To repeat the rule one more time:
When
(1) both parties enter a contract under a mistaken assumption that is
(2) basic to the contract,
(3) the contract is voidable by the party adversely affected by the mistake, unless
(4) that party ought to bear risk of the mistake.
The mistaken assumption that that the cow was infertile was basic to the contract. A fertile cow is worth much more than an infertile one, so Walker would not have sold the cow at the contract price if he had known that the cow was pregnant. So, the contract is voidable by Walker unless he ought to bear the risk of the mistake.
How do we assess whether Walker ought to bear the risk of the mistake?
Recall that a party bears the risk of the mistake if
(1) the contract assigns the risk to that party; or
(2) the party was aware at the time of contracting that he or she had only limited knowledge of the facts relevant to assessing the risk of the mistake and decided to enter the contract anyway; or
(3) it is reasonable, in the circumstances for the party to bear the risk of the mistake.