Is it permissible to offer a female animal as an udhiyah?

With regard to the udhiyah, it is stipulated that it
should be an an‘aam animal (i.e., a camel, cow, sheep or goat), free of
defects, and of the appropriate age as ordained by Islamic teaching.
There is no difference in that regard between male and female, so it is
permissible to offer either as an udhiyah. 

An-Nawawi (may Allah have mercy on him) said in al-Majmoo‘
(8/364): 

The condition of the udhiyah being valid is that it should be
one of the an‘aam animals, which are camels, cattle and sheep (and goats).
All types of camels, all types of cattle and all type of sheep and goats are
acceptable. It is not acceptable to offer anything other than an‘aam
(livestock) animals, such as wild cows, wild donkeys, and so on, and there
is no difference of scholarly opinion concerning this point. It makes no
difference whether the animal is male or female, of any of these types.
There is no difference of opinion among us concerning any of these matters.
End quote. 

The scholars of the Standing Committee for Issuing Fatwas
were asked:

Tell us about the udhiyah: is it acceptable to offer a
six-month old sheep, as they say that it is not acceptable to offer a sheep
or a lamb unless it is one year old. 

They replied:

With regard to the udhiyah, it is not acceptable to offer a
lamb unless it is over six months old, regardless of whether it is male or
female. It is called jadh‘an because of the report narrated by Abu Dawood
and an-Nasaa’i from the hadith of Mujaashi‘ who said: I heard the Messenger
of Allah (blessings and peace of Allah be upon him) say: “A jadha‘
(six-month-old sheep) is acceptable (as a sacrifice) for what a two-year-old
goat is acceptable for.” A goat, cow or camel is not acceptable unless it is
of the appropriate age, regardless of whether it is male or female. In the
case of goats, it is that which has passed the age of one year. In the case
of cattle, it is that which has passed the age of two years. In the case of
camels, it is that which has passed the age of five years, because the
Prophet (blessings and peace of Allah be upon him) said: “Do not slaughter
anything but a two-year-old animal, unless it is too difficult for you, in
which case slaughter a jadh’ah sheep.” Narrated by Muslim.

End quote. 

Fataawa al-Lajnah ad-Daa’imah
(11/414)

For more information on the conditions of the udhiyah, please
see the answer to question no. 36755 

And Allah knows best.