dependent dropdowns via #ajax for unlimited option
Use drupal_html_id() to generate unique ids. That is:

$html_id_first = drupal_html_id('dropdown-first-replace');
$form['field_mycollection']['und'][$key]['field_field1']['und']['#ajax'] = array(
  'callback' =&gt; 'custom_dependent_dropdown_callback_year',
  'wrapper' =&gt; $html_id_first,
);
...
$form['field_mycollection']['und'][$key]['field_make']['und']['#prefix'] = '&lt;div id="' . $html_id_first . '"&gt;';
$form['field_mycollection']['und'][$key]['field_make']['und']['#suffix'] = '&lt;/div&gt;';