Could Old Slavic крава (krava) "cow" and серна (serna) "horn" be cognate?
Could Old Slavic трава (trava) "grass" and терн (tern) "thorn" be cognate?

I specially write Old Slavic forms because I think that:

reconstructed Proto-Slavic *trava is wrong and should be *torva
Proto-Slavic *korva is not a Proto-Celtic borrowing
Proto-Slavic *torva "grass" and *tьrnъ "thorn" are cognates
Proto-Slavic *korva "cow" and *sьrna "horn" are cognates
Proto-Slavic *gorxъ "pea" and *zьrno "corn" are cognates

The first Sanscrit meaning of तृण is трава (trava) "grass"!!!

Also I want to say that Proto-Slavic (Russian) has a large number of words with -вa suffix:

ясти > яства, пасти > паства, лихой > лихва , клять > клятва, жрети > жертва, бити > битва and so on