A Gutte Voch,
Despite it being better to use sayros (hairs) from davka an egel (calf) when wrapping tefillin parshiyos, most sofrim I know use from an older cow because the egel hairs are generally too short to wrap around properly (esp by vehoya im shamoah where the sayros must be long to extend externally).
I have recently been told about a species of calf that have longer tails despite being very young and this is ideal for a sofer to use when wrapping tefillin. Can anyone provide more info on this? Does anyone know where such tails can be obtained?