Why was it important that the cattle have 'never yet felt the goad?'

Question

In the Iliad, book 6, there is a line were Hector is speaking to his mother.

"...Get the matrons together, and go with offerings to the temple of Athena driver of the spoil; there, upon the knees of Athena, lay the largest and fairest robe you have in your house - the one you set most store by; promise, moreover, to sacrifice twelve yearling heifers that have never yet felt the goad, in the temple of the goddess if she will take pity on the town, with the wives and little ones of the Trojans, and keep the son..."

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Note: Bold is my own format.

Goad has the meaning of a cattle prod. Why was it important for the young heifers to have never been steered with a goad?

Answer 1

Sacrifices are usually expected to be of high quality and worth, pure, strong or unusual in some way. That's why virgins tend to be sacrificed in many myths and stories or rams, bulls and stallions (i.e. strong examples of masculinity).
This is easily explained as sacrificing something of great value shows the strength of your devotion or desire.

A heifer is a young female cow that hasn't calved, and specifying a yearling enforces the youth aspect. So you're basically talking about a virgin cow. This fits the expected mould of the sacrifice being pure.

The sacrificed cows should be perfect and pure, untouched and unsullied in any way.
Goading usually means using a sharp implement, which is likely to scar or mark the cow's hide in some way. This could be seen to mar their purity or perfection and I would assume this is the reason it was specified the cows should not have felt the goad.