In 'Atlas for identification of foetal skeletal elements of Cattle, Horse,
Sheep and Pig.' Archaeozoologia (1987 I.1 pp23-30.) Wietske Prummel
discusses the fusion of 3rd and 4th metapodia from several breeds of cattle,
quoting Regli (1963) the diaphyses of 3 and 4 begin to fuse at day 200
after conception and at day 210 this process of fusion should be finished.
In Part 2 of the paper (1987 Vol I2, pp11-42) there are drawings, and in
this part Wietske casts doubt on his findings saying that in the youngest
cattle foetus she had studied the metatarsalia 3 & 4 were already in the
process of fusion at c 194 days whereas the metacarpalia 3 & 4 were still
totally unfused. Other examples: both mt & mc fused at 255 days. Cattle
with age est of 259 days mt in process of fusion while mc still totally UF.
Any way, they do seem to fuse before birth as you suggest.
Hope this helps,