Castration is the method by which animals, both male and female, are deprived of parts of their generative organs ( testicles and ovaries ). The term does seem to be used more often to apply to removal of the testicles from a male animal, whereas 'spaying' is used, to refer to castration of a female.

Chambers's Encyclopaedia 1935 makes the following statement, about the castrated animals, which might amuse feminists; "They are in consequence rendered more valuable for working purposes and also tend to grow and fatten much more quickly for the butcher" (italics mine).

A castrated horse is called a gelding.
A castrated bull is called a bullock, a steer, or a stot.
A castrated boar is called a hog.
A castrated ram is called a wether.
A castrated cock is called a capon.

A spayed sow is called a gelt.