If you're using Bootstrap Sass, here's another way that avoids having to add extra classes to your element markup:
@import "bootstrap/mixins/_border-radius";
@import "bootstrap/_variables";
.your-class {
$r: $border-radius-base; // or $border-radius-large, $border-radius-small, ...
@include border-top-radius($r);
@include border-bottom-radius($r);
}