Spell:Fertility

Birthright spell

The target becomes very fertile for a brief period with conception several times more likely than usual if they undertake the necessary procreative activities.
The spell is typically used during hires of a prime bull or stallion to ensure that they impregnate as many cows/mares as possible during the hire period.
The spell is used on people usually only when an elderly noble has trouble conceiving. The spell has about a 90% success rate when used on humans, but is less effective on non-humans, particularly sidhe and karamhul who have very low natural rates of conception.

The wizard can aid a female animal to conceive when mated. As prime bulls and stallions are very expensive to hire even for brief periods it can be cost effective to cast this spell on a herd to maximise the number of offspring sired.

The spell is a hedge-wizard spell with no combat value. It should not count towards the maximum number of spells that a wizard can know, kind DMs might even allow a sorcerer to cast it with time to meditate on the desired effect.

Invented by the Basarji for use on there herds of mares this spell has a long history amongst the Khinasi, the Brecht and Anuireans. Amongst the Rjurik it is used solely by druids - often as part of a wedding ceremony! The Vos make no use of this spell.
It is said that Namir ibn Gerad el-Arrasi made frequent use of this spell - on himself - and certainly his harem was noted for its fecundity. Such a use of the spell is deeply frowned upon - a multitude of heirs is no blessing when only one can inherit the bloodline...

If you could add an AD&D conversion (i.e. clerical sphere of the spell, magic resistance %age not Spell resistance, impact on proficiency slot not skill DC) it would increase the potential number of users.