What is foetal ageing?
Foetal ageing is the assignment of an estimated age of a foetus (in days) at the time of ultrasound scanning within the first trimester (within the first 2-3 months) of pregnancy. During this period the foetus undergoes a uniform growth and development pathway that can be easily visualised through real-time ultrasonography. With practice and regular calibration, ultrasound scanners can assess foetal age from the real-time images of the foetus and associated tissues of pregnancy.
What is the value of foetal ageing?
An assessment of foetal age at time of scanning provides a valuable piece of information on the hind: an estimate of the date she conceived. This is calculated simply by subtracting the foetal age (in days) from the date of scanning. In the case of stud herds, if entered for the hind in question into DEERSelect, DEERSelect can rank her on her early conceiving/calving potential and assign an estimated birth date to her progeny.
The data can also be used to check the success of artificial insemination (‘AI’) by enabling a producer to determine whether AI carried out on a particular day was successful, and whether particular sire stags have performed in their duties during the rut, by identifying which stag out of a sequence (where records were kept of stag movements) actually served the hind.
Assigning conception date of hinds in DEERSelect
This will become increasingly important for genetic selection programmes for two reasons:
(1) There is a genetic basis to reproductive seasonality in deer; conception date in hinds is moderately heritable. It is therefore possible to select our breeding herds for an earlier calving season to better match pasture growth with feed demand by deer. The only reliable phenotypic measure of this trait is conception date based on foetal ageing for each breeding hind. This requires that hinds are joined early (i.e. before 10 March) so that they can express their true phenotype (i.e. hinds may cycle early but if stags are not present the hinds will not show evidence of this trait).
(2) Birth date – which is more reliably estimated from estimated conception date than attempted physical monitoring of birthing - is valuable for accurate determination of breeding values for traits such as growth rate. Where a birth date is estimated without reference to a conception date for a progeny crop, calculated growth rates related to weaning date favourably bias the calves born earlier than the estimated date and unfavourably bias the calves born later than the estimated birthing date. Conception date profiling of hinds by foetal ageing will provide a more reliable proxy birth date for each calf, such that other DEERSelect data on the calf will be more reliable as a whole. While this may not be 100% accurate (because there is a degree of natural variation in gestation length), it is a considerable improvement over the present situation of assigning a standard birth date.
Conception date profiling of the herd for calving management
Commercial farms can benefit from improved management of the breeding herd over calving through the use of conception date data. The ability to draft pre-calving hinds into management groups based on expected calving dates (calculated simply by adding 234 days to the estimated conception date for red deer and 240 days for red x wapiti hybrids) facilitates improved feed management by enabling rotational grazing of hind/calf mobs. Delayed set stocking of later calving hinds allows pasture harvesting options and optimisation of feed quality for this mob. Separation of early and late calving hind mobs improves calf survival. A farmer planning precise rotational grazing will know when a mob of hinds and calves can safely be moved between paddocks if a small range of calving dates applies to the hind mobFoetal ageing at scanning provides the necessary data for the farmer to make these decisions.
Checking the success of AI programmes
Although ultrasound scanning does not influence the success of AI programmes, it does give an early measure of success and facilitates future herd management (such as drafting of non-conceiving hinds). All deer AI programmes involve the artificial synchronisation of hind oestrus (heat) for fixed-time insemination. Hinds that fail to conceive to AI will generally return to oestrus and be mated to the back-up stag 18-21 days later. This time interval between AI conception and follow-up mating is crucial to the use of ultrasound scanning to assess the effectiveness of the AI. Scanning is normally performed 40-60 days after AI, with success of AI being based on the presence of an appropriately aged foetus. Hinds conceiving to back-up stags will either be not detectably pregnant (i.e. if pregnant the pregnancy is too young to detect by ultrasound: <20 days) or there will be a very clear age gap of about 20 days.
How accurate is foetal ageing?
With practice, good scanning operators will achieve an accuracy of +/- 5 days of actual conception date. However, this accuracy is dependent on the timing of scanning; it can realistically be achieved when the foetus is between 30 and 70 days of age (late April to early June). Before this period, early stage pregnancies can be difficult to detect and the actual foetus is easily overlooked. After this period, foetal growth and development becomes more variable and it also becomes difficult to acquire suitable images of diagnostic features. Under rectal scanning, larger foetuses often drop below the field of view after 70-80 days. Within the first 70-80 days of gestation there is little effect of foetal genotype on the rate of foetal development, therefore no adjustments are required for wapiti-type animals.
What training is required by scanning operators to do accurate foetal ageing?
The deer farming industry is well serviced by commercial ultrasound scanner operators undertaking routine pregnancy diagnostics. In most cases, additional training and practice will enable most existing operators to accurately perform foetal ageing . The purpose of this manual is to assist scanning operators in assigning foetal ages through calibration of their observations with images collected from known-age foetuses. As foetal ageing is conducted early in pregnancy, it is most likely to be commercially adopted by operators using rectal ultrasound equipment rather than by those that use flank scanners.
How much will it cost compared to normal pregnancy diagnoses?
It is important to negotiate the costs of foetal ageing according to need. Accurate assignment (i.e. +/- 5 days) for purposes of recording data on DEERSelect (i.e. hind conception date and calf birth date) takes longer than a simple pregnancy diagnosis and will likely cost more ($3.50-$5.00 per hind). However, less accurate assessments for purposes of assigning hinds to ‘early’ and ‘late’ calving groups can generally be performed more quickly and at lower cost.
How is it done?
Rectal ultrasound imaging of the pregnancy is performed between late May and mid June, when pregnancies range from 30-80 days of age. To avoid mis-
scanning of late conceiving hinds (i.e. <25 days) as non-pregnant hinds it is advisable to scan at least 30 days after stag removal. Scanning should not extend beyond mid June as the larger foetuses (>80 days) become difficult to clearly visualise.
The scanner operator will estimate foetal age based on foetal size and stage of development, including head and limb development and bone hardening.
These characteristics of the red deer foetus have been well documented for red deer.
Generally, most operators that offer a foetal ageing service will allocate the foetal age to within a 5-day grouping, although for early stage pregnancies (i.e. <45 days) the accuracy of ageing can be as high as 1-2 days.
Tips for foetal ageing
- Try to organise the scanning date to occur during the optimal window of between 30-80 days of pregnancy. This will usually mean scanning mid-May to early June.
- Diagnosis of non-pregnant hinds is most accurate from 30 days after stag removal (all ‘empty’ hinds should be rescanned 20-30 days later).
- Age foetuses into 5-day groupings (as presented in this manual) rather than trying to assign a specific age to each foetus. At best, operator accuracy is ± 5 days anyway.
- Placentome size varies enormously, even within an individual hind, so don’t rely on this for ageing. Try to visualise at least part of the foetus.
- Sometimes it is not possible to obtain a clear image of the foetus (e.g. due to faeces or gas). In such case remove the probe from the rectum, clean faecal matter off the probe, apply more lubricant and try again. Do not apply excessive downward force to the probe as this may damage the rectum.
Show me the science
Revol, B., Wilson, P.R. (1990) Rectal ultrasonographic pregnancy diagnosis and foetal ageing of red deer. Department of Veterinary Clinical Sciences, Massey University, Palmerston North, NZ.
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