allows a shift in metabolic expenditure so that other physiological functions can develop.
- The excess internal energy is converted to fat stores, and a priority is then placed on reproduction so the onset of puberty begins.
- Fat accumulation alone does not determine the onset of puberty, but it is an important regulator.
- All female mammals must reach a certain body size before the onset of puberty can be initiated.
- Metabolic signals affect production of GnRH.
- Massive influence on pubertal onset. The best example is in dairy cattle.
- A major goal in management is to achieve a successful, uncomplicated birth by 24 months of age.
- In order for this to occur, appropriate nutrition and adequate body size must be achieved.
- When fed a high plane of nutrition in order to gain 2.0 lbs/day for the first 12 months of life, heifers reach puberty at 6-8 months. If this diet is continued into the second year of life, it can result in over-conditioning which will lead to problems during parturition and subsequent lactation.
- A second level of nutrition allows heifers to reach the same target weight (1200 lbs at 3 months), but they grow at a uniform weight of 1.5 lbs/day for the entire 24 months. They reach puberty at 9-11 months.
- Heifers fed a restricted diet or low quality feed to gain 1.2 lbs/day mostly reach puberty by 12 months. However, they are too small for successful pregnancy and parturition.. Therefore, they will not calf until 2-3 years of age and are less productive in terms of milk yield.
- Thus, feeding a higher plane of nutrition causes animals to reach the desired weight and exhibit oestrus much faster. But, this is not more economical because milk production during the first and second lactations will be reduced due to incomplete mammary development.
The month of birth will influence the onset of puberty, especially in seasonal breeders.
- Short day breeders (begin oestrous cycles in response to short day length)
- Spring born lambs (February-March) reach puberty in the subsequent autumn (September-October) if they have adequate nutrition. The age at puberty is 5-6 months.
- Autumn born lambs do not reach puberty until 10-12 moths of age.
- This is because, seasonal cues (long days of spring/summer) delay reproductive cyclicity.
- Thus, oestrus in both spring and autumn born lambs is synchronised.
- This maximizes the chance that all ewes become pregnant in the fall and give birth in the spring when nutritional and environmental conditions are favourable.
- An adequate degree of nutrition ('fatness') is required before photoperiod can exert an effect.
- There are sex differences that also have an effect. Ram lambs reach puberty at ~10 weeks of age during summer, but ewe lambs born at the same time (spring) will not reach puberty until 25-35 weeks of age.
- Heifers born in the autumn generally reach puberty before those born in the spring.
- Exposure to long photoperiods and warm temperatures during the second 6 months of life hastens the onset of puberty.
- The sequence of photoperiod exposure makes a difference. Exposure to short days during the first 6 months of life, followed by increasing day lengths in the second 6 months of life is associated with the earliest puberty. This is true of calves born in the autumn with exposure to natural photoperiods.
- Season of birth does not affect the onset of puberty in bull calves.
- Little seasonality associated with the onset of puberty.
- Long day breeders
- Increasing photoperiod prompts onset of puberty.
- Onset of puberty occurs in January-February in the Northern Hemisphere, where there is increasing day length.
- Queens born in February-March do not reach puberty until the following spring.
- Queens born in summer/autumn generally display their first oestrus the following January.
- Before this can have effect, there must be adequate nutrition and growth.
- Breed has an influence on the age of puberty in both males and females.
- Due to this, puberty is not reached at the same age for all males or females of a particular species.
- Dairy cattle reach puberty earlier than beef cattle, as dairy breeds are selected to become productive as early as possible. The emphasis on selectively breeding beef cattle is to maximize growth.
- High yielding dairy cattle reach puberty earlier than lower yielding cattle.
- Larger breeds reach puberty earlier than smaller breeds.
- Small breeds reach puberty at 6-10 months.
- Medium breeds reach puberty at 9-14 months.
- Large breeds reach puberty at 18-20 months.
- There is also a link to growth hormone secretion. Some breeds secrete higher levels of growth hormone, and thus attain puberty faster.
Environmental and Social Conditions
- Pheromonal substances in the urine act as a stimulus to enhance the onset of puberty.
- Females reaching puberty in the presence of a male (producing the pheromones) have a greater opportunity of becoming pregnant.
- Pubertal onset cannot be accelerated using pheromones in animals that have not achieved the appropriate metabolic body size to trigger hypothalamic responsiveness to Oestradiol (oestrogen).
- The size of the social group in which females are housed has an influence over pubertal onset.
- Gilts housed in small groups will have delayed pubertal onset compared to gilts housed in larger groups.
- The presence of a male in either visual or direct physical contact will hasten the onset of puberty in gilts.
- Visual and olfactory environmental information is perceived by sensory neurons of the optic and olfactory systems.
- Stimuli are processed by the central nervous system and converted into neural inputs to the GnRH neurons of the hypothalamus.
- The hypothalamus then gains the ability to produce high frequency and high amplitude pulses of GnRH at an earlier age.