Order posts by custom field

Hi!
I try to order all post by a custom field called “jahr”. I tryed the following code within the function.php of my child theme:

add_filter('posts_join', 'add_join' );
function add_join($pjoin){  
    global $wpdb;
    $pjoin .= "LEFT JOIN (
    SELECT *
    FROM $wpdb->postmeta
    WHERE meta_key =  'jahr' ) AS postmeta ON $wpdb->posts.ID = postmeta.post_id";
    return ($pjoin);
}
add_filter('posts_orderby', 'change_sortorder' );
function change_sortorder( $orderby ){
    global $wpdb;
    $orderby = "postmeta.meta_value+0 DESC, $wpdb->posts.post_date DESC";
    return $orderby;
}

The mentioned code fits but also rss feed ist sorted by the custom field. Therefore new posts are not listed as “new post” in the rss feed when the custom field value is smaller then values of other posts (e.g. “1988” < “2016”). The “all posts” list in the backend is also affected by the custom post order.

How is it possible to have the desired result only visible for visitors and the standard order in the rss feed and the backend?

Thanks for your support!

BR, Patrick