Any claim that hybrids can be produced from this highly disparate cross would require confirmation.
Two turtle-cow hybrids were reported on page 3, columns 4 and 5, of the October 28, 1902, issue of the Evening Times-Republican, a newspaper published in Marshalltown, Iowa (source). The following is a transcript of the report:
A Freak Cow
Waterloo, Oct. 28.—Circus men and museum managers will be after a cow owned by Nicholas Apple, east of Gilbertsville, when her wonderful freak-bearing record is made public. A year ago she gave birth to a calf bearing a mud turtle’s head. The calf was dead, and while it was looked upon as a strange freak of nature no mention was made of the fact beyond the neighborhood. She has now given birth to a second calf more wonderful than the other. The head is precisely that of a turtle, and, stranger still, the back is covered with the same horny shell that grows on a turtle. The calf was dead as in the case of the first, but the skin, head and hoofs were saved and will be placed in some museum.
So then could a mud turtle mate with a cow? Mud turtles are tiny. The Yellow Mud Turtle (Kinosternon flavescens) native to Iowa, is 4 to 6 in (10.2 to 15.2 cm) long. So it seems a Herculean task for one of these little fellows to mate with a cow even once, let alone twice.